Domain Wall Velocities in Thin Iron-Nickel Films

Abstract

An apparatus based on the Kerr magneto-optical effect has been used to measure the velocity of domain walls in thin iron-nickel films. Polarized light, after being reflected by a film, is analyzed and detected by a photomultiplier tube. The slope of an oscilloscope trace resulting from the photomultiplier signal is directly proportional to the domain wall velocity, as the wall traverses a light spot of known size. Wall velocities are found to be well described by an equation of the form v = m(H-H0), where m is the wall mobility and H-H0 the excess applied field. Measurements have been made on films varying in thickness from 4000 A to 700 A with corresponding mobilities of 0.5×104 cm/sec-oe for the thicker film and 4.9×104 cm/sec-oe for the thinner film. The wall mobility is found to be inversely proportional to Bsσd where σ is the electrical conductivity and d the film thickness. This is the dependence that is predicted on the basis of a simple eddy current model indicating that, even in films as thin as 700 A, the dominant loss mechanism in wall motion is caused by eddy currents rather than intrinsic damping of the Landau-Lifshitz type. © 1960 The American Institute of Physics.