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.