Growth of ferromagnetic Zn _1-xMn _xO thin films on Al ₂O ₃ (0001) by reactive RF magnetron sputtering

Zn _1-xMn _xO thin films were grown on Al ₂O ₃ (0001) substrates by a reactive rf magnetron sputtering method. The films grown by employing a 40 nm buffer layer showed a significantly clean surface with highly smooth morphology. The formation mechanism of mirror-like Zn _1-xMn _xO thin films might be considered to be a result of the effect of suppressing the columnar growth mode for the initial film growth stage because it was observed that the buffer layer grown at lower temperature has a relaxed c-axis preference. For the measurement of magnetic field-dependent magnetization the Zn _0.91Mn _0.09O thin films clearly showed a hysteretic behavior, which is obvious evidence of ferromagnetism. The easy axis for magnetization was in the plane of the film surface resulting from the compressive strain-dependent anisotropy, and this indicates that the Zn _0.91Mn _0.09O thin films grown on Al ₂O ₃ (0001) substrates are well merged laterally because the wurtzite lattice structure oriented with c-axis has compressive strain. The transition temperature to paramagnetism from ferromagnetism was estimated at about 110 K for the measurement of magnetization as a function of temperature.