21-01-2013, 03:00 PM
Spin Valve Transistor
[attachment=47884]
ABSTRACT
In a world of ubiquitous presence of electrons can you imagine any other
field displacing it? It may seem peculiar, even absurd, but with the advent of
spintronics it is turning into reality.
In our conventional electronic devices we use semi conducting materials
for logical operation and magnetic materials for storage, but spintronics uses
magnetic materials for both purposes. These spintronic devices are more versatile
and faster than the present one. One such device is spin valve transistor.Spin valve transistor is different from conventional transistor. In this for
conduction we use spin polarization of electrons. Only electrons with correct spin
polarization can travel successfully through the device. These transistors are used
in data storage, signal processing, automation and robotics with less power
consumption and results in less heat. This also finds its application in Quantum
computing, in which we use Qubits instead of bits.
INTRODUCTION:
Two experiments in 1920’s suggested spin as an additional property of the
electron. One was the closely spaced splitting of Hydrogen spectralines, called
fine structure. The other was Stern –Gerlach experiment, which in 1922 that a
beam of silver atoms directed through an inhomogeneous magnetic field would be
forced in to two beams. These pointed towards magnetism associated with the
electrons.
Spin is the root cause of magnetism that makes an electron tiny magnet.
Magnetism is already been exploited in recording devices. Where data is
recorded and stored as tiny areas of magnetized iron or chromium oxide. To
access that information the head detects the minute changes in magnetic field.
This induces corresponding changes in the head’s electrical resistance – a
phenomenon called Magneto Resistance.
EVOLUTION OF SPINTRONICS:
Spintronics came into light by the advent of Giant Magneto Resistance
(GMR) in 1988. GMR is 200 times stronger than ordinary Magneto Resistance.
It results from subtle electron – spin effects in ultra multilayers of magnetic
materials that cause a huge change in electrical resistance.
The discovery of Spin Valve Transistor (GMR in magnetic multilayers)
has let to a large number of studies on GMR systems. Usually resistance of
multilayer is measured with the Current in Plane (CIP). For instance, Read back
magnetic heads uses this property. But this suffers from several drawbacks such
as; shunting and channeling, particularly for uncoupled multilayers and for thick
spaced layers diminish the CIP magneto resistance. Diffusive surface scattering
reduces the magneto resistance for sandwiches and thin multilayers.
To erase these problems we measure with Current Perpendicular to the
Plane (CPP), mainly because electrons cross all magnetic layers, but a practical
difficulty is encountered; the perpendicular resistance of ultra thin multilayers is
too small to be measured by ordinary techniques.
The use of Micro fabrication techniques for CPP measurements, from 4.2
to 300k was first shown for Fe/Cr multilayers, where the multilayers were etched
into micropillars to obtain a relatively large resistance (a few milli ohms). These
types of measurements have confirmed the larger MR for the CPP configuration,
but they suffer from general complexity of realisation and measurement
techniques. Experiments using electro deposited nanowires showed CPP MR up
to 15% at room temperature, such multilayers find an application in Spin Valve
Transistors.
WORKING:
The collector barrier height about 0.7eV while the emitter barrier height is
0.6eV.The emitter and collector Schottky barrier are in forward and reverse bias
respectively as illustrated by the CB configuration in Fig. 1. The emitter bias
accelerates the electrons towards the emitter barrier, after which they constitute
the hot “Ballistic” electrons in the base. The probability of passing the collector
barrier is limited by the collisions in the base which effect their energy and
trajectory by optical phonon scattering in the semiconductor and by quantum
mechanical reflections at the base collector interface. For a base transistor with a
single metal base film, this can be expressed by the CB current transfer ratio or
current gain.
MAGNETIC SENSITIVITY:
The barrier height of collector and emitter as determined at room
temperature by the current voltage method are 0.7 and 0.6 eV. Because of the low
barrier heights and large area of the collector the leakage current is quiet large
(30μA) and exceeds the magneto current for an injection current of 100mA.
Magneto current measurements have been performed at 77 K reducing the
leakage current to acceptable values, magneto current measurements have been
performed with the CB setup of fig.1, Ie = 100 mA and VBC = 0V. the collector
current Vs the applied magnetic field is plotted in fig. 3 as large current change
with field is observed, with typical GMR characteristics of a second peak Co/Cu
multilayer, such as saturation field and hysterisis. The corresponding CIP-MR
value of implemented multilayer was only 3% in 10K Oe.
TEMPERATURE EFFECTS:
Transport property of hot electron is not fully understood at very low
energy regime at finite temperatures. So, It is necessary to probe the temperature
dependence of the hot electron transport property in relation to the SVT. The
collector current across the spin valve changes its relative orientation of magnetic
movements at finite temperature. Surprisingly the collector current showed
different behaviors depending on the relative spin orientation in Ferro Magnetic
layers. The parallel collector current is increasing up to 200 K and decreasing
after that, while anti-parallel collector current is increasing up to room
temperature. Actually in ordinary metals, the scattering strength increases with
temperature T. This implies that any thermally induced scattering process
enhances the total scattering. As a result measured current should be decreased,
but it is happening so, increasing of collector current with temperature T may not
be related to the ordinary scattering events in the metallic base. Two different
mechanisms are suggested. One of them is spatial distribution of Schottky barrier
diode. This may explain the behaviors of both parallel and antiparallel collector
current upto 200K because thermal energy contributes to over come the Schottky
barrier height at Collector side with the increasing temperature T.
[attachment=47884]
ABSTRACT
In a world of ubiquitous presence of electrons can you imagine any other
field displacing it? It may seem peculiar, even absurd, but with the advent of
spintronics it is turning into reality.
In our conventional electronic devices we use semi conducting materials
for logical operation and magnetic materials for storage, but spintronics uses
magnetic materials for both purposes. These spintronic devices are more versatile
and faster than the present one. One such device is spin valve transistor.Spin valve transistor is different from conventional transistor. In this for
conduction we use spin polarization of electrons. Only electrons with correct spin
polarization can travel successfully through the device. These transistors are used
in data storage, signal processing, automation and robotics with less power
consumption and results in less heat. This also finds its application in Quantum
computing, in which we use Qubits instead of bits.
INTRODUCTION:
Two experiments in 1920’s suggested spin as an additional property of the
electron. One was the closely spaced splitting of Hydrogen spectralines, called
fine structure. The other was Stern –Gerlach experiment, which in 1922 that a
beam of silver atoms directed through an inhomogeneous magnetic field would be
forced in to two beams. These pointed towards magnetism associated with the
electrons.
Spin is the root cause of magnetism that makes an electron tiny magnet.
Magnetism is already been exploited in recording devices. Where data is
recorded and stored as tiny areas of magnetized iron or chromium oxide. To
access that information the head detects the minute changes in magnetic field.
This induces corresponding changes in the head’s electrical resistance – a
phenomenon called Magneto Resistance.
EVOLUTION OF SPINTRONICS:
Spintronics came into light by the advent of Giant Magneto Resistance
(GMR) in 1988. GMR is 200 times stronger than ordinary Magneto Resistance.
It results from subtle electron – spin effects in ultra multilayers of magnetic
materials that cause a huge change in electrical resistance.
The discovery of Spin Valve Transistor (GMR in magnetic multilayers)
has let to a large number of studies on GMR systems. Usually resistance of
multilayer is measured with the Current in Plane (CIP). For instance, Read back
magnetic heads uses this property. But this suffers from several drawbacks such
as; shunting and channeling, particularly for uncoupled multilayers and for thick
spaced layers diminish the CIP magneto resistance. Diffusive surface scattering
reduces the magneto resistance for sandwiches and thin multilayers.
To erase these problems we measure with Current Perpendicular to the
Plane (CPP), mainly because electrons cross all magnetic layers, but a practical
difficulty is encountered; the perpendicular resistance of ultra thin multilayers is
too small to be measured by ordinary techniques.
The use of Micro fabrication techniques for CPP measurements, from 4.2
to 300k was first shown for Fe/Cr multilayers, where the multilayers were etched
into micropillars to obtain a relatively large resistance (a few milli ohms). These
types of measurements have confirmed the larger MR for the CPP configuration,
but they suffer from general complexity of realisation and measurement
techniques. Experiments using electro deposited nanowires showed CPP MR up
to 15% at room temperature, such multilayers find an application in Spin Valve
Transistors.
WORKING:
The collector barrier height about 0.7eV while the emitter barrier height is
0.6eV.The emitter and collector Schottky barrier are in forward and reverse bias
respectively as illustrated by the CB configuration in Fig. 1. The emitter bias
accelerates the electrons towards the emitter barrier, after which they constitute
the hot “Ballistic” electrons in the base. The probability of passing the collector
barrier is limited by the collisions in the base which effect their energy and
trajectory by optical phonon scattering in the semiconductor and by quantum
mechanical reflections at the base collector interface. For a base transistor with a
single metal base film, this can be expressed by the CB current transfer ratio or
current gain.
MAGNETIC SENSITIVITY:
The barrier height of collector and emitter as determined at room
temperature by the current voltage method are 0.7 and 0.6 eV. Because of the low
barrier heights and large area of the collector the leakage current is quiet large
(30μA) and exceeds the magneto current for an injection current of 100mA.
Magneto current measurements have been performed at 77 K reducing the
leakage current to acceptable values, magneto current measurements have been
performed with the CB setup of fig.1, Ie = 100 mA and VBC = 0V. the collector
current Vs the applied magnetic field is plotted in fig. 3 as large current change
with field is observed, with typical GMR characteristics of a second peak Co/Cu
multilayer, such as saturation field and hysterisis. The corresponding CIP-MR
value of implemented multilayer was only 3% in 10K Oe.
TEMPERATURE EFFECTS:
Transport property of hot electron is not fully understood at very low
energy regime at finite temperatures. So, It is necessary to probe the temperature
dependence of the hot electron transport property in relation to the SVT. The
collector current across the spin valve changes its relative orientation of magnetic
movements at finite temperature. Surprisingly the collector current showed
different behaviors depending on the relative spin orientation in Ferro Magnetic
layers. The parallel collector current is increasing up to 200 K and decreasing
after that, while anti-parallel collector current is increasing up to room
temperature. Actually in ordinary metals, the scattering strength increases with
temperature T. This implies that any thermally induced scattering process
enhances the total scattering. As a result measured current should be decreased,
but it is happening so, increasing of collector current with temperature T may not
be related to the ordinary scattering events in the metallic base. Two different
mechanisms are suggested. One of them is spatial distribution of Schottky barrier
diode. This may explain the behaviors of both parallel and antiparallel collector
current upto 200K because thermal energy contributes to over come the Schottky
barrier height at Collector side with the increasing temperature T.