Ferromagnetic Shape Memory

Ferromagnetic Shape Memory in Ni2MnGa

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
martensitic transformation.

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.

              Single crystal boule properties

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.

           Tetragonal variant structure
        and associated magnetizations

The figure below shows a schematic representation of the field-induced variant rearrangement process. Consider a rectangular bar specimen with <100> edges with two variants of martensite. A uniaxial layered magnetic substructure is present within each variant as shown at zero applied field. When a field is applied along the long axis of the bar, the variant with horizontal magnetization is favored and its volume fraction increases at the expense of the other variant. For large enough fields, the specimen may completely detwin, resulting in a large shape change as shown.

The micrograph below shows field-induced variant rearrangement occuring in a rectangular bar specimen with <100> edges like the one pictured above. The applied field is along one of the short <100> directions of the bar. The twinned microstructure is made visible using polarized microscopy on a polished (100) surface of the specimen.

left image: 6000 Oe
middle image: 10000 Oe
right image: 12000 Oe

The research presented here represents the first systematic effort to characterize the magnetic and magnetomechanical properties of both austenitic and martensitic phases of Ni2MnGa. This includes anisotropy and magnetostriction constants for both phases as well as characterization of the work output and blocking stress for a range of specimen geometries. The procedures developed here have been used to produce strains on the order 5% repeatably. Click below for details of the individual experiments:

Magnetic and Magnetomechanical Properties of Ni2MnGa

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