A structural, magnetic and Mössbauer spectral study of the magnetocaloricMn1.1Fe0.9P1−xGex compounds

Abstract

The structural, magnetic and Mössbauer spectral properties of the magnetocaloricMn1.1Fe0.9P1−xGex compounds,with 0.19<x<0.26, have been measured between 4.2 and 295 K. The 295 K unit-cell volume increases fromx = 0.19 to 0.22 and is substantially smaller in the ferromagneticMn1.1Fe0.9P0.74Ge0.26.The temperature dependence of the magnetization reveals a ferromagnetic to paramagnetic transitionwith a Curie temperature between approximately 250 and 330 K and hysteresis width of 10 to 4 K, for0.19<x<0.25. Thecomposition Mn1.1Fe0.9P0.78Ge0.22 shows the largest isothermal entropy change of approximately10 J/(kgKT) at 290 K. The Mössbauer spectra have been analysed with a binomial distribution ofhyperfine fields correlated with a change in isomer shift and quadrupole shift, a distributionthat results from the distribution of phosphorus and germanium among the nearneighbours of the iron. The coexistence of paramagnetic and magnetically ordered phasesin ranges of temperature of up to 50 K around the Curie temperature is observedin the Mössbauer spectra and is associated with the first-order character of theferromagnetic to paramagnetic transition. The temperature dependence of the weightedaverage hyperfine field is well fitted within the magnetostrictive model of Bean andRodbell. Good fits of the Mössbauer spectra could only be achieved by introducinga difference between the isomer shifts in the paramagnetic and ferromagneticphases, a difference that is related to the magnetostriction and electronic structurechange.