Phase-Sensitive Detection of Spin Pumping via the ac Inverse Spin Hall Effect

Abstract

An intriguing feature of spintronics [1] is the use of pure spin-currents to manipulate magnetization [2], e.g., spin-currents can switch magnetization in spin-torque MRAM [3], a next-generation DRAM alternative. Giant spin-currents via the spin Hall effect [4][5][6] greatly expand the technological opportunities [7]. Conversely, a ferromagnet/normal metal junction emits spin-currents under microwave excitation, i.e. spin-pumping [8][9][10]. While such spin-currents are modulated at the excitation frequency, there is also a non-linear, rectified component that is commonly detected using the corresponding inverse spin Hall effect (iSHE) dc voltage [11][12][13][14]. However, the ac component should be more conducive for quantitative analysis, as it is up to two orders of magnitude larger and linear [15]. But any device that uses the ac iSHE is also sensitive to inductive signals via Faradays Law and discrimination of the ac iSHE signal must rely on phase-sensitive measurements.We use the inductive signal as a reference for a quantitative measurement of the magnitude and phase of the ac iSHE.

Current-induced spin torque resonance of a magnetic insulator

“…From a different perspective, our results also show that ac spin pumping [36][37][38] in magnetic insulators is reciprocal, as predicted by Onsager symmetry in the linear-response regime. Spin transfer torque therefore provides an efficient link between pure magnonic and conventional electronic circuits.…”

“…The origin of the phase offset δ is not known. It could originate from our particular measurement geometry, electromagnetic wave propagation through the magnetic medium, a frequency dependent spin Hall angle as recently proposed by Weiler et al [38], or a significant current-induced effective field torque [40,41].…”

Minimal Model of Spin-Transfer Torque and Spin Pumping Caused by the Spin Hall Effect

“…37), so the observed dc spin current may be well explained by the damping-like component alone which however does not rule out the possibility of nonzero fieldlike component. Moreover, the field-like component also contributes to the ac measurements [30][31][32] which identify a much larger ac spin current than the dc one. Thus none of the known experiments contradicts the possibility of having a field-like component.…”

Nanomechanical detection of the spin Hall effect

“…Since the inverse SHE is used to measure spin transport due to the spin Seebeck effect and spin pumping, accurate knowledge of the spin Hall angle is important for metrology. One intriguing new result implies that the spin Hall angle is complex-valued, resulting in a phase shift between an applied AC charge current and the resulting AC spin current [33]. Additionally, the spin diffusion length in spin Hall materials may be correlated with the spin Hall angle, indicative of the same physics affecting both quantities.…”

“…Measurements and characterization of Θ SH have, as yet, been limited to methods based on optical, electrical, and magnetic effects, which require knowledge of Kerr rotation coupling, metallic interfaces, magnetic properties, and spin diffusion parameters to quantify Θ SH accurately [15,26]. As such, reported values of Θ SH span orders of magnitude for materials such as Pt and Pd [19,[27][28][29][30][31][32][33][34][35]. Open questions such as the dependence of Θ SH on growth conditions, film thickness, impurity level, frequency, and other systematic parameters must be addressed both experimentally and theoretically.…”