Spin caloritronic nano-oscillator

Abstract

Energy loss due to ohmic heating is a major bottleneck limiting down-scaling and speed of nano-electronic devices, and harvesting ohmic heat for signal processing is a major challenge in modern electronics. Here, we demonstrate that thermal gradients arising from ohmic heating can be utilized for excitation of coherent auto-oscillations of magnetization and for generation of tunable microwave signals. The heat-driven dynamics is observed in Y₃Fe₅O₁₂/Pt bilayer nanowires where ohmic heating of the Pt layer results in injection of pure spin current into the Y₃Fe₅O₁₂ layer. This leads to excitation of auto-oscillations of the Y₃Fe₅O₁₂ magnetization and generation of coherent microwave radiation. Our work paves the way towards spin caloritronic devices for microwave and magnonic applications.Harvesting ohmic heat for signal processing is one of major challenges in modern electronics and spin caloritronics, but not yet well accomplished. Here the authors demonstrate a spin torque oscillator device driven by pure spin current arising from thermal gradient across an Y₃Fe₅O₁₂/Pt interface.

Fig. 1

YIG/Pt nanowire magnetoresistance. a Sketch of the YIG/Pt nanowire spin torque oscillator. Arrows across the YIG/Pt interface illustrate the flow of spin Hall j _SH and spin Seebeck j _SS currents with corresponding smaller arrows representing the spin current polarization. The directions of the temperature gradient ∇T perpendicular to the YIG/Pt interface, the direct current I _dc, and the magnetic field H are depicted by arrows. b Magnetoresistance R of the YIG/Pt nanowire measured at low (I _dc = 0.15 mA) and high (I _dc = 2.75 mA) direct current bias for a magnetic field applied in the sample plane at the field angle ϕ = 90° with respect to the wire axis. c Spin Seebeck voltage V _SS induced in the nanowire by a large microwave current (microwave power P _rf = 2 dBm) as a function of magnetic field

Fig. 2

YIG/Pt nanowire microwave signal generation. a Spectra of normalized microwave power P/P _max generated by the nanowire at the frequency f = 3.2 GHz and magnetic field angle ϕ = 70° as a function of magnetic field at several direct current biases (vertically offset for clarity). b Color plot of microwave power generated by the nanowire at 3.2 GHz as a function of magnetic field and direct current bias

Fig. 3

Angular dependence of the critical current. Critical current for the onset of auto-oscillations as a function of in-plane magnetic field direction ϕ. The line shows the expected behavior due to only a spin Hall current (with a fitting parameter I ₀) in the absence of a spin Seebeck current

Fig. 4

ST-FMR measurements. a A single spin torque ferromagnetic resonance spectrum measured at a microwave frequency f = 3.5 GHz and magnetic field angle ϕ = 70°. Low-frequency (LF) and high-frequency (HF) modes are observed. b ST-FMR spectra of the YIG/Pt nanowire measured as a function of magnetic field and drive frequency at the field angle ϕ = 70°. c Micromagnetic simulation of the spin wave eigenmode spectra in the YIG/Pt nanowire with a top view of the spatial dependence of the LF and HF mode amplitudes. d ST-FMR spectra measured as a function of magnetic field and direct current bias current I _dc for microwave power P _rf = −3 dBm. e Linewidth of the LF₁ measured as a function of the microwave drive power P _rf. f Linewidth of the LF₁ mode as a function of direct current I _dc for ϕ = 65° and P _rf = −3 dBm. g Linewidth of the LF₁ mode as a function of direct current for ϕ = 15° and P _rf = 1 dBm