Annular Uniaxially Magnetized Domains in Thin Ni-Fe Films

Abstract

Stable annular uniaxially magnetized domains have been observed in thin circular Ni-Fe films using both the Kerr and Faraday magneto-optic effects. The films exhibit unusual magnetic behavior in both the“easy” and “hard” directions. Generally the magnetization reversal begins at the circumference of the filmwith the appearance of a uniaxially magnetized annular domain whose inner radius then decreases with increasing applied field. In a number of films, the mechanism of magnetization reversal in an outer annular region appears to be distinctly different from that in the central region. In the annular region, the low-frequency magnetization reversal in the easy direction proceeds by wall motion with the Bloch walls approximately parallel to the direction of the applied field, but there is evidence that the reversal in the hard direction occurs by a rotational process. After the annular region is completely reversed, the center region is reversed by a radially inward motion of the circular boundary between the two concentric uniaxially magnetized domains. The magneto-optic observations are qualitatively correlated with 60 cps dφM/dt vsH and φM vs H loop data. Based on the assumptions of uniform magnetization per unit volume, M, and uniform film thickness, quantitative correlation of φM vs H as obtained from Kerr effect photographs and from normalized 60 cps hysteresis loop measurements have been made and are reported.