Ferromagnetic shape memory (FSM) materials
are a new class of active materials which combine
the properties of ferromagnetism with those of
a diffusionless, reversible martensitic transformation.
These materials have been the subject of recent study
due to the unusually large magnetostriction exhibited
in the martensitic phase. Our research has centered
on the ferromagnetic shape memory alloy Ni2MnGa,
which has a cubic Heusler structure in the high temperature
austenitic phase and undergoes a cubic-to-tetragonal
The Heusler lattice structure and a table with the material
properties of the crystal boule from which all experimental
specimens were cut are shown on the right.
In Ni2MnGa a cubic to tetragonal transformation occurs when the material is cooled below
a characteristic martensite start temperature Ms. In this transformation the cubic unit
cell is contracted along one <100> axis and extended along the other two, as shown in the figure on the right. Cubic symmetry
permits three possible tetragonal structures called variants to form, depending on which
axis contracts. A typical martensitic microstructure consists of mixtures of the three
variants in which two adjacent variants meet at one of two possible well-defined interfaces
called twin planes. While each of these variants has a unique orientation defined by its
c-axis, the martensitic phase is essentially a polycrystalline state composed of variable
volume fractions of the three variants.
The ferromagnetic shape memory (FSM) effect refers to either the reversible field-induced austenite to martensite transformation, or the rearrangement of martensitic variants by an applied field leading to an overall change of shape. Our research has concentrated on the large magnetostrictive strains due to the latter effect. Our experiments have demonstrated the ferromagnetic shape memory effect using procedures based on the assumption that each variant has a strong uniaxial magnetic anisotropy in which the easy axis is aligned with the c-axis (as shown in figure on right). From the twinning orientation relationship in Ni2MnGa it can be seen that easy axes of neighboring twin bands are nearly perpendicular to each other, so that a suitable pair of fields or a suitable arrangement of field and stress can be used to bias the material toward one variant of martensite or another, leading to a large change of shape.
left image: 6000 Oe
middle image: 10000 Oe
right image: 12000 Oe