 |
 |
|||||||||||
 |
 |
 |
|
 |
|
 |
|
 |
|
 |
|
 |
|
||||||||||||||||||||||||||
| Domain Walls and Twin Boundaries in Ferromagnetic Shape Memory Alloys [NSF] | |
Alloys of the type Ni2MnGa exhibit a phenomenon known as ferromagnetic shape memory, meaning that they can change shape under an applied magnetic field. These alloys belong to the class of the multiferroic materials, which can respond to multiple externally applied fields. Upon cooling from the melt, this alloy undergoes two ordering transformations (disordered to B2 and B2 to L21, also known as Heusler ordering), a paramagnetic to ferromagnetic transition at around 380 K, and a martensitic transformation from cubic to tetragonal or orthorhombic at a temperature that depends sensitively upon the alloy composition.  The crystal structures of the cubic (left) and tetragonal (right) phases are shown in the figure on the left; Ni atoms are green, Mn red, and Ga blue. The arrows indicate the atomic magnetic moments, with a dominant contribution from the Mn atoms. The unit cell parameter is about 0.56 nm for the cubic phase, and the tetragonal phase, which has a large magnetocrystalline anisotropy, has a c/a ratio of about 0.94. |
|
|
In this project, we employ Lorentz transmission electron microscopy (LTEM) to study the nature of magnetic domain walls, and the interactions of those walls with other microstructural features, such as Recently, we have added Fe-Pd alloys to our systems of interest. The images below show examples of magnetic domain configurations in these materials. We are currently working on models and image interpretations...
|
|
Recent Publications:
|
anti-phase boundaries and martensite variant boundaries (twin boundaries). The image to the right shows a magnetization map of an austenitic sample (at room temperature); the width of the image corresponds to a distance of about 4 microns. The colors indicate the direction of the local magnetization vector: red indicates a magnetization pointing from left to right, green from right to left, yellow from top to bottom, and blue from bottom to top. The curved boundaries between the colored regions correspond to anti-phase boundaries, i.e., defects in the lattice ordering of this alloy. The interactions between these anti-phase boundaries and the magnetic domain walls affect the magnetic coercivity of this alloy, and form the subject of intense micro-structural investigations.
