Magnetocaloric effect in Mn5Ge3−xSix pseudobinary compounds

Abstract

Structure and magnetic properties of Mn5Ge3−xSix pseudobinary compounds with x=0–1.5 were studied by x-ray diffraction and magnetic measurements. The compounds retain the hexagonal D88-type structure with a space group P63∕mcm, and their lattice parameters decrease from a=7.204(2)Å and c=5.028(2)Å to a=7.084(2)Å and c=4.945(3)Å with increasing Si content. Wigner-Seitz cell volume calculations indicate that Mn site volumes and Mn-Mn atomic distances decrease with increasing x, which influences the exchange interaction. All the compounds are ferromagnetic and their Curie temperatures, TC, decrease from 298 to 252K as the Si content is increased. In addition, the average atomic magnetic moment of Mn (at 5K) decreases from 2.64 to 2.35μB with increasing x from 0 to 1.5. The magnetocaloric effect is evaluated by measuring the isothermal magnetic entropy change, ΔSm, using the appropriate thermodynamic Maxwell relation. The peak value of ΔSm ranges from about 9.0to6.3J∕kgK for x=0–1.5 near their TC under an external magnetic field change of 5T. On the other hand, ΔSm peaks broaden with increasing x, consistent with the less sharp change of magnetization near TC. We show that the phenomena can be ascribed to the weakening of the exchange interaction as the Mn-Mn separation is reduced upon the addition of Si atoms.