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Mn-Bi-Al thin films. were produced by sequential evaporation of the constituents, followed by an anneal at 300 °C. The temperature and composition dependencies of the Kerr rotation angle, absolute reflectivity, and magnetic anisotropy were measured. The results show that, up to 30 at. % Al concentration, the thin films retain the pure MnBi hexagonal structure. Further, for suitable Al content, the films have the same large Kerr rotation as MnBi. Pure MnBi films exhibit perpendicular anisotropy at room temperature and in-plane anisotropy for temperatures below 142 K. In contrast,. the Al-doped films prepared here have perpendicular anisotropy down to at least 85 K. The increased coercivities of the Al-doped films are attributed to the occupation of grain-boundary and interstitial sites of the NiAs-type hexagonal structure by the Al-atoms.