Symposium EM9 : Materials and Nanostructures for Magnetic Skyrmions

Many of structurally-chiral cubic magnets host the triangular-lattice skyrmion crystal (SkX) as the thermodynamic equilibrium state. However, this state exists only in a narrow temperature and magnetic-field region just below the magnetic transition temperature Tc, while a helical or conical magnetic state prevails at lower temperatures. We report that the relatively rapid cooling by way of the thermodynamical skyrmion lattice phase in chiral lattice magnets can suppress the transition to the helical or conical state, instead realizing robust metastable SkX states that survive over a very wide temperature and magnetic-field region, including down to zero temperature and up to the critical magnetic field of the ferromagnetic transition. Furthermore, the lattice form of the metastable SkX is found to undergo reversible transitions between a conventional triangular lattice and a novel square lattice upon varying the temperature. These findings exemplify the topological robustness of the once-created skyrmions, and establish metastable skyrmion phases as a fertile ground for technological applications.

This work was done in collaboration with, H. Oike,K. Karube, J.S. White, N. Reynolds, J.L. Gavilano, A. Kikkawa, Y. Tokunaga, H.M. Rønnow, X.Z. Yu, D. Morikawa, T. Nakajima, T.Arima, F. Kagawa, and Y. Taguchi.

Magnetic skyrmions are a perfect example for the ensuing complexity of mesoscale magnetism stemming from competitions between interactions crossing many lengthscales [1]. The interplay between applied magnetic fields, magnetic anisotropies, as well as symmetric and antisymmetric exchange interactions, can stabilize topologically distinct spin textures known as magnetic skyrmions. Due to their topology magnetic skyrmions can be stable with quasi-partcile like behavior, where they can be manipulated with very low electric currents. This makes them interesting for extreme low-power information technologies [2], where data is envisioned to be encoded in topological charges, instead of electronic charges as in conventional semiconducting devices. Towards the realization of this goal we demonstrated at room temperature stable magnetic skyrmions in magnetic heterostructures, which can be manipulated using spin Hall effects [3]. Furthermore, using inhomogeneous electric charge currents even allows the generation of skyrmions in a process that is remarkably similar to the droplet formation in surface-tension driven fluid flows [4]. Micromagnetic simulations reproduce key aspects of this transformation process and suggest that a possible second mechanism at higher currents that does not rely on preexisting magnetic domain structures [5]. Indeed, we demonstrated this second mechanism experimentally using non-magnetic point contacts. Lastly, we demonstrated that the topological charge gives rise to a transverse motion on the skyrmions, i.e., the skyrmion Hall effect [6], which is in analogy to the ordinary Hall effect, which is due to the motion of electrically charged particles in the presence of a magnetic field.
This work was supported by the U.S. Department of Energy, Office of Science, Materials Sciences and Engineering Division. Lithographic patterning was carried out at the Center for Nanoscale Materials, an Office of Science user facility, which is supported by DOE, Office of Science, Basic Energy Sciences under Contract No. #DE-AC02-06CH11357.

References
[1] A. Hoffmann and H. Schultheiss, Curr. Opin. Solid State Mater. Sci. 19, 253 (2015).
[2] A. Hoffmann and S. D. Bader, Phys. Rev. Appl. 4, 047001 (2015).
[3] A. Hoffmann, IEEE Trans. Magn. 49, 5172 (2013).
[4] W. Jiang, et al., Science 349, 283 (2015).
[5] O. Heinonen, et al., Phys. Rev. B 93, 094407 (2016).
[6] W. Jiang, et al., arXiv:1603.07393.

Magnetic skyrmions are topologically non trivial collection of spins which collectively behave as a single particle during their movement under the application of electric field [1]. MnSi is one such material which hosts skyrmion phase or A-phase for certain range of magnetic field just below its magnetic transition from paramagnetic to helimagnetic phase. The formation of such unusual spin structure comes from the competing interactions between Dzyaloshinskii-Moriya interaction strength (D) and Heisenberg exchange interaction (J). The period of helical magnetic structure is determined by ratio of these two strengths (D/J) [2]. By now it is clear that both chemical substitution on Mn site and application of positive hydrostatic pressure modify the electronic/magnetic behavior of this compound [3-4]. However, it is relatively unknown how these properties are modified during chemical substitution on Si site and/or under the application of negative pressure. Recently, we are working toward exploring the effect of chemical substitution on Si site and the effect of negative pressure by substituting Si with Ga and Al. Our study shows that the transition temperature, saturation moment and temperature range of stability of skyrmion phase are increased due to such substitution. We will discuss our findings within the context of magnetization and small angle neutron scattering measurements.
References:
Nagaosa, Naoto, and Yoshinori Tokura. "Topological properties and dynamics of magnetic skyrmions." Nature nanotechnology 8.12 (2013): 899-911.
Mühlbauer, S., et al. "Skyrmion lattice in a chiral magnet." Science 323.5916 (2009): 915-919.
Pfleiderer, C., et al. "Non-Fermi liquid metal without quantum criticality."Science 316.5833 (2007): 1871-1874.
Bauer, A., et al. "Quantum phase transitions in single-crystal Mn_1−xFe_xSi and Mn_1−xCo_xSi: Crystal growth, magnetization, ac susceptibility, and specific heat." Physical Review B 82.6 (2010): 064404.

GaV₄S₈, a member of the lacunar spinel family, is the first known example of skyrmion-host materials with non-chiral but polar crystal structure. This compound is a magnetic semiconductor with a rhombohedral crystal symmetry (R3m) and an easy-axis magnetic anisotropy. In this compound we observed the formation of a Néel-type skyrmion lattice, which exists over a broad temperature range. This is in contrast to Bloch-type skyrmions found in chiral magnets, which are stabilized by thermal fluctuations in small pockets of the phase diagram. We found that the orientation of these Néel-type skyrmions is not controlled by the external magnetic field, but instead confined to the magnetic easy axis. Another isostructural compound of this family is GaV₄Se₈, which is characterized by a weaker anisotropy. In this material the Néel-type skyrmion state is even more extended, namely it is the first bulk compound where the skyrmion lattice is stable down to zero temperature. We found that the skyrmion phase can also be stabilized in GaV₄S₈ by moderate hydrostatic pressures. Due to the polar nature of these crystals, the Néel-type magnetic skyrmions wear a ferroelectric dressing and exhibit strong static and dynamic magnetoelectric effects.

Magnetic skyrmion is a topological spin texture observed in several helimagnets. Inspired by the novel physics tracing back to its nontrivial topology and promising applications in next generation memory device and ultra-dense data storage, a full understanding of these materials is an urgent subject. We have analyzed the symmetry of a typical helimagnet family, the B20 compounds, and derived an effective description of the electron conduction therein, which explains recent experiments and suggests other predictions. On the other hand, a similar symmetry analysis leads to a full classification and search strategy of helimagnets. Based on this guideline, we discovered a new helimagnet family, the molybdenum nitride, harboring skyrmions, which is confirmed by magnetic imaging. Many interesting properties of molybdenum nitrides will also be addressed.

We predict that magnon motive force leads to nonlinear chiral damping in both conducting and insulating ferromagnets¹. Our theory is analogous to the electronic feedback damping, where an emergent electric field generates a backflow of electrons and affects the magnetization dynamics dissipatively. However, we find that the strength of the magnonic effect grows with temperature and is inversely properties to Gilbert damping parameter. Similar to its electronic counterpart, the influence of magnon motive force is enchanced in the presence of Dzyaloshinskii-Moriya interaction.

We estimate that magnonic damping can strongly influence the motion of domain walls and skyrmions at finite temperatures, which are relevant for new magnetic data storage devices. In particular, we find that in systems with low Gilbert damping and strong Dzyaloshinskii-Moriya interaction, moving chiral magnetic textures results in magnon motive forces can induce large spin and energy currents in the transverse direction, leading to observable temperature drops across the sample.

The predictions of our theory can be tested experimentally by observing the skyrmion hall angle or studying domain wall dynamics in materials with small Gilbert damping such as CoFeB multilayers at room temperature.

This work was supported by the DOE Early Career Award DE-SC0014189, and in part by the NSF under Grants Nos. Phy-1415600, and DMR-1420645 (UG).



(1) Utkan Güngördü, Alexey A. Kovalev, Phys. Rev. B 94, 020405(R) (2016)

The talk is on individual skyrmions induced by interface Dzyaloshinskii-Moriya Interactions (DMI) in thin magnetic films or multilayers. I will present:
Ab-initio calculations of the characteristic features of interface DMI [1]: extension of the DMI away from the interface in the magnetic film, thickness dependence, influence of the existence of proximity-induced magnetism in neighbor layers, influence of interface roughness, perspective with new materials…
Experimental characterization of small skyrmions at room temperature in multilayers such as (Ir/Co/Pt)x10 [2].
Experimental results on the current-induced motion and electrical detection of small skyrmions in multilayers.
References:
[1] H. Yang et al, Phys. Rev. Letters 115, 267210, 2015
[2] C. Moreau-Luchaire et al, Nature Nanotechnology 11, 444, 2016

We study how skyrmions in insulating chiral magnets can be manipulated by thermal currents. Thermal magnons couple efficiently to skyrmions via an emerging magnetic field arising both from Berry phase effects and relativistic spin-orbit coupling. This coupling gives rise to an intrinsic damping mechanism for the skyrmion and to forces both parallel and perpendicular to thermal gradients. Semiclassical approaches are used to investigate the crossover from the ballistic regime to a regime of diffusive magnons. We show why and under what conditions stochastic Landau Lifshitz Gilbert equations drastically fail to describe thermal transport and the coupling of skyrmions to heat currents. Time permitting, we will also advertize a new design principle for a skyrmion-based racetrack memory [1] where skyrmions move on two (or more) parallel lanes and information is encoded in the lane number.
[1] Magnetic Skymrions on a Two-Lane Racetrack, Jan Muller, arXiv:1606.07412

Most theoretical studies in the past have focused on B20 materials with broken bulk inversion symmetry, where skyrmions are stabilized by easy-axis anisotropy. We will describe here our recent theoretical work on systems that break surface inversion (or reflection) symmetry [1,2], in addition to bulk inversion. This leads to two distinct Dzyaloshinskii-Moriya interactions (DMI) arising from Rashba spin-orbit coupling (SOC) and from Dresselhaus SOC. We show [3] that skyrmions become progressively more stable with increasing ratio of Rashba to Dresselhaus DMI, and extend into the regime of easy-plane anisotropy. We find that the spin texture and topological charge density of skyrmions develops a nontrivial spatial structure with quantized topological charge in a unit cell given by a Chern number. Our results give a design principle for tuning Rashba SOC and magnetic anisotropy to stabilize skyrmions in thin films, surfaces, interfaces, and bulk magnetic materials that break reflection symmetry.

[1] S. Banerjee, O. Erten, and M. Randeria, Nature Phys. 9, 626 (2013)
[2] S. Banerjee, J. Rowland, O. Erten, and M. Randeria, Phys. Rev. X 4, 031045 (2014)
[3] J. Rowland, S. Banerjee and M. Randeria, Phys. Rev. B 93, 020404(R) (2016)

Skyrmions, novel topologically stable spin vortices, hold promise for next-generation magnetic storage due to their nanoscale domains to enable high information storage density and their low threshold for current-driven motion to enable ultralow energy consumption. One-dimensional (1D) nanowires are ideal hosts for skyrmions since they not only serve as a natural platform for magnetic racetrack memory devices but also can potentially stabilize skyrmions. We have developed methods to synthesize free standing nanowires of many silicides, including the B20 monosilicides (MnSi, FeSi, CoSi) and their alloys (Fe_xCo_1-xSi), many of which display exotic helimagnetic and skyrmion magnetic orderings. We have already recently synthesized cubic B20 Mn_xFe_1-xSi and FeGe nanowires that can host skyrmions with a variety of domain size from 10 to 230 nm and stability up to about 280 K. In collaboration with Prof. Y. Tokura, Prof. Mingliang Tian, and Prof Martino Poggio, we have used Lorentz TEM, magnetotransport measurements, as well as dynamic magnetic cantilever measurements to confirm that magnetic Skyrmions are stable over a larger magnetic field-temperature range in MnSi nanowires compared to bulk crystal and thin films. We have developed a general method to measure Hall effect in nanowires. Using the topological Hall effect (THE) of MnSi nanowires, we further confirmed the extended phase stability and demonstrated the current-driven motion of skyrmions in this extended skyrmion phase region. These results open up the exploration of nanowires as an attractive platform for investigating skyrmion physics in 1D systems and exploiting skyrmions in magnetic storage concepts.
1) Schmitt, A. L.; Higgins, J. M.; Szczech, J. R.; Jin, S., Synthesis and Applications of Metal Silicide Nanowires; J. Mater. Chem. 20 (2010), 223.
2) Higgins, J. M.; Ding, Ruihua; DeGrave, J. P.; Jin, S., Signature of Helimagnetic Ordering in Single-Crystal MnSi Nanowires; Nano. Lett. 10 (2010), 1605.
3) Yu, X.; DeGrave, J. P.; Hara, Y.; Hara, T.; Jin, S.; Tokura, Y., Observation of the Magnetic Skyrmion Lattice in a MnSi Nanowire by Lorentz TEM; Nano Lett. 2013, 13, 3755.
4) Du, H.; DeGrave, J. P.; Xue, F.; Liang, D.; Ning, W.; Yang, J.; Tian, M.; Zhang, Y.; Jin, S., Highly Stable Skyrmion State in Helimagnetic MnSi Nanowires; Nano Lett. 2014, 14, 2026.
5) Mehlin, A.; Xue, F.; Liang, D.; Du, H. F.; Stolt, M. J.; Jin, S.; Tian, M. L.; Poggio, M., Stabilized Skyrmion Phase Detected in MnSi Nanowires by Dynamic Cantilever Magnetometry; Nano Lett. 2015, 15, 4839.
6) DeGrave, J. P.; Liang, D.; Jin, S.; A General Method to Measure Hall Effect in Nanowires: Examples of FeS2 and MnSi; Nano Lett. 2013, 13, 2704.
7) Liang, D.; DeGrave, J. P.; Stolt, M. J.; Tokura, Y.; Jin, S., Current-driven dynamics of skyrmions stabilized in MnSi nanowires revealed by topological Hall effect; Nature Commun. 2015, 6, 8217.

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 one-dimensional nanowire or nanostripes. In this talk, we report experimental observation of individual skyrmions in confined helimagnets. We will first discuss the formation and stability of skyrmion cluster states in MnSi nanowires by using conventional magneto-resistant measurement. Then, the observation of single skyrmion chain in FeGe nanostripes is outlined by using high resolution Lorentz transmission electron microscopy.
References:
1: Du H. F., et al., Nano Lett. 14. 2026 (2014)
2: Du H. F., et al., Nature Communications. 6. 7637 (2015)
3: Du H. F., et al., Nature Communications. 6. 9504 (2015)

Magnetic skyrmions have garnered much interest over their potential use as information carriers in racetrack memory systems. Nanowires (NWs) that can host skyrmions provide a natural platform to implement racetrack memory devices while also serving as a means of stabilizing the skyrmion phase. Herein we report the synthesis of cubic B20 structured FeGe synthesized for the first time through a chemical vapor deposition (CVD) process involving a Ge (100) substrate seeded with finely ground particles of cubic FeGe powder. This critical seeding step results in a preferential growth of the cubic FeGe nanostructures over other phases of the Fe-Ge binary system especially the hexagonal FeGe polymorph, which would be the product without the seeding. Diameters and lengths of the cubic FeGe NWs can be tuned by adjusting the CVD reaction conditions. X-ray diffraction and extensive transmission electron microscopy analyses both confirm the growth of the single crystal cubic FeGe NWs on the seeded samples. Lorentz transmission electron microscopy studies reveal a skyrmion lattice phase with a greater stability in the cubic FeGe NWs than that of the bulk counterpart and more similar to ultra-thin films of FeGe. Application of magnetic fields around 0.4 to 0.5 T at 93 K transforms the skyrmion lattice into a skyrmion chain state. Four probe magnetoresistance measurements carried out on the NWs showed transitions that can be related to the critical fields for the skyrmion phase. The successful synthesis of these cubic FeGe NWs creates opportunities for more intensive studies on the creation, manipulation, and detection of skyrmions to be carried out in the future.

Skyrmions are a topologically non-trivial spin texture, and they are distinguished from a ferromagnetic or helimagnetic state by the integer topological charge Q. For information processing, the topological charge Q serves as the state variable representing Boolean logic 0 and 1, and creation, annihilation, control, and sensing of individual skyrmions is required. I will describe our theoretical investigations of Skyrmion creation and annihilation with polarized spin current and spin waves and our discovery the topological spin Hall effect (TSHE) [1-3].

Vertically injected spin-polarized electron current into a helimagnetic thin film is an efficient mechanism to create Skyrmions. The critical switching current density is 10 MA/cm², which decreases with the easy-plane type uniaxial anisotropy and thermal fluctuations. In-plane spin polarization of the injected current provides ultrafast switching times (100 ps) and reliable switching outcomes. Once formed, the Skyrmion is protected by an energy barrier determined by the spin geometry at the point of transition between a trivial and nontrivial spin topology of ~2J.

Skyrmions in a cross-bar geometry can be created, destroyed, and sensed using spin waves and electrical current. With the Dzyaloshinskii-Moriya (DM) interaction localized to the center region of the cross bar, the injection of a spin wave from one of the leads can both create and destroy a Skyrmion, but the optimum frequency for creation is below the critical frequency for Skyrmion annihilation. If a skyrmion already exists in the cross bar region, a spin wave below the critical frequency causes the Skyrmion to circulate within the central region. The effective field resulting from the DM interaction and the emergent field of the Skyrmion acting on the spin wave drive the creation and annihilation processes.

With a Skyrmion localized in the center of a crossbar, readout could be performed using the topological Hall effect. The Skyrmion generates an emergent gauge field acting on a band electron of one flux quantum, and the direction of the gauge field is opposite for opposite electron spins. Therefore, electrons with opposite spins are deflected in opposite directions. This, we found, could separate the spin current from the charge current and generate an unconventional topological spin Hall effect (TSHE). This effect applies equally well to a Skyrmion lattice. The THSE is similar in magnitude to the SHE in Pt thin films. However, the physical mechanism giving rise to the TSHE is fundamentally different from the one leading to the spin Hall effect in strong spin-orbit coupled (SOC) systems. The TSHE results from the topological property of the Skyrmion spin texture in real space.

1. G. Yin et al., Phys. Rev. B, 93, 174403 (2016). DOI: 10.1103/PhysRevB.93.174403
2. Y. Liu et al., Appl. Phys. Lett., 107, 152411, (2015). DOI: 10.1063/1.4933407
3. G. Yin et al., Phys. Rev. B, 92(2), 024411 (2015). DOI: 10.1103/PhysRevB.92.024411

The nitrogen-vacancy (NV) center in diamond is an atomic scale defect with long spin coherence times from cryogenic to room temperature. These properties make the NV an ideal and versatile sensor for applications where high spatial and magnetic field resolution is required. In this presentation, I will first introduce the concepts of NV center magnetometry and describe how a single NV sensor is incorporated into a robust scanning probe microscope that operates from room temperature down to 5 K [1]. Then I will present images of skyrmion structures in the multilayer stack Ta/CoFeB/Pt/MgO/Ta [2]. Magnetic skyrmions are topologically stabilized magnetization structures with a number of characteristics that make them appealing for use in future high-density, low-power memory and logic devices. We use our NV microscope to investigate the magnetization texture of skyrmion bubble domain walls. The bubble domain wall type has important implications for the functionality of skyrmions in real devices, in particular the topological stability of the skyrmions and the current density required for their manipulation. I will also describe our exploration of skyrmion pinning effects and discuss the relevance of these effects for manipulating skyrmions.

References
1. Matthew Pelliccione, Alec Jenkins, Preeti Ovartchaiyapong, Christopher Reetz, Eve Emmanouilidou, Ni Ni, Ania C. Bleszynski Jayich, “Scanned probe imaging of nanoscale magnetism at cryogenic temperatures with a single-spin quantum sensor”, Nature Nano. AOP, DOI: 10.1038/nnano.2016.68 (2016).
2. Guoqiang Yu, Pramey Upadhyaya, Xiang Li, Wenyuan Li, Se Kwon Kim, Yabin Fan, Kin L. Wong, Yaroslav Tserkovnyak, Pedram Khalili Amiri and Kang L. Wang, “Room-Temperature Creation and Spin−Orbit Torque Manipulation of Skyrmions in Thin Films with Engineered Asymmetry”, Nano Letters 16 (3), 1981-1988, DOI: 10.1021/acs.nanolett.5b05257 (2016).

Nanoscale skyrmions in metallic ultrathin films and multilayers [1,2], stabilized by interfacial Dzyaloshinskii-Moriya interactions [3], have recently become of significant interest due to their great potential for future magnetic memory and logic devices [4]. Based on the first observation and manipulation of individual skyrmions in Pd-Fe bilayers epitaxially grown on Ir substrates [5-7] by spin-polarized scanning tunneling microscopy (SP-STM) techniques [8], a large number of skyrmion-based device concepts have been proposed which profit from the small size, enhanced stability and easy movement of skyrmions in nanostructured magnetic films and multilayer structures.

Atomic-resolution three-dimensional spin maps of nanoscale skyrmion lattices [9-11] as well as individual skyrmions [5-7] by SP-STM were found to be in excellent agreement with early pioneering theoretical predictions of chiral magnetic skyrmions [12,13]. By locally injecting spin-polarized electrons from an atomically sharp SP-STM tip, writing and deleting of individual skyrmions has been demonstrated, making use of spin-transfer torque exerted by the injected high-energy spin-polarized electrons [5]. Alternatively, individual skyrmions can be created and deleted by local electric fields [14], which can be of great advantage in view of energy-efficient skyrmionic device concepts. The subsequent detection of the written skyrmions can also be achieved by electrical means rather than by using a magnetic sensing element [15]. Recently, it has been demonstrated that it is also possible to drive trains of individual room-temperature skyrmions along magnetic nanowire tracks at speeds exceeding 100 m/s using short current pulses [16]. These results highlight the potential of current-driven skyrmions for future racetrack-type memory applications [17].

References
[1] R. Wiesendanger, Nature Reviews Materials 1, 16044 (2016).
[2] Focus Issue on Magnetic Skyrmions, eds. A. Fert, N. Nagaosa, M. Thorwart, and
R. Wiesendanger, New J. Phys. (2016).
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[10] A. Sonntag et al., Phys. Rev. Lett. 113, 077202 (2014).
[11] J. Brede et al., Nature Nanotechnology 9, 1018 (2014).
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[14] P.-J. Hsu et al., Nature Nanotechnology (2016), in press.
[15] C. Hanneken et al., Nature Nanotechnology 10, 1039 (2015).
[16] S. Woo et al., Nature Materials 15, 501 (2016).
[17] S. Krause and R. Wiesendanger, Nature Materials 15, 493 (2016).

Single defects in diamond, especially the nitrogen vacancy impurity (NV center), can serve as sensitive probes for magnetic fields with nanoscale spatial resolution. Our group’s goal is to utilize diamond NV centers for the imaging of nanoscale magnetic phenomena in condensed matter physics.

In this talk I will give an introduction into nanoscale diamond magnetometry, and attempt to highlight the pros and cons of the technique for the investigation of skyrmion materials. In a second part I will present a case study where diamond magnetometry was applied to the helimagnetic phase of bulk FeGe [1].

[1] A. Dussaux, P. Schoenherr et al., Nature Communications, in press (2016); arXiv:1503.06622

A magnetic skyrmion is a topological twist of two-dimensional spin texture which exhibits many fascinating properties. In experiment, magnetic skyrmions were realized in 2009 in several experimental systems as a result of Dzyaloshinsky-Moriya interactions (DMI). An alternative approach is to produce non-collinear spins in magnetic vortex states. With this motivation, we fabricated single crystalline Co disks on perpendicularly magnetized Ni/Cu(001) film to create artificial skyrmions whose topology can be tailored by changing the relative orientation between the vortex core polarity and the surrounding perpendicular magnetization. In this way, we studied the topological effect of the skyrmion using Photoemission Electron Microscopy (PEEM). By applying an in-plane magnetic field pulse of various strength, we find strong evidence that the annihilation of the skyrmion core depends on the topological skyrmion number of the system.

The twisted magnetization textures in chiral magnets are inherently frustrated, similarly to the mesophases in chiral liquid-crystals. The twisted basic texture can become dramatically altered by the penetration of secondary twists over larger lengths and the formation of defects. Hence, a well-ordered and smooth texture like a simple spiral may be twisted or defected. In chiral liquid-crystal systems, the frustration results in the formation of defects like the disclination networks of blue phases or twisted-grain-boundary phases. Such states can easily be shaped and transformed under the influence of competing anisotropies, e.g., by applied fields in the bulk and by anchoring the molecules of a liquid crystal at surfaces.
We investigated surfaces of FeGe single crystals with the cubic B20 structure using various magnetic imaging techniques and found a ferromagnetic order above the magnetic ordering transition in the bulk. We discover a static defect-ordered state with a network of line-defects emerging near the surface under the influence of a particular surface-magnetic ordering transition. These defects of the helical magnetic order are topologically necessary lines where the magnetic order becomes singular or passes through zero at elevated temperatures. This ferromagnetic skin has a strong uniaxial anisotropy and frustrates the helimagnetic texture by anchoring it to the surface. In the spiral, below the Neel temperature at 279 K, conical modulations in the ferromagnetic surface layer are observed that prove the formation of a network of dislocations. This is because the propagation direction of the surface-modulation deviates from the propagation direction in the bulk. Near the magnetic ordering temperature, a coexistence of bubble-like circular domains and stripes is observed in the surface layer. This illustrates the appearance of complex three-dimensional textures with defects, double-twists and spiral-like kinks near the surface and related to the particular surface-magnetic ordering. Hence, at the first-order transition between the precursor state and spiral order in zero magnetic field of FeGe, a co-existence of helical and skyrmionic textures is revealed. Ab initio calculations have been used to address the existence of enhanced spin-moments at the surfaces of FeGe and an increased effect of spin-orbit coupling. This explains the experimental observations of a surface-magnetic ordering in FeGe, which acts like a strongly uniaxial ferromagnetic film with an Ising-like character on the underlying spiral bulk state.
Similar experiments on the isostructural compound FeSi give evidence of a fragile magnetic ordering at the surface of this anomalous paramagnetic semiconductor too. This may mean that the FeSi surfaces possibly behave like strongly anisotropic ultrathin magnetic films, while no magnetic long-range ordering takes place in the bulk.

In the past decade the study of skyrmions [1,2] has been intense, and with the renewed emphasis on interfacial Dzyaloshinskii-Moriya interactions in thin film multilayers [3,4] the possibility of using them in devices [5] is more imminent. Several techniques have been crucial in uncovering the potential of skyrmions, notably neutron reflectometry [6,7], Lorentz-TEM [8], STM [9]. Other techniques can provide in-situ information of hidden layers supporting the magnetic structures, in particular XMCD-PEEM [10], and STXM [4]. In this contribution we show results of magnetic force microscopy (MFM) measurements of skyrmion multilayers which reveal several outstanding features of the technique. These include high spatial and magnetic resolution, and the ability to obtain quantitative measures of the stray fields of the magnetic textures, with the important added benefit of being readily available.

Specifically, we show we can maintain a constant 12nm-tip-sample distance by MFM in vacuum, and use the technique to obtain high-resolution images of the stray fields over multilayers comprising 5 repeats of Ir/Co(0.6nm)/Pt. In them we find that skyrmions coexist with domains, and have sizes that are inconsistent with a single value of the Dzyaloshinskii-Moriya interaction (DMI), as e.g. the average. We show how we can use measured magnetic structures to obtain an accurate transfer function that allows a comparison between measured and simulated images of skyrmion stray fields.

References

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Skyrmions are stable topological defects that occur at the nanoscale, forming a close-packed hexagonal lattice. They are of great interest for spintronics because of their potential application to ultradense memory devices. Experimental bulk methods for exploring Skyrmions are limited to measurements of magnetic, electric, and thermal properties; ac susceptibility being one of the most reliable methods to detect them.
Recently, measurements of the elastic stiffness and ultrasonic absorption using conventional pulse-echo technique were realized, revealing the reversal of c11 and c33 in the skyrmion phase relative to the conical or helical phases. Other ultrasound techniques such as Resonant Ultrasound Spectroscopy (RUS) have not been used yet to detect and study the skyrmion phase. RUS offers complimentary information, obtaining the full elastic tensor with one measurement. It is sensitive to changes in symmetry variation as function of temperature and field as the material traverses different phases. Here, we present studies of MnSi, Co-doped MnSi and Co-Zn-Mn using RUS and discuss our results.

The interaction between conducting electrons and spin textures in magnetic materials and heterostructures gives rise to many important phenomena, including giant magnetoresistance and spin transfer torque, that are essential for spintronics. In addition, the spin-orbit interaction that the electrons experience in solids also plays essential role. In cubic B20 chiral magnets (FeGe, MnSi, Fe_1-xCo_xSi, etc.), in addition to the exchange coupling in usual ferromagnets, the presence of Dzyaloshinskii-Moriya (D-M) interaction due to broken inversion symmetry and spin-orbit coupling gives rise to a helical ground state with a spiral spin texture. At finite temperatures and with a magnetic field, the resultant conical and Skyrmion spin textures exhibit spectacular static and dynamic properties.
In this work, we show the unconventional spin-dependent transport in B20 (Fe-Co)Si and the manipulation/detection of spin textures including Skyrmions. As a result of the broken inversion symmetry, the 4-fold rotation symmetry is also broken in the cubic B20 materials. We discuss the unusual anisotropic magnetoresistance inherent to the broken 4-fold rotation symmetry and the spin-orbit coupling. We have determined the intrinsic resistivities of helical spin texture (i.e., spin helix), which is a macroscopic Bloch domain wall, with current parallel and perpendicular to the helix. The ratio of the intrinsic resistivities of spin helix for current parallel and perpendicular to the helix, independent of temperature and composition, is much smaller than those observed in magnetic domain wall. We also describe the creation and detection of the magnetic kink state, a new spin texture that has been proposed in theory. The transport signature of Skyrmions and Skyrmions' response to a rotating magnetic field will be discussed.

We report the synthesis and measurements of magnetic and transport properties of single crystalline Ru_1-xCo_xGe (x = 0.15 & 0.20, nominal). RuGe is diamagnetic and a small band gap insulator which crystallizes in B20 cubic structure. Interestingly CoGe is also non-magnetic and crystallizes in B20 Structure when grown under pressure. Doping Co in Ru sites nucleates magnetic moment and results in a magnetic ground state. Single crystals of Ru_0.85Co_0.15Ge and Ru_0.8Co_0.2Ge were synthesized and both powder XRD and single crystal XRD performed on those crystals confirmed the B20 structure. From the ac susceptibility measurement under ambient pressure, a magnetic transition was observed at around 5.5 K for nominal Ru_0.85Co_0.15Ge and 8.5 K for nominal Ru_0.8Co_0.2Ge. Magnetization measurements suggest a low saturation moment where the overall behavior is akin to other B20 compounds such as MnSi and Fe_1-xCo_xSi. Ru_1-xCo_xGe is only the second example after Fe_1-xCo_xSi, in which magnetism was found by chemical substitution between a nonmagnetic insulator (RuGe) and a non-magnetic semimetal (CoGe). Unlike the Fe_1-xCo_xSi compounds, Ru_1-xCo_xGe demonstrates conclusively that the presence of Fe is not necessary for nucleating a magnetic state in an FeSi-like system suggesting a more itinerant mechanism. Transport measurements are similar to the behvaior of Fe_1-xCo_xSi where a two step increase in resistivity was observed with decreasing the temperature. The Magnetoresistance is positive and decreases with temperature. These measurements suggest that the introduction of Co has induced a small density of charge carriers as well as magnetic moments. The magnetization and susceptibility data suggest a weak ferromagnetic or helimagnetic behavior where the DM interaction may be important.

Advancements of scanning probe microscopy have been contributing to broaden fundamental understating of surface physics. Low temperature scanning tunneling microscopy (STM) combined with synchrotron based X-rays provides a new tool to probe magnetic states on surfaces at high spatial resolution. The recent studies demonstrated the technique has capabilities to extract chemical information with sensitivity at the atomic limit [1] and localized magnetic contrast by utilizing polarized beams [2]. Here, a combination of STM and X-ray magnetic circular dichroism (XMCD) techniques is utilized to image skyrmions on conducting silicon substrate. Such a new type of measurement technique will benefit the communities to study localized surface magnetic properties of materials at atomic resolution.

References
[1] N. Shirato et al., Nano Letters 14, 6499 (2014).
[2] A. DiLullo et al., J. Synchrotron Rad. 23, 574 (2016).

Magnetic skyrmions are promising for magnetic storage and maybe even computation owing to their nanometer-scale size and the very-low electrical current required to move them compared to magnetic domain walls. In this talk, we present our most recent progress toward measurements of the depinning current of skyrmions in mesoscopic samples. We present transport and magnetization measurements on high quality single crystals from the MnSi compounds family. Some of the samples are prepared using Focused Ion Beam (FIB) to make few-micrometer thick lamellas extracted from a single crystal. This process enables us to have almost perfect control of the Hall geometry and to fabricate samples thin enough for extremely uniform irradiation damage. LA-UR-16-27877

The Dzyaloshinskii-Moriya interaction (DMI) arises in situations where structural inversion symmetry is broken in a magnetic material. It favours chiral magnetic states. Whilst considered a curiosity for many decades, it has recently become a topic of intense interest due to its ability to stabilise spin textures with non-trivial topology, most notably skyrmion states. In order to realise skyrmion-based spintronics, thin films showing strong DMI are needed [1].

Structural inversion symmetry is broken in bulk in the B20 lattice, which is possessed by the helimagnetic metal FeGe. We have grown epilayers of this material show interesting transport properties [2], can have their chiral states controlled by ferromagnetic capping layers [3], and show an inversion of the sign of the DMI on doping with Co.

On the other hand, structural inversion asymmetry is also naturally present at an interface, and ultrathin (sub-nm) magnetic layers, which are often perpendicularly magnetized, will also show DMI. This leads to homochiral domain walls that are topologically protected against mutual annihilation unless the applied field is large [4]. We have shown that the DMI of sputtered Pt/Co/Pt layers can be inverted by the insertion of an Ir overlayer [5], that the sign and magnitude of the DMI in Pt/Co/Pt can be controlled by varying differences in interfacial roughness above and below the Pt, and that the DMI can be made to oscillate by varying the electron count of the top layer in Pt/Co/Pt_1-x-yIr_xAu_y trilayers. Small skyrmion bubbles have been observed in perpendicularly magnetised {Pt/Co/Ir}×N multilayers by both scanning X-ray transmission microscopy using XMCD contrast (in patterned dots) and Lorentz transmission electron microscopy (in sheet films). In both cases they are stabilised at zero field by pinning centres in the polycrystalline sputtered films.


[1] Nagaosa, N. and Tokura, Y. (2013). Topological properties and dynamics of magnetic skyrmions. Nature Nanotech. 8, 899.

[2] Porter, N. A., Gartside, J. C, and Marrows, C. H. (2014). Scattering mechanisms in textured FeGe thin films: Magnetoresistance and the anomalous Hall effect. Phys. Rev. B 90, 024403.

[3] Porter, N.A., Spencer, C. S., Temple, R. C., Kinane, C. J., Charlton, T. R., Langridge, S., and Marrows, C.H. (2015). Manipulation of the spin helix in FeGe thin films and FeGe/Fe multilayers. Phys. Rev. B 92, 144402.

[4] Benitez, M. J., Hrabec, A., Mihai, A. P., Moore, T. A., Burnell, G., McGrouther, D., Marrows, C. H., and McVitie, S. (2015) Magnetic microscopy and topological stability of homochiral Néel domain walls in a Pt/Co/AlO_x trilayer. Nature Commun. 6, 8957.

[5] Hrabec, A., Porter, N. A., Wells, A., Benitez, M. J., Burnell, G., McVitie, S., McGrouther, D., Moore, T. A., and Marrows, C. H. (2014). Measuring and tailoring the Dzyaloshinskii-Moriya interaction in perpendicularly magnetized thin films. Phys. Rev. B 90, 020402.

The non-trivial topological winding of skyrmions in chiral magnets distinguishes them from conventional forms of magnetic order and micromagnetic textures. This topological protection of skyrmions promises a new route to advanced non-volatile high density data storage devices. We report a detailed study of the stability and the decay of skyrmion lattices in chiral magnets when prepared in a metastable state. We consider possible decay mechanisms and discuss the underlying energy scales associated with the topological protection.

21st century has witnessed a dramatic transformation of magnetism research and device development from passive structures to electronically driven magnetic (spintronic) nanostructures that can find applications in information and microwave technologies. Skyrmions - nanoscale topologically nontrivial magnetization textures – are particularly suitable for such active nanostructures, because the expected magnitudes of currents required for their manipulation are considerable smaller than for the usual quasi-uniform magnetization states. While the initial studies of skyrmions focused on their thermodynamic phases, much of the recent research has been dedicated to developing new methods for the control and detection of individual skyrmions.
I will describe several approaches to electronic control of the magnetization states that have been developed by us and other groups in recent years, as well as their observation by electronic and optical techniques. I will briefly discuss magnetic nanostructures driven by spin-polarized electrical current, and then describe the recent demonstrations of devices driven by pure spin currents, generated either by the spin Hall effect or by the nonlocal spin injection. I will show that for in-plane magnetized films, the dynamical states are generally localized due to some combination of dynamical mode nonlinearity, effects of multiple-mode excitation, dipolar and Oersted fields. For perpendicularly magnetized films, both magnetoelectronic measurements and micromagnetic simulations indicate that by varying the applied magnetic field, it is possible to achieve three different dynamical states: a propagating spin-wave at large fields, a nonlinear self-localized spin-wave bullet at intermediate fields, or a nanoscale magnetic bubble with the skyrmion topology, and chirality that periodically evolves in time, at small fields. Our results suggest a straightforward route for the generation and manipulation of magnetization textures in thin magnetic films.

This work was supported by the NSF grants ECCS-1305586, ECCS-1509794 and DMR-1504449.

Magnetic skyrmions have shown promise in future magnetic racetrack memory devices, with ultra high storage density and low power consumption. A significant portion of the current research on magnetic skyrmions is dedicated to the development of techniques for the creation, detection and manipulation of skyrmions as information bits. Nanowires can act as a guiding track for directing skyrmions motion, which is ideal for implementation of racetrack memory. Furthermore, skyrmions can have a higher phase stability in nanowires in comparison to bulk and thin films. Nanowires of cubic B20 silicide Fe_xCo_1-xSi can host skyrmions of size that range from ~ 40nm to 230nm by controlling the alloy composition. We have synthesized Fe_xCo_1-xSi nanowires with higher cobalt concentration (x>0.3) than previously reported via chemical vapor deposition (CVD). X-ray diffraction patterns confirms the alloying of cubic B-20 phase of FeSi and CoSi. The relative cobalt compositions in these nanowires range from ~ 10-50% confirmed by energy dispersive spectroscopy, which can be tuned by providing different reaction conditions for CVD reaction. The diameter of the nanowires range from 80nm to 300 nm. By controlling the skyrmion size in these nanowires through alloying, we could explore interesting device physics by hosting a single chain of skyrmions which could be interesting for racetrack memory devices. We will also report our preliminary results from the device studies using these newly available skyrmion-hosting nanowires.

The ternary Fe_(1-x)Co_xSi B20 phase was formed by implanting Fe and Co consecutively into Si(100) substrate at 50 keV energy each at fluences of _1.0 × 10¹⁷ atoms/cm².. An external magnetic field was used to enhance the formation of the ternary phase in the Si substrate during the implantation process. The external magnetic field of 0.053 T was applied perpendicular to the incoming ion beam direction and parallel to the substrate surface, which is much stronger compared to the earth’s magnetic field. The samples were subsequently annealed in vacuum at 500 ^oC for 60 minutes. XRD and XPS characterization results indicate the formation of Fe_(1-x)Co_xSi B20 structures in the implanted layer with a lattice parameter of 0.453±0.003 nm. Magnetic property measurements indicate a typical diamagnetic response for the as-implanted sample with a weak ferromagnetic contribution. This diamagnetic behavior comes from the silicon substrate and the implanted Fe and Co forms the ferromagnetic structures with the implantation process to contribute the weak ferromagnetic contribution in the as-implanted sample. Further, magnetization of the as-implanted sample does not show a strong temperature dependence, which indicates strong diamagnetic behavior in the sample. After the heat treatment, the sample shows ferromagnetic behavior at 3 K and 300 K temperatures. Also, the magnetic measurement shows coercivity which corresponds to typical ferromagnetic materials. Earlier studies have shown that the Fe_(1-x)Co_xSi B20 phase has a helical magnetic ordering. However, our experimental observations contradict the previous claims on the Fe_(1-x)Co_xSi B20 phase structure by showing coercivity values. Moreover, a magnetic phase transitions was observed around ~ 50 K.