Magnetic and magnetocaloric properties of MnFe 1-xCoxGe compounds.

0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.85, 0.9 and 1.0 by arc melting stoichiometric amounts of the pure (better than 3N) elements Mn, Fe, Co and Ge. In order to obtain a homogeneous phase, the samples were annealed for one week at 1123 K under an Ar atmosphere of 100 mbar, and slowly cooled to room temperature. The crystal structure of the compounds is determined by means of x-ray diffraction taken on the powder samples at room temperature. The MnFe 1-x Co x Ge compounds crystallize in the hexagonal Ni 2 In-type crystal structure for x ≤ 0.8 and in the orthorhombic NiTiSi-type structure for x > 0.8. The phase and composition of the two samples with x = 0.3 and 0.8 were checked by using a JEOL JXA-8621 electron probe micro analyzer (EPMA). For a compound with nominal composition MnFe 0.3 Co 0.7 Ge, the EPMA data led to an average composition of 33.5 at.% Mn, at.10.1% Fe, at.23.7% Co and 32.7 at.% Ge, which corresponds to an actual formula Mn 1.00 Fe 0.30 Co 0.71 Ge 0.98 . The average composition of the compound with nominal composition MnFe 0.2 Co 0.8 Ge is 32.9 at.% Mn, 6.6 at.% Fe, 26.9 at.% Co and 33.6 at.% Ge, corresponds to an actual composition Mn 0.99 Fe 0.19 Co 0.81 Ge 1.00 . It is clear that the deviation between the nominal composition and the actual average composition is very small. The dc magnetization was measured in a Quantum Design MPMS2-type SQUID magnetometer, operating in fields up to 5 T and in temperature range 5 to 400 K. The magnetization measurements show that the MnFe 1-x Co x Ge compounds exhibit a complex magnetic behavior. The composition dependence of the Curie temperature and the saturation magnetization of the compounds are shown in Fig. 1. An anomalous comparatively high magnetic moment is observed in MnFe 0.2 Co 0.8 Ge. Furthermore, we investigated the MCE in these compounds by means of magnetization measurements. The magnetic-entropy change is derived from the magnetization data by using the Maxwell relations. Fig. 2 shows the magnetic-entropy change of MnFe 1-x Co x Ge compounds in a field change from 0 to 2 and from 0 to 5 T, respectively. The magnetic-entropy changes in these compounds, whether in the Ni 2 In-type structure or in the NiTiSi-type, increases with increasing x. This is probably due to the increase of magnetic moment. But an anomalous behavior is found for the composition x = 0.8. As can be seen in Fig. 2, the maximum magnetic-entropy