Report on Giant Magnetoresistance(GMR) & Spintronics

GIANT
MAGNETORESISTANCE
A SEMINAR REPORT BY
ECE Department
3rd year
APRIL 22, 2014
A report submitted to the Respective Faculty in partial fulfillment
of the Seminar on
GIANT MAGNETORESISTANCE

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ABSTRACT
Giant magnetoresistance (GMR) is a quantum mechanical
magnetoresistance effect observed in thin film structures composed of
alternating ferromagnetic and nonmagnetic layers.
The effect manifests itself as a significant decrease (typically 10-80%)
in electrical resistance in the presence of a magnetic field. In the absence
of an external magnetic field, the direction of magnetization of adjacent
ferromagnetic layers is antiparallel due to a weak anti-ferromagnetic
coupling between layers. The result is high-resistance magnetic scattering
as a result of electron spin.
When an external magnetic field is applied, the magnetization of the
adjacent ferromagnetic layers is parallel. The result is lower magnetic
scattering, and lower resistance(3).
The effect is exploited commercially by manufacturers of hard disk
drives. The 2007 Nobel Prize in physics was awarded to Albert Fert and
Peter Grünberg for the discovery of GMR.

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Contents
1 Introduction 4
2 Background
A. Ferromagnetic Metals………………………………………………..... 7
B. Resistance …………………………..…………………………………… 9
C. Growth of Superlattices……………………………………………….. 10
D. Interlayer Coupling……………………………………………………… 12
3 Giant Magnetoresistance
A. Introduction………………………………………………………………. 14
B. Mathematicl Model…………………………………………………….. 14
C. Carrier transport through a magnetic superlattice………………. 16
D. CIP & CPP Geometry………………………………………………….... 17
E. Experimental Findings…………………………………………………… 18
4 Spintronics
A. What is Spintronics? ……………………………………………………….... 20
B. Logic of Spin…………………………………………………………………. 20
C. Half Metals…………………………………………………………………..… 21
5 Other improvements on GMR
A. Tunneling Magnetoresistance …………………………………………….. 23
B. Colossal Magnetoresistance ………………………………………………. 24
6 Recent Developments & Future works
A. Work so far………………………………………………………………………. 25
A.1. Magnetic switching and microwave generation by spin
Transfer…………………………………………………………………… 25
A.2. Field Effect Spin Transistors……………………………………………. 26
7 Concluding Remarks 28
8 References 29

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1. Introduction
The phenomenon called magnetoresistance (MR) is the change of resistance of a
conductor when it is placed in an external magnetic field. For ferromagnets like iron,
cobalt and nickel this property will also depend on the direction of the external field
relative to the direction of the current through the magnet. Exactly 150 years ago W.
Thomson (1) (Lord Kelvin) measured the behaviour of the resistance of iron and nickel
in the presence of a magnetic field. He wrote “I found that iron, when subjected to a
magnetic force, acquires an increase of resistance to the conduction of electricity
along, and a diminution of resistance to the conduction of electricity across, the lines
of magnetization”. This difference in resistance between the parallel and
perpendicular case is called anisotropic magnetoresistance (AMR) (2). It is now known
that this property originates from the electron spin-orbit coupling. In general
magnetoresistance effects are very small, at most of the order of a few per cent.
The MR effect has been of substantial importance technologically, especially in
connection with read-out heads for magnetic disks and as sensors of magnetic fields.
The most useful material has been an alloy between iron and nickel, Fe20Ni80
(permalloy). In general, however, there was hardly any improvement of the
performance of magnetoresistive materials since the work of Kelvin. The general
consensus in the 1980s was that it was not possible to significantly improve on the
performance of magnetic sensors based on magnetoresistance.
Therefore it was a great surprise when in 1988 two research groups independently
discovered materials showing a very large magnetoresistance, now known as giant
magnetoresistance (GMR). These materials are so called magnetic multilayers, where
layers of ferromagnetic and non-magnetic metals are stacked on each other (figure
1). The widths of the individual layers are of nanometre size – i.e. only a few atomic
layers thick. In the original experiments leading to the discovery of GMR one group, led
by Peter Grünberg (3), used a trilayer system Fe/Cr/Fe, while the other group, led by
Albert Fert (4), used multilayers of the form (Fe/Cr)n where n could be as high as 60.

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Figure 1. Schematic figure of magnetic
multilayers. Nanometre thick layers of iron
(green) are separated by nanometre thick
spacer layers of a second metal (for example
chromium or copper). The top figure illustrates
the trilayer Fe/Cr/Fe used by Grünberg´s group
(3), and the bottom the multilayer (Fe/Cr)n ,
with n as high as 60, used by Fert´s group (4).
In figure 2 the measurements of Grünberg´s group are displayed (left) together with
those of Fert´s group (right). The y-axis and x- axis represent the resistance change and
external magnetic field, respectively. The experiments show a most significant negative
magnetoresistance for the trilayer as well as the multilayers. The systems to the right,
involving large stacks of layers, show a decrease of resistance by almost 50% when
subjected to a magnetic field.
Figure 2. After refs. (3) and (4).
Left: Magnetoresistance measurements (3) (room temperature) for the trilayer system Fe/Cr/Fe. To the
far right as well as to the far left the magnetizations of the two iron layers are both parallel to the external
magnetic field. In the intermediate region the magnetizations of the two iron layers are antiparallel. The
experiments also show a hysterisis behaviour (difference 1 and 4) typical for magnetization
measurements.
Right: Magnetoresistance measurements (4) (4.2K) for the multilayer system (Fe/Cr)n . To the far right
(>HS , where HS is the saturation field) as well as to the far left (< – HS ) the magnetizations of all iron layers
are parallel to the external magnetic field. In the low field region every second iron layer is magnetized
antiparallel to the external magnetic field. 10 kG = 1 Tesla.

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The effect is much smaller for the system to the left, not only because the system is
merely a trilayer but also because the experiments led by Grünberg were made at
room temperature, while the experiments reported by Fert and co-workers were per-
formed at very low temperature (4.2K).
Grünberg (3) also reported low temperature magnetoresistance measurements for
a system with three iron layers separated by two chromium layers and found a
resistance decrease of 10%.
Not only did Fert and Grünberg measure strongly enhanced magnetoresistivities, but
they also identified these observations as a new phenomenon, where the origin of the
magnetoresistance was of a to-tally new type. The title of the original paper from
Fert´s group already referred to the observed effect as “Giant Magnetoresistance”.
Grünberg also realized at once the new possibilities for technical applications and
patented the discovery. From this very moment the area of thin film magnetism
research completely changed direction into magnetoelectronics.
The discovery of giant magnetoresistance immediately opened the door to a wealth
of new scientific and technological possibilities, including a tremendous influence on
the technique of data storage and magnetic sensors. Thousands of scientists all
around the world are today working on magnetoelectronic phenomena and their
exploration. The story of the GMR effect is a very good demonstration of how a totally
unexpected scientific discovery can give rise to completely new technologies and
commercial products.

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2. Background
A. Ferromagnetic metals
Among the d transition metals (Sc…Cu, Y…Ag, Lu…Au, i.e. 3d, 4d, and 5d
transition elements), the 3d metals iron, cobalt and nickel are well-known to be
ferromagnets. Among the lanthanides (the 4f elements, La-Lu) gadolinium is also a
ferromagnet. The origin of magnetism in these metals lies in the behaviour of the 3d
and 4f electrons, respectively. In the following it is mainly the magnetism in the 3d
elements that will be discussed.
In the free atoms, the 3d and 4s atomic energy levels of the 3d transition
elements are hosts for the valence electrons. In the metallic state these 3d and 4s
levels are broadened into energy bands. Since the 4s orbitals are rather extended in
space there will be a considerable overlap between 4s orbitals belonging to
neighbouring atoms, and therefore the corresponding 4s band is spread out over a
wide energy range (15–20 eV). In contrast to this, the 3d orbitals are much less
extended in space. Therefore the energy width of the associated 3d energy band is
comparatively narrow (4–7 eV). In practice one cannot make a clear distinction
between the 3d and 4s orbitals since they will hybridize strongly with each other in the
solid. Nevertheless for simplicity this two band picture will be used here and the 3d
electrons will be considered as metallic – i.e. they are itinerant electrons and can carry
current through the system, although they are still much less mobile than the 4s
electrons.
A useful concept in the theory of solids is the electron density of states (DOS),
n(E), which represents the number of electrons in the system having energy within the
interval (E, E+dE). According to the exclusion principle for fermions (in this case
electrons), only one electron can occupy a particular state. However each state is
degenerate with respect to spin and can therefore host both an electron with spin up
and an electron with spin down. In the ground state all the lowest energy levels are
filled by electrons and the highest occupied energy level is called the Fermi energy,
EF. In figure 3 (left) the density of states is illustrated schematically for a non-magnetic
3d metal, sometimes referred to as a paramagnet, where there are equally many
electrons with spin up as with spin down, i.e. there is no net magnetization. The so
called spin polarization, P, [ P = (N↑ – N↓)/(N↑ + N↓), where N↑ ( N↓) = number of electrons
with spin up (down)], is here equal to zero.
For a ferromagnet N↑ is larger than N↓, so that there is a net spin polarization, P >
0. In order to com-pare the energy for the ferromagnetic state with the energy for the
paramagnetic state one can start from the paramagnetic state and allow for a small
imbalance in the number of spin up and spin down electrons. A transfer of spin down
electrons from the spin down band into the spin up band leads to more exchange
energy in the system, which means a lowering of the total energy (a gain) On the other
hand such a process requires a transfer of electrons from spin down levels below the
initial Fermi energy, into spin up levels situated just above the initial Fermi energy.

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Figure 3. To the left a schematic plot is shown for the energy band structure of a d transition metal. The
density of states N(E) is shown separately for the spin up and down electrons and where a simplified
separation has been made between the 4s and 3d band energies. For the non-magnetic state these
are identical for the two spins. All energy levels below the Fermi energy are occupied states (orange
and blue). The coloured area (orange + blue) corresponds to the total number of valence electrons in
the metal. To the right the corresponding picture is illustrated for a ferromagnetic state, with a spin-
polarization chosen to be in the up direction (N↑ > N↓; blue area > orange area). This polarization is
indicated by the thick blue arrow at the bottom figure to the right.
This will necessarily lead to a loss of band energy, “kinetic energy” and thus to an
increase of the total energy (a loss). Thus there is a competition between two opposite
effects. This can be formulated as the so called Stoner criterion (5) for magnetism,
namely that when
I N(EF) > 1,
the system will be a ferromagnet. Here I is called the Stoner exchange parameter and
N(EF) is the density of states at the Fermi energy. The Stoner parameter has a specific
value for the individual element, while N(EF) depends much more on the particular
spatial arrangements of the atoms relative to each other (like crystal structure).
Furthermore, and most important, N(EF) tends to be high for systems with narrow
energy bands as is the case for the heavier 3d transition elements (Fe, Co and Ni). This
is the explanation for the ferromagnetism among the d transition metals.
The situation for a ferromagnetic spin polarization is illustrated to the right in figure
3 (with a direction chosen to be upwards). The vertical displacement between the spin
up and spin down densities of states exemplifies the exchange energy splitting
between the spin up and spin down energy bands, which is relevant for the metals Fe,
Co and Ni. In particular the density of states at the Fermi energy N(EF) can now be very
different for the two spin bands. This also means that for a ferromagnet the character
of the state at the Fermi energy is quite different for spin up and spin down electrons.
This is an important observation in connection with the GMR effect. This picture of 3d
energy bands (figure 3 to the right) for the ferromagnetic metals is often referred to as
the itinerant model (6), also known as the Stoner-Wohlfarth model (5).

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One important property of ferromagnets is that at high temperature their
magnetism is lost. This hap - pens at a well-defined temperature, the so called Curie
temperature, TC. For the present systems (Fe, Co and Ni) these critical temperatures
are far above room temperature and can be neglected.
B. Resistance
An electrical current of electrons sent through a metallic system will always
experience a resistance R. (Exceptions are the so called superconductors where
below a certain temperature the current can flow without resistance). There are a
number of reasons for this. In a crystal the atoms will always vibrate (phonons) around
their equilibrium positions, thereby deviating from the perfect lattice positions, and the
conduction electrons may be scattered by these deviations (electron – phonon
interaction). Other important contributions to the resistance of a metal are scattering
of electrons against impurities and defects. The only electrons that participate in the
electrical conduction process are those at (or very close to) the Fermi level. For
paramagnetic metals there is no difference between the spin up and spin down
electrons, and they contribute equally to the resistance.
Figure 4. To the left is the scattering event for spin dependent electrons due to impurities and defects.
The spin is shown on the individual electrons by either up or down. The scattering event is shown by a
star.
To the right is the Mott’s equivalent circuit for the two current model. One part(Green) is due to the spin
up electron and the other(Purple) is due to the spin down electron
Already in 1936 Sir Nevil Mott (7) considered the electrical conductivity of d
transition elements. He suggested that the conductivity was mainly determined by the
4s electrons which are easily mobile due to the wide energy range of the bands
derived from the 4s-states. However in a scattering process the s electrons can scatter
into the many d states which are available at the Fermi level. Therefore they
experience a strong scattering giving rise to a considerable resistance. On the other
hand for Cu, the element following Ni in the Periodic Table, all the 3d states are
situated below the Fermi level and therefore not available for scattering processes.

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This explains the particularly high conductivity of Cu.
In the 1960s and 1970s Fert together with Campbell studied in great detail the
conductivity of 3d ferromagnetic materials (3, 8). They carried out extensive
investigations of resistivity changes which occur when low concentrations of alloying
elements, like Cr and other transition metals, are put as scattering centres into for
example Fe and Ni. From these studies they could confirm that in a ferromagnet like
iron there are two types of carriers, one made up from spin up electrons and one from
spin down electrons. Since the density of states at the Fermi surface is quite different
for the two spin states it follows that there is a significant difference in resistance for the
spin up electrons and the spin down electrons. There could also be contributions to
the resistance from scattering processes where the spins are flipped. This could for
example be due to scattering against spin waves or from the spin orbit coupling.
However these effects are small and will be neglected here. Thus the picture which is
emerging is that the electrical current in a ferromagnet like iron, cobalt and nickel
consists of spin up and spin down carriers, which experience rather different
resistances.
C. Growth of superlattices
From the beginning of the 1970s the development in physics, chemistry and
materials science had led to new experimental techniques allowing scientists to
manufacture completely novel materials. Using what was called epitaxial growth one
could start to produce artificial materials building one atomic layer after the next.
Techniques that were introduced at this time involved for example sputtering, laser
ablation, molecular beam epitaxy and chemical vapour deposition. Molecular beam
epitaxy was already being used in the late 1960s to make thin semiconducting
materials and at the end of the 1970s nanometre thick metallic layers could be
produced. This was first applied to non-magnetic metals, but later also to metallic
ferromagnets. At the same time, a number of characterization techniques had been
largely improved, utilizing for example the magneto-optic Kerr effect (MOKE) and light
scattering from spin waves. Using these methods it was possible to grow metallic
multilayers involving for example iron and study their magnetic properties.
In order to produce well-defined materials the choice of substrate on which to
grow the material is of great importance. Commonly used materials are silicon, silicon
dioxide, magnesium oxide and aluminium oxide. To obtain well-behaved metallic
multilayers it is important that the lattice parameters for the different metallic layers
match each other (figure 5) and it is also an advantage if the two metals forming the
multilayer have the same crystal structure. This is the case for chromium and iron, where
both metals adapt the bcc (body-centred cubic) crystal structure and where in
addition they have very similar lattice spacing. This was important for the studies for
which this Nobel Prize is awarded under-taken by the groups of Fert and Grünberg. In
addition it was also extremely important that it was now possible to grow multilayers
where the spatial separation between the magnetic layers is of the order of
nanometres. In order to exhibit the GMR effect the mean free path length for the

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conduction electrons has to greatly exceed the interlayer separations so that the
electrons can travel through magnetic layers and pick up the GMR effect. Without the
new experimental growth techniques this requirement could not have been fulfilled
and the GMR effect would have remained unknown. In this connection it should be
mentioned that, in several publications prior to the work of Fert and Grünberg, there
were reports of observations of substantial (of the order of a few per cent)
magnetoresistance effects (9, 10, 11, 12). In none of them were the observations
recognized as a new effect.
Figure 5: Molecular Beam Epitaxy facility at Forschungszentrum Jülich - Peter Grünberg Institute in
Julich at the University of Germany.

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D. Interlayer coupling
It has been known for a long time that disturbances like defects and impurities in
metallic systems become screened by the surrounding conduction electrons. The
disturbance gives rise to decaying oscillations of the electron density as a function of
the distance from the disruption (so called Friedel oscillations). Similarly, a magnetic
impurity atom in a metallic surrounding gives rise to an induced spin polarization of the
electron density. With increasing dis-tance from the magnetic impurity there will be an
oscillation in the sign of the polarization and the disturbance will also decay in
magnitude with distance. As a consequence, the magnetic moment of a second
impurity placed relatively close to the first one, will become aligned parallel or
antiparallel to the magnetic moment of the first moment depending on the sign of the
induced polarization at that particular distance. This coupling (exchange coupling)
between magnetic moments (schematically shown in figure 6) was well-known for the
rare-earth metals where each atom possesses a magnetic moment originating from
the very tightly bound (and localized) 4f electronic configuration positioned deep
inside the atom. In fact the magnetism of the heavier lanthanide metals originate from
this interaction.
As already mentioned gadolinium is a ferromag-
net where the magnetic moments originate from
the localized 4f electrons on each atom having a
4f7 configuration. That is, all the 4f magnetic mom-
ents point in the same direction and surrounding
these moments there are three conduction elec-
trons per atom which mediate the interaction
between the 4f magnetic moments. In 1986
Majkrzak et al. (13) published work on a super-
lattice of Gd/Y/Gd where they reported an anti-
parallel mag-netic moment alignment between
the Gd layers for the case of 10 monolayers of Y.
This could be understood from the way that a
ferromagnetic Gd layer induces an oscillatory spin
polarization of the normally non- magnetic Y metal
and that the second Gd layer happens to be at a
distance where an antiferro-magnetic alignment is
preferred. Practically simultaneously Grünberg et al.
(14) discovered an antiferromagnetic coupling
between the iron layers for the Fe/Cr/Fe tri-layer.
This can be explained in a similar way to the Gd/Y/Gd case. It should be
remarked, however, that in both cases, due to the geometry, there are important
contributions to the interlayer exchange coupling from quantum interference of the
Figure 6. Schematic illustration of
the behaviour of the exchange
coupling as a function of
distance.

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electron waves reflected at the magnetic layers (15). In the present context it is
however sufficient to conclude that the important role of the electrons of the non-
magnetic layer(s) is that they provide the coupling mechanism between the magnetic
layers.
The next step was to investigate the dependence of the coupling on the
thickness of the intermediate non-magnetic layers. Several groups identified a
change of sign with increasing thickness (13, 15, 16, 17,18). A thorough study of the
dependence of the oscillatory behaviour on the thickness of the non-magnetic layer,
its dependence on the material of the non-magnetic layer as well as on the
dependence of the material of the magnetic layer itself was made by Parkin (1). Here
he actually utilized the GMR effect as a tool to study this dependence. In the
preparation of the multilayers Parkin used a magnetron sputter deposition technique.
With this method it was possible to prepare a large number of samples under
comparable conditions. This extensive work was important for the further
development of the GMR effect into a working device (20, 21,22).

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3. Giant Magnetoresistance
A.Introduction
In 1988, the Giant
Magnetoresistance (GMR) effect was
discovered independently by two
groups led by Albert Fert and Peter
Grünberg. In recognition of their
achievement, Albert Fert and Peter
Grünberg and were awarded with the
Nobel Prize in 2007. Both groups found
that the electric conductance of a
layer system consisting of ferromag-
netic and non-magnetic layers would
depend on the relative orientation of
the magnetization in neighbouring
ferromagnetic layers. The resistance
was small when the magnetization was
parallel and vice versa.
B. Mathematical Model
The resistance of a GMR device can be understood from the following
somewhat simplified picture. In figure 7 a plot of the magnetic configuration for the
FM/NM/FM (ferromagnetic/non-magnetic/ferromagnetic) multilayer is made together
with the corresponding electron density of state for the two ferromagnetic sides (FM).
In the absence of a magnetic field (at the top) the two FM layers are separated from
each other in such a way that they have opposite magnetization directions. In the
presence of a magnetic field the magnetizations of the two FM layers will be parallel
(at the bottom). An electrical current is now sent through the system for both
configurations. As already mentioned above the current through the FM layer is
composed of two types – one spin up current and one spin down current – and the
resistance for these two currents will differ. When an electron leaves the first iron layer
and enters the non-magnetic metal there will be additional scattering processes giving
rise to extra resistance. Since the spin up and spin down particles have different density
of states at the Fermi level (or rather, they originate from energy levels having different
character), the resistance not only within the FM layers, but also that originating from
the FM/NM interface will be different for the two spins. Inside the NM layer the up and
down spins will experience the same resistance, but generally this is low compared to
those in the FM layers and FM/NM interfaces and can here be neglected.

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Figure 7. Schematic illustration
of the electronic structure of a
trilayer system with two
ferromagnetic layers (light
green) on both sides separated
by nonmagnetic material
(grey). The top figure is for the
case without external magnetic
field (H=0), i.e. when the two
magnetic layers have opposite
magnetizations (indicated by
the thick blue and orange
arrows at the top of the
topmost figure). The bottom
figure is for the case when an
external magnetic field (H ≠ 0)
has forced the two
magnetizations to be parallel
(two thick blue arrows at the
bottom of the lower figure.).The
magnitude of the four
magnetizations is the same.
When the electrons enter the second iron layer they will again experience spin
dependent scattering at the NM/FM interface. Finally the spin up and spin down
electrons go through the second iron layer with the same resistance as in the first iron
layer, which still of course differs for the two spins. For simplicity the resistance for the
spin up (down) electrons through the FM layer and the scattering at the interface to
the NM layer will be called R↑ (R↓). Thus when the two layers have parallel spin
polarizations (magnetizations), i.e. in the presence of an external magnetic field (H),
the resistance for the spin up channel is 2 R↑ and for the spin down channel it is 2 R↓.
Standard addition of resistances for a parallel current configuration gives the following
total resistance, RH, in the presence of an external magnetic field;
RH = 2R↑R↓/(R↑ + R↓).
In the case of no external magnetic field, (H=0), the configuration between the
two magnetic layers is antiparallel (top part of figure 8). In this case the first scatterings
in the left part of the multilayer system are exactly the same as before for the lower
part of the figure. However, when a spin up electron enters into the second FM layer it
finds itself in a totally upside-down situation where the conditions are now exactly the
same as they were for the spin down electron in the initial FM layer. Thus the spin up
particle will now experience a total resistance of R↑ + R↓. The spin down particle will be
affected in the same (but opposite) way and its resistance will be R↓ + R↑. The total
resistance will accordingly be
𝑅0 =
𝑅↑+𝑅↓
2

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Figure 8. The same physical system as in figure 6.
The magnetic layers are now represented by
resistances R↑ and R↓. This shows very clearly
that the total resistance for the two cases are
different, i.e. there is a magnetoresistance
effect. In case R↑ >> R↓ it is practically only the
lowest of the four possibilities which will permit a
current. In the lower picture with parallel
magnetizations the resistance for the spin up
(spin down) electrons will be R↑ (R↓) in both
magnetic layers. In the upper picture with
antiparallel magnetizations the spin up (spin
down) electrons will have a resistance R↑ (R↓) in
the first magnetic layer to the left. In the second
magnetic layer the resistance for the spin up
(spin down) electron will be R↓ (R↑), since the
magnetization environment has here become
totally opposite compared to the first magnetic
layer.
Thus the difference in resistance between the two cases (magnetic field or not)
becomes:
R = RH – R0 = -
(𝑅↑−𝑅↓)2
2((𝑅↑+𝑅↓)
Thus the larger the difference between R↑ and R↓ the larger the negative
magnetoresistance. This expression clearly shows that the magnetoresistance effect
arises from the difference between the resistance behaviour of the spin up and down
electrons.
C. Carrier transport through a magnetic superlattice
Magnetic ordering differs in superlattices with ferromagnetic and anti-
ferromagnetic interaction between the layers. In the former case, the magnetization
directions are the same in different ferromagnetic layers in the absence of applied
magnetic field, whereas in the latter case, opposite directions alternate in the
multilayer. Electrons traveling through the ferromagnetic superlattice interact with it
much weaker when their spin directions are opposite to the magnetization of the
lattice than when they are parallel to it. Such anisotropy is not observed for the
antiferromagnetic superlattice, as a result, it scatters electrons stronger than the
ferromagnetic superlattice and exhibits a higher electrical resistance.(31)

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Figure 9: Carrier transport through a structured FM/NM/FM layer. To the left we can see that the current
consisting of spin up and spin down electron faces low resistivity when the spins are parallel due to
external applied magnetic field. To the right the current is facing more resistance due to the anti-parallel
alignment of the ferromagnetic layer and gets scattered away.
Applications of the GMR effect require dynamic switching between the parallel
and antiparallel magnetization of the layers in a superlattice. In first approximation, the
energy density of the interaction between two ferromagnetic layers separated by a
non-magnetic layer is proportional to the scalar product of their magnetizations:
𝜔 = −𝒥(𝑀1. 𝑀2)
The coefficient J is an oscillatory function of the thickness of the non-magnetic
layer ds; therefore J can change its magnitude and sign. If the ds value corresponds
to the antiparallel state then an external field can switch the superlattice from the
antiparallel state (high resistance) to the parallel state (low resistance).
D. CIP and CPP geometries
Electric current can be passed through magnetic superlattices in two ways. In
the current in plane (CIP) geometry, the current flows along the layers, and the
electrodes are located on one side of the structure. In the current perpendicular to
plane (CPP) configuration, the current is passed perpendicular to the layers, and the
electrodes are located on different sides of the superlattice.(31) The CPP geometry
results in more than twice higher GMR, but is more difficult to realize in practice than
the CIP configuration

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Figure 10: To the left is shown the current in plane (CIP) geometry where the contact electrode is
connected parallel to Structure
To the right is shown the current perpendicular to the plane (CPP) where the contact electrode is
connected perpendicular to Structure. CPP results in better performance than CIP
Image courtesy Seagate
E. Experimental findings
The trilayer system of Grünberg et al. consisted of a Fe-Cr-Fe sandwich(3) that
was grown epitaxially on a GaAs substrate. The 12 nm thick iron film grow epitaxially.
The thickness of the chromium layer amounted to 1nm. With that particular thickness,
the magnetization in the two iron layer assume an anti-parallel orientation due to
interlayer coupling(52). In a
strong magnetic field parallel
to the direction in the film
plane the magnetizations in
the iron film turn parallel. The
electrical resistance
measured parallel to the layer
is 1.5 smaller in that case.
The reason for smaller
resistance is that conducting
electrons traversing the
chromium layer from one iron
layer into the other carry their
spin orientation along. If the
magnetization of both iron
films point in the same
direction, a spin-up electron
originating in one iron layer
easily finds matching states
concerning its k-vector and
Figure 11: Magnetoresistance in a three layer sandwich from Grunberg et al. In zero field,
the magnetization in the two iron layers are held antiparallel via interlayer coupling
through the antiferromagnetic chromium. On application of external magnetic field the
magnetization are forced to become parallel hence electrical resistance decreases.

18
energy in the second to continue its path without being scattered. In case of anti-
parallel orientation, the spin-up states in the second layer are shifted by the exchange
energy. In general there will be no matching states for electronic the second iron layer.
The electron is hence scattered to loose its preferential direction, which increases the
electric resistance.

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4. Spintronics
A. What is Spintronics?
To enhance the multifunctionality of devices (for example, carrying out
processing and data storage on the same chip), investigators have been eager to
exploit another property of the electron—a characteristic known as spin. Spin is a
purely quantum phenomenon roughly akin to the spinning of a child’s top or the
directional behavior of a compass needle. The top could spin in the clockwise or
counterclockwise direction; electrons have spin of a sort in which their compass
needles can point either “up” or “down” in relation to a magnetic field.
Spin therefore lends itself elegantly to a new kind of binary logic of ones and zeros. The
movement of spin, like the flow of charge, can also carry information among devices.
One advantage of spin over charge is that spin can be easily manipulated by
externally applied magnetic fields, a property already in use in magnetic storage
technology.
Another more subtle (but potentially significant) property of spin is its long
coherence, or relaxation, time—once created it tends to stay that way for a long time,
unlike charge states, which are easily destroyed by scattering or collision with defects,
impurities or other charges.
B. Logic of spin
Spin relaxation (how spins are created and disappear) and spin transport (how
spins move in metals and semiconductors) are fundamentally important not only as
basic physics questions but also because of their demonstrated value as phenomena
in electronic technology.
Researchers and developers of spintronic devices currently take two different
approaches. In the first, they seek to perfect the existing GMR-based technology either
by developing new materials with larger populations of oriented spins (called spin
polarization) or by making improvements in existing devices to provide better spin
filtering. The second effort, which is more radical, focuses on finding novel ways both
to generate and to utilize spin-polarized currents—that is, to actively control spin
dynamics. The intent is to thoroughly investigate spin transport in semiconductors and
search for ways in which semiconductors can function as spin polarizers and spin
valves. This is crucial because, unlike semiconductor transistors, existing metal-based
devices do not amplify signals (although they are successful switches or valves). If
spintronic devices could be made from semiconductors, however, then in principle
they would provide amplification and serve, in general, as multi-functional devices.
Perhaps even more importantly, semiconductor-based devices could much more
easily be integrated with traditional semiconductor technology. Although
semiconductors.
In addition to the near-term studies of various spin transistors and spin transport properties
of semiconductors, a long-term and ambitious subfield of spintronics is the application of
electron and nuclear spins to quantum information processing and quantum computation. The
late Richard Feynman and others have pointed out that quantum mechanics may provide
great advantages over classical physics in computation. However, the real boom started after

20
Peter Shor of Bell Labs devised a quantum algorithm that would factor very large numbers into
primes, an immensely difficult task for conventional computers and the basis for modern
encryption. It turns out that spin devices may be well suited to such tasks, since spin is an
intrinsically quantum property.
Figure 12: Spins can arrange themselves in a variety of ways that are important for spintronic devices.
They can be completely random, with their spins pointing in every possible direction and located
throughout a material in no particular order (upper left). Or these randomly located spins can all point
in the same direction, called spin alignment (upper right). In solid state materials, the spins might be
located in an orderly fashion on a crystal lattice (lower left) forming a nonmagnetic material. Or the
spins may be on a lattice and be aligned as in a magnetic material (lower right).
C. Half-metals
Since magnetoresistance deals with electrical conductivity it is obvious that it
is the behaviour of the electrons at the Femi surface (defined by the Fermi energy)
which is of primary interest. The more spin-polarized the density of states (DOS) at the
Fermi energy, i.e., the more N↑ (EF) deviates from N↓ (EF), the more pronounced one
expects the efficiency of the magnetoelectronic effects to be. In this re-spect a very
interesting class of materials consists of what are called half-metals, a concept
introduced by de Groot and co-workers (23). Such a property was then predicted
theoretically for CrO2 by Schwarz in 1986 (24). The name half-metal originates from
the particular feature that the spin down band is metallic while the spin up band is
an insulator. This is shown schematically in figure 13, and it is clear that there is a 100%
spin polarization at the Fermi level. The theoretical prediction for CrO2 was later
confirmed by experiment (25,26).

21
Figure 13. Schematic illustration
of the density of states for non-
magnetic (left) and
ferromagnetic CrO2 (right). As
can be seen immediately for
the ferromagnetic case (right),
the spin down electrons
(orange) are placed in a
metallic band, while the spin up
electrons (blue) show an
electronic structure that is
typical for an insula-tor. The net
spin polarization is shown by a
thick blue arrow.
In figure 14 it is shown the two DOS (spin up and spin down) for the ferromagnetic
state. For a trilayer of two ferromagnetic half-metals with one non- magnetic metallic
layer between them, it becomes very easy to appreciate the mechanism behind the
GMR effect. When the magnetizations of the two half-metals are parallel there will be
a current made up exclusively of spin down electrons. However, for an antiparallel
magnetization, the spin down channel will be totally blocked for conduction of
electricity. Hence a magnetic field which can switch between these two
configurations will give rise to a large change in resistance, i.e. will show a strong
magnetoresistance behaviour. An enhanced magnetoresistance for the half-metal
CrO2 was confirmed experimentally by Hwang and Cheong (27).
Figure 14. Illustration of the
magnetoresistance effect for a half-
metal. Two ferromagnetic half-metallic
layers (light green) separated by a non-
magnetic metal (grey). In the absence of
an external magnetic field (H = 0) the two
ferromagnets have antiparallel spin
polarizations (blue and orange thick
arrows at the top). In the presence of an
external field (H ≠ 0) the two magnets
have parallel spin polarization (two thick
blue arrows at the bottom of the figure).
As can be immediately understood, there
will be no current for the upper case. For
the lower case there will only be a spin
down current.

22
6. Other improvements on GMR
A. Tunnelling magnetoresistance
Another variation of multilayers in the present context is to grow layered materials
with an alternation between metallic and insulating layers. Here the insulating material
should be only a few atomic layers thick so that there is a significant probability that
electrons can quantum mechanically tunnel through the insulating barrier (figure 15).
In this manner a current can be sent through the multilayer. The first publication on such
a system was made by Julliere (28). This work was done for a trilayer junction with the
following structure Fe/amorphous Ge/Co. The experiments were done at low
temperature and an effect of about 14% was reported.
Figure 15. Illustration of
tunneling magnetoresistance
(TMR). Two ferromagnetic
layers separated by an
insulating layer (i = electron
current).
The next work of this type was carried out by Maekawa and Gäfvert (29). They
investigated junctions of the type Ni/ NiO /FM, where FM stands for Fe, Co or Ni. The
magnetoresistance they found was of the order of a few per cent, again at low
temperatures. These two reports remained essentially unnoticed for a long time. In fact
it was only after the discovery by Fert and Grünberg that attention focussed on these
types of systems again. The breakthrough came in 1995 when two groups reported
significant progress. Thus Moodera and his group (30,31) measured tunneling layers on
CoFe / Al2O3 /Co (or NiFe) and found resistance changes of 24% at 4.2 K and 12% at
room temperature. Similarly, Miyazaki and Tesuka (32) used a Fe /Al2O3/ Fe junction
and found resistance changes of 30% and 18% at 4.2 K and room temperature,
respectively. Today it is rather common to find changes of the order of 50% at room

23
temperature. This is indeed higher than the resistance changes found in “standard”
GMR materials. Recently barriers of Fe/MgO/Fe have been shown to give rise to TMR-
values that sometimes exceeded 200% (33,34,35).
Due to the better performance of the magnetic tunnel junctions they are
expected to become the material of choice when it comes to technical
applications. Their use in connection with non-volatile magnetic random access
memories (MRAM) is of particular interest and MRAM systems based on TMR are
already on the market. One expects that TMR based technologies will become
dominant over the GMR sensors. However, the discovery of the GMR effect paved
the way for the TMR technology.
B. Colossal magnetoresistance
The discovery of the GMR effect for magnetic multilayers gave rise to an
increased interest in finding related effects among bulk materials (36). Thus von
Helmolt and his group (37) found even larger magnetoresistance effects than for GMR
in certain manganese perovskites. These materials are some-times referred to as mixed
valence systems. Jin and co-workers (38) also found these effects, where the
resistance change in an applied magnetic field could be several magnitudes higher
than for GMR. Hence the observed effect became known as colossal
magnetoresistance (CMR). These extraordinary systems exhibit a very rich variety of
exceptional properties, where electron correlations play a very central role. However
it is unlikely that they will become of technological interest, mainly because the
required magnetic fields are very high.

24
7. More recent developments & Future work
A.Work so Far
Here I just mention a few of the vast number of different research areas which
represent more recent trends regarding spin materials and their applications. One such
area is for example magnetic semiconductors, where the Ohno’s group demonstrated
the potential of such materials (39,40) using the semiconductor (Ga, Mn)As.
Another area concerns spin injection. Here the early work by Johnson on metallic
systems should be mentioned (41,42). Injection of spin from a metallic ferromagnet into
a semiconductor was successfully accomplished by Zhu et al. (43) and Hanbicki et al.
(44), using Fe and GaAs.
Injection of spins from a magnetic semiconductor to a non-magnetic
semiconductor was shown by Ohno et al. (45) and by Fiederling et al. (46). The question
of how far spin-polarized electrons can travel in a material while maintaining their spin
polarization is of great importance and promising work has been reported by
Awshalom and his colleagues (47,48,49).
Very intense work is now being directed towards magnetic switching induced
by spin-currents. This interest started from two theoretical papers where it was shown
that a spin-current through a magnetic multilayer can lead to a magnetization
reversal (50,51). This prediction was soon verified experimentally (52,53,54). The
realization of current induced domain wall motions (55,56,57) forms the basis for the
idea of a magnetic “Race-Track Memory” (58).
A.1 Magnetic Switching And Microwave Generation By Spin Transfer
The study of the spin transfer phenomena is one of the most promising new
directions in spintronics today and also an important research topic in our CNRS/Thales
laboratory. In spin transfer experiments, one manipulates the magnetic moment of a
ferromagnetic body without applying any magnetic field but only by transfer of spin
angular momentum from a spin-polarized current. The concept, which has been
introduced by John Slonczewski and appears also in papers of Berger, is illustrated in
Fig. 16. As described in the caption of the figure, the transfer of a transverse spin current
to the “free” magnetic layer F2 can be described by a torque acting on its magnetic
moment. This torque can induce an irreversible switching of this magnetic moment or,
in a second regime, generally in the presence of an applied field, it generates
precessions of the moment in the microwave frequency range. The first evidence that
spin transfer can work was indicated by experiments of spin injection through point
contacts by Tsoi et al.(23) but a clear understanding came later from measurements
performed on pillar-shaped metallic trilayers (Fig. 11a). In Fig.11b-c, I present examples
of the experimental results in the low field regime of irreversible switching, for a metallic
pillar and for a tunnel junctions with electrodes of the dilute ferromagnetic

25
semiconductor Ga1-xMnxAs. For metallic pillars or tunnel junctions with electrodes
made of a ferromagnetic transition metal like Co or Fe, the current density needed for
switching is around 106-107 Amp/cm2, which is still slightly too high for applications,
and an important challenge is the reduction of this current density. The switching time
has been measured in other groups and can be as short as 100 ps, which is very
attractive for the switching of MRAM
The spin transfer phenomena will have certainly important applications.
Switching by spin transfer will be used in the next generation of MRAM and will bring
great advantages in terms of precise addressing and low energy consumption. The
generation of oscillations in the microwave frequency range will lead to the design of
Spin Transfer Oscillators (STOs). One of the main interests of the STOs is their agility that
is the possibility of changing rapidly their frequency by tuning a DC current. They can
also have a high quality factor.
Figure 16. Illustration of the spin transfer concept introduced by John Slonczewski45 in 1996. A spin-
polarized current is prepared by a first magnetic layer F with an obliquely oriented spin-polarization with
respect to the magnetization axis of a second layer F2. When this current goes through F2, the exchange
interaction aligns its spin-polarization along the magnetization axis. As the exchange interaction is spin
conserving, the transverse spin polarization lost by the current has been transferred to the total spin of
F2, which can also be described by a spin-transfer torque acting on F2.This can lead to a magnetic
switching of the F2 layer or, depending on the experimental conditions, to magnetic oscillations in the
microwave frequency range
A.2 Field effect Spin Transistor
The first scheme for a spintronic device based on the metal-oxide-
semiconductor technology familiar to microelectronics designers was the field effect
spin transistor proposed in 1989 by Supriyo Datta and Biswajit Das of Purdue University.
In a conventional field effect transistor, electric charge is introduced via a source
electrode and collected at a drain electrode. A third electrode, the gate, generates
an electric field that changes the size of the channel through which the source-drain
current can flow, akin to stepping on a garden hose. This results in a very small electric
field being able to control large currents. In the Datta-Das device (60), a structure
made from indium-aluminum-arsenide and indium-gallium-arsenide provides a

26
channel for two-dimensional electron transport between two ferromagnetic
electrodes. One electrode acts as an emitter, the other a collector (similar, in effect,
to the source and drain, respectively, in a field effect transistor). The emitter emits
electrons with their spins oriented along the direction of the electrode’s magnetization,
while the collector (with the same electrode magnetization) acts as a spin filter and
accepts electrons with the same spin only. In the absence of any changes to the spins
during transport, every emitted electron enters the collector. In this device, the gate
electrode produces a field that forces the electron spins to precess, just like the
precession of a spinning top under the force of gravity. The electron current is
modulated by the degree of precession in electron spin introduced by the gate field:
An electron passes through the collector if its spin is parallel, and does not if it is
antiparallel, to the magnetization. The Datta-Das effect should be most visible for
narrow band-gap semiconductors such as InGaAs, which have relatively large spin-
orbit interactions (that is, a magnetic field introduced by the gate current has a
relatively large effect on electron spin).
Another interesting concept is the all-metal spin transistor developed by Mark
Johnson at the Naval Research Laboratory. Its trilayer structure consists of a
nonmagnetic metallic layer sandwiched between two ferromagnets. The all-metal
transistor has the same design philosophy as do giant magnetoresistive devices: The
current flowing through the structure is modified by the relative orientation of the
magnetic layers, which in turn can be controlled by an applied magnetic field. In this
scheme, a battery is connected to the control circuit (emitterbase), while the direction
of the current in the working circuit (base-collector) is effectively switched by changing
the magnetization of the collector. The current is drained from the base in order to
allow for the working current to flow under the “reverse” base-collector bias
(antiparallel magnetizations). Neither current nor voltage is amplified, but the device
acts as a switch or spin valve to sense changes in an external magnetic field.
Figure 17: In the spin transistor invented by Mark Johnson, two ferromagnetic electrodes - the emitter
and collector—sandwich a nonmagnetic layer—the base. When the ferromagnets are aligned, current
flows from emitter to collector (left). But when the ferromagnets have different directions of
magnetization, the current flows out of the base to emitter and collector (right). Although it can act as
a spin valve, this structure shares the disadvantage of allmetal spintronic devices in that it cannot be an
amplifier.

27
8. Concluding remarks
The discovery by Albert Fert and Peter Grünberg of giant magnetoresistance
(GMR) was very rapidly recognized by the scientific community. Research in
magnetism became fashionable with a rich variety of new scientific and technological
possibilities. GMR is a good example of how an unexpected fundamental scientific
discovery can quickly give rise to new technologies and commercial products. The
discovery of GMR opened the door to a new field of science, magnetoelectronics (or
spintronics), where two fundamental properties of the electron, namely its charge and
its spin, are manipulated simultaneously. Emerging nanotechnology was an original
prerequisite for the discovery of GMR, now magnetoelectronics is in its turn a driving
force for new applications of nanotechnology. In this field, demanding and exciting
scientific and technological challenges become intertwined, strongly reinforcing
progress.
The field of spintronics already has promising future for future electronics with the
most important being the Quantum computing. Although much remains to be
understood about the behaviour of electron spins in materials for technological
applications, but much has been accomplished. A number of novel spin-based
microelectronic devices have been proposed, and the giant magnetoresistive
sandwich structure is a proven commercial success, being a part of every computer
coming off the production line. In addition, spintronic-based non-volatile memory
elements may very well become available in the near future. But before we can move
forward into broad application of spin-based multifunctional and novel technologies,
we face the fundamental challenges of creating and measuring spin, understanding
better the transport of spin at interfaces, particularly at ferromagnetic/semiconductor
interfaces, and clarifying the types of errors in spin-based computational systems.
Tackling these will require that we develop new experimental tools and broaden
considerably our theoretical understanding of quantum spin, learning in the process
how to actively control and manipulate spins in ultra-small structures.

28
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