Low-field magnetoresistance and domain drag in ferromagnets

Abstract

Despite common misconceptions, domain walls are too thick to ’’scatter’’ electrons appreciably. However, electrons crossing a wall apply a torque to it, which tends to cant the wall spins. This could be used to measure the conduction electron spin polarization. Most of the low-field resistive anomalies observed in pure Fe, Ni and Co at low temperature are caused by the Lorentz force associated with the internal field B=Ms present inside each domain. The existence of low-resistivity paths extending over many domains accounts for still unexplained magnetoresistance data in iron whiskers. In uniaxial materials, a d.c. eddy-current loop caused by the Hall effect runs around each wall. The field Hz generated by these loops tends to ’’drag’’ the whole domain structure in the direction of the carrier drift velocity. Also, the Joule dissipation of the eddy currents manifests itself as an excess Ohmic resistance. As predicted, this excess resistance decreases as the square of the field, in amorphous Gd25Co75 films, in MnBi films, and in pure bulk cobalt, when the walls are removed by an external field. The excess resistance can also be changed by reorienting the walls.