Symposium NM07 : Nanoscale Magnetic Structures and Materials

Over the past few years there have been remarkable discoveries in spin-based phenomena that rely on spin-orbit coupling that could spur the development of advanced magnetic memory devices1-3. These include the formation of chiral spin textures in the form of Néel domain walls and topological spin textures, skyrmions, that are stabilized by a Dzyaloshinskii-Moriya exchange interaction. The Dzyaloshinskii-Moriya exchange interaction is derived from broken symmetries and spin-orbit interactions at interfaces or within the bulk of materials. Recently magnetic antiskyrmions have been discovered in a tetragonal Heusler compound using Lorentz transmission electron microscopy. The antiskyrmion are stable over a wide range of temperature and magnetic field4. We show that the anti-skyrmions become more stable as the thickness of the slab in which the antiskyrmions are imaged is increased. We compare the properties of antiskyrmions with those of skyrmions and chiral domain walls and their possible use in Racetrack Memory2,3.
1 Parkin, S. S. P. & Yang, S.-H. Memory on the Racetrack. Nat. Nano. 10, 195-198, (2015).
2 Yang, S.-H., Ryu, K.-S. & Parkin, S. S. P. Domain-wall velocities of up to 750?ms?1 driven by exchange-coupling torque in synthetic antiferromagnets. Nat. Nano. 10, 221-226, (2015).
3 Garg, C., Yang, S.-H., Phung, T., Pushp, A. & S.P.Parkin, S. Dramatic influence of curvature of nanowire on chiral domain wall velocity. Sci. Adv. 3, e1602804, (2017).
4 Nayak, A. K. et al. Discovery of Magnetic Antiskyrmions Beyond Room Temperature in Tetragonal Heusler Materials. arXiv:1703.01017 (2017).

Topological insulators (TIs), Weyl and Dirac semimetals are new quantum states of matter, which have attracted considerable interest from the condensed matter community. Heusler compounds are a remarkable class of materials which exhibit a wide range of extraordinary multi-functionalities including tunable topological insulators, half metallic ferromagnets and non collinear topological spin structures [1]. Weyl and Dirac semimetals open up new research directions and applications that result from the large Berry phases that they exhibit: these lead to giant anomalous Hall effect (AHE), spin Hall effects (SHE) and topological spin structures. In the C1b Heusler compounds, the inclusion of rare earth atoms allows the use of magnetic exchange fields to induce Weyl points in magnetic fields, which break time-reversal symmetry.
Recently Co₂TiSn and other Co₂-Heusler compounds were found to be Weyl semimetals [2-4]: these materials have an energy-gap for one spin orientation and crossing points in the other spin direction. The Berry phase induces a giant AHE in these ferromagnets [5]. However, even antiferromagnetic Manganese-rich Heusler compounds can be designed with frustrated spins, large Berry curvature as a consequence of Weyl points close to the Fermi energy [6]: this has recently been proven via a giant AHE for single crystals of Mn₃Sn and Mn₃Ge [6,7]. In general Mn-rich Heusler compounds with heavy transition metals such as Mn₂RnSn show a large Dzyaloshinskii-Moriya interaction and therefore non-collinear spin structures. Skyrmions, topologically stable spin textures, are of great interest for new generations of spintronic devices. Depending on the crystal symmetries, two distinct types of swirling of the skyrmions, named Bloch and Neel types. In a family of acentric tetragonal Heusler compounds with D_2d crystal symmetry Skyrmions with a special type of spin-swirling, called antiskyrmions, can even realized. The interplay between the anisotropic exchange and DMI modifies a helical magnetic phase that propagates in the tetragonal basal plane into antiskyrmions arranged on a hexagonal lattice. The flexibility of their manipulation in the present system is demonstrated by the achievement of antiskyrmions up to 400 K and their zero field metastable state at low temperatures [8]. The family of tetragonal Heusler materials including non-collinear spin structures and Skyrmions opens a new spintronics direction including the realization of skyrmionics.
[1] Graf, et al., Progress in Solid State Chemistry 39 1 (2011).
[2] Wang et al., Physical review letters 117, 236401 (2016)
[3] Kübler and Felser, Europhys. Lett. 114, 47005 (2016)
[4] Chang et al., Scientific Reports 6, 38839 (2016)
[5] Kübler and Felser, EPL 108 (2014) 67001 (2014)
[6] Nayak, et al., Science Advances 2 e1501870 (2016)
[7] Nakatsuji, Kiyohara and Higo, Nature 527 212 (2015)
[8] Nayak et al., Nature 548 561 (2017) preprint arXiv:1703.01017

Skyrmions [1] are topologically non-trivial vortex-like spin textures, anticipated for application in spintronic technologies, referred to as skyrmionics, in next generation magnetic data processing and storage due to their facile manipulation by spin-polarized currents of very low magnitude [2, 3]. Moreover, the unique features of skyrmions, e.g., their dynamics, topological structure, competing magnetic interactions, are generally of great interest from a fundamental physics point of view, understanding emerging magnetic field-like interactions induced by topologically non-trivial chiral spin structures. Envisaged applications of skyrmions in magnetic memory and logic devices crucially depend on the stability and mobility of these topologically non-trivial magnetic textures in thin films. In particular knowledge about the full three-dimensional spin texture including its coupling to surfaces and interfaces, ubiquitous in thin film technology, is of fundamental importance.

We combine transport of intensity (TIE) holography, focal series inline electron holography (EH), and off-axis EH to quantitatively reconstruct the projected magnetic field pertaining to both the helical and the skyrmion lattice phase of chiral magnet Fe_0.95Co_0.05Ge. By combining these investigations with electron tomography and magnetostatic simulations of the fields, we extract quantitative information on the 3D spin texture of skyrmions. Experiments are conducted in a double corrected FEI Titan³ 80-300 microscope operated in imaging corrected Lorentz mode. A single crystal Fe_0.95Co_0.05Ge nanoplates is cooled to 90 K using a Gatan double tilt liquid nitrogen cooling holder. The objective lens is excited in such a manner, that an out-of-plane magnetic field of 0 mT and 43 mT is present at the sample, to study the helical and skyrmion phase respectively.

Our experiments yield an in-plane magnetic flux of up to 0.3 T in the skyrmions, which does not agree with the field values expected for z-invariant Bloch skyrmions. The analysis of a cryogenic tilt series of the helical phase results in the observation of a sinusoidal modulation of the Lorentz contrast [4]. Both findings provide for the first time experimental evidence for a characteristic 3D modulation of the skyrmionic spin texture that was theoretically predicted [5]. Instead of a z-invariant Bloch skyrmion, Néel like surface states surround an unmodulated Bloch skyrmion core. Our findings highlight the relevance of surfaces for the formation of skyrmions in thin film geometries and may pave the way towards a surface-induced tailoring of the skyrmion structure.

[1] A.N. Bogdanov and A. Hubert, J. Magn. Magn. Mater. 138, 255 (1994).
[2] N. Nagaosa and Y. Tokura, Nature Nanotech 8, 899 (2013).
[3] N. Kanazawa et al., “Noncentrosymmetric Magnets Hosting Magnetic Skyrmions” (2017).
[4] S. Schneider et al., arXiv:1710.08322 [cond-mat.mtrl-sci] (2017)
[5] F.N. Rybakov et al., New J. Phys. 18, 045002 (2016).

Advances in the development of materials exhibiting the magnetic skyrmion phase have made their use in real-world device applications more practicable. Skyrmions are topologically protected chiral spin textures found in materials with out-of-plane magnetization stabilized by competition between the exchange interaction and the Dzyaloshinskii-Moriya interaction (DMI). While much of the early skyrmion work has focused on bulk B20 materials with broken inversion symmetry, recent studies have used interfacial DMI in thin- and ultrathin film heterostructures to stabilize skyrmions at or above room temperature and zero applied magnetic field. Here, we study the effect of interfacial Rashba DMI on skyrmion stability using in situ aberration corrected Lorentz transmission electron microscopy (LTEM) and differential phase contrast scanning TEM (DPC-STEM).
To vary the effect of Rashba DMI, metals with varying strengths of spin-orbit coupling (SOC) are deposited on thin films of FeGe grown on (111)Si by molecular beam epitaxy. Using in situ LTEM, the influence of proximity is studied as a function of temperature, applied magnetic field, strength of the SOC in each metallic layer, and specimen thickness in the direction of the applied field. By introducing metallic capping layers, it is predicted that skyrmion stability in field/temperature space may be enhanced due to the increase in Rashba DMI.
High spatial resolution electron microscopy enables in situ imaging of nanoscale magnetic structures, helping to further our understanding of the role of defects on the formation and stability of important phases such as skyrmions. In addition to this, complementary compositional, chemical, and structural information can be collected to fully characterize film quality, especially at interfaces. We also correlate in situ imaging experiments with traditional magnetic measurements and theory to more rapidly develop materials systems for skyrmion device applications.

Hybrid materials allow the engineering of new material properties by creative uses of proximity effects. When two dissimilar materials are in close physical proximity the properties of each one may be radically modified or occasionally a completely new material emerges. In the area of magnetism, controlling the magnetic properties of ferromagnetic thin films without magnetic fields is an on- going challenge with multiple technological implications for low- energy consumption memory and logic devices. Interesting possibilities include ferromagnets in proximity to dissimilar materials such as antiferromagnets or oxides that undergo metal-insulator transitions. The proximity of ferromagnets to antiferromagnets has given rise to the extensively studied Exchange Bias[1].
In a series of recent studies, we have investigated the magnetic properties of different hybrids of ferromagnets (Ni, Co and Fe) and oxides, which undergo metal-insulator and structural phase transitions. Both the static as well as dynamical properties of the ferromagnets are drastically affected. Static properties such as the coercivity, anisotropy and magnetization [2-3] and dynamical properties such as the microwave response are clearly modified by the proximity effect and give raise to interesting perhaps useful properties.

The oxide growth and characterization supported by the US-AFOSR and the magnetic studies funded by US-DOE

Selected References:
[1] Exchange Bias, Josep Nogues and Ivan K. Schuller, J. Magn. Magn. Mater. 192, 203 (1999).
[2] Control of Magnetism Across Metal to Insulator Transitions, J. de la Venta, Siming Wang, J. G. Ramirez, and Ivan K. Schuller, App. Phys. Lett. 102, 122404 (2013).
[3] Coercivity Enhancement in V₂O₃/Ni Bilayers Driven by Nanoscale Phase Coexistence, J. de la Venta, Siming Wang, T. Saerbeck, J. G. Ramirez, I. Valmianski, and Ivan K. Schuller, Appl. Phys. Lett. 104, 062410 (2014).
[4] Collective Mode Splitting in Hybrid Heterostructures, Juan Gabriel Ramírez, J. de la Venta, Siming Wang, Thomas Saerbeck, Ali C. Basaran, X. Batlle, and Ivan K. Schuller, Phys. Rev. B, 93, 214113 (2016).

The magnetic structure of magnetite nanoparticles has been studied by polarized neutron small angle scattering. The nanoparticles were prepared by a surfactant method¹ and then washed to remove all but a thin capping layer of oleic acid producing powder-like samples with minimum agglomeration. Transmission electron microscopy (TEM) data reveal a narrow (0.2 nm) distribution of particle sizes about a mean value of 9.0 nm. The nanoparticles self-assemble into face-centered cubic (FCC) crystalline nanoparticle arrays of lattice parameter 13.6 nm with a coherence of about a micron. The existence of this crystalline array is confirmed from small-angle neutron scattering (SANS) measurements that yielded a Bragg peak at a Q = 0.080 A^-1 arising from the dominant (111) reflection of the FCC array.
The addition of incident neutron polarization and post-scattering polarization analysis to the SANS data (PASANS) provides four scattering cross sections, two of them reflect scattering for which the incident neutron spin is preserved in direction (non-spin-flip cross sections) and two for which the spin direction is inverted during the scattering process (spin-flip cross sections). Data were taken both in a remnant (0.005 T) applied magnetic field as well as in a 1.2 T saturating field and at temperatures between 300K and 10K². Analysis of the four scattering cross sections for data taken in a 1.2T field at 200K revealed that the particles consist of a core-shell structure exhibiting an ≈7.4 nm core with ferrimagnetic spin alignment (similar to bulk magnetite) surrounded by a 0.8 - 1.2 nm shell in which the spins are canted ferrimagnetically away from the direction of the applied field and the core spins. The shell thickness and the effective spin canting angle vary with temperature in the range 160K – 300K. The core-shell structure does not form in zero applied field or when cooled to low temperature in zero applied field demonstrating that the shell is of magnetic and not chemical origin. The surface layer canting angle is determined by a competition of exchange, dipolar, anisotropy, and Zeeman energies.³
In an effort to explore the dynamics of these unusual magnetic states, recent inelastic scattering measurements of spin wave energies versus wave vector transfer have been made on a related system consisting of magnetite core|manganese ferrite shell nanoparticles.⁴ These results revealed a quadratic dispersion of the spin excitations with a spin stiffness parameter D = 62 meV-A² significantly reduced from bulk ferrites. Remarkably these data demonstrate that the spin waves originate from dipolar coupling between nanoparticles and not from exchange coupling within individual nanoparticles. The nanoparticles are then effectively modeled as dipolar-coupled superspins.
¹S. Sun et al. J. Am Chem. Soc. 126, 273 (2004)
²K.L. Krycka et al., Phys. Rev. Letters 104, 207203 (2010)
³K.L. Krycka et al., Phys. Rev. Letters 113, 147203 (2014)
⁴K.L. Krycka et al., (submitted)

Sm₂Fe₁₇N₃ was discovered about seven years after Nd₂Fe₁₄B [1,2], and it seemed to offer intrinsic magnetic properties that were superior (T_C, K₁) or comparable (M_s) to those of its famous predecessor. However, the promise of the new material to challenge Nd₂Fe₁₄B did not materialize. The 2:17 nitride powder, prepared by a low-temperature gas-phase interstitial modification process was difficult to orient and was not stable at the temperatures needed to process dense sintered magnets [3]. An early result was zinc-bonded Sm₂Fe₁₇N₃ [4] with an energy product of 84 kJm³ but a rather low coercivity of 480 kAm^-1, less than 5 % of the anisotropy field (2K₁/M_s ≈ 11 MAm^-1). Improvements were later obtained in polymer-bonded magnets produced by injection moulding [5]. Work continued both in Japan and China to develop a coercive powder suited for isotropic bonded magnets. Attempts to make fully-dense magnets by explosive compaction [6] or spark sintering [7] yielded interesting results, but these methods are not suited to large-scale production.

Here we report on the magnetic properties of melt-spun Sm-Fe-N powder, which has superior corrosion resistance and thermal stability compared to melt-spun Nd-Fe-B. The powder, with a crystallite size of approximately 15 nm deduced from Scherrer broadening of the X-ray reflections was in the form of flakes 10 µm thick and up to 100 µm in diameter. Composition was checked by EDX microprobe analysis. The Nitroquench powder exhibits a room-temperature coercivity of 690 kAm^-1after saturation in 14 T, with an isotropic remanence of 700 kAm^-1 in zero applied field and an extrapolated saturation magnetization of 1230 kAm^-1. The remanence ratio M_r/M_s = 57 % (63% if the remanence is measured in zero internal field), is reflected in a preferred orientation seen in ⁵⁷Fe Mössbauer spectra of magnetized powder; spectra obtained after saturating an immobilized powder absorber either in-plane or perpendicular to the sample plane exhibit distinctly different relative intensities of the ΔM = 0 absorption lines. The remanence enhancement is attributed to fact that the nanocrystallite size is not very much greater than the exchange length. The maximum energy product for the powder, assuming full density, is 162 kJm^-3. The Nitroquench powder may be used to produce isotropic polymer-bonded magnets with an energy product > 100 kJm^-3.

Keywords: Permanent magnets; nanocomposites; interstitial intermetallics.

[1] J. M. D. Coey and Sun Hong, J Magn Magn Mater 87 L251-254 (1990)
[2] T. Iriyama et al, IEEE Trans. Magn., 28 2326 (1992)
[3] R. Skomski, Ch 4 in Rare-Earth Iron Permanent Magnets (J. M. D. Coey, editor) Clarendon Press, Oxford 1996, p178
[4] Y Otani et al, J Appl Phys 69 6735-6737 (1991)
[5] K. Ohmori et al. J. Alloys and Compounds 408–412 1359 (2006)
[6] W. Kaszuwara et al, J. Mater. Sci; Materials in Electronics 9, 17 (1998)
[7] T. Saito, J. Magn. Magn. Mater 369 184 (2014)

Recently the computational search for new permanent magnetic phases using data mining and machine learning became popular. However, in addition to the intrinsic magnetic properties the microstructure critically influences the hysteresis properties of permanent magnet. Here we present an automated method for microstructure optimization in permanent magnets based on an active learning algorithm. We synthetically generate a set of suitable microstructures, compute the hysteresis properties for the selected structures using a finite element method and maximize coercivity or energy density product. The results are collected to form a response surface which relates the coercivity or energy density product with key microstructural features. Key design parameters include the grain size, the grain sphericity and the grain aspect ratio. We also introduced grain boundary phases of specified thickness and specific ferromagnetic properties. The simulations show that the grain boundary phase is of utmost importance. The properties of the grain boundary phase influence both the nucleation of reversed domains and the pinning of domain walls. These two typical magnetization processes determine the coercivity of the magnet.

The grain boundary phases in NdFeB are Fe rich. We optimized the composition, 0.1 ≤ x ≤ 0.7, and the thickness, 1.5 nm < t < 6 nm, of the Nd_xFe_1-x grain boundary phase. Thin grain boundaries with a thickness of 1.5 nm and a Nd content of 30 percent maximize the energy density product, whereas the maximum coercive field is reached for the same grain boundary thickness but a Nd content of 70 percent.

Similarly, we optimized the grain shape and the magnetization of the grain boundary phase for a hypothetical magnet made of L1₀ FeNi grains. Interestingly the maximum energy density product of 400 kJ/m³ was found for a relatively high magnetization of the grain boundary phase of µ₀M_s = 1.8 T. The computed values for the coercive field were µ₀H_c = 1 T and 0.63 T for a grain boundary thickness of 3 nm and 6 nm, respectively. This result is promising for the realization of a compact magnet of L1₀ FeNi as it shows that a non-magnetic grain boundary phase is not required to achieve excellent hysteresis properties. Indeed, a difference in the domain wall energy between the grain boundary phase and the bulk is sufficient to establish pinning of the domain walls at the grain boundaries.

In MnAl based magnets crystallographic defects such as twin boundaries and anti-phase boundaries limit the maximum possible coercive field. The coercive field of defect free structures was µ₀H_c = 1.9 T which was reduced to µ₀H_c = 0.36 T under the presence of twin boundaries. Anti-phase boundaries favor demagnetization and thus deteriorate the loop shape and reduce the energy denstiy product.

Work supported by the EU H2020 project Novamag (686056).

Rare-earth-free magnets are highly demanded by clean and renewable energy industries because of the supply constraints and environmental issues. A promising permanent magnet should possess a high remanent magnetic flux density (Br), a large coercivity (Hc), and consequently, a large energy product ((BH)max). BCT phase Fe16N2 has been emerging as one of the promising candidates because of its recently double-confirmed large magnetocrystalline anisotropy (Ku > 1.0x107 erg/cc), giant saturation magnetization (4πMs>2.4 T) and the enough availability of Fe and N on the earth. However, there is no report on the fabrication of bct phase Fe16N2 magnet with high Br and large Hc in bulk form yet. In this talk, I will report our group’s effort in recent ten years to prepare bct phase Fe16N2 permanent magnets by using different approaches, such as ion implantation method, ball milling method and strained-wire method, which can be looked as the dawn light of FeN rare-earth-free permanent magnet. At the beginning of my talk, I will review the mystery of Fe16N2 in our magnetic materials community, following by introducting our effort in past decade to clarify fundamental uncertaities related with this material, including 1) the discovery of the origin of the giant saturation magnetization of Fe₁₆N₂, 2) fabrication of partially ordered Fe₁₆N₂ samples with giant saturation magnetization; 3) finding of the local 3d electron states in Fe₈N and Fe₁₆N₂; 4) finding of the role of Fe6-N octahedral clusters; 5) first-principle simulation based on a model of clusters (Fe₆N) + atoms (Fe);

Novel schemes for controlling the ferromagnetic state at the femtosecond time scale by pulsed laser excitation have received great current interest recently. Driving systems into the strongly non-equilibrium regime, it has been shown possible not only to quench magnetic order by femtoseond laser pulses, but also to drive systems through a ferromagnetic phase transition, and even switch the magnetic moment by single pulses of circularly polarized light. More recently, it has been proposed that pulsed laser excitation can also induce spin currents over several to tens of nanometers, which can act as an additional source of sub-picosecond magnetization dynamics. In this presentation I will focus on two different processes in which spin currents induced by femtosecond laser pulses in especially engineered multilayered magnetic thin films are of relevance: emission of THz spin waves and all-optical switching of magnetization.

I will start with a brief review of the field of fs control of the magnetic state by pulsed laser excitation. Proposed mechanisms for ultrafast loss of magnetic order upon fs laser heating as well as all-optical switching will be discussed. Next, different processes that give rise to laser-induced spin currents will be distinguished. In particular I will address recent experiments that have demonstrated laser-induced spin transfer torque on a free magnetic layer, using a collinear multilayer configuration consisting of a free in-plane layer on top of a PMA injection layer and separated by a nonmagnetic spacer. As it will be shown, these non-collinear fs spin currents are absorbed within a few nanometers, and thereby provide ideal conditions for exciting THz spin waves, their quantum mechanical manifestation called magnons. This allowed us in recent experiments to map out the dispersion of the frequency and the Gilbert damping of thin Co and CoB layers.

In the final part of this presentation, it will be shown how the magnetization of synthetic ferromagnetic thin films systems can be reversed using single fs pulses. Although toggle switching of magnetization in ferromagnetic transition metal/rare earth alloys is known now for almost a decade, for synthetic (multilayered) systems only all-optical switching in a multi pulse scenario was found. Here we show fully deterministic single pulse toggle switching of Pt/Co/Gd trilayers. Exposing the system to an even number of pulses results in the original magnetization, while an odd number of pulses leads to the reversed magnetic state in a reducible way and even after thousands of laser pulses. Threshold fluences are determined as a function of Co thickness and record low efficiencies corresponding to below 50 fJ needed to switch a 50x50 nm² are found. It is argued that also in this case fs laser induced spin currents may provide the driving force for the exciting dynamics.

Increasing attention is being attracted to the physics and materials for spin-orbit torques due to the promise of spin-orbitronics. One popular technique for studying spin-orbit torques is the spin-torque ferromagnetic resonance that uses the intermixing between microwave current and magneto-resistance. I will first mention some aspects related to this technique, including device geometry, angular dependence, out-of-plane tipping field, and their subsequent impacts on the analysis. I will then talk about how antiferromagnets and magnetic interface effects can offer new opportunities for various spin current effects. I will show large yet anisotropic spin-orbit torques measured in metallic antiferromagnets [1] and effective spin harvesting using a spin-flopped, insulating antiferromagnet [2], as well as why these experiments are expected from analytical theory or ab initio calculations. Finally, we want to highlight the fact that, despite the various interesting spin current phenomena revealed in antiferromagnetic insulators, metallic antiferromagnets still offer a fascinating platform for exploring rich effects in spin-orbitronics [3]. Examples of such include a recent result of spin current amplification by a metallic antiferromagnet [4].
[1] W. Zhang et al, PRB 92, 144405 (2015); [2] S. Wu, W. Zhang et al, PRL 116, 097204 (2016); W. Zhang et al, MSE-R 105, 1 (2016); [4] H. Saglam, W. Zhang et al, PRB 94, 140412 (2016).

Long-range ferromagnetic order in emerging two-dimensional atomic crystals, is significant in both fundamental science and technological applications. For the first time, we observed intrinsic ferromagnetism in pristine van der Waals layers Cr₂Ge₂Te₆,¹ via magneto-optic Kerr effect. The prominent thickness-dependent Curie temperatures reveal the strong dimensionality effect and the significant interlayer magnetic coupling across the van der Waals spacing. Remarkably, we showed that, due to the very small intrinsic magnetocrystalline anisotropy and the negligible spurious magnetic anisotropies, Cr₂Ge₂Te₆ exhibits as the first close-to-ideal 2D Heisenberg ferromagnet. It therefore enables an unprecedented access to a regime in which a small magnetic field can effectively open up the spin wave excitation gap, thus substantially controlling Curie temperatures. The fundamental understanding in this work will shed lights on possible avenues of seeking high Curie temperature 2D magnets for practical applications.

[1] C. Gong et al. Nature 546, 265-269 (2017).

Magnetic phenomena are determined by delicate energy balances which determine the magnitude and orientation of magnetic moments as well as domain structures and topology. In transition metal alloys, varying composition and structural parameters allows tuning magnetic anisotropy[1], magnetic phase transitions (e. g. in FeRh)[2], chiral skyrmion structures [3], and many other magnetic characteristics. Here we are focusing on two examples: Tuning the stoichiometry in Fe_xCo_yMn_1-x-y to enhance/ the magnetic moment and varying the Co layer thickness in a (Cu_3MLCo_3-10MLPt_2ML)₁₀ stack to engineer the transition from a chiral magnetic stripe phase to non-chiral and closure domain structures.

The average magnetic moment in binary 3d transition magnetic alloys is known/has been shown to follow the Slater Pauling Curve. A peak magnetic moment of 2.4 μ_B/atom is achieved for Fe₇₀Co₃₀.
Here we have explored the average magnetic moment of ternary alloys Fe_xCo_yMn_1-x-y. Moreover, using epitaxial strain, we have locked-in a bcc crystal structure up to unexpectedly high Mn concentrations, up to 35%, whereas the bulk material converts to the non-magnetic fcc structure above ~12% Mn. [4] Using x-ray magnetic circular dichroism we determine the magnetic moment for each element for a variety of compositions covering around 80% of the ternary phase diagram and find that for some compositions the average magnetic moment collapses, whereas for others, which are unstable in bulk form, it reaches ~3.25 μ_B/atom, exceeding the moment of Fe₇₀Co₃₀.

Another intriguing example of magnetic engineering are (Cu_3MLCo_3-10MLPt_2ML)₁₀ multilayers with varying Co thickness. Changing the Co thickness, changes the impact of the interface induced Dzialochinsky-Moriia interaction (DMI): The interface anisotropy energy is unaltered, whereas the dipolar energy of the system and the cost of a domain wall increases. This leads to a transition from a chiral magnetic stripe phase through a non-chiral stripe phase to the formation of Néel closure domains. We follow this transition using x-ray spectroscopy and scattering, and x-ray PEEM.

These are two intriguing examples show important magnetic characteristic can be carefully engineered using compositions and layer thicknesses in 3d transition metal systems.

WTe₂ has been heavily researched due to its extremely large non-saturating magnetoresistance, quantum spin hall state in the monolayer limit, and other novel properties. In this work we demonstrate the successful growth and characterization of single-crystalline WTe₂ nanowires and report the electrical properties of the synthesized nanowires. WTe₂ has a crystal structure that leads to many anisotropic properties, and by studying WTe₂ at limited dimensions, we can isolate interesting physical phenomena. Our nanowires approximate a one-dimensional system, providing a platform for the study of fundamental physics and development of future technological applications.

Flexible spin valves were fabricated with single-crystalline two-dimensional (2D) MX₂ (M = Mo, W; X = S, Se) monolayers as spacer layer. Ferromagnetic metals were used as top and bottom layers in the flexible spin valve devices. The spin valve effects are studied at room- and variable -temperatures. The temperature dependence tunneling magnetoresistance of the spin valves is studied and analyzed. This research paves the way for possible flexible spin valves composed of all 2D layers (2D ferromagnetic layers and spacer layer) in the future towards ultra- thin and small memory devices in flexible electronics.

Chiral magnets are a series of magnets with broken inversion symmetry. A new type of spin interaction therein, the Dzyaloshinskii-Moriya interaction, stimulates the formation of many novel topological spin textures. One typical example is the emergence of magnetic skyrmion, whose nontrivial topology enables unique dynamical property and thermal stability and gives out promise on future magnetic memory devise. Inspired by skyrmions, in this talk, I will present three other relevant spin textures in chiral magnets. One is the target skyrmion we recently observed, both theoretically and experimentally, in ultra-small nanodisks of chiral magnets. Zero-field target skyrmions and their polarization switch will be discussed. Putting in heterostructures, we also found a new type of topological configuration dubbed the Hopfion therein. Finally, I will discuss emergent topology driven by thermal fluctuations. This work is supported by the grant DE-SC0016424 funded by the U.S. Department of Energy, Office of Science.

The emergence of a topologically nontrivial vortex-like magnetic structure, the magnetic skyrmion, has launched new concepts for memory devices. Extensive studies have theoretically demonstrated the ability to encode information bits by using a chain of skyrmions in confined geometries. So far it is generally assumed that skyrmion is two-dimensional (2D) localized magnetic structure. In this talk, we report experimental evidence of three-dimensional (3D) skyrmions in confined helimagnets. The 3D skyrmion will give rise lots of novel phenomena that can not be observed in 2D one. Moreover, we report the discovery of another 3D localized spin texture, termed as magnetic bobber, in chiral magnets.

Topology, both in real and momentum space, is the source of some of the most interesting phenomena in condensed matter physics. For example, chiral spin textures can give rise to the topological Hall effect [1]. The underlying interactions giving rise to chiral spin textures and skyrmions are however less well understood. For example, Dzyaloshinskii-Moriya (DM) interaction may not be needed as helimagnetic phases are also reported in centrosymmetric structures. As will be shown here, electron doped EuTiO3 is an interesting material to study the influence of topology on a wide range of transport phenomena. Stoichiometric EuTiO₃ is a quantum paraelectric with cubic perovskite structure at room temperature, similar to SrTiO₃. Spins on the Eu site [4f⁷ (s=7/2)] order antiferromagnetically at the Neel temperature of ~5.5 K.
Here, we investigate high quality, single crystal EuTiO₃ thin films grown by hybrid molecular beam epitaxy. We report the results of low temperature magnetotransport and magnetization measurements. While the stoichiometric EuTiO₃ films are highly resistive, Sm-doped samples (n_RT=1.2, 3.4, 6.5 and 8.7×10²⁰ cm^-3) show metallic behavior [2]. The topological Hall effect (THE) is observed in all samples, regardless of the carrier concertation. Furthermore, THE peak experiences a sign change with carrier concentration. The sign change is concurrent with a drastic change in the magnetic properties of doped EuTiO₃ thin films. Dilute samples show a carrier-controlled itinerant metamagnetism with a very narrow magnetic hysteresis. Such metamagnetic transitions can be the source of very interesting phenomena in condensed matter physics including quantum critical fluctuations [3]. Here, the results are not conclusive whether there is a cross-over, or a first-order phase transition accompanied by a low temperature critical point. However, the resistivity data from 2 to 5 K under various magnetic fields (0-9 T) using the general expression R(T)=R₀+ATⁿ for transport, shows evidence of “n” dropping and “A” increasing. The temperature coefficient is directly proportional to carrier mass and an increase in “A” hints a carrier mass enhancement.

[1] N. Nagaosa and Y. Tokura, Nat. Nanotechnol. 8, 899 (2013).
[2] K. Ahadi, L. Galletti, and S. Stemmer, Appl. Phys. Lett. 111, 172403 (2017).
[3] S. A. Grigera, R. S. Perry, A. J. Schofield, M. Chiao, S. R. Julian, G. G. Lonzarich, S. I. Ikeda, Y. Maeno, A. J. Millis, and A. P. Mackenzie, Science 294, 329 (2001).

The Rashba effect, a spin splitting in electronic band structure, attracts much attention for the potential applications in spintronics with no requirement of external magnetic field. Realizing one-dimensional (1D) Rashba system is a big challenge due to the difficulties of growing high-quality heavy-metal nanowires or introducing strong spin-orbit coupling (SOC) and broken inversion symmetry in flexible materials. Here, based on first-principles calculations, we propose a pathway to realize the Rashba spin-split by adsorbing Gd atom on zigzag graphene nanoribbons (Gd-ZGNR) and further investigate the magnetic anisotropy energy (MAE). Perpendicular MAE and unconventional MAE contributions in k-space are found in the self-assembled Gd-ZGNR system, which presents a remarkable Rashba effect (the estimated strength is 1.89 eV Å) due to the strong SOC (~65.6 meV) and the asymmetric adsorption site at nanoribbons edge. Moreover, first-order MAE is connected to the intrinsic Rashba effect beyond the traditional second-order MAE, which is confirmed based on the analysis of electronic structures perturbed with SOC in comparison with metastable Gd-ZGNR at central symmetric adsorption site. The dependence on the ribbon width of first-order MAE as well as Rashba effect in Gd-ZGNRs are also examined. This work not only opens a new gate for designing 1D Rashba system but also provides insight into the unconventional MAE due to the intrinsic Rashba effect, which would be of great significance for searching Majorana fermions and promoting the potential applications in spintronics.

Conductive atomic force microscopy (C-AFM) enables scanning magnetoresistance measurements of magnetic tunnel junction (MTJ) nanostructures. It can be used to image the tunnel current in an array of MTJs, which is beneficial both for studying size-dependent behavior and also for characterizing the distribution of switching properties among devices of the same size. The resistance of individual MTJs can be controlled either by a magnetic field, or electrically with a bias voltage or current.

Voltage-controlled magnetic anisotropy is a feature of MTJs with perpendicular magnetization due to thin CoFeB layers on either side of the MgO tunnel barrier. MTJs ranging from 18 to 500 nm were characterized as a function of magnetic field to determine the effective anisotropy of the free layer. When the free layer was metastable, the tunnel current showed random telegraph noise. Reversal occurs by nucleation and domain wall motion. Due to the magnetostatic field of the fixed layer, nucleation for an antiparallel to parallel switch occurs most often near the edge of the free layer.

A charge current passing through a heavy metal beneath the MTJ generates a spin orbit torque that can switch the adjacent magnetic layer. While magnetic random access memory (MRAM) devices based on this switching mechanism would ideally be sub-100 nm and would have a high thermal stability factor (60-80), measurements of spin orbit torque switching have focused on larger structures with lower thermal stability. Here we show how CAFM can detect reversal of a 20 nm nanomagnet with a thermal stability factor of 85. From our results, the charge current density needed for spin orbit torque switching is estimated to by only 15% of that needed for spin transfer torque reversal, and has an estimated write energy of 0.1 fJ. This approach is promising for low power MRAM.

A third type of tunnel junction uses thicker CoFeB layers and has in-plane magnetization. These MTJs are interesting for probabilistic computing because an applied bias voltage or current can stimulate superparamagnetic behavior with zero applied magnetic field. We show how a voltage can tune the time-averaged resistance of the MTJ, and how output from multiple MTJs can be used in logic gates and simple arithmetic.

Spin Transfer Torque Magnetic Random Access Memory (STT-MRAM) is a type of emerging memory which holds the promise of high speed, high endurance, non-volatility, and good scalability. Since the theoretical prediction of the STT switching mechanism in 1996, significant progress has been made in the field, largely through materials innovations. In this talk, I will review the key materials discoveries that enabled the advancement of STT-MRAM technology. This includes the theoretical prediction and experimental realization of large tunneling magneto-resistance (TMR) with MgO tunnel barrier and the discovery of CoFeB based materials with interfacial perpendicular magnetic anisotropy (iPMA). I will also discuss the critical materials parameters impacting STT-MRAM device switching performance, for example, the Gilbert damping constant and exchange stiffness of the free layer material.

Reversibly controlling the nanoscale magnetization at room temperature by electric field means would enable the development of various spintronic devices, in particular, novel magnetic information storage devices. Although several approaches have been developed so far to achieve electrical modulation of magnetization reversal, most of these methods suffer from practical issues that hinder them from direct applications. In this talk, we will present our recent progresses on nanoscale magnetization reversal caused by electric field-induced ion migration in oxide thin films. We observed that in ferrite films the nanoscale magnetization can be reversibly and nonvolatilely reversed at room temperature via an electrical ion-manipulation approach, wherein the application of electric fields with appropriate polarity and amplitude can modulate the size of magnetic domains with different magnetizations up to 70 % [1, 2]. We also utilized the high-throughput synthesis approach, namely, combinatorial substrate epitaxy, to understand the degree of ionic migration in different orientations [3]. It was determined from the analysis that the [110] crystal direction exhibits the maximum nanoscale magnetization reversal ratio. This is mainly attributed to the ease Co²⁺ migration in the [110] direction under the electric field assisted by a Fe³⁺ and oxygen vacancies.
Reference:
Xinxin Chen, Xiaojian Zhu, Wen Xiao, Gang Liu, Yuan Ping Feng, Jun Ding, and Run-Wei Li, ACS Nano. 9, 4210–4218 (2015)
Xiaojian Zhu, Jiantao Zhou, Lin Chen, Shanshan Guo, Gang Liu, Run-Wei Li, and Wei D. Lu, Adv. Mater. 28, 7658–7665 (2016)
Pravarthana Dhanapal, Shanshan Guo, Baomin Wang, Huali Yang, Sandeep Agarwal, Qingfeng Zhan, and Run-Wei Li, Appl. Phys. Lett. 111, 162401 (2017)

Barium hexaferrite (BaFe₁₂O₁₉ - HF) nanoplatelets display a high uniaxial magnetocrystalline anisotropy with an easy axis perpendicular to the platelet. Due to this unique property, they show tremendous potential in innovative applications, especially in relation to their ability to be effectively aligned with an applied magnetic field. However, for some applications their saturation magnetization M_S should be increased. As a hard-magnetic material, HF displays a rather modest M_S, 72 Am²/kg in the bulk, while in the form of nanoplatelets the M_S is significantly smaller, up to 35 Am²/kg. The M_S of the nanoplatelets can be effectively increased by coating them with a shell of soft-magnetic iron oxide maghemite (γ-Fe₂O₃). If the two phases are magnetically exchange-coupled the formed composite nanoplatelet homogeneously magnetizes like a single-phase particle.
In this work the composite nanoplatelets were synthesized using the deposition of epitaxial maghemite (M) layers onto the HF nanoplatelet cores dispersed in an aqueous suspension. The layers were deposited by a heterogeneous nucleation and growth on the surfaces of the core nanoplatelets during the precipitation of Fe²⁺/Fe³⁺ ions. The composite nanoplatelets were characterized using an aberration-corrected scanning-transmission electron microscope (HAADF STEM) in combination with other methods (TEM, EDXS, XRD, Raman). The magnetic properties were thoroughly analyzed.
Atomic-resolution imaging revealed that the synthesized composite nanoplatelets display an incredibly uniform structure of the “sandwich”-type, with a HF core in between two M layers. As the core nanoplatelets adopt a distinct structure and composition, which are significantly different to the bulk, they can be considered as novel structural variations of hexaferrite stabilized on the nanoscale. The HF structure is characterized by two alternating structural blocks, i.e., a hexagonal “R” block ((BaFe₆O₁₁)^2-) and a cubic “S” block ((Fe₆O₈)²⁺), stacked along the c-direction of their hexagonal structure (RSR*S*, * denotes the rotation of 180 °around the c-axis). The nanoplatelets contain only two, or rarely three, R blocks and always terminate with the full S block at the basal surfaces. The vast majority of the nanoplatelets showed the SR*S*RS stacking, corresponding to the composition of Ba₂Fe₃₀O₄₆. Given the cubic S block termination of the platelets, layers of maghemite with a cubic spinel structure, can be easily grown epitaxially on the basal surfaces of the platelets. The epitaxial M layers deposited on the cores are of very uniform thickness.
Magnetic measurements proved the exchange coupling between the two magnetic materials. The exchange-coupled composite nanoplatelets exhibit a remarkably uniform structure, with an enhanced M_S of more than 50 emu/g while essentially maintaining the out-of-plane easy axis. The enhanced M_S could pave the way for their use in diverse platelet-based magnetic applications.

Great efforts have been taken to reveal the intrinsic origins of emerging ferromagnetism (FM) in strained LaCoO₃ (LCO) films, different from LCO bulks. However, even macro magnetic performances of LCO are still not well understood, such as magnetic anisotropy. Understanding magnetic anisotropy might help to find the true causes of FM in turn.
Perpendicular magnetic anisotropy (PMA) is the first time to be directly observed in high quality LCO films with different thickness. The in plane (IP) and out of plane (OOP) remnant magnetic moment ratio of 30 unit cell (u.c.) films is as large as 20. The easy axis lays in the OOP direction with an IP/OOP coercive field ratio of 10. What`s more, the PMA could be simply tuned by changing the thickness. With the thickness increasing, the IP/OOP magnetic moment ratio remarkably decrease with magnetic easy axis changing from OOP to IP. Such a huge and tunable PMA performance exhibit strong potentials in fundamental researches or applications.

What causes PMA is the first concern. More OOP orbitals occupation may be one of the micro reasons of PMA. A cluster-like magnetic domain pattern was found in 30 u.c. with no obvious color contrasts, similar to that of LaAlO₃/SrTiO₃ films. And the nanosize domains couldn`t be totally switched even at a large OOP magnetic field of 23 T. It indicates strong IP characters or none OOP magnetism of some clusters. The IP magnetic domains might influence the magnetic performance and help to form PMA. Meanwhile some possible nonmagnetic clusters might be the reason why the measured moments of LCO films are smaller than the calculated values 2 μ_B/Co, one of the biggest confusions in LCO films.

What tunes PMA seems much more interesting. Totally different magnetic domain patterns were found in 180 u.c. films with cluster magnetic domains surrounded by <110> cross-hatch lines. These lines were regarded as structure domain walls (DWs) determined by 3D reciprocal space mapping (RSM). Two groups of in-plane features with fourfold symmetry were observed near the film diffraction peaks in (002) 3D-RSM. One is along <110> directions with a larger intensity, which is well match the lines on the surfaces. The other is much weaker and along <100> directions, which is from the normal lattice titling of films deposited on cubic substrates. The <110> domain features obtained from (103) and (113) 3D-RSMs exhibit similar evolution of the DWs percentages and magnetic behavior. Structure domains and domain walls are believed to tune PMA performances by transform more IP magnetic moments to OOP.

Last but not the least, thick films with lots of structure domains exhibit novel electrical transport behaviors, different from those of thin films. A metal-to-insulator transition (MIT) and an angular dependent negative magnetic resistivity were observed near 150 K, a temperature higher than FM transition temperature (90 K) but similar to that of spin-orbital coupling related 1/4 order diffraction peaks.

The quest for the search of multiferroic materials with transition temperatures above room temperature are due to their technological importance and novel applications such as in sensors and logic, memories and multifunctional devices. Gallium Ferrite (GaFeO₃, GFO) is known to be piezoelectric and it is near room temperature ferrimagnetic with significant magnetoelectric (ME) coupling (10⁻¹¹ s/m at 4.2 K). Herein phonons, magnetic ferroelectric ordering in perovskite GFO have been investigated using magnetization, dielectric and Raman scattering measurements as a function of temperature. Single phase GFO ceramics is prepared by solid state reaction method at optimized calcined temperature of 1573 K. Studies of X-rays patterns indicated that the titular compound stabilizes in orthorhombic phase (space group C_2v⁹). Stoichiometry of the elemental composition is confirmed using energy dispersive X-rays analysis and X-ray photoelectron spectroscopic techniques. Temperature dependent magnetization behavior studied at both field-cooled and zero-field-cooled conditions between 5- 395 K using several static magnetic fields (such as 100 Oe, 500 Oe and 1000 Oe) identify the Néel transition (T_N) at around 225 K. Evidence of spin-glass like magnetic ordering is observed at low temperature. Phonon frequencies and its line-widths exhibit anomaly across T_N. The changes in phonon frequencies observed in Raman scattering are explained based on spin-lattice coupling in the low temperature ferrimagnetic phase. Examination of the phonon behavior indicates that three phonon anharmonicity process is dominant. The temperature and frequency dependent dielectric measurements indicated a dielectric anomaly at around 590 K suggesting that the system exhibits ferroelectric relaxor behavior. The detailed experimental results and their physical correlations will be presented in the meeting.

Single-crystal iron germanium nanowires were synthesized on germanium wafer via high-pressure chemical vapor deposition without the assistance of any catalysts. Magnetic measurements of FeGe2 nanowire arrays show strong anisotropic behavior: temperature dependent magnetization and field dependent magnetization and ac susceptibility show the mixture of ferromagnetism and antiferromagnetism. Nanowires integrated nanodevices with four electrodes were successfully fabricated using optimizd e-beam lithography process with e-beam dose correction and double layer geometry. The resistivity of isolated nanowires shows two resistive anomalies around 250 K and 200 K which correspond to abnormal resistivity changes at the magnetic transitions of the SDW state in bulk FeGe2, but are greatly enhanced by dimensionality effects under confinement. The ability to control and isolate these dimensionality effects in an antiferromagnetic system thus offers a tantalizing path forward in understanding and creating novel spin-dependent applications in the emerging fields of antiferromagnetic storage, resistive switching, and spintronics.

We report a new strategy to prepare isotropic and anisotropic SmCo₅-based nanocomposite magnets.

First we coated Fe nanoparticles (NPs) with a layer of SiO₂ and co-assembled the Fe/SiO₂ NPs with the pre-synthesized Sm(OH)₃ nanorods and Co(OH)₂ nanoplates. Reductive annealing of the assembled mixtures at 850^oC in the presence of Ca led to the formation of SmCo₅-Fe/SiO₂ composites. Washing the composites with aqueous NaOH solution followed by compaction under 1.5 GPa at room temperature yielded the exchange-coupled SmCo₅-Fe nanocomposites with the SmCo₅ phase having an average size of 68 nm and Fe NPs preserved at 12 nm. The isotropic nanocomposites show Fe NP content dependent magnetic properties with their M_s tunable from 43 emu/g (for the pure SmCo₅) to 78 emu/g (for the SmCo₅-Fe(20 wt%)) while H_c from 20.1 to 11.2 kOe.

The Sm(OH)₃ nanorods were further explored as the starting precursor to prepare anisotropic SmCo₅. In the process, the pre-synthesized Co NPs were assembled onto the Sm(OH)₃ nanorods. The composite was embedded in CaO matrix and annealed at 850^oC in the presence of Ca to form SmCo₅ nanoplates with an average lateral dimension of 200 nm. Washing with water to remove CaO and mixing SmCo₅ in resin under an external magnetic field led to the desired alignment of SmCo₅ with room temperature H_c ~30 kOe and M_s ~55 emu/g. We are working to prepare anisotropic SmCo₅-Fe with tunable magnetic properties for high performance permanent magnet applications.

Our group has been studying the magnetic and structural properties of novel magnetic nanoparticles for the past few years. Our specific focus is on nanometer-length-scale and real structure control of new or metastable structures as a means of creating materials with high magnetization, high spin polarization, large magnetocrystalline anisotropy and high ordering temperatures using non-equilibrium fabrication techniques. In the last couple of years our work has been focused on nanoclusters made by the cluster beam deposition (Co₂Ge, Mn₅Si₃, Fe₅Si₃, Co₃Si) [1-2] and L1₀ FePt (CoPt) made by chemical synthesis. These types of materials may have potential impact in ultra-strong permanent magnets and extremely high density magnetic recording media.
Our experimental and theoretical results show unique and interesting properties in nano-size particles which are drastically different from bulk. For example, in Co₂Ge and Mn₅Si₃ nanoparticles, high Curie temperatures of around 820 K and 590 K, respectively were found as compared to bulk systems which are paramagnetic at room temperature. The enhancement of Curie temperature in Co₂Ge is believed to be due to both size effects and surface segregation of Co atoms while in the case of Mn₅Si₃ is attributed to a different electronic structure in the nanoparticle surface. Also, Fe₅Si₃ nanoparticles show a higher Curie temperature (by about 50% of bulk) and enhanced magnetization which are attributed to a large spin polarization at the cluster surface. In Co₃Si nanoparticles, a large coercivity of 4.3 kOe at room temperature was obtained, despite the easy-plane anisotropy in bulk, which can be explained by the combined effects of exchange interactions between the particles and their field alignment during deposition. L1₀ FePt and CoPt nanoparticles were fabricated at low temperatures (350 ^oC) by liquid phase synthesis using Bi doping without the need of post annealing. FePt and CoPt nanoparticles exhibit coercivities of 13.6 kOe and 1.4 kOe, respectively. Surface segregation of Bi atoms was found to enhance the low temperature ordering of the L1₀ structure. The Curie temperature of the fcc and L1₀ phase particles in both systems show an unexpected trend; increasing ordering in FePt leads to an increase in Curie temperature, while an increase in ordering in CoPt leads to decrease in Curie temperature.
This work is supported by DOE DE-FG02-04ER46152 and DE-FG02-90ER45413
References
[1] B. Das, B. Balasubramanian, P. Manchanda, P. Mukherjee, R. Skomski, G. C. Hadjipanayis, and D. J. Sellmyer, Nano Letters 16 (2), 1132 (2016)
[2] Balamurugan Balasubramanian, Priyanka Manchanda, Ralph Skomski, Pinaki Mukherjee, Shah R. Valloppilly, Bhaskar Das, George C. Hadjipanayis, and David J. Sellmyer, applied physics letters 108, 152406 (2016)

Monodisperse ferrimagnetic Co_xFe_3-xO₄ nanoparticles (NPs) are synthesized through chemical methods which are readily dispersed in hexane, forming stable ferrimagnetic NP dispersion and allowing easy NP self-assembly. When assembled under an external magnetic field, these NPs show the preferred magnetic alignment with their H_c reaching 2.49 kOe. ^{1, 2}
Chemical synthesis of L1₀-FePtAu NPs will be also introduced. The highest Hc of 12.2 kOe can be achieved, which is much higher than the Hc reported by the previous studies on solution-synthesized FePt NPs. The phase separation between Au and FePt phase likely is the main reason for the ordering of FePt phase. The reported one-pot synthesis of L1₀-FePtAu NPs may provide an ideal class of building blocks for magnetic energy and data-storage applications.³
In addition, we also report a new approach for preparation of exchange-coupling L1₀-FePt/Fe nanocomposite magnets with gradient interface between hard phase and soft phase⁴ and L1₀-FePd/Fe nanocomposite magnets with controllable morphology and compositions⁵. These works provide a general approach to L1₀-FePt(Pd)/Fe nanocomposite magnets for understanding exchange-coupling in nanoscale.
1.Y. S Yu et al. Advanced Materials 2013, 25, 3090-3094
2.Y. S Yu et al. Nanoscale 2015, 7, 2877-2882
3.Y. S Yu et al. Nanoscale 2014, 6, 12050-12055
4. Y. S Yu et al. Journal of Materials Chemistry C, 2015, 3, 7075-7080
5.Y. S Yu et al. Nano Letters, 2013, 13, 4975-4979

With increasing demand for high performance permanent magnets in energy generation and conversion systems, the quest to develop novel processing routes to produce such materials has become urgent. Nd-Fe-B based permanent magnets, due to their superior magnetic properties, have attracted intensive attention. In this alloy, cobalt substitution of iron can increase the Curie temperature. However, conventional physical processing methods are associated with high cost, and chemical synthesis methods have limited scalability. Hence, we report a cost-effective and scalable mechanochemical process for synthesis of high coercivity Nd₂(Fe,Co)₁₄B nanoparticles. Nd₂(Fe,Co)₁₄B nanoparticles with a size of 40 – 150 nm and coercivity up to 8.8 kOe was obtained by milling Nd₂O₃, Fe₂O₃, CoO and B₂O₃ in the presence of Ca and CaO diluent, followed by annealing and removal of the by-product. The formation mechanism was investigated for the first time to understand the process and to facilitate process parameter optimization. The formation mechanism changed with increasing CaO diluent content, the average crystal size of the Nd₂(Fe,Co)₁₄B nanoparticles also increased, resulted in an enhancement in coercivity values. The reduction kinetics of the mechanochemcial process were also studied by determining the change in the concentration of reactants and products as a function of milling time. It was found that unlike self-propagating reactions, this reduction reaction during milling requires continuous input of mechanical energy to reach a steady state. The impact energy of the milling event activates the chemical reactions and was found to play a role analogous to that of thermal energy in the reduction-diffusion process. Our experimental data was found to fit well with a model of the kinetics of the mechanochemical process.

Nanoparticle (NP) clusters generated via a facile microemulsion method has proven promising for a wide variety of applications. Herein, we demonstrate how nanoclustering of CoO and CoFe₂O₄ NPs can lead to synergistic interfacial effects. Different from core/shell methods, our method can optimize magnetic NPs as individual components before clustering, facilitating the improvement of novel magnetic materials. The advantage of this method is showed through studying the magnetic properties of nanoclusters combining antiferromagnetic CoO and superparamagnetic CoFe₂O₄ NPs with tunable size and ratio. It shows that close interparticle interactions is achieved with an enhancement of coercivity compared with pure CoFe₂O₄ NPs. In addition, a large exchange bias field of 0.32 T was observed after annealing, which is more than twice larger than that in any other related systems. Furthermore, Scanning electron microscope (SEM) images reveal a compartmentalized microstructure of nanoclusters that may help maintain magnetic coercivity after the annealing process. Overall, this work develops a general approach for studying the magnetic interactions within NPs, functionalizing NP assemblies, and constructing high energy product, low cost bulk magnets.

Technical approaches have been developed to produce cobalt nanowires (NWs) with high coercivity. The magnetic hardening of the NW assemblies originates from the magnetocrystaline anisotropy and the shape anisotropy, which strongly depends on the aspect-ratio and uniformity of the NWs. Single crystalline hcp Co NWs with the aspect ratio from 10 to 66 are synthesized via a solvothermal method by controlling the growth process. The increased aspect ratio leads to enhanced coercivity of aligned Co nanowire assemblies up to a 12.5 kOe at 300 K, which is closed to the theoretical limit of the coercive field (14.3 kOe). As a result, the energy product of the wires reaches 44 MGOe. To further understand the coercivity mechanism in the NWs, angular dependence of the coercivity has been experimentally investigated for the aligned NW assemblies and the corresponding magnetization reversal mode is determined to be a coherent reversal mode according to an analytical simulation based on the Stoner-Wohlfarth model. The as-synthesized Co nanowires are susceptible to oxidation when exposed to air and thus they form the shell of native oxides (CoO) on the Co nanowire surface. Owing to the exchange coupling between the ferromagnetic (FM) Co core and the antiferromagnetic (AFM) CoO shell of the NWs, a giant exchange-bias field (H_EB) up to 2.4 kOe is observed below a blocking temperature (T_EB∼150 K). This study shows that single domain ferromagnetic nanowires (NWs) can be promising building blocks for next generation high-performance permanent magnets, which opens a new pathway to the preparation of hard magnetic materials that fill the gap between the ferrites and the rare-earth magnets.

Iron-rhodium ordered alloy with B2 (CsCl-type) crystal structure has a first-order phase transition from the low-temperature antiferromagnetic (AF) phase to the high-temperature ferromagnetic (FM) phase near room temperature. It is also found that this transition is accompanied by a volume increase of 1–2%, and a reduction in resistivity. Much interest has been recently paid to this material mainly owing to its potential technological applications, such as data storage media using thermally controlled AF–FM coupling phenomena, micro-electromechanical systems and spin valve-based devices.
We have ever reported that the ion beam irradiation at various energies ranging from GeV to keV orders can induce the FM state in FeRh bulk and film samples without any structural changes below room temperature at which they are originally in the AF state. In addition, further ion beam irradiation causes the change from the FM state to the paramagnetic (PM) state accompanied by a structural change from the B2-type to A1 (disordered fcc)-type structures. During the course of these studies, the change in the magnetization of the ion-irradiated FeRh is considered to be mainly dominated by elastic collisions between the ions and the samples, which generate lattice defects in B2-type FeRh. On the basis of such knowledge, we have attempted to fabricate micro scale magnetic patterns using the ion microbeam technique. If such magnetic patterns can be easily fabricated in samples by ion microbeam irradiation, novel applications for patterned media may be feasible.
In addition, we also realized that ferromagnetic layered structure has been made at sub-surface of the antiferromagnetic FeRh bulk samples by high energy He ion beam irradiation, because the beam energy solely determines the regions where the elastic energy deposited. The possibility three dimensional magnetic patterning for the FeRh thin films and bulks by high and low energy ion beam irradiation, such as 1 keV Ar, 30 keV Ga, 2-6MeV H and He ion beam will be discussed based on the experimental results of SQUID, MFM, XMCD and XMCD-PEEM. We will also discuss the depth directional magnetic modification at ultra-surface region of the samples by using the energetic cluster ion beam irradiation such as C2, C3 and C60. The results of the depth-directional XMCD analysis techniques was used for such communication.

Heusler alloys are known for their functionalities related to the magnetic shape memory, giant magnetoresistance, barocaloric, magnetocaloric, and exchange bias effects [1]. The stoichiometry of the materials showing these effects lies in a narrow compositional range where a martensitic transition takes place from a high-temperature cubic austenite state, with long-range ferromagnetic (FM) ordering, to a low temperature modulated tetragonal martensite state, where the ordering is usually short-range frustrated antiferromagnetic (AF).
Recently, a new functional property has been observed in an off-stoichiometric Heusler-based alloy [2]. An AF martensitic Heusler Ni₅₀Mn₄₅In₅, when annealed at high temperatures under a magnetic field of about 1 T, segregates and forms Ni₅₀Mn₂₅In₂₅ precipitates embedded in a NiMn matrix. The precipitates are paramagnetic (PM) at the annealing temperature, whereas the matrix is AF. The spins at the interface with the NiMn matrix align with the field during their growth and become strongly pinned in the direction of the applied field during annealing, whereas the core spins become PM. This gives rise to a PM precipitate with a FM shell. The remanent pinning exists below 600 K and an estimated field of 20 T is required to reverse the magnetization of the shell. The occurrence of such permanent memory, despite the PM core, provides possibilities for new applications related both to non-volatile memory withstanding high temperatures at reduced dimensions and to permanent magnets.
Small-angle neutron scattering (SANS) is one of the most important methods for microstructure investigation, which is utilized in a wide range of scientific disciplines, and particularly in magnetism. SANS allows characterizing structures or objects on the nanometer scale, typically between a few and a few hundred of nanometers. By means of magnetic SANS, we have investigated the presumed shell-ferromagnetism in Ni₅₀Mn₄₅In₅ samples. By combining unpolarized and polarized SANS (POLARIS), we demonstrate that a number of important conclusions regarding the microscopic spin structure can be made. In particular, the analysis of the magnetic neutron data suggests that precipitates with an effective ferromagnetic component form on annealing. We also provide a real-space analysis of the data by computing the correlation function of the total cross section, giving information on the characteristic length scales (both nuclear and magnetic). We show that the annealing treatment of the samples results in a decrease of the correlation length, the latter is related to the change of microstructural properties of these samples.
[1] A. Planes, L. Mañosa, and M. Acet, J. Phys. Condens. Matter 21, 233201 (2009).
[2] A. Çakir, M. Acet, and M. Farle, Sci. Rep. 6, 28931 (2016)

Ability to tailor the nature of the magnetic coupling between two ferromagnetic electrodes can enable the realization of new spintronics device systems. This paper discusses our finding that deposition of an ultrathin tantalum (Ta) on the NiFe top electrode reversed the nature of inter-ferromagnetic electrode coupling. We observed that the deposition of ~ 5 nm Ta on the top of a magnetic tunnel junction with Ta( 2 nm)/Co(5 nm)/NiFe (5 nm)/AlOx( 2 nm)/NiFe (10-15 nm) configuration changed the magnetic coupling between two ferromagnetic electrodes from antiferromagnetic to ferromagnetic. We investigated Ta effect using multiple magnetic characterizations like ferromagnetic resonance, magnetometry, and polarized neutron reflectometry. Ferromagnetic resonance characterization was very sensitive for detecting the changes in magnetic coupling via the insulating spacer. This simple approach of adding Ta film to alter the magnetic coupling can impact the other burgeoning areas like molecular spintronics. We found that preexisting magnetic coupling between two ferromagnetic electrodes impacted the resultant magnetic properties of magnetic tunnel junctions based molecular spintronics devices.

We study dynamic and thermodynamic properties of a classical spin chain driven by imaginary magnetic field, which is shown to correctly describe the action of Slonczewski spin-transfer torque (STT). We consider the particular case of a parity-time (PT) symmetric system with mutually perpendicular applied real and imaginary magnetic fields, perdicting a new type of non-equilibrium phase transitions. It corresponds to a transition from precessional to exponentially damped dynamics of the spin chain and is associated with SST-driven spontaneous PT symmetry breaking. We show how the properties of this new phase transitions can be derived from the system’s partition function, demonstrating the remarkable connection between the system’s dynamics and its statistical properties. Our results establish the physical role of imaginary magnetic fields in the description of dynamical and statistical properties of open non-equilibrium spin systems.

Anionic Interstitial electrons (IAEs) which strongly localized in the interstitial space are a form of electrons occupying structural cavities in condensed phase of matters such as solvated electrons in liquid and trapped electrons in solid. Recent theoretical studies predict that dense metals exhibit magnetism as constructing an electride crystal encompassing IAEs in structural cavities. However, such a topology of localized electrons has hardly been realized in a tubular or planar space due to the delocalization nature of electrons. Here, we report the anisotropic magnetism along the direction of crystalline originated from IAEs that occupy a specific crystallographic site in two-dimensional (2D) space. Furthermore, the spin-alignment of IAEs in 2D interlayer spacing can be tuned by chemical pressure that controls the magnetic properties of 2D electrides. The centimeter-scale single crystalline Y₂C electride were syntheized by floating-zone method and characterized its anisotropic electrical and magnetic properties. The origin of magnetism was clarified from the isovalent Sc substitution on Y site that the localization degree of IAEs at interlayer becomes stronger as the unit cell volume and c-axis lattice parameter were systematically reduced by increasing the Sc contents, thus eventually enhancing superparamagnetic behavior originated from the increase in ferromagnetic particle concentration. It is clarified from the theoretical calculations that the anisotropic nature originates from strongly localized IAEs with an inherent magnetic anisotropy in the interlayer spaces. These results indicate that the physical properties of 2D electrides can be tailored by adjusting the localization of IAEs at interlayer spacing via structural modification that controls the spin instability as found in three-dimensional elemental electrides of pressurized potassium metals.

Converting the advantages of nanostructured permanent magnets into bulk magnets remains a challenge, as the time-at-temperature typically required for consolidation/densification of nanostructured material produced by rapid solidification or attrition results in grain growth and other deleterious effects. Further, crystallographic alignment of nanocomposite (i.e., exchange-spring) permanent magnets remains elusive. Spark plasma synthesis (SPS) is capable of densification of materials with limited exposure to elevated temperatures, providing a means of retaining nanostructured grain sizes or limiting grain growth. In addition, hot deformation, necessary to generate crystallographic alignment, can be accomplished utilizing SPS. Here, we report the consolidation of melt-spun Nd-Fe-B-based materials with a Nd-rich composition. Initially, abnormal grain growth was observed, with subsequent deleterious effects on the properties. Grain-boundary infiltration created a much more uniform and fine microstructure. In Fe-rich compositions, we were similarly able to create highly coercive, fully dense material. Further, two-step hot deformation was used to create significant texture in the Fe-rich compositions, leading to energy densities approaching 40 MGOe. Finally, different feedstock material for the SPS was investigated, varying from lightly ground ribbon to ball-milled, micron scale powder. The behavior of these during SPS and hot deformation will also be presented.

Bulk nanocrystalline RCo₅ (R=Sm, Pr) permanent magnets with high coercivity and Curie temperature are promising candidates for practical applications at elevated temperatures. Recently, strong c-axis crystallographic texture and magnetic anisotropy have been successfully developed in such kind of magnets (SmCo₅ for example) by using severe hot deformation method^[1]. However, mechanism of crystallographic texture development in nanocrystalline RCo₅ permanent magnets during hot deformation process is still unknown. In present study, the electron backscattered diffraction (EBSD) has been applied to investigate the microstructure and crystallographic texture evolution in hot deformed RCo₅ permanent magnets. Increase of height reduction rate of deformed RCo₅ magnets leads to the formation of platelet shape grains perpendicular to the press direction; correspondingly c-axis crystallographic texture is gradually enhanced. As a result, the remanence of the magnets increases substantially. For the first time, it is observed that the grain boundary planes are also textured in the magnet^[2]. Therefore, it is expected that the grain boundary (GB) sliding and grain rotation are responsible for the plastic deformation. On the other hand, crystal texture and magnetic property heterogeneity were observed in hot deformed magnets^[3]. In hot deformed PrCo₅ magnets, for example, the remanence of the magnets decreases from central position to the edge position, while the coercivity exhibits opposite variance behavior. Microstructure and crystal texture analysis shows that the central position of the magnets has its grain size larger than that in the edge positions, but its c-axis alignment is weaker than the latter.

References
M. Yue, J. H. Zuo, W. Q. Liu, W. C. Lv, D. T. Zhang, J. X. Zhang, Z. H. Guo, and W. Li. Magnetic anisotropy in bulk nanocrystalline SmCo5 permanent magnet prepared by hot deformation. J. Appl. Phys., 2011, 109(7), 07A711
X.K. Yuan, M. Yue, D.T. Zhang, T. N. Jin, Z. R. Zhang, J. H. Zuo, J. X. Zhang, J. Zhu, X. X. Gao, Orientation textures of grains and boundary planes in a hot deformed SmCo₅ permanent magnet. Cryst. Eng. Comm., 2014, 16(9), 1669-1674.
D.T. Zhang, X.K. Yuan, M. Yue, Q. Ma, J. Zhu, J.X. Zhang, Orientation texture of local habit planes and its relevance to local magnetic performance in a hot deformed PrCo₅ bulk permanent magnet. RSC Adv., 2015, 5(110): 90976-90982.

With unique magnetic properties and feasible synthesis, magnetic nanoparticles (MNPs) have been widely used in cancer theranostics. Herein, we first introduce general protocle to produce monodisperse magnetic nanoparticles, and then highlight the synthetic process and theranostic applications of iron carbides, such as Fe₅C₂, Fe₂C, even multifunctional Au-Fe₂C nanoparticles (NPs). It’ worth noting that the presence of halides is critical to produce pure phase iron carbides, while the growth thermodynamic and kinetic processes also affect the final products. In addition, through modification of affinity proteins (Z_HER2:342), Fe₅C₂ NPs can selectively bind to HER2 overexpressing cancer cells. T₂-weighted MRI and PAT signals are readily observed, and tumors are effectively ablated by PTT under NIR irradiation. Likewise, the Z_HER2:342-binding Au-Fe₂C NPs are capable of MRI/multispectral photoacoustic tomography/computed tomography tri-modal imaging-guided PTT. To enhance cancer therapeutic efficiency, anticancer drug doxorubicin is loaded into bovine serum albumin coated Fe₅C₂ NPs, combining PTT with chemotherapy. Such nanoplatform can respond to NIR and acidic environments, and exhibit burst drug release. In summary, we will present their chemical synthesis and application potential of multimodal imaging-guided PTT for precise diagnosis and efficient cancer treatment of iron carbide NPs.
REFRENCES
[1] R. Hao, R. Xing, Z. Xu, Y. Hou, S. Gao, S. Sun,, Advanced Materials, 22 (2010), 2729–2742.
[2] Z. Xu, C. Shen, Y. Hou, H. Gao, S. Sun. Chemistry of Materials, 21(2009), 1778-1780.
[3] C. Yang, J. Wu, Y. Hou, Chemical Communication, 47(2011), 5130-5141.
[4] C. Yang, H. Zhao, Y. Hou, D. Ma, Journal of the American Chemical Society, 134 (2012), 15814-15821.
[5] J. Yu, C. Yang, J. Li, Y. Ding, L. Zhang, M.Z. Yousaf, J. Lin, R. Pang, L. Wei, L. Xu, Advanced Materials, 26 (2014), 4114-4120.
[6] J. Yu, Y. Ju, L. Zhao, X. Chu, W. Yang, Y, Tian, F. Sheng, J. Lin, F. Liu, Y. Dong and Y. Hou, ACS Nano, 10(2016), 159−169.
[7] Y. Ju, H. Zhang, J. Yu, S. Tong, N. Tian, Z. Wang, X. Wang, X. Su, X. Chu, J. Lin, Y. Ding, G. Li, F. Sheng, Y. Hou, ACS Nano, 11, (2017), 9239-9248.

Development of new cancer therapies rely on investigation of new drug classes to exploit cellular targets that may be used in single-agent or combination therapy and selective delivery of drugs to tumors. NDnano has developed magnetoelectric nanoparticles (ME-NPs) loaded with drugs that can be spatially directed to specifically penetrate malignant cells while sparing normal tissues. This approach not only expands the arsenal of single agents available to the clinical oncologist but also broadens the scope of combination therapy.
Combination therapy is critical for dealing with drug resistance mechanisms and multiple cancer cell mutations that typify difficult to treat tumors. In this study, we loaded potent inhibitors of vacuolar ATPase (V-ATPase) along with standard chemotherapeutics such as paclitaxel to specifically target metastatic breast cancer cells. Metastasis of breast cancer cells results in >50% fatalities. V-ATPase proton pumps have been directly implicated in aiding metastasis. V-ATPase and cancer cell-specific isoforms are overexpressed in many tumor types and are involved in cellular signaling, membrane trafficking, cancer cell survival, cell migration, cell invasiveness, metastasis, and drug resistance. Thus, the hypothesis is that the multidrug carrying ME-NPs will target the primary tumor with standard chemotherapeutics and simultaneously prevent metastasis by inhibiting the proton pumps, thus improving the overall prognosis. Additionally, using targeted ME-NPs mitigates the debilitating side-effects of current chemotherapeutic regimens by using the ME-NPs as a nanocarrier for delivering low doses of therapeutics with increased accumulation of the therapeutics at the tumor site through magnetic field guidance.
In this abstract we will (a) discuss the synthesis and conjugation of multiple therapeutics to ME-NPs; (b) discuss multiple modes of drug delivery from the ME-NPs (ON-Demand, stimuli dependent, enzymatic release, etc.) and (c) in vitro cell and in vivo mouse studies that show selective targeting of ME-NPs to metastatic MDAMB-231 breast cancer cell and improved survivability in the sample group.

Advances in medical and science technologies have been prolonging the human life expectancy. But much more effort is needed in diagnosis and treatment of illnesses like cancer. Current methods for treating cancer often destroy both unhealthy and healthy cells. So, it is important to distinguish unhealthy cells from healthy cells for accurate diagnosis and treatment. Nanomaterials, such as nanowires, are promising for labeling cells so that unhealthy cells can be identified and/or selected for specific therapy delivery. In this work, we have developed magnetic nanowire labels that can be used to identify a variety of cell types using ferromagnetic resonance (FMR) which can be engineered to produce unique signatures for each type of nanowire label. Ni, Fe, and Co nanowires were made by template-directed electrodeposition and characterized with vibrating sample magnetometer and scanning electron microscope for magnetic and physical properties, respectively. The nanowire samples were then placed onto a coplanar waveguide for application of a fixed microwave frequency while sweeping a DC magnetic field. Three different trends of FMR absorption vs. applied field were successfully measured for Ni, Fe, and Co nanowires. When measuring mixtures of nanowires with this technique, distinct absorption peaks were evident. In addition, this technique is shown to have the potential for nanowires coated with cell-specific antibodies that can be successfully utilized to tag cells in drug delivery, hyperthermia, or cellular barcoding and manipulation.

The design of anisotropic-shaped iron oxide nanoparticles has been a fascinating strategy to enhance the magnetic resonance transverse relaxivity (r₂) owing to their larger effective radii than that of their spherical counterparts. But the effect on r₂ when the radii of spherical nanoparticles are equal or larger than that of the anisotropic-shaped remains still elusive. Here, we unveil this issue via a comparative study of the isotropic and anisotropic-shaped iron oxide nanoparticles. We observed that the 12 nm edge-sized cubic iron oxide nanoparticles exhibit almost 5 times and 3 times the r₂ that exhibited by the spherical nanoparticles with the diameter of 12 nm and 21 nm respectively. But the underlying mechanism on it is not well-understood. After grafting the nitrodopamine-PEG, the cubic nanoparticles also show relatively larger hydrodynamic diameter than the spherical nanoparticles–i.e., 56 nm, 31 nm, and 45 nm respectively, while the zeta potentials are comparable. Although the cubic iron oxide nanoparticles outperform the spherical nanoparticles for r_2, they possess lower cellular uptake in the breast cancer cells (MDB-MB-231). The findings from this study indicate the anisotropic-shaped iron oxide nanoparticles yet hold great promise for the superior performance magnetic resonance imaging (MRI) contrast agents.

Magnetic resonance imaging (MRI) is a powerful non-invasive technique which becomes considerably more potent when contrast agents are introduced. Magnetic iron oxide nanoparticles have potential in biomedicine and have seen application as negative MRI contrast agents clinically, though their popularity has plummeted recently due to their low efficacy and safety concerns, including haemagglutination. There is therefore a real need for new CAs with excellent MRI contrast capabilities and good biocompatibility. Herein, we seek to explore the effect of a clinically-approved templating agent on protic relaxation behaviour – assessing the contrast agent behaviour of both reversible and permanent 1-D arrays of magnetic nanoparticles.

An in situ procedure is used to prepare colloidal magnetite nanoparticles, exploiting the clinically approved anti-coagulant, heparin, as a templating stabiliser. This preparative procedure provides control over vital interparticle interactions in ferrite nanocomposites, yielding stable aqueous colloids with exceptionally strong MRI contrast capabilities, particularly at low fields (r₁ values of 37.2 mM^-1s^-1 and r₂ values of 264.9 mM^-1s^-1 at 13.2 MHz), which outperform the current clinical standards. Relaxometric investigations using nuclear magnetic resonance dispersion (NMRD) techniques demonstrate that this behaviour is due to interparticle interactions, thanks to the templating effect of heparin, resulting in strong magnetic anisotropic behaviour. The stable colloidal nanoparticles have also been shown to prevent protein-adsorption triggered thrombosis, which causes unexpected (and potentially fatal) problems in the clinic. These species therefore show strong potential for in vivo MRI diagnostics.

The vital importance of shape anisotropy on resulting MR contrast has been further investigated through the production of permanent 1-D magnetic nanowires, using a novel magnetically-driven nanoparticle assembly preparative technique. These 1-D nanowires boast excellent relaxivity values (r₂=278 mM^-1s^-1 at 13.1 MHz and r₂/r₁=16.7), showing strong potential for next-generation negative MRI contrast agents.

We report on the exploration of perpendicular exchange bias in iron platinum/nickel oxide (FePt/NiO) bilayer thin films grown using pulsed laser deposition (PLD) on MgO (100) substrates. Exchange bias is an important property for giant magnetoresistance, and, as such has promise for applications in spin valves, magnetic sensors and magnetic random access memory. The magnetic L1₀ phase of FePt is known for having high perpendicular magnetic anisotropy, tunable coercivity/grain size and large magnetic storage density. The FePt layer was first deposited directly on MgO, followed by the deposition of the NiO layer on top of the FePt layer. The coercivity of the L1₀ FePt layer was tuned during growth to form a hard or soft magnetic layer. The FePt/NiO thin films grown for this study exhibit perpendicular exchange bias at 5K, as quantified using our SQUID measurements. XRD confirms parallel plane ordering between the MgO (200), FePt (002) and NiO (111) atomic planes while cross sectional TEM confirms the epitaxial growth of L1₀-FePt(001)<100>//MgO(100)<001> and the preferential growth of NiO on top of the FePt. Films of only FePt were grown to examine the surface architecture of the ferromagnetic layer and thus the interface of the FePt/NiO bilayer. The results from our XRD, TEM and magnetometry characterization of the FePt films and FePt/NiO bilayer thin films will be discussed.

The M-type Ba/Sr-hexaferrites with magnetoplumbite structure have attracted wide attention. Owning to differences in magnetic ordering of five distinct Fe³⁺ sublattice sites, hexaferrite offers rich opportunity to tune its magnetic and electrical properties via doping. Usually rare-earth (RE) elements are doped at Sr²⁺ site and magnetic and non-magnetic atoms at Fe³⁺ sites. RE distribution at Sr²⁺ also influences solubility of magnetic ions at the Fe³⁺site. It is, known in the literature that the Cu²⁺ has similar ionic radius and replacement ability as Co²⁺ but lower cost, we have attempted to understand the influence of rare-earth on Cu²⁺ doping in SrRE_xFe_12-xCu_xO₁₉(RE: La, Pr, x = 0.0-1.0). While La-Cu doped samples were single phase whereas Pr-Cu doped sample showed presence of secondary phase with increasing x value. La-Cu and Cu doped samples projected a slower decline in Ms reaching a value of 68.53 emu/g and 60 emu/g, respectively. The rapid decline in Pr-Cu Ms value is due to increased secondary phase in the compound. The coercivity of La-Cu and Pr-Cu doped samples were observed to be higher than Cu doped samples due to higher anisotropy K1 offered by RE elements. Furthermore, Tc decreases at the rate of 58.3, 37.1 and 7.3^oC per x content. The RT Mossbauer spectral study indicate that Cu²⁺ prefers 12 k site with broad hyperfine field distribution. The quadrupole shift value of 2b site, was observed to decrease with increase in x indicating improved site symmetry and concomitantly reducing anisotropy of the compound. In Due 2b sites proximity to Sr²⁺, QS of RE-Cu was most affected with the RE substitutin. Furthermore, Mossbauer results indicate that La (without 4f charge) is easily accommodated at Sr²⁺ site as compared to Pr³⁺(4f oblate charge distribution). The study clearly demonstrate that solubility of rare-earth in SrFe₁₂O₁₉ is much dependent on RE shape of 4f charge distribution and hence allows one to choose RE for desired modification of SrFe₁₂O₁₉.

The hot deformed Nd-Fe-B permanent magnets have attached much attention due to their good magnetic properties, excellent thermal stability, desirable corrosion and strong fracture toughness. The microstructure, crystallographic alignment, magnetic domain evolution, magnetic properties and magnetic hardening mechanism are studied for the hot deformed Nd-Fe-B permanent magnets. Three distinct regions, namely fine-grain (FG), coarse-grain (CG) and large-grain (LG) regions were identified in the hot deformed Nd-Fe-B magnets. Further investigations indicate that the heterogeneous microstructure plays a key role in determining the magnetic reversal and magnetic properties of the hot deformed magnets. The FG regions are composed of fine and uniform plate-like grains with strong c-axis crystal texture. The FG regions provide the strong pinning force of the magnetic domains, leading to high coercivity and good squareness of the demagnetization curve. The CG regions are composed of partially aligned equiaxed grains, with sub-micron size diameters. Most reversal domains grow from the CG region and propagate into the FG region, resulting in magnetic reversal and contribute to the observed deviations from squareness in the demagnetization curves. The LG regions are composed of randomly aligned equal-axis grains with micron size diameters. Most grains in the LG regions are multi-domains, which reverse even in their remanent magnetization states, lowering the remanence of the magnets.
By changing the technical parameter, the optimization of microstructure and magnetic properties of the hot deformed Nd-Fe-B magnets are realized. The proportion of FG, CG and LG regions change from 78.2 %, 20.0 % and 1.8 % to 88.7 %, 11.0 % and 0.3 %, respectively. Correspondingly, the magnetic properties have also been improved. The Jr and (BH)max of the optimized HD magnets are 14.23 kGs and 50.70 MGOe, which are 6.3 % and 16.7 % higher than the original HD magnets, respectively.

Due to the high saturation magnetization and coercivity, permanent magnet with rare-earth components have been used in the industrial applications such as not only mechanical parts but also electrical and electronic products. However, rare-earth components have strong dependency on rare-earth elements limited and regionally concentrated supply. To overcome this problem, investigation of rare-earth free magnets are necessary. In this study, among rare-earth free magnetic materials, Fe16N2 films shown significantly high saturation magnetization on several previous papers[1-3] were deposited by RF magnetron sputtering. Depending lattice mismatch between substrate and films affect to the formation of the Fe16N2 phases and saturation magnetization. so that we investigated magnetic properties of Fe-N films with various substrate. Crystallization and the ratio of Fe16N2 in the Fe-N film were measured by X-Ray Diffraction (XRD) and X-Ray Photoelectron Spectroscopy (XPS). Magnetic properties were measured by Vibrating Sample Magnetometer (VSM). Surface morphology and a structure were shown with Field Emission Scanning Electron Microscope (FE-SEM).

Reference
[1] S. Yamashita et al. J. Solid. State. Chem. 194 (2012) 76
[2] M. Takahashi et al. J. Mag. Mag. Mater. 208 (2000) 145
[3] H. Takahashi, et al. J. Mag. Mag. Mater. 174 (1997) 57

The last few decades have seen a rapid increase in the energy product of permanent magnets mainly because of the inclusion of rare earth elements. 3d/4d structures involving heavy transition elements have long been known to possess good permanent magnetic properties because of their ability to induce large anisotropy in structures such as L1₀ magnets. However, the use of heavy transition metals such as palladium and platinum is very expensive. Thus, a comprehensive understanding and desire to develop other itinerant 3d/4d permanent magnets is of utmost importance to materials scientists. There have been reports which suggest that low content of Tantalum (W) has the potential to induce large magneto crystalline anisotropy and increase magnetization in iron (Fe) and cobalt (Co) as a result of large spin-orbit coupling of 4d elements which can lead to strong 3d/4d hybridization. Fe₉₀Ta₁₀ was arc-melted and used as target during pulsed laser deposition (PLD) experiments. The films exhibited enhanced coercivities (259.76 Oe) and saturation magnetization (425 emu/cm³). The effects of post annealing temperature, and deposition temperature on the magnetic properties in-plane and out-of-plane of the film was also investigated. The results have shown that the easy magnetization direction was in-plane which was structurally determined by x-ray diffraction to be (111). Deposition at room temperature followed by post-annealing at 400 C showed the highest coercivities. The anisotropy energy was calculated to be in the films deposited at room temperature, a value which is larger than many known ferromagnetic materials. We have also observed Extra-ordinary Hall Effect (EHE) effect in Fe-Ta films which establishes the scaling behavior of the extraordinary Hall constant, R_s. An involved analysis of high-resolution Hall (B, T) data recorded at temperature from 5 to 300 K shows that the scaling exponent, n in R_s ~ ρⁿ, where ρ is the Ohmic resistivity, is ~ 1.1 ± 0.1. Theoretically, for homogeneous ferromagnets, n = 1 corresponds to Smit classical asymmetric scattering while n = 2 is attributed to the quantum mechanical side-jump scattering.

Imaging-guided photothermal therapy (PTT) has been emerging as a novel therapeutic method for precision therapy through combination of imaging and PTT together. However, most nanoplatforms both for PTT and multiple-model imaging lack stable structure. So, it is necessary to develop nanomaterial with high stability. Iron carbide nanoparticles (NPs) have been reported by our group as an excellent candidate for imaging-guided PTT with no side effects.
Here, based on the previous results, we incorporated Au into iron carbide to form a truly “multifunctional entities” monodispersed Au-Fe₂C Janus nanoparticles (JNPs) with 12 nm for the first time. We further optimized its performance as MR/MSOT/CT imaging agents with high photothermal transduction efficiency relying on its intrinsic properties, which can both provide accurate information for precision diagnosis and ablate tumors for therapy. Due to their brand absorption in the near-infrared range, Au-Fe₂C JNPs showed excellent photothermal effect under 808nm laser irradiation with high photothermal transduction efficiency value. We conjugated Au-Fe₂C JNPs with affibody Z_HER2:342 and found Au-Fe₂C-Z_HER2:342 JNPs have longer tumor retention times and deeper tumor penetration than Au-Fe₂C-PEG JNPs in vivo. Moreover, our results verified Au-Fe₂C-Z_HER2:342 JNPs can have selectively high therapy efficacy against tumors with low cytotoxicity. All in all, monodispersed Au-Fe₂C JNPs can combine multiple-model imaging techniques and high therapeutic efficacy and had great potential for precision theranostic nanomedicines.

Magnetic nanomaterials with small coercivity have a potential application related to the dynamic magnetization including magnetic hyperthermia (kHz) and microwave devices (GHz). One-dimensional (1D) magnetic nanostructures with the high aspect ratio have drawn considerable attention due to their control on the magnetic moment reversal by the large shape anisotropy. Herein, we propose a facile approach to obtain well-dispersed highly crystalline Fe₃O₄ nanorods (NRs) by the solvent-thermal method. The aspect ratios of Fe₃O₄ NRs can be tuned from 4.5 to 10. Magnetic induction fields within and around a single Fe₃O₄ nanorod in the remanence state were obtained by off-axis electron holography. The induction fields indicated a single domain state of the highly anisotropic Fe₃O₄ nanorod due to its strong magnetic shape anisotropy. Quantitative magnetic moment analysis of the obtained phase image yielded an average magnetization of 0.53 T of a single Fe₃O₄ nanorod. We demonstrate that the specific absorption rate (SAR) of Fe₃O₄ NRs with an aspect ratio of 4.5 can be enhanced by tuning have a much higher SAR as compared with 15 nm Fe₃O₄ nanoparticles and Fe₃O₄ NRs counterparts with an aspect ratio of 10. The highest SAR is greatly increased up to 1072 W g⁻¹ for an AC field of 33 kA m⁻¹ and a concentration of 5 mg mL⁻¹. These findings provide a new strategy to improve the heating efficiency of magnetic nanomaterials at the lower particles concentration with minimal invasiveness for the patient. Moreover, Moreover, the real part of the permeability (μ’) of magnetic-oriented Fe₃O₄ NRs is obviously higher than that of random Fe₃O₄ NRs in the GHz range. The oriented Fe₃O₄ NRs exhibit a higher resonance peak at 4.75 GHz compared to the bulk counterpart (1.2 GHz) in the frequency dependence of μ in the range of 1–10 GHz. These results could play a guiding significance in the development of an effective method to improve the permeability of magnetic nanomaterials at GHz working frequency.

Magnetic nanoparticles can convert electromagnetic energy into thermal energy in an ac magnetic field. Normal cells have a higher temperature tolerance than cancer cells, which makes it possible to treat tumors without harming normal tissue by employing magnetic nanoparticles (NPs). Therefore, to obtain magnetic nanoparticles with excellent hyperthermia performance has become one of the intensively studied topic. In this paper, the hyperthermic properties of Zn_0.5Co_xFe_2.5-xO₄ (x=0.0, 0.05, 0.1, 0.15, 0.2) (ZCFO) NPs were systematically studied. The ZCFO NPs were synthesized by high-temperature solution phase reaction method and the ZCFO@SiO₂ core/shell NPs were obtained by the reverse microemulsion method. We fixed the magnetic core to be 22 nm and shell thickness to be 4 nm, as shown by the transmission electron microscopy (TEM) images (Fig. 1 and Fig. 2). The saturation magnetization of the series decreases with the increase of Co²⁺ substitution, which is presumably due to the lower spin magnetic moment of Co²⁺ than the Fe²⁺ (Fig. 3 and Fig .4). On the other hand, the specific loss power (SLP) of ZCFO NPs increases with increasing Co concentration, reaching a maximum value of 1795 W/g_metal for x=0.1, and then gradually decreases to 1160 W/g_metal for x=0.2, in an AMF of 430 kHz and 31 kA/m (Fig. 5 and Fig. 6). The increase of SLP despite a decrease in magnetization at x ≤ 0.1 is due to the increase of effective magnetic anisotropy with Co substitution, thereby affecting the Néel relaxation process.

Development of a rapid and sensitive method for infectious disease diagnosis is highly important to prevent the further spread of disease and to enable effective clinical treatment. Herein, an ultrasensitive colorimetric approach combining the advantage of immunomagnetic nanobeads and the enzyme mimic activity of gold nanoparticles has been developed. Two kinds of amplification processes are used to enhance the detection sensitivity. The increased surface area and the magnetic properties enable the magnetic nanobead to catch a large number of antibodies and target viruses, thus very small amounts of the virus can be easily detected. And the signal amplification of naonzyme causing the enhancing of the optical signal. This approach could avoid complicate instruments and allowed detecting Influenza virus only by naked eyes as well as microplate reader.
In addition, we introduced a novel practical approach to develop a robust sensing system, named magnetoplasmonic ELISA (magplas-ELISA), which collects and concentrates target antigen, and amplifies signal simultaneously. Gold nanoparticles (Au NPs) were decorated with the magnetic nanoparticles (MNPs) based on its ease of synthesis and bio-compatibility. It can replace peroxidase in colorimetric biosensor owing to its outstanding peroxidase-like activity. Our choice of MNP-Au NP has a triple function which is a capture probe, magnetic concentrator and signal amplifier in this system. Triple-functional- magnetoplasmonic NPs described here provides direct monitoring of biomarker in a clinical sample as urine and serum.

Locomotion adaptability is one of the most significant evolutionary steps in the story of life. Organism self-adjusted displacement in a controlled manner is essential for survival, reproduction, morphogenesis, and to respond to pathological processes. For example, Escherichia coli (E. coli) swim by rotating or tumbling their helical flagella to seek prey or nutrients.¹ In their quest to fertilize the ovum, sperm cells propel with rotating and undulatory locomotion in a bulk and a confined fluid environment, respectively.² This implies that the ability to modulate the locomotion mechanism provides the critical advantages when designing artificial swimmers.

Recent advances in template-assisted electrochemical manufacturing have enabled to create a plethora of advanced functional inorganic nanoarchitectures, which have been exploited as locomotion building blocks in mobile nanoswimmers.³ In this presentation, we capitalize on our wide expertise in template-assisted electrochemical processing to fabricate the simplest artificial swimmers, hard-magnetic CoPt nanowires. The CoPt nanowires are magnetized with a 60offset direction with respect to the nanowire’ long axis by a pre-magnetization process. Specifically, the pre-magnetized nanowires can show selective locomotion in a fluid, namely tumbling, precession, and rolling, by adjusting only the rotation speed of an external rotating magnetic field. This choice of displacement mode using a single input is achieved due to the intrinsic memory effects and dynamics of our developed hard-magnetic materials that constitute our swimmers.

Our results not only trigger a substantial body of research in the magnetism of nanomaterials, but also open new avenues in sensing and delivering cargos at nanoscale, performing intelligent behaviour in a confined fluid environment.

References
1. Darnton, N. C.; Turner, L.; Rojevsky, S.; Berg, H. C. J. Bacteriol. 2007, 189, 1756-1764.
2. Nosrati, R.; Driouchi, A.; Yip, C. M.; Sinton, D. Nat. Commun. 2015, 6, 8703.
3. Jang, B., et al. Nano Lett. 2015, 15, 4829-4833.

The development of two-dimensional (2D) materials with the coexistence of intrinsic half-metallic and Dirac features will be of great realistic significance for next-generation spintronic nano-devices. Using first-principles calculations, we demonstrate that the pristine 2D MX₃ are the highly desired Dirac half metals and feasible in experiment. All proposed MX₃ monolayers possess Dirac cone in the conducting spin channel with high Fermi velocities, while the other spin orientations have large energy gaps. In addition, these monolayers favor ferromagnetic ground states with large exchange energies as well as about 4 μ_B spin-polarization originated from the 3d orbitals of M atoms. The evaluated Curie temperature indicate MX₃ monolayers can maintain their ferromagnetism beyond room temperature. Remarkably, our results show that some monolayers exhibit large magnetocrystalline anisotropy (MCA). When the spin−orbit coupling is involved, the Dirac cones open small gaps and give rise to topologically nontrivial states with nonzero Chern number (-1), indicating MX₃ monolayers are Chern insulators. This work not only highlight the promising candidates for future spintronic applications, but also paves the way for the realization of integrating the long-craved qualities and quantum anomalous Hall effect (QAHE) in pristine 2D layers.

We have studied magneto-PL from ZnTe QDs embedded in ZnMnSe matrix grown by molecular beam epitaxy. The PL is due to radiative, spatially indirect transitions between holes confined in the ZnTe QDs and electrons that remain in the ZnMnSe matrix [1]. The peak energy at T = 7 K exhibits a red shift with increasing magnetic field and saturates around 4 tesla, mirroring the magnetization of the paramagnetic ZnMnSe matrix. The observed red shift is due to the exchange interaction between the manganese and the carrier spins. The hole-Mn interaction comes from the hole wavefunction tail that penetrates into the ZnMnSe matrix. The saturation value of the red shift exhibits a strong dependence on the photon energy of the exciting laser beam. The red shift with excitation using 3.06 eV photons (energy above the matrix gap), has a saturation value of 4.5 meV; in contrast the saturation value using excitation with 2.54 eV (energy below the matrix gap but above the QD gap) is 14.5 meV. This strong dependence on energy of the exciting photon is understood as follows: Excitation with 3.06 eV photons generates electron-hole pairs in the ZnMnSe matrix. The holes are then captured by the ZnTe QDs. Once a QD becomes occupied by a hole, a barrier is created due to the Coulomb repulsion that prevents other holes to occupy the QD. Excitation with 2.54 eV photons do not have this limitation and result in multiple hole occupancy of the QDs. The Coulomb repulsion between holes in the same QD modifies the wavefunction in such a way that we have increased penetration into the magnetic ZnMnSe matrix, and therefore stronger Mn-hole spin-exchange interaction that gives us the larger PL energy red shift. We have calculated the hole wavefunction for single and double occupation using the linear variational method by constructing a matrix representation of the Hamiltonian operator in a basis of slater determinants of single particle harmonic oscillators. All matrix elements are solved analytically. The matrix is diagonalized to yield the hole wave function. The results of the calculation are in agreement with the experimental observations.
[1]. B. Barman et al, Phys. Rev. B, 92, 035430 (2015).
This work is supported by NSF DMR 1305770.

Partial hydrogenation of graphene has profound effects on its material properties. For example, graphene undergoes a metal-insulator transition upon dosing with small amounts of atomic hydrogen in the low-coverage monomer regime. Recently, it was experimentally demonstrated that the adsorption of atomic hydrogens on graphene induces local magnetic moments around the adsorption sites. However, a ferromagnetic spin-spin interaction for a pair of H adatoms is energetically favored only when the H adatoms sit on a same sublattice of graphene. In this presentation, we present our first-principles spin-polarized calculations of hydrogenated graphene and discuss how to obtain a ferromagnetic phase through atomic-scale control of the H adsorption or desorption processes. We propose a new method to make the hydrogenation occur preferentially on one of the two equivalent sublattices of graphene, paving the way for transition-metal-free magnetism in graphene.

The mixed-valence oxide Co₃O₄ nanoparticles, having the normal spinel structure, possess large surface area, active-site surface adsorption properties, and fast ion diffusivities. Consequently, they are widely used in lithium-ion batteries, as well as for gas sensing and heterogeneous catalysis applications. In our research, we use a two-step method to synthesize Co₃O₄–based core-shell nanoparticles (CSNs). Cobalt oxide (Co₃O₄) nanoparticles were successfully synthesized using a wet synthesis method employing KOH and cobalt acetate. Manganese was incorporated into the Co₃O₄ structure to synthesize inverted Co₃O₄@Mn_xCo_3-xO₄ CSNs using a hydrothermal method. By adjustment of pH value, we obtained two different morphologies of CSNs, one resulting in pseudo-spherical and octahedron-shaped nanoparticles (PS type) whereas the second type predominantly have a nanoplate (NP type) morphology. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and x-ray photoelectron spectroscopy (XPS) have been performed in order to determine the morphological and structural properties of our CSNs, whereas the magnetic properties have been characterized using a superconducting quantum interference device (SQUID) magnetometer. XRD and TEM results show that the CSNs have the same spinel crystal structure throughout the core and shell with an average particle size of ~19.8 nm. Our Co₃O₄ nanoparticles, as measured prior to CSN formation, are shown to be antiferromagnetic (AFM) in nature as shown by the magnetization data. Our SQUID data indicate that the core-shell nanoparticles have both AFM (due to the Co₃O₄ core) and ferrimagnetic properties (of the shell) with a coercivity field of 300 Oe and 150 Oe at 5 K for the PS and NP samples, respectively. The magnetization vs temperature data show a spin order-disorder transition at ~33 K and a superparamagnetic blocking temperature of ~90 K for both batches.

Ferrofluids are colloidal fluid suspensions of superparamagnetic nanoparticles. Under certain conditions, ferrofluids are unstable in the presence of magnetic fields. They undergo symmetry-breaking transitions that manifest themselves in extraordinary patterns. These include hexagonal, square array, labyrinth and so forth. These patterns collapse upon removal of the magnetic field. In our work, we establish methods to make permanent patterns from ferrofluidic media. We show that ferrofluids, when doped with compatible organosilicon polymer, can be cured to yield permanent records of the interaction of magnetic fields with the rheological response of the host. Dilution of the hybrid guest-host magnetic nanocomposite with volatile organic solvents (heptane, octane or nonane) yields stunning solvent-dependent hierarchies of patterns over length scales spanning nanometers to millimeters. The potential applications of our findings are in the creation of new kinds of flexible diffractive optics and new strategies for exploring and developing new theories of pattern formation based on our discoveries of this new kind of coupling of chemistry with applied magnetic fields.

Polymer nanocomposites have been intensively investigated because they exhibit both the properties of the polymers and the properties of the nanoparticles. Of particular importance are superparamagnetic iron oxide polymer nanocomposites. Their properties can be modified by application of an external magnetic field.^[1-4] It has been shown, that the properties of superparamagnetic nanocomposite materials depend strongly on the distribution of the nanoparticles in the nanocomposite and on cluster formation. A method to protect those nanoparticles from clustering during the preparation of polymer nanocomposites is the encapsulation of the nanoparticles.^[5] We encapsulated monodisperse superparamagnetic iron oxide nanoparticles (SPIONs) with an amphiphilic diblock copolymer. For this purpose, we used a polymer with carbon-carbon double bonds which we crosslinked in a thiol-ene clickreaction to stabilize the polymer shell surrounding the particles. We dispersed those stabilized nanoparticles into a polymeric matrix and characterized the nanocomposites with small angle x-ray scattering (SAXS). Those investigations demonstrated the presence of mostly single particles and a negligible amount of dyads. By applying an external magnetic field during SAXS measurements we obtained anisotropic scattering images due to an agglomeration of the particles induced by magnetic interactions between the SPIONs. By removal of the external magnetic field, the agglomerates are dispersed, leading to an isotropic scattering image with again mostly single particles.

References
1. J. Jestin et al., Adv. Mater. 2008, 20, 2533-2540.
2. A.-S. Robbes et al., Macromolecules 2011, 44, 8858-8865.
3. C. Chevigny et al., Macromolecules 2011, 44, 122-133.
4. A.-S. Robbes et al., Macromolecules 2012, 45, 9220-9231.
5. A. Feld, R. Koll et al., ACS Nano 2017, 11, 3767-3775.

The catalysis market is responsible for more than 35% of the world’s GDP and it is involved in the most successful industrial sectors: energy generation, chemicals, and pharmaceuticals. Despite the remarkable advances in catalytic technologies, the industry still faces thermal management issues and accumulation of heat on reactor walls. To overcome thermal transport problems, heat can be generated in situ with the utilization of iron oxide (Fe₃O₄) nanoparticles. Traditionally, Fe₃O₄ is heavily used in magnetic hyperthermia studies where radio waves (RF) are used to generate heat within the particle, which has not yet been applied to catalysis. The utilization of Fe₃O₄ nanoparticles in catalysis is beneficial since its purpose would be twofold: an improvement in energy transport and the utilization of the particles as catalysts as well. For dehydrogenation reactions, for example, Fe₃O₄ acts as a great reducing agent, and its structure can be tuned such that it is functionalized according to the catalytic necessities of each reaction.
In this work, ~20 nm anisotropic iron oxide nanoparticle spheres, cubes, and truncated octahedrons of tunable sizes are investigated for RF induced catalysis. By varying surfactant to precursor ratio in thermal decomposition reactions, exposed Fe₃O₄ facets are controlled. These facets allow surface activity control and heat generation, key parameters in the development of high selectivity catalysis. Specific Absorption Rate (SAR) measurements of these nanoparticles were performed in organic solvents achieving values as high as 200 W/gFe, which is 34% higher than commercially available nanoparticles tested under similar conditions. After surface functionalization, particles were also dispersed in alcohols to perform the catalytic production of acetaldehydes and ketones. These reactions were performed in the presence of RF and the results were compared to thermally activated systems. This RF driven catalytic reaction has also been investigated via in situ neutron scattering to identify the reaction pathway and particle-molecule interactions for the dehydrogenation of ethane. Furthermore, the Fe₃O₄ valence levels are probed via ultraviolet photoelectron spectroscopy to identify particle-molecule interactions. Although further optimization is required, future work includes surface functionalization to apply this technology to a range of catalytic reactions.

MoO₂-Fe₂O₃ nanoparticle system was successfully synthesized by mechanochemical activation of MoO₂ and α-Fe₂O₃ equimolar mixtures for 0-12 hours of ball milling time. The study aims at exploring the formation of magnetic oxide semiconductors at the nanoscale, which is of crucial importance for catalysis, sensing, energy-related and electrochemical applications. X-ray powder diffraction (XRD), Mössbauer spectroscopy and magnetic measurements were used to study the phase evolution of MoO₂-Fe₂O₃ nanoparticle system under the mechanochemical activation process. Rietveld refinement of the XRD patterns yielded the values of the crystallite size and lattice parameters as function of milling times and indicated the presence of Mo-substituted hematite and Fe-doped molybdenum dioxide at long milling times. The Mössbauer studies showed that the spectrum of the mechanochemically activated composites evolved from a sextet for hematite to sextet and a doublet upon duration of the milling process with molybdenum dioxide. Recoilless fraction was determined using our dual absorber method and was found to decrease with increasing ball milling time. Magnetic measurements recorded at 5 and 300 K in an applied magnetic field of 50,000 Oe showed the magnetic properties in the antiferromagnetic and canted ferromagnetic states. The Morin transformation was evidenced by zero-field cooling-field cooling (ZFC-FC) measurements in 200 Oe and the transformation characteristic temperatures were shifted to lower values.

With the recent advances on the nanotechnology science, nanoencapsulation is rapidly growing. Nanocapsules are nanostructured materials composed of a core and a protective layer [1]. The core is typically solid or liquid, and the protective layer is usually a non-toxic polymer membrane, which makes the delivery of the drug controllable. Today researchers are interested in novel drug delivery systems that are able to release drugs in a specific area without affecting other non-contaminated areas, preventing overdoses and leveraging the effect of the drug and its use in the diagnosis of the diseases as marker or contrast agent at the same time [1], [2], [3].
In this work, we show the nanoencapsulation of hydrophobic ION's within an oleic acid nanoemulsions using sodium dodecyl sulfate (SDS) as surfactant coated with chitosan and its characterization. Due to the properties of toxicity, biocompatibility, and biodegradability, exhibited by the materials used for the nanocapsules synthesis, is believed that they could be a good alternative to the state-of-the-art ones due to the possibilities of use the nanocapsules with the iron oxide nanoparticles within as contrast agents, thermo-therapies, and delivery vectors for bioactive ingredients.

References
[1] R. W. Kelsall, I. W. Hamley, and M. Geoghegan, Nanoscale Science and Technology. 2005.
[2] C. A. Mirkin, Single Molecule Detection in Solution Biomineralization – From Biology to Biotechnology and Medical Application. 2004.
[3] J. Melorose, R. Perroy, and S. Careas, Advanced Biomaterials and Biodevices, vol. 1. 2015.

Soft magnetic materials have been studied extensively in the recent years due to their applications in micro-transformers, micro-inductors, spin dependent memories etc. The unique features of these materials are the high frequency operability and high magnetic anisotropy. High uniaxial anisotropy is one of the most important properties for these materials. There are many methods to achieve high anisotropy field (H_k) which include sputtering with presence of magnetic field, exchange bias and oblique angle sputtering without the presence of the magnetic field, etc. There are multiple ways to increase the ferromagnetic resonance frequency which is proportional to coercivity H_k and magnetic saturation M_s. The ferromagnetic resonance depends on softness of the magnetic material to the applied field. With a highly soft magnetic material, one can operate the devices made out of these films at giga hertz regions without loss of efficiency. of the magnetic films is also highly desirable for many applications, such as power conversion and radio frequency (RF) devices operating at high frequencies. This work focuses on analyzing different growth conditions of thin films of CoZrTaB and the resulting magnetic and electrical properties of the films. Thin films are grown by oblique-angle sputtering method, where the sputtering gun forms an angle with respect to the sample substrate normal, ranging from 0 to 75 degrees. External magnetic field normally applied in order to form magnetic anisotropy is not used during film sputtering process. Using vibrating sample magnetometer, it was observed that films resulting from small oblique angles have no clear magnetic anisotropy developed; whereas in samples deposited at large angles close to 60 degree, there is clear magnetic anisotropy observed. Scanning electron microscopy imaging of the cross-section views of such film suggests the formation of tilted columns, which is likely to be the reason for magnetic anisotropy. Resistivity of the films was measured systematically and found to increase as the magnitude of oblique angle during sputtering increased.

Room temperature ferromagnetism in oxide based semiconductors can be attributed to the alignment of localized electron spins,due to defects and/or dopants.In this work, we synthesized pure and manganese doped zinc oxide by a solid-state reaction method, and the defect type and concentration were varied using two different processes, conventional furnace annealing and spark plasma sintering. These defects were characterized by a variety of techniques and correlated with the measured magnetic behaviour in the material. Ferromagnetic like behaviour was observed in the doped samples prepared by furnace annealing, due to the manganese present in multiple oxidation states. Ferromagnetic behaviour was also observed for pure zinc oxide, after spark plasma sintering, where the oxygen vacancies mediated the magnetism. Thus, the magnetic origin in zinc oxide can be controlled by both defects and transition metal dopants. ZnO based inks for depositing thin films were developed by dispersing the respective nanoparticles in an ethylene glycol based solvent with polymeric stabilizer additive using mechanical milling approach.Comparative study of magnetic properties of bulk and thin films was done. Our work shows, this approach can be helpful in designing dilute magnetic semiconductor based devices.

Light-matter interaction is a key photonics phenomenon that is essential to quantum-information technology. In particular, the coupling of microwave photons to magnons (known as photon-magnon coupling) has received much attention due to its potential applications in coherent information storage. In most studies on magnon-photon coupling, three-dimensional (3D) structures consisting of a cavity resonator and an yttrium iron garnet (YIG) sphere have been used in real experiments wherein the photon and magnon modes were excited by the photon cavity boundary and the spin precession under an external static magnetic field, respectively. However, the main disadvantage of the 3D cavity approach is that the technology is relatively complex and impractical for applications. In the present study, we observed magnon-photon coupling at room temperature in a compact planar-geometry hybrid system consisting of an inverted split-ring resonator (ISRR) and an YIG film.
We experimentally demonstrated strong anti-crossing effects of the ISRR’s photon mode and YIG film’s magnon modes with an effective photon-magnon coupling strength (g_eff/2π) of 90 MHz at a microwave frequency of f = 3.79 GHz [1]. The spin-number-normalized coupling strength was determined to be g_eff/2π√N = 0.194 Hz, higher than any reported to date, which result highlights a great potential for quantum information processing. Furthermore, we found that g_eff/2π can be increased up to 64% simply by changing φ from 90° to 0°, where φ is the angle between the in-plane magnetic field and the transverse direction of the device.
We also observed, in the anti-crossing region, fine photon-magnon coupling features originating from the excitation of different spin-wave modes such as magnetostatic surface waves (MSSWs) and backward-volume magnetostatic waves (BVMSWs). In order to elucidate the underlying physics of such features, we performed delicate measurements of the |S₂₁| power on the f-H plane with 1 Oe fine scans for different field directions ( φ = 0 to 90°). It was demonstrated the coherent magnon-photon coupling for BVMSW modes (0 ≤ φ ≤ 30°) and MSSW modes (30° ≤ φ ≤ 90°) as well as for Kittel modes. Using the coupled-oscillator model, the coupling strength corresponding to each specific spin-wave mode was estimated quantitatively and found to decrease with the mode index.
Our experimental results provide a means for the design of new types of high-gain magnon-photon coupling systems in planar geometry and establish, furthermore, a new approach to the exploration of magnetostatic modes in magnetic systems. Such multi-mode coupling is very interesting in both the fundamental and practical aspects of signal processing, magnon conversion, and quantum memory.
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2015R1A2A1A10056286).
[1] B. Bhoi, et al., Sci. Rep., 7, 11930 (2017).

Magnetic nanoparticles are of increasing interest due to their unique physical properties such as superparamagnetism, the exchange-bias effect, and particle-size-dependent static and dynamic properties [1-3]. These novel characteristics make magnetic nanoparticles very attractive for bio-applications including magnetic hyperthermia and MRI contrast agents. As an example, soft magnetic nanoparticles in single-domain states exhibit collective Larmor precession of individual spins. In cases where the frequency of time-varying magnetic fields equals the Larmor precession frequency, individual magnetic moments efficiently absorb energies that are transferred from externally applied AC magnetic fields, after which those energies dissipate into other forms due to their intrinsic damping of given materials. Such energy dissipations of magnetic nanoparticles are of crucial importance in hyperthermia bio-applications for high specific loss power (SLP). Larmor precession motions of individual spins in magnetic particles excited by relatively high-frequency (several hundred MHz) AC magnetic fields can give rise to a higher efficiency of energy dissipation than those by Brownian rotation of nanoparticles and/or by Néel relaxation of nanoparticles’ magnetizations.
In the present study, we explored robust non-linear magnetization dynamics and the associated high-efficiency energy-dissipation effect using soft single-domain-state magnetic nanospheres excited by oscillating magnetic fields of different frequencies and amplitudes under given static magnetic fields. We conducted micromagnetic simulations to explore the novel magnetization dynamics of soft magnetic particles and additional analytical derivations of the energy-dissipation rate for the steady-state regime by varying the frequency and strength of rotating magnetic fields for different Gilbert damping constants and static magnetic field strengths. All of the simulation results and analytical calculations agree well quantitatively. The dynamic origin of such a high-efficiency energy-dissipation mechanism is completely different from those of the typical ones used in bio-applications. This work provides further insights into the fundamentals of magnetization dynamics in magnetic particles and the associated energy dissipation effect, and suggests a highly efficient means of magnetic-hyperthermia-applicable energy dissipation.
The research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2015R1A2A1A10056286).

[1] M. –K. Kim et al. Appl. Phys. Lett. 232402 (2014) 105.
[2] S.-K. Kim et al. Sci. Rep. 11370 (2015) 5.
[3] S.-K. Kim et al. Sci. Rep. 31513 (2016) 6.

MnBi has attracted much interest within the past decade for its potential use in rare-earth free magnets. When stabilized in the so-called low temperature phase, MnBi exhibits an intriguing magnetic behavior with an unusual positive-dependence of anisotropy constant on temperature, reaching a maximum of 2.2 MJ.m^-3 at 490 K [1]. On the other hand, two magnetic phase nanocomposites, made of a fine mixture of nanosized grains with respectively high magnetization and high anisotropy, are good candidates to build high performance magnets [2]. In this context, we studied the structural and magnetic properties of MnBi nanosized grains.

MnBi nanoclusters, with a mean size of 5 nm, were produced by gas aggregation in a low energy cluster beam deposition system, and protected with an amorphous carbon layer. We investigated their structure in a transmission electron microscope and by X-ray diffraction, and their magnetic response in a superconducting quantum interference device (SQUID) magnetometer. In this study, we focused on the onset of magnetic ordering after annealing at various temperatures, ranging from 250°C to 500°C.

In the as-deposited state, MnBi clusters, with nearly equiatomic composition, present a low degree of crystallinity. They show a paramagnetic-like response, with relatively large magnetic moment, which contrasts with a recent report on as-prepared MnBi 10-nm clusters made by sputtering [3]. Low temperature annealing leads to magnetic ordering with temperature independent magnetization below room temperature. Annealing at a temperature ranging between 300°C and 400°C leads to the formation of a hard magnetic phase with low thermal stability, which disappears after annealing at 500°C. We will discuss the evolution of structural and magnetic properties upon annealing, considering finite size effects.

[1] Chen, T., and Stutis, W. E., IEEE Trans. Magn. (1974) 10, 581
[2] Kneller, E. F., and Harwig, R., IEEE Trans. Magn. (1991) 27, 3588
[3] Mukherjee, P., Balamurugan, B., Shield, J. E., and Sellmyer, D., RSC Adv. (2016) 6, 92765-92770

Magnetic nanocomposites are a class of material that will allow for rational inductor design for power electronics. Zero-valent iron nanoparticles are a promising candidate as the magnetic fraction, due to the emergence of the superparamagnetic phenomenon at small sizes. Such superparamagnetic nanoparticles possess zero magnetic coercivity and are too small to support eddy currents, thereby removing two of the largest sources of energy loss in inductor applications.

We present the systematic growth of zero-valent iron nanoparticles with tight size and shape distribution via a reversible agglomeration method. We then demonstrate the scalability of this approach by synthesizing gram scale amounts of well-defined zero-valent iron nanoparticles. To ensure uniform switching and to prevent ferromagnetic domains from forming within the resulting nanocomposite the interparticle spacing of the nanoparticles must be closely controlled. To achieve this we have developed a ‘matrix-free’ approach in which a ligand exchange procedure is performed on the nanoparticles to express a chosen functionality at the surface. A cross-linking agent is then introduced to form a cross-linked network that reduces the organic fraction and maximizes particle loading.

The nanocomposite has good workability enabling it to be cast into custom molds, or directly 3D printed, leading to a number of different inductor form factors. Upon curing we obtain a mechanically strong, magnetically active material which shows promise for the next generation of inductors for power electronics.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525.

Magnetocaloric materials (MCMs) show great potential for utilization in magnetic refrigeration techniques and energy conversion methods in thermomagnetic generators (TMGs).^[1] A thermomagnetic motor (TMM) prototype utilizing Gd was built by Swiss Blue Energy and is able to convert low-temperature waste heat to electrical or mechanical energy.^[2] In order to enhance the efficiency appropriate advanced magnetocaloric materials having strong magnetization changes with low hysteresis should be used to replace Gd. In contrast to the refrigeration application, energy conversion with MCMs requires a low latent heat for high efficiencies.
Materials with suitable Curie temperatures and small hysteresis can be found in the quaternary (Mn,Fe)₂(P,Si) system. In order to find suitable compounds, a large fraction of the phase diagram of Mn_xFe_2-xP_i-ySi_y ^[3] (0.5≤Mn≤1.8, 0.33≤Si≤0.6) has been investigated. Polycrystalline samples of Mn_xFe_2-xP_i-ySi_y have been prepared by the ball-milling method. After ball-milling, the samples were sintered at 1373K for 25h then quenched into water.
Suitable materials for applications having Curie temperatures around room temperature, large magnetization changes and small hysteresis can be found in the Mn-rich (x>1.2) and Fe-rich (x<0.7) region. Furthermore, we observe an antiferromagnetic (AFM) to paramagnetic (PM) phase transition for Mn_xFe_2-xP_2/3Si_1/3 with 1.1≤x≤1.7. X-ray diffraction shows that all the Mn_xFe_2-xP_2/3Si_1/3 compounds with 0.5≤x≤1.8 crystallize in Fe₂P-based hexagonal structure. The lattice parameter a (in plane) expands and c shrinks (out of plane) with increasing manganese content, while only a small volume expansion is observed. In the Fe₂P type region (0.5≤x≤0.9), T_c and hysteresis rise with increasing manganese concentration. In contrast, in the region with AFM-PM transition (1.1≤ x≤1.7), T_N and hysteresis drop while the manganese concentration increases.

References

[1] Kitanovski, A., et al., Magnetocaloric Energy Conversion. 2015: Springer International Publishing.
[2] http://www.swiss-blue-energy.ch/
[3] Dung, N.H., et al., Mixed Magnetism for Refrigeration and Energy Conversion. Advanced Energy Materials, 2011. 1(6): p. 1215-1219.

Magnetic cooling has attracted considerable attention since it is an energy efficient and green thermal management technology. However, the cost of the magnetocaloric material, which is usually Gd based, has severely curtailed commercialization of this technology. Additionally, most commercial applications have service temperatures near room temperature. Hence, we studied the structural and magnetocaloric (MCE) properties of low cost Fe_3-xM_xAl (where M = Cr, Mn and x = 0.6 to 1) alloys with high relative cooling power (RCP) and Curie temperature (T_c) near room temperature. The Cr and Mn doped Fe₃Al alloys exhibited either a L2₁ or B2 crystal structure. With increasing Cr content from x = 0.76 to x = 1, the Bohr magnetron per formula unit (µ_f.u.) decreased from 2.2 µ_B to 1.6 µ_B while coercivity remained low, no thermal hysteresis was observed. Among the Fe_3-xCr_xAl (x = 0.76, 0.84, 0.92 and 1) alloys studied, a maximum value of RCP of 300 Jkg^-1 at 5 T and T_c of 300 K was exhibited by the Fe_2.25Cr_0.76Al alloy composition. A comparison of MCE based on the RCP/US$ performance metric suggested that these Fe_3-xCr_xAl alloys show higher RCP/$ values (~ 2.1) compared to other iron based and Gd based second order MCE materials (RCP/$ typically less than 1). In the case of Fe_3-xMn_xAl alloys, an RCP value of 425 Jkg^-1 at 5 T and Tc = 260 K was observed for x = 0.6. For x = 0.7, Tc = 292 K and a RCP of 400 Jkg^-1 at 5 T was observed. The RCP values of Fe_3-xMn_xAl alloys are comparable to those of the best known magnetocaloric materials. The low cost, high corrosion resistance, ready availability, good MCE values and established manufacturing technology makes Fe_3-xMn_xAl and Fe_3-xCr_xAl alloys attractive for near room temperature magnetic cooling applications.

Iron oxide nanoparticles (IONPs) are widely investigated due to their chemical tunability and great potential as diagnostic and therapeutic agents. In magnetic particle imaging (MPI), which is an emerging imaging modality that enables the direct mapping of IONP tracers, the signal generation relies heavily on the magnetization reversal of the IONP tracers. As such, it is essential to tune the IONP’s magnetic properties in order to achieve good MPI image resolution. To date, most studies have focused in optimizing spherical magnetite IONPs in MPI applications. In this presentation, a systematic investigation of the effects of chemical doping and shape anisotropy on the MPI performance of IONP tracers will be discussed. Moreover, the demonstration of focused hyperthermia through an MPI-guided approach (hMPI) will be presented.

Magnetic materials have played a significant role in the development of automotive engineering from internal combustion to hybrid and electric vehicle powertrain technologies. Now advanced magnetic materials are enabling the future of electric vehicles (EVs). The newly developed alternatives to traditional rare-earth permanent magnets and electrical steels are driving the development of electric machines to operate at increasingly higher speeds with higher efficiency. In this talk, three types of magnetic materials in the EV applications will be introduced: permanent magnets for permanent-magnet motors, lamination materials such as electrical steel and amorphous/nanocrystalline ribbons, and soft magnetic composites for higher frequency applications. Currently, there are continuing demands for all types of magnetic materials with higher energy density and higher energy conversion efficiency. The future trend of the magnetic material applications in automotive industry will be discussed as well.

We have explored a new mechanism for switching magnetism and superconductivity in a magnetically frustrated iron-based superconductor using spin-polarized scanning tunneling microscopy (SPSTM) [1]. Our SPSTM study on single crystal Sr₂VO₃FeAs made of alternating self-assembled FeAs monolayer and Sr₂VO₃ bilayers shows that a spin-polarized tunneling current can switch the FeAs-layer magnetism into a non-trivial C₄ (2×2) order, which cannot be achieved by thermal excitation with unpolarized current. Our tunneling spectroscopy study shows that the induced C₄ (2×2) order has characteristics of plaquette antiferromagnetic order in the Fe layer and strongly suppresses superconductivity. Also, thermal agitation beyond the bulk Fe spin ordering temperature erases the C₄ state. These results suggest a new possibility of switching local superconductivity by changing the symmetry of magnetic order with spin-polarized and unpolarized tunneling currents in iron-based superconductors [2]. We also performed high-resolution quasiparticle interference (QPI) measurements, self-consistent BCS-theory-based QPI simulations and a detailed e-ph coupling analysis to provide direct atomic-scale proofs of enhancement of iron-based superconductivity due to the BCS mechanism based on forward-scattering interfacial phonons [3].

[1] J.-O. Jung et al., Rev. Sci. Instrum. 88, 103702 (2017)
[2] S. Choi et al., Phys. Rev. Lett. 119, 227001 (2017)
[3] S. Choi et al., Phys. Rev. Lett. 119, 107003 (2017)

Sm-Co 2:17 permanent magnets have been used in many applications including power generation, high performance industrial motors, travelling wave tubes for satellite communications, oscillators, insulators and other equipment in which the magnets operate at temperatures above 200^oC, or require high thermal stability of the magnetic properties. Sm-Co 2:17 permanent magnets with their unique cellular subgranular microstructure offer property advantages that can’t be matched by any other category of magnets. In this paper we will review some of the lesser known applications of Sm-Co 2:17 permanent magnets, including magnetocaloric refrigeration systems for high temperature applications, watt balances, and gyro guidance systems. The magnetic circuit design and numerical optimization process will be reviewed for each application.

We present an ab-initio study of the effects of partial atomic disorder on the static and dynamic magnetic properties of the full-Heusler alloys Co₂MnSi, Co₂MnSn, Co₂FeAl and Co₂MnAl. These compounds are interesting for spintronic applications because they could possess most of the characteristics required for performant magnetic electrode materials in spin valves or magnetic tunnel junctions. Their exceptional characteristics must however remain in the presence of the alloy disorders that may exist in these compounds.
The spin-polarized relativistic Korringa-Kohn-Rostoker (KKR) method was used with the coherent potential approximation (CPA) for describing partial random disorders intermediate between the L2₁, D0₃, B2 and A2 phases of these full-Heusler alloys.
We describe the modification of their magnetization, chemical species spin and orbital magnetic moments, spin polarization at the Fermi level and Gilbert damping parameter, as a function of the disorder rates. These results could be useful for experimentalists, helping them understanding the impact of partial alloy disorders on most of the physical properties that they measure for these compounds.

MoS₂ nanosheets were doped with vanadium (V) with a variety of concentrations using a hydrothermal method. Raman, X-ray photoelectron spectroscopy and electron paramagnetic resonance results indicate the effective substitutional doping in MoS₂. Without V doping, oxides such as MoO₂ and MoO₃ have been observed, whereas, with 5 at% V doping, the oxide disappears. Magnetic measurements show that room temperature ferromagnetism has been induced by V doping. Magnetization tends to increase with the increased V doping concentration. A very large coercivity up to 1.87 kOe has been observed in 5 at% vanadium doped MoS₂, which may attribute to a combination effect of localized charge transfer between V and S ions, pinning effect due to the in-between defects, stress induced by doping and shape anisotropy due to two-dimensional nature of MoS₂ ribbons

Magnetic spinel ferrites including Fe₃O₄, CoFe₂O₄, (Mn-Zn)- and (Ni,Zn)-ferrites form an important group in engineering magnets particularly for soft-magnetic applications. Though oxide-based magnets have the advantages such as excellent chemical stability, low cost of raw materials and fabrication, but intermetallic counterparts have high saturation magnetization (for example Fe-Co alloys) and high magnetic crystalline anisotropy (for example SmCo₅ and Nd₂Fe₁₄B). High saturation magnetization has been reported in Fe₃O₄ thin films [1,2]. However, their magnetization decreases quickly with increasing film thickness, and no magnetization enhancement was found in films with a thickness > 20-30 nm. Recently, we have used hydrothermal method for preparation of thick Fe₃O₄ films (> 100 nm). Enhanced magnetization over 1 Tesla has been found, and magnetization is sensitively dependent on MgO substrate orientation. Our further study has revealed that enhanced magnetization is due to spin flipping which is due to lattice expansion form cubic to tetragonal stabilized by C atoms and MgO substrate. Doping with 10% Co can result in high magnetocrystalline anisotropy that can lead in high coercivity with a energy product over 10 MGOe (100% enhancement compared to energy product achieved by hexaferrite). High electric resistance and very low coercivity have been obtained after doping with 10% Ni, showing a promising high-performance soft magnet.

[1] S. K. Arora, et.al, Phys. Rev. B 77 (2008) 134443.
[2] T. S. Herng,W. Xiao, S. M. Poh, F. He, R. Sutarto, X. J. Zhu, R. W. Li, X. M. Yin, C. Diao, Y. Yang, H. L. Huang, X. J. Yu, Y. P. Feng, A. Rusydi and J. Ding_, , NanoResearch 8 (2015) 2935.

Ferrites thin films have a number of technological applications in areas such as telecommunications (microwave and millimeter wave devices), magneto-electric coupling devices and are also promising candidates for future spintronic devices. However, spinel ferrite thin films such as NiFe₂O₄ (NFO) suffer from a number of structural and magnetic drawbacks, e.g. formation of antiphase boundaries and high magnetic saturation fields. We show that by using substrates having similar crystal structure and low lattice mismatch, one can avoid formation of antiphase boundaries and thereby obtain magnetic properties comparable to bulk single crystal. We used spinel MgGa₂O₄ and CoGa₂O₄ substrates, which have 0.8% and 0.2% lattice mismatch, respectively, with NFO to grow epitaxial films that are essentially free of antiphase boundaries and exhibit sharp magnetic hysteresis characteristics. Moreover, ferromagnetic resonance linewidths similar to those in single crystals are obtained. We have compared these results with NFO film grown on another spinel substrate MgAl₂O₄, which has 3.1% lattice mismatch, that has antiphase boundaries and clearly exhibits degraded properties. We also investigated spin transport properties of the films grown on the three substrates via the longitudinal spin Seebeck effect (LSSE). An increase in the spin voltage signal with reduction in lattice mismatch is observed, which is in correspondence with similar improvements in structural and magnetic properties. Thickness and temperature dependence of the LSSE for NFO films grown on different substrates have been investigated.

Since its inception almost six decades back, magnetic recording technology has seen over a million-fold increase in areal storage density together with a million-fold decrease in price/GB. These factors have helped magnetic recording based hard-disk drives (HDD’s) to remain the dominant auxiliary storage devices. Today, HDD’s with areal densities of ~1 Tbits/in² are in commercial production. In the mid-2000’s, magnetic recording technology evolved from longitudinal recording media that had in-plane magnetization to perpendicular magnetic recording (PMR) media that had out-of-plane magnetization. This shift meant that higher anisotropy materials were necessary to stabilize the grains against shape-based demagnetizing effects. In this regard, the invention of the CoCrPt-oxide material was a major breakthrough. Another major breakthrough was the introduction of the exchange-spring (ES) media. Originally proposed more than two decades back in permanent magnet design, the ES magnet was composed of both hard magnetic and soft magnetic phases, finely dispersed and in mutual contact with each other. Whereas the ES effect allowed permanent magnets to realize high energy products, the objective in PMR media design is rather different. In particular, the exchange-coupled composite (ECC) PMR media design consists of a stack of several magnetic layers, each of which has a different magnetocrystalline anisotropy and intergranular exchange coupling strength. Due to an incoherent reversal that begins in the soft(est) layer which then propagates across the interface and switches the hard layer(est) layer, ECC media allowed for improved media writability without sacrificing thermal stability. At the same time, the ES effect also facilitated narrower switching field distributions in ECC media, which enabled high levels of Signal-to-Noise Ratio. The ES concept, together with other innovations, have helped achieve areal densities of ~ 1Tbit/in².

While PMR has successfully transformed the areal density perspective, its further advancement faces significant challenges due to the superparamagnetic limit. Within the next few years, PMR is expected to transition to Heat-Assisted Magnetic Recording (HAMR) technology. The structurally ordered L1₀ phase of FePt has among the highest known magnetocrystalline anisotropies. Due to the high anisotropy, the smallest thermally stable grain sizes can be of the order of 3 to 4nm. At these grain sizes, the FePt medium is capable of supporting several Tbits/in² of data. This makes it the material system of choice for next-generation magnetic recording media. However, the high anisotropy also means that the FePt medium has to be heated with a laser to above its Curie temperature, and the recording bits written during the re-freezing process. Briefly, this is the concept underlying HAMR technology.

This tutorial style talk will review progress made in recent years on current & future magnetic recording media technologies.

The challenge of developing ultradense magnetic data storage technologies is the production of nanoscale magnetic bits, capable of being individually switched, and which can be assembled in a close-packed array with minimum bit-bit coupling. It has been shown that magnetic coupling can occur between neighboring magnetic nanoparticles (MNPs) in self-assembled nanoring structures, generating chiral, bistable domains, known as flux closure (FC) states. The magnetostatic fields within these FC states are confined within the ring structure, thus minimizing stray fields, and making MNP nanoring structures appealing as magnetic bits. Reported self-assembled MNP ring structures, however, are produced by evaporative techniques, and produce low yields of oblong, polydisperse rings with relatively large diameters, which cannot be assembled into organized arrays or layers. Constructing nanoscale assemblies with positional control at the nanometer level requires a templating method with high specificity and monodispersity, such as that offered by tobacco mosaic virus (TMV) coat protein, which, as a biomolecule, is inherently monodisperse in size, shape and surface chemistry. As a scaffold for nanoparticle self-assembly, TMV disks not only produce well-defined nanostructures, but close-packed structures wherein coupling between neighboring nanoparticles, which is essential to forming FC states, can occur. The EDC coupling reaction is utilized to conjugate propargylamine to the surface of superparamagnetic iron oxide nanoparticles (IONP) coated in 3,4-dihydroxyphenylacetic acid (DOPAC), to yield solution exposed alkyne groups on the IONP surface (IONP-alkyne). The solution exposed N-terminal amines of the TMV disks are similarly used to conjugate 5-azidopentanoic acid to the outer surface of the TMV disk, yielding solution exposed azide groups (TMV-azide), through EDC coupling. The copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction is then applied to covalently bind the IONP-alkyne to the TMV-azide disks, in order to form templated self-assembled ring structures for further characterization and FC studies. These structures not only open novel potential avenues for the assembly of ultradense data storage devices through the self-assembly of virus-templated superparamagnetic IONPs, but also provide an assembly, upon which, the fundamentals of FC states can be studied.

Cylindrical nanowires synthesized by controlled electrodeposition constitute excellent strategic candidates to engineer magnetic domain configurations. In this work, multisegmented CoNi/Ni nanowires are synthesized for tailoring a periodic magnetic structure determined by the balance between magnetocrystalline and magnetostatic energies. High-resolution Transmission Electron Microscopy confirms the segmented growth and the sharp transition between layers. Although both CoNi and Ni segments have similar fcc cubic crystal symmetry, their magnetic configuration is quite different as experimentally revealed by Magnetic Force Microscopy (MFM) imaging. While the Ni segments are single domain with axial magnetization direction, the CoNi segments present two main configurations: a single vortex state or a complex multivortex magnetic configuration, which is further interpreted with the help of micromagnetic simulations. This original outcome is ascribed to the tight competition between anisotropies. The almost monocrystalline fcc structure of the CoNi segments, as revealed by the electron diffraction patterns, which is atypical for its composition, contributes to balance the magnetocrystalline and shape anisotropies. The results of MFM measurements performed under in-plane magnetic field demonstrate that it is possible to switch from the multivortex configuration to a single vortex configuration with low magnetic fields.

The spin-transfer-torque (STT)-based magnetic random-access memory (MRAM) is one of the most promising nonvolatile memory technologies. [1,2] Prospective materials for the STT-MRAM devices should possess high magnetic anisotropy, high Curie temperature, high spin polarization at the Fermi level, and moderately low saturation magnetization. Additionally, the prospective materials should be grown as epitaxial thin films on MgO, a commonly used tunneling barrier. Mn-based Heusler compounds have emerged as one of the most promising classes of high magnetic-anisotropy materials. Mn₂PtSn is one such compound, which is predicted to have an inverse tetragonal crystal structure and high spin polarization with very large magnetic anisotropy of about 3.04 meV/atom (50.0 Merg/cm³).[3] Here, we report that epitaxial thin films of Mn₂PtSn have been grown on a MgO (001) substrate. The structure of these epitaxial films is inverse tetragonal with lattice constants a = 0.4505 nm, and b = 0.6124 nm with space group I-4m2, No. 119. The calculated ground-state spin polarization at the Fermi energy is around 90%. Measured saturation magnetization is 3.2 μ_B/f.u.. The magnetization measurements revealed an in-plane anisotropy energy of 10.0 Merg/cm³. The longitudinal resistivity shows an approximately linear increase with increasing temperature up to 300 K, showing a metallic behavior with the residual resistivity ρ₀ of 250 µΩcm. The magnetoresistance ratio is 1.0 % with a magnetic field of 70 kOe applied out of the plane. These results show the potential of this material for spin valves or magnetic tunnel junctions as high-performance magnetic memory elements. This research is supported by NSF-DMREF: SusChEM (1436385), NSF-NNCI (1542182), NCMN.

Multiferroic heterostructure has drawn a great amount of interest due to robust coupling between magnetic and electric orders and potential spin-based applications. Magnetoelectric coupling in multiferroic heterostructure involves several mechanisms including strain-mediated effect, charge-driven screening effect and exchange bias coupling. In particular, La_1-xSr_xMnO₃ (LSMO)/PbZr_xTi_1-xO₃ (PZT) bilayered heterostructure is extensively investigated for its high sensitivity of electronic and spin state. Charge-mediated screening effect is limited to a few unit cells in half-metallic manganite. Strain effect and layer thickness may also affect magnetization. The choice of dominant coupling mechanism in bilayered multiferroic heterostructure depends on film thickness. In bilayered multiferroic heterostructure, a better understanding of the effects of film layer thickness on charge and strain and corresponding properties warrants further exploration.
In this work, we report a systematic study on the effects of PZT thickness on the LSMO (~14 unit cells)/PZT bilayered multiferroic heterostructure. It was found that magnetic properties of LSMO changed consistently as a function of PZT thickness. Reciprocal space mapping and half-integer x-ray diffraction were used to probe the crystal lattice of LSMO and local structure of MnO₆ octahedron. Out-of-plane PFM was used to directly measure the spontaneous polarization of as-grown PZT with different thicknesses. Interfacial charge driven screening effect of valence modulated magnetic moment was demonstrated by x-ray absorption near edge spectroscopy. Local measurements of strain- and charge-mediated magnetization changes were investigated by scanning transmission electron microscopy. In summary, thickness-dependent control of magnetism in LSMO/PZT bilayered heterostructure is observed. The strain- and charge-mediated intrinsic multiferroic coupling in LSMO/PZT is carefully investigated.