5 Spintronics and Synchrotron Radiation
135
trilayered structures with two distinct coercive fields, later called spin valves. In
these structures, one of the layers switches using low fields while the second is stable
up to large fields. It is to be emphasized that the time between the first discovery
in a lab and the use of the GMR effect in cutting-edge technology devices has been
extremely fast as less than 10 years after the discovery of GMR, and IBM introduced
in 1997 spin valves in read heads of hard-disk drives. GMR-based magnetic sensors
is now used in a multitude of applications such as monitoring wheel speed, detecting
charge current, and fluid flow.
The research on magnetic multilayers and GMR became rapidly a very hot topic.
It is not our purpose here to make here a review of all experimental and theoretical
results that followed up the pioneer results. A complete review can be consulted
in [8].
5.1.1.3 A Simple Model to Describe the GMR
The two-current model, described previously, was developed to explain the spindependent resistivity in magnetic materials. It can, in a rather simple way, be adapted
to describe the giant magnetoresistive effect in these magnetic multilayers. This
model is based on two assumptions: 1) α > 1: the minority-spin electrons (opposed
to local magnetization) are more scattered than those of majority spin (aligned with
local magnetization); 2) the spin is conserved when the electrons are scattered. These
two conditions are fulfilled at low temperature.
Two geometries can be considered to evaluate the resistance of such a structure:
either the current flows in the direction of the layer planes (known as "current-inplane GMR", CIP-GMR), or the current flows in a direction perpendicular to the
layer plane (known as "current-perpendicular-to-the-plane GMR", CPP-GMR). A
similar description can be used to account for the magnetoresistive properties for
both geometries, without entering into the fine discussion about the actual physical
spin-transport mechanisms, as long as the layer thickness remains small compared
to a characteristic length associated with each geometry: the mean free path for the
CIP case and the spin-flip length for the CCP case [11].
Let us note r the resistance of a FM layer for the majority-spin channel and R the
resistance for the minority-spin channel, with r < R. Here the resistance of the NM
separating layer is neglected. As illustrated in Fig. 5.4, an electron will have to pass
through adjacent FM layers. Depending on whether these layers have parallel (P) or
anti-parallel (AP) magnetizations, the resulting resistance shall differ.
In the (P) configuration, the electrons with spin (↑) and (↓) are either the electron
spin majority or minority in all magnetic layers. Spin (↑) electrons then experience identical resistances r ↑ = r when crossing each magnetic layer, while spin (↓)
electrons experience identical resistances r ↓ = R. The mean resistance then writes
r P =
Rr
r +R
. In case of materials with large spin asymmetry (α 1 and r R), the
multilayer is short-biased by the spin (↑) channel and the total resistance is r P ≈ r .
For the (AP) configuration, the electrons of the two (↑) and (↓) channels correspond alternately to electron spin majority and minority in consecutive magnetic
135
trilayered structures with two distinct coercive fields, later called spin valves. In
these structures, one of the layers switches using low fields while the second is stable
up to large fields. It is to be emphasized that the time between the first discovery
in a lab and the use of the GMR effect in cutting-edge technology devices has been
extremely fast as less than 10 years after the discovery of GMR, and IBM introduced
in 1997 spin valves in read heads of hard-disk drives. GMR-based magnetic sensors
is now used in a multitude of applications such as monitoring wheel speed, detecting
charge current, and fluid flow.
The research on magnetic multilayers and GMR became rapidly a very hot topic.
It is not our purpose here to make here a review of all experimental and theoretical
results that followed up the pioneer results. A complete review can be consulted
in [8].
5.1.1.3 A Simple Model to Describe the GMR
The two-current model, described previously, was developed to explain the spindependent resistivity in magnetic materials. It can, in a rather simple way, be adapted
to describe the giant magnetoresistive effect in these magnetic multilayers. This
model is based on two assumptions: 1) α > 1: the minority-spin electrons (opposed
to local magnetization) are more scattered than those of majority spin (aligned with
local magnetization); 2) the spin is conserved when the electrons are scattered. These
two conditions are fulfilled at low temperature.
Two geometries can be considered to evaluate the resistance of such a structure:
either the current flows in the direction of the layer planes (known as "current-inplane GMR", CIP-GMR), or the current flows in a direction perpendicular to the
layer plane (known as "current-perpendicular-to-the-plane GMR", CPP-GMR). A
similar description can be used to account for the magnetoresistive properties for
both geometries, without entering into the fine discussion about the actual physical
spin-transport mechanisms, as long as the layer thickness remains small compared
to a characteristic length associated with each geometry: the mean free path for the
CIP case and the spin-flip length for the CCP case [11].
Let us note r the resistance of a FM layer for the majority-spin channel and R the
resistance for the minority-spin channel, with r < R. Here the resistance of the NM
separating layer is neglected. As illustrated in Fig. 5.4, an electron will have to pass
through adjacent FM layers. Depending on whether these layers have parallel (P) or
anti-parallel (AP) magnetizations, the resulting resistance shall differ.
In the (P) configuration, the electrons with spin (↑) and (↓) are either the electron
spin majority or minority in all magnetic layers. Spin (↑) electrons then experience identical resistances r ↑ = r when crossing each magnetic layer, while spin (↓)
electrons experience identical resistances r ↓ = R. The mean resistance then writes
r P =
Rr
r +R
. In case of materials with large spin asymmetry (α 1 and r R), the
multilayer is short-biased by the spin (↑) channel and the total resistance is r P ≈ r .
For the (AP) configuration, the electrons of the two (↑) and (↓) channels correspond alternately to electron spin majority and minority in consecutive magnetic