We studied the entropy change and the shift of the martensitic transformation temperatures with magnetic field in samples of a polycrystalline Ni–Co–Mn–In alloy having different degrees of long-range atomic order due to different heat treatments. We found, for the samples of the same composition, strong variations of the entropy change with the degree of atomic order, mediated by the difference between the Curie and martensitic transformation temperatures. Calculations of the field-induced shift of the transformation using data of entropy variations show good agreement with experimental results.

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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Application of a new method—high-sensitivity measurements of periodic stress-induced induction (reversible Villari effect, RVE)—allows us to uncover new effects in the low-field behaviour of different magnetic phases of polycrystalline Dy. A loss of magnetoelastic coupling (Villari critical point) is found around 166 K, close to the temperature which was supposed to separate the helical structure from the possible vortex one. We show that the low-field magnetic hysteresis emerges immediately below the Néel temperature and, below the Villari critical point, demonstrates a qualitative difference for cooling and heating from the ferromagnetic state. It has been found that, below the Villari critical point, in the helical phase, polycrystalline Dy is in essentially non-equilibrium state, revealed as a time-dependent relaxation of RVE. We relate the effects observed in the helical phase with thermal internal stresses and existence of lattice defects which inherit ferromagnetic order upon heating from the ferromagnetic state.