Sign of inverse spin Hall voltages generated by ferromagnetic resonance and temperature gradients in yttrium iron garnet platinum bilayers

Abstract

We carried out a concerted effort to determine the absolute sign of the inverse spin Hall effect voltage generated by spin currents injected into a normal metal. We focus on yttrium iron garnet (YIG)|platinum bilayers at room temperature, generating spin currents by microwaves and temperature gradients. We find consistent results for different samples and measurement setups that agree with theory. We suggest a right-hand-rule to define a positive spin Hall angle corresponding to the voltage expected for the simple case of scattering of free electrons from repulsive Coulomb charges.The bon mot that the sign is the most difficult concept in physics since there are no approximate methods to determine it has been ascribed to Wolfgang Pauli. Indeed, the struggle to obtain correct signs permeates all of physics. While the negative sign of the electron charge is just a convention, that of derived properties, such as the (conventional) Hall voltage, has real physical meaning. Often it is much easier and sufficient to determine sign differences between related quantities. However, a complete understanding requires not only the relative but also the absolute sign. Here we address the sign of the (inverse) spin Hall effect [(I)SHE] [1-10] and related phenomena. The characteristic parameter is the spin Hall angle, defined as the ratio θ SH ∝ J s /J c of the transverse spin current J s caused by an applied charge current J c (a more precise definition is given below). The sign of θ SH may differ for different materials. Since the spin Hall angle for Pt is generally taken to be positive, θ SH of Mo [11], Ta [12], and W [13] must be negative.The sign of θ SH governs the direction of the spin transfer torque on a magnetic contact relative to that caused by the Oersted magnetic field induced by the same current J c [12,13]. It also determines the sign of the induced transverse voltage in experiments in which the ISHE is used to detect spin currents [7]. This technique is now widely used to study spin current injection by a magnetic contact, through "spin pumping" induced by ferromagnetic resonance (FMR) [11,[14][15][16][17][18] or by temperature differences [19][20][21][22][23] ("spin Seebeck effect", SSE). * michael.schreier@wmi.badw.de However, the pitfalls that can affect the determination of the sign of the θ SH , such as the sign of the spin currents [24] and magnetic field direction are often glossed over in experimental and theoretical papers. Moreover, a mechanism for a sign reversal of the longitudinal spin Seebeck effect has recently been proposed [25]. A careful analysis of experimental results with respect to the signs of of FMR and thermal spin pumping voltages generated by the inverse spin Hall effect is therefore overdue.In this letter we present the results of a concerted action to resolve the sign issue by comparing experiments on microwave-induced spin pumping and spin Seebeck effect for a bilayer of the magnetic insulator yttrium iron garnet (YIG) and platinum (Pt) at room temperature. Samples grown by differe...

Thermographic measurements of the spin Peltier effect in metal/yttrium-iron-garnet junction systems

“…The SPE appears as a result of a spin current generated by the spin Hall effect (SHE) in the PM [64][65][66][67][68][69]. When a charge current J c is applied to the PM with strong SOI, a spin current J s with a spin vector σ is generated by the SHE, which satisfies the following relation [37]:…”

Thickness and power dependence of the spin-pumping effect inY3Fe5O12/Pt heterostructures measured by the inverse spin Hall effect

“…After the discovery of the spin-pumping effect in ferrimagnetic insulator (yttrium iron garnet, Y 3 Fe 5 O 12 , YIG)/nonmagnetic metal (platinum, Pt) heterosystems by Kajiwara et al [7], there was rapidly emerging interest in the investigation of these structures [6,7,[9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. Since Y 3 Fe 5 O 12 is an insulator with a band gap of 2.85 eV [24] no direct injection of a spin-polarized electron current into the Pt layer is possible.…”

Microwave-induced spin currents in ferromagnetic-insulator|normal-metal bilayer system

“…For example, in Ref. 10, the direction of the spin current in the SP and SSE processes has been determined by combining the FMR technique with additional Peltier or dc/ac based heating techniques. Our results reveal that in a ferromagnet|normal metal (paramagnet) system , the spin current flows from the ferromagnet (FM) to the normal metal (NM) in the case of SP process, while the flow reverses for the LSSE provided that the NM is hotter than the FM.…”