Shape memory ferromagnets

TL;DR

This paper discusses ferromagnetic alloys with shape memory, focusing on how magnetic fields can reversibly alter their structure and shape near room temperature, especially in Ni2+xMn1-xGa alloys, enabling significant strains.

Contribution

It introduces the potential for magnetic control of shape and size in ferromagnetic martensitic alloys near room temperature, highlighting Ni2+xMn1-xGa as a key example.

Findings

Reversible strains of about 6% achieved in Ni2+xMn1-xGa single crystals.

Magnetic fields of approximately 1 Tesla can induce shape changes.

High magnetocrystalline anisotropy facilitates domain wall movement and shape memory effects.

Abstract

In ferromagnetic alloys with shape memory large reversible strains can be obtained by rearranging the martensitic domain structure by a magnetic field. Magnetization through displacement of domain walls is possible in the presence of high magnetocrystalline anisotropy, when martensitic structure rearrangement is energetically favorable compared to the reorientation of magnetic moments. In ferromagnetic Heusler alloys Ni$_{2+x}$Mn$_{1-x}$Ga the Curie temperature exceeds the martensitic transformation temperature. The fact that these two temperatures are close to room temperature offers the possibility of magnetically controlling the shape and size of ferromagnets in the martensitic state. In Ni$_{2+x}$Mn$_{1-x}$Ga single crystals, a reversible strain of $\sim 6$% is obtained in fields of $\sim 1$ T.