We report the observation of strong coupling between the exchange-coupled spins in a gallium-doped yttrium iron garnet and a superconducting coplanar microwave resonator made from Nb. The measured coupling rate of 450 MHz is proportional to the square root of the number of exchange-coupled spins and well exceeds the loss rate of 50 MHz of the spin system. This demonstrates that exchange-coupled systems are suitable for cavity quantum electrodynamics experiments, while allowing high integration densities due to their spin densities of the order of one Bohr magneton per atom. Our results furthermore show, that experiments with multiple exchange-coupled spin systems interacting via a single resonator are within reach.

We perform a quantitative, comparative study of the spin pumping, spin Seebeck and spin Hall magnetoresistance effects, all detected via the inverse spin Hall effect in a series of over 20 yttrium iron garnet/Pt samples. Our experimental results fully support present, exclusively spin currentbased, theoretical models using a single set of plausible parameters for spin mixing conductance, spin Hall angle and spin diffusion length. Our findings establish the purely spintronic nature of the aforementioned effects and provide a quantitative description in particular of the spin Seebeck effect.Pure spin currents present a new paradigm in spintronics [1, 2] and spin caloritronics [3]. In particular, spin currents are the origin of spin pumping [4,5], the spin Seebeck effect [6,7] and the spin Hall magnetoresistance (SMR) [8][9][10]. Taken alone, all these effects have been extensively studied, both experimentally [6-9, 11-13] and theoretically [4,[14][15][16][17][18]. From a theoretical point of view, all these effects are governed by the generation of a current of angular momentum via a non-equilibrium process. The flow of this spin current across a ferromagnet/normal metal interface can then be detected. The relevant interface property that determines the spin current transport thereby is the spin mixing conductance. Nevertheless, there has been an ongoing debate regarding the physical origin of the measurement data acquired in spin Seebeck and SMR experiments due to possible contamination with anomalous Nernst effect [19][20][21] or anisotropic magnetoresistance [22,23] caused by static proximity polarization of the normal metal [23]. To settle this issue, a rigorous check of the consistency of the spin-current based physical models across all three effects is needed. If possible contamination effects are absent, according to the spin mixing conductance concept [24], there should exist a generalized Ohm's law between the interfacial spin current and the energy associated with the corresponding non-equilibrium process. This relation should invariably hold for the spin pumping, spin Seebeck and spin Hall magnetoresistance effects, as they are all based on the generation and detection of interfacial, nonequilibrium spin currents. We here put forward heuristic arguments that are strongly supported by experimental evidence for a scaling law that links all aforementioned spin(calori)tronic effects on a fundamental level and allows to trace back their origin to pure spin currents. (c) The spin Hall magnetoresistance is due to the torque exerted on M by an appropriately polarized Js which yields a change in the reflected spin current J r s . The interconversion between Js (J r s ) and the charge currents Jc (J r c ) are due to the (inverse) spin Hall effect in the normal metal.[schematically depicted in Fig. 1(a)], we place YIG / Pt bilayers in a microwave cavity operated at ν = 9.85 GHz to resonantly excite magnetization dynamics. The emission of a spin current density J s across the bilayer interface into the Pt provides...

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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...

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.