Bidirectional Regulation Of Magneto-structural Transformation And Magnetocaloric Effect In MnCoGe-Based Alloys

The MM'X alloys will undergo a martensitic structural transition from the high-temperature hexagonal Ni₂In phase to the low-temperature orthogonal TiNiSi phase.Tuning the structural transition into the temperature interval between the Curie temperatures of hexagonal and orthorhombic phases in MM'X alloys can acquire coincident magneto-structural transformation with abundant physical effects due to a large change in magnetization.At present,the first-order magneto-structural transformation of MM'X alloys can be obtained by adjusting chemical composition,heat-treatment,and applying hydrostatic pressure.But most researchers just focus on the unidirectional regulation and propose various physical mechanisms to explain their results.Based on this,the magnetic and structural transitions have been controlled to realize the magnetostructural transformation via designing the appropriate composition by bidirectionally adjusting the element ratio in MnCoGe-based alloys,which aims to arrive at a uniform explanation.In addition,due to large volume change across the martensitic transition,the extremely poor mechanical properties of the MM'X alloy severely limit the research and application of these alloys.Taking MnNiGe-based alloys as the research object,the improvement of mechanical properties in MM'X alloys has been studied by using the hot-pressing method with low melting point metals.Firstly,magneto-structural transformation with an increased magnetization difference is simultaneously realized through bidirectionally controlling the ratio of Mn/Ge in MnCoGe-based alloys.The magnetic-field-induced metamagnetic martensitic transformation is observed in Mn_0.98CoGe_1.02 alloy,which resulted in a huge magnetocaloric effect at room temperature,and the-?35?S_M,max,max under the magnetic field chage of?₀?35?H=0-5 T is³1.60 Jkg^-1K^-1.The combined effect of e/a and chemical pressure explains the physical mechanism of transition regulation in both directions.Secondly,magneto-structural transformation is obtained through bidirectionally controlling the ratio of Co/Ge in MnCoGe-based alloys.The Co₂MnGe secondary phase appears in the Co-rich system MnCo_1+yGe_1-y,which makes Co-vacancy exist in both two systems.It is the instability of the Co-Ge hexatomic ring due to Co-vacancies accounts for the achievement of magneto-structural transformation in both systems.Likewise,large magnetocaloric effect around room temperature has also been obtained in both systems.Finally,the preparation of Mn_0.84Fe_0.16NiGe/Sn composite is proposed to solve the fragile problem in MM'X alloys by using the hot-pressing method with low melting point metals.It is found that the composite is dense,showing a metallic luster on the surface and exhibiting good processability and corrosion resistance.A large linear thermal expansion coefficient a_L^-164.7ppm/K is obtained during the negative thermal expansion temperature range?35?T=76 K?124-200K?in Mn_0.84Fe_0.16NiGe/Sn composite.At the same time,a maximum magnetostrictive value reaches up to⁶28 ppm at 185 K under?₀H=0-7 T.To conclude,the first-order magneto-structural transformation has been realized and accordingly large magnetocaloric effect near room temperature by bidirectionally controlling the element ratio in the MnCoGe-based alloys.The unified mechanism has been given for Mn/Ge-and Co/Ge-adjusting systems,respectively.A theoretical basis is provided to expand the research on the magneto-structural transformation and associated physical properties in MM'X alloys.Dense MnNiGe-based alloys bulk composites have been prepared by using the hot-pressing method with low melting point metals,which effectively enhances the mechanical properties of MM'X alloys and expands the related researches.