Myung-Geun Han

Magnetic spinel oxides are high-performance spintronic materials due to their high Curie temperature, high spin polarization, fast spin dynamics, and strain-tunable magnetic anisotropy. Epitaxial strain and disorder can significantly modify the electronic and magnetic energy landscapes, while their interplay remains elusive. Here, we use epitaxial NiCo₂O₄ thin films and freestanding NiCo₂O₄ membranes as model systems to reveal the complex roles of strain and defects in determining the metallicity and magnetotransport properties of the ferrimagnetic spinel. Unlike the perpendicular magnetic anisotropy and 2-fold sinusoidal anisotropic magnetoresistance (AMR) observed in metallic NiCo₂O₄ films on spinel substrates, NiCo₂O₄ on perovskite substrates and NiCo₂O₄ membranes exhibit insulating behaviors and spin canting, with an additional 4-fold AMR component emerging due to disorder-induced spin scattering and strain-induced tetragonal magnetocrystalline anisotropy. The amplitude ratio between the 4-fold and 2-fold AMR components provides critical information on the disorder types that contribute to the AMR. Electron microscopy studies reveal structural and chemical phase separation in the membranes similar to those in disordered films, which explains the highly consistent magnetotransport properties for NiCo₂O₄/Sr₃Al₂O₆ films and NiCo₂O₄ membranes. Our study provides effective material strategies for engineering spin transport and magnetic anisotropy in NiCo₂O₄ and presents a promising venue for designing flexible magnetic memory, sensor, and spintronic applications.

Phase selecting nearly degenerate crystalline polymorphs during epitaxial growth can be challenging yet critical to targeting physical properties for specific applications. Here, we establish how phase selectivity of altermagnetic and noncentrosymmetric polymorphs of MnTe can be programmed by subtle changes to the surface of lattice-matched InP substrates in molecular beam epitaxy growth. Bulk altermagnetic MnTe is thermodynamically stable in the hexagonal NiAs-structure and is synthesized here on the polar (111)A surface (In-terminated) of InP, while the noncentrosymmetric, cubic ZnS-structure with wide band gap (>3 eV), which epitaxially matches III-V materials, is stabilized on the (111)B surface (P-terminated). Electron microscopy, X-ray photoemission spectroscopy, and reflection high-energy electron diffraction indicate that phase selection is triggered at the interface and proceeds along the growing surface. First-principles calculations suggest that interfacial termination and strain have a significant effect on the interfacial energy; stabilizing the NiAs polymorph on the In-terminated surface and the ZnS structure on the P-terminated surface. Selectively grown, high-quality, phase pure films of both MnTe polymorphs will enable our understanding of the novel properties of these materials, thereby facilitating their use in new applications ranging from spintronics to microelectronic devices.

Mott materials are archetypal quantum systems actively explored as next-generation electronic and photonic platforms, with potential applications spanning non-Von Neumann computing, robotics, energy storage, and microwave technologies. Among these, vanadium dioxide (VO ₂ ) has emerged as one of the most intensively studied compounds, owing to its sharp, near-room-temperature insulator-to-metal phase transition. VO ₂ also serves as a benchmark system for testing cutting-edge theories and experimental techniques. Here, we directly visualize the electrically driven transition dynamics in VO ₂ using a microwave-driven, frequency-tunable pulsed transmission electron microscope that combines nanometer spatial and picosecond temporal resolution. Under high-frequency (MHz–GHz) excitation, we capture the ultrafast nucleation, propagation, and dissolution of metallic domains within an operating device over millions of reversible cycles. We observe the ultrafast formation of consistent metallic nuclei beneath the electrodes, followed by the propagation of a structural phase front at 4.54 nm/ns. Our experiments show that phonon-mediated structural recovery ultimately limits reversible switching of VO ₂ at GHz frequencies, and that a tunable regime for reversible operation spans from kHz to GHz through device engineering. Beyond VO ₂ , our approach provides a powerful framework for probing non-equilibrium structural transformations in correlated and functional materials under realistic electrical stimuli.

Conductivity modulation via ferroelectric polarization coupling with LaNiO₃ (LNO) is demonstrated at an epitaxial ferroelectric-LNO interface. Conductivity measurements, varying the thickness of the LNO channel, show that this phenomenon is confined to a few atomic layers at the interface. Combining in situ biasing and off-axis holography, we mapped out electrostatic potentials at the PbZr0_.2Ti_0.8O₃(PZT)/LNO/SrTiO₃(STO) heterostructure upon polarization switching. Using aberration-corrected STEM, the interfacial atomic structures were investigated for the two different PZT polarization states. Polarization in PZT induces a significant change in the in-plane O-Ni-O bond angles, with a 37° modulation in the topmost 1 or 2 LNO unit cells, driven by strain in the oxygen sublattice for the two opposite polarization directions in PZT. Both oxygen and cation sublattices exhibit strain responses exceeding 10% upon switching. This atomic-layer structural modulation highlights a mechanism for functional oxide heterostructure development, offering pathways for advancements in nonvolatile memory, sensors, and energy-efficient transistors.

Topochemical intercalation is widely used to access metastable phases with novel electronic properties, but the reverse reaction (deintercalation) typically restores the original state, limiting practical use. Here, we present a topochemical approach that employs a sacrificial intercalant that thermally decomposes to irreversibly lock in the new electronic state. Using 2-aminobutane as the sacrificial intercalant, we convert the van der Waals (vdW) material 1T-TiSe₂ into a superconductor and the vdW superconductor 2H-NbSe₂ into a nonsuperconducting metal, while preserving the ability to exfoliate the resulting crystals. We find that this transitory intercalation increases the electron density in both materials and partially suppresses the CDW in TiSe₂. By tuning the thermolysis temperature, we can systematically vary the carrier density in TiSe₂, enabling us to map its phase diagram. The superconductivity in TiSe₂ is retained in exfoliated flakes, although with a lower critical temperature. This transitory topochemical strategy enables access to new electronic states with precisely tuned carrier densities that are otherwise inaccessible through direct solid-state synthesis.

The ultimate scaling limit in ferroelectric switching has been attracting broad attention in the fields of materials science and nanoelectronics. Despite immense efforts to scale down ferroelectric features, however, only few materials have been shown to exhibit ferroelectricity at the unit-cell level. Here we report a controllable unit-cell-scale domain in brownmillerite oxides consisting of alternating octahedral/tetrahedral layers. By combining atomic-scale imaging and in situ transmission electron microscopy, we directly probed sub-unit-cell-segmented ferroelectricity and investigated their switching characteristics. First-principles calculations confirm that the phonon modes related to oxygen octahedra are decoupled from those of the oxygen tetrahedra in brownmillerite oxides, and such localized oxygen tetrahedral phonons stabilize the sub-unit-cell-segmented ferroelectric domain. The unit-cell-wide ferroelectricity observed in our study could provide opportunities to design high-density memory devices using phonon decoupling.

The emergence of ultrafast electron microscopy (UEM) has enabled the discovery of strongly correlated dynamic mechanisms, including electron–phonon coupling, structural phase transitions, thermal transport, and electromagnetic deflection. Most UEM systems operate stroboscopically, meaning that the technique is susceptible to artifacts, mistakes, and misinterpretation of the data due to extensive experimental effort. In contrast to the ultrafast designation, data acquisition is extraordinarily slow because the electron beam has significantly reduced signal compared to traditional transmission electron microscopy due to pulsing the electron beam. Consequently, the sample may drift, tilt, or undergo irreversible structural changes that are independent of the time-resolved dynamics throughout the experimental time frame. Furthermore, these datasets require significant user interpretation that can be problematic when proper controls are not implemented thoroughly. Here, we demonstrate a new algorithm designed to separate ultrafast structural dynamics from long-term artifacts using a LiNbO3 sample experiencing electrically driven surface acoustic wave propagation. Additionally, we provide examples of the impact of user bias when analyzing the data and provide a methodology, which enables the extraction of time-resolved responses when the image signal is extraordinarily low. Overall, the goal of this publication is to provide methods that validate the experimental results and reduce researcher biases during UEM data interpretation.

In materials with 1D electronic bands, electron–electron interactions can produce intriguing quantum phenomena, including spin‐charge separation and charge density waves (CDW). Most of these systems, however, are non‐magnetic, motivating a search for anisotropic materials where the coupling of charge and spin may affect emergent quantum states. Here, chemical intercalation of the van der Waals magnetic semiconductor CrSBr yields Li_0.17(2)(tetrahydrofuran)_0.26(3)CrSBr, which possesses an electronically driven quasi‐1D CDW with an onset temperature above room temperature. Concurrently, electron doping increases the magnetic ordering temperature from 132 to 200 K and switches its interlayer magnetic coupling from antiferromagnetic to ferromagnetic. The spin‐polarized nature of the anisotropic bands that give rise to this CDW enforces an intrinsic coupling of charge and spin. The coexistence and interplay of ferromagnetism and charge modulation in this exfoliatable material provide a promising platform for studying tunable quantum phenomena across a range of temperatures and thicknesses.

The intrinsic magnetic topological insulator (IMTI) family [MnTe][Bi₂Te₃]_n has demonstrated magneto-topological properties dependent on n, making it a promising platform for advanced electronics and spintronics. However, due to technical barriers in sample synthesis, their properties in the large n limit remain unknown. To overcome this, we utilized the atomic layer-by-layer molecular beam epitaxy (ALL-MBE) technique and achieved IMTIs with n as large as 15, far beyond that previously reported in bulk crystals or thin films. Then, we discover that the "single-layer magnet (SLM)" phase, primarily determined by intralayer ferromagnetic coupling, emerges for n > ∼4 and remains little affected up to n = 15. Nonetheless, still, nonzero, interlayer ferromagnetic coupling is necessary to stabilize the SLM phase, suggesting that the SLM phase eventually disappears in the n → ∞ limit. This study uncovers the secrets of IMTIs beyond the thermodynamic limit and opens a door to diverse magneto-topological applications.

Two-dimensional van der Waals magnets hosting topological magnetic textures, such as skyrmions, show promise for spintronics and quantum computing. Electrical control of these topological spin textures is crucial for enhancing operational performance and functionality. Here, using electron microscopy combined with in situ electric and magnetic biasing, we show that the skyrmion helicity in insulating Cr₂Ge₂Te₆ can be controlled by the direction of the external electric field applied during the field cooling process. The electric field-tuned helicity remains stable at low temperature, even amid variations in magnetic and electric fields. Our theoretical investigation reveals that nonzero Dzyaloshinskii-Moriya interactions between nearest neighbors, induced by the electric field, change their sign upon reversing the electric field direction, thereby facilitating helicity selection. The electrical control of magnetic helicity demonstrated in this study can be extended to other centrosymmetric skyrmion-hosting magnets, paving the way for future device designs in topological spintronics and quantum computing.