A structural, magnetic, and Mössbauer spectral study of the magnetocaloric Mn1.1Fe0.9P1-xGex compounds

The structural, magnetic and Mossbauer spectral properties of the magnetocaloric Mn1.1Fe0.9P1-xGex compounds, with 0.19 < x < 0.26, have been measured between 4.2 and 295 K. The 295 K unit-cell volume increases from x = 0.19 to 0.22 and is substantially smaller in the ferromagnetic Mn1.1Fe0.9P0.74Ge0.26. The temperature dependence of the magnetization reveals a ferromagnetic to paramagnetic transition with a Curie temperature between approximately 250 and 330 K and hysteresis width of 10 to 4 K, for 0.19 < x < 0.25. The composition Mn1.1Fe0.9P0.78Ge0.22 shows the largest isothermal entropy change of approximately 10 J/(kgKT) at 290 K. The Mossbauer spectra have been analysed with a binomial distribution of hyperfine fields correlated with a change in isomer shift and quadrupole shift, a distribution that results from the distribution of phosphorus and germanium among the near neighbours of the iron. The coexistence of paramagnetic and magnetically ordered phases in ranges of temperature of up to 50 K around the Curie temperature is observed in the Mossbauer spectra and is associated with the first-order character of the ferromagnetic to paramagnetic transition. The temperature dependence of the weighted average hyperfine field is well fitted within the magnetostrictive model of Bean and Rodbell. Good fits of the Mossbauer spectra could only be achieved by introducing a difference between the isomer shifts in the paramagnetic and ferromagnetic phases, a difference that is related to the magnetostriction and electronic structure change