A structural, magnetic and Mössbauer spectral study of the magnetocaloric Mn_1.1Fe_0.9P_1−<i>x</i>Ge_<i>x</i>compounds

Abstract

The structural, magnetic and Mssbauer spectral properties of the magnetocaloric Mn 1.1 Fe 0.9 P 1-x Ge x 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 x = 0.22 and is substantially smaller in the ferromagnetic Mn 1.1 Fe 0.9 P 0.74 Ge 0.26 . The temperature dependence of the magnetization reveals ferromagnetic to paramagnetic transition with a Curie temperature between ca. 250 and 330 K and hysteresis width of 10 to 4 K, for 0.19 < x < 0.25. The composition Mn 1.1 Fe 0.9 P 0.78 Ge 0.22 shows the largest isothermal entropy change of ca. 10 J/(kgKT) at 290 K. The Mssbauer 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 nearneighbours 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 Mssbauer 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 fit within the magnetostrictive model of Bean and Rodbell. Good fits of the Mssbauer 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.