Co-resonant enhancement of spin-torque critical currents in spin-valves with synthetic-ferrimagnet free-layer

TL;DR

This paper demonstrates that using synthetic ferrimagnet free-layers in spin-valves can significantly increase the critical current for spin-torque instability, with the effect being polarity-dependent and explained by a co-resonance model.

Contribution

It introduces a novel approach to enhance critical currents in spin-valves using synthetic ferrimagnet free-layers and explains the phenomenon with a two-macrospin resonance model.

Findings

Critical current can be strongly enhanced with synthetic ferrimagnet free-layers.

Enhancement occurs only for one bias polarity due to co-resonance effects.

The model explains the phenomenon through energy transfer between coupled modes.

Abstract

It is experimentally shown that the critical current for onset of spin-torque instability in current-perpendicular-to-plane spin-valves can be strongly enhanced using "synthetic ferrimagnet" free-layers of form FM1/Ru/FM2 (FM=ferrromagnet). However, this enhancement occurs for only one polarity of bias current. A two-macrospin model is shown to reproduce the observations. The model suggests that this phenomenon is related to a polarity-dependent, spin-torque induced co-resonance between the two natural dynamic modes of the FM1/FM2 couple. The resonance condition facilitates energy transfer out of the spin-torque destabilized mode into the other stable mode whose effective damping is actually enhanced by spin-torques, thereby delaying the onset of instability of this coupled system to larger critical currents.