The structural, electronic and magnetic properties for the transition process between nonmagnetic and magnetic states in CoFe1+xTi1−xAl

We investigated the evolution process of equilibrium lattice parameter (ELP), magnetic properties and electronic structure from nonmagnetic semiconductive CoFeTiAl to ferrimagnetic CoFeFeAl in theory and experiment. The theoretical results show that CoFe 1+x Ti 1−x Al compounds have a diluted magnetism with a 100% spin-polarized ratio at the Fermi level when X is less than 30%. When X is in the range of 30%–60%, CoFe 1+x Ti 1−x Al compounds have also a high spin-polarized ratio. The ELP and magnetization have a gradual and linear change with the increasing Fe level in the ranges of X = 0–50% and 70%–100%. An unexpected sharp jump in ELP and magnetization occurs near the composition point of X = 65%, which was attributed to band Jahn-Teller effect. In experiment, CoFe 1+x Ti 1−x Al (X = 0, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% and 100%) compounds with Heusler structure were successfully synthesized. The experimental results are in agreement with the theoretical results when X is less than 30% or higher than 70%. The sharp jump in ELP and magnetization are experimentally observed near the composition point of 65%. The deviation of the experimental magnetization from theoretical results in the range of 50%–70% is attributed to the theoretical underestimation to ELP. We predict that by the same mechanism of the band Jahn-Teller effect, the sharp change in the magnetization and lattice parameter should also be possible to occur with the changing temperature for a certain composition of CoFe 1+X Ti 1−X Al compounds. A new type of ferromagnetic shape memory alloys based on a martensitic transformation with an invariant crystallographic symmetry is proposed.