Most common metals and many ceramics are known as
"polycrystals," materials consisting of a collection of randomly
oriented crystals. When such materials are exposed to a magnetic
field, each individual grain moves in a different direction,
cancelling out the overall deformation, so that the alloy ends
up not moving at all. A single crystal of the new Ni-Mn-Ga alloy
deforms at a rate of approximately 10% when exposed to the force
of a magnetic field. The alloy retains its new shape but returns
to its original formation if the magnetic field is rotated by 90
degrees. This is what scientists refer to as "magnetic shape
memory".† While the single crystals morph 10%, the foam as a
whole moved by only 0.12%. ”It was very exciting because we went
from zero to an actual value, albeit small, but comparable to
the best competitor,” said Professor David Dunand from
Northwestern University. Prof. Dunand hopes to be able to
optimize the Ni-Mn-Ga foam through casting it differently or
perhaps by heat-treating it different