23-04-2014, 03:55 PM
Spin Valve Transistor
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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.
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.
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. The large values of MC
(%) and Je/Jc indicate a short λ↑ (↓) (of order
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.
CONCLUSION:
Now it is clear that, Spinvalve transistor is more versatile and more robust but it
needs further fabrication methods to improve magnetic sensitivity of collector
current. The greatest hurdle for spintronic engineers may be controlling all that
spin. To do it on a single transistor is already feasible while to do it on a whole
circuit will require some clever ideas. How ever the key question will be whether
any potential benefit of such technology will be worth the production cost. Spin
valve transistors and other spin devices will become affordable by using common
metals.