  The dependence of the critical current of spin transfer torque-driven
magnetization dynamics on the free-layer thickness was studied by taking into
account both the finite penetration depth of the transverse spin current and
spin pumping. We showed that the critical current remains finite in the
zero-thickness limit of the free layer for both parallel and anti-parallel
alignments. We also showed that the remaining value of the critical current of
parallel to anti-parallel switching is larger than that of anti-parallel to
parallel switching.
