Atomic structure and electronic properties of MgO grain boundaries in tunnelling magnetoresistive devices

Granular vortex spin-torque nano oscillator for reservoir computing

In this paper, we investigate the granularity in the free layer of the magnetic tunnel junctions (MTJ) and its potential to function as a reservoir for reservoir computing where grains act as oscillatory neurons while the device is in the vortex state. The input of the reservoir is applied in the form of a magnetic field which can pin the vortex core into different grains of the device in the magnetic vortex state. The oscillation frequency and MTJ resistance vary across different grains in a non-linear fashion making them great candidates to be served as the reservoir's outputs for classification objectives. Hence, we propose an experimentally validated area-efficient single granular vortex spin-torque nano oscillator (GV-STNO) device in which pinning sites work as random reservoirs that can emulate neuronal functions. We harness the nonlinear oscillation frequency and resistance exhibited by the vortex core granular pinning of the GV-STNO reservoir computing system to demonstrate waveform classification.

Underlying Mechanisms and Tunability of the Anomalous Hall Effect in NiCo2 O4 Films with Robust Perpendicular Magnetic Anisotropy

Their high tunability of electronic and magnetic properties makes transition-metal oxides (TMOs) highly intriguing for fundamental studies and promising for a wide range of applications. TMOs with strong ferrimagnetism provide new platforms for tailoring the anomalous Hall effect (AHE) beyond conventional concepts based on ferromagnets, and particularly TMOs with perpendicular magnetic anisotropy (PMA) are of prime importance for today's spintronics. This study reports on transport phenomena and magnetic characteristics of the ferrimagnetic TMO NiCo2 O4 (NCO) exhibiting PMA. The entire electrical and magnetic properties of NCO films are strongly correlated with their conductivities governed by the cation valence states. The AHE exhibits an unusual sign reversal resulting from a competition between intrinsic and extrinsic mechanisms depending on the conductivity, which can be tuned by the synthesis conditions independent of the film thickness. Importantly, skew-scattering is identified as an AHE contribution for the first time in the low-conductivity regime. Application wise, the robust PMA without thickness limitation constitutes a major advantage compared to conventional PMA materials utilized in today's spintronics. The great potential for applications is exemplified by two proposed novel device designs consisting only of NCO films that open a new route for future spintronics, such as ferrimagnetic high-density memories.

Controlling the Formation of Conductive Pathways in Memristive Devices

Resistive random-access memories are promising candidates for novel computer architectures such as in-memory computing, multilevel data storage, and neuromorphics. Their working principle is based on electrically stimulated materials changes that allow access to two (digital), multiple (multilevel), or quasi-continuous (analog) resistive states. However, the stochastic nature of forming and switching the conductive pathway involves complex atomistic defect configurations resulting in considerable variability. This paper reveals that the intricate interplay of 0D and 2D defects can be engineered to achieve reproducible and controlled low-voltage formation of conducting filaments. The author find that the orientation of grain boundaries in polycrystalline HfO_x is directly related to the required forming voltage of the conducting filaments, unravelling a neglected origin of variability. Based on the realistic atomic structure of grain boundaries obtained from ultra-high resolution imaging combined with first-principles calculations including local strain, this paper shows how oxygen vacancy segregation energies and the associated electronic states in the vicinity of the Fermi level govern the formation of conductive pathways in memristive devices. These findings are applicable to non-amorphous valence change filamentary type memristive device. The results demonstrate that a fundamental atomistic understanding of defect chemistry is pivotal to design memristors as key element of future electronics.

Atomic structures of twin boundaries in CoO

The twinning plane of crystals with a face-centered-cubic (FCC) structure is usually the (111) plane, as found in FCC metals and oxides with FCC sublattices of oxygen, like rock-salt-type NiO and spinel-type Fe₃O₄. Surprisingly, we found in this work that the twinning plane of rock-salt-type CoO is the (112) plane, although Co is adjacent to Ni in the periodic table. The atomic and electronic structures of the CoO(112) twin boundary with in-plane shift vector 1/2[111] have been studied combining aberration-corrected scanning transmission electron microscopy (STEM), electron-energy-loss spectroscopy (EELS), and density functional theory (DFT) calculations. It was found that the atoms at the twin boundary have nominal oxidation states, and the twin boundary remains insulating and antiferromagnetically coupled. Importantly, through the electronic structures and the crystal orbital Hamilton population (COHP) analyses, the (112) twin boundary is found to be more stable than the (111) twin boundary.

Local electronic structure variation resulting in Li 'filament' formation within solid electrolytes

Solid electrolytes hold great promise for enabling the use of Li metal anodes. The main problem is that during cycling, Li can infiltrate along grain boundaries and cause short circuits, resulting in potentially catastrophic battery failure. At present, this phenomenon is not well understood. Here, through electron microscopy measurements on a representative system, Li₇La₃Zr₂O₁₂, we discover that Li infiltration in solid oxide electrolytes is strongly associated with local electronic band structure. About half of the Li₇La₃Zr₂O₁₂ grain boundaries were found to have a reduced bandgap, around 1-3 eV, making them potential channels for leakage current. Instead of combining with electrons at the cathode, Li⁺ ions are hence prematurely reduced by electrons at grain boundaries, forming local Li filaments. The eventual interconnection of these filaments results in a short circuit. Our discovery reveals that the grain-boundary electronic conductivity must be a primary concern for optimization in future solid-state battery design.

Moringa oleifera Leaf Extract-Mediated Green Synthesis of Nanostructured Alkaline Earth Oxide (MgO) and Its Physicochemical Properties

The magnesium oxide nanoparticles (MgO NPs) were prepared from Moringa oleifera leaf extract. Phytochemicals are derived from plant extract which are served as stabilizing and capping agents. This green route has been attracted owing to speed, reliable, and eco-friendly and cost-effective one. The synthesized magnesium oxide nanoparticles were taken into three different calcination temperatures (500, 600, and 700°C). The powder X-ray diffraction (PXRD) study shows a pure phase of face-centered cubic structure. Periclase MgO nanoparticles were prepared. The optical band gap of MgO nanoparticles is 4.5 eV, and its absorption in the UV region was observed by UV-visible spectroscopy (UV-Vis). Photoluminescence spectra have exhibited multicolor emissions were being at UV and visible region due to defect centers (F centers) of MgO nanoparticles. EDX (energy dispersive X-ray spectrum) has given the stoichiometric ratio of Mg and O. The functional groups have been studied by Fourier transformed infrared spectroscopy (FTIR), surface morphology transformation has been identified by scanning electron microscopy (SEM) studies, and VSM measurements have given the information of diamagnetic nature of MgO nanoparticles. H-R TEM micrographs have confirmed that particles were in nanorange matched with XRD report. Polycrystalline nature has been observed pattern information. TG-DSC characterization revealed phase transition and weight loss information. D-band and G-band of MgO nanoparticles are studied by micro-Raman analysis. Dielectric analysis has proven that MgO nanoparticles will be a promising candidate for linear dielectric ceramics, thermistor. The present resent studies have revealed that MgO powder will be an economical and promising candidate in superconductor, optoelectronic device, and energy storage applications.

An origin of excess vibrational entropies at grain boundaries in Al, Si and MgO: a first-principles analysis with lattice dynamics

First-principles lattice dynamics is applied to symmetric tilt grain boundaries (GBs) in Al, Si and MgO, with the goal of revealing critical factors in determining excess vibrational entropies at the atomic level. Excess vibrational entropies at GBs are found to vary depending on the substances. Al GBs tend to show larger excess entropies and hence larger temperature dependence of the GB free energies than those in Si and MgO. Most of the Si GBs show small excess entropies. For Al and MgO, atom-projected vibrational entropies are well correlated with bond-length changes at GB cores, and have large positive values as bond lengths increase for GB atoms. This demonstrates that a similar mechanism likely dominates excess vibrational entropies of GBs for both substances, despite their dissimilar bonding nature. For Si GBs, atoms with threefold coordination do not simply follow such a correlation, implying the importance of other factors that are different from bond-length changes. These systematic comparisons will be a foothold for understanding a physical origin of excess entropies at GBs even in more complex substances.

Ceramic phases with one-dimensional long-range order

Solids are generally classified into three categories based on their atomic arrangement: crystalline, quasicrystalline and amorphous^1-4. Here we report MgO and Nd₂O₃ ceramic phases with special atomic arrangements that should belong to a category of solids different from these three well known categories by combining state-of-the-art atomic-resolution scanning transmission electron microscopy and first-principles calculations. The reported solid structure exhibits a one-dimensional (1D) long-range order with a translational periodicity and is composed of structural units that individually have atomic arrangements similar to those observed in coincidence-site lattice configurations present at grain boundaries. Regardless of the insulating nature of the bulk MgO, the bandgap of which is measured to be 7.4 eV, the MgO 1D ordered structure is a wide-bandgap semiconductor with a bandgap of 3.2 eV owing to this special atomic arrangement. The discovery of 1D ordered structures suggests that the structural categories of solids could be more abundant, with physical properties distinct from their regular counterparts.

The interplay of structure and dynamics at grain boundaries

Molecular simulations are carried out to address the structure and atomic diffusion at grain boundaries. We use an inherent structure approach, which maps each configuration in a molecular dynamics trajectory to the potential energy minimum ("inherent structure") it would reach by a steepest descent quench. Dynamics are then decomposed into a combination of displacements within an inherent structure and transitions between inherent structures. The inherent structure approach reveals a simple structural picture of the grain boundary that is normally obscured by the thermal motion. We apply our methodology to polycrystalline MgO. Grain boundary atoms are identified as atoms that are undercoordinated in the inherent structure, relative to those in the perfect crystal. Our method enables the calculation of grain boundary diffusion coefficients without arbitrary assumptions about which atoms or spatial regions belong to the grain boundary, and the results are shown to be consistent with estimates from experiments. The inherent structure approach also enables the elementary steps in the diffusion process to be elucidated. We show that the process in MgO grain boundaries primarily involves vacancy hops, but that there is also significant motion of other nearby atoms during such a hop.