Variable spin-charge conversion across metal-insulator transition

Abstract

The charge-to-spin conversion efficiency is a crucial parameter in determining the performance of many useful spintronic materials. Usually, this conversion efficiency is predetermined by the intrinsic nature of solid-state materials, which cannot be easily modified without invoking chemical or structural changes in the underlying system. Here we report on successful modulation of charge-spin conversion efficiency via the metal-insulator transition in a quintessential strongly correlated electron compound vanadium dioxide (VO₂). By employing ferromagnetic resonance driven spin pumping and the inverse spin Hall effect measurement, we find a dramatic change in the spin pumping signal (decrease by > 80%) and charge-spin conversion efficiency (increase by five times) upon insulator to metal transition. The abrupt change in the structural and electrical properties of this material therefore provides useful insights on the spin related physics in a strongly correlated material undergoing a phase transition.

Fig. 1. Structural, electrical, and magnetic properties of VO2 (68 nm)/YIG bilayer.

a θ−2θ X-ray diffraction scan (Cobalt source). b AFM image of VO₂ (68 nm) surface. c Resistivity vs. temperature showing a metal-insulator transition between 62 and 77 °C. d Temperature dependence of the magnetization in VO₂ (68 nm)/YIG(3 μm) at 1 kOe as measured from SQUID.

Fig. 2. Schematic illustration of the spin pumping measurement and results.

a Schematic of experimental setup for ISHE voltage measurements. b V_SP vs. H_ext spectra of VO₂(68 nm)/YIG (3 μm) bilayer at θ_H = 90° (blue) and 270° (red) at T = 20 °C. c V_SP spectra of VO₂ (68 nm)/YIG(3 μm) across the transition; the plots for different temperatures are shifted upwards by 30 μV for clarity. Temperature dependence of d V_SP (stars) and resistivity (line) of theVO₂(68 nm)/YIG(3 μm) sample (e) Resonant field from experiment (red) and calculated from Kittel’s formula (blue) at 8 GHz. (f) V_SP vs. H_ext for Pt(6 nm)/YIG bilayers sample at different temperatures; plots for different temperatures are shifted by 10 μV for clarity.

Fig. 3. Variation of V_SP and charge-spin conversion across transition.

Temperature dependence of a spin pumping voltage and resistivity of a VO₂ (68 nm)/YIG (100 nm) sample, b Gilbert damping coefficient of bare YIG (blue) and VO₂ (68 nm)/YIG (100 nm) (red) (the dash-dotted lines are drawn to guide the eye), and c variation of θ_SHλ_SD of VO₂ (68 nm)/YIG (100 nm) sample across the metal-insulator transition. Error bars in this figure reflect uncertainties (standard error) in linear fitting of the damping coefficient.