3.5 Physical Origin of GMR: Qualitative Explanation
87
the electrodes are positioned on one side of the device structure and the current is
supposed to flow along the layers (Fig. 3.9b). The GMR results from CPP geometry
have been found to be more than double that of CIP configuration. However, CPP is
more difficult to understand in practice.
To summarize, the following considerations can be made:
(1) Since the ↑ and ↓ spin channels are independent, they can be regarded as two
parallelly connecting wires.
(2) Additionally, the majority and minority spin electrons (already discussed in
Chap. 2), during their passage through a ferromagnetic layer, are scattered
in different probabilities. However, the spin orientations of electrons would
not change in this case. This phenomenon is referred to as spin-dependent
scattering.
3.5.1 What Is Spin-Dependent and Spin-Flip Scattering
of Electrons in Multilayers?
Before going into the details of origin of GMR, first we will discuss various categories
of scatterings that electrons might suffer in magnetic multilayers. In our approach to
understand the phenomenon considering Boltzmann equation, let us mainly suppose
the elastic, i.e., energy conserved scattering, where in each scattering process electrons undergo change of its direction of propagation only. However, in real situation,
the electrons may experience different types of scattering, which might flip the spin
of electrons from up to down and vice versa. This type of scattering is generally
referred to as spin-flip scattering. Now, distinction between spin-dependent scattering causing GMR and spin-flip scattering that plays detrimental role to GMR
needs to be discussed to understand GMR properly. Such scattering processes are
illustrated in Fig. 3.10.
Let us first give simple definition of two scattering processes:
Spin-dependent scattering: In spin-dependent scattering phenomenon, during
each scattering event suffered by electrons, the scattering probabilities of ↑ and ↓
spin electrons are different. Noteworthy, in this scattering process, the orientation of
the electrons’ spin should be conserved.
Spin-flip scattering: In spin-flip scattering phenomenon, during each scattering
event suffered by electrons, there is change of spin orientation of electrons from ↑
s z =
2
to ↓
s z = −
2
or vice versa.
Now, let us discuss the scattering processes one by one.
I. Spin-dependent Scattering
As already discussed, the key features of spin-dependent scattering process, encountered by conduction electrons, when they flow through the ferromagnetic layers are:
(a) the scattering process is elastic; (b) the electron spin is conserved during scattering; (c) electrons having ↑ or ↓ spin orientations are scattered at different probabilities. First, we will discuss some general features of electron scattering, irrespective
87
the electrodes are positioned on one side of the device structure and the current is
supposed to flow along the layers (Fig. 3.9b). The GMR results from CPP geometry
have been found to be more than double that of CIP configuration. However, CPP is
more difficult to understand in practice.
To summarize, the following considerations can be made:
(1) Since the ↑ and ↓ spin channels are independent, they can be regarded as two
parallelly connecting wires.
(2) Additionally, the majority and minority spin electrons (already discussed in
Chap. 2), during their passage through a ferromagnetic layer, are scattered
in different probabilities. However, the spin orientations of electrons would
not change in this case. This phenomenon is referred to as spin-dependent
scattering.
3.5.1 What Is Spin-Dependent and Spin-Flip Scattering
of Electrons in Multilayers?
Before going into the details of origin of GMR, first we will discuss various categories
of scatterings that electrons might suffer in magnetic multilayers. In our approach to
understand the phenomenon considering Boltzmann equation, let us mainly suppose
the elastic, i.e., energy conserved scattering, where in each scattering process electrons undergo change of its direction of propagation only. However, in real situation,
the electrons may experience different types of scattering, which might flip the spin
of electrons from up to down and vice versa. This type of scattering is generally
referred to as spin-flip scattering. Now, distinction between spin-dependent scattering causing GMR and spin-flip scattering that plays detrimental role to GMR
needs to be discussed to understand GMR properly. Such scattering processes are
illustrated in Fig. 3.10.
Let us first give simple definition of two scattering processes:
Spin-dependent scattering: In spin-dependent scattering phenomenon, during
each scattering event suffered by electrons, the scattering probabilities of ↑ and ↓
spin electrons are different. Noteworthy, in this scattering process, the orientation of
the electrons’ spin should be conserved.
Spin-flip scattering: In spin-flip scattering phenomenon, during each scattering
event suffered by electrons, there is change of spin orientation of electrons from ↑
s z =
2
to ↓
s z = −
2
or vice versa.
Now, let us discuss the scattering processes one by one.
I. Spin-dependent Scattering
As already discussed, the key features of spin-dependent scattering process, encountered by conduction electrons, when they flow through the ferromagnetic layers are:
(a) the scattering process is elastic; (b) the electron spin is conserved during scattering; (c) electrons having ↑ or ↓ spin orientations are scattered at different probabilities. First, we will discuss some general features of electron scattering, irrespective