Magnetic and electrical properties of postannealed Co2MnSi Heusler alloy films

Abstract

The Co2MnSi (CMS) Heusler alloy is known as a strong ferromagnet with high Curie temperature (TC~985 K) and saturation magnetization Ms of about 5.1 B/f.u. at 4 K. The material provides increasing scientific and technological interest due mainly to full spin polarization of carriers and thus it is promising for the fabrication of magnetic tunneling junctions, spin filters, and other spintronics devices operating at room temperature [1]. In this work, the CMS films (d=100-120 nm) were grown by dc magnetron sputtering of a stoichiometric target on MgO(100) and Si(100) followed by annealing in vacuum at T=Ta=300-500 oC. Formation of nanocrystalline structure with typical grain size 20-40 nm has been indicated from SEM surface images for the films grown either on Si or MgO. XRD investigations revealed amorphous state for the as prepared films while partially ordered B2 structure and traces of highly ordered L21 structure have been indicated for the films annealed at T>300 oC. The alternating current (AC) magnetic susceptibility (x~dM/dH) was investigated at RT and 78 K to reveal evolution of magnetic properties of the prepared films with annealing. Increase of saturation magnetisation and reduced electrical resistance have been indicated with Ta increasing from 300 to 400 oC. Meanwhile, long term annealing of the films at T> 450 oC resulted reduced saturation magnetization valus (Figure 1). We point out 3 major competing processes having great influence on magnetic and electrical properties of the films, namely, nucleation of nanometric grains in the beginning of the annealing, diffusioncontroled formation of the ordered B2 and L21 structures and dissociation of highly ordered L21structure with annealing at T> 450 oC. Instability of the L21 structure at the highest annealing temperatures we associate to a possible Si phase segregation at grain boundaries of the prepared nanocrystalline CMS films.