Driving mechanisms of conventional field-driven FMR (a) and STT–FMR (b). An oscillating magnetic field (Brf ) exerts an oscillating torque (τ B ) on a magnetic moment (a). An oscillating current (Irf ) which flows between the fixed (bottom) and free (top) layers of a spin valve generates an oscillating STT torque (τ STT ) on the magnetic moment of the free layer (b).
STT-driven magnetodynamics in a simple spin-valve structure where a free magnetic layer s is separated from a fixed magnet s* by a nonmagnetic spacer N (a) and (b). When electron current crosses the spin valve from left to right, electrons transmitted through the fixed polarizer will generate torque τ+ on the free layer (a). When the current flows from right to left, electrons reflected from the polarizer will generate torque τ− on the free layer (b). Magnetic precession driven by STT oscillating in sync with the precession (c). Schematic diagram of a STT–FMR experiment with dc and rf electronics connected to an STT device via a bias tee. Both dc and rf currents can be applied to the device, and the resulting dc voltage across the device can be measured (d).
Typical experimental results of STT–FMR. The rectified dc voltage (at zero dc bias) as a function of applied magnetic field B for different frequencies of applied rf current ω/2π = 9, 10, 11, 12, 13, 15, 16, 17, 18, 19, 20, 21, 22, 23, and 23.7 GHz. The spectra are shifted vertically for clarity (a). The experimental resonance frequency vs field relation (solid circles) fitted by the Kittel equation (b). DC bias dependence of FMR linewidth (c).
Examples of devices used in STT–FMR. EBL patterned nanopillars of spin valves (MTJs) with two magnetic—fixed and free—layers (dark grey) separated by a nonmagnetic metallic (insulating) spacer (a). Metallic point-contact devices to EBSV films (b). Magnetic nanowires electrodeposited into porous membrane (c). Ferromagnetic (dark grey)/Pt (light grey) metal bilayer used in SHE driven FMR experiments. A transverse spin current IS is generated by a longitudinal charge current I in the Pt layer via SHE (d).
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