Magneto-Optic Properties of Quenched Thin Films of MnBi and Optical Memory Experiments

Abstract

Magnetic, magneto-optic, and optical properties of thin films of MnBi prepared in the quenched high-temperature phase have been measured. The saturation magnetization is reduced from normal phase by 23% and the coercive force is more than doubled at room temperature. In the visible spectrum region, there is no significant change in the optical absorption, but the strong wavelength dependence of the magneto-optic Faraday rotation in the normal films is essentially eliminated. At the 6327-Å HeNe-laser wavelength the room-temperature Faraday rotation is reduced to one-half of that of the normal films. These properties coupled with the low Curie temperature of 180°C make the quenched MnBi films well suited for optical memory applications. Experimental results of laser Curie-point writing, erasing, and reading using the quenched films have been obtained. In comparing with the normal films, the laser writing power is reduced by a factor of four; unheated spots are not disturbed by the erasure field; and the margin between the writing temperature and the decomposition temperature is much increased. The measured activation energy and annealing time constant associated with the transformation of this high-temperature phase to the normal phase indicates that the thermal stability of the quenched MnBi films is lacking for memory applications.