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

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 m t o t 1 = 0.9999 μ B/f.u. and spin-polarization degree of Fermi electrons P 1 = 100 %, for atomic orders of second and third types, m t o t and P are equal to m t o t 2 = 1.0186 μ B/f.u., P 2 = 89 % and m t o t 3 = 6.8673 μ B/f.u., P 3 = 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 E c o h 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 ( 300 K ) = 0.442 μ B/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.