Giant magnetoresistance (GMR) is a quantum mechanical magnetoresistance effect observed in multilayers composed of alternating ferromagnetic and non-magnetic conductive layers. The 2007 Nobel Prize in Physics was awarded to Albert Fert and Peter Grünberg for the discovery of GMR. The effect is observed as a significant change in the electrical resistance depending on whether the magnetization of adjacent ferromagnetic layers are in a parallel or an antiparallel alignment. The overall resistance is relatively low for parallel alignment and relatively high for antiparallel alignment. The magnetization direction can be controlled, for example, by applying an external magnetic field. The effect is based on the dependence of electron scattering on the spin orientation.

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Giant Magnetoresistance And Their Applications
PREMASHIS KUMAR
CENTRAL UNIVERSITY OF RAJASTHAN, CONDENSED MATTER PHYSICS II
Submitted: 12/11/2017
1. Introduction
Magnetoresistance is a property of
material by virtue of which there is a
change in electrical resistance when it is
subjected to external magnetic field.
Giant magnetoresistance, one of the four
types of magnetoresistance, is a purely
quantum mechanical magnetoresistance
effect mostly seen in the magnetic
multilayer ultrathin film consists of
alternating ferromagnetic layers and
nonmagnetic conducting layers. It is
basically a large change (more than 10-
20%) in electrical resistance of above
mentioned multilayer film due to large
external field.
As multilayers are embedded in ultrathin
film so it could be considered as one of
the first purposeful application of newly
emerging field of nanotechnology then.
Now-a-days GMR is considered as the
backbone of new application of
nanotechnology.
Discovery of GMR leads to development
of new field of science known as
spintronics. Spintronics is a new type of
electronics which exploits not only the
charge of electron like classical
electronics but also consider the intrinsic
spin of electrons.
In spintonics we basically consider how
the orientation of spin modifies the
magnetic property generated by charge
part. Spintronics is also known as
magneto electronics. But generally
change in resistance of passive elements
belongs to magneto electronics and
change of resistance of active elements
belongs to spintronics.
2. A Brief History
Around 1980s it was believed that we
can’t improve the performance of
magnetic sensor much using
Abstract
Giant magnetoresistance is quantum mechanical phenomenon which was discovered
very unexpectedly but then lead to the development of new science field named
Spintronics. It has now become a backbone of storage devices. It was developed
independently by two different groups of scientists by similar but little different
techniques and experiment was performed at different temperature. Spin dependent
scattering of conduction electrons is the origin of the GMR. There are different types
of GMR has been developed on the basis of current flowing and structure of
Multilayers.

2. A.Premashis Kumar GMR And its applications
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magnetoresistance property. So
discovery of GMR was completely
unexpected.
In 1986 by Brillouin scattering
experiment Grunberg showed that two
layers of Fe separated by nonmagnetic
layer of Cr is actually coupled
antiferromagnetically.
In 1988 GMR was measured and
identified independently by two
different groups-Albert Fert and co. at
University of Paris Sud,Orsay,France and
Peter Grünberg and co. at
Jülich,Germany.
Stuart Parkin and his co-workers showed
GMR in multilayer ultrathin film made
by sputtering technique in 1990 by
exploring a long range of thickness.In
1991 spin valve GMR, best way to obtain
AF configuration between two
ferromagnetic layers was invented. After
five years of its discovery GMR sensors
was first exploited commercially by IBM
in 1994 for read out in hard disk drives.
In 1995 TMR was rediscovered by
scientists.
Discovery of GMR and its practical
importance has been recognised with the
award of the 2007 Nobel physics prize to
Albert Fert and Peter Grünberg.
3. Origin of GMR
The phenomenon which is behind the
origin of giant magnetoresistance is
known as spin dependent quantum
mechanical scattering. Spin is getting
scattered from the interface between
single domain ferromagnet and
nonmagnetic ultrathin layer depending
on relative orientation of electron’s spin
and magnetic moment of spin. This
situation is analogous to light passing
through polarizer in optics.
Density of state is defined as number of
states present between energy E and
E+dE . In the ground state Fermi level is
highest occupied energy state. For
nonmagnetic material number of spin up
and spin down electrons are equal so if
we represent this by band structure then
both spin up and spin down electron
energy band will be equal. But in the case
of magnetic materials like Fe or Co, as
there is different number of spin up and
spin down electrons so there is a splitting
in energy band of spin up and spin down
electrons. If any case we consider that
spin up electron is majority spin and spin
down electrons as minority
FIGURE 1: ENERGY BAND IN THE CASE OF NON
MAGNETIC MATERIAL
FIGURE 2: SPLITTING OF LEVEL IN THE CASE OF
FERROMAGNETIC MATERIAL
spin then spin up bands will be filled up
to the maximum energy level. So Fermi
energy in the case of ferromagnet is
different for two different spin
orientation.

3. A.Premashis Kumar GMR And its applications
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Electrical resistance is generated
because of scattering of conduction
electron in material. In a simplest
multilayer system If we consider spin
ordering in both ferromagnetic layers are
same which is spin up, then when a
conducting electron with spin up
orientation tries to go from one layer to
another layer it will not be scattered as it
has same spin orientation. But if the spin
orientation of conducting electron is
down then it will be scattered by two up
spin in the ferromagnetic layers because
of relatively opposite spin orientation.
Because of this spin dependent
scattering spin up conduction will go
through easily as it is freeway for it but
spin down electron will be slows down.
4. Resistive Model of Spin dependent
Conduction
Spin dependent of scattering and
conductivity during the passing of
conducting electron through the
ferromagnetic material was first
suggested by Mott around 1936 and it
was demonstrated experimentally by
Fert and co. around 1968.
If we consider two antiparallel
orientation of spin in the two
ferromagnetic layers then as a first
approximation we can consider this two
layers create two independent channels
for transport of conducting electrons.
Now if we dope the magnetic material
with selected impurity then depending
upon resonant scattering on impurity
level there will be different resistivity at
different channel.
Experiments by Fert using series of Fe
and Ni alloys shows different results. We
can translate the experimental results by
‘resistivity model’ or ‘Two current
model’.
In this two current or resistivity model
electrical resistivity of ferromagnetic
conductor is simply given by:
In the above case we neglect the spin
mixing resistivity term as in the case of
low temperature spin flip scattering is
too small.
Spin asymmetry coefficient of two
channel is given by:
↓
↑
Relation between this and another
coefficient is given by following
equation:
Resistivity is linearly proportional to the
effective mass and inversely
proportional to number of states n,
relaxation time and number of states
↑ ↓
↑ ↓
Where ↑ is resistivity of spin up channel
Where ↓ is resistivity of spin down channel

4. A.Premashis Kumar GMR And its applications
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near Fermi level. So we can say that spin
dependence of the above mentioned
factor namely effective mass, relaxation
time, number of states is actually the
reason of spin dependency of resistivity
of ferromagnet.
If in the case of Ni ferromagnetic layers
we use Co as impurity then experiments
shows that resistivity of spin down
channel is 20 times larger than spin up
channel. As doping creates a blocking
down condition in spin down channel
and there is shunting by spin up channel.
Quantum mechanically we can say that
resonant scattering on impurity level
there is a pick in spin down channel
which is the origin of this kind of blocking
down condition.
On the other hand if we use we use 1% V
as doping element then it results in
completely opposite situation. Resistivity
of spin down channel is very low now.
Now if we add both Co and V impurities
at the same time then there will be
blocking down condition in both
channels of Ferromagnets due to which
conduction electrons will be significantly
slowed down in both channels. So the
resistance of the alloy will be very large.
Whereas if we use Co and Fe as two
impurities then conduction in spin down
channel will be stopped but conduction
electron moves freely through spin up
channel. That’s why resistivity of alloy is
so small. This is the basic idea that leads
to find out the phenomenon called Giant
magnetoresistance.
5. Theoretical Model
The physical model of GMR was
developed on the basis of above
discussed model of ferromagnetic
material. The idea was instead of using
impurities if one puts a nonmagnetic
ultrathin layer of the order of nm on the
way of conduction electron passing
through one layer of ferromagnet to
another layer of ferromagnet.
We consider that in the case of simplest
possible multilayer if spin up and spin
down electron scattered differently when
passing through magnetic layers. Now if
we take two ferromagnetic materials such
that they have spin magnetic moment in
the same direction i.e ferromagnetic
orientation then when conduction
electron is passing through the channel
then one of the spin direction is slowing
down by magnetic layers but for opposite
spin orientation of conduction electron
there will be always a shunting condition
and it will be subjected to very small
scattering. That is why electrical
resistance is very low in the case of
ferromagnetic orientation of the magnetic
layers.
But if we take two layers such that they
have antiparallel spin magnetic moment
then one channel will strongly slows
down the majority spin orientation of
conduction electrons and other channel
will block the minority spin direction of
conduction electron.So both spin
direction is strongly scattered due to
effect of both layers oriented like

5. A.Premashis Kumar GMR And its applications
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antiferromagnet. This results in high
electrical resistance in the case of AF
orientation of two magnetic material.
This theoretical model given by Albert
Fert & co. can explain the phenomenon
properly only if the thickness(t) of
ultrathin nonmagnetic layer << the mean
free path( of conduction electrons.
This is the reason behind selecting
nonmagnetic layer of the order of nm.
In the case of ferromagnetic orientation
we can visualize a basic electronic circuit
consists of two small resistances r in one
branch and two large resistances R in
other branch. So effective resistance of
the whole circuit is given by
So the resistance is dominated by shunt.
On the other hand, we can visualise the
antiparallel orientation of magnetic
layers as a basic electronic circuit which
have r and R resistance in each of the two
branches.
So effective resistance of the circuit is
given by
So as we can see .So
finally the GMR ratio is given as
In the case of parallel orientation
electron of one layer can’t see what is
going on the other magnetic layer as the
magnetic resistance decreases
exponentially.
In more complicated and realistic model
We need to consider wave function and
intrinsic and extrinsic potential in which
strong scattering will be visualised as
spike in potential field.
6. Effect Of External Magnetic Field
We will now see how the above mention
GMR ratio changes in the presence and
absence of large magnetic field. Let us
consider in ferromagnetic the absence of
field if the layers are arranged in AF
orientation. Due to antiferromagnetic
coupling it should have high
magnetoresistance ratio. But by applying
a strong magnetic field we can align all
the spin magnetic moment in certain
arbitrary direction.
As now all the spin moment is in the
same direction so it is similar to
ferromagnetic orientation. In the case of
this orientation as spin have same
direction so current can easily tunnel
through the layers and as a consequence
of this electrical resistance is very low. So
by using magnetic field we can induce a
large change in electrical resistance. The
large change in the resistance is given by
⁄
∗
≫
= =

6. A.Premashis Kumar GMR And its applications
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Which is the amplitude of GMR.
So in experiments there is as large as 80%
variation of resistance have been
observed between AF and ferromagnetic
configuration with ultrathin spacer of 0.9
nm. GMR amplitude is decreased very
rapidly with increase in the thickness of
the spacer as the exchange coupling
between two magnetic layers become
very weak.
If we consider initially multilayer of the
Ferromagnetic orientation then there
will be no effect of applied magnetic
field.
7. GMR Spin Valves
During the application of external
applied field we can’t allow slow
saturation of GMR amplitude which is
common in general case. As in digital
application where GMR will be exploited
for 0 and 1 bit reading , it is required that
GMR amplitude reaches the saturation
point very sharply. Then it will be very
sensitive to small field change. To fulfil
this requirement GMR spin valve was
developed.
The first layer of spin valve is free
magnetic layer which acts as valve. We
can change the magnetic moment of this
free magnetic layer using a small
magnetic field.
After that top layer there is an
antiferromagnetic layer followed by a
multilayer composed of two
ferromagnetic (Co) layers with a
nonmagnetic layer known as spacer.
Magnetic moment of upper (Co) layer is
broken or pinned by the
antiferromagnetic layer above this
because of exchange bias. As the net
magnetization of the upper layer of Co is
zero so we can only switch the second Co
level. By applying external magnetic field
we can rotate the top free layer in either
directions by which we can have
antiparallel configuration with high
resistivity or we can have parallel
configuration of very low resistivity.
Hysteresis curve in this case is aligned
more vertically than the normal case.
GMR amplitude of GMR spin valve can
vary up to 5% even for a very small
magnetic field like 10 Oe. Such a huge
spin magnetic moment sensitivity is very
important for application of GMR in
Digital field.
8. CPP GMR:
Till now we only consider those CIP GMR
with current flowing along the
intermediate layers of multilayers
element. Whereas in the case of CPP
GMR current is flowing perpendicular to
the surfaces. Although this phenomenon
is hard to realise but it is very interesting
one.

7. A.Premashis Kumar GMR And its applications
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Experimental procedure of this is difficult
one as we now need to measure
resistance of a multilayer having very
small geometrical factor and first
experiment of CPP GMR was conducted
at Michigan State University.
CPP GMR works properly if thickness of
ultrathin intermediate layer is smaller
than the spin diffusion length. Spin
accumulation effect at the interface of
nonmagnetic and magnetic layers leads
to occurence of GMR in CPP GMR.
9. Granular GMR
Stuart Parkin and his two co-workers
reproduced the result of Fert and
Grunberg at IBM’s Algmaden research
centre ,San Jose by not using the slow
and precise expensive technique but
they rather exploited faster but not so
precise technique of sputtering.
Co and Cu are immiscible elements so if
we try to add Co to Cu, then Co will form
a cluster like structure over the Cu
matrix. In this case depending on the
cluster size of Cu resistance in this type
of material varies. So in this case there is
no need of multilayer or continuous
layer. So nanoparticle of Co embedded in
Cu matrix is considered as an excellent
candidate to generate GMR because of
spin dependent scattering from interface
of this two elements.
10. Applications
1. By applying GMR magnetic sensors
were developed initially.
2. More difficult applications of GMR was
found eight years later. Multilayers that
produces GMR has been used in read
head of hard disks .It is very difficult to
detect very small magnetic field of bits
but by using GMR is possible to measure
this small bit magnetic field easily. It is
possible to make disk of coin size using
multilayers which is used in ipod classic.
3. Using the similar principle of magnetic
storage devices, magnetonano ‘blood
scanner’ has been developed to detect
blood cancer. In this scanner
nanoparticle is used to ‘tag’ antigen of
cancer and after that this is read out by
using Magnetic sensor.
Acknowledgements: I would like to
thank our course instructor DR.AJIT
KUMAR PATRA.
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Pan; “A brief introduction to giant
magnetoresistance”

8. A.Premashis Kumar GMR And its applications
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[5]Giant Magnetoresistance
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[8]Pratts,Andrew.“So what is Giant
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[9]Ennen,Inga;Kappe,Daniel;Thomas,Rem
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