Structural Variation and Control of Magnetic Properties of MnBi Thin Films by Kr^+ Ion Irradiation
Details
MnBi (15nm) thin films were prepared by the magnetron sputtering of Mn/Bi multilayers followed by vacuum annealing of 350℃, and the variation of their structures and magnetic properties with 30keV Kr^+ ion irradiation was studied. The Mn_<52>Bi_<48> (15nm) film exhibited a magnetization of 180emu/cc, which was 1/3 of the reported value, but it exhibited a large coercivity of 10kOe resulting from large perpendicular anisotropy of this film. The magnetization and coercivity of the Mn_<52>Bi_<48> film were disappeared after a low ion dose of 3.1 x 10^<14>ions/cm^2. However, the existence of NiAs type MnBi phase and uni-directional anisotropy was confirmed by X-ray diffraction and torque magnetometer, respectively, even after the ion irradiation of 1 x 10^<15>ions/cm^2. From atomic force microscope and magnetic force microscope observations, there existed convex surface structures with a height of 〜50nm on the MnBi films, and MnBi phase underneath the convex structures exhibited ferromagnetism even after the ion irradiation of 1 x 10^<15>ions/cm^2. The surviving MnBi phases were considered to be magnetically isolated, and the existence of the isolated MnBi phases explained the variation of the structure and magnetic properties with the ion irradiation. From these results, transformation between ferromagnetic and nonmagnetic MnBi alloys by a low dose ion irradiation of 〜3 x 10^<14>ions/cm^2 was expected.
