Structure, magnetic anisotropy and magnetocaloric effect of Mn5Ge3-xSix single crystals

Abstract

Magnetocaloric polycrystalline materials based on Mn5Ge3 have been widely studied due to their potential application for room temperature magnetic refrigeration. However, many investigations are performed on polycrystalline materials and thus the strong anisotropy in this layered structure is usually neglected. In this study, Mn5Ge3-xSix (x = 0, 0.7) single crystals were successfully synthesized via Sn flux and characterized by single-crystal X-ray diffraction and magnetic measurements. The Si substitution does not change the structural symmetry, but reduces the shortest Mn − Mn distances. As described by the Bethe-Slater curve, the reduction of the shortest Mn − Mn distances leads to weaker magnetic exchange, accounting for the lower ordering temperature. The magnetocrystalline anisotropy was evaluated by the Sucksmith and Thompson method, and became smaller upon substitution. The anisotropy constants K1 and K2 of Mn5Ge3-xSix (x = 0, 0.7) single crystals are positive, and decrease with the temperature. The electron paramagnetic resonance spectra suggest high spin d5 configuration of manganese in both crystals. The easy magnetization direction yields much larger maximum entropy changes in both crystals: 6.0 J/kg K for the pristine and 5.4 J/kg K for the substituted under an applied field of 30 kOe.