Spin-torque-driven ferromagnetic resonance in point contacts

Abstract

We demonstrate the technique of spin-torque-driven ferromagnetic resonance (ST-FMR) in point contacts, which enables FMR studies in sample volumes as small as a few cubic nanometers. In our experiments, we use point contacts ∼10 nm in size to inject both dc and microwave currents into F/N/F/AFM exchange-biased spin valves where two ferromagnetic (F) layers are separated by a nonmagnetic (N) metal spacer and one of the Fs is pinned by an adjacent antiferromagnetic (AFM) layer. High current densities produce the spin-transfer torque on magnetic moments in a small contact region and drive it to resonance at appropriate frequency of the applied microwaves. The resulting magnetodynamics are detected electrically via a small rectified dc voltage, which appears across the contact at resonance. The width of the resonance varies linearly with the applied dc bias as expected for spin transfer in spin valves. Potentially, the point-contact technique extends the applicability of ST-FMR to higher/lower frequencies, smaller sample volumes, and a broader range of materials.