Curie temperature effect on the inverse MCE in Ni2MnIn based Heusler alloys near room temperature.

We investigate the magneto caloric properties in Ni2MnIn based Heusler alloys having the martensitic to austenitic transition temperatures close to the Curie temperature of the austenitic phase. We tuned the structural transition by composition in order to encounter three possible cases as a function of the increasing temperature: i) the martensite to austenite transition completes before the drop of magnetization associated to the Curie temperature of the austenite, ii) the martensite to austenite transition evolves across the Curie temperature of the austenite and iii) the martensite to austenite transition lies almost completely above the Curie transition of the austenite. When the martensitic transition temperature overlaps with the Curie temperature of the austenite we observe, at constant temperature, the coexistence of direct and inverse magneto caloric effects. In order to understand this behaviour we design a layered structure composed by a 1st order inverse MC material and a 2nd order direct MC material. In this structure, part of the heat exchanged by the direct MCE cooperates with the magnetic field to drive the structural transition further.

本研究针对Ni₂MnIn基赫斯勒合金（Heusler alloys）的磁热性能开展系统探究，这类合金的马氏体-奥氏体转变温度与奥氏体相居里温度（Curie temperature）较为接近。我们通过成分调控实现结构转变的精准定制，以此得到随温度升高时的三类典型场景：i) 马氏体-奥氏体转变在对应奥氏体相居里温度的磁化强度下降之前完成；ii) 马氏体-奥氏体转变恰好跨越奥氏体相的居里温度；iii) 马氏体-奥氏体转变几乎完全高于奥氏体相居里温度。当马氏体转变温度与奥氏体相居里温度重合时，我们在恒温条件下观测到正磁热效应与反磁热效应的共存现象。为阐明该现象的内在机制，我们设计了一种层状复合结构，该结构由一级相变反磁热材料（inverse magnetocaloric material）与二级相变正磁热材料（direct magnetocaloric material）构成。在此结构中，正磁热效应（Magnetocaloric Effect, MCE）所交换的部分热量可与外磁场协同作用，进一步推动结构转变的进程。