Effect of microwaves on domain wall motion in thin Ni wires

We report new results on domain wall motion in very narrow ($\sim 400\;{\rm \AA}$ diameter) Ni wires. In previous work we have presented evidence that the magnetoresistance can be used to observe the pinning and depinning of domain walls from what we believe are structural defects. Those results suggest that at low temperatures the depinning can occur via quantum tunneling of a wall through the associated pinning barrier. In the present paper we describe new results for the effects of microwave radiation on this behavior. This radiation alters the tunneling probability in a manner which suggests that the energy levels of a domain wall are quantized, much like those of a particle in a potential well