Hyperfine fields and site occupation of the Fe atoms in the Mn1−xCoFexGe giant magnetocaloric materials

The crystal properties, site occupation and hyperfine fields of Mn _1−xCoFe _x Ge (x = 0.01, 0.02, 0.05, 0.10) alloys after melting are presented. The X-ray diffraction and Mössbauer spectroscopy, without and with external magnetic field of B = 1.3 T parallel to beam direction, were carried out. X-ray diffraction shows that Mn _1−xCoFe _xGe (x = 0.01 and 0.02) alloys are double phase with Ni ₂In-type (space group P6 ₃ /mmc) hexagonal structure and TiNiSi-type (space group Pmna) orthorhombic structure. For higher Fe concentration (x = 0.05 and 0.10) the single hexagonal phase is observed. The Mössbauer spectra are superposition of two doublets which come from the Fe atoms located in hexagonal structure and additional broad sextet originated from Fe in orthorhombic structure for samples x = 0.01 and 0.02. The analysis of Mössbauer spectra shows that doped Fe atoms are located in both Mn and Co crystallographic positions in hexagonal phase with almost equal probabilities. The results show that values of isomer shift and quadrupole splitting do not change with increasing iron concentration. For hexagonal structure the isomer shift is equal to (0.30 ± 0.05) mm/s for Fe in Co and (0.49 ± 0.05) mm/s for Fe in Mn position. The quadrupole splitting for hexagonal structure are equal to (0.79 ± 0.05) mm/s and (0.87 ± 0.05) mm/s for Fe in Co and Mn site, respectively. The Mössbauer measurements carried out in external magnetic field for iron atoms located in hexagonal structure show, that observed induced hyperfine magnetic field on Fe in Mn position is much higher while on Fe in Co position smaller then applied external magnetic field.