Computational study of microwave oscillations in absence of external field in nonstandard spin valves in the diffusive transport limit

TL;DR

This paper investigates microwave oscillations in nonstandard spin valves without external magnetic fields, revealing conditions for out-of-plane precession driven by anomalous spin accumulation and torque effects.

Contribution

It provides a numerical analysis of magnetization dynamics in nonstandard spin valves, confirming the wavy torque dependence and explaining experimental features through micromagnetic modeling.

Findings

Out-of-plane precession can occur without external magnetic fields.

Wavy-like torque angular dependence aligns with experimental data.

Nonlinear frequency-current behavior requires micromagnetic analysis.

Abstract

An anomalous (inverse) spin accumulation in the nonmagnetic spacer may build up when the spin valve consists of magnetic films having different spin symmetries. This leads to wavy-like dependence of spin-transfer torque on the angle between magnetizations, as predicted by spin-dependent diffusive transport model, and also confirmed experimentally. Making use of these predictions, we have numerically studied the magnetization dynamics in presence of such a wavy-torque in Co(8 nm)/Cu(10 nm)/Py(8 nm) nanopillar, considering geometry with extended and etched Co layer. In both cases we specify conditions for the out-of-plane precession to appear in absence of external magnetic field and neglecting thermal fluctuations. We prove the assumption of wavy-like torque angular dependence to be fully consistent with experimental observations. We also show that some features reported experimentally, like nonlinear slope of frequency vs. current behavior, are beyond the applicability range of macrospin approximation and can be explained only by means of full micromagnetic analysis.