Magnetic and electrical properties of postannealed Co2MnSi Heusler alloy films

Abstract

The Co2MnSi (CMS) Heusler alloy compound with ordered crystalline structure is attractive for a number of applications due to soft ferromagnetic properties, high Curie temperature (TC ≅ 985 K), high saturation magnetization (Ms ≅ 5.1 µB/f.u), and predicted full spin polarization of carriers. In this work, we report preparation and properties of nanocrystalline CMS films with thickness d = 120 nm magnetron sputtered onto Si(100) substrates followed by annealing in vacuum at Ta = 300 ÷ 500 °C. Nanocrystalline structure of partially ordered cubic B2 phase with grain diameter d = 15 ÷ 30 nm increasing gradually with Ta has been indicated after annealing at Ta ≥ 400 °C. The angle-dependent differential susceptibility measurements revealed an interesting evolution of magnetic properties of the films in a course of postdeposition annealing at Ta=500 °C. Formation of isolated FM particles demonstrating the angle-dependent coercivity Hc(α) following the well-known Stoner–Wohlfarth model has been indicated after 0.5 h annealing. However, the Kondorsky low Hc ~ 1/cosα demonstrating a strong pinning of domain walls has been certified for similar films after 1.5 h annealing. At the same time, long-term (~ 5 h) annealing at 500 °C revealed reduced magnetization of the films due to a decomposition of the material. Nevertheless, electrical transport measurements revealed low field magnetoresistance of these films of about 0.1% and 0.05% at 80 and 295 K, respectively, demonstrating tunneling of spin-polarized carriers between nanocrystalline grains.