A structural, magnetic and Mössbauer spectral study of the magnetocaloric Mn_1.1Fe_0.9P_1−xGe_xcompounds

Abstract

The structural, magnetic and Mössbauer 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 Mössbauer 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 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 fit 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.

Kinetic-arrest-induced phase coexistence and metastability in(Mn,Fe)2(P,Si)

“…1 and 2). The derived average quadrupole splitting and hyperfine field at 4.2 K for the FM phase is in good agreement with those reported for the isostructural (Mn,Fe) 2 (P,Ge) compounds [48]. However, the (Mn,Fe) 2 (P,Si) compounds show smaller average isomer shifts than the (Mn,Fe) 2 (P,Ge) compounds, which is probably due to the smaller unit-cell volumes for the (Mn,Fe) 2 (P,Si) compounds.…”

Effect of changing P/Ge and Mn/Fe ratios on the magnetocaloric effect and structural transition in the (Mn,Fe)2 (P,Ge) intermetallic compounds

“…Four compositions 1, 2, 4, 5 are paramagnetic at room temperature, whereas sample 3 is ferromagnetic at room temperature. Its smaller c lattice parameter in comparison with sample 1 and 2, and its smaller c/a ratio results from the magnetostriction effect occurring at the Curie temperature, what stays in agreement with the previous results [4,5]. The temperature dependent powder X-ray diffraction patterns in 2 theta range from 12°to 20°o f the samples 1, 3 and 5 are shown in Fig.…”

“…In all the studied compounds first order magnetic transition (FOMT) has been reported. Until now, several papers describing magnetocaloric effect in the MnFePGe series with respect to the composition changes have been published [4][5][6][7][8][9][10][11].…”

Tunable thermal hysteresis in MnFe(P,Ge) compounds

“…Also, a small amount ͑ϳ4 vol %͒ of secondary phase Mn 2 O 3 is detected. 17 The temperature dependence of the magnetization ͑M-T͒ for Mn 1.1 Fe 0.9 P 1−x Ge x ͑x = 0.19, 0.22, 0.25͒ is shown in Fig. 1͑a͒.…”