Symposium MT07 : In Situ/Operando Studies of Dynamic Processes in Ferroelectric, Magnetic and Multiferroic Materials

In order to better understand magnetic behaviour in naturally occurring or synthetic samples, it is often necessary to investigate the underlying processes on the nano-scale. Transmission electron microscopy (TEM) allows atomic spatial resolution imaging and combining in situ TEM experiments with techniques like off-axis electron holography or differential phase contrast (DPC) imaging allows for imaging of magnetisation in nanostructures whilst under the influence of external stimuli; e.g. gas atmospheres, biasing, temperature, etc. In this context, several examples of the use of in situ TEM and magnetic imaging will be presented.

Fe₃O₄ is the most magnetic naturally occurring mineral on Earth, carrying the dominant magnetic signature in rocks and providing a critical tool in palaeomagnetism. The oxidation of Fe₃O₄ to maghemite (γ-Fe₂O₃) influences the preservation of remanence of the Earth's magnetic field by Fe₃O₄. Further, the thermomagnetic behaviour of Fe₃O₄ grains directly affects the reliability of the magnetic signal recorded by rocks. Through combining electron holography with environmental TEM, in situ heating and liquid cell TEM, the effects of oxidation [1] and temperature [2] on the magnetic behaviour of Fe₃O₄ NPs are visualised successfully, as well as the magnetism within hydrated magnetotactic bacteria [3].

Equiatomic iron-rhodium (FeRh) has attracted much interest due to its magnetostructural transition from its antiferromagnetic (AF) to ferromagnetic (FM) phase and is considered desirable for potential application in a new generation of novel nanomagnetic or spintronic devices. Several scanning TEM techniques are performed to visualise the localised chemical, structural and magnetic properties of a series of FeRh films. The quantitative evolution of the growth and co-existence of AF and FM phases in the FeRh films are observed directly during in situ heating using DPC imaging [4]. In addition, combining DPC imaging with both heating and the application of electrical current pulses in situ within the TEM provides direct visualisation of the current-driven motion of magnetic DWs within planar FeRh thin films.

[1] T. P. Almeida et al., Nature Communications 5, 5154 (2014).
[2] T. P. Almeida et al., Science Advances 2, e1501801 (2016).
[3] T. Prozorov at al., Journal of the Royal Society: Interface 14, 20170464 (2017).
[3] T. P. Almeida et al. Scientific Reports 7, 17835 (2017).

The main concerns during the polarization switching in nanosize ferroelectrics are the space charges on the oxide-electrode interfaces, trapping of the charged defects and strong domain wall pinning which readily increase the required switching field. To better understand and solve these problems, dynamic experiments are needed. In situ (heating and biasing) transmission electron microscopy (TEM) offers imaging and analytical tools at the relevant length-scales.
Thin lamellae samples with optimized geometry, prepared from a single crystal BTO (100), exhibit unusually stable and ordered 180° domain walls at room temperature [1]. Heating up to ~50 °C induces highly ordered 90° walls. Further heating above the Currie temperature (Tc) and then cool-down reintroduces the 180° domain structure. The transition from 180° to the 90° domain structure could originate due to the thermal strain release as the sample is heated. The appearance of the 180° domain-state is independent of the cooling rate below the Tc.
Further to the heating studies, we applied cyclic triangle wave-shape potential with a slope of 0.035 V/s. Due to the complex geometry of the sample, the electric field at the centre of the lamella was estimated by finite element calculations. The results show a homogeneous electric field distribution within the electron transparent region of ~3.1 kV/cm per 1 V applied to the electrodes. The operando response of weakly charged domain walls on the applied electric field revealed a tail to tail configuration [2]. Theoretically, in a bulk sample this tail-to-tail configuration is hindered by the polarizing surface effect, which leads the preferable direction of the polarization to point perpendicularly to the normal of the electrode [3]. In a thin lamella sample polarization direction pointing out of the lamella surface is electrostatically highly unfavourable, which could explain the appearance of the weakly charged domain walls.
By following the dynamics of the domain walls with respect to the applied voltage, clear hysteretic and non-linear behaviour is revealed. By recording the domain growth/shrinkage and nucleation, “P-E hysteresis-like” loops can be produced by appropriate image analysis. Additionally, application of cyclic voltage with various amplitudes reveals quadratic response of the domain growth with respect to the applied maximal voltage. This direct measurement complies with Rayleigh’s law P(E)~αE² [4] and shows its fulfilment even at the local scale.

[1] T. Matsumoto and M. Okamoto, J. Appl. Phys. 109, 014104 (2011).
[2] P. S. Bednyakov, T. Sluka, A. K. Tagantsev, D. Damjanovic, and N. Setter, Sci. Rep. 5, 15819 (2015).
[3] M. Y. Gureev, A. K. Tagantsev, and N. Setter, Phys. Rev. B 83, 184104 (2011).
[4] L. Mitoseriu, V. Tura, D. Ricinschi, and C. Harnagea, Ferroelectrics 240, 1317 (2000).

Oxygen defects can have a profound effect on the physical properties of transition metal oxides. In complex oxides where magnetic, ferroelectric and superconducting phases emerge from strong correlations between localized transition metal electrons, oxygen vacancies can radically alter a plurality of quantum phenomena via valance changes and structural phase transitions [1]. The ability to control the concentration and profile of oxygen vacancies in oxide nanostructures would thus open up comprehensive prospects for new functional ionic devices. Here, we use in situ transmission electron microscopy (TEM) to demonstrate reversible switching between uniform structural phases in epitaxial La_2/3Sr_1/3MnO₃ films. In our experiments, we employ a piezo-controlled probing holder to apply short voltage pulses and local strain. Simultaneous high-resolution imaging and resistance probing under zero strain reveals reproducible voltage-induced transformations between a low-resistance perovskite phase, a high-resistance La_2/3Sr_1/3MnO_2.5 brownmillerite structure, and an intermediate-resistance perovskite-like phase [2]. Reversible horizontal migration of oxygen vacancies within the manganite film, driven by combined effects of Joule heating and bias voltage, predominantly triggers the structural and resistive phase transitions. Concurrent application of perpendicular strain and voltage pulses produces an entirely new structural phase whereby oxygen vacancies order in regular 3D rather than 2D patterns.

[1] S. V. Kalinin and N. A. Spaldin, Science 341, 858 (2013).
[2] L. Yao, S. Inkinen, and S. van Dijken, Nature Commun. 8, 14544 (2017).

Transition metal oxides (TMOs) undergo a plethora of phase transitions in response to external perturbations (temperature (T), pressure...), and vice versa outstanding physical phenomena in TMOs can be fine-tuned under controlled external stimuli (T, electrical or magnetic fields, light...). For instance, two types of insulator-to-metal transitions (IMTs) can be activated in Mott insulators or correlated systems such as (V_1-xCr_x)₂O₃ in response to external perturbations such as pressure, electronic doping or temperature ^[1]. V₂O₃ presents a T-driven IMT from an antiferromagnetic insulator (AFI) to a paramagnetic metallic (PM) state at around 160 K associated with a symmetry change. A different paramagnetic Mott insulating (PI) state can be usually destabilized by pressure or chemical doping into a correlated metallic state via the isostructural Mott transition. An out of equilibrium Mott transition can even be electrically activated in Cr-doped systems at 300K by applying short electric field pulses to induce a non-volatile ultrafast resistive switching creating conductive filaments nanodomains in the insulating matrix ^[2]. This electrically activated RS has revealed its potential for non-volatile memories used in high-performance resistive random-access memories. So far, the insulating/metallic (I/M) domain coexistence and the electronic phase separation in such oxides have only been investigated in situ by ARPES, nano-IR and STM/STS but with 10nm spatial resolution at best and not in cross-section so as to reveal buried interfaces. Moreover, additional properties can emerge solely in nanostructured TMOs where strain engineering and interfacial mechanisms play a key role. However the local mechanisms governing I/M domain formation, percolation, and dissolution may arise at boundaries as thin as an atomic column and be due to local structural defects, chemical clustering or interfacial mechanisms – all atomic scale phenomena.
Very recently, advanced monochromated electron spectromicroscopes (UHR-STEM/EELS) emerged as real game-changers for nanomaterials characterization where the improved EELS resolution, i.e. 5meV, extending a larger range of relevant electronic excitations (from IR to soft X-ray) available at the nanometer scale and below. Combining such remarkable instrumental capabilities with in situ (tuneable temperature and electrical biasing) options open an entirely new avenue of experiments for monitoring TMO’s phase transitions, and provide an unique in situ characterization tool capable of mapping the I/M nanodomains at sub-nm scale. Hence the requirement to monitor the IMT switching effect by probing and mapping in situ their electronic, atomic and chemical structures at the finest scale under thermal and/or electrical stimuli to gain a full understanding of these nanostructures’ functionalities.
Here we will first present recent variable-T UHR-STEM/EELS experiments performed on a Cs-corrector monochromated NION microscope, CHROMATEM, equipped with a HennyZ stage holder providing tuneable temperature (from liquid nitrogen to 1000°C) and biasing capabilities. New insigths into nanostructured V₂O₃ and Cr-doped systems are unveiled by probing their PM and AFI spectroscopic features upon low-T thermal cycling through both low-loss and core-loss signatures (interband plasmon excitation and V-L_2,3 and O-K fine-structures) ^[3]. Mapping such electronic excitations down to the nm-scale could highlight nanoscale electronic phase separation mechanisms further correlated with their structural and chemical properties down to the atomic scale. Finally, first perspectives on the electrically activated IMT in Mott insulator will be introduced.

References:
[1] D. B. McWhan et al. Phys. Rev. Lett. 1969
[2] E. Janod et al. Adv. Funct. Mater. 2015
[3] H. Abe et al. Jpn. J. Appl. Phys. 1998
[4] The authors acknowledge funding from the National Agency for Research under the program of future investment TEMPOS-CHROMATEM (No. ANR-10-EQPX-50)

The recent discovery of a two-dimensional electron gas (2DEG) at the interface between insulating perovskite oxides SrTiO₃ (STO) and LaAlO₃ (LAO) was made possible by advances in atomic layer-controlled growth. It has been demonstrated that the 2DEG is localized within a few nm of the interface and that the carrier concentration can be altered with an electric field and/or lattice strain which allows possibility of device structures such as field effect transistor, diode and nonvolatile memory devices. Eventually, seeing such changes during the operation of devices helps us to improve our understanding and also provides a technical breakthrough for further optimization of devices.
In this study, in-situ inline electron holography biasing experiments have been carried out on the epitaxially grown LAO (10 u.c.)/STO (15 u.c.)/LAO (3 u.c.) devices to observe charge density modulation by electrostatic gating. TEM samples for in-situ biasing experiment were prepared by using focused ion beam (FIB). A thin TEM lamella prepared by FIB was attached to a MEMS biasing chip (DENSsolutions^TM) by depositing Pt to ensure electrical contact of the thin film electrodes of TEM sample to the MEMS chip. The top and bottom electrodes were electrically isolated by milling trenches. While negative/positive DC biases are applied to the SrRuO₃ top electrode, inline electron holography was carried to visualize the modulation of local charge density, and STEM imaging and EELS line profile were also carried out at the same position to investigate the associated ionic displacement and change of the valence state of Ti ions using an aberration-corrected STEM operated at 300 kV (GRAND ARM 300 CF).
The in-situ TEM I-V curves of the device showed very low current level of only a few nA, indicating the leakage current through the sample surfaces has been suppressed sufficiently. The charge density maps obtained by inline electron holography clearly showed strong peaks at the interfaces of LAO/STO, which arises from both mean inner potential difference between the two materials and the interfacial space charges such as 2DEG. Therefore, to visualize the modulation of 2DEG induced by the applied bias, the charge density profiles were calibrated by subtracting that of 0 V to remove the contribution from the mean inner potential variation across the interfaces. The zero-bias calibrated charge density profiles obtained at each voltage showed an increase and decrease of 2DEG in response to negative and positive voltage, respectively. In addition to the charge density modulation, the change of valence state of Ti ion assessed by in-situ EELS and the related lattice strain by quantitative image processing of STEM images will be discussed in greater detail.

BaTiO₃ is one of the most promising candidate oxides for various devices due to ferroelectric polarization and/or resistive switching capability. For large data storage applications, the polarization of low dimensional material forms, especially the surface polarization, needs be characterized precisely to control its device characteristics. Recently, more attention has been paid to the (001) surface of tetragonal phase of BaTiO₃ with perpendicular polarization to understand the evolution of surface deadlayer due to depolarization effects and the associated near-surface domain structure. Previously, STEM-HAADF and STEM-ABF have been used for unit-cell by unit-cell polarization mapping but for TEM study of the intrinsic surface properties a clean surface without contamination or protection should be prepared, which poses technical challenges for implementation.
Here, by means of in-situ high resolution transmission electron microscopy we observed the surface polarization of BaTiO₃ (001) single crystal on the unit cell scale from 1000 °C to room temperature using a Cs-corrected TEM (Grand ARM300F). TEM samples were prepared from a (001) BaTiO₃ single crystal along the [100] cross-sectional zone axis by using FIB and attached to a DENSsolutions MEMS heating chip. By using negative spherical aberration imaging (NCSI) technique, all atoms (including oxygen columns) were observed clearly with minimal contrast delocalization, so that complete and quantitative polarization maps were obtained for the BaTiO₃ (001) surface of cubic and tetragonal phases.
From HRTEM surface profile imaging with minimal contrast delocalization, we found that the surface is stabilized by the BaO termination in most temperature ranges we studied. Interestingly, the surface BaO layer showed a rumpling with 4-5 unit cells period. At the peak of rumpling, the HRTEM image showed that the barium (Ba) atoms move upward from the surface plane and nearby oxygen columns are preferentially populated by oxygen vacancies. We also found that there are Ti ions in the surface BaO layer, particularly occupying the oxygen-oxygen bridge sites (i.e. corresponding to the octahedral interstitial sites in bulk BaTiO₃). The polarization maps showed that at the peak of surface rumple the polarization mostly points toward bulk region (P_down) and the in-plane polarization (P_in) dominates the neighboring unit cells with oxygen vacancies. This finding reveals that oxygen vacancies play a key role in the evolution of surface polarization. Although the surface structure and polarization observed after high temperature annealing in TEM and by using electron beam may deviate from the genuine surface properties of BaTiO₃, the observed periodic array of oxygen vacancies along in-plane direction is closely related to stabilizing mechanism of the polarization at surface. We will further show the real-time imaging of the dynamic processes of surface reconstruction occurring at various temperatures, which can be used to study how surface polarization and point defects intricately interplay with each other to result in distinct surface reconstruction, ferroelectric domain structure and ferroelectric size effects.

In the field of oxide electronics, there has been tremendous progress in recent years in atomic engineering of functional oxide thin films with controlled interfaces at the unit cell level. The functionality of such perovskite oxides depends mostly on the electronic configuration of the B-site cations. Relevant devices such as tunable ferroelectric microwave capacitors (varactors) are based on dielectric Ba_xSr_1-xTiO₃ and are stymied by the absence of suited compatible, very low resistive oxide electrode materials on the micrometer scale. This has been overcome by implementing highly conducting SrMoO₃ (SMO) thin-film electrodes as enabling material for all-oxide ferroelectric varactors [1]. By using scanning transmission electron microscopy (STEM) in combination with electron energy-loss spectroscopy (EELS) we were able to reveal the atomic and electronic structure in an all oxide ferroelectric epitaxial varactor heterostructure with a SMO thin-film bottom electrode. The remarkable result is that in spite of the contradicting thermodynamic oxidation conditions, the dielectric Ba_xSr_1-xTiO₃ and the highly conducting SrMoO₃ can be effectively coupled under preservation of their physical properties in an epitaxial heterostructure with negligible cation intermixing and valence change. The resulting interface is atomically sharp down to the unit cell scale. Surprisingly, a few unit cells of SrTiO₃ are sufficient to engineer the interface, allowing the utilization of the Pt-like conducting SrMoO₃ in oxide electronics applications. Furthermore, we are currently exploring the use of microelectromechanical systems (MEMS) based in situ biasing chips to enable controlled ionic displacement studies directly inside a transmission electron microscope. We aim to link the atomic structure to the physical properties under working or so called operando conditions in electrically contacted electron transparent lamellae of the metal-insulator-metal (MIM) ferroelectric varactor structure by directly tuning the dielectric layer.
Reference:
[1] P. Salg et al. APL Materials 7, 051107 (2019).

Understanding the short timescale dynamic response of materials to electric fields is essential for the development of next generation electronic & photonic devices. Specifically, in ferroelectrics, a transient measurement of lattice response to E-field encodes key information about the nucleation and growth of domains, domain wall motion, and coupling to strain. Despite its scientific and technological importance, most previous work has focused on the separate characterization of electrical transport and structure.
Here, we report the development of a time-resolved technique for the simultaneous measurement of electronic transport and atomistic structure in response to an applied E-field. This is based on in situ diffraction using ~100 fs electron pulses with energies of ~3 MeV. Our setup has demonstrated a momentum transfer range of 9 A^-1, q-resolution of <0.17 A^-1 and a temporal resolution limited only by the rise time of the E-field pulse across the device. Devices are electrically excited using short voltage pulses while structural dynamics are probed in a stroboscopic manner as a function of delay time after the pulse is turned on. As a demonstration, we use this technique to investigate the dynamics of the electrically-induced insulator-metal phase transition in VO₂. Through a simultaneous measurement of structure and electrical transport, we correlate the nucleation and growth of rutile domains with metallization of the channel. These experiments provide new insights into the fundamental processes governing the switching speed of VO₂ devices, and elucidate the role played by nucleation, growth and percolation of phases.

By symmetry, all 90^o domain walls in Cu-Cl boracite (Cu₃B₇O₁₃Cl) must be associated with either head-to-head or tail-to-tail polar discontinuities and are hence charged. As in other ferroelectric systems, charged walls have either enhanced or reduced electrical conductivities, compared to bulk, and so controlling their injection and movement could be of interest in the context of future domain wall nanoelectronics [1].
In this work, we show that repositioning these walls using electric fields can lead to an increase in the global electrostatic energy: changes in the microstructure can be such that regions with polarisation aligned against the field coarsen at the expense of those aligned with the field. Field-induced changes in polarisation which increase polar populations which are anti-aligned, imply a contribution to the effective permittivity from domain wall motion that is negative. This is extremely unusual and worthy of much more exploration and discussion [2].
The notion of this kind of anomalous domain wall motion seems impossible in proper ferroelectrics. However, in the case of improper ferroelectrics, such as the Cu-Cl boracites, the low spontaneous polarisation associated with strong coupling to the spontaneous strain makes the idea plausible, as local electrostatic energy increases might be offset by global strain energy decreases in the system.
After presenting evidence for the correct attribution of the nature of the domains, relying on several scanning probe techniques, the classical and anomalous motion of domain walls in Cu-Cl boracite studied through optical microscopy will be presented. The complexity of the energy landscape in this specific system will be discussed and finite element models quantifying the electrostatic work done will be presented, along with potential avenues to be pursued which could shed more light on the unique behaviour observed.
References
[1] R.G.P. McQuaid, et al. Injection and controlled motion of conducting domain walls in improper ferroelectric Cu-Cl boracite. Nat. Commun. 8, 15105 (2017)
[2] J. Guy et al. Anomalous Motion of Charged Domain Walls in Copper-Chlorine Boracite and Implications for Negative Capacitance. Submitted for publication (2019).

Efficient energy conversion and deterministic controllability are long-standing demand of novel miniaturized electromechanical devices [1]. Recent advancements in ferroelectric thin film synthesis provide a potential pathway to apply interface engineering for favorable electrostatic and strain conditions for promotion of ideal ferroelastic domain switching and leads to enhanced electromechanical response in tetragonal PbZr_1-xTi_xO₃ (T-PZT)/ rhombohedral PbZr_1-xTi_xO₃ (R-PZT) bilayer thin films [2-3]. However, the dynamic evolution of their domain switching mechanism remains elusive.

In this work, we utilize in situ transmission electron microscopy (TEM) to explore the interaction between two dissimilar ferroelectric layers (R-PZT and T-PZT). During the real-time domain switching process, the domain wall orientation in T-PZT can be determined by the external electric field. The interface of the PZT bilayer serves as the nucleation site for the controllable domain wall reorientation, which is revealed by both atomic resolution electron microscopy imaging and phase field simulation. Thus, the interplay between two ferroelectric polymorphs is the underlying physical mechanism for unprecedented electromechanical properties in the PZT bilayer, i.e., ferroelastic domain switching and its wall reorientation. The T-PZT/R-PZT heterostructure acts as a model system, which can stimulate further advances of elctromechanical device design.
References
[1] G. Catalan et al., Reviews of Modern Physics, 84, 119-156 (2012)
[2] L.W. Martin, A.M. Rappe, Nature Reviews Materials, 2, 16087 (2017)
[3] H. H. Huang et al., Advanced Materials Interfaces, 2, 1500075 (2015)

Two years after the invention of modern prototype solar cells, it was found that BaTiO₃ exhibits a photovoltaic effect distinct from that of p-n junctions, later called the bulk photovoltaic (BPV) effect. Under uniform illumination, a homogeneous ferroelectric material gives rise to a short-circuit current and produces an anomalously large photo-voltage well exceeding the bandgap energy. The microscopic origins of this effect supposed to originate from the asymmetric distribution of photoexcited non-equilibrium carriers in k-space, caused by absence of centrosymmetry.
The present talk will present the basics of the bulk photovoltaic effect, tip enhancement, as well as the electronic origin of the anomalous BPV in some materials such as BiFeO₃.I will show how the tip-enhanced effect may be used in optical switching of ferroelectric polarization. Finally, we will discuss the new flexo-photovoltaic effect which turns the BPV effect into a universal effect allowed in all semiconductors under strain gradient and how this effect does affect the local photoelectric effects at domain walls. [1,2]

References
[1] M.-M. Yang, D. J. Kim, & M. Alexe, Flexo-Photovoltaic Effect, Science 360, 904 (2018).
[2] M.-M. Yang et al., Strain-Gradient Mediated Local Conduction in Strained BiFeO3 Film, Nature Comms, in press.

Coupling between different degrees of freedom, i.e. charge, spin, orbital, and lattice, is responsible for emergent phenomena in complex oxide heterostrutures. A notable example is formation of a two-dimensional electron gas (2DEG) at the polar/nonpolar LaAlO₃/SrTiO₃(LAO/STO) interface due to the polar discontinuity and as a means to counteract the electrostatic potential build-up across the LAO film. The polar discontinuity can also be produced by a ferroelectric polarization at the domain walls and ferroelectric/insulator interface. In this case, such a 2DEG can be controlled by switchable ferroelectric polarization, thus providing a new functionality. Depending on the polarization orientation, either electrons or holes are transferred to the interface, to form either a 2DEG or two-dimensional hole gas (2DHG). While the recent first-principles modeling predicts the formation of 2DEGs at the ferroelectric/insulator interfaces, experimental evidence of a ferroelectrically-induced interfacial 2DEG still remains elusive. Here, we report the emergence of strongly anisotropic polarization-induced conductivity at a ferroelectric/insulator interface, which shows a strong dependence on the polarization orientation. By probing the local conductance and ferroelectric polarization over a cross-section of the BiFeO₃/TbScO₃(BFO/TSO) (001) heterostructure, we demonstrate that this interface is conducting along the 109^odomain stripes in BFO, whereas it is insulating in the direction perpendicular to these domain stripes. Electron energy-loss spectroscopy (EELS) and theoretical modeling suggest that this extraordinary anisotropy of interfacial conduction is caused by alternating polarization associated with the ferroelectric domains, producing either electron or hole doping of the BFO/TSO interface. Similarly, we observed a spin-polarized 2DEG forms at the PZT/STO interface, which is strongly localized at the interfacial Ti atoms, due to the interplay between Coulomb interaction and band bending, and can be tuned by the ferroelectric polarization. Finally, we found that the intrinsic conductivity of domain walls in BiFeO₃thin film grown (110) TbScO₃is strongly dependent on the polarization configurations. We show that the 71° domain walls exhibit conductivity, which is about an order of magnitude larger than that of the 109° domain walls. The 71° domain wall conductivity is strongly anisotropic: more conductive along the [010]_pdirection than along the [001]_pdirection. High resolutions electron energy loss spectroscopy reveals that the origin of the anisotropic conductivity of the 71° walls stems from the potential discontinuity of the 71° domain walls along the [110]_pdirection. Our findings in the ferroelectric interfaces and domain walls open a door for engineering ferroelectric/insulator interfaces toward the creation of tunable ferroic orders for magnetoelectric device applications and provide the opportunities for designing multiferroic materials in heterostructures.

Ferroelectric domain walls are spatially mobile interfaces that naturally occur in materials that develop a spontaneous electric polarization. Because of their unique electronic properties, such walls hold great promise as functional 2D systems, but the characterization of their intrinsic transport properties remains a challenging task.

Despite the significant progress in experiment and theory, most investigations on ferroelectric domain walls still fall into the basic research sector, aiming to understand their complex nano-physics and tailor their local electronic properties. One of the main challenges lies in the reliable characterization of emergent transport phenomena, which commonly is realized in terms of conductive atomic force microscopy (cAFM) measurements. However, cAFM is a two-probe measurement and, as such, susceptible to contributions from contact resistance that can obscure the data. Furthermore, as the resolution limit of cAFM (≥ 20 nm) is much larger than the width of typical ferroelectric domain walls (≈ 10 Å), it is often difficult to evaluate whether measured signals arise at the wall center, in adjacent accumulation and depletion regions, or due to displacement currents associated with domain wall movement.

In this talk, we will discuss how the physical property extraction from local conductance measurements can be improved by combining local I(V)-spectroscopy measurements with machine learning, employing a neural network autoencoder. Using the doped hexagonal manganite (Er_0.99,Zr_0.01)MnO₃ as a model system, we conduct a comprehensive characterization, using different methods such as density functional theory (DFT), dielectric spectroscopy, piezoresponse force microscopy (PFM) and cAFM. To highlight different limitations associated with standard cAFM scans, we first discuss the observation of surprising conductance phenomena at charged head-to-head (h-t-h) and tail-to-tail (t-t-t) domain walls. We then use the neutral network autoencoder to disentangle different conductivity contributions, revealing the intrinsic electronic domain wall properties in (Er_0.99,Zr_0.01)MnO₃.

The scaling behavior of ferroelectrics and related materials are of both fundamental and practical interest. In particular, there has been significant progress in the understanding of scaling behavior for ferroelectrics over the past few decades. In stark contrast to ferroelectrics, understanding of size effects in relaxor ferroelectrics (relaxors), a special class of ferroelectrics, has been lacking, despite the close similarity between ferroelectrics and relaxors. In this work, we use a combination of thin-film epitaxy, X-ray diffuse scattering, scanning transmission electron microscopy, dielectric and ferroelectric characterization, and molecular-dynamics simulations to investigate the size-dependent evolution of structures and properties in the prototypical relaxor ferroelectric 0.68PbMg_1/3Nb_2/3O₃-0.32PbTiO₃. Here we demonstrate that reducing the size (i.e., thickness of the films), contrary to prior reports, first enhances relaxor behavior until a threshold thickness below which the critical temperatures that characterize various relaxor phases (i.e., dynamic, static, and frozen) collapse together, indicating destabilization of the relaxor state. Using temperature-dependent diffuse scattering, scanning transmission electron microscopy, and dielectric and ferroelectric measurements, we demonstrate that relaxors lose their defining characteristics below this threshold thickness. The mechanism for destabilization of relaxor behavior below the critical thickness is discussed in terms of faster dynamics of polarization fluctuations in ultrathin relaxor films.

Relaxor ferroelectrics are one of the most widely used functional materials as they exhibit low coercive field and giant piezoelectric coefficients [1]. These can be distinguished from classical ferroelectrics based on their diffuse phase transitions and dielectric properties. These properties are commonly associated with the existence of polar nanoregions in a non-polar matrix [2]. Recently, a model of slush-like response of a nanoscale multi-domain state is proposed to explain relaxor behavior, but these models are not are fully predictive [3]. In order to design relaxor ferroelectric materials, a fundamental understanding of their behavior under an applied electric field needs to be found-from the micro to atomic length scale. Recent advances in in situ microscopy and STEM detector technologies, such as 4D STEM and differential phase contrast (DPC) imaging [4], have allowed to reveal the structural information required to explain the origin of relaxor behavior.

We show that DPC provides robust imaging of the relaxor ferroelectric domain structure as a function of voltage bias. We apply the technique to Pb(Mg_1/3Nb_2/3)O₃-28PbTiO₃(PMN-PT) single crystals prepared using conventional polishing methods pair with photolithography to define the electrodes. First, influence of electron beam on the domain structure is studied by examining a series of DPC images without an applied field. The electron beam causes the domain walls to flicker in the time-resolved dataset, but large-scale changes are not observed at this field of view. A dramatic change in domains is observed at different applied electric fields. The domain structure at the start and end of a field cycle is explained in the context of remnant polarization. Effect of waveform shape on switching behavior is also determined. To quantify changes at the atomic scale, we demonstrate how in situ integrated differential phase contrast (iDPC) provides high signal-to-noise ratio imaging of oxygen atom columns, even in the presence of heavier atoms (Pb). Further, we demonstrate how the simultaneous acquisition of annular dark field (ADF) can be used to simplify the quantification and removal of drift and scan distortion from iDPC images [5]. From the drift-corrected iDPC images, the projected polarization is then quantified at different applied fields. It is observed that the polar nanoregions try to align to the electric field, but reside within a microscale ferroelectric domain that does not switch. With a combination of data across length scales for the same sample, we discuss how the resulting details provide new insights into the dynamics of relaxor ferroelectric domains.

References:
[1] F. Li et al., Nature Materials 17 (2018)
[2] F. Li et al., Nature Communications 7 (2016)
[3] H. Takenaka et al., Nature 546(2017)
[4] N.Shibata et al., Nature Physics 8 (2012)
[5] X. Sang and J. M. LeBeau, Ultramicroscopy 138(2014)

Time resolved X-ray diffraction measurement was performed for rhombohedral lead zirconate titanate (PZT) with synchrotron X-ray source for various frequencies and pulse widths. The PZT is the most widely used ferroelectric and piezoelectric material due to its outstanding electric properties. In particular, their excellent electro-mechanical responses have been attracted a great deal of interest, because they enable us to produce the various applications, such as sensors and actuators. Because of the recent increase in the demand for these devices using micro-electro-mechanical-systems, it is required to understand the nature of the piezoelectric and dielectric properties of PZT more deeply. It is well recognized that non-180° domain switching plays essential roles for the piezoelectric responses in PZT. The frequency dispersion in piezoelectric and dielectric responses often become a problem to use PZT at high frequency. Because the dispersion is known to relate with the domain switching motions, the time-resolved measurement with broadband frequency is needed to understand the piezoelectric responses. In particular, remarkable frequency dispersion is reported for rhombohedral PZT.
The PZT films prepared by metal-organic chemical vapor deposition were used for the measurements. The in-situ X-ray diffraction was performed at SPring-8 BL15XU using the focused X-ray beam on area 2 × 3 μm² in width and height using a two-dimensional focusing refractive lens. To observe the domain switching for various frequencies and duty ratios, the pulse width and repetition frequency of the electric field, were varied.
The domain switching was confirmed by applying electric field irrespective of the frequencies and duty ratios. We observed marked delay of the domain switching back when removing the electric field. The high frequency of 1 MHz electric field gave rise to a small amount of domain switching motion compared to low frequencies electric field. In addition, the larger duty ratio also smaller changes in domain fraction by the electric field even with the low frequency of 10 kHz. Interestingly, the domain fraction without electric field during electric cycling (i.e. in-situ measurement) is different from the original domain fraction before starting measurements if we used at high frequencies or large duty ratios. This means that the longer time required to recover original domain structure in rhombohedral PZT, which is quite different from tetragonal PZT responding to both applying and removing the electric field.

Conducting ferroelectric domain walls (DWs) are an emerging research focus in nano-electronics and potential quantum technologies [1,2]. DWs have their own unique electronic properties and most excitingly can be moved by an applied stimulus.[3] However, due to the high energetic cost of creating charged DWs in conventional ferroelectrics they are not stable long term. Improper ferroelectrics circumvent this issue as their driving force is not a polar instability, but instead is the symmetry-breaking non-polar primary mode. With this in mind there has been a recent surge in improper ferroelectric DW research. In this study boracite is the material we have chosen to focus on as it has already shown very promising results in terms of charged DW injection and movement. [5] Boracite is an improper ferroelectric material where the primary order parameter is the physical quantity spontaneous shear strain. As a ferroelastic material the conductivity of the charged DWs in boracite is governed by the shear strain direction of the neighbouring domains. Thus traditional atomic resolution Scanning Transmission Electron Microscopy (STEM) polarity mapping will not give the true picture of the ferroelectric properties. In order to understand the mechanism of the DW movement and the fundamental physics governing conducting DW formation strain analysis at the DW was completed by 4D-STEM [6] strain mapping. This strain analysis was then compared to the atomic resolution STEM imaging and polarity mapping. The dynamic nature of the DWs was investigated by utilising the applied electric field of the STEM probe itself, allowing for controlled angle of the incoming applied electric field and a corresponding STEM data set.

1. Jiang, J. et al., Nature Materials, 2017
2. Ma, J., et al., Nature nanotechnology, 2018
3. Catalan, G., et al., Reviews of Modern Physics, 2012
4. Småbråten, D. R., et al., Physical Review Materials, 2018
5. McQuaid, R. G. P., et al., Nature Communications, 2017
6. Ophus, C., et al., Microscopy and Microanalysis, 2019

The secondary nature of polarization in improper ferroelectrics promotes functional properties beyond those of conventional ferroelectrics. For example, in the hexagonal manganites, the non-ferroelectric distortive origin of electric polarization enables topologically protected ferroelectric vortex domain-patterns, conducting domain walls and multiferroicity. In technologically relevant ultrathin films, however, the improper ferroelectric behavior remains largely unexplored. Here, we probe the emergence of the coupled improper polarization and primary distortive order parameter in thin films of hexagonal YMnO₃. Combining state-of-the-art in-situ characterization techniques separately addressing the improper ferroelectric state and its distortive driving force, we reveal a pronounced thickness dependence of the improper polarization, which we show originates from the strong modification of the primary order at epitaxial interfaces. Nanoscale confinement effects on the primary order parameter reduce the temperature of the phase transition, which we exploit to visualize its order-disorder character with atomic resolution. Our results lay the foundation for understanding the evolution of improper ferroelectricity within the confinement of ultrathin films – essential for the successful implementation in nanoscale applications.

For the last several decades Time-of-flight Secondary Ion Mass Spectrometry (ToF-SIMS) became a prime tool for chemical characterization of the materials at sub-micrometer and nanometer scales. However, most of the investigations published so far lacks information about physical properties of the studied samples and their time evolution. The solution can be found in combination of mass spectrometry with Atomic Force Microscopy (AFM). Such combined multimodal AFM/ToF-SIMS platform enables a nanoscale correlated studies of both chemical and physical properties of the sample. In this case, chemical phenomena characterized by the ToF-SIMS can be complemented with functional properties measured by the AFM. Furthermore, continuous data acquisition in both ToF-SIMS and AFM modes supplemented by multivariate statistical analysis allows characterization of time evolving processes.
Here we utilize commercial AFM/ToF-SIMS platform to explore interplay of chemical and physical phenomena in ferroelectrics and photovoltaics. In particular, our research allowed to reveal chemical phenomena associated with polarization switching in ferroelectric thin films and study ion migration in hybrid organic-inorganic perovskites. Developed approach enables direct characterization of interplay between chemical and functional response in functional materials and aids in the development and optimization of novel functional devices.
This research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility and using instrumentation within ORNL's Materials Characterization Core provided by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy.

The ultimate dynamic electrical performance of functional ceramic devices depends on the nanoscale materials properties in 3-dimensions. Thickness effects and engineered heterogeneities are of particular interest, requiring advances in nanoscale functional measurements. Accordingly, emerging methods for micromachining and Tomographic AFM, along with Kelvin Probe Force Microscopy (KPFM), Piezo Force Microscopy (PFM), and Conductive AFM (cAFM), are used to study dielectric and ferroelectric heterostructures, including as a function of thickness. Dynamic charging and discharging processes are notably mapped with KPFM as a function of time and energy, providing novel insight into local dielectric behavior and especially the voltage dependencies of surface or grain boundary defect states. These are related to sub-surface features and thickness dependencies via tomographic PFM and cAFM, volumetrically revealing size effects, 3-dimensional heterogeneities, and charge dissipation pathways. By considering in-situ charging and discharging phenomena, the engineered performance for micro- and nano- scale electronic devices such as multi-layer-chip-capacitors can therefore be optimized.

Interfaces in oxide materials offer amazing opportunities for fundamental and applied research, giving a new dimension to functional properties, such as magnetism, multiferroicity and superconductivity. Ferroelectric domain walls recently emerged as a new type of interface, where the dynamic characteristics of ferroelectricity introduce the element of spatial mobility, allowing for the real-time adjustment of position, density and orientation of the walls. This mobility adds an additional degree of flexibility that enables domain walls to take an active role in future devices and hold great potential as functional 2D systems for electronics. For example, domain walls can readily be injected and deleted to control electric conductivity, enabling multi-level data storage. However, while this approach clearly achieves a step beyond conventional interfaces by utilizing the wall mobility, it does not break the mould of classical device architectures.

In this talk, I will present a conceptually different approach, utilizing ferroelectric domain walls to emulate key electronic components such as digital switches and diodes. In particular, I will discuss three experimental studies, showing how advanced microscopy strategies helped us to understand the complex nanoscale physics that give rise to the functional electronic domain wall properties. (i) The diode-like behavior that arises at electrode-wall junctions, for instance, became accessible due to innovative conductive atomic force microscopy (cAFM) studies performed under a.c. voltages in the kilo- to megahertz range [1]. (ii) By combining cAFM, transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS), we demonstrated and explained the gating of electronic currents at domain walls [2], and (iii) correlated cAFM, electrostatic force microscopy (EFM) and X-ray photoemission electron microscopy (PEEM) provided novel insight into their electrostatic properties [3]. Finally – going beyond just imaging (i-iii) – I will discuss the possibility to combine cAFM, focused ion beam (FIB) methodologies, and in-situ scanning electron microscopy (SEM) imaging to achieve domain walls with well-defined charge states. This possibility brings us an important step closer to electronic components based on individual domain walls, following the ultimate goal to achieve domain-wall based circuitry and networks.

[1] J. Schaab et al., Nature Nano. 13, 1028 (2018)
[2] J. Mundy et al., Nature Materials 16, 622 (2017)
[3] P. Schoenherr et al., Nano Lett. 19, 1659 (2019)

Benefits of utilising the 2-D properties and the dynamic nature of ferro-electric domain walls are increasingly being explored. The combination of their sub-nm thickness and agility permits so far un-achieved applications in nano-electronic and –photonic devices, e.g., as ultrafast electrical switches, a.o., to pump for single photon emitters, or as mobile nano-mirrors in nano-scale lasers. To achieve this a fundamental understanding of these materials and their functionalities in terms of their physical, chemical and electronic structure down to the atomic level is required.
Using atomic resolution transmission electron microscopy (TEM) methods, pico-meter sifts of individual atoms were observed (e.g., in boracite and lead titanate), resulting in the formation of electric dipoles. Comparison of the contrast of the atoms in (individual) dipoles to crystal models showed the relative shifts of cations with respect to anions, revealing the dipole orientation. Maps of atom shifts, signifying the formation orientation distributions of dipoles, were obtained over 1000 nm² areas, using the Atomap[1] program. Changes of dipole orientations along certain line directions then revealed domain boundaries and walls, as well as dipole (e.g., head-to-head or tail-to-tail) constellations and angles (e.g., 90⁰) along these walls. The shifts of atoms as well as the orientations of individual dipoles are revealed via high angle annular dark field (Z-contrast) and annular bright field imaging, the latter method depicting also the lighter atoms (e.g., O). Positions of all atoms together with their elemental nature are disclosed with sub-Å accuracy in areas on either side of domain walls and in the domain wall itself. These measurement are accompanied by differential phase contrast imaging to reveal the local electric field directions. The relative change in the distance of cations with respect to anions along the walls can furthermore give insight into whether these walls are expected to be conducting or insulating. Moreover, the movement of the walls and control over this movement are investigated and demonstrated via atomic-scale in-situ TEM.

Atomic force microscopy (AFM) provides a wide range of methods which allow mapping of electrostatic (EFM, SCM, KPFM), piezoelectric (PFM) and magnetic (MFM) properties of samples with high spatial resolution. The performance of these methods is always a trade-off between signal to noise ratio, sensitivity, and speed. The latter is limited by the parameters of the probe sensor and the bandwidth of the feedback control system [1]. Scanning speed increment is an important direction for development of in-situ studies of dynamic properties at nanoscale.
The nature of tip-sample interaction forces detected by AFM is different for various techniques. But in all cases of feedback-driven methods, the major contribution in the final result is brought by the derivative of the tracked value: surface potential in KPFM, frequency shift in MFM, etc. In the assumption of linear disturbances and taking into account that errors in trace and retrace scanning directions have odd symmetry, the following transform can be applied to AFM data:
(1)where p_r and p_b are trace and retrace profiles of tracked parameter and p_r is a reconstructed profile.
The example of the described approach is shown in fig. 1 representing the map of surface potential (both trace) distribution over the charged line induced by the AFM tip on Si/SiO₂ surface. Scanning was done in two-pass AM-KPFM mode consequently at maximal speed which was fitting the KPFM feedback loop response and at an order higher rate (fig. 1 a and b). Fig. 1 c represents the reconstructed surface potential map. It is seen that the reconstruction result fits well with the image taken under appropriate conditions.
The proposed image reconstruction approach can be applied to images acquired by other feedback-driven AFM methods.
Fig. 1 AM-KPFM images of charged line on Si/SiO₂ substrate. a and b correspond to scanning speeds of 10 and 140 µm/s. c – corrected image. d introduces corresponidng scan profiles. Image size: 7,7×9,7 µm. Color scale: -2,3..-7,8 V
Image was acquired by means of Ntegra AFM system, NT-MDT Spectrum Instruments Co. The authors acknowledge funding from the European Union's Horizon 2020 research and innovation program, OYSTER under grant agreement No 76082
References
[1] T. Sulchek, G.G. Yaralioglu, C.F. Quate, and S.C. Minne, “Characterization and optimization of scan speed for tapping-mode atomic force microscopy,” Rev. Sci. Instrum. 73: 2928−2936, 2002.

Layered low-dimensional perovskites have emerged as a popular system for a wide range of optoelectronic applications due to their enhanced environmental stability and tunability relative to three-dimensional hybrid organic-inorganic perovskites. However, to date there have been few studies linking the photoinduced dynamics in these layered systems with their topographic features. Here, we use two complementary “big data” electrical scanning probe microscopy methods, fast free time-resolved electrostatic force microscopy (FF-trEFM) and general mode Kelvin probe force microscopy (G-KPFM), to probe light-induced dynamics in thin films comprising Ruddlesden-Popper phases of the layered 2D perovskite (C₄H₉NH₃)₂PbI₄ (BAPI) as a function of position, time, and illumination. These scanning probe techniques capture the raw AFM cantilever motion and use advanced signal processing of the large (~50 GB) four-dimensional datasets (x, y, z, t) to extract microsecond-timescale dynamics. We use G-KPFM and unsupervised clustering to show that the photovoltage equilibrates over long timescales (hundreds of microseconds) that we attribute to ion motion. Surprisingly, this effect is actually slower at boundaries between adjacent grains as opposed to grain centers. With FF-trEFM, we extract faster dynamics at grain centers but with timescales consistent with electronic motion (70 - 100 μs). We therefore propose that the slower evolution at grain boundaries is due to a combination of ion migration occurring between PbI₄ planes, as well as electronic carriers moving through traps at grain boundaries, thereby changing the time-dependent band unbending even within a micron-scale perovskite grain. Beyond perovskite optoelectronics, these experiments lay the groundwork for data-driven analysis of large microscopy datasets on other dynamic systems requiring sensitive detection of transient phenomena in scanning probe microscopy.

Nb₃X₈ (X=Cl or Br) are insulating, cluster-based, layered materials that exhibit an antiferromagnetic to non-magnetic transition [1]. In Nb₃Cl₈, the loss of magnetic order occurs at temperatures below 90 K and has been coupled to a symmetry-lowering crystallographic distortion and layer stacking change from a 2-layer (α-phase) to a 6-layer (β-phase) unit cell [2]. The transition temperature, however, depends strongly on composition with Nb₃Br₈ exhibiting the β-phase at 293 K. While tuning of the magnetic ordering temperature through composition is appealing for materials design, the mechanism by which the layer stacking reorganizes is not understood, and pinpointing the emergence of such structural changes and their relationship to magnetic order necessitates atomic-resolution imaging across the transition.

Atomic-resolution, cryogenic scanning transmission electron microscopy (cryo-STEM) has enabled precise mapping of structural responses related to underpinning low-temperature physics, such as periodic lattice displacements and associated charge density waves [3-5]. Most cryo-STEM, however, is performed at the absolute temperature of the holder, precluding access to intermediate temperatures, and thus limiting the ability to track structural transformations and the emergence of correlated phases over temperature. To overcome this, we have optimized a novel approach to cryo-STEM using a liquid-N₂ specimen holder equipped with a 6-pin MEMS system for local heating [6]. This has expanded the realm of cryo-STEM to any desired temperature from ~95-500 K. Thus, we can use controlled-temperature cryo-STEM to explore the effects of temperature on the layer stacking of van der Waals Nb₃Br₈ and its relationship to magnetic order.

Employing controlled-temperature cryo-STEM we have observed a distinct structural phase transformation in Nb₃Br₈ with atomic-resolution. Plan-view imaging demonstrates a clear, reversible transformation from the α-phase to β-phase at ~250 K upon cooling and β-phase to α-phase at ~425 K upon heating through a series of apparent intermediate phases. Imaging of cross-sectional specimens, however, does not show a reorganization of layer stacking. Rather, persistent stacking faults are observed, some consisting of α-phase inclusions accompanying the β-phase unit cell. The lack of an observed transition in this geometry may indicate that the transition in thin, cross-sectional STEM samples is suppressed. Coupling the plan-view results with Multislice image simulations provides a clearer picture of the temperature-induced atomic scale changes in Nb₃X₈ systems. Developing controlled-temperature cryo-STEM has allowed investigation into the implications of composition and temperature on layer stacking in a single materials system. Understanding the mechanism of stacking changes and their effect on magnetic ordering will afford handles for new materials with tailored transition temperatures.

[1] J. Kennedy, et al., Mater. Sci. Forum 91-93 (1992), p. 183.
[2] J.P. Sheckelton, et al., Inorg. Chem. Front. 4 (2017), p. 481.
[3] R. Hovden, et al., Proc. Natl. Acad. Sci. U.S.A. 113 (2016), p. 11420.
[4] I. El Baggari, et al., Proc. Natl. Acad. Sci. U.S.A. 115 (2018), p. 1445.
[5] B.H. Savitzsky, et al., Ultramic. 191 (2018), p. 56.
[6] Hotz, et al. Microsc. Microanal. 24 (2018), p. 1132-1133.

Phase transitions in transition metal oxides, such as metal-Insulator transition, ferromagnetic, ferroelectric materials, are useful phenomena to realize novel electrical/magnetic/thermal switching and sensing devices. The dynamical behavior of their nano- to micro- scale domain structures play an important role for phase transitions, and their scaling behavior would create superior functionally of steep changes in resistive switching magnetoresistance in VO₂, mangantite [1] and so on.
Cathodoluminescence (CL) imaging technique realizes the multi-scale from nano to several hundred micro level observation for the spatial distribution of electronic proerties via the optical excitation. So far, CL mapping was not applicable for the non-fluorescent samples. Electron transport cathode luminescence microscope (ETCLM) has been developed by the energy transfer (ET) ratio visualization in the dye-assist CL technique, where ET between luminescent dye and objective materials is applied in imaging for optical conductivity dispersion over the sample based on a conventional scanning electron microscope with a CL system. This ETCLM technique enables the simple and contractless image mapping to characterize the optical conductivity, i.e., metallic/insulating area distribution based on the contrast of the ETCL intensity image. The principle of visualization for optical conductivity dispersion is derived from the radiative/nonradiative emission reflecting ET coupling between the dye and objective materials. As a proof on the principle, the ETCL mapping for the prototype Au disk patterned SrTiO₃ sample was attributed to the ET process substantiated by significant ETCL change of the dye. The capability of our technique for mapping metal/insulator property distribution is demonstrated for the observation of the phase separation during insulator-metal transition process in strongly-electron correlated VO₂ and Magnetoresistive manganite. Temperature dependent ETCL images adequately capture the characteristic of metal/insulator electronic domain formation across the phase transition phenomena.
Ref) [1] A. N. Hattori, H. Tanaka et al, Nano Lett. 15 (2015) 4322-4328.

As the dimensions of magnetic materials decrease to the nanoscale, novel distributions of spin can be created. We are exploring the behavior of these novel magnetic nanostructures and ways to control the behavior through gaining a quantitative understanding of their local energy landscape as a function of parameters such as structural confinement. We use Lorentz transmission electron microscopy (LTEM) and in-situ magnetizing experiments, to elucidate the micromagnetic behavior at the sub-micron scale in magnetic nanostructures such as artificial spin ice (ASI) arrays, in addition to the behavior of nanoscale spin structures such as skyrmions in layered thin films. Quantitative analysis of the LTEM data is carried out using the transport of intensity equation (TIE) approach, which we have extended to allow us to visualize the magnetic structure in three dimensions. By comparing these data with the results of micromagnetic simulations, we are able to gain a fuller understanding of the various energy terms that contribute to the behavior that we observe.

For example in artificial spin ices, we have explored the way in which aperiodicity leads to local variations in the energy landscape and to a difference in behavior with respect to periodic ASIs. We are able to calculate the energy at each vertex motif for each possible magnetic configuration from our simulations and thus determine which vertices play the most dominant role in influencing behavior in response to magnetic fields [1,2] and to changes in temperature. Applied to magnetic skyrmions we used in situ LTEM to observe skyrmions in a Co/Pt multilayer film as a function of applied magnetic field, and by combining with simulations we were able to determine the strength of the Dzyaloshinskii-Moriya interaction [2].

[1] V. Brajuskovic, F. Barrows, C. Phatak, A.K. Petford-Long Scientific Reports 6, 34384 (2016).
[2] V. Brajuskovic, A. Addi, C. Phatak, A. K. Petford-Long, Phys. Rev. B 98, 094424 (2018).
[3] W. Jiang, S. Zhang, X. Wang, C. Phatak, Q. Wang, W. Zhang, M.B. Jungfleisch, J.E. Pearson, Y. Liu, J. Zang, X. Chang, A. Petford-Long, A. Hoffmann, S.G.E. te Velthuis, Phys. Rev. B 99, 104402 (2019).

We have newly developed hard X-ray photoelectron spectroscopy (HAXPES) under an applied magnetic field of 1 kOe to study the electronic and magnetic states of the interface-induced perpendicular magnetic anisotropy (PMA), in particular for MgO/Fe interfaces [1]. In this work, we used MgO(2 nm)/Fe(1.5 and 20 nm)/MgO(001) structures to reveal the interface-induced electronic states of the Fe film. Perpendicular magnetization of the 1.5-nm-thick Fe film without extrinsic oxidation of the Fe film was detected by the Fe 2p core-level magnetic circular dichroism (MCD) in HAXPES under a magnetic field, and easy magnetization axis perpendicular to the film plane was confirmed by ex-situmagnetic hysteresis curve measurements. The valence-band HAXPES spectrum of the 1.5-nm-thick Fe film revealed that the Fe 3d electronic states were strongly modified from the thick Fe film and a reference bulk Fe sample due to the lifting of degeneracy in the Fe 3d states near the MgO/Fe interface. We found that the tetragonal distortion of the Fe film by the MgO substrate also contributes to the lifting of degeneracy in the Fe 3d states and PMA, as well as the Fe 3d-O 2p hybridization at the MgO/Fe interface, by comparing the valence-band spectrum with density functional theory calculations for MgO/Fe multilayer structures. Thus, we can conclude that the Fe 3d-O 2p hybridization and tetragonal distortion of the Fe film play important roles in PMA at the MgO/Fe interface. HAXPES with in-situ magnetization thus represents a powerful new method for studying spintronic structures. This is a first step for HAXPES under a coexistence of electrical and magnetic fields to understand the phenomena driven by electric and magnetic fields from the electronic structures.

[1] S. Ueda, M. Mizuguchi, M. Tsujikawa, and M. Shirai, Sci. Technol. Adv. Mater. (under review).

The National Synchrotron Light Source II (NSLS-II) houses the world leading coherent soft x-ray (CSX) beamline, which was designed to exploit a high coherent x-ray flux (x10¹³photons s^-1) to study structure and dynamics of novel materials and model devices using resonance elastic x-ray scattering (REXS). By selecting incident energies of x-ray absorption edges (200 - 1800 eV), researchers use the in-vacuum diffractometer and liquid helium cryostat to investigate single crystals, thin films, and engineered nano-arrays. This energy range is particularly exploited by 3d transition metal oxides, but it is not limited to these materials. The variable setup of CSX offers scientists a variety of techniques including, but not limited to, nano-diffraction, ptychography, resonant reflectivity / diffraction, and x-ray photon correlation spectroscopy (XPCS). In particular, coherent diffraction produces a signal with x-ray speckles caused by domain boundaries and defects that act as phase objects, and XPCS uses speckles to characterize dynamics of electronic and magnetic ordering on the 10s of milliseconds to hours time scale. The x-ray speckles may also be used to study phenomena like return point memory or material degradation. Our main experimental-station also has some limited in-situ capability with permanent magnets and 4-wire measurements. This is in addition to our new holography experimental-station that is outfitted with an electromagnet (0.8 Tesla maximum) with advanced application of current/voltage to samples. We will present these capabilities, some recent science from the first 4 years of operation, and our future plans.

Here, we present in situ and operando in a transmission electron microscope (TEM) studies where the micro-structure is characterized at atomic scale across phase transitions characterized by specific temperatures, i.e., Curie temperature, critical superconductive temperature (Tc). We use a dedicated low temperature double-tilt holder able of atomic-scale characterization in a 100 K to 1,000 K temperature range while applying electrical stimulus. We will present a few cases study where the combination of low temperature and atomic resolution imaging allow to get deeper understanding on the superconductor, ferroelectric and ferromagnetic materials properties. We will present the nano-confinement effect on the Curie temperature of the (MnNiSi)_1-x(Fe₂Ge)_x system for magneto-caloric applications with the aim to tune the ferromagnetic transition both by a control of the composition (x) and of the crystal size. The effect of the x, accounting for MnNiSi to Fe₂Ge phase ratio have been shown to control the Curie temperature by bulk characterization techniques but so far the size effects have not been investigated. The transition is monitored both by real-space imaging techniques inside a TEM combined with phase contrast techniques that allow the measurement of magnetic moments aligned perpendicularly to the electron beam propagation direction. We will also present our recent results on the imaging of high-temperature superconductor, Bi₂Sr₂Ca₂Cu₃O₁₀, around its critical temperature (~110 K). The use of scanning (S-)TEM techniques combined with multi-slice simulations at different collection angles allowed to follow how the micro-structure affects the superconducting mechanism. We measured operando the resistivity drop inside the TEM at micro-scale using standard IV characterization techniques and electron holography for local mapping of the electrostatic potential. Multiferroic, magnetoelectric, piezoelectric and lead-free – epitaxially strained bismuth ferrite (BFO) thin films exhibites both exciting physics and fascinating characteristics for tuning their functionality. Using SEM and FIB techniques, we identified pre-written AFM regions and milled cross-sectional lamellae across distinct regions which included an area of native polymorphs, an area which was electrically written into the pure T phase and an area which was transformed back into the mixed-phase via stress. We present direct measurements of the atomic and electronic structure of the native and post-external stimuli-written phases using nano-beam electron diffraction (NBED) and electron energy-loss spectroscopy (EELS) acquired using aberration-corrected STEM, accompanied by EEL spectra calculated using the program FEFF.

Large susceptibilities in the PbZr_1-xTi_xO₃ system have historically been attained by selecting materials in the compositional vicinity of the morphotropic phase boundary. This typically comes as a trade-off with saturation polarization, as polarization diminishes and susceptibility rises at the phase transition. Here, we take advantage of unit-cell precise growth to introduce ferroelectric-ferroelectric interfaces as an additional design parameter wherein the material response can be tuned by both composition and superlattice periodicity. We likewise focus on heterostructures with overall chemistry near this boundary, but built from compositions far from the phase boundary itself: rhombohedral PbZr_0.8Ti_0.2O₃ and tetragonal PbZr_0.4Ti_0.6O₃. Using reflection high-energy electron diffraction (RHEED)-assisted pulsed-laser deposition, we create atomically-precise (PbZr_0.8Ti_0.2O₃)_n/(PbZr_0.4Ti_0.6O₃)_2n (n = 2, 4, 6, 8, and 16 unit cells) superlattices with overall film chemistry PbZr_0.53Ti_0.47O₃. As compared to uniform films of the parent materials, the superlattice structures exhibit both large saturation polarization (P_s = 64 µC/cm²) and dielectric susceptibility (ε_r = 776 at 10 kHz for n = 4 superlattices). AC field-dependent dielectric measurements suggest the presence of both phase-boundary- and parent-like switching events. First-order reversal curve studies<span style="font-size:10.8333px"> </span>reveal separate switching events for each of the parent layers in addition to an emergent phase-boundary-like interfacial layer. Ultimately, this reveals that artificial superlattices can be deterministically designed as an effective pathway for enhanced responses to external biases.

In composite multiferroics, magneto-electric (ME) coupling – e.g., altering the magnetic properties by applying an electric field and vice versa– can be achieved by creating an interface between a ferromagnetic and a ferroelectric compound. The induced magnetoelectric coupling originates from strain transfer and strongly depends on the structural and chemical features at the interface. In order to realize structurally stable metal/oxide interfaces for electronic device applications or to realize composite multiferroics with significant ME coupling, it is therefore important to understand the interface properties and to identify how the interface is modified in the presence of an external electric field.

In our work, we have studied the evolution of the magnetic spin structure and the Fe oxidation state at the interface using Fe/LNO (LiNbO₃) and Fe/BTO (BaTiO₃) as two model systems for ferromagnetic (FM)/ferroelectric (FE) interfaces. To this end, we relied on the isotope-sensitivity of Mössbauer spectroscopy and nuclear resonant scattering (NRS) of synchrotron radiation, which allows to isolate the signal of an approx. 1 nm thin Fe layer at (or near) the FM/FE interface. Doing so, the chemistry and magnetism at the interface can be in situ probed, while an electric field is applied.

The results show that application of a large enough electric field induces the formation of a thin magnetically dead layer at the interface between the metal and the FE oxide due to ion transport across the interface. Remarkably, the final interface state depends on the polarization history of the system. In turn, this magnetically dead layer results in an irreversible decrease of the magneto-electric coupling. With recent NRS measurements, we have identified the values of the critical field above which interface mixing occurs, for both Fe/BTO and Fe/LNO systems, to be of the order of 400 kV/m [1].

Based on our experimental findings, a model was proposed [1,2] for the effect of an applied electric field on the metal/FE oxide interface. Due to the work function difference between the metal and the oxide, a built-in electric field emerges at the interface during the growth of the metal layer onto the FE oxide. Depending on the direction of this built-in field (hence, depending on the sign of the work function difference), the direction of the applied electric field either favors ion transport across the interface or opposes it until the external field overcomes the built-in field.

We recently extended and validated this model by including a wider range of oxides. Therefore, we studied by Mössbauer spectroscopy the chemistry and magnetic state of the interfaces between an Fe layer and a number of complex oxides (with a wide range of permittivity values), before and after the application of external electric fields. Besides Fe/LNO and Fe/BTO, also Fe/BiFeO₃, Fe/SrTiO₃ and Fe/MgO were investigated. This allows us to compare metal/ferroelectric interfaces with metal/non-ferroelectric oxide interfaces to identify the effect of the ferroelectric polarization charges on the electric field induced ion transport across the interface [3]. It is found that the nature of the oxide (ferroelectric or non-ferroelectric) determines if an external electric field will induce interfacial oxidation/reduction or not.

From our investigation, it is evident that for multiferroic studies, electric fields below the threshold values should be used to avoid significant interfacial changes due to ionic diffusion, which may deteriorate the magneto-electric coupling. These findings may have important implications not only for the further development of composite multiferroics, but also for complex oxide heterostructures in general.

References
[1] Manisha Bisht et al., Advanced Materials Interfaces 3 (2016) 1500433.
[2] Sébastien Couet et al., Adv. Funct. Mat. 24 (2014) 71–76.
[3] Manisha Bisht et al., to be published.

Correlated oxide thin films often show spatial inhomogeneity in the phase states in the vicinity of the critical temperature, and consequently, the phase transitions become rather gradual. To extract the intrinsic phase transition properties of correlated oxides in device structures, therefore, spatial characterization of the phase domain sizes and the device scaling down to the domain size are necessary. Vanadium dioxide (VO₂) is an archetypal correlated oxide that shows a metal-insulator transition (MIT) around 340 K and of great interest for electronic device applications because the resistance changes by up-to 5 orders of magnitude across the MIT.
Some studies have shown that VO₂ thin films consist of insulating and metallic phase domains near the critical temperature, whose sizes depend on the crystallinity determined by the growth substrates. Recently, we have demonstrated the growth of high-quality polycrystalline VO₂ thin films on hexagonal boron nitride (hBN) that is an insulating layered material. However, the phase domain sizes of VO₂/hBN have yet to be characterized.
Here, we investigate the phase domain sizes and the scaling behavior of the resistance-temperature characteristics of VO₂/hBN. VO₂ thin films were grown on mechanically-exfoliated thin flakes of hBN by pulsed laser deposition. First, we employed temperature-dependent optical microscopy to characterize the domain sizes of VO₂/hBN, since insulating and metallic VO₂ are known to show different optical contrasts under visible light illumination. By optical microscopy, we found that the metallic phase domains were emerged near the critical temperature. The sizes of metallic domains were observed to range from several hundreds of nanometers up to a few micrometers, which are comparable to the grain sizes of VO₂/hBN measured by atomic force microscopy.
Next, we measured the temperature-dependent resistances of VO₂/hBN of various sizes. We found that micrometer-scale VO₂/hBN showed a step-like behavior in the resistance-temperature characteristic, which reflects the MIT of individual domains in the thin film. The step-like resistance changes became more marked with scaling down to a few micrometers. Such step-like resistance changes can never be seen in a similar size of polycrystalline VO₂ on other substrates such as Al₂O₃ and is a unique feature of VO₂/hBN that consists of micrometer-scale domains.

Ferroelectric materials hold great potential for a wide range of technological advances and are an actively researched class of materials. Ferroelectric switching is governed by domain nucleation and domain wall motion. These dynamic processes are difficult to measure. Although fine-scale and local probes have provided much insight and understanding of the localized switching mechanisms, exploration on a global scale, taking into account both intrinsic and extrinsic effects on switching and interfacial phenomena, remains a challenge. This talk presents results from in-situ transmission electron microscopy (TEM) that allows for the visualization of ferroelectric domain switching at high spatial and temporal resolution. In-situ TEM allows for domain dynamics to be quantified in terms of factors such as local dislocation content, charge and point defect behavior, and strain enhanced phenomena. Many of these factors occur in one experiment, however, presenting difficulties for tracking their motion or measuring changes. Recent developments in direct electron detection for both imaging and spectroscopy allow for rapid acquisition of time-resolved spectral data and images at rates over 1000 frames per second. At these frame rates, signal to noise ratio is not necessarily optimal. Moreover, events that occur simultaneously can easily be “missed” with conventional image processing or data analysis tools. For these reasons, machine learning approaches are implemented to glean critical information that dictates ferroelectric and multiferroic behavior in datasets deemed too “noisy” for the naked eye. These advances present the possibility of understanding multiple factors in domain dynamics simultaneously, and thus the opportunity to tune these complex materials more predictively.

In complex oxide thin-film heterostructures, a pulsed laser deposition (PLD) technique has been widely employed for the epitaxial film growth. In PLD, we note that the plasma ions generated by the laser ablation of the ceramic target straightly move to the opposite side and then, adhere to the surface of a heated single-crystal substrate forming an epitaxial film layer eventually. Then, understanding such a dynamic process in PLD film growth is of practical interest in aspect of the high-crystalline thin-film fabrication. And, the physical properties of the as-grown films are significantly affected by the crystallinity of the initial ceramic target used for the PLD. However, the detailed studies on the relationship between the ceramic-target crystallinity and the linked physical properties have been rare. In this work, we demonstrate the effect of ceramic-target crystallinity on the metal-to-insulator transition in rare-earth nickelate thin films epitaxially grown by PLD. We first prepared two LaNiO₃ ceramic targets with different crystallinity and then, visualized their surface morphology and the spatial distribution of local stoichiometry in these two LaNiO₃ targets. Using various spectroscopic analyses, we microscopically examined how the target crystallinity directly affects the electrical transport properties in epitaxial LaNiO₃ thin films. More details of our experimental results will be presented in conjunction with a discussion about the correlation between the observed metal-to-insulator transitions and the degree of disproportionation in the Ni charge valence state.

PbZr_1-xTi_xO₃ (PZT) is widely used for ferroelectric and piezoelectric applications like MEMS. PZT has a morphotropic phase boundary (MPB) where rhombohedral and tetragonal phases coexist. PZT of MPB composition leads to excellent dielectric and piezoelectric properties by free rotation of spontaneous polarization in structural gradient regions (SGR) at domain walls and phase boundaries¹⁾. The density of the SGR is expected to increase by controlling size and density of domains. We have previously elucidated that MPB compositional range of Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PMN-PT) and PZT thin film deposited on SrTiO₃(001) (STO) single crystal was extended in comparison with PMN-PT and PZT bulk crystals, and that nano-sized domains were formed due to residual strain induced by misfit dislocations ^3,4). This revealed that we could control the phase stability and domain structure by utilizing elastic field. The objective of this research is to elucidate the effect of elastic field on phase stability and domain structure in MPB composition of PZT thin films from the viewpoint of microstructure. The target of this research is to elucidate the effect of the local strain around defects on the formation of the SGR and MPB compositional range, which leads to intensify the properties of the films.
XRD 2θ-ω profiles and an electron diffraction patterns of PbZr_0.3Ti_0.7O₃(PZT30/70) on KTaO₃(KTO) thin films prepared by CSD method using MOD solutions with excessive 10 at% Pb indicated the cube-on-cube epitaxial relationship between the film and the substrate. The film involves only a perovskite phase with no pyrochlore phase. HAADF-STEM image and the strain map by geometric phase analysis of PZT30/70 on KTO. The interface between PZT30/70 and KTO interface is coherent without misfit dislocation owing to the little lattice mismatch of about 0.01 for the rhombohedral phase and about 0.005 for the a-axis of the tetragonal phase of the film. The volume dilatation map, which means the average of the normal strain both of the in- and out-of-plane directions, showed that two phases with different volume coexisted. The normal strain map in out-of-plane direction revealed that 90°domains of tetragonal phase were formed around the interface and that two-layer structure in out-of-plane direction. The tetragonal and the rhombohedral phases were dominant at the bottom region the upper region of the film, respectively. Then, MPB composition is around Zr/Ti=30/70. From our early studies on PMN-PT and PZT epitaxial films on STO, 90°domains of tetragonal phase nucleated at misfit dislocations in the semicoherent interface. However, the result on the PZT epitaxial films on KTO shows that the growth of 90°domains needs no misfit dislocation, which is corresponding to the PTO epitaxial films on STO with little lattice mismatch of 0.015 for a-axis. The strain map of normal strain in the in-plane direction shows that tetragonal phase grew in the out-of-plane direction from the edge of 90°domains. This result suggests that the edges of 90°domains also act as the nucleation sites of phase boundaries which plays the role of the SGR, indicating that the density of two kinds of SGR, i.e. phase boundaries and domain walls, can be extended by increasing 90°domain population.
Acknowledgemen
This study was conducted with the support of JSPS KAKENHI Grant Numbers JP19H02421, JP19H04531, JP17K18970, JP17H05327 and Kato Foundation for Promotion of Science.
References
1) H. Fu et al., Nature, 403, 281 (2000).
2) S. Wada et al., Bri. Cera. Trans., 103, 2 (2004).
3) T. Kiguchi et al., 56,10PB12 (2017).
4) T. Kiguchi, T. Shimizu et al., The 66th Spring meeting of Jpn. Soc. Appl. Phys. (2019).

HfO₂-based fluorite-type oxides are well-known as polymorphic materials (Monoclinic, Tetragonal, and Cubic phases). In recent years, ferroelectricity has been discovered in HfO₂-based thin films containing a metastable orthorhombic phase (O-phase), and these materials have attracted much attention for future ferroelectrics.
Since the O-phase is obtained by rapid thermal annealing of amorphous HfO₂-based films deposited at room temperature, the crystallization and formation mechanisms of the film has been of great interest. In fact, the selection of crystalline phases depends on the condition of rapid thermal annealing due to polymorphism of HfO₂-based oxide family. It is, hence, important to clarify the relationship between the annealing conditions and the resultant crystalline phase, including the O-phase. In this study, we investigated the influence of the annealing conditions on the crystalline phase(s) of HfO₂-based thin films, with a particular emphasis on the formation of O-phase.
20 nm-thick (Hf,Ce)O₂ films were deposited on (001) yttria-stabilized zirconia (YSZ) substrates by ion-beam sputtering. The deposition was carried out at room temperature in Ar atmosphere, then as-deposited films were annealed under various conditions (annealing time and temperature) in N₂ atmosphere. The crystal structure of the deposited films was investigated by X-ray diffraction (XRD) and scanning transmission electron microscope (STEM).
XRD measurement confirmed that as-deposited films are amorphous, while in-situ XRD measurement showed that as-deposited films completely crystallized at a temperature above 600 °C. The lattice constants estimated from XRD 2θ-ω patterns changed discontinuously with decreasing temperature, suggesting that a phase transition has occurred. STEM observation for the annealed films revealed that the crystallization began at the film/substrate interface. We also found that the crystalline phase of the annealed films varied with annealing temperature, and most importantly, observed that the number of O-phase domains increased with increasing annealing time. The resulting (Hf,Ce)O₂ films exhibited multidomain structures composed of the O-phase. We show that the optimum microstructure can be controlled by tuning the annealing condition.

This research was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant No. 19K15282. In addition, this work was supported by the Murata Science Foundation and the Nippon Sheet Glass Foundation for Materials Science and Engineering.

Investigation of reliable or non-volatile information storage has become an irresistible attraction in the field of modern electronic technology [1,2]. As ferroelectric domain walls are one to two orders of magnitude thinner than magnetic domain walls, exploration of switching properties and stabilization of ferroelectric domains can open a way from microelectronics to nanoelectronics, that is, it can help achieve higher information storage density [3].
Here, we focus on the (110) orientated BFO film, where the compression coming from the STO substrate can suppress certain ferroelastic switching. In order to observe the real-time domain evolution, in situ electrical biasing in transmission electron microscope (TEM) is applied. During the experiment, the switching processes including nucleation at the interface, propagation and growth has been captured. Moreover, using double-spherical aberration corrected TEM, the activation of domain switching near the antiphase boundary (APB) and the stabilization of 180^o domain walls at the final state maintained by flux closure are also revealed down to atomic scale. Our study sheds light on switching performance of the BFO film grown on (110) orientated substrate and is expected to provide a potential pathway for future design of information storage devices.

References
[1] Seidel, J. Nanoelectronics based on topological structures. Nat Mater 2019, 18, 188-190
[2] Sharma, P.; Zhang, Q.; Sando, D.; Lei, C. H.; Liu, Y.; Li, J.; Nagarajan, V.; Seidel, J. Nonvolatile ferroelectric domain wall memory. Science advances 2017, 3, e1700512.
[3] Catalan, G.; Seidel, J.; Ramesh, R.; Scott, J. F. Domain wall nanoelectronics. Reviews of Modern Physics 2012, 84, 119-156.

Rare earth doped lead zirconate titanate (PZT) nano crystalline films have excellent dielectric, ferroelectric, and piezoelectric behavior and are useful for memory and energy storage devices. Herein we report rare earth La³⁺ and Sc³⁺ cation doped PZT thin films and studied their ferroelectric properties and energy storage capacity. The nano films of (1-y)[PbZr_0.53Ti_0.47] y[La_1-xSc_x]O_3-δ with y = 0.10 and 0 ≤ x ≤1, were fabricated by using pulse deposition method on La_0.67Sr_0.33MnO₃ (LSMO) coated MgO (100) substrates employing a KrF excimer laser of wavelength 248 nm. The thickness of the grown films, measured using profilometer, was ~ 350 nm. We observed (100) oriented thin films stabilized in tetragonal perovskite phase from analysis of x-ray diffraction data. The Raman spectra of these thin films at different temperatures suggest the nucleation and evolution of the polar nano-regions dispersed in a nano polar matrix attributed to their relaxor behavior. We observed the surface roughness of films between 3 - 10 nm from analysis of Atomic force microscopy. The film capacitors fabricated as LSMO/PLZTS/Pt were investigated for its dielectric and ferroelectric behavior. The temperature dependent (100-650 K) dielectric measurement in the frequency ranges 10²-10⁶ Hz, revealed dielectric constant ε’ ∼ 700-2000 with a comparatively low loss. Ferroelectricity of these thin films were evident well saturated hysteresis loops at room temperature recorded at several frequencies. The high dielectric constant, low losses and excellent ferroelectricity of nanostructured film capacitors suggest their potential application in electronic devices.

Colloidal dispersions of magnetic nanoparticles (NP) were synthesized at the University of São Paulo, Brazil, and at Charotar University of Science and Technology, India, based on manganese-zinc ferrites, (Mn_0.5Zn_0.5Fe₂O₄). Solutions containing nanoparticles with spherical (MZS) and cubic (MZC) nanoparticles were studied by means of the Z-Scan technique in the open-aperture configuration, allowing the determination of the two-photon absorption cross-section (σ_2PA). Since the nanoparticles are in the superparamagnetic state and free to rotate within the liquid carrier, external magnetic fields, with magnitudes between 0 Oe and 3100 Oe, were applied in order to orient the particles in solution, aligning the axis of easy magnetization of each NP to the external field. These experiments were performed with the incident beam polarization in the same direction to the magnetic field lines, herein called parallel configuration, and in the case in which the polarization and the magnetic field directions were orthogonal, the perpendicular configuration. To better understand the nonlinear absorption, verified during the Z-Scan measurements, the electron dynamics were investigated by a spectrally resolved femtosecond transient absorption (without the presence of external magnetic field). Samples were pumped at 390 nm (region in which the samples present a large absorption) with different pulse energy and the relaxation dynamic was probed by using an ultrashort white-light continuum pulse (450 nm to 750 nm). In addition to these measurements, the pump beam was changed for 780 nm in order to check the dynamic induced by two-photon absorption. It is important to say that in the absence of magnetic field, linear absorptions of both samples are the same, as expected since the NP's have the same constituents. The magnetic field effects on the linear absorption was also evaluated and no changes were verified regardless the field presence, indicating that the nanoparticles are being oriented but not forming bigger structures that would scatter the light. The two-photon absorption cross-section in the absence of external magnetic field was measured as σ_2PA^○=18.0(6) GM for spherical nanoparticles and 17.0(7) GM for the cubes. The measured values are compatible with each other within the experimental errors. For both studied nanoparticles, the 2PA cross-section measured in the parallel configuration presented an increment in the value, while in the perpendicular configuration, the measured value decreased when compared to H=0. In the presence of magnetic fields, nanoparticles tend to align their magnetic momentum to the external field and, in consequence, there is an alignment of crystallographic planes of the material. Thus, these experiments put in evidence the optical anisotropy in the two-photon absorption of our ferrite nanoparticles, since in the parallel configuration components of the third-order nonlinear tensor was measured alongside the magnetic momentum direction, while in the perpendicular configuration, in the two dimensions perpendicular to that direction, that are equivalent. When there is not magnetic field applied, the nanoparticles are randomly oriented and the measurements correspond to an average over the three orthogonal directions. Femtosecond transient absorption measurements revealed an ultrafast relaxation process containing at least two separated dynamical processes, one shorter than 5 picoseconds, possible related to the hot electron relaxation or exciton-exciton annihilation, and a second one, much longer, may be related to the trapped electron on the conduction band. Also, the longer lifetime may be associated to the relaxation from the conduction to the valence band. Same behavior was observed by pumping the samples with 775 nm, indicating that the electronic states excited by two-photon absorption may be the same as the ones excited by one-photon absorption.

Colloidal dispersions of magnetic nanoparticles were synthesized at the University of São Paulo, Brazil, and at Charotar University of Science and Technology, India, based on manganese-zinc ferrites, (Mn_0.5Zn_0.5Fe₂O₄). Solutions containing nanoparticles with distinct shapes (MZS, containing spherical nanoparticles, and MZC, composed of cubic NP's) were studied by means of the hyper-Rayleigh scattering (HRS) technique, allowing the determination of the optical second harmonic generation, defined as β. The size distribution and shape of nanoparticles were determined through small angle x-rays scattering (SAXS). Since the produced nanoparticles are free to rotate within the liquid carrier, an external magnetic field was applied in order to orient the particles in solution. Experiments with applied magnetic field were performed both in the parallel configuration (when the incident laser beam polarization was parallel to the magnetic field lines) and in the perpendicular case (where the polarization and the field directions were orthogonal). Furthermore, the linear attenuation spectrum was measured in the presence and absence of external magnetic field and it shows no changes regardless the field presence, indicating that the nanoparticles are being oriented but not forming bigger structures that would scatter the light. This is supported by the SAXS data, also measured in the presence of magnetic field, that demonstrate the formation of small linear aggregates, composed by a few nanoparticles. In the absence of magnetic field the linear attenuation spectrum of both samples is the same, as expected since the NP's have the same constituents. In this case, the hyperpolarizability was measured, for spherical nanoparticles, as β^○=9.5(2)×10^-28 cm⁵/esu, while for cubic ones, β^■=7.8(1)×10^-28 cm⁵/esu. For both studied nanoparticles, optical second harmonic experiments were performed in the presence of external field in the parallel configuration. In this condition, an increase in the value was verified, that is, for spherical nanoparticles (MZS), β^○_//=10.1(2)×10^-28 cm⁵/esu, and for MZC sample, β^■_//=8.1(2)×10^-28 cm⁵/esu. On the other hand, a slight decrease on the hyperpolarizability value was measured for experiments performed on the perpendicular configuration, β^○_⊥=9.3(3)×10^-28 cm⁵/esu for MZS and β^■_⊥=7.4(2)×10^-28 cm⁵/esu for the cubes. For cubic nanoparticles the anisotropy in the first-order hyperpolarizability δβ = (β_// - β_⊥)/β_// was 8.2 while for the spherical nanoparticles, 7.6%. In the presence of magnetic fields, nanoparticles tend to align their magnetic momentum to the external field, in consequence, there is the alignment of crystallographic planes of the material. Therefore, in the parallel configuration the first-order hyperpolarizability was measured alongside the magnetic momentum direction, while in the perpendicular configuration, in the two dimensions perpendicular to that direction, that are equivalent. When there is not magnetic field applied, the nanoparticles are randomly oriented and the measured hyperpolarizability corresponds to an average over the three orthogonal directions. This can be verified calculating the weighted average of the hyperpolarizability in the presence of external field, that is, <β> = 1/3(β_// + 2β_⊥), so <β^○>=9.6(3)×10^-28 cm⁵/esu and <β^■>=7.7(2)×10^-28 cm⁵/esu, in both cases compatible with the measured value for the system without magnetic field. This demonstrates the shape dependent anisotropy in the second-order nonlinear properties of ferrite NP's, opening the possibility of the development of optofluidic devices, associating magnetic nanoparticles in solution and optical elements as i.e. optical fibers, as optical magnetometers, that would allow the determination of the field magnitude and direction, as well as in information propagation since there are three possible states and light intensities for a system composed by magnetic NP and an external magnetic field.

The effect of Y2O3 doping on the structure, transport, dielectric, and relaxation properties of BT ceramics will be investigated. The possibility of formation of an extensive range of solid solution regions and the formation of a polymorphism of Y-doped BaTiO3 Will be studied. In this study, coarse-grained and nano-sized grain BT ceramics will be synthesized by spark plasma sintering. Conventional sintering is expected to produce micro-sized grained ceramics, whereas spark plasma sintering of BT nanopowder could produce nanoceramics/fine grained ceramics. The structure of the BT nanoceramics will be investigated by X-ray diffraction, whereas FE-SEM, Raman, and AFM will determine the grain size and the chemical composition of the ceramics. The photocurrent characteristics of BT nanoceramics will be studied by using Quantum efficiency and solar systems. The transport dielectric and relaxation properties will be studied by impedance spectroscopy measurements over wide ranges of temperatures and frequencies. Thermally stimulated depolarization current (TSDC) measurements are interesting and helpful technique that will be used to study the relaxation properties due to the defects in the studied ceramics. We will also perform investigations using theoretical calculations based on density functional theory (DFT) to study the role of cation substitution on structural and electronic properties in BaTiO3.

Lanthanide doped-titanium-containing perovskite nanocrystals, M_1-χLn_χTiO₃ (where Ln = La³⁺, Nd³⁺, Sm³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺ and M = Ba²⁺, Sr²⁺), have have been synthesized via a solvothermal route at relatively low temperatures, T 〈 180 ^oC. The versatility of the synthesis route has made it possible to synthesize similar sizes (20 nm) and shapes (cuboidal) for all Ln-dopant compositions and as such, trends in their electronic and optical properties can be explored. Without the need for high temperature annealing, X-ray diffraction patterns of the as-synthesized nanocubes such as Ba_0.97Tm_0.03TiO₃, Ba_0.97Er_0.03TiO₃ and Ba_0.97Dy_0.03TiO₃ powders synthesized at 150 ^oC showed highly crystalline, single phase and pure nanocrystals. Transmission electron microscope coupled with an EDS-detector was used to map and quantify the actual composition and stoichiometric ratios of the lanthanide ions in the crystal lattice. Also, a fluorescence emission spectroscopy of the M_1-χLn_χTiO₃ nanocubes colloidal solution showed interesting emission spectra such as an up-conversion of the excitation wavelength for some specific Ln-dopant compositions. Temperature dependent Raman spectroscopy was used to analyze the local structural distortions within the crystal lattice as well as the tetragonality of the ferroelectric phase. We observed that for certain compositions such as Ba_0.95Yb_0.05TiO₃, the tetragonality increases with increasing temperature but decreases sharply after the transition temperature; 80 ^oC. Advanced microscopy techniques such as atomic force microscopy (AFM), piezoelectric force microscopy (PFM) and magnetic field PFM were used to study the electric and magnetic field response of the nanocrystals. With increasing percent compositions of the same Ln-dopant type, we observed a general trend in the ferroelectric and piezoelectric responses of the as-synthesized M_1-χLn_χTiO₃ nanocubes. The general trend and values of piezoelectric displacement coefficient (d₃₃) for all sample compositions has been measured at ambient and high temperature conditions.

The fabrication of smaller, denser, and higher frequency electronic devices has resulted in the need to completely understand and control thermal transport at the nanometer scale [1, 2]. At the nanoscale shorter than the heat diffusion length, thermal transport in semiconductor devices is dominated by phonon scattering across interfaces between the semiconductor heterojunctions and is quantified by the thermal boundary conductance (TBC). We investigated the kinetics of thermal transport across a GaAs/AlGaAs epitaxial interface using timeresolved Refection High Energy Electron Diffraction (TR-RHEED). In the experiments, the temperature imbalance is created by ultrafast heating the top GaAs nanofilm selectly with 800-nm femtosecond optical pulses and monitored by tracing its temperature evolution with TR-RHEED. The thermal transport kinetics oberserved with TR-RHEED was also simulated using two seperate models: a heat conduction and three-temperature model, providing a method of measuring and extracting the thermal boundary conductance in semiconductor heterostructures. The TBC was found to agree well with the Diffuse Mismatch Model at lower temperature imbalance across the interface, but start showing different behavior when the temperature of the top GaAs nanofilm is higher than the Debye temperature, opening up questions about the mechanisms governing interface heat transfer under highly non-equilibrium conditions.

References
[1] D. G. Cahill, W. K. Ford, et al., Journal of Applied Physics 93, 793-818 (2003).
[2] D. G. Cahill, P. V. Braun, et al., Applied Physics Reviews 1, 011305 (2014).