  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.
