Evolution of the magnetic properties of Co2MnGa Heusler alloy films:: From amorphous to ordered films

We have investigated the evolution of the structural ordering and the crystal structure of polycrystalline Co2MnGa Heusler alloy (HA) films and the structural effect on their magnetic properties. It is shown that the chemical and the structural ordering can be changed by the postannealing temperatures from an amorphous to more ordered structures typical for the HA. The magnetic properties of the films also depend strongly on the postannealing temperature. The as-deposited amorphous Co2MnGa films show a weak ferromagnetic behavior due to the presence of a small amount of relatively highly magnetized regions in the nonmagnetic alloy matrix. Low-temperature (up to 513 K) annealing of the amorphous films results in the formation of a mixture of Co2MnGa and metastable epsilon-Mn3Ga and/or Mn3Co7 phases and causes a substantial increase in both magnetic moment and resistivity. Annealing at higher temperatures (up to 753 K) leads to a structural order of A2/B2 type. The amorphous -> A2 (B2) structural transformation in Co2MnGa films induces an increase of about 30% in the alloy resistivity, qualitative changes in the shape of magneto-optical spectra, and an increase in the magnetic moment per f.u. up to (2-2.5)mu(B). These results are discussed in terms of the band structure of the alloy. The best Co2MnGa films, deposited on heated (753 K) substrates, exhibit the B2 (or L2(1)) structure and nearly the bulk value of magnetization of 3.5 mu(B).