| Magnetic refrigeration, based on the magnetocaloric effect(MCE) of magnetic materials, is a new kind of cooling technology. Compared with the traditional gas compression refrigeration, magnetic refrigeration has been attracted much attention for the environment friendliness and energy efficiency. In many types of magnetic phase transformation alloys, Mn-based alloys have became one of the best candidate materials for the application of magnetic refrigeration around room temperature owing to the giant magnetocaloric effect, low-cost raw materials and simple preparation means. However, the related studies in association with ferrimagnetic phase transformation alloys are still very little compared with those of ferromagnetic materials up to date, though both belong to strong magnetism. In addition, as ferromagnetic transition materials, MnCoGe-based bulks are easily pulverized after several heating or magnetic cycles. Compared to bulks, ribbons with excellent magnetic properties are more stable, and they do not need or just need a short annealing time to form phase. In this paper, we focus on the preparation, the regulation of magnetic transformation and the related MCE for Mn2Sb-based bulks and MnCoGe-based ribbons.Mn2-xCoxSb(x=0.15, 0.20) and Mn2-xCrxSb0.95In0.05(x=0.05, 0.09, 0.13) alloys were prepared by arc-melting under high-purity argon atmosphere with base pressure of 10-4 Pa. All the samples reveal the tetragonal Cu2Sb-type structure with a small amount of MnSb second phase near room temperature. The substitution of Co/Cr for Mn results in the appearance of antiferromagnetic state at low temperature region. Therefore, the first order magnetic transition occurs between antiferromagnetic and ferrimagnetic states, which can be induced by temperature or magnetic field. With the increase of Co/Cr content, the transition temperature increases. Under an applied filed change of 0-10 kOe, the values of maximal magnetic entropy change for Mn2-xCoxSb(x=0.15, 0.20) are 3.4, 1.8 Jkg-1K-1, respectively by standard process methods. And the values of maximal magnetic entropy change for Mn2-xCrxSb0.95In0.05(x=0.05, 0.09, 0.13) are 0.95, 1.63, 1.29 Jkg-1K-1, respectively. Another interesting phenomenon is that one spin-reporientation transition and two spin-reporientation transitions in the ferrimagnetic region are observed in Mn2-xCoxSb, Mn2-xCrxSb0.95In0.05 alloys, respectively. In addition, almost no thermal hysteresis suggests the second-order character of spin-reporientation transition. The substitution of In for Sb has little effect on their magnetic properties in Mn1-xCrxSb1-yIny alloys, but introducing small amount of In(0≤y≤0.05) could effectively suppress the MnSb precipitation.Mn1-xCrxCoGe(x=0.01, 0.015, 0.02, 0.03) alloy ribbons were prepared successfully adopting melt spinning technique. Stoichiometric MnCoGe alloy is a collinear ferromagnet with the orthorhombic TiNiSi-type crystal structure at room temperature, the Curie temperature TC≈345 K, it will suffer a structure transformation from low-temperature orthorhombic TiNiSi-type structure to high-temperature hexagonal Ni2In-type structure at Tt≈650 K. The structure temperature was decreased effectively by the substitution of transition element Cr for Mn. A first-order magnetostructural transformation between TiNiSi-type ferromagnetic and Ni2In-type paramagnetic is achieved at 321 K, which accompanied with a sharp change of the magnetization in the annealed Mn0.98Cr0.02 CoGe ribbons. When the content of Cr increases to x=0.03, the magnetostructure coupling decouples. Under an applied filed change from 0 to 20 kOe, the values of maximal magnetic entropy change for annealed Mn0.98Cr0.02 CoGe ribbons near room temperature is 9.7 Jkg-1K-1, it is worth pointing out that the average magnetic hysteresis losse of this alloy is very small, being only 0.7% of the total refrigerant capacity, which is important for the practical application. |
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