Presence of atomic disorder and its effect on the magnetic and electronic properties of NiCrGa half Heusler alloy

Abstract Polycrystalline NiCrGa half Heusler alloy, which is predicted to be a half-metallic ferromagnet from first principles calculations, has been synthesized by arc meting technique and its structural, magnetic as well as the electronic properties have been studied. The measured x-ray diffraction (XRD) pattern shows the signature of a disordered structure. Magnetization and magnetoresistance measurements indicate a competition between ferromagnetic and antiferromagnetic correlations in the sample, with no evidence of long range magnetic order down to the lowest temperature (2 K) and highest magnetic field (70 kOe) studied. The experimentally observed structural and magnetic properties are found to be significantly different from the theoretically predicted properties of the ordered cubic C1 b structure. To probe into the possibility of the presence of atomic disorder in the system, we have performed theoretical calculations using the spin-polarized-relativistic Korringa-Kohn-Rostoker method (SPR-KKR). By comparing the experimental data from XRD, magnetization, photoelectron spectroscopy measurements and theoretical calculations, we conclude that NiCrGa has significant amount of atomic disorder. Although, the ordered structure is energetically more stable than the disordered structures, we find that after synthesis, the system tends to stabilize in a disordered structure. The nature of the disorder has been ascertained from experiments and the corresponding electronic structure and magnetic properties have been obtained. With this atomic disorder present in the sample, the ferromagnetic ordering is disturbed and the spin polarization is consequently reduced.