Session Q67: Tools and Techniques for Exploring Materials Physics at the Frontier of Time and Length Scales

Magneto-optical and magnetic force microscopy of Al-deficient LaAlO3/SrTiO3 interfaces

The nature of magnetism at the LaAlO3/SrTiO3 (LAO/STO) interface is not well understood. Magnetic force microscopy (MFM) imaging has revealed electronically tunable domains at the LAO/STO interface near the insulating transition point[1]. Recent reports indicate that room temperature magnetic ordering can be achieved in LAO/STO heterostructure by controlling the Al deficiency of the LAO upper layer during growth [2]. By taking advantage of optically-induced magnetism in STO[3], scanning magneto-optical Kerr effect (MOKE) microscopy can be used in addition to MFM to map magnetic domains spatially. Ultrafast MOKE can then be used to study magnetic domain dynamics and transitions with picosecond resolution using optical pump and probe measurements. Observing apparent ferromagnetism at this interface will offer new insights into the nature of magnetism and the source of this phenomenon.

 

[1] F. Bi, et al. Nat Commun 5, 5019 (2014).

[2]D.S. Park, et al., Nat Commun 11 3650 (2020) 

[3] W. Rice, Ambwani, P., Bombeck, M. et al. Nature Mater 13, 481–487 (2014).   

Depth-resolved x-ray characterization of interfacial ferromagnetism in oxide superlattices

Origins of the emergent interfacial ferromagnetism in CaMnO3/CaRuO3 heterostructures have been investigated for nearly two decades. Several experimental and theoretical studies suggest that this phenomenon is stabilized by the charge transfer from Ru to Mn, but they differ in predicting the thicknesses of the resultant ferromagnetic layers [1-4]. In this study, we have carried out polarization-dependent x-ray absorption spectroscopy with circular dichroism and x-ray resonant magnetic scattering (XRMS) measurements to probe the depth-resolved magnetic profile in a CaMnO3/CaRuO3 superlattice. Self-consistent x-ray optical fitting of both non-resonant and resonant XRMS spectra near the Mn L2,3 absorption edge revealed significantly different magnetic moments localized within one unit-cell of the top and bottom interfaces of the CaMnO3 layer. Complementary hard x-ray photoemission spectroscopy was utilized to probe bulk-sensitive core-level and valence-band electronic structure. The experimental results were rationalized using first-principles theoretical calculations.

[1]  K. S. Takahashi et al., Appl. Phys. Lett. 79, 1324 (2001); [2]  B. R. K. Nanda et al., Phys. Rev. Lett. 98, 216804 (2007); [3]  J. W. Freeland et al., Phys. Rev. B 81, 094414 (2010); [4]  A. J. Grutter et al., Nano Lett. 16, 5647 (2016).   

Bilayer-single-layer competition in artificial hybrid superlattices of square-lattice iridates

Controlling fundamental electronic and magnetic degrees of freedom via artificial heterostructure engineering is expected to provide a new way for obtaining new emergent quantum states. We successfully designed and synthesized a new hybrid superlattice (SL) of (SrIrO₃)₁/(SrTiO₃)₁/(SrIrO₃)₂/(SrTiO₃)₁ that combines single-layer and bilayer of Jeff = 1/2 square lattices which cannot be realized in the bulk materials. X-ray diffraction and resistivity measurements suggest that the hybrid SL hosts an intermediate electronic and structurally distorted state of the single-layer (SrIrO₃)₁/(SrTiO₃)₁ and bi-layer (SrIrO₃)₂/(SrTiO₃)₁ SLs. More interestingly, resonant x-ray magnetic scattering and magnetization measurements show that the hybrid SL stabilizes a unique magnetic state with a transition temperature similar to the bilayer SL but with a planar anisotropy similar to that of the single-layer SL. This may suggest that the hybrid state is closer to the quantum phase transition boundary between ab plane and c-axis magnetic states.   

Using Bragg CDI to Study Phase Transitions in Binary Alloys

Nimish Prashant Nazirkar¹, Edwin Fohtung<sup>1

1</sup>Department of Materials Sciences and Engineering, Rensselaer Polytechnic Institute, Troy, NY

Abstract:

During directional solidification of a binary alloy, the solid–liquid interface exhibits a variety of patterns that are due to the Mullins–Sekerka instability and governed by the growth conditions [1]. It is well known that properties of the grown material are largely controlled by the microstructures left in the solid during processing. Thus, a precise quantification of the microstructure and mastering of the solidification is essential to tailor products in a reproducible fashion to a specified quality. Bragg coherent diffraction imaging (BCDI), a well-established technique for imaging the internal strain of nanocrystals and grains in polycrystalline films. I intend to use BCDI to image the internal compositional distribution of binary alloys in thermal equilibrium. Our results will be used to relate displacement fields to compositional distributions in various binary alloy nanoparticles.

References:

1. Nguyen Thi, H.; Jamgotchian, H.; Gastaldi, J.; Härtwig, J.; Schenk, T.; Klein, H.; Billia, B.; Baruchel, J.; Dabo, Y, Journal of Physics D Applied Physics, 2003, 36, A83-A86,    doi:10.1088/0022-3727/36/10A/317.

2. D. Karpov & E. Fohtung J. of Appl. Phys. 125, 121101 (2019)   

Functional properties of the multiferroic spinel FeV2O4

The ferrimagnetic spinel class of materials is known for anomalous magnetoresponsive behaviors stemming from strong spin-lattice coupling and orbital ordering effects. In particular, the material FeV2O4, which has two-orbital active ions, is one of a few spinels to display type-II multiferroicity. The connection between symmetry lowering orbital-ordering transitions on the Fe/V-sites and the development of a ferroelectric polarization has yet to be understood. Furthermore, single-crystal samples demonstrate giant magnetostrictive and magnetodielectric couplings related to the behavior of magnetoelastic/multiferroic domains on the nanoscale in the bulk of the material. Here we present our ongoing work which characterizes this material on a variety of length-scales using macroscopic and scattering probes. Our results are comprised of neutron diffraction and diffuse scattering, small-angle neutron scattering, and macroscopic response measurements. By correlating these complementary datasets, we can comprehensively understand the functional properties of the material, ranging from the unit cell-level behavior to the field-response of complex domain-wall patterns observed on the mesoscale.   

Novel Self-Assembled Two-Dimensional Layered Oxide Structure Incorporated with Au Nanoinclusions towards Multifunctionalities

Two-dimensional (2D) layered oxides have recently attracted wide attention owing to the strong coupling effect between charge, spin, lattice and strain, which opens up great flexibility and opportunities in structure construction as well as multifunctionality exploration. In parallel, plasmonic hybrid nanostructures exhibit combined physical properties coming from different material components and the broad range of applications in nanophotonic and electronic devices. In this work, novel self-assembled bismuth-based supercell (SC) structures incorporated with Au nanoinclusions are integrated via one-step pulsed laser deposition (PLD) technique. Comprehensive microstructural characterization results demonstrate the highly epitaxial quality and particle-in-matrix morphology of the nanocomposite thin films. The SC oxide matrix exhibits typical anisotropic ferromagnetic and ferroelectric properties while the Au nanoparticle phase results in enhanced localized surface plasmon resonance (LSPR) in the visible wavelength region. This study opens a new gate for developing novel 2D layered complex oxides incorporated with plasmonic metal or semiconductor phases showing great potential applications in multifunctional nanoelectronics devices.   

Multilayer phase-coherent tunneling through amorphous MoO3 barriers in heteromorphic In2O3/MoO3 superlattices

Multilayer phase-coherent tunneling has been demonstrated in a heteromorphic superlattice structure with alternating polycrystalline In₂O₃ conducting layers and amorphous MoO₃ insulating barriers. Unconventional periodic wavefunction with coherent Bloch-like states through multiple disordered barriers is demonstrated. Two methods of transport characterization verify multilayer coherence of this state in samples with varying thicknesses of the MoO₃ layers, namely magnetoconductance (MC) with magnetic field parallel or perpendicular to the superlattice and temperature-dependent conductance. The MC reveals three superlattice weak localization (SLWL) behaviors with decreasing barrier thickness from uncoupled 2D multilayers to “scatter-first-then-tunnel” diffusive Fermi-surface regime, to “tunnel-first-then-scatter” propagative Fermi-surface regime (2D-DFS-PFS). The temperature dependence confirms quasi-2D logarithmic and quasi-3D exponential behaviors consistent with these regimes. Fit parameters including phase coherent length, mean free path, anisotropy coefficient, single-barrier tunnel time, vertical coupling energy are distinguished in each regime, from which evidence of multilayer phase-coherent tunneling is revealed with proper MoO₃ layers thicknesses.   

Phase transition pathway by a co-operative pseudo-Jahn teller effect under microwave radiation

We will present unstable material phase retention under microwave radiation experimentally and theoretically discuss the underlying physics of the process. The intrinsic dielectric loss in a material can be explained by lifetime broadened two-phonon difference processes. At temperatures near the phase transition, this absorption can lead to disruption of long-range order between different localized Jahn teller active sites leading to order-disordered phases and phase retention in the crystal structure. In this work, we investigate the effect of microwave radiation on structural phase transitions and present the results for the case of bismuth oxide. There have been five polymorphs of Bi₂O₃ reported, including α, β, δ, γ, and ε, with α-Bi₂O₃ being the stable phase at room temperature. However, under microwave radiation, we can synthesize the metastable β phases at room temperature. The sample is characterized using X-ray diffraction, Raman spectroscopy, DSC, and UV-Vis spectroscopy to evaluate the structure and different phases in the sample. These metastable states are structurally related, and different phases can emerge depending on the temperature and intensity at which the sample is irradiated with the microwave.   

Real-Time 3D Materials Analysis During Electron Tomography using tomviz

Three-dimensional (3D) characterization across the nanoscale is now possible using scanning transmission electron microscopes. Unfortunately, tomographic reconstructions can take one to several days to complete depending upon the dataset size or algorithm(s) employed. Even worse, the reconstruction occurs offline, long after all the data has been collected, preventing immediate interpretation during an ongoing experiment.

Here we present interactive 3D visualization that seamlessly runs while experimental projections are collected in an electron microscope using the tomviz platform (www.tomviz.org). Tomviz presents 3D materials structure to scientists in real-time enabling high-throughput specimen interpretation with immediate tomogram visualization. During the experimental acquisition, tomviz monitors for new projections which are continuously appended into the reconstruction process. After a reconstruction updates, tomviz immediately renders the 3D volume. Tomograms are reconstructed in parallel with data acquisition and a high-quality 3D reconstruction is available before the experiment ends.

Tomviz offers unparalleled electron tomography throughput with real-time 3D analysis—it is an open-source repository that is available to all institutions for download at www.tomviz.org   

Tracking twin boundary jerky motion at nanometer and microsecond scales

The jerky motion of twin boundaries in the ferromagnetic shape memory alloy Ni-Mn-Ga is studied by simultaneous measurements of stress and magnetic emissions (ME). A careful design of the experimental conditions results in an approximately linear relationship between the measured ME voltage and the nm-scale volumes exhibiting twinning transformation during microsecond-scale abrupt 'avalanche' events. This work shows that the same distributions of ME avalanches, related to features of jerky twin boundary motion, are found both during and between stress drop events. Maximum likelihood analysis of statistical distributions of several variables reveals a good fit to power laws truncated by exponential functions. Interestingly, the characteristic cutoffs described by the exponential functions are in the middle of the distribution range. Further, the cutoff values can be related to physical characteristics of the studied problem. Particularly, the cutoff of amplitudes of ME avalanches matches the value predicted by high rate magnetic pulse tests performed under much larger driving force values. This observation implies that avalanches during slow rate twin boundary motion and velocity changes observed by high rate tests represent the same behavior and can be described by the same theory.   

Enhanced MOKE in the Otto Configuration: What are the implications in magnetometry and related technology?

We demonstrate the enhancement of the transverse magneto-optical Kerr effect (TMOKE) signal, due to surface plasmon resonance in the Otto configuration. This way, a separation of the magnetic sample from the plasmonic device and the modulation of the plasmon resonance with an enhancement of the MOKE signal is possible. We have achieved this by using air as a low index dielectric, with variable thickness, where the evanescent wave extends, preceding to excitation of surface plasmons. The magnetic sample under consideration is a thin layer of cobalt coated by an ultrathin silver layer, on a silicon substrate (Ag/Co/Si). The sample is brought close enough to the prism/Air interface, allowing surface plasmon excitation in the air/Ag interface. This leads to an increase of the TMOKE signal up to ~ 2 per mil with respect to the incident light. This is about 7 times the traditional MOKE signal in the absence of plasmons and comparable to the Kretschmann-Raether configuration. Furthermore, the fact that the plasmon field generated at the metal-air interface substitutes the laser light used in traditional MOKE, allows new functionalities such as controlling the penetration depth of the plasmonic field into the sample. This should find applications in magnetometry and related technology.   

Second harmonic generation in AB-type LaTiO3/SrTiO3 superlattices

Conventional generation of second harmonic generation (SHG) is by using non-central symmetric materials, or a combination of them. However this is limited by the material itself, which is usually of bulk size. In this work, we employ the surface and interfaces to generate and control the SHG from a series of central symmetric oxide LaTiO₃/SrTiO₃ superlattices. We demonstrate that the nonlinear optical susceptibility χ⁽²⁾ and the SHG pattern can be tuned by tailoring the number of superlattice interfaces. Significantly, unlike the conventional ABC-type nanostructures, our AB-type nanostructures can also produce SHG signals. The χ⁽²⁾ value (24.3 pm/V) is even one to two orders of magnitude greater than the conventional ones. Hence, we demonstrate that SHG is a local microscopic process—it does not rely on macroscopic broken inversion symmetry. Moreover, we show that temperature dependence or structural phase transition can further enhance the SHG intensity. Our results introduce a scheme for SHG, extending the generation and control of SHG in nano-photonics to nearly all the accessible centrosymmetric materials.