The bright and dark areas can be regarded at the north and south poles of these nano-magnets. islands with a single distinct bright/dark pair
are known as single domains, while those that have more complex structures are commonly referred to
as "multidomains". The islands have dimensions of 200nm x 400nm x 20nm. As such, the magnetization
lies predominantly on the film plane. Furthermore, because
the islands are twice as long as they are wide, the "easy" axis (or the preferred axis where the
magnetization would orient) is more or less parallel to the long dimension (horizontal). At H=0, most of the islands are oriented nominally
in N-S direction (dark-bright) while a few are oriented in the opposite sense.
The apparent dispersion of the easy axis about the horizontal axis is
due to the crystalline anisotropy of the grains the make up the individual islands.
This has been predicted by theory. [1].
Islands which are nominally aligned with the field exhibit gradual rotation of the moment.
(Best seen is topmost left island.)
Single domain islands switch at specific field. The fact that the switching event is
observed is because the tip induces the event. Typically, the reversal occurs when the tip
is roughly on the middle of the island.
Multidomain islands show multistep switching, i.e., different subdomains switch at different
fields.
In this case, each island can be seen as having a square hysteresis loop, with well defined
coercivity transition fields. As an aggregate however, the transition field broadens due to the
variance in the switching fields of the individual islands. The normal notion of "domain wall motion" does not apply
to this system.
(1) R.M.H. New, R.F.W. Pease, R.L. White, J. Appl. Phys. 79, 5851 (1996).
For in depth reading, see: R.D. Gomez, M.C. Shih, R.M.H. New, R.F.W. Pease
and R.L. White,"Switching characteristics of sub-micron cobalt islands", J. Appl. Phys.
80, 6441-6446 (1996); R.L. White, Data Storage Magazine Sept. 1997.
Also checkout
Permalloy Animation to see domain wall motion.
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