arXiv subjects

Saburo Takahashi

Publications and source records attributed to Saburo Takahashi.

At least 19 recordsLinked to original sources

arXiv2024

Field Dispersion and Strong Coupling of Nuclear-Electron Spin Excitation in MnCO$_3$

Hybridized nuclear and electron spin excitation in a MnCO$_3$ crystal, a weakly-anisotropic antiferromagnet, has been investigated. In this material, the hyperfine interaction is strong enough to form a nuclear spin wave. We measure the microwave absorption by a bulk MnCO$_3$ and observe the dispersion representing strong frequency repulsion between electron and nuclear modes due to their hybridization, the signature of nuclear spin wave. Additionally, we observe that the nuclear spin resonance enters a nonlinear regime above a certain excitation power, attributed to the excitation of finite wavenumber nuclear spin waves.

cond-mat.str-el↗

arXiv2023

Mechanism of paramagnetic spin Seebeck effect

We have theoretically investigated the spin Seebeck effect (SSE) in a normal metal (NM)/paramagnetic insulator (PI) bilayer system. Through a linear response approach, we calculated the thermal spin pumping from PI to NM and backflow spin current from NM to PI, where the spin-flip scattering via the interfacial exchange coupling between conduction-electron spin in NM and localized spin in PI is taken into account. We found a finite spin current appears at the interface under the difference in the effective temperatures between spins in NM and PI, and its intensity increases by increasing the density of the localized spin $S$. Our model well reproduces the magnetic-field-induced reduction of the paramagnetic SSE in Pt/Gd$_3$Ga$_5$O$_{12}$ experimentally observed when the Zeeman energy is comparable to the thermal energy, which can be interpreted as the suppression of the interfacial spin-flip scattering. The present finding provides an insight into the mechanism of paramagnetic SSEs and the thermally induced spin-current generation in magnetic materials.

cond-mat.mes-hall↗

arXiv2021

Spin-current-related magnetoresistance in epitaxial Pt/Co bilayers in the presence of spin Hall effect and Rashba-Edelstein effect

We analyze the experimentally obtained spin-current-related magnetoresistance in epitaxial Pt/Co bilayers by using a drift-diffusion model that incorporates both bulk spin Hall effect and interfacial Rashba-Edelstein effect (REE). The magnetoresistance analysis yields, for the Pt/Co interface, a temperature-independent Rashba parameter in the order of 1e-11 eV m that agrees with theoretical calculations, along with an effective interfacial REE thickness of several angstroms which is in overall consistency with our previous spin-orbit torque analysis. In particular, our results suggest that both bulk and interface charge-spin current inter-conversions need to be taken into account for the spin-current-related magnetoresistance analysis in highly-conductive magnetic hetero-structures such as the epitaxial Pt/Co bilayers.

cond-mat.mtrl-sci↗

arXiv2018

Efficient spin transport in a paramagnetic insulator

The discovery of new materials that efficiently transmit spin currents has been important for spintronics and material science. The electric insulator $\mathrm{Gd}_3\mathrm{Ga}_5\mathrm{O}_{12}$ (GGG) is a superior substrate for growing magnetic films, but has never been considered as a conduit for spin currents. Here we report spin current propagation in paramagnetic GGG over several microns. Surprisingly, the spin transport persists up to temperatures of 100 K $\gg$ $T_{\mathrm{g}} = 180$ mK, GGG's magnetic glass-like transition temperature. At 5 K we find a spin diffusion length ${λ_{\mathrm{GGG}}} = 1.8 \pm 0.2 μ$m and a spin conductivity $σ_{\mathrm{GGG}} = (7.3 \pm 0.3) \times10^4$ $\mathrm{Sm}^{-1}$ that is larger than that of the record quality magnet $\mathrm{Y}_3\mathrm{Fe}_5\mathrm{O}_{12}$ (YIG). We conclude that exchange coupling is not required for efficient spin transport, which challenges conventional models and provides new material-design strategies for spintronic devices.

cond-mat.mtrl-sci↗

arXiv2018

Anomalous spin Hall magnetoresistance in Pt/Co bilayers

We have studied the spin Hall magnetoresistance (SMR), the magnetoresistance within the plane transverse to the current flow, of Pt/Co bilayers. We find that the SMR increases with increasing Co thickness: the effective spin Hall angle for bilayers with thick Co exceeds the reported values of Pt when a conventional drift-diffusion model is used. An extended model including spin transport within the Co layer cannot account for the large SMR. To identify its origin, contributions from other sources are studied. For most bilayers, the SMR increases with decreasing temperature and increasing magnetic field, indicating that magnon-related effects in the Co layer play little role. Without the Pt layer, we do not observe the large SMR found for the Pt/Co bilayers with thick Co. Implementing the effect of the so-called interface magnetoresistance and the textured induced anisotropic scattering cannot account for the Co thickness dependent SMR. Since the large SMR is present for W/Co but its magnitude reduces in W/CoFeB, we infer its origin is associated with a particular property of Co.

cond-mat.mes-hall↗

arXiv2018

Spin-current coherence peak in superconductor/magnet junctions

Coherence peak effects in a superconductor induced by a thermal spin current are reported. We measured inverse spin Hall effects induced by spin injection from a ferrimagnetic insulator Y$_3$Fe$_5$O$_{12}$ into a superconductor NbN using longitudinal spin Seebeck effects. In the vicinity of the superconducting transition temperature of the NbN, a large enhancement of the spin Seebeck voltage is observed, whose sign is opposite to that for the vortex Nernst effect, but is consistent with a calculation for a coherence peak effect in the superconductor NbN.

cond-mat.mes-hall↗

arXiv2017

Enhanced Magneto-optical Kerr Effect at Fe/Insulator Interfaces

Using density functional theory calculations, we have found an enhanced magneto-optical Kerr effect in Fe/insulator interfaces. The results of our study indicate that interfacial Fe atoms in the Fe films have a low-dimensional nature, which causes the following two effects: (i) The diagonal component $σ_{xx}$ of the optical conductivity decreases dramatically because the hopping integral for electrons between Fe atoms is suppressed by the low dimensionality. (ii) The off-diagonal component $σ_{xy}$ of the optical conductivity does not change at low photon energies, but it is enhanced at photon energies around 2 eV, where we obtain enhanced orbital magnetic moments and spin-orbit correlations for the interfacial Fe atoms. A large Kerr angle develops in proportion to the ratio $σ_{xy}/σ_{xx}$. Our findings indicate an efficient way to enhance the effect of spin-orbit coupling at metal/insulator interfaces without using heavy elements.

cond-mat.mtrl-sci↗

arXiv2017

Evaluation of bulk-interface contributions to Edelstein magnetoresistance at metal/oxide interfaces

We report a systematic study on Edelstein magnetoresistance (Edelstein MR) in Co25Fe75/Cu/Bi2O3 heterostructures with a strong spin-orbit interaction at the Cu/Bi2O3 interface. We succeed in observing a significant dependence of the Edelstein MR on both Cu layer thickness and temperature, and also develop a general analytical model considering distinct bulk and interface contributions on spin relaxation. Our analysis, based on the above model, quantitatively illustrates a unique property of the spin transport near the Rashba interface, revealing a prominent role of the spin relaxation process by determining the ratios of the spin relaxation inside and outside the interface. We further find the characteristic spin transport is unaffected by temperature. Our results provide an essential tool for exploring the transport in a system with spin-momentum-locked two-dimensional states.

cond-mat.mes-hall↗

arXiv2017

The Spin Nernst effect in Tungsten

The spin Hall effect allows generation of spin current when charge current is passed along materials with large spin orbit coupling. It has been recently predicted that heat current in a non-magnetic metal can be converted into spin current via a process referred to as the spin Nernst effect. Here we report the observation of the spin Nernst effect in W. In W/CoFeB/MgO heterostructures, we find changes in the longitudinal and transverse voltages with magnetic field when temperature gradient is applied across the film. The field-dependence of the voltage resembles that of the spin Hall magnetoresistance. A comparison of the temperature gradient induced voltage and the spin Hall magnetoresistance allows direct estimation of the spin Nernst angle. We find the spin Nernst angle of W to be similar in magnitude but opposite in sign with its spin Hall angle. Interestingly, under an open circuit condition, such sign difference results in spin current generation larger than otherwise. These results highlight the distinct characteristics of the spin Nernst and spin Hall effects, providing pathways to explore materials with unique band structures that may generate large spin current with high efficiency.

cond-mat.mes-hall↗

arXiv2017

Magnetic-proximity-induced magnetoresistance on topological insulators

We theoretically study the magnetoresistance (MR) of two-dimensional massless Dirac electrons as found on the surface of three-dimensional topological insulators (3D TIs) that is capped by a ferromagnetic insulator (FI). We calculate charge and spin transport by Kubo and Boltzmann theories, taking into account the ladder-vertex correction and the in-scattering due to normal and magnetic disorder. The induced exchange splitting is found to generate an electric conductivity that depends on the magnetization orientation, but its form is very different from both the anisotropic and spin Hall MR. The in-plane MR vanishes identically for non-magnetic disorder, while out-of-plane magnetizations cause a large MR ratio. On the other hand, we do find an in-plane MR and planar Hall effect in the presence of magnetic disorder aligned with the FI magnetization. Our results may help understand recent transport measurements on TI|FI systems.

cond-mat.mes-hall↗

arXiv2016

Theory of spin Hall magnetoresistance (SMR) and related phenomena

We review the recently discovered spin Hall magnetoresistance (SMR) and related effects from a theoretical point of view. The SMR is observed in bilayers of a magnetic insulator and a metal, in which spin currents aregenerated in the normal metal due to the spin Hall effect. The associated angular momentum transfer to the ferromagnetic layer and thereby the electrical resistance is modulated by the angle between the applied current and the magnetization direction. The SMR provides a convenient tool to non-invasively measure the magnetization direction and spin-transfer torque to an insulator. We introduce the minimal theoretical instruments to calculate the SMR, i.e. spin diffusion theory and quantum mechanical boundary conditions. This leads to a small set of parameters that can be fitted to experiments. We discuss the limitations of the theory as well as alternative mechanisms such as the ferromagnetic proximity effect and Rashba spin-orbit torques, and point out new developments related to the SMR.

cond-mat.mtrl-sci↗

arXiv2016

Spin Hall magnetoresistance in metallic bilayers

Spin Hall magnetoresistance (SMR) is studied in metallic bilayers that consist of heavy metal (HM) layer and a ferromagnetic metal (FM) layer. We find nearly a ten-fold increase of SMR in W/CoFeB compared to previously studied HM/ferromagnetic insulator (FI) systems. The SMR increases with decreasing temperature despite the negligible change in the W layer resistivity with temperature. A model is developed to account for the absorption of the longitudinal spin current to the FM layer, one of the key characteristics of a metallic ferromagnet. We find that the model not only quantitatively describes the HM layer thickness dependence of SMR, allowing accurate estimation of the spin Hall angle and the spin diffusion length of the HM layer, but also can account for the temperature dependence of SMR by assuming a temperature dependent spin polarization of the FM layer. These results illustrate the unique role a metallic ferromagnetic layer plays in defining spin transmission across the HM/FM interface.

cond-mat.mes-hall↗

arXiv2015

Modelling of the Peltier effect in magnetic multilayers

We model the charge, spin, and heat currents in ferromagnetic metal$|$normal metal$|$normal metal trilayer structures in the two current model, taking into account bulk and interface thermoelectric properties as well as Joule heating. Results include the temperature distribution as well as resistance-current curves that reproduce the observed shifted parabolic characteristics. Thin tunneling barriers can enhance the apparent Peltier cooling. The model agrees with experimental results for wide multilayer pillars, but the giant effects observed for diameters $\lesssim 100\,$nm are still under discussion.

cond-mat.mes-hall↗

arXiv2015

Magnetization damping in noncollinear spin valves with antiferromagnetic interlayer couplings

We study the magnetic damping in the simplest of synthetic antiferromagnets, i.e. antiferromagnetically exchange-coupled spin valves in which applied magnetic fields tune the magnetic configuration to become noncollinear. We formulate the dynamic exchange of spin currents in a noncollinear texture based on the spindiffusion theory with quantum mechanical boundary conditions at the ferrromagnet|normal-metal interfaces and derive the Landau-Lifshitz-Gilbert equations coupled by the static interlayer non-local and the dynamic exchange interactions. We predict non-collinearity-induced additional damping that can be sensitively modulated by an applied magnetic field. The theoretical results compare favorably with published experiments.

cond-mat.mes-hall↗

arXiv2015

Current-induced spin torque resonance of magnetic insulators affected by field-like spin-orbit torques and out-of-plane magnetizations

The spin-torque ferromagnetic resonance (ST-FMR) in a bilayer system consisting of a magnetic insulator such as Y3Fe5O12 and a normal metal with spin-orbit interaction such as Pt is addressed theoretically. We model the ST-FMR for all magnetization directions and in the presence of field-like spin-orbit torques based on the drift-diffusion spin model and quantum mechanical boundary conditions. ST-FMR experiments may expose crucial information about the spin-orbit coupling between currents and magnetization in the bilayers.

cond-mat.mes-hall↗

arXiv2014

Current-induced spin torque resonance of a magnetic insulator

Pure spin currents transport angular momentum without an associated charge flow. This unique property makes them attractive for spintronics applications, such as torque induced magnetization control in nanodevices that can be used for sensing, data storage, interconnects and logics. Up to now, however, most spin transfer torque studies focused on metallic ferromagnets, while magnetic insulators were largely ignored, in spite of superior magnetic quality factors. Here, we report the observation of spin torque-induced magnetization dynamics in a magnetic insulator. Our experiments show that in ultrathin magnetic insulators the spin torque induced magnetization dynamics can be substantially larger than those generated by the Oersted field. This opens new perspectives for the efficient integration of ferro-, ferri-, and antiferromagnetic insulators into electronic devices.

cond-mat.mes-hall↗

arXiv2014

Current-induced spin torque resonance of magnetic insulators

We formulate a theory of the AC spin Hall magnetoresistance (SMR) in a bilayer system consisting of a magnetic insulator such as yttrium iron garnet (YIG) and a heavy metal such as platinum (Pt). We derive expressions for the DC voltage generation based on the drift-diffusion spin model and quantum mechanical boundary condition at the interface that reveal a spin torque ferromagnetic resonance (ST-FMR). We predict that ST-FMR experiments will reveal valuable information on the current-induced magnetization dynamics of magnetic insulators and AC spin Hall effect.

cond-mat.mes-hall↗

arXiv2014

Anomalous temperature dependence of current induced torques in CoFeB|MgO heterostructures with Ta based underlayers

We have studied the underlayer thickness and temperature dependences of the current induced effective field in CoFeB|MgO heterostructures with Ta based underlayers. The underlayer thickness at which the effective field saturates is found to be different between the two orthogonal components of the effective field, i.e. the damping-like term tends to saturate at smaller underlayer thickness than the field-like term. For large underlayer thickness films in which the effective field saturates, we find that the temperature significantly influences the size of the effective field. A striking difference is found in the temperature dependence of the two components: the damping-like term decreases whereas the field-like term increases with increasing temperature. Using a simple spin diffusion-spin transfer model, we find that all of these results can be accounted for provided the real and imaginary parts of an effective spin mixing conductance are negative. These results imply that either spin transport in this system is different from conventional metallic interfaces or effects other spin diffusion into the magnetic layer need to be taken account in order to model the system accurately.

cond-mat.mes-hall↗