Observations on MnBi Films during Heat Treatment

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Abstract
Observations of magnetic surface phenomena, which were made during the vacuum heat treatment necessary to develop ferromagnetic MnBi films from their nonmagnetic constituents, are described. A vacuum hot stage for a metallurgical microscope was constructed in order to accomplish by means of the Kerr magneto‐optic effect this visual observation of incipient ferromagnetism. MnBi films with some areas already magnetic were first observed. The phenomena occurring during evolution of the magnetic state in the areas adjacent to the already magnetic areas could be observed readily and the experiments could be reproduced consistently. Attempts to nucleate this transformation in the vacuum hot stage were only partly successful because reproducibility could not be achieved. Growth of a new magnetic area on MnBi film manifests itself first as an essentially structureless area. This structureless area then becomes an area of antiparallel domains. The size of the domains depends mainly on the duration of the existence of the structureless state. Large domains tend to develop if the structureless state is kept in existence for a short time and vice versa. In addition to the immediate practical goal of improving reproducibility in the preparation of MnBi films such visual observation of the nascent state of ferromagnetism may also be of general interest to the physics of magnetism.

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