Speaker
Description
In my presentation I will review the latest progress in understanding elastomagnetic multiferroic behavior of Ni-Mn-Ga Heusler alloys called magnetic shape memory effects [1]. These phenomena are underlined by displacive and diffusionless phase transformation from cubic to lower symmetry phase called martensitic transformation. Apart from temperature the transformation can be induced by magnetic field, and this usually provides large field induced stress, but the main merit is for magnetocaloric applications. The field induced transformation is not strictly multiferroic as only magnetization difference between phases is needed. The effect usually requires large magnetic field of order several Teslas.
The other effect is magnetically induced (structural) reorientation (MIR) providing giant, up to $12\%$ deformation in moderate magnetic field. The effect takes place in single ferromagnetic phase after martensitic transformation establishing ferroelastic order. Coupling between ferromagnetic and ferroelastic order is provided by large magnetocrystalline anisotropy. Together with high mobility of twin interfaces or very low twinning stress compared to other shape memory alloys these provide necessary conditions for the effect.
Here I will focus on new recent discoveries concerning MIR in Ni-Mn-Ga. I will examine material demands; in particular the partial doping of Ni-Mn-Ga by transition metals in attempt to increase transformation temperatures, the twin hierarchy from macro to nanoscale important for twin mobility, and recently discovered extreme shear instability of the lattice, which may be a key for the supermobility of twin boundaries [2]. Moreover, the nature of antiphase boundaries in ordered Heusler alloys, which can enhance the functionalities of the material, will be discussed. All these characteristics observed experimentally will be put to the perspective of theoretical calculation which is still regrettably not to date for such interesting materials [3]. From experimental point of view the phenomenology of the effect is well known, however, the underlining physical principles and the role of magnetism in the existence of modulated structures is not clear.
Acknowledgements
We thank for support OP JAK Ferroic Multifunctionalities No. CZ.02.01.01/00/22_008/0004591 and GAČR No. 23-04806S projects.
References
[1] O. Heczko et al., “Coupling between ferromagnetic and ferroelastic transitions and ordering in Heusler alloys produces new multifunctionality,” MRS Bulletin, vol. 47, no. 6. Springer Science and Business Media LLC, pp. 618–627, Jun. 2022. https://doi.org/10.1557/s43577-022-00354-x
[2] K. Repček et al., “Compliant Lattice Modulations Enable Anomalous Elasticity in Ni–Mn–Ga Martensite,” Advanced Materials. Wiley, Aug. 12, 2024. https://doi.org/10.1002/adma.202406672
[3] H. Seiner et al., “Experimental Observations versus First‐Principles Calculations for Ni–Mn–Ga Ferromagnetic Shape Memory Alloys: A Review,” physica status solidi (RRL) – Rapid Research Letters, vol. 16, no. 6. Wiley, Mar. 30, 2022. https://doi.org/10.1002/pssr.202100632
