Influence of Site Occupancy on the Structure, Microstructure and Magnetic Properties of Ternary and Quasi-ternary Alloys of Ni-Mn-Ga

摘要

A systematic study of the effect of cobalt, selectively substituted for Ni and Mn, in the modulated orthorhombic (7M) Ni_(50)Mn_(29)Ga_(21) alloy has led to interesting correlations between the resultant structure and microstructure. Substitution of Co for Mn resulted in the stabilization of a non-modulated tetragonal (NM) phase at higher Co content and caused suppression of long-range twin deformation leading to sporadic islands within which twin variants were confined. On the other hand, substitution of Co for Ni does not to alter either the superstructural ordering or the long-range twin deformation. A study of the compositional dependence of saturation magnetization measured at 5 K is shown to throw light on the site preference of cobalt and iron atoms substituted in Mn-rich alloys of quasi-ternary Ni-Mn-Ga-(Fe,Co) system. The study reveals that the dopant atoms occupy the regular Mn site, rather than the vacant Ga site, with ferromagnetic exchange relative to the moments on Ni and Mn sub-lattices. These effects are attributed to have their origin in minimizing the stresses generated by the corresponding atomic volume changes incorporated by doping.