Thermal spin torques in magnetic insulators

Abstract

The damping of spin waves transmitted through a two-port magnonic device implemented on a yttrium iron garnet thin film is shown to be proportional to the temperature gradient imposed on the device. The sign of the damping depends on the relative orientation of the magnetic field, the wave vector, and the temperature gradient. The observations are accounted for qualitatively and quantitatively by using an extension of the variational principle that leads to the Landau-Lifshitz equation. All parameters of the model can be obtained by independent measurements. DOI: 10.1103/PhysRevB.95.104432 The discovery of giant magnetoresistance (GMR) revolutionized information storage technology [1,2] and the spin-transfer torque (STT), predicted two decades ago by Slonczewski [3] and Berger [4], may reshape once again the magnetic memory industry [5]. The concept of a heat-driven spin torque, or thermal spin-transfer torque (TST), has been suggested [6][7][8] and opened the world of spin caloritronics. Magnetic insulators are ideal for studying the fundamentals of spin caloritronics, because they are free of the effect of heat on charge transport. Here, we demonstrate that a spin torque can be induced in magnetic insulators by applying a thermal gradient. The effect is not linked to spin-dependent transport at interfaces since we observe a heat-driven contribution to damping of magnetization waves on a millimeter scale. We show that by adding to M(r) the bound magnetic current (∇ × M) as state variable, the variational principle that yields the Landau-Lifshitz equation predicts the presence of a thermal spin torque, from which we derive an expression for spin currents in insulators. Our experiments verify the key predictions of this model. Thermodynamics can predict a link between heat and magnetization, but cannot determine the strength of the effect [9].Spin caloritronics studies the interplay of spin, charge, and heat transport [10]. As the spin dependence of the electrical conductivity proved to be important since it gives rise to GMR, the spin dependence of other transport parameters has been investigated, such as that of the Seebeck [11] and Peltier coefficients [12]. The combination of heat with spin and charge transport gained widespread attention owing to studies of the spin Seebeck effect [13,14]. The STT effect which uses a spinpolarized electrical current has shown promising applications, e.g., in magnetic memories (STT-MRAM). It was already established that heat flowing through a ferromagnetic metal can generate a diffusive spin current [15] which induces a spin torque when flowing through a magnetic nanostructure [6]. Experimentally, this effect was studied in Co/Cu/Co spin valve nanowires by observing the change in the switching * haiming.yu@buaa.edu.cn † jean-philippe.ansermet@epfl.ch field of magnetization due to a local thermal gradient [7]. It was later shown that heat couples to magnetization dynamics [16][17][18]. The effect of heat on magnetization was also found in magnetic tunnel junctions [19] and...

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