Hysteresis in
Magnetic Materials
Hysteresis in Magnetic Materials |
 Candice
A. Viddal
Roy M. Roshko Department of Physics and Astronomy 303 Allen Building University of Manitoba Winnipeg, Manitoba, Canada |
 Hysteresis
is perhaps the most distinctive and widely
recognized experimental signature
of magnetism. A
material with hysteresis is a material with persistent memory, and
its magnetic
response is path dependent, in the sense
that the magnetic moment
M(t) of the material at time t depends not only
on the current value of the excitation field H(t), but on the
complete history of the field
H(t') at all previous times t'< t. One of the experimental
fingerprints of
this memory is the hysteresis loop,
illustrated on the upper right, for a
thin film of nanodimensional particles of Fe embedded in Al2O3.
The loop traces the response M of the material to variations in the
applied field Ha,
starting from a demagnetized
initial state (Ha=0,M=0), and then proceeding to positive
saturation (Ha~4000 Oe), and then to
negative saturation, and back to positive saturation. Hysteresis in the
field dependence of the response is
always accompanied by thermal hysteresis, which is typically observed
as a bifurcation of the temperature dependence of the field cooled (FC)
and zero field cooled (ZFC) moment into two branches,
as illustrated on the lower
right for the same system of Fe nanoparticles in Al2O3.Hysteresis is a property which is of considerable interest from a technological perspective, since essentially all practical applications of magnetism, from the hard permanent magnets in electric motors to the soft magnets in transformer cores, from electronic devices to information storage and recording technology, exploit some aspect of the history-dependence of the magnetic response. However, from a fundamental perspective, hysteresis is also a fascinating and complex physical and mathematical problem, for which there is currently no convincing general interpretive theoretical framework. |
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