Lattice site occupation effect on the electronic structure and physical properties of quaternary CoMnCrAl Heusler alloy

Abstract

Theoretical and experimental approaches have been employed for studying the lattice site occupation effect on the electronic structure and some physical properties of CoMnCrAl Heusler alloy (HA). Among possible variants of the ordered stoichiometric CoMnCrAl HA lattice site occupation by Co, Mn, Cr, and Al atoms, the first-principle calculations have been performed for three nonequivalent ones (marked by us as type 1, type 2, and type 3, respectively). CoMnCrAl HA with atomic order of the first type is half-metallic ferromagnet (FM) with resulting magnetic moment of mtot1=0.9999 μB/f.u. and spin-polarization degree of Fermi electrons P1=100%, for atomic orders of second and third types, mtot and P are equal to mtot2=1.0186 μB/f.u., P2=89% and mtot3=6.8673 μB/f.u., P3=77%, respectively. Changes in the order type on the way T1→T2→T3 cause a decrease in the spatial electron density, weakening of the covalent chemical interatomic bonds, lead to a decrease in the cohesion energy Ecoh and hence to loss of the thermodynamic stability of the CoMnCrAl alloy. Despite various high-temperature heat treatments, only B2- and A2-types of atomic order have been obtained in bulk and film CoMnCrAl alloy samples. Experimentally, it was found that bulk B2-type ordered CoMnCrAl alloy demonstrates significantly lower magnetization [M(300K)=0.442 μB/f.u.] and spin-polarization degree of Fermi electrons [P≈30%] than theoretically predicted. More disordered crystalline CoMnCrAl films with A2-type of atomic order practically are non-FM at T=300 K with P=0.