Low-damping magnetic insulators are essential for pure spin current-based electronics as they can generate and transfer spin currents without associated charge currents. Nanometer-thick epitaxial thin films of low-damping magnetic insulators are particularly important in order to control and switch the magnetization via spin transfer torques. We have recently developed films of the ferromagnetic insulator MgAl0.5Fe1.5O4 (MAFO) with a low Gilbert damping parameter (∼0.001). In contrast to Y3Fe5O12 (YIG), MAFO films can be grown on a variety of substrates and have significant in-plane magnetic anisotropy, leading to higher spin-wave frequencies. Here, we demonstrate efficient spin current injection from MAFO into adjacent Pt and β-W layers by ferromagnetic resonance (FMR) broadening and inverse spin Hall effect measurements. Angular dependent magnetoresistance (ADMR) measurements indicate that the proximity effect magnetoresistance is small compared to the spin Hall magnetoresistance associated with spin pumping. FMR and ADMR measurements indicate that MAFO/Pt interfaces have a spin-mixing conductance of ∼2 × 1014 Ω−1 m−2, comparable to that of YIG/Pt. These measurements also show that the spin transport can be described by Dyakonov-Perel spin relaxation combined with an extrinsic spin Hall effect (from skew scattering). These results demonstrate the promise of spinel ferrites for spin current-based spintronics.
We present measurements of the spin Seebeck effect (SSE) by a technique that combines alternating currents (AC) and direct currents (DC). The method is applied to a ferrimagnetic insulator/heavy metal bilayer, Y3Fe5O12(YIG)/Pt. Typically, SSE measurements use an AC current to produce an alternating temperature gradient and measure the voltage generated by the inverse spin-Hall effect in the heavy metal at twice the AC frequency. Here we show that when Joule heating is associated with AC and DC bias currents, the SSE response occurs at the frequency of the AC current drive and can be larger than the second harmonic SSE response. We compare the first and second harmonic responses and show that they are consistent with the SSE. The field dependence of the voltage response is used to characterize the damping-like and field-like torques. This method can be used to explore nonlinear thermoelectric effects and spin dynamics induced by temperature gradients.A central theme in spintronics is the interconversion of charge and spin currents [1]. Recently, a focus has been on magnetic insulators where spin transport occurs through spin-wave propagation and spin currents can be generated by either spin injection [2] or by thermal gradients [3,4]. These phenomena can be studied in simple bilayer films consisting of a ferrimagnetic (FIM) insulator, such as Y 3 Fe 5 O 12 (YIG), and a heavy metal (HM) with large spin-orbit coupling such as Pt. Spin to charge current conversion in such bilayers occurs by the inverse spin-Hall [5][6][7] and Rashba-Edelstein effects [8,9].Spin to charge conversion enables determination of the spin Seebeck effect (SSE) [10][11][12][13]. A thermal gradient across the FIM film produces a spin current into a neighboring heavy metal film, resulting in a transverse charge current or a voltage across the heavy metal film in an open circuit situation. This leads to a convenient route to characterize the spin transport as well as a means to study the inverse effects, such as the spin torque on the FIM magnetization in response to spin currents associated with charge current flow in the HM. In fact, the SSE also enables detection of the FIM magnetization direction by relatively simple electrical measurements.In this article, we present first harmonic measurements of the SSE by a technique that combines AC and DC currents in a YIG/Pt bilayer. The temperature gradient is created by Joule heating in a Pt strip and both the linear and nonlinear responses in the longitudinal and transverse voltages are determined as a function of the angle between the AC current and an in-plane external magnetic field. An analysis of the responses shows that the SSE accounts for the main component of the second harmonic voltage response, corroborating results in the literature [14]. However, when a DC current is present, * These two authors contributed equally † Electronic address: andy.kent@nyu.edu several new features are observed. First, we detect fieldinduced switching of the YIG magnetization in both the first and second harm...
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