Review: On the complex magnetic phase diagram of the MnRuxRh1−xAs system,crystal, a.c. susceptibility, magnetization and electronic structure characterizations

Abstract

The crystal structure of MnRuxRh1−xAs system belongs to the hexagonal Fe2P-type structure (SG: P6̅ 2 m). Mn atoms locate at pyramidal (3 g) sites, Ru and Rh atoms occupy tetrahedral (3 f) sites and As atoms occupy 1b and 2c positions. For x > 0.75 the compounds exhibits ferromagnetic (F) ordering with a maximum magnetization saturation value of 3.96 μB/f.u. for MnRuAs. Increase of the Rh concentration results in a marked drop of the Curie temperature. At the opposite side, for x ≤ 0.2, the MnRuxRh1−xAs system undergoes successive AF arrangements. For x < 0.1 complex AF2 followed by a non collinear (AF2 +F) arrangements when cooling down, followed by a pure antiferromagnetic state AF1 at the lowest temperatures. Systematic XRD data collected in the 80–550 K temperature range were used to refine the crystal structure for intermediate compositions. Since the magnetic phase transitions are accompanied by structural changes, systematically the thermal variations of both cell parameters and atomic positions were analyzed in terms of magneto-elastic trends. It results that a local anisotropic distortion parameter δ affects the (3 f) tetrahedral site, depending on temperature and composition, i.e. magnetic state. Effectively, both low field a.c. susceptibility and high field (up to 10 T) magnetization measurements, have allowed evidence new phase transitions in the intermediate range of compositions, all exhibiting a magneto-elastic character. Electronic structure calculations were performed using the Korringa-Kohn-Rostoker method in the coherent potential approximation (KKR-CPA) to account for the Ru/Rh disorder. Special attention was paid to the total energy difference between AF and F orderings versus the Ru to Rh content. On the basis of the present experimental and theoretical characterizations, a more detailed (x,T) phase diagram was built for the MnRuxRh1−xAs system. It was found that a small magnetic polarization of the 4d states related to the marked anisotropic distorsion affecting the TET site, appears to play a significant role on the sucessive magnetic arrangements Ferro dominant with Ru-rich, AF and complex with Rh-rich compounds.