Spin-transfer torque in disordered weak ferromagnets

We study theoretically the spin transfer effect on a domain wall in disordered weak ferromagnets. We have identified the adiabatic condition for the disordered case as $λ\gg λ_{\rm D}\equiv \sqrt{{\hbar D}/{\spol}}$, where $D$ and $\spol$ are the diffusion constant and the spin splitting energy due to the $s$-$d$ type exchange interaction, respectively, and found out that perfect spin-transfer effect occurs even in weak ferromagnets as long as this condition is satisfied. The effective $β$ term arising from the force turns out to govern the wall dynamics, and therefore, the wall motion can be as efficient as in strong ferromagnets even if $\spol$ is small.

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