Crystal structure and hyperfine fields in Mn1-xCoFexGe magnetocaloric alloys

Abstract

The phase analysis, site occupancy, and hyperfine fields of Mn1-xCoFexGe alloys (x = 0.01, 0.02, 0.05, 0.10) after annealing procedure are presented. Mössbauer spectroscopy was performed with and without an external magnetic field of B = 1.3 T parallel to the beam direction at room temperature. RT X-ray diffraction shows that Mn1-xCoFexGe compound is two phases of Ni2In type hexagonal structure and TiNiSi type orthorhombic structure. Samples with × = 0.01, 0.02 and 0.05 exhibit (pre)dominant contribution of the orthorhombic structure while for × = 0.10 the amount of hexagonal phase increases significantly. In the series of alloys, the lattice parameters and the unit cell volume remain weakly sensitive to Fe concentration at both phases. The measured Mössbauer spectra are superpositions of two doublets which come from the Fe atoms located in hexagonal structure and two broad sextets of Fe orthorhombic structure origin. The values of isomer shift and quadrupole splitting do not change with increasing iron content. Using information about the composition of local environment and measured values of isomer shift, we identify the origin of individual subspectra, and site occupation of Fe atoms in the alloy. The Mössbauer measurements carried out in external magnetic field shows that induced hyperfine magnetic field in the hexagonal structure is much higher for Fe at Mn sites than the observed for Fe at Co sites. Analysis of sextets shows that magnetic moments of Fe tend to arrange along the external field direction with almost the same values of about 0.87 µB.