MnBi hard magnetic films optimised through the correlation between resistivity, morphology and magnetic properties

Hard magnetic MnBi films were fabricated using pulsed laser deposition (PLD). The films consisted of Mn/Bi bilayers and the thickness of each sample constant was 185 nm. The number of bilayers (5 to 80 bilayers) and consequently, the thickness of the Mn and Bi layers changed among the different samples, although the proportion of Mn and Bi remained constant in each bilayer. The structural and morphological changes in the Mn/Bi multilayered films were produced by thermal treatments and were monitored by measuring the change in their resistance as a function of the temperature. These measurements, along with the magnetic measurements, allowed us to determine the optimum heat treatments that induced the formation of the low temperature (LT) alpha-MnBi phase. A maximum value of both the magnetisation (460 emu/cm(3)) and the maximum BH product (46 kJ/m(3)) was observed in the sample with 40 bilayers, showing, in addition, a magnetic anisotropy, wherein the easy magnetisation direction was predominantly perpendicular to the plane of the sample. A detailed explanation of these changes and of the atomic diffusion processes present for such optimisation has been achieved.