High coercivity of Alnico thin films: effect of Si substrate and the emergence of a novel magnetic phase

In our research work to study the spinodal decomposition of Alnico thin films prepared by sputtering on Si substrates, it has been discovered that Si diffuses into the films and gives rise to a new magnetically hard phase with T _C = 305 °C. As-made thin films with the “α phase” of the Alnico were heat treated by annealing at different temperatures in the range of 600–900 °C. The heat treatment gave a room temperature coercivity of 6.5 kOe after annealing at 900 °C. The maximum coercivity observed is approximately ten times larger than the bulk Alnico V value. Planar electron diffraction patterns can be mostly indexed to an FCC spinel phase with a = 7.79 Å. On the other hand, 80 nm deep inside the film a separate layer is observed with smaller grains having BCT structure with lattice constants a = 2.65 and c = 3.19 Å. We strongly believe that this phase is related to the observed high coercivities in the samples. This discovery is very important in view of the recent “rare earth problem” and may lead to alternative to rare earth materials for the development of high performance magnets.