Temperature dependence of the giant magnetostrain in a NiMnGa magnetic shape memory alloy

The temperature dependence of the magnetostrain was investigated in the $Ni50Mn27.5Ga22.5$ magnetic shape memory alloy with a five-layer martensitic (5M) structure in the temperature range from $110Kto300K$⁠. A temperature threshold at $166K$ was found for the magnetostrain. A giant magnetostrain of 6.3% was achieved above the temperature, while no magnetostrain was monitored below the temperature. No intermartensitic transformation was detected around the temperature threshold. The lattice parameter $a$ slightly increases, $c$ largely decreases, and the tetragonality $(a∕c−1)$ drastically increases with decreasing the temperature. The increase of the tetragonality is thought to be related to the temperature threshold of the magnetostrain by inducing a change of the electronic structure, twin structure, or the type of the variant with the same 5M martensitic structure below the temperature threshold. The interpretation is reasonably understood by the fact that only few samples with the same 5M martensitic structure exhibit a giant magnetostrain.