Antiferromagnetism in Ru2MnZ (Z=Sn, Sb, Ge, Si) full Heusler alloys: effects of magnetic frustration and chemical disorder

We present systematic theoretical investigations to explore the microscopic mechanisms leading to the formation of antiferromagnetism in Ru2MnZ (Z= Sn,Sb,Ge,Si) full Heusler alloys. Our study is based on first-principles calculations of inter-atomic Mn-Mn exchange interactions to set up a suitable Heisenberg spin-model and on subsequent Monte-Carlo simulations of the magnetic properties at finite temperature. The exchange interactions are derived from the paramagnetic state, while a realistic account of long-range chemical disorder is made in the framework of the Coherent Potential Approximation. We find that in case of the highly ordered alloys (Z=Sn and Sb) the exchange interactions derived from the perfectly ordered L21 structure lead to N\'eel temperatures in excellent agreement with the experiments, whereas, in particular, in case of Si the consideration of chemical disorder is essential to reproduce the experimental N\'eel temperatures. Our numerical results suggest that improving a heat treatment of the samples to suppress the intermixing between the Mn and Si atoms, the N\'eel temperature of the Si-based alloys can potentially be increased by more than 30%. Furthermore, we show that in strongly disordered Ru2MnSi alloys a distinct change in the antiferromagnetic ordering occurs.