Angular Dependence of Spin-Torque Critical Currents in CPP-GMR Read Heads

TL;DR

This paper derives theoretical expressions for the critical current in CPP-GMR read heads considering angular dependence of spin-transfer torque and validates them with experimental measurements on nanoscale devices, revealing implications for device operation.

Contribution

It introduces a generalized model for critical current dependence on angle and experimentally verifies it on fabricated nanoscale CPP-GMR devices.

Findings

Critical current depends on both B(q) and its derivative dB/dq in non-collinear configurations.

Experimental data aligns with models where B(q) ~ 1/[1+cnst*q].

Results suggest new considerations for read head stability and operation.

Abstract

This paper derives expressions for the critical current at the onset of spin-transfer-torque (STT) instability in CPP-GMR read heads, as a function of the relative angle (theta) between the free and reference layer magnetizations. Including a general angular dependent STT coefficient B(q=cos(theta)) exclusive of the angular momentum conserving sin(theta) factor, the critical current is found to depend on both B(q) and dB/dq in the non-collinear case |q| < 1. The paper also details the experimental measurement of the angular dependent critical currents on 50-nm sized CPP-GMR devices with synthetic antiferromagnet pinned layers, and fabricated using e-beam lithography. The measurements are consistent with prior theoretical models of the form B(q) ~ 1/[1+cnst*q], and indicate perhaps unanticipated implications for read head operation due to the critical current dependence on dB/dq.