Interdiffusion Processes in High-Coercivity RF-Sputtered Alnico Thin Films on Si Substrates

Alnico V thin film samples with the thickness of 100 nm were prepared by radio-frequency sputtering on Si substrates with and without a SiO2 layer. Heat treatment of the as-deposited thin films in above ambient pressure in an Ar atmosphere at a temperature range of 600–900°C, followed by quenching and/or slow cooling, leads to higher coercivity values compared to bulk Alnico magnets. Annealing at 800°C followed by quenching results in the highest coercivity reported here of 1.8 kOe. The formation of several triangular-shaped features along the interface between the substrate and the film were observed. A high-resolution transmission electron microscope analysis showed these to be formed via interdiffusion of Fe, Co and Ni atoms into the Si substrate. These features show a large difference in lattice parameters compared with the magnetically soft bulk Fe-Co-Si alloys, and a heterogeneous or layered magnetic ion distribution inside these features could be the origin of the high coercivity observed in the heat-treated films.