CHAPTER-1
INTRODUCTION
1.1 GENERAL INTRODUCTION
Spintronics is a rapidly emerging field of
science and technology that will most likely have a significant impact on the
future of all aspects of electronics as we continue to move into the 21st
century. Conventional electronics are based on the charge of the electron.
Attempts to use the other fundamental property of an electron, its spin, have
given rise to a new, rapidly evolving field, known as spintronics, an acronym for spin transport electronics that was
first introduced in 1996 to designate a program of the U.S. Defense Advanced
Research Projects Agency (DARPA). Initially, the spintronics program involved
overseeing the development of advanced magnetic memory and sensors based on
spin transport electronics Studies of spin-polarized transport in bulk and
low-dimensional semiconductor structures show promise for the creation of a
hybrid device that would combine magnetic storage with gain—in effect, a spin
memory transistor.
Magnetic
materials and magnetic devices have occupied a major place in science and
technology for most of the twentieth century and played a very important role
in the emergence of the digital computer by providing both ferrite core and
plated wire memories. It was not until the early 1980s that thin-film magnetism
was applied to higher-density nonvolatile random access memory .A new path
leading to the integration of magnetic devices into computer technology began
to emerge with the discovery of giant magnetoresistance (GMR at low
temperatures and high magnetic fields. Although it was known for quite some
time that the current from a magnetic metal is spin-polarized and that current
transport through adjacent magnetic layers depends on the spin-polarization of
those layers, neither the magnitude of the current nor the temperature at which
it was observed were of technological significance. Discoveries in this
new field were quite rapid, and the path toward a new technology started to
appear quite early. The first significant GMR device was the spin valve
CHAPTER-2
THEME: SPIN VALVE TRANSISTOR
2.1 Giant Magnetoresistance
Magnetic field sensors have found many applications:
read heads in audio/video/computer systems,magnetic random access memories
(MRAMs), position/ rotation/ velocity sensors in cars/ aircrafts/satellites,
electronic compass applications, measurement of currents and scientific
measurement instruments. Other applications for magnetoresistive sensors
include coin evaluation, non-contact switching, and measurement of currents. An
important issue in digital magnetic recording is the bit density and several
new technologies have pushed this density forward. Future high density
recording systems will depend increasingly on more sensitive field sensors,
because of the shrinking bit sizes and magnetic fluxes. The thin film head, the
thin film media and subsequently the introduction of magnetoresistance heads
enhanced the annual bit density increase drastically.Due to tailoring of the
magnetic materials in the base, the spin valve transistor shows a broad measurable
field range and may further enhance bit densities.
A major advantage of using
magnetoresistive sensing of magnetic fields as compared to inductive sensing is
the static measurement mode of the MR sensor: a static magnetic field can be detected
in contrast to inductive pick-up coils for which a voltage is only generated by
a temporal flux change. For magnetic recording, the increase of density leads
to a corresponding reduction in magnetic signal. Inductive head designs have
compensated for the weakening signal by increasing the number of turns in the
coil, but each turn adds approximately 0.5 ohm of resistance to the circuit,
with a corresponding increase in thermal noise. Beyond 1 Gbit/in2, this thermal noise
of the coil becomes the main limitation preventing signal detection
This new magnetoresistance called
"Giant Magnetoresistance", was discovered in 1988 in magnetic
multilayers. It was soon called the spin valve effect because the magnetic
layers act as valves for electrons with different spin moments (spin up and
spin down). The spin-valve transistor consists of three regions: a spin-valve
base, a hot electron injector such as a Schottky barrier or a tunneling barrier
and a collector barrier which discriminates between scattered and ballistic
(not scattered) electrons. The base can be made of any magnetoresistive metal
system.
2.2 The spin valve effect
GMR effect can be observed in
the conduction process in magnetic materials,
particularly the transition metals Fe, Co and Ni.Conduction electrons are divided in to two classes , those whose spin
is parallel to the local magnetization and those whose spin is anti parallel
.The resistance to the flow of an electronic current in a metal is determined
by the scattering processes to which the electrons are subject. If the
scattering processes are strong and effective, the mean free path (mfp) of an
electron between scattering processes is small and the resistance is
large.Conversely, weak scattering processes lead to a long mfp and a low
resistance.
Fig
2.1: Graph
of conduction in multilayer magnetic film array, showing how differential spin
scattering produces a different resistance for antiparallel (a) and parallel
(b) film magnetizations.
Consider now electronic conduction in a multilayer
array such as shown in Fig. 2.1 In Fig. 2.1a the magnetic moments of successive
ferromagnetic layers (Co) are antiparallel due to antiferromagnetic coupling
across the spacer layer (Cu). In (b) they are parallel due to an external magnetic
field which is strong enough to overcome the antiferromagnetic coupling. In
case of Fig.2.1a , antiparallel moments,
no electron can traverse two magnetic layers without becoming unfavored , highly
scattered species. An electron conserves its spin orientation as it traverses a
solid .Therefore if it was the favored 'up' electron in an 'up' magnetization
layer it becomes the unfavored 'down' electron in an 'up' magnetization layer
as soon as it traverses the few Ångstroms of the spacer layer. In the case
depicted in Fig.2.1a , by contrast, an electron having the favored 'up' spin
orientation in one magnetic layer has the same favored orientation in all
layers, and can traverse the array relatively freely. For configuration (a) no
electron traverses the array freely; for (b) half of the electron species can
traverse the array relatively freely, and a significant difference in
resistance is measured between the parallel and anti-parallel arrays.
2.3 Spin valve
transistor
2.3.1 Spin valve transistor principle
The perpendicular electron
transport and exponential mean free path dependence in metal base transistors allows
for fundamental detection of the perpendicular spin valve effect by
incorporating a spin valve into the base. The base is formed by a spin valve. A
Co44Å/Cu88Å/Co8Å/Pt88Å sandwich base is sputtered onto a Si(100) collector
substrate. The emitter is negatively biased (forward) using a DC current
source, the collector substrate is in reverse (positive voltage bias), in common
base.
Fig
2.2 Schematic cross section of the spin-valve
transistor. A Co44Å/Cu88Å/Co8Å/Pt88Å sandwichbase is rf-sputtered onto the Si(100) collector substrate.
A Pt capping layer on top of the spin valve is used
to make the emitter Schottky barrier larger than the collector barrier, in
order to decrease quantum mechanical reflections at the collector barrier. This
can also be seen in the schematic energy band diagram of the bonded Co/Cu
spin-valve transistor in Fig. 2.2
Fig. 2.3 Schematic energy band diagram of the spin-valve
transistor under forward bias.
2.4
Current Transfer
The emitter bias accelerates the
electrons over the emitter barrier, after which they constitute the hot,
quasi-ballistic electrons in the base. The probability of passing the collector
barrier is limited by collisions in the base, which affect their energy and
trajectory (momentum), by optical phonon scattering in the semiconductors and
by quantum mechanical reflections at the base-collector interface. For a metal
base transistor with a single metal base film the relationship between the
collector current density Jc and the injected emitter current density Jinj is
where W is the base width (=thickness) and l the mean free path of the injected hot
electrons in the base. ae
represents the emitter efficiency, aqm represents quantum mechanical transmission and ac represents the collector efficiency. Jleakage is the
collector leakage current, determined by the reverse biased collector Schottky
barrier and Je is the injected emitter current. The
avalanche multiplication factor M depends on device design but if impact
ionization is absent, equals one. The leakage current of the collector may also
contribute to the total collector current.
The emitter to collector current transfer ratio, or
current gain is defined as:
where the collector leakage current has been
neglected. Here a0 is the
common base current gain and a* is the common base current gain extrapolated to zero base thickness. The
factor
represents the probability of
transmission of the hot electrons through the base. Jc is the
total collector current. In the spin-valve transistor under consideration, the
collector current of the Co/Cu spinvalve
transistor depends exponentially on the spin dependent hot electron mean
free paths in the base. Neglecting spin-flip scattering, we may consider the
spin up and spin
down electrons to carry the current in
parallel (two current model). Following this idea, the collector current of the
Co/Cu spin-valve transistor is
expressed as:
……………(2.3)
Õ +(-) denotes
the product of transmission probabilities of spin up (+) and down (-)
electrons through each layer and interface. In first
approximation we take ae, ac and aqm similar for the two species of electrons
since these quantities reflect the properties of the semiconductors and Schottky
barriers. At saturation, all Co layers have their magnetization parallel.
The sum of the transmission probability factors for the
two spin channels can then be written as:
……………… (2.4)
………………(2.5)
where WCo expresses the sum of all Co layer
widths (total Co thickness) which is valid for equally thick layers, ½W is half
of the total Co thickness, WCu is the total Cu thickness , l the majority (minority) MFPs in the Co layers and l Cu the MFP in the Cu layer. The
factor 2 in eqn appears because the two parallel channels are equal for magnetizations
antiparallel. The values of the collector current in the parallel (P) and
antiparallel (AP) magnetic configurations are then obtained .
The typical properties in the spin valve transistor are
thus:
- Perpendicular
GMR can be measured down to tri-layers
- Exponential
amplification of the magnetoresistance occurs because the transfer is exponentially
dependent on the electron mean free path in the base
- Electron
energy can be varied so electron spectroscopy can be performed by
changing emitter Schottky barrier
height (or tunnel bias)
- Measurements
can be done at cryogenic and room temperature
- Since the scattering
processes appear as products in the transfer equation., the spin dependent scattering centers can be
located accurately and, in contrast to common - CPP MR, the relative change in
collector current CC(%) is not decreased by spin independent scattering processes such
as in the Cu layers or in the semiconductors
- As
a consequence of the direct MFP dependence of the transmission across the
base, the spinvalve transistor allows quantification of spin dependent
electron MFPs l
7. The
output is a high impedance current source.
2.5 Resistance measurement
Resistance
of the multilayer can be measured with
Current In Plane (CIP) or Current Perpendicular to the Plane (CPP)
configurations. CIP is the easiest experimental approach of electrical
transport in magnetic multilayers. But the drawback of CIP configuration is that the spin valve
effect is diminished by shunting because
many electron travel within one layer
because of channeling. Uncoupled multilayers or sandwitches with thick spacer layer suffer from this
problem .Spin independent boundary scattering reduces the CIP magnetoresistance
largely in thin sandwiches. Also, fundamental parameters of the effect, such as
the relative contributions of interface and bulk spin dependent scatterings are
difficult to obtain using the CIP geometry. Measuring with the Current
Perpendicular to the Planes (CPP) solve most of
these
problems, mainly because the electrons cross all magnetic layers, but a
practical difficulty is encountered: the perpendicular resistance of the ultra
thin multilayers is too small to be measured by ordinary techniques
.
.
Fig. 2.4 a. CIP-GMR: shunting and channeling of electrons
in the magnetic and nonmagnetic layers versus b. CPP-GMR: perpendicular
electrons cross all magnetic layers, no shunting at antiparallel alignment.
As shown in Fig. 2.4, a high resistant state (in
zero field) can only be obtained if electrons cross at least two magnetic
layers with antiparallel orientation.
Because many electrons travel almost parallel to the layers in the CIP-GMR, and
do not cross many layers, the adjacent
layers must have the antiparallel orientation, i.e. they need an antiferromagnetic
coupling. In the case of CPP-GMR the electrons cross all layers, and a random
orientation of the layers produces the same high resistant state as the
AF-coupled state (“self averaging”). In CIP-GMR the electric field is
independent of position in the film, but the current density depends on the
perpendicular direction to the film. The characteristic length scale is the
longest mean free path. For CPP transport, the electric field depends on the
perpendicular position in the film, but the current density is independent of
position in the film. The spin diffusion length is the new length scale.
2.6 Scattering Mechanism
To stress the difference with Fermi transport, we
demonstrate the electron energy dependence of the scattering mechanisms in it.
Three important transport processes affect GMR:
1. spin dependent bulk scattering in the
magnetic layers
2. spin dependent scattering at the
interfaces
3. reflection at the interfaces due to band
mismatch between the layers.
The scattering processes leading
to bulk scattering have quasi elastic phonon, magnon and elastic defect
scattering. The scattering processes leading to diffusive interface scattering
are mainly temperature independent elastic defect and impurity scattering.
Inelastic electron-electron interactions are neglected both for Fermi transport
and hot electrons .Also, phonon and magnon scattering are neglected (low
temperature restriction), but may be included when finite temperatures are
considered. Since defect and impurity scattering are of the same nature, both
interface and bulk scattering may be included in one picture, taking different relaxation
times only. The third process, quantum
mechanical reflection at the layer interfaces, is entirely different. CPP
transport incorporating the interface and bulk diffusive scattering has been
modeled by the series resistor model which was used very effectively to
describe the resistance in CPP experimentsThe total resistance is
Here rP and rAP are the
CPP resistances per unit area and per superlattice period in the parallel (P) and
antiparallel (AP) magnetic configurations respectively. .
and
. are used
for the majority and minority spin directions in a magnetic layer. r*F is the experimentally measured bulk
resistivity of the ferromagnetic film, r* N is the
non-magnetic bulk resistivity and r* b is the spin averaged interface resistance.
2.7 Electron transport in the spin valve
transistor
2.7.1 Schottky
(Thermionic) injection: emitter efficiency e
The
various ways in which electrons can be transported across a metal-semiconductor
junction under forward bias are shown schematically for an n-type semiconductor
. The mechanisms are:
(a) emission of electrons from the
semiconductor over the top of the barrier into the metal
(b) temperature assisted tunneling through
the barrier: thermionic field emission
(c) direct tunneling through the barrier:
field emission
(d) recombination in the space charge region
(e) recombination in the neutral region
(hole injection)
It is possible to make Schottky barrier diodes in
which (a) is the most important transport mechanism and such diodes are
generally referred to as “nearly ideal”. Processes (b) and (c) may contribute under
high doping and low temperature conditions. Under normal conditions, (c) and
(e) hardly contribute. The relative contributions of the other transport
processes depend mainly upon temperature, doping and applied bias To analyze
the injection of electrons into the base, the electron potential energy as a
function of distance from the metal is schematically drawn in Fig. 2.5.
In Fig. 2.5 qfe is the
barrier height and Dqfe is the
emitter barrier lowering due to the electric field and the image force. xl is the
point where an electron at rest in the emitter has enough energy to surmount a
collector barrier of height fc. Dqfec is the energy difference between the emitter and collector
barriers of the full metal base transistor structure.
Fig. 2.5 Electron
potential energy q as a
function of distance in a metal semiconductor Schottky barrier and electron
transport processes under forward bias condition.
As shown in Fig.2.5 the barrier maximum is not at
x=0 but at xm. This deviation is due to the image force
correction . According to the thermionic emission-diffusion theory the forward
transport of electrons according to process (a) can be described as:
where J is the forward current density, IRs is the
voltage drop due to series resistance and
is the saturation current density and A** is the
effective Richardson
The ideality factor n is defined as
which is
reflected by the slope of the forward
current response. The contribution of transport processes (b), (c), (d)
and (e) to the total injection current causes the n-factor to become larger
than 1.
2.7.2 qm:Quantum
mechanical transmission factor at the collector barrier
Quantum mechanics allows particles to
penetrate an energy barrier larger than its own energy. Also , a particle with
energy larger than a potential barrier, may be partly reflected. Because the
average electron kinetic energy in the metal is much larger than in the
semiconductor due to the addition of the Fermi energy of the metal.This energy
is lost in when the electron enters the conduction band of the collector
semiconductor. A simple step potential model of the collector Schottky barrier
gives some insight in the relative importance of parameters. The relatively
large electron energy loss justifies the use of a step potential to model the
Schottky barrier. For smaller energy losses when using metals with small Fermi
energies such as Cs (1.5eV) would require more correct potential shapes, as
presented in Fig. 2.6.
Fig. 2.6
Metal semiconductor barrier models
2.7.3 Semiconductor transport: collector efficiency αc
The angle of acceptance in the collector is
quite small
When electrons are transmitted into the
collector within the angle of acceptance, there is a further limitation to
collection: electron-phonon scatterings before the collector barrier
maximum may throw back the electron into the metal. As in the emitter, within
the collector, electrons
can scatter by emission of optical phonons. As shown in Fig. 4.6 the position
of the Schottky
barrier maximum is not at the metallurgical M-S interface but is shifted by a
few nm into the semiconductor due to the image potential.
Electrons with energies just over the threshold for transmission that
excite phonons in the region before the Schottky barrier maximum are expected
to have a high probability of reentering the metal. Beyond the Schottky barrier
maximum, the internal electric field in the depletion region accelerates the
electrons toward the interior of n-type semiconductors. Therefore the effect of
phonon scattering on the magnitude of Ib in the region beyond the Schottky
barrier maximum depends on the doping density of the semiconductor, since this
defines the length of the depletion region and thus the acceleration rate.
Fig. 2.7 Collector
Schottky barrier under reverse bias showing the maximum of the barrier at xm resulting
from image force lowering and reverse transport mechanisms (a) thermionic
emission (b) thermionic field emission and (c) field emission.
2.7.4 Impact
ionization: avalanche multiplication
Once the kinetic energy of the electrons in the collector semiconductor
exceeds Eg, electron-hole pair generation, or impact
ionization, becomes possible, see Fig. 4.7. This process is usually employed in
Avalanche Photodiodes (APDs) to increase the detector current. This process can
also take place in metal base transistor structures, and has recently been
observed as a parasitic process in BEEM experiment. In reverse biased Schottky
diodes ,breakdown may occur due to tunneling or avalanche breakdown. When the
electric field in a semiconductor is increased above a certain value, the
carriers gain enough energy so that they can excite electron-hole pairs by
impact ionization.
Fig. 2.8 Electron-hole
pair generation in the reverse biased collector barrier.
The electron-hole pair generation rate G for impact
ionization is given by
Where an is the electron ionization rate defined
as the number of electron-hole pairs generated by an electron per unit distance
traveled. Similarly ap is the
analogously defined ionization rate for holes. a n,p is strongly field dependent as can
be observed in the physical expression for the ionization rate
2.7.5 Schottky reverse saturation current: collector
leakage current Jbc
The reverse
current of the collector barrier can be considered to be a parasitic current
which limits detection of the hot electron current in the collector under
certain conditions. The principal leakage current is determined by electrons
which have a thermal energy larger than the barrier height. Obviously this
current is very sensitive to temperature and was deduced from the thermionic
emission theory as
where A** is the
effective Richardson
A plot of the calculated saturation current versus barrier
height for Si is shown in
Fig 2.9
Fig.
2.9 saturation
current density Js versus barrier height, at T=77, 200 and 295K. A** has
been taken 112 (A cm²K²) for Si.
2.8 Vacuum bonding: Spin valve transistor
preparation
2.8.1 Schematic process flow
In vaccum
bonding initially cleaning process is
done.For this 1 micron tetra ethyl ortho
silicate (TEOS) SiO2 is used as protecting layer and the Si fragments are
etched away using HF/HNO3 at room temperature isotropic etch.Preparation scheme
is shown in figure 2.10.
Fig2.10
Schematic process flow for the preparation of
vacuum bonded spin valve transistors
2.8.2 Deposition of the base
layers
For deposition of the base layers
a DC-RF magnetron sputtering machine is used. The robot is inserted into loadlock
F and transported using beam G to the main chamber A after approximately 1 hour
pumping. Multilayers can be deposited using a computer controlled rotating
table and deposition shutters.
Fig. 2.11 High vacuum DC/RF magnetron sputter system.
The properties of the system are: background
pressure typically 10-9 mbar, three magnetron sputter guns,
variable substrate-target distance, heated substrate table, RF and DC power
supplies. Twelve different samples can be sputtered in one run using the
specially designed substrate rotator, of which a schematic picture is shown in
Fig. 4.9.
Fig.2.12 Substrate rotator for multiple in-situ sample
preparation.
Spring 1 is wound up using manipulator
2. Samples 6 are mounted on rotating table 4. Deposition occurs via 5.
Substrate selection is via magnetically coupled beam 3. In this way optimized
GMR multilayers and sandwiches can be found quickly.
2.8.3 Emitter wafer thinning
After bonding, the emitter substrate has to be
thinned down to dimensions which allow definition of transistors to micron
dimensions. For this reason, the emitter has to be thinned down to about 1 to 5
micron. A major requirement is that the emitter substrate needs a highly doped
region for ohmic contact formation (the emitter barrier contact is reverse
biased, in contrast to the collector contact). In so called BESOI (Bond and
Etch back Silicon On Insulator) several techniques are known to come to a small
device layer: 1. Grinding and polishing 2. Etch stop layers
Grinding and polishing is a
possibility for the required device layer thickness, and would be the most
obvious way for standard wafer size. Thickness variations of about 0.5 microns
are achievable. In grinding one has to be careful with subsurface damage and
the final etching has to be performed chemo-mechanically. However for small samples chemo mechanical polishing is not used.
Using etch stop layers thickness variation of about 5 microns is
obtainedEtch stops using HF anodic etching usually provide fast etching of p
type and n++ type, so in this case an etch stop on n++ is not possible.
Moreover, it is difficult to grow defect free device n-layers wafers on a
buried n++ layer, sufficiently high doped for ohmic contact formation. This
problem also plays a role in etch stops using highly B doped p++ Si and KOH,
TMAH or EPW. Another disadvantage of this technique is that it is difficult to
grow defect free layers on top of this layer .Addition of larger Ge to the B
atoms provides stress free etch stop layers without misfit dislocations.
Electrochemical etch stops using P/N junctions require KOH etching at elevated temperatures
with the additional buried n++ layer problem.
2.8.4 Completed spin valve transistor structure
Following emitter thinning, the
base region is defined using photolithography: photoresist prebaked at 900°C was
used to protect the base either during wet etching (10:H₂O₂/1:HF) during 20 seconds (for Co/Cu) or using ion beam etching during 30
minutes. To reduce the large sputter induced leakage currents after ion beam
etching, a short TMAH silicon etch is necessary to remove the damaged silicon
surface next to the base. For the H₂O₂/HF base
etch this is not required since it does not introduce defects and grows a
surface passivating SiO₂ automatically.
Since the H₂O₂/HF tends to attack the photoresist, care has to be
taken not to etch longer than 1 minute. After the base etching procedure, the
substrate is glued using conducting room temperature curing epoxy with its
backside ohmic contact to a printed circuit board, aluminum wires are
ultrasonically bonded to the base and emitters and is ready for electrical
characterization .
2.8.5 Processing other semiconductors
Experiments
with Germanium collectors have also been performed. The difference in
preparation before metal deposition with Si is that Ge can be etched using HF/H₂O₂ (1/10). This etchant does not attack photoresist and consequently, the surface can be
protected with a single (hardbaked) photoresist layer. First experiments with
epitaxially grown n-GaAs films on n+ GaAs substrates have also provided
excellent bonds. Photoresist was employed to offer protection during a H₂SO₄/H₂O₂/H₂0 fragment etch. There are new ways to obtain both a very clean GaAs
surface and very good Schottky barriers: an AlAs layer has been grown in situ
over the epitaxial GaAs layer, providing protection of the GaAs surface. (it is
even possible to use a buried AlAs layer as an etch stop with citric acid/H₂O₂ etchants .This top layer is removed using a very selective HF 2% (1 min)
dip as a final cleaning step prior to bonding. We found nearly ideal Schottky barriers
using this method. Another technique for preparing ideal Schottky barriers on
GaAs involves substrate heating (5500°C)
before deposition .
2.9 Other spin-valve configurations
2.9.1 Granular GMR
As indicated, it is very difficult to measure
perpendicular transport GMR. An intermediate between in-plane and perpendicular
GMR can be created by making small ferromagnetic grains in a metallic matrix. A
sketch of such a system is given in Fig.
6.1
Fig.
2.13 Sketch of granular GMR (G2MR). The high
resistant state can be the uncoupled random state of magnetic granule
In granular systems the ferromagnetic
particles are assumed to be single domain and uncoupled. In this case the
resistance will be largest for randomly oriented particles, which occurs when
the total magnetization of the sample is zero, and lowest at applied saturation
field Hs. The GMR in granular systems in independent of the direction of the
applied field, unlike in multi layers where the saturating field is larger for
perpendicular fields, than for in-plane fields. The MR is mostly due to spin
dependent grain-matrix interface scattering and to a lesser extent, from spin dependent
scattering within the magnetic grains. The major disadvantage of granular
systems is the saturation field which is usually very large .. However, this
causes local pinholes in the nonmagnetic spacer layers to be filled with
ferromagnetic material, inducing ferromagnetic bridging between the magnetic
layers. This effect is decreased when the ferromagnetic layers consist of clusters
,because the ferromagnetic bridging does not proceed across the whole magnetic layer,
but is limited to one granule. For the spin-valve transistor this separation of
magnetic layers in clusters to avoid ferromagnetic bridging is not required,
since the spacer layer can be made thick enough
2.9.2 Inverse GMR
When the resistance of a spin
valve system increases under
application of a magnetic field, it is said to be inverse GMR. However, this
effect may be related to three principles: The first is the anisotropic
magnetoresistance (AMR) which may lead to parasitic effects, and which may,
depending on current-field angle, produce an inverse behaviour. The second is
the simple observation that the magnetisation of the magnetic layers orient
more into antiparallel directions, like in Co/Cu multilayers going from zero
field to the coercive field.. The third effect is of more relevance: it is
related to the band structure of certain materials that cause inverse spin
asymmetry. The inverse GMR effects can be accounted for
qualitatively by opposite scattering spin asymmetries in Co (positive spin
asymmetry) and A (negative spin asymmetry) (A is an alloy). For two layers with
both positive spin asymmetry, high resistance is observed, ergo normal GMR. If
however the lower magnetic layer is replaced by an alloy with negative spin
asymmetry, electrons that passed freely through the first layer can also pass
the second layer, and now the antiparallel situation represents a low
resistance state, hence inverse GMR. Quantitatively the inverse effect can be
accounted for by introducing bulk and interface spin asymmetry coefficients as
in CPP-GMR, and respectively.
2.9.3 Magnetic tunnel junctions: MTJs
Electrons are able to pass from one conducting
electrode (initial electrode) to another (final electrode) via a thin insulator
(1-5 nm) with an energy barrier larger than the electron energy. This quantum
transport phenomenon is called electron tunneling. Instead of a Schottky
barrier to create hot electrons in the spin valve base, the spin valve
transistor can also be made using incorporation of a tunneling emitter A tunnel
collector could also be opted for, as in MOMOM (M=metal, O=oxide) high speed
devices. A magnetic tunnel junction acting as emitter could add extra magnetic
field dependence to the collector current change, since the transmission of the
tunnel structure as well as the transfer characteristics in the base spin valve
add constructively. In case of an antiferromagnetic tunnel barrier such as NiO
or CoO, the tunnel injection could be combined with the pinning abilities, to
construct exchange biased spin valves in the base.
In the normal case magnetic tunnel
junctions (MTJs) consist of a ferromagnetic initial electrode, an oxidic
barrier and again a ferromagnetic final electrode.With no voltage applied the
Fermi levels of the two electrodes must align. Under application of a bias, a
voltage drop V and energy level difference eV across the insulator is found.
The current is found using Fermi’s golden rule: the number of electrons
tunneling is given by the product of the density of filled states at a given
energy in one electrode and the density of empty states in the other electrode
at the same energy multiplied by the square
of a matrix element describing the probability of
tunneling. For this model the total current from the initial electrode to the
final electrode is proportional to
………….(2.14)
In the model proposed by Julliere, it is assumed that the spin is
conserved in the tunneling process and that the conductance of each spin
direction is proportional to the density of states of that spin in each
electrode. In this model one expects the tunneling current to be larger when magnetizations
are parallel as compared to antiparallel orientations. The conductances of the
junctions are then:
I(ap)par the current in the (anti)parallel
case, where Ni the number of available electrons on the
injecting electrode and Df the number of available empty
sites in the DOS of the collecting electrode.
This situation may be illustrated graphically:
Fig.2.14
Illustration of effect in magnetic tunnel
junction (MTJ) consisting of two ferromagnetic electrodes separated by an insulator.
In the case of parallel magnetisations, Ni,up*Df,up>0, hence acurrent may be flowing. In the
antiparallel case, Df,up=0 and Ni,down=0, consequently zero current.
2.10 Advantages
- Traditional transistors use on and off currents to
create bits the binary zero and one of computer information, quantum spin
valve transistor will use up and down spin states to generate the same
binary data.
- Currently logic is usually carried out
using conventional electrons, while spin is used for memory. Spintronics
will combine both.
- In most semi
conducting transistors the relative
proportion of up and down carrier
types are equal. If ferromagnetic
material is used as the carrier source then the ratio can be deliberately
skewed in one direction.
- Amplification
and/or switching properties of the device can be controlled by the external magnetic field applied to
the device.
- One of the
problems of charge current electrons is that we pack more devices
together, chip heats up. Spin current releases heat but it is rather less
2.11 Applications
1
Spin valve
transistors have huge potential for incoporatio
in stable, high sensitivity magnetic
field sensors for automotive , robotic , mechanical engineering and data
storage applications.
2
It finds its
application towards quantum computer, a new trend in computing here we use
qubits instead of bits.Qubit exploit spin up and spin down states as super
positions of zero and one.
3
They have the
advantage over conventional semi conductor chips that do not require power to
maintain their memory state.
4
This may also be used as Magnetically controlled
parametric amplifiers and mixers, as magnetic signal processors for control of
brush less dc motors as magnetic logic elements.
2.12 Related work
Scientists have recently proposed
new class of spin transistors, referred to as spin-filter transistor (SFT) and
spin metal-oxide-semiconductor field-effect transistor (spin MOSFET), and their
integrated circuit applications. The fundamental device structures and
theoretically predicted device performance are theoretically calculated
predicted. The spin MOSFETs potentially exhibit significant
magnetotransport effect such as large magneto-current and also satisfy
important requirements for integrated circuit applications such as high
transconductance, low power-delay product, and low off-current. Since the
spin MOSFETs can perform signal processing and logic operations and can store
digital data using both of the charge transport and the spin degree of freedom,
they are expected to be building blocks for a memory cell and logic gates on
spin-electronic integrated circuits. Novel spin-electronic integrated
circuit architectures for nonvolatile memory and reconfigurable logic employing
spin MOSFETs are also proposed
Now researcher Christian Schoenenberger and
colleagues at the University of Basel,
Switzerland, describe a carbon nanotube transistor operating on a same
principle, opening a promising avenue toward the introduction of spin-based
devices into computer chips. A device
consisting of a single carbon nanotube connected to two magnetic electrodes that control the
orientation of the electrons’ spins have
been developed.
2.13 Future scope
There
are major efforts on going at ibm, Motorola in developing RAM based on spin
valves, such devices called MRAMs have demonstrated faster speeds, high density
,low power consumptions and nonvolatility. They are promising replacement for
semi conducting rams currently used
Also
reserches are going on to replace Pt with suitable combinations
of metal (low cost alloys ) in order to make it affordable at minimum cost.
CHAPTER-3
CONCLUSION
Now it is clear that spin valve 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 cicuit will require some clever ideas. In the spin-valve transistor perpendicular
GMR can be measured down to tri-layers. Exponential amplification of the
magnetoresistance occurs because the transfer is exponentially dependent on the
electron mean free path in the base. Electron energy can be varied so electron
spectroscopy can be performed by changing emitter Schottky barrier height (or
tunnel bias). Measurements can be done at cryogenic and room temperature. Since
the scattering processes appear as products in the transfer equation., the spin
dependent scattering centers can be located accurately and, in contrast to common
CPP-MR, the relative change in collector current CC(%) is not decreased by spin
independent scattering processes such as in the Cu layers or in the
semiconductors.However 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.
REFERENCES
1. Dr S.S. Verma ,” Spintronics for the
Ultimate in Performance” Electronics
for
You , VOL. 34 NO. 8.August 2002, Pages 110-11
