Ti substitution in MnBi rare-earth-free magnetocaloric compound.

The MnBi compound exhibits a first order magneto-structural phase transition around 630 K. This material could be suitable as a rare-earth-free magnetocaloric material provided that one can shift the transition temperature towards room temperature. In this work the effect of a partial substitution of Mn with Ti is investigated. In particular we study the transformation kinetics of the magneto-structural transition, in samples prepared with different techniques: powder metallurgy and rapid quenching from the melt. The samples show a logarithmic dependence of the transition temperature on the temperature scan rate upon cooling. This suggests that the transition occurs with the help of thermal fluctuations. The linear size of the activation volume v0 of these thermally activated processes is found to pass from 2.5 nm in MnBi to around 1.8 nm in Ti substituted samples.

MnBi化合物在630 K左右发生一级磁结构相变。若能将其相变温度调整至室温附近，该材料有望成为一种无稀土磁热材料（magnetocaloric material）。本研究探讨了Ti部分取代Mn的影响，具体考察了采用不同制备工艺（粉末冶金法与熔体快淬法）的样品中磁结构相变的转变动力学。样品在冷却过程中，相变温度与温度扫描速率呈对数依赖关系，这表明相变是在热涨落的辅助下发生的。研究发现，这些热激活过程的激活体积v0的线性尺寸从MnBi中的2.5 nm降至Ti取代样品中的约1.8 nm。