Kinetics of Magnetic Skyrmion Crystal Formation from the Conical Phase.

The particle-like magnetic skyrmion or skyrmion lattice (SkX) formation has promoted strong application and fundamental science interests. Despite extensive research, the kinetic of the SkX development is much less understood because of the ultrafast spin rotation and high sensitivity to external perturbations. Here, using in situ Lorentz transmission electron microscopy, we successfully measured the dynamics of SkX formation from the conical phase with precise control of both the temperature and the magnetic field. We discovered that the Avrami equation can accurately describe the transition process with an initial Avrami constant around 1, suggesting that the rate-limiting step for the quasiparticle lattice formation is one-dimensional heterogeneous nucleation of individual skyrmions. A modified Arrhenius rate law is established, with an energy barrier that has a square-root dependence on temperature and a quadratic dependence on the magnetic field. This study paves the way toward precise and predictable manipulation of topological spin structures.

Predictive Materials Design of Magnetic Random-Access Memory Based on Nanoscale Atomic Structure and Element Distribution.

Magnetic tunnel junctions (MTJs) capable of electrical read and write operations have emerged as a canonical building block for nonvolatile memory and logic. However, the cause of the widespread device properties found experimentally in various MTJ stacks, including tunneling magnetoresistance (TMR), perpendicular magnetic anisotropy (PMA), and voltage-controlled magnetic anisotropy (VCMA), remains elusive. Here, using high-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy, we found that the MTJ crystallization quality, boron diffusion out of the CoFeB fixed layer, and minimal oxidation of the fixed layer correlate with the TMR. As with the CoFeB free layer, seed layer diffusion into the free layer/MgO interface is negatively correlated with the interfacial PMA, whereas the metal-oxides concentrations in the free layer correlate with the VCMA. Combined with formation enthalpy and thermal diffusion analysis that can explain the evolution of element distribution from MTJ stack designs and annealing temperatures, we further established a predictive materials design framework to guide the complex design space explorations for high-performance MTJs. On the basis of this framework, we demonstrate experimentally high PMA and VCMA values of 1.74 mJ/m2 and 115 fJ/V·m-1 with annealing stability above 400 °C.

Direct Visualization of Li Dendrite Effect on LiCoO2 Cathode by In Situ TEM.

Nonuniform and highly localized Li dendrites are known to cause deleterious and, in many cases, catastrophic effects on the performance of rechargeable Li batteries. However, the mechanisms of cathode failures upon contact with Li metal are far from clear. In this study, using in situ transmission electron microscopy, the interaction of Li metal with well-defined, epitaxial thin films of LiCoO2 , the most widely used cathode material, is directly visualized at an atomic scale. It is shown that a spontaneous and prompt chemical reaction is triggered once Li contact is made, leading to expansion and pulverization of LiCoO2 and ending with the final reaction products of Li2 O and Co metal. A topotactic phase transition is identified close to the reaction front, resulting in the formation of CoO as a metastable intermediate. Dynamic structural and chemical imaging, in combination with ab initio simulations, reveal that a high density of grain and antiphase boundaries is formed at the reaction front, which are critical for enabling the short-range topotactic reactions and long-range Li propagation. The fundamental insights are of general importance in mitigating Li dendrites related issues and guiding the design principle for more robust energy materials.

Formation, Structural Variety, and Impact of Antiphase Boundaries on Li Diffusion in LiCoO2 Thin-Film Cathodes.

Antiphase boundaries (APBs) are observed in as-synthesized and processed LiCoO2, which is used widely as a cathode material. Using a combination of scanning transmission electron microscopy and ab initio simulations, we investigate APB formation, structure, stability, and impact on Li ion diffusion. It is found that APB low-coordinated sites give rise to cation defects which, in turn, provide a variety of APB structures. Li diffusion along an APB can enter a correlated mode, leading to a ∼40% decrease in activation energy with respect to that for the uncorrelated hopping of Li ions. We propose that APBs function as additional mass-transfer channels that couple in-plane Li ion diffusion pathways, thus facilitating Li transfer from one two-dimensional basin to another, potentially enabling new energy storage architectures.

An Ultrathin Single Crystalline Relaxor Ferroelectric Integrated on a High Mobility Semiconductor.

The epitaxial growth of multifunctional oxides on semiconductors has opened a pathway to introduce new functionalities to semiconductor device technologies. In particular, the integration of gate materials that enable nonvolatile or hysteretic functionality in field-effect transistors could lead to device technologies that consume less power or allow for novel modalities in computing. Here we present electrical characterization of ultrathin single crystalline SrZrxTi1-xO3 (x = 0.7) films epitaxially grown on a high mobility semiconductor, Ge. Epitaxial films of SrZrxTi1-xO3 exhibit relaxor behavior, characterized by a hysteretic polarization that can modulate the surface potential of Ge. We find that gate layers as thin as 5 nm corresponding to an equivalent-oxide thickness of just 1.0 nm exhibit a ∼2 V hysteretic window in the capacitance-voltage characteristics. The development of hysteretic metal-oxide-semiconductor capacitors with nanoscale gate thicknesses opens new vistas for nanoelectronic devices.

Low-Dimensional Oxygen Vacancy Ordering and Diffusion in SrCrO3-δ.

We investigate the formation mechanisms of vacancy-ordered phase and collective mass transport in epitaxial SrCrO3-δ films using ab initio simulations within the density functional theory formalism. We show that as the concentration of oxygen vacancies (VO) increases, they form 1D chains that feature Cr-centered tetrahedra. Aggregation of these 1D VO chains results in the formation of (111)-oriented oxygen-deficient planes and an extended vacancy-ordered phase observed in recent experiments. We discuss atomic-scale mechanisms enabling the quasi-2D VO aggregates to expand along and translate across (111) planes. The corresponding lowest activation energy pathways necessarily involve rotation of Cr-centered tetrahedra, which emerges as a universal feature of fast ionic conduction in complex oxides. These findings explain reversible oxidation and reduction in SrCrO3-δ at low temperatures and provide insights into transient behavior necessary to harness ionic conductive oxides for high-performance and low-temperature electrochemical reactors.