Non-collinear antiferromagnetic states in Ru-based Heusler compounds induced by biquadratic coupling

Abstract

We investigate the magnetic properties of Ru2MnZ (Z=Sn,Sb,Ge,Si) chemically ordered, full Heusler compounds for zero as well as finite temperatures. Based on first-principles calculations we derive the interatomic isotropic bilinear and biquadratic couplings between Mn atoms from the paramagnetic state. We find frustrated isotropic couplings for all compounds and, in the case of Z=Si and Sb, a nearest-neighbor biquadratic coupling that favors perpendicular alignment between the Mn spins. By using an extended classical Heisenberg model in combination with spin dynamics simulations we obtain the magnetic equilibrium states. From these simulations we conclude that the biquadratic coupling, in combination with the frustrated isotropic interactions, leads to noncollinear magnetic ground states in the Ru2MnSi and Ru2MnSb compounds. In particular, for these alloys we find two distinct, noncollinear ground states which are energetically equivalent and can be identified as 3Q and 4Q states on a frustrated fcc lattice. Investigating the thermal stability of the noncollinear phase we find that, in the case of Ru2MnSi, the multiple-Q phase undergoes a transition to the single Q phase, while in case of Ru2MnSb the corresponding transition is not obtained due to the larger magnitude of the nearest-neighbor biquadratic coupling.