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We theoretically study the tunnel magnetoresistance (TMR) effect in (111)-oriented magnetic tunnel junctions (MTJs) with SrTiO$_{3}$ barriers, Co/SrTiO$_{3}$/Co(111) and Ni/SrTiO$_{3}$/Ni(111). Our analysis combining the first-principles calculation and the Landauer formula shows that the Co-based MTJ has a high TMR ratio over 500%, while the Ni-based MTJ has a smaller value (290%). Since the in-plane lattice periodicity of SrTiO$_{3}$ is about twice that of the primitive cell of fcc Co (Ni), the original bands of Co (Ni) are folded in the $k_x$-$k_y$ plane corresponding to the $ab$ plane of the MTJ supercell. We find that this band folding gives a half-metallic band structure in the $\Lambda_1$ state of Co (Ni) and the coherent tunneling of such a half-metallic $\Lambda_1$ state yields a high TMR ratio. We also reveal that the difference in the TMR ratio between the Co- and Ni-based MTJs can be understood by different $s$-orbital weights in the $\Lambda_1$ band at the Fermi level., Comment: 7 pages, 5 figures, 3 tables

We study the tunnel magnetoresistance (TMR) effect and magnetocrystalline anisotropy in a series of magnetic tunnel junctions (MTJs) with $L1_1$-ordered fcc ferromagnetic alloys and MgO barrier along the [111] direction. Considering the (111)-oriented MTJs with different $L1_1$ alloys, we calculate their TMR ratios and magnetocrystalline anisotropies on the basis of the first-principles calculations. The analysis shows that the MTJs with Co-based alloys (CoNi, CoPt, and CoPd) have high TMR ratios over 2000$\%$. These MTJs have energetically favored Co-O interfaces where interfacial antibonding between Co $d$ and O $p$ states is formed around the Fermi level. We find that the resonant tunneling of the antibonding states, called the interface resonant tunneling, is the origin of the obtained high TMR ratios. Our calculation of the magnetocrystalline anisotropy shows that many $L1_1$ alloys have large perpendicular magnetic anisotropy (PMA). In particular, CoPt has the largest value of anisotropy energy $K_{\rm u} \approx 10\,{\rm MJ/m^3}$. We further conduct a perturbation analysis of the PMA with respect to the spin-orbit interaction and reveal that the large PMA in CoPt and CoNi mainly originates from spin-conserving perturbation processes around the Fermi level., Comment: 11 pages, 9 figures, 2 tables

The coexistence of a low magnetic Gilbert damping constant and large perpendicular magnetic anisotropy (PMA) in ferromagnetic thin films is critical for high-speed and energy-efficient spintronics devices. To clarify the spin-resolved contributions of damping and PMA, a flat, lattice-matched interface is developed in an $\mathrm{Fe}$(0.7 nm)/${\mathrm{Mg}\mathrm{Al}}_{2}{\mathrm{O}}_{4}$(oxide)(3 nm) bi-layer film by varying the ex situ annealing temperature, which acts as a catalyst to vary the PMA energy densities according to the oxidation degree. Here, the optimized procedure for interface engineering is implemented to achieve an extremely low magnetic Gilbert damping constant (0.013) and a strong interfacial PMA energy $(0.8\phantom{\rule{0.1em}{0ex}}\mathrm{MJ}\phantom{\rule{0.1em}{0ex}}{\mathrm{m}}^{\ensuremath{-}3})$ for epitaxial thin films. By employing different interfacial atomic configurations in a first-principles calculation, this study explains the origin of the PMA energy and damping constant. The d(yz) and d(zx) orbitals of the interfacial $\mathrm{Fe}$ atoms in the minority-spin states play a role in the orbital moment and its anisotropy. Furthermore, the matrix elements between these two orbitals in the non-spin-flip term predominately contribute to damping. These detailed findings provide a clear insight into the development of materials with significantly improved PMA energies and low damping characteristics, thereby facilitating promising future spintronic applications.

$\mathrm{C}{\mathrm{o}}_{2}\mathrm{MnAl}$ has been predicted to have Weyl points near the Fermi level in $L{2}_{1}$-ordered structure, which is expected to give rise to exotic transverse transport properties such as large anomalous Hall (AHE) and Nernst effects (ANE) due to large Berry curvature. In this study, the effects of Fermi level position and atomic ordering on AHE and ANE in $\mathrm{C}{\mathrm{o}}_{2}\mathrm{MnA}{\mathrm{l}}_{1\ensuremath{-}x}\mathrm{S}{\mathrm{i}}_{x}$ were studied systematically. The $\mathrm{C}{\mathrm{o}}_{2}\mathrm{MnAl}$ film keeps $B2$-disordred structure regardless of annealing temperature, which results in much smaller anomalous Hall conductivity ${\ensuremath{\sigma}}_{xy}$ and transverse Peltier coefficient ${\ensuremath{\alpha}}_{xy}$ than those calculated for $L{2}_{1}$-ordered $\mathrm{C}{\mathrm{o}}_{2}\mathrm{MnAl}$. Our newly performed calculation of ${\ensuremath{\sigma}}_{xy}$ with taking $B2$ disordering into account well reproduces the experimental result; thus it was concluded that Berry curvature originating from the Weyl points is largely reduced by $B2$ disordering. It was also revealed Al substitution with Si shifts the position of the Fermi level and greatly improves the $L{2}_{1}$ atomic ordering, leading to strong enhancement of ${\ensuremath{\alpha}}_{xy}$, which also agreed with our theoretical calculation. The highest thermopower of ANE of 5.7 \ensuremath{\mu}V/K, which is comparable to the recent reports for $\mathrm{C}{\mathrm{o}}_{2}\mathrm{MnGa}$, was observed for $\mathrm{C}{\mathrm{o}}_{2}\mathrm{MnA}{\mathrm{l}}_{0.63}\mathrm{S}{\mathrm{i}}_{0.37}$ because of dominant contribution of ${\ensuremath{\alpha}}_{xy}$. This study clearly shows the importance of both Fermi level tuning and high atomic ordering for obtaining the effect of topological features in Co-based Heusler alloys on transverse transport properties.

The flow of in-plane current through ultrathin magnetic heterostructures can cause magnetization switching or domain-wall nucleation owing to bulk and interfacial effects. Within the magnetic layer, the current can create magnetic instabilities via spin transfer torques (STT). At interface(s), spin current generated from the spin Hall effect in a neighboring layer can exert torques, referred to as the spin Hall torques, on the magnetic moments. Here, we study current-induced magnetization switching in perpendicularly magnetized CoFeB/MgO heterostructures with a heavy metal (HM) underlayer. Depending on the thickness of the HM underlayer, we find distinct differences in the in-plane field dependence of the threshold switching current. The STT is likely responsible for the magnetization reversal for the thinner underlayer films whereas the spin Hall torques cause the switching for thicker underlayer films. For the latter, we find differences in the switching current for positive and negative currents and initial magnetization directions. We find that the growth process during the film deposition introduces an anisotropy that breaks the symmetry of the system and causes the asymmetric switching. The presence of such symmetry-breaking anisotropy enables deterministic magnetization switching at zero external fields.

The electron transport through an array of tunnel junctions consisting of an STM tip and a granular film is studied both theoretically and experimentally. When the tunnel resistance between the tip and a granule on the surface is much larger than those between granules, a bottleneck of the tunneling current is created in the array. It is shown that the period of the Coulomb staircase(CS) is given by the capacitance at the bottleneck. Our STM experiments on Co-Al-O granular films show the CS with a single period at room temperature. This provides a new possibility for single-electron-spin-electronic devices at room temperature.

We report enhanced tunnel magnetoresistance (TMR) ratios of 188$%$ (308$%$) at room temperature and 328$%$ (479$%$) at 15 K for cation-site-disordered MgAl${}_{2}$O${}_{4}$-barrier magnetic tunnel junctions (MTJs) with Fe (Fe${}_{0.5}$Co${}_{0.5}$ alloy) electrodes, which exceed the TMR ratios theoretically calculated and experimentally observed for ordered spinel barriers. The enhancement of TMR ratios is attributed to the suppression of the so-called band-folding effect in ordered spinel MTJs [Phys. Rev. B 86, 024426 (2012)]. First-principles calculations describe a dominant role of the oxygen sublattice for spin-dependent coherent tunneling, suggesting a mechanism of coherent tunneling occurring even in the disordered systems.

The spinmotive force associated with a moving domain wall is observed directly in Permalloy nanowires using real time voltage measurements with proper subtraction of the electromotive force. Whereas the wall velocity exhibits nonlinear dependence on magnetic field, the generated voltage increases linearly with the field. We show that the sign of the voltage reverses when the wall propagation direction is altered. Numerical simulations explain quantitatively these features of spinmotive force and indicate that it scales with the field even in a field range where the wall motion is no longer associated with periodic angular rotation of the wall magnetization.

Spin-dependent coherent tunneling has been experimentally observed in high-quality sputtered-deposited ${\text{Co}}_{2}\text{FeAl}/\text{MgO}/\text{CoFe}$ epitaxial magnetic tunneling junctions (MTJs). Consequently, the microfabricated MTJs manifest a very large tunnel magnetoresistance (TMR) at room temperature and an unexpectedly TMR oscillation as a function of MgO barrier thickness. First-principles electronic band calculations confirm the pronounced coherent tunneling effect and are in good agreement with the experimental data. The present work demonstrates the importance of coherent tunneling for large TMR with Heusler alloys

The effect of chemical ordering of a half-metallic full-Heusler alloy ${\text{Co}}_{2}\text{MnSi}$ (CMS) on interlayer exchange coupling (IEC) behavior was investigated in Co-Mn-Si(20 nm)/Cr(1.2 nm)/Co-Mn-Si(7 nm) trilayers. Two types of experimental investigations were employed, namely, the variations in IEC with changing the annealing temperature of the bottom CMS from room temperature to $500\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ and with changing the Co concentration in top and bottom Co-Mn-Si layers. The $90\ifmmode^\circ\else\textdegree\fi{}$ coupling parameter ${J}_{2}$ and the bilinear coupling parameter ${J}_{1}$ were estimated from the numerical simulations of magnetization curves. Only the strong $90\ifmmode^\circ\else\textdegree\fi{}$ coupling with almost the same strength was observed in both the completely $B2$ ordered and in a mixture of ordered $L{2}_{1}$ and $B2$ structures. The degree of $B2$ ordering as well as the strength of $90\ifmmode^\circ\else\textdegree\fi{}$ coupling was observed to decrease simultaneously with the Co concentration in Co-Mn-Si and $180\ifmmode^\circ\else\textdegree\fi{}$ coupling appeared in highly Co antisite defective $(A2)$ structures, i.e., for the Co concentration over $73\text{ }\text{at}\text{.}\text{ }\mathrm{%}$. This observation showed a clear relationship between the degree of $B2$ ordering and the IEC behavior in full-Heusler Co-Mn-Si-based structures.

Fermi level tuning has been successfully demonstrated in Co-based full-Heusler alloy ${\mathrm{Co}}_{2}{\mathrm{FeAl}}_{0.5}{\mathrm{Si}}_{0.5}$ (CFAS). The half-metallic band gap of CFAS was proved by the behavior of differential conductance of $\mathrm{CFAS}/({\mathrm{MgAl}}_{2}){\mathrm{O}}_{x}/\mathrm{CoFe}$ magnetic tunneling junctions with an unexplored crystalline $({\mathrm{MgAl}}_{2}){\mathrm{O}}_{x}$ barrier. CFAS exhibits the highest effective spin polarization (${P}_{\mathrm{eff}}$) at 300 K and the weakest temperature dependence of ${P}_{\mathrm{eff}}$ among all known half metals. Further study shows that ${P}_{\mathrm{eff}}$ of CFAS decays with increasing temperature ($T$) following ${T}^{3/2}$ law perfectly, which indicates that the depolarization of CFAS is determined by spin wave excitation only.

We report the first observation of a large pressure-induced enhancement of giant magnetoresistance (GMR) in magnetic multilayers (MML). In Fe/Cr MMLs with the Cr layer thickness of approximately 30 A, a crossover from biquadratic to bilinear interlayer exchange coupling (IEC) was observed by applying pressure, and simultaneously the GMR under high pressure (>2 GPa) was enhanced to be twice as large as that at ambient pressure. The enhanced GMR is attributed to the suppression of the biquadratic IEC by applying pressure, and the electrical resistivity in parallel alignment of magnetization also showed a crossover behavior, suggesting an electronic origin for the observed pressure effects.

Magnetoresistance (MR) for currents perpendicular to planes is calculated within the ballistic limit for spring ferromagnets in which an artificial domain wall is formed by the external magnetic fields. We consider two contributions to the MR: one is caused by a twisting of the magnetization and the other is due to a mismatch of the electronic structure between the two ferromagnets comprising the spring ferromagnets. We show that the resulting MR may show a nonmonotonic dependence on the width of the domain walls and can be either positive or negative according to the magnitude of these two contributions. We further show that oscillatory behavior appears in the MR when the soft ferromagnet is sandwiched between hard ferromagnets.

Effects of high pressure on tunnel transport and magnetoresistance have been investigated in ${\mathrm{Co}}_{52}{\mathrm{Al}}_{20}{\mathrm{O}}_{28}$ insulating granular films. It is found that the temperature dependence of electrical resistivity $\ensuremath{\rho}(T)$ is affected strongly by applying pressure while the ${T}^{\ensuremath{-}1/2}$ dependence in $\ensuremath{\rho}(T)$ is still observed at high pressures. Furthermore, tunnel magnetoresistance is enhanced by more than 2% at 3.1 GPa. The results are discussed briefly on the basis of higher-order tunneling theory.

The anisotropic nonlinear magneto-optical response from Fe/Au~001! superlattices is studied. Perfect singlecrystalline order through the whole thickness of the multilayer is observed. The magneto-optical signals are measured in both the longitudinal and transversal configurations. A strong fourfold anisotropy of nonlinear Kerr rotation angle is demonstrated, in addition to the anisotropic second harmonic intensity changes. The results are described in terms of a simple phenomenological model, that involve both dipole and quadrupole nonlinear-optical interactions. Furthermore, general, model-independent symmetry properties of the nonlinear magneto-optical response are established.