What would be the Curie temperature of a perfectly two dimensional
magnet ? What would be the properties of an alternating stack of
superconducting material and magnetic material ? What would happen
if you could inject a spin into a two dimensional electron gas ?
Nature does not provide us with systems in which to study such
curiosity driven questions, so ways have been devised to make materials
in which the relevant phenomena can be studied. The layer-by-layer
growth of materials has yields a variety of new physical phenomena
ranging from the Quantum Hall Effect (QHE) and the Fractional QHE in
layered semiconductors to Giant Magnetoresistance in layered magnetic
materials. The "cleanest" way of performing such growth is by molecular
beam epitaxy illustrated in the figure on the left. By alternately
opening and
closing the shutters of different atomic sources, a new material whose
composition is changed atomic layer by atomic layer can be made. This
has brought the materials scientist's dream of manipulating materials at
will on the atomic scale an important step closer. Examples of devices
resulting from these activities are quantum-well semiconductor lasers,
heterojunction bipolar transistors and magnetoresistive sensors.
After the great success achieved with the epitaxial deposition of
semiconductors, multilayers comprising a repeated stack of a magnetic
metal (such as Fe, Co or Ni) alternating with a non-magnetic metal
(such as Cu or Pd) were investigated for possible applications as
magneto-optical recording media and to study low dimensional magnetic
phenomena. The result of this work was the discovery of perpendicular
magnetism and enhanced magnetooptical effects in novel layered materials,
of oscillatory exchange coupling as a function of the thickness of a
layer of spacer materials separating two magnetic films and the
giant magnetoresistance in similar layered materials.
The topics of interest in magnetic multilayers are:
Magnetic Anisotropy Energy
Oscillatory Exchange Coupling
Giant Magnetoresistance