MACROSCOPIC QUANTUM TUNNELING OF A DOMAIN-WALL IN A FERROMAGNETIC METAL

The macroscopic quantum tunneling of a planar domain wall in a ferromagnetic metal is studied based on the Hubbard model. An effective action of the magnetization for the slowly varying case is obtained to be the same as that of a ferromagnetic Heisenberg model with a term non-local in time that describes the dissipation due to the itinerant electron. The crucial difference from the case of an insulator is the presence of the Ohmic dissipation even at zero temperature due to the gapless Stoner excitation. The dissipative effect is calculated by use of the instanton method, and is found to be very large for a thin domain wall with thickness of a few times the lattice spacing, but is negligible for a thick domain wall. Dissipation due to eddy current is shown to be irrelevant for a wall of mesoscopic size. The results are discussed in the light of recent experiments on ferromagnets with strong anisotropy.