Direct Observation of Domain-Wall Movements in MnBi Films

MnBi films epitaxially grown on mica substrates possess an easy axis perpendicular to the film plane. The easy-direction magnetic hysteresis exhibits a sharp magnetization change near Hc but requires a much higher field for saturation. This behavior was studied by direct observation of the dynamic domain-wall formation and wall movement on a microscopic scale using the magneto-optic Faraday effect. The results were recorded on photographic films, using a polarized light source, an analyzer, a lens system of 1.3-μ resolution, and a high-speed movie camera. In the demagnetized state, a mosaic domain pattern consisting of closely packed hexagonal-like units was formed with the magnetization inside a ring within each unit remaining unswitched. After the entire film was covered with this mosaic pattern, further switching was accomplished mainly by the wall motion within each unit. A domain theory calculation based on the above observation gives detailed explanation of the idealized magnetization process.