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.

Ferromagnetic resonance driven by an ac current: A brief review

“…High current densities associated with their extremely small size were instrumental for the first demonstration of STT in spin-valve multilayers [10] and for highfrequency magnetodynamics studies [15,24,25]. STT-FMR was demonstrated in mechanical point contacts to ultrathin films of exchanged bias spin valves (EBSV) [15]. A standard mechanical system [10,23] can be used to establish an …”

“…Metallic point contacts are convenient tools for experimental studies of electron kinetics in metals [21][22][23]. High current densities associated with their extremely small size were instrumental for the first demonstration of STT in spin-valve multilayers [10] and for highfrequency magnetodynamics studies [15,24,25]. STT-FMR was demonstrated in mechanical point contacts to ultrathin films of exchanged bias spin valves (EBSV) [15].…”

“…A direct outcome from the STT-FMR linewidth analysis is the determination of Gilbert damping in individual nanomagnets with only a million of Bohr magnetons and scalable to even smaller dimensions. The critical dc current of STT-induced dynamics can also be extracted from the linear fit for the bias dependence of the resonance linewidth [14,15].…”

“…The spin transfer torque-driven FMR technique (STT-FMR) enables FMR studies in sample volumes smaller by a factor of 1000 compared to the conventional methods with resonant microwave cavities. Moreover, the electrical FMR detection based on rectification phenomena associated with anisotropic magnetoresistance (AMR) in single-layer structures, giant magnetoresistance (GMR) in metallic spin valves and magnetic multilayers, or tunneling magnetoresistance (TMR) in magnetic tunnel junctions provide the means to both excite and probe electrically the dynamical magnetic phenomena on the nanoscale [13][14][15][16]. This makes STT-FMR a unique and powerful tool for characterization of magnetodynamics in nanoscale devices.…”

“…When an ac current is flowing between the magnets, the resulting STT torque also oscillates and thus can be used to drive FMR. The combination of STT with magnetic resonance techniques extends the applicability of FMR to magnetic nanostructures, thanks to the development of sub-micron fabrication and electrical probing techniques [13][14][15]. The spin transfer torque-driven FMR technique (STT-FMR) enables FMR studies in sample volumes smaller by a factor of 1000 compared to the conventional methods with resonant microwave cavities.…”