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

The structural, magnetic and Mössbauer 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 M¨ossbauer 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
Mössbauer 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 Mössbauer
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.