Magnetic mosaics in crystalline tiles: The novel concept of polymagnets (invited)

This article presents our recent results on magnetic shape memory alloys (SMAs) and reviews some of our relevant past work. By treating the martensite structure as an assembly of microscopic crystalline tiles (twins), the observed micromagnetic structure or 'magnetic mosaic' in these alloys can be explained in terms of a spatially varying axis of the magnetocrystalline anisotropy and magnetostatic interactions across the tiles. A systematic investigation of ferroelastic structural domains of increasing structural complexity leads to a novel concept of 'polymagnets'. Similar to the idea of hierarchical ferroelastic polydomains, polymagnets are hierarchical micromagnetic structures, and parallels the complexity of the underlying ferroelastic polydomains. Preliminary results on the dynamics of the transformation pathways reveal a strong correlation between the underlying microstructure and achievable strain. In addition, reversible shape memory effect has been observed in sputter-deposited Ni-Mn-Ga thin films, and Co-Ni films with controlled composition have been successfully made by electrodeposition.