High-energy-product MnBi films with controllable anisotropy

High-anisotropy NiAs-type MnBi films are produced by in situ annealing of Bi/Mn/Bi trilayers, [Bi/Mn/Bi]_n multilayers, and subsequent magnetic field annealing. Phase components, crystallographic anisotropy, and magnetic properties of the Mn_xBi_100-x thin films exhibits strong dependence on Mn concentration. High-purity MnBi thin films with perfect c-axis orientation are obtained by carefully controlling the Mn/Bi ratio. An energy product of 16.3MGOe, which is the highest value reported so far, is achieved for the x=50 film of thickness t=100nm. The MnBi thick film (t=2μm) changes from isotropic to anisotropic after magnetic field annealing. Depending on the direction of the applied field during magnetic field annealing, the MnBi thick film may have out-of-plane or in-plane anisotropy. This control of anisotropy direction enables applications of MnBi films in permanent-magnet, spintronic devices, or magnetic micro-electro-mechanical systems. In addition, the room-temperature magnetocrystalline anisotropy constant and saturation polarization of the hard magnetic MnBi phase are determined to be K=K₁+K₂=15.0Mergscm^-3 and J_s=8.2kG, respectively.