The magneto-structural transition in Mn_1−xFe_xCoGe

Abstract

Large refrigeration capacities, between 212(30) J kg-1 and 261(40) J kg-1 for a magnetic field change from 0 T to 5 T, were obtained in Mn1-xFexCoGe (x = 0.01, 0.02, 0.03 and 0.04) compounds. A partial magnetic phase diagram has been derived on the basis of magnetic transition and martensitic transformation temperatures determined from differential scanning calorimetry (200 K to 450 K), variable temperature x-ray diffraction (20 K to 310 K) and magnetisation measurements (5 K to 340 K; 0.01 T). Mn1-xFexCoGe compounds with compositions in the range x = 0.01 to 0.03 exhibit magneto-structural transitions. Neutron diffraction experiments were carried out on the Mn0.98Fe0.02CoGe sample over the temperature range of 5 K to 450 K. The diffraction patterns were analysed based on irreducible representation theory which confirms a ferromagnetic structure in the sample with an atomic magnetic moment of 3.7(1)μB at 5 K on the Mn sublattice, oriented along the orthorhombic c axis. More significantly, a magneto-structural transition around TM ∼ 297(1) K with a full width at half maximum of 29 K is demonstrated directly via neutron diffraction. Larger magnetic entropy changes are obtained for the Mn1-xFexCoGe (x = 0.01, 0.02 and 0.03) samples than for Mn0.96Fe0.04CoGe which has separate structural and magnetic transitions. In addition, it is noted that standard Arrott plots do not provide unambiguous insight to the nature of the magneto-structural transition in the Mn1-xFexCoGe compounds. Abstract:Large refrigeration capacities, between 212(30) J kg -1 and 261 (40) samples than for Mn0.96Fe0.04CoGe which has separate structural and magnetic transitions. In addition, it is noted that standard Arrott plots do not provide unambiguous insight to the nature of the magneto-structural transition in the Mn1-xFexCoGe compounds.

Magnetism and magnetocaloric effect of Mn0.98Fe0.02CoGe

“…we would assume a value of 3.67 µ B for Mn moment again using the assumption of a zero Co moment value. This inconsistency is the subject of continuing analysis . As expected, the main crystallographic phase of the Mn 0.98 Fe 0.02 CoGe sample at 365 K is hexagonal and paramagnetic (see Fig.…”

“…b) indicates that Mn 0.98 Fe 0.02 CoGe adopts a ferromagnetic structure below T MS . As discussed fully elsewhere , all of the models for the space group Pnma were simulated and results show that the representation model Γ 3 (collinear ferromagnetic structure aligned along c ‐axis) gives the best refinement of the data. The initial neutron diffraction refinements show that the sample adopts a magnetic structure consisting of Mn moments aligned ferromagnetically along the c ‐axis, with a magnetic moment of 3.98(6) µ B .…”

First-order magneto-structural transition and magnetocaloric effect in Mn(Co0.96Fe0.04)Ge

“…4(a). Similar to other MnCoGe-based compounds [10,28], the relationship between the lattice parameters of the orthorhombic and hexagonal structures are: a orth ~ c hex , b orth ~ a hex , c orth /√3 ~ a hex and V orth /2 ~ V hex . In addition, a volume reduction by 4.0% (ΔV/V = (V orth /2 -V hex )/(V orth /2)) occurs from the low-temperature orthorhombic structure to the hightemperature hexagonal structure.…”

“…Compared with a model that considers that both the Mn and Co sublattices in the orthorhombic structure carry a magnetic moment, the model in which only the Mn sublattice carries a magnetic moment along the c orth -axis is found to be consistent with the results from the saturated magnetisation measurements presented in Section 3.4 (Table 3). This indicated that the magnetic moment in Mn(Co 0.96 Fe 0.04 )Ge mainly comes from the Mn sublattice and that magnetic moment on the Co sublattice is so small that it cannot be resolved by powder neutron diffraction, similar to the case of as-prepared (Mn 0.98 Fe 0.02 )CoGe [28]. Thus only the magnetic moment on the Mn sublattice in the orthorhombic phase was considered in the Rietveld refinement of the neutron diffraction patterns.…”

Effect of doping with rare-earth element La in MnCoGe system

“…元素化学替代: 元素替代法是最为常见、也是较容易 控制和调整相变温度的方法. 用Si [13] , Sn [14,15] , Al [16,17] , Fe [18,19] , Cr [20] , V [21] , Co [22] , Ti [23] , Cu [24][25][26] , Ni [27,28] , Zn [29] , Ga [30] , In [12] , Si [31] , Zr和Pd [32] 等元素替代过渡金属(Mn, Co)或主族元素(Ge)都被证明是有效的. Liu等人 [33] [34] 及Fe-Ga磁致伸缩材料 [35]…”