Elastic multiferroics - Magnetic shape memory phenomena


Here we focus on the multiferroic materials combining ferromagnetic and ferroelastic ordering. Such material can exhibit new kinds of functional behavior as e.g. colossal strains up to 12% percent in relatively modest magnetic field. They are sometimes described as magnetic muscles driven by magnetic field. The response in field is conditioned by the coupling of strong magnetic anisotropy with ferroelastic twinned martensite microstructure. Thorough research of magnetic properties and martensite structures is crucial for the understanding the effect and its further development. The most of these compounds belong to specific group of Heusler alloys in which Ni-Mn-Ga alloys are the best prototype material.

In 2016 we made an important finding that the basic structure of Ni-Mn-Ga can become nanotwinned near martensitic transformation, with the size of individual twins being less than 20 nm. Our finding fills the knowledge gap and scale between the previously investigated adaptive concept with size of twins being about 0.4-1.4 nm and a better known microscopic and macroscopic twinning with the size of twins between 20 nm up to milimeters. Later on in 2017 we found the nanotwining also in martensite at low temperatures and identified a region in phase diagram where it occurs. The nanotwinning is of large relevance for proper understanding of the true structure of Ni-Mn-Ga magnetic shape memory alloys and consequently of their unique properties and the existence of magnetic shape memory effect. Previously the structure of Ni-Mn-Ga was described as incommensurate based on the specific features in diffraction patterns. Our new hypothesis which will be further investigated is that the specific features in diffraction are actually caused by nanotwinning; theoretical calculations to test the hypothesis are now in progress.

In connection with it we were successful to directly observed a/b laminate using painstaking SEM observation. Developed method helps on turn to understand specific changes due to nanotwinning. Moreover, thanks to expertise in observing magnetic domain walls in steels the magnetic domains in a magnetic shape-memory Ni-Mn-Ga alloy were for the first time observed by magneto-optical Kerr microscopy using monochromatic blue LED light. The domains were observed for both single- and multivariant ferroelastic states of modulated martensite and confirm the indirect observation. The simultaneous observation of magnetic domains and ferroelastic domains (twins) opens new venue for investigation of mutual interactions between ferromagnetic and ferroelastic ordering.

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