  To make a useful STT-MRAM (spin-transfer torque magnetoresistive
random-access memory) device, it is necessary to be able to calculate switching
rates, which determine the error rates of the device. In a single-macrospin
model, one can use a Fokker-Planck equation to obtain a low-current thermally
activated rate $\propto \exp(-E_{eff}/k_B T)$. Here the effective energy
barrier $E_{eff}$ scales with the single-macrospin energy barrier $KV$, where
$K$ is the effective anisotropy energy density and $V$ the volume. A
long-standing paradox in this field is that the actual energy barrier appears
to be much smaller than this. It has been suggested that incoherent motions may
lower the barrier, but this has proved difficult to quantify. In the present
paper, we show that the coherent precession has a magnetostatic instability,
which allows quantitative estimation of the energy barrier and may resolve the
paradox.
