Magnetostriction of martensite: new materials that combine shape-memory and ferromagnetism

Dick James (Minnesota)


Magnetostriction is the spontaneous deformation of a solid caused by a change of its state of magnetization. The speaker will describe recent research on a new class of magnetostrictive materials: these combine ferromagnetism and shape-memory. In these materials a martensitic phase transformation leaves a mobile microstructure of twin bands that is subsequently redistributed by the field, or else, the field induces an austenite-to-martensite phase transformation. The existence of this class of "ferromagnetic shape memory" materials was evident from theoretical research in the early 1990s on the behavior of the giant magnetostrictive material $Tb_{0.3}Dy_{0.7}Fe_2$. I shall describe the evidence, together with recent work on a "constrained theory of magnetostriction" that is guiding the search for new ferromagnetic shape memory materials. This theory involves, in a crucial way, modern concepts in the mathematical modeling of materials. Recently, interesting examples of ferromagnetic shape memory materials have been found in the $FePd$ and $NiMnGa$ systems which exhibit, under moderate fields, magnetostrictive strains that are 50 times that of the best giant magnetostrictive materials. Further improvements await the solution of a critical scientific question which I describe. Future directions of research are indicated: new systems of interest, thin film research, applications to MEMS and biomedicine.

[Newton Institute] [Seminars on the Web] [Monday Seminars] [James, 1999-11-22] abstract.html

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