Defense Intelligence Reference Document Metallic Spintronics

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studies (Reference 30} on sputtered cobalt-copper (Co/Cu) multilayers revealed
magnetoresistances at room temperatures 3 to 4 times larger than those for iron-
chromium and 13 times greater than those for the permalloy films that were used as
magnetoresistive sensors in magnetic reading heads at that time. The much higher
numbers observed in magnetic multilayers predetermined the fate of GMR in magnetic
recording technology.
The current understanding is that GMR
observed in magnetic multilayers arises
from the dependence of the resistivity
on their internal magnetic configuration
and the role of the external magnetic
field to change this configuration. Figure
2b illustrates GMR in the simple limit
where the electron mean-free-path is
much longer than the layer thicknesses.
The electrical transport properties of the
system are described in terms of the so-
called two-current model (Reference
31), based on the suggestion by Mott
(Reference 32) that, at temperatures
lower than the Curie temperature, the
spin-up and spin-down electrons will be
almost independent and carry current in
parallel. Electrons are much more
strongly scattered by a magnetic layer if
they and the local magnetization spin in
opposite rather than the same direction
(R > r). For simplicity, the figure is
drawn with scattering only at interfaces;
however, there is also scattering within
the layers. At zero magnetic field,
where the magnetizations of adjacent
magnetic layers are aligned
antiparallel-for example, because of
exchange coupling between the layers
(Reference 29)-the spin-down
electrons are weakly scattered in layer
F1 F2
f t ....
..
(i '
Magnetic Field
F1 F2
I t
----· .......... _
. ►
F1 F2
I __t
Figure 2. (a} Resistance of a magnetic multilayer R
versus magnetic field. {b) Origin of GMR in terms of
spin-dependent electron scattering: Fl and F2 are
ferromagnetic layers with a nonmagnetic layer in
between. At zero magnetic field, the magnetizations
in Fl and F2 are aligned antiparallel (center panel)
and can be switched to parallel orientation by an
applied field. (c) The equivalent resistance circuits
corresponding to the three magnetic configurations
shown in (b). See text for details.
Fl but strongly scattered in F2. In contrast, the spin-up electrons are weakly scattered
in layer F2 but strongly scattered in Fl. As a result, two channels are equivalent,
leading to a total resistance in this "antiferromagnetic" configuration RAF= (R+r)/2 (see
the corresponding resistance circuit in Figure 2c).
When the magnetizations of the two F layers are set into parallel configuration by an
applied magnetic field1 the spin-up electrons are weakly scattered in both layers and
form a low-resistivity channel, whereas the spin-down electrons are strongly scattered
in all the layers and form a high-resistivity channel. The reversal of magnetic field just
interchanges the roles of spin-up and spin-down channels. The current's shunting by
the low-resistivity channel produces a low total resistance RF;;; 2Rr/(R+r) in this
"ferromagnetic" configuration. The size of the GMR is defined as (RAF-RF}/RAF = (R-
2
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