Biomedical Engineering Reference
In-Depth Information
Fig. 7.1
Electrons and
corresponding scattering
events as they pass through a
GMR multilayer film stack
with associated circuit model:
(
a
) antiparallel state
(
b
) parallel state
upper layer causing it to align with the external field, minimizing the total energy of
the system.
Qualitatively, the operation of the device can be understood by examining the two
extreme cases. In the antiparallel state, as an electron in a spin-up state (designated
with an arrow pointing to the right) passes through the film stack, it will scatter
when it travels through each ferromagnetic layer (Fig.
7.1
a). As it travels through
the first ferromagnetic layer, the scattering is relatively small and leads to a low
resistance since the spin of the electron is in the same direction as the majority
spin of this layer. As the electron continues into the second ferromagnetic layer
of the opposite magnetization, it will again scatter. This scattering event, however,
is relatively large and leads to a higher resistance because the spin of the electron
is in the same direction as the minority spin of this layer. The electron in a spin-
down state (designated with an arrow pointing to the left), traveling through a GMR
sensor in the antiparallel state, will have a similar resistance to the spin-up electron
except in a reversed sequence, where the first layer it travels through is of high
resistance and the second layer it travels through is of low resistance. In contrast, in
the parallel state, the electron in the spin-up state passes through the first layer and
the second layer with relatively few scattering events and thus has a low resistance
in its entire path because the spin of the electron is always in the same direction as
the majority spin of the layers. The electron in the spin-down state passes through
both layers with relatively high resistance because the spin of the electron is always
in the same direction as the minority spin of the layers. The overall resistance in
each state can be understood using a circuit model where the resistance of the path
taken depends on the spin polarization of the electron. For the antiparallel state,
each path has a high resistance in series with a low resistance. In the other extreme
where a large external magnetic field has caused the two layers to be in the parallel
state, the spin-up electron will pass through the structure with minimal scattering.
The spin-down electron will undergo significantly more scattering in both layers and
thus have a higher resistance as seen in the equivalent circuit model (Fig.
7.1
b). The
parallel state has two paths: one with two low resistances in series and one with two
high resistances in series. If the distribution of conducting electron spins is equal in
spin-up and spin-down states, the circuit in the parallel state has an overall lower
resistance than that in the antiparallel state.
