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We experimentally investigated the magnetic properties of ${\mathrm{NiCo}}_{2}{\mathrm{O}}_{4}$ epitaxial films known to be conductive oxides with perpendicular magnetic anisotropy (PMA) at room temperature. Both magnetotorque and magnetization measurements at various temperatures provide clear experimental evidence of the spin reorientation transition at which the MA changes from PMA to easy-cone magnetic anisotropy (ECMA) at a certain temperature (${T}_{\mathrm{SR}}$). ECMA was commonly observed in films grown by pulsed laser deposition and reactive radio frequency magnetron sputtering, although ${T}_{\mathrm{SR}}$ is dependent on the growth method as well as the conditions. The cone angles measured from the $c$ axis increased successively at ${T}_{\mathrm{SR}}$ and approached a maximum of 45--50 degrees at the lowest measurement temperature of 5 K. Calculation with the cluster model suggests that the ${\mathrm{Ni}}^{3+}$ ions occupying the ${T}_{d}$ site could be the origin of the ECMA. Both the magnetic properties and the results of the calculation based on the cluster model indicate that the ECMA is attributable to the cation antisite distribution of ${\mathrm{Ni}}^{3+}$, which is possibly formed during the growth process of the thin films.

We investigated the temperature dependence of the saturation magnetization ${M}_{S}(T)$ and magnetic anisotropy constant ${K}_{u}(T)$ of ${\mathrm{CoMnO}}_{3}$ with an ilmenite structure, which is known as an orbital ferrimagnet. Because of strong antiferromagnetic coupling between the ${\mathrm{Co}}^{2+}$ spin $({d}^{7}\text{:}\phantom{\rule{0.16em}{0ex}}S=3/2)$ and ${\mathrm{Mn}}^{4+}$ spin $({d}^{3}\text{:}\phantom{\rule{0.16em}{0ex}}S=3/2)$ in ${\mathrm{CoMnO}}_{3}$, the net magnetic moment is considered to originate only from the orbital angular momentum [(OAM); $\ensuremath{\langle}L\ensuremath{\rangle}\ensuremath{\approx}1]$ of ${\mathrm{Co}}^{2+}$. Experimental results for ${\mathrm{CoMnO}}_{3}$ epitaxial films clearly show that ${K}_{u}(T)$ is proportional to ${M}_{S}(T)$ for a wide temperature range up to the transition temperature, suggesting that ${K}_{u}(T)$ is proportional to $\ensuremath{\langle}L\ensuremath{\rangle}$. An electron theory based on the cluster model for ${K}_{u}$ and the OAM of ${\mathrm{Co}}^{2+}$ at 0 K also indicates that ${K}_{u}$ is proportional to the OAM and to the orbital angular anisotropy of ${\mathrm{Co}}^{2+}$.

In order to elucidate the origin of the temperature ($T$) dependence of spin-dependent tunneling conductance ($G$) of magnetic tunnel junctions (MTJs), we experimentally investigated the $T$ dependence of $G$ for the parallel and antiparallel magnetization alignments, ${G}_{\mathrm{P}}$ and ${G}_{\mathrm{AP}}$, of high-quality $\mathrm{C}{\mathrm{o}}_{2}\mathrm{MnSi}$ (CMS)/MgO/CMS MTJs having systematically varied spin polarizations ($P$) at 4.2 K by varying the Mn composition $\ensuremath{\alpha}$ in $\mathrm{C}{\mathrm{o}}_{2}\mathrm{M}{\mathrm{n}}_{\ensuremath{\alpha}}\mathrm{Si}$ electrodes that exhibited giant tunneling magnetoresistance ratios. Results showed that ${G}_{\mathrm{P}}$ normalized by its value at 4.2 K exhibited a notable, nonmonotonic $T$ dependence although its variation with $T$ was significantly smaller than that of ${G}_{\mathrm{AP}}$ normalized by its value at 4.2 K, indicating that an analysis of the experimental ${G}_{\mathrm{P}}(T)$ is critical to revealing the origin of the $T$ dependence of $G$. By analyzing the experimental ${G}_{\mathrm{P}}(T)$, we clarified that both spin-flip inelastic tunneling via a thermally excited magnon and spin-conserving elastic tunneling in which $P$ decays with increasing $T$ play key roles. The experimental ${G}_{\mathrm{AP}}(T)$, including its stronger $T$ dependence for higher $P$ at 4.2 K, was also consistently explained with this model. Our findings provide a unified picture for understanding the origin of the $T$ dependence of $G$ of MTJs with a wide range of $P$, including MTJs with high $P$ close to a half-metallic value.

The current fluctuation in normal metal / d-wave superconductor junctions are studied for various orientation of the crystal by taking account of the spatial variation of the pair potentials. Not only the zero-energy Andreev bound states (ZES) but also the non-zero energy Andreev bound states influence on the properties of differential shot noise. At the tunneling limit, the noise power to current ratio at zero voltage becomes 0, once the ZES are formed at the interface. Under the presence of a subdominant s-wave component at the interface which breaks time-reversal symmetry, the ratio becomes 4e, 13 pages, 3 figures

Understanding the effect of the interface on electrical spin injection is of great importance for the development of semiconductor spintronics. Fe/GaAs(001) is one of the leading systems for exploring these effects due to the small lattice mismatch. We report on the correlation between the experimentally observed Fe/GaAs(001) interface with the spin-transport properties. Using high-angle annular dark-field scanning transmission electron microscopy, we observe a predominantly abrupt interface with some regions of partial mixing also observed in the same film. We report that reproducible behavior with no bias-dependent polarization inversion was achieved for three-terminal devices. Using ab initio calculations of the experimentally observed interfaces, we show that the contribution to the transport from minority carriers is strongly dependent on the interface structure.

For a magnetic multilayer system, the physical origin of the difference between the giant magnetoresistances for currents parallel and perpendicular to the layer planes is studied. In order to take into account the two characteristics of a multilayer system, interfacial roughness and layered structure, we adopt the single-cell coherent-potential approximation. The spin-dependent resistivities are calculated for currents parallel and perpendicular to the layer planes by using a single-band tight-binding model and the Kubo formalism. It is shown that the magnetoresistance ratio for perpendicular current is larger than that for parallel current. The difference between magnetoresistances for parallel and perpendicular currents is attributed to the fact that the effects of the anisotropy of the effective mass and the two-dimensional distribution of randomness on the anisotropy of the resistivity depend on the spin-dependent potential. It is also shown that interfacial roughness is favorable to magnetoresistance for parallel currents.

Exchange coupling (EC) in Fe/Fe${}_{3}$O${}_{4}$ junctions containing magnetic impurities and in-gap states at the interface is calculated using a formula obtained by a cleaved layer method. The model for EC is constructed by performing first-principles calculations of the electronic and magnetic states of Co, Mn, and Cr impurities on the Fe surface and those of in-gap states in a bulk $\ensuremath{\gamma}$-Fe${}_{2}$O${}_{3}$, which has the same lattice structure as Fe${}_{3}$O${}_{4}$ but contains Fe defects. We show that the effect of Co impurities on EC is opposite to that of Cr and Mn impurities and that in-gap states tend to cause parallel magnetization alignment of two ferromagnets. These results are attributed to the change in electronic states caused by the presence of impurities. Further, we compare calculated results with experimental ones obtained in Fe/Fe${}_{3}$O${}_{4}$ junctions and suggest that doping magnetic impurities at the interface could be a useful way to control the magnitude and sign of the EC.

The electronic and magnetic structures and magnetic moments of ${\text{Co}}_{2}\text{MnSi}\text{ }(\text{CMS})/\text{Cr}/{\text{Co}}_{2}\text{MnSi}$ (001) trilayers are calculated by the first-principles method and compared to those calculated for Fe/Cr/Fe trilayers. The magnetic moments of Co are found to be parallel to those of Cr, whereas those of Mn are antiparallel to Cr moments. The results suggest that an antiphase structure in CMS layers may also bring about a frustration in the interlayer exchange coupling in multilayers. We also study the effects of antisite atoms at interfaces on the electronic and magnetic structures and find that the antiferromagnetic ordering of Cr moments is more robust than that in Fe/Cr/Fe trilayers. We propose a possible scenario in which the robustness of the antiferromagnetic ordering of Cr moments and the frustration in the interlayer exchange coupling, which is caused by antiphase or step structures, are responsible for the biquadratic coupling observed in CMS/Cr multilayers.

Based on a numerical study of a full-orbital tight-binding model, we show that magnetoresistance (MR) in ferromagnetic metal (FM)/graphene/FM lateral junctions originates from a spin-dependent shift of the Dirac points (DPs) caused by the presence of FM electrodes. Graphene with finite length $L$ is in contact with FM at a zigzag edge and the spin-dependent conductance is calculated in the ballistic transport regime. We find that the tunneling conductance near DPs influences the features of the MR considerably. We further show that a slight change in the electronic structure near the DPs strongly affects the spin-dependent conductance resulting in a large MR ratio for an FM made of certain ferromagnetic transition metal alloys.

We perform numerical simulations of conductance and conductivity for magnetic multilayers and magnetic granular films which show giant magnetoresistance (GMR). The system size to which the recursive Green's function method is applied is sufficiently large to eliminate incidental effects caused by the system boundary reported previously. It is shown that the resistivity change in the magnetic granular films is proportional to the inverse of the grain size.

We study the symmetry of Cooper pair in a two-dimensional Hubbard model with the Rashba-type spin-orbit interaction as a minimal model of electron gas generated at a heterointerface of ${\text{SrTiO}}_{3}/{\text{LaAlO}}_{3}$. Solving the \'Eliashberg equation based on the third-order perturbation theory, we find that the gap function consists of the mixing of the spin-singlet ${d}_{xy}$-wave component and the spin-triplet $({p}_{x}\ifmmode\pm\else\textpm\fi{}i{p}_{y})$-wave one due to the broken inversion symmetry originating from the Rashba-type spin-orbit interaction. The ratio of the $d$-wave and the $p$-wave component continuously changes with the carrier concentration. We propose that the pairing symmetry is controlled by tuning the gate voltage.

We study the spin Hall effect (SHE) in graphene using a realistic multi-orbital tight-binding model that includes the atomic spin-orbit interaction. The SHE is found to be induced by the spin-dependent Aharonov-Bohm phase. In the metallic case, the calculated values for the spin Hall conductivity (SHC) are much smaller than the quantized Hall conductivity for realistic parameter values of metallic graphene. In the insulating case, quantization of the SHC is violated due to the multi-orbital effect. The present study suggests that the SHE in a honeycomb lattice is enhanced by chemical doping, such as the substitution of carbon atoms with boron atoms., 5 pages, 4 figures

We study the intrinsic spin Hall conductivity (SHC) in various $5d$ transition metals (Ta, W, Re, Os, Ir, Pt, and Au) and $4d$ transition metals (Nb, Mo, Tc, Ru, Rh, Pd, and Ag) based on the Naval Research Laboratory tight-binding model, which enables us to perform quantitatively reliable analysis. In each metal, the obtained intrinsic SHC is independent of resistivity in the low resistive regime $(\ensuremath{\rho}l50\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\ensuremath{\Omega}\phantom{\rule{0.2em}{0ex}}\mathrm{cm})$ whereas it decreases in proportion to ${\ensuremath{\rho}}^{\ensuremath{-}2}$ in the high resistive regime. In the low resistive regime, the SHC takes a large positive value in Pt and Pd, both of which have approximately nine $d$ electrons per ion $({n}_{d}=9)$. On the other hand, the SHC takes a large negative value in Ta, Nb, W, and Mo, where ${n}_{d}l5$. In transition metals, a conduction electron acquires the trajectory-dependent phase factor that originates from the atomic wave function. This phase factor, which is reminiscent of the Aharonov--Bohm phase, is the origin of the SHC in paramagnetic metals and that of the anomalous Hall conductivity in ferromagnetic metals. Furthermore, each transition metal shows huge and positive $d$-orbital Hall conductivity (OHC), independent of the strength of the spin-orbit interaction. Since the OHC is much larger than the SHC, it will be possible to realize an orbitronics device made of transition metals.

The interfacial density of states and conductance of $\mathrm{Gd}\mathrm{As}∕\mathrm{Ga}\mathrm{As}∕\mathrm{Gd}\mathrm{As}$ junctions, where GdAs and GaAs are a semimetal and a semiconductor, respectively, are calculated using a full-orbital tight-binding model. The tunnel conductance of the junction is calculated using the Kubo formula at zero temperature assuming coherent tunneling of electrons. Emphasizing the role of interfacial states on the conductance, the following results are obtained. The interfacial states extend 2-4 atomic layers from the interface; however, the effect of the interfacial states on the conductance extends up to nearly 10 atomic GaAs layers, and nonmonotonic dependence of the tunnel conductance on the barrier thickness appears for thin GaAs layers. The height of the interfacial density of states does not necessarily correlate with the magnitude of the tunnel conductance. The matching between the Fermi surface of the leads and interfacial states governs the magnitude of the tunnel conductance. It is also found that the nonmonotonic dependence of the tunnel conductance is caused by an interference effect of the interface states existing at the two interfaces of GaAs layer.

Cuprate ${(\mathrm{La}\text{\ensuremath{-}}R)}_{4}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{2}{\mathrm{O}}_{10}$ shows ferromagnetic to antiferromagnetic phase transition by replacing La ions with rare earth ($R=\mathrm{Nd}$, Sm, Eu, and Gd) ions at low temperature. We propose that the transition is caused by a competition between antiferromagnetic superexchange interaction of Cu spins via La ions and ferromagnetic superexchange interaction due to Hund's rule coupling of spins of holes on oxygen ions. It is shown that the ferromagnetic interaction becomes antiferromagnetic with increasing overlap between the wave functions of oxygen and $\mathrm{La}(R)$ ions.

The interlayer exchange coupling constant of ferromagnetic trilayers made of perovskite oxides has been calculated by using a tight-binding model for ${e}_{g}$ orbitals of Mn and Ni ions in $\mathrm{La}\mathrm{Ba}\mathrm{Mn}{\mathrm{O}}_{3}∕\mathrm{La}\mathrm{Ni}{\mathrm{O}}_{3}∕\mathrm{La}\mathrm{Ba}\mathrm{Mn}{\mathrm{O}}_{3}$. We show that the extended ${e}_{g}$ band model, which includes the second nearest neighbor hoppings, reproduces experimental results well for suitable settings of the Fermi level. However, both oscillation period and intensity are shown to be sensitive to the position of the Fermi level. The results may be examined experimentally by replacing La with Sr or Ca to alter the electron number in $\mathrm{La}\mathrm{Ni}{\mathrm{O}}_{3}$.

A mechanism for the large magnetoresistance observed recently in Co-Al-O granular magnetic films is presented. It is shown that the resistivity decreases with increasing applied magnetic field because the spin-dependent tunneling increases as the relative orientation of the magnetization between grains becomes parallel. With this mechanism we are able to account for the dependence of the magnetoresistance on the magnetization and temperature.

We study the linear response spin Hall conductivity of a two-dimensional electron gas (2DEG) in the presence of the Rashba spin orbit interaction in the diffusive transport regime. When defect scattering is modeled by isotropic short-range potential scatterers the spin Hall conductivity vanishes due to the vertex correction. A non-vanishing spin Hall effect may be recovered for dominantly forward defect scattering., Submitted to The Physical Review B

We make a theoretical study of the quantum oscillations of the tunneling magnetoresistance (TMR) as a function of the spacer layer thickness. Such oscillations were recently observed in tunneling junctions with a nonmagnetic metallic spacer at the barrier-electrode interface. It is shown that momentum selection due to the insulating barrier and conduction via quantum well states in the spacer, mediated by diffusive scattering caused by disorder, are essential features required to explain the observed period of oscillation in the TMR ratio and its asymptotic value for thick nonmagnetic spacer., 4 pages, 5 figures, two column, REVTex4 style

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.

A magnetic phase diagram of the ground state of layered perovskite manganites is studied in a mean-field approximation for a two-orbital double exchange model with the superexchange interaction between localized spins andJahn-Teller effect. We take into account the G-type and C-type spin alignment and their canted spin ordering. It is shown that with increasing electron dopant the ground state changes from the G-type state to ferromagnetic state via a G-type canted spin state. The C-type state appears only for the intermediate electron dopant with stronger superexchange interaction. The Jahn-Teller distortion stabilizes 3y 2 -r 2 orbital ordering in the C-type state while x 2 - y 2 ordering occurs in other cases. It is also shown that rather complex electronic phase separation occurs, which is compared with experimental observations.

We theoretically study effects on tunnel magnetoresistance (TMR) of spin configurations localized within insulating barriers. Two cases are treated herein: interaction between a tunneling electron and an isolated classical spin being canted at an angle to the magnetization axis of the ferromagnetic leads, and interaction between a tunneling electron and many quantum spins within the insulating barrier. The characteristic features of TMR observed in δ-doped ferromagnetic tunnel junctions and in grain boundaries of metallic manganites are discussed in view of the results obtained., 6pages

The stability of the magnetic phase of quarter-filled manganites was studied using the two-orbital double-exchange model with the effect of lattice distortion and Coulomb repulsion in a mean field approximation for two-dimensional systems. A magnetic phase diagram is depicted in a plane of the lattice distortion and the superexchange interaction between localized ${t}_{2g}$ spins. When the ratio of the superexchange interaction to the hopping integral is small, a ferromagnetic ground state with ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ orbital ordering is stabilized. With increases in the ratio, the ferromagnetic ground state changes to a charge-ordered CE type antiferromagnetic state irrespective of the lattice distortion. The respective roles of the various ingredients included in the model are discussed, and theoretical and experimental results are compared. We also discuss the effects of Fe and Cr substitution for Mn ions with regards to the stability of the CE phase.

By using an optimization variational Monte Carlo method, we study the half-filled-band Hubbard model on anisotropic triangular lattices, as a continuation of the preceding study [T. Watanabe, H. Yokoyama, Y. Tanaka, and J. Inoue, J. Phys. Soc. Jpn. 75, 074707 (2006)]. We introduce two new trial states: (i) A coexisting state of ${\ensuremath{\Psi}}_{Q}^{\text{co}}$-antiferromagnetic (AF) and a $d$-wave singlet gaps, in which we allow for a band renormalization effect, and (ii) a state with an AF order of $120\ifmmode^\circ\else\textdegree\fi{}$ spin structure. In both states, a first-order metal-to-insulator transition occurs at smaller $U/t$ than that of the pure $d$-wave state. In insulating regimes, magnetic orders always exist; an ordinary $(\ensuremath{\pi},\ensuremath{\pi})$-AF order survives up to ${t}^{\ensuremath{'}}/t\ensuremath{\sim}0.9$ $(U/t=12)$, and a $120\ifmmode^\circ\else\textdegree\fi{}$-AF order becomes dominant for ${t}^{\ensuremath{'}}/t$. The regimes of the robust superconductor and of the nonmagnetic insulator the preceding study proposed give way to these magnetic domains.

The magnetic phase diagram of metallic kagome´ and pyrochlore lattices at the ground state have been studied theoretically in the double-exchange model using a mean-field approximation. The model includes the hopping of itinerant electrons, which is expressed by a single tight-binding band, antiferromagnetic coupling between localized spins, and ferromagnetic exchange interaction between the localized and itinerant electron spins. The phase diagram was found to be rather insensitive to the ferromagnetic exchange interaction but to show a rich variety in magnetic structure with doping. A crossover between ferromagnetic and spin glass phases observed in RE_2Mo_2O_7 pyrochlore lattices, where RE denotes rare-earth ions, is discussed in view of the calculated results.

Based on a previously reported double-resonance mechanism, we present an analytical expression of the effective exchange interaction between impurity moments of transition-metal elements introduced in semiconductors. It is shown that the exchange interaction is enhanced by several orders of magnitude as compared with the ordinary RKKY interaction when the Fermi level and impurity states are close to the top of the valence band. Although the interaction is found to decay exponentially, the magnitude is large even when the distance between two impurities is several times as long as the lattice constant. The concentration dependence of the Curie temperature is calculated by using a percolation theory, and the calculated results are compared with the experimental findings.

We have calculated the energy of the resonating-valence-bond (RVB) state made of nearest-neighbor singlet pairs in the two-dimensional Hubbard model at half filling and the energy of holes in the RVB state near half filling. The energy of the RVB state was obtained by extrapolating the result in a 6\ifmmode\times\else\texttimes\fi{}6 square lattice with the periodic boundary condition to the infinite lattice. The energy of holes was obtained by examining the first five moments of the density of states of holes in the RVB state. We find that the holes are more stabilized in the RVB state than in the N\'eel state for J/t\ensuremath{\gtrsim}0.1, where J and t are the exchange interaction and the nearest-neighbor hopping integral, respectively. We discuss the possibility of the RVB state in the high-${T}_{c}$ Cu oxides.