Abstract
We consider theoretically the impact of Rashba spin–orbit coupling on spin torque oscillators (STOs) in synthetic ferromagnets and antiferromagnets that have either a bulk multilayer or a thin film structure. The synthetic magnets consist of a fixed polarizing layer and two free magnetic layers that interact through the Ruderman-Kittel-Kasuya-Yosida interaction. We determine analytically which collinear states along the easy axis that are stable, and establish numerically the phase diagram for when the system is in the STO mode and when collinear configurations are stable, respectively. It is found that the Rashba spin–orbit coupling can induce anti-damping in the vicinity of the collinear states, which assists the spin transfer torque in generating self-sustained oscillations, and that it can substantially increase the STO part of the phase diagram. Moreover, we find that the STO phase can extend deep into the antiferromagnetic regime in the presence of spin–orbit torques.
Highlights
A suitable choice for this material could be a heavy normal metal such as Au or Pt
By comparing the new terms introduced by having RSOC present, we find that the spin–orbit torques effectively can be described by a modification of the Gilbert damping α,to the extent where we can get an anti-damping term in the LLGS equation
The precise value of these parameter values has little influence on the size of the spin torque oscillator (STO) phase as long as their magnitude is chosen reasonably, as we will briefly discuss later
Summary
We follow the procedure by Zhou et al.[18] and consider when the collinear states of m1 and m2 along the easy axis are stable or not. We start off with the ansatz that there is a slight perturbation ui from the collinear state, such that mi = λink + ui (λi =±1). Plugging this ansatz into (1) and performing a Fourier transform ui(t) = ∫ u i(ω) exp(−iωt) dω/2π, we get a result that is on the form (Aω + V ) uu 12 = 0
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