During the 1990-91 program at the workshop on Microstructure
and Phase Transition, a great deal of interest was generated by
a
series of micrographs presented Donald Lord of Ford Motor
Company.
The micrographs display beautiful and mysterious domain
patterns in
the alloy Terfenol, which had been developed in the 1950's and
found to
exhibit magnetostriction, i.e., change of shape in response to
a magnetic
field.
The micrographs captured the attention of workshop participants
R. James and D. Kinderlehrer, who began working on mathematical
models
for magnetostrictive materials that explained the
observations. They
noticed that there was excellent agreement with the experiment
results if
they assumed a kind of free energy that was then emerging in
models
of other types of phase transformations, namely a free energy
with
symmetry—related "energy wells," combined with terms
accounting for
magnetostatic energies. This insight immediately suggested a
further
step: if a material could be found which was both martensitic
and also
ferromagnetic properties, the theory indicated that it could be
made to
undergo much larger field-induced-strains than Terfenol. Their
theory
directly led, in the mid-1990s, to the discovery of a new
family of
alloys of nickel, magnesium, and gallium, which are now termed
ferromagnetic shape memory materials or FMSAs. A decade
later these
materials have been developed to the point of exhibiting
magnetic
field-induced strains 100 times those of Terfenol, so that a
magnetic
field of less than 1 Tesla can induces shape changes of up to
10%.
An industry has grown around the production and use of such
FMSAs.
Many applications involve FMSAs as actuators in compact
precision
engineered instruments such as valves and pumps, for example in
medical
device applications, for latches, motors, and sonar
transducers.
FSMAs are also used in sensing, for example to detect magnetic
fields
or vibrations, and for passive energy absorption. Research in
the field
remains very active with entire conferences devoted to their
study.
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