Microscopic origin of ferro-antiferromagnetic transition upon non-magnetic substitution in Ru2(Mn1−xVx)Ge full Heusler alloys

On the basis of first-principle simulations of interatomic magnetic exchange interactions, we show that the transition from antiferromagnetic order in Ru2MnGe to ferromagnetic order in vanadium substituted Ru2Mn1−xVxGe is due to a progressive increase of the first-nearest neighbor (NN) ferromagnetic coupling between Mn atoms. The revealed mechanism is quite unusual since commonly one would expect, and indeed it has been proposed earlier, that the transition scenario is due to the suppression of some relevant interactions by the non-magnetic substitution or chemical disorder effects. Here, using our ab-initio calculated exchange parameters and performing finite temperature Monte Carlo statistical simulations on a disordered lattice, we describe the experimental magnetic phase diagram for the full range of vanadium concentrations, including the variation of the ordering temperature and the onset of finite magnetization in the ground state. Since the observed changes in the MnMn magnetic interactions are continuous with increasing V content, we argue that there is a threshold concentration of V substitutions where the critical fluctuations associated with frustration effects on the fcc lattice could be studied experimentally.