Thermomagnetic writing in homogeneous MnBi films

Abstract

The specific Faraday rotation of the homogeneous low-temperature phase (LTP) and quenched high-temperature phase (HTP) MnBi films used for the writing experiments was measured to be <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9.5\times10^{5}</tex> degrees per centimeter and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4.3\times10^{5}</tex> degrees per centimeter, respectively, at 25°C and a light wavelength of 632.8 nm. The coercive field of the films decreased exponentially with increasing film thickness for both phases. Thermomagnetic writing experiments showed that the written spots (1 to 10 μm in diameter) were not satisfactorily stable for films thicker than 50 nm. If thicker films are to be used for magnetooptic memory applications, they should possess larger wall-motion fields than the films investigated, in order to achieve stable spots. The erasure field of the spots was found to depend on the strength of the applied writing field and on the duration and the power of the heating laser pulse. A writing-erasing hysteresis was recorded by means of the Faraday effect. A thermal saturation field is introduced and measured in dependence on film thickness for LTP films. With the help of this field and wall-pinning effects, thermomagnetic writing in MnBi films is discussed.