Research On The Magnetocaloric Effect And Magnetic Transition Of Mn-based Alloys

The magnetocaloric effect (MCE) is of great interest nowadays, especially the MCE near ambient temperature, for its potential application as an alternative for traditional refrigeration technology. In this big family, different kinds of materials have been studied, including first order transition (FOT) materials and second order transition (SOT) materials, and distinct properties have been observed in FOT and SOT materials. Generally, FOT materials provide big entropy changes with limited temperature spans as well as large hysteresis losses, while SOT materials reveal small entropy changes with wide temperature spans. Both of FOT and SOT materials are studied in our work.As a SOT material, Mn5Ge3is adopted in our research. A maximum entropy change of-4.92J/kg K and a refrigerant capacity of-201.3J/kg were observed at an external field change of30kOe. The critical exponents were obtained by a modified Arrott plots (MAPs) technique and the Kouvel-Fisher (KF) method. Finally,β=0.372±0.003and γ=1.05±0.01were determined. The exponent δ=3.88±0.02was obtained from the magnetization isotherm of Tc. These components fulfill the Widom scaling relation. Moreover, with these exponents, the magnetization curves collapse into two independent curves following a single equation around Tc. These results confirm the reliability of the obtained critical exponents. However, this set of exponents belongs to none of the classical theoretical models. This phenomenon is probably caused by the anisotropy of the Mn ions. The influence of Al adoption is also studied. Curie temperatures decrease from297K to283K as the Al content increases, and the peak entropy changes also decrease from-4.92J/kg K to-3.0J/kg K under an external magnetic field change of30kOe. Similar values of critical exponents β-0.37,γ-1and δ-3.8were obtained for all compositions of Mn5Ge3_xAlx. Moreover, rescaled entropy data collapse into a universal curve, revealing universal behavior for the magnetocaloric effect in this series of compounds.For FOT materials, we construct multiple transitions and tunable magneto structural transitions in the CoMnGe-NiMnGe system. Generally, MM’X (M, M’=transition metals, X=Si, Ge, Sn) compounds can be crystallized in two different structures, and each structure reveals a distinct magnetic phase. In this CoMnGe-NiMnGe system, three different magnetic phases are possible, including a TiNiSi-type FM-PM phase (CoMnGe), a TiNiSi-type AFM-PM phase (NiMnGe) and a Ni2In-type FM-PM phase, which indicate possible multiple magnetic transitions. On the other hand, tunable magneto structural transitions are also possible around the Curie-temperature window (TW) of the system as the existence of two different structures. Consequently, AFM-FM-FM(magnetostructural transition)-PMtransitions or AFM-FM-PM(magnetostructural transition) transitions were obtained, where giantmagnetic entropy changes were also observed around the magnetostructural transitions. What’s more,magnetic-field-induced transitions are also obtained according to the magnetization curves.