Effects of composition variation and site disorder on the magnetic interactions in Ru2Fe(Si1-xGex) alloys.

[Display omitted] • Ru 2 FeSi 1- x Ge x alloys transform from antiferro-ferromagnetic for x > 0.15. • ab initio study of magnetic interactions in Ru 2 FeSi 1- x Ge x alloys. • B2 disorder favours antiferromagnetic interaction over ferromagnetic. • DOS at E F of L 2 1 and B 2 show non-half-metallic nature. X-ray diffraction studies on arc melted Ru 2 Fe(Si 1-x Ge x) (0 ≤ x ≤ 1) alloys did not show the characteristic (1 1 1) and (2 0 0) super-lattice peaks of the stable L2 1 structure of full Heusler alloys. The alloy with x = 0 exhibited antiferromagnetic behaviour. However, with increase in x, the alloys became ferromagnetic. In order to understand the origin of the novel magnetic behavior of these alloys and its correlation with their crystal structure, ab initio study was carried out using spin-polarized relativistic Korringa-Kohn-Rostoker Green's function method. The study shows that the antiferomagnetism in the alloy with x = 0 is due to the presence of B2 disorder. Due to this, the 2nd nearest neighbour (NN) exchange interaction between the Fe atoms at proper and improper sites, respectively, becomes strongly antiferromagnetic which dominates over the 1st NN ferromagnetic exchange interaction. With increase in x, the alloys become ferromagnetic with an improvement in L 2 1 ordering which in turn makes the 1st NN interaction strongly ferromagnetic. The electronic structure calculations show no half metallic gap in either spin direction for all the alloys. Comparison of experimental and theoretically estimated properties of Ru 2 Fe(Si 1-x Ge x) alloys provides insight to the variation in the magnetic interaction with the composition and the disorder present in the alloys. [ABSTRACT FROM AUTHOR]