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Thayer Watkins Silicon Valley USA |
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Source: L.M. Falicov,
"Metallic Magnetic Superlattices,"
Physics Today, (October 1992), pp. 46-51
Metallic magnetic superlattices (also known as multilayers) are
alternating thin layers of magnetic and nonmagnetic metals constructed
with precise dimensions and composition. They have interesting properties
with respect to resistance and magneto-optics. The magnetic layers
may have their direction of magnetization parallel (ferromagnetic coupling
or antiparallel (antiferromagnetic coupling). The nature of the
coupling between the magnetic layers depends upon the thickness and
nature of the nonmagnetic spacers between the magnetic layers. For very
thin nonmagnetic layers the natural arrangement is antiferromagnetic
coupling. This is related to the tendency for a ferromagnetic material
to partition itself into domains with antiparallel orientation of the
directions of magnetization. It is energetically advantageous for a
ferromagnetic material to divide into such domains. But there is a
natural unit of length for such domains and if the thickness of the
nonmagnetic spacer layers gets to twice the natural width of the
ferromagnetic domains the coupling of the magnetic layers will be
parallel rather than antiparallel.
The nature of the coupling of the magnetic layers affects resistance
as well as other properties of the metallic superlattice. Thus the
magnetic arrangement or coupling can be monitored by several means
including:
Falicov cites the creation of a wedge-shaped spacer with a
very low wedge angle as an important technical achievement. The
slope of the wedge is between 10nm per mm and 40 nm per mm or
1x10-5 and 4x10-5. The reciprocal of this
slope is the ratio of the accuracy required in the horizontal
dimension to achieve a given accuracy in the vertical dimension;
i.e., 105 to 2.5x104.