Synthetic ferrimagnet spin transfer torque oscillator: model and non-linear properties

TL;DR

This paper models the non-linear dynamics of synthetic ferrimagnet spin transfer torque oscillators, showing how coupling and layer asymmetry influence frequency shift and spectral linewidth, with implications for oscillator performance.

Contribution

It provides analytical expressions for non-linear parameters and demonstrates control over frequency shift and linewidth through layer coupling and asymmetry in synthetic ferrimagnets.

Findings

Only one mode (acoustic or optical) is excited at a time.

Strong coupling and asymmetry reduce non-linear frequency shift.

Linewidth can be minimized or reach linear linewidth values.

Abstract

The non-linear parameters of spin-torque oscillators based on a synthetic ferrimagnet free layer (two coupled layers) are computed. The analytical expressions are compared to macrospin simulations in the case of a synthetic ferrimagnet excited by a current spin-polarized by an external fixed layer. It is shown that, of the two linear modes, acoustic and optical, only one is excited at a time, and therefore the self-sustained oscillations are similar to the dynamics of a single layer. However, the non-linear parameters values can be controlled by the parameters of the synthetic ferrimagnet. With a strong coupling between the two layers and asymmetric layers (different thicknesses), it is demonstrated that the non-linear frequency shift can be reduced, which results in the reduction of the linewidth of the power spectral density. For a particular applied field, the non-linear parameter can even vanish; this corresponds to a transition between a red-shift and a blue-shift frequency dependence on the current and a linewidth reduction to the linear linewidth value.