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Contains fulltext : 175573.pdf (publisher's version ) (Open Access)

We theoretically study the effects of strong magnetoelastic coupling on the transport properties of magnetic insulators. We develop a Boltzmann transport theory for the mixed magnon-phonon modes ("magnon polarons") and determine transport coefficients and the spin diffusion length. Magnon-polaron formation causes anomalous features in the magnetic field and temperature dependence of the spin Seebeck effect when the disorder scattering in the magnetic and elastic subsystems is sufficiently different. Experimental data by Kikkawa [Phys. Rev. Lett. 117, 207203 (2016)PRLTAO0031-900710.1103/PhysRevLett.117.207203] on yttrium iron garnet films can be explained by an acoustic quality that is much better than the magnetic quality of the material. We predict similar anomalous features in the spin and heat conductivity and nonlocal spin transport experiments.

The tunnel magneto-Seebeck effect is the dependence of the thermopower of magnetic tunnel junctions on the magnetic configuration. It is conventionally interpreted in terms of a thermoelectric generalization of the tunnel magnetoresistance. Here, we investigate the heat-driven electron transport in these junctions associated with electron-magnon scattering, using stochastic Landau-Lifshitz phenomenology and quantum kinetic theory. Our findings challenge the widely accepted single-electron picture of the tunneling thermopower in magnetic junctions.

Sharp structures in the magnetic field-dependent spin Seebeck effect (SSE) voltages of Pt/Y3Fe5O12 at low temperatures are attributed to the magnon-phonon interaction. Experimental results are well reproduced by a Boltzmann theory that includes magnetoelastic coupling. The SSE anomalies coincide with magnetic fields tuned to the threshold of magnon-polaron formation. The effect gives insight into the relative quality of the lattice and magnetization dynamics.

Magnons are the elementary excitations of a magnetically ordered system. In ferromagnets, only a single band of low-energy magnons needs to be considered, but in ferrimagnets the situation is more complex owing to different magnetic sublattices involved. In this case, low lying optical modes exist that can affect the dynamical response. Here we show that the spin Seebeck effect (SSE) is sensitive to the complexities of the magnon spectrum. The SSE is caused by thermally excited spin dynamics that are converted to a voltage by the inverse spin Hall effect at the interface to a heavy metal contact. By investigating the temperature dependence of the SSE in the ferrimagnet gadolinium iron garnet, with a magnetic compensation point near room temperature, we demonstrate that higher-energy exchange magnons play a key role in the SSE., QN/Quantum Nanoscience, Applied Sciences

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. The 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 the experimental results for wide multilayer pillars, but the giant effects observed for diameters <100 nm are still under discussion, Kavli Institute of Nanoscience, Applied Sciences

We model the injection of elastic waves into a ferromagnetic film (F) by a nonmagnetic transducer (N). We compare the configurations in which the magnetization is normal and parallel to the wave propagation. The lack of axial symmetry in the former results in the emergence of evanescent interface states. We compute the energy-flux transmission across the N|F interface and sound-induced magnetization dynamics in the ferromagnet. We predict efficient acoustically induced pumping of spin current into a metal contact attached to F., QN/Quantum Nanoscience, Applied Sciences

We report on the spin-Hall magnetoresistance (SMR) and spin Seebeck effect (SSE) in multiferroic CoCr2O4 (CCO) spinel thin films with Pt contacts. We observe a large enhancement of both signals below the spin-spiral (Ts=28K) and the spin lock-in (Tlock?in=14K) transitions. The SMR and SSE responses in the spin lock-in phase are one order of magnitude larger than those observed at the ferrimagnetic transition temperature (Tc=94K), which indicates that the interaction between spins at the Pt|CCO interface is more efficient in the noncollinear magnetic state. At T>Tc, magnetic-field-induced SMR and SSE signals are observed, which can be explained by a high interface susceptibility. Our results show that the spin transport at the Pt|CCO interface is sensitive to the magnetic phases but cannot be explained solely by the bulk magnetization., QN/Quantum Nanoscience, Applied Sciences

We report the observation of current-induced spin torque resonance in yttrium iron garnet/platinum bilayers. An alternating charge current at GHz frequencies in the platinum gives rise to dc spin pumping and spin Hall magnetoresistance rectification voltages, induced by the Oersted fields of the ac current and the spin Hall effect-mediated spin transfer torque. In ultrathin yttrium iron garnet films, we observe spin transfer torque actuated magnetization dynamics which are significantly larger than those generated by the ac Oersted field. Spin transfer torques thus efficiently couple charge currents and magnetization dynamics also in magnetic insulators, enabling charge current-based interfacing of magnetic insulators with microwave devices., QN/Quantum Nanoscience, Applied Sciences

We formulate a scattering theory to study magnetic films in microwave cavities beyond the independent-spin and rotating-wave approximations of the Tavis-Cummings model. We demonstrate that strong coupling can be realized not only for the ferromagnetic resonance mode, but also for spin-wave resonances; the coupling strengths are mode dependent and decrease with increasing mode index. The strong-coupling regime can also be accessed electrically by spin pumping into a metal contact., QN/Quantum Nanoscience, Applied Sciences

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 fieldlike 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., QN/Quantum Nanoscience, Applied Sciences

We apply Mie scattering theory to study the interaction of magnetic spheres with microwaves in cavities beyond the magnetostatic and rotating wave approximations. We demonstrate that both strong and ultrastrong coupling can be realized for stand alone magnetic spheres made from yttrium iron garnet (YIG), acting as an efficient microwave antenna. The eigenmodes of YIG spheres with radii of the order mm display distinct higher angular momentum character that has been observed in experiments., QN/Quantum Nanoscience, Applied Sciences

We compute the thermoelectric transport parameterized by the Seebeck coefficient and thermal/electric conductance of random-alloy FeCo/MgO/FeCo(001) magnetic tunnel junctions (MTJs) from first principles using a generalized Landauer-Büttiker formalism. The thermopower is found to be typically smaller than those of Fe/MgO/Fe(001) MTJs. The (magneto-)Seebeck effect is sensitive to the details of the FeCo/MgO interfaces. Interfacial can greatly enhance the thermoelectric effects in MTJs. We also compute angular-dependent Seebeck coefficients that provide additional information about the transport physics. We report large deviations from the Wiedemann-Franz law at room temperature., QN/Quantum Nanoscience, Applied Sciences

We study the dynamics of spin valves consisting of two layers of magnetic insulators separated by a normal metal in the macrospin model. A current through the spacer generates a spin Hall current that can actuate the magnetization via the spin-transfer torque. We derive expressions for the effective Gilbert damping and the critical currents for the onset of magnetization dynamics including the effects of spin pumping that can be tested by ferromagnetic resonance experiments. The current generates an amplitude asymmetry between the in-phase and out-of-phase modes. We discuss superlattices of stacked films of metals and magnetic insulators., QN/Quantum Nanoscience, Applied Sciences

We report the observation of the spin Peltier effect (SPE) in the ferrimagnetic insulator yttrium iron garnet (YIG), i.e., a heat current generated by a spin current flowing through a platinum (Pt)|YIG interface. The effect can be explained by the spin transfer torque that transforms the spin current in the Pt into a magnon current in the YIG. Via magnon-phonon interactions the magnetic fluctuations modulate the phonon temperature that is detected by a thermopile close to the interface. By finite-element modeling we verify the reciprocity between the spin Peltier and spin Seebeck effect. The observed strong coupling between thermal magnons and phonons in YIG is attractive for nanoscale cooling techniques., QN/Quantum Nanoscience, Applied Sciences

We formulate a theory of the ac spin Hall magnetoresistance in a bilayer system consisting of a magnetic insulator such as yttrium iron garnet and a heavy metal such as platinum. 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 the ac spin Hall effect., QN/Quantum Nanoscience, Applied Sciences

Nonresonant circularly polarized electromagnetic radiation can exert torques on magnetizations by the inverse Faraday effect (IFE). Here, we discuss the enhancement of IFE by spin-orbit interactions. We illustrate the principle by studying a simple generic model system, i.e., the quasi-one-dimensional ring in the presence of linear/cubic Rashba and Dresselhaus interactions. We combine the classical IFE in electron plasmas that is known to cause persistent currents in the plane perpendicular to the direction of the propagation of light with the concept of current and spin-orbit-induced spin transfer torques. We calculate light-induced spin polarization that in ferromagnets might give rise to magnetization switching., QN/Quantum Nanoscience, Applied Sciences

We measure the low-frequency thermal fluctuations of pure spin current in a platinum film deposited on yttrium iron garnet via the inverse spin Hall effect (ISHE)-mediated voltage noise as a function of the angle ? between the magnetization and the transport direction. The results are consistent with the fluctuation-dissipation theorem in terms of the recently discovered spin Hall magnetoresistance (SMR). We present a microscopic description of the ? dependence of the voltage noise in terms of spin-current fluctuations and ISHE., QN/Quantum Nanoscience, Applied Sciences