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We show that in metamagnetic shape memory alloys exhibiting a magnetostructural first order phase transition the direct transition from ferromagnetic austenite to nonmagnetic martensite is isothermal. In contrast to the direct transformation, the reverse one (nonmagnetic martensite–ferromagnetic austenite) is athermal, just as are athermal both direct and reverse martensitic transformations in conventional ferromagnetic shape memory alloys. The observed asymmetry of properties of the direct and reverse phase transitions in metamagnetic alloys, together with the data on entropy change during the magnetostructural transition, evidences that the magnetostructural transition is driven by the first order lattice modification. The change in magnetic ordering is an effect accompanying the lattice modification, opposing the direct transformation and promoting the reverse one. It has been shown that relaxation effects in metamagnetic shape memory alloys are intrinsic in the direct transformation itself and do not require the “arrest” of the transformation.

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