A technical report based on the power point presentation of Spintronics

1. SPINTRONICS
INTRODUCTION
Imagine a data storage device of the size of an atom working at the speed of light. Imagine a
microprocessor whose circuits could be changed on the fly. Imagine a computer memory
thousands of times denser and faster than today's memories.
The above mentioned things can be made possible with the help of spintronics.
Spintronics is an amalgamation of Physics, Electronics and nanotechnology which deals with
spin dependent properties of an electron instead of or in addition to its charge dependent
properties.
Conventional electronics devices rely on the transport of electric charge carriers-electrons.
But there are other dimensions of an electron other than its charge and mass i.e. spin. This
dimension can be exploited to create a remarkable generation of spintronic devices. It is
believed that in the near future, spintronics could be more revolutionary than any other thing
that nanotechnology has stirred up so far.
WHY IS IT GOING TO BE ONE OF THE RAPIDLY EMERGING FIELDS?
Moore’s law states that the number of transistors on a silicon chip doubles every 18 months.
As there is rapid progress in the miniaturization of semiconductor electronic devices leads to
a chip features smaller than 100 nm in size, device Engineers and Physicists will face various
problems as to how to reduce the size of their devices without hampering the performance of
their device. The solution of this problem of theirs would be given by spintronics.
With the help of spintronics, complex yet compact devices can be easily manufactured.
THE ELECTRON SPIN
On close analysis of quantum theory the hydrogen atom, we find out that one of the solutions
of the Schrodinger wave equation is the spin of the electron. This takes values +1/2 and -1/2.
If the electron rotates from west to east, it is denoted by ‘Spin Up’ and conversely, if the
electron rotates from east to west, the spin is denoted by ‘Spin Down’. In a magnetic field,
electrons with "spin up" and "spin down" have different energies. Spintronic devices create
spin-polarized currents and use the spin to control current flow.

2. SPINTRONICS
GIANT MAGNETO RESISTANCE
Magnetism is the integral part of the present day's data storage techniques. Right from the
gramophone disks to the hard disks of the super computer, magnetism plays an important
role. Data is recorded and stored as tiny areas of magnetized iron or chromium oxide. To
access the information, a read head detects the minute changes in magnetic field as the disk
spins underneath it. In this way the read head detect the data and sends it to the various
succeeding circuits. The magneto resistant devices can sense the changes in the magnetic
field only to a small extent, which is appropriate to the existing memory devices. When we
reduce the size and increase data storage density, we reduce the bits, so our sensor also has to
be small and maintain a very, very high sensitivity. This thought gave rise to the powerful
effect called "GIANT MAGNETORESISTANCE". Giant magnetoresistance (GMR) came
into picture in 1988, which lead the rise of spintronics. It results from subtle electron-spin
effects in ultra-thin ' multilayer' of magnetic materials, which causes huge changes in their
electrical resistance when a magnetic field is applied. GMR is 200 times stronger than
ordinary magnetoresistance. It was soon realized that read heads incorporating GMR
materials would be able to sense much smaller magnetic fields, allowing the storage capacity
of a hard disk to increase from 1 to 20 Gigabits.

3. SPINTRONICS
CONSTRUCTION OF GMR
The basic GMR device consists of a three-layer sandwich of a magnetic metal such as cobalt
with a nonmagnetic metal filling such as silver. Current passes through the layers consisting
of spin-up and spin-down electrons. Those oriented in the same direction as the electron spins
in a magnetic layer pass through quite easily while those oriented in the opposite direction are
scattered. If the orientation of one of the magnetic layers can easily be changed by the
presence of a magnetic field then the device will act as a filter, or 'spin valve', letting through
more electrons when the spin orientations in the two layers are the same and fewer when
orientations are oppositely aligned. The electrical resistance of the device can therefore be
changed dramatically. In an ordinary electric current, the spin points at random and plays no
role in determining the resistance of a wire or the amplification of a transistor circuit.
Spintronic devices, in contrast, rely on differences in the transport of "spin up" and "spin
down" electrons. When a current passes through the Ferro magnet, electrons of one spin
direction tend to be obstructed. A Ferro magnet can even affect the flow of a current in a
nearby nonmagnetic metal. For example, in the present-day read heads in computer hard
drives, wherein a layer of a nonmagnetic metal is sandwiched between two ferromagnetic
metallic layers, the magnetization of the first layer is fixed, or pinned, but the second
ferromagnetic layer is not. As the read head travels along a track of data on a computer disk,
the small magnetic fields of the recorded 1's and 0`s change the second layer's magnetization
back and forth parallel or antiparallel to the magnetization of the pinned layer. In the parallel
case, only electrons that are oriented in the favoured direction flow through the conductor
easily. In the antiparallel case, all electrons are impeded. The resulting changes in the current
allow GMR read heads to detect weaker fields than their predecessors; so that data can be
stored using more tightly packaged magnetized spots on a disk.

4. SPINTRONICS
Important Applications of Spintronics:-
MRAM
An important spintronic device, which is supposed to be one of the first spintronic devices
that have been invented, is MRAM. Unlike conventional RAMs, MRAMs do not lose stored
information once the power is turned off. Current generation PCs use SRAM and DRAM -
both volatile memories. They can store information only if we have power. DRAM is a series
of capacitors, a charged capacitor represents 1 where as an uncharged capacitor represents 0.
To retain 1, you must constantly feed the capacitor with power because the charge you put
into the capacitor is constantly leaking out.
MRAM is based on integration of magnetic tunnel junction (MTJ). Magnetic tunnel junction
is a three-layered device having a thin insulating layer between two metallic Ferro magnets.
Current flows through the device by the process of quantum tunnelling; a small number of
electrons manage to jump through the barrier even though they are forbidden to be in the
insulator. The tunnelling current is obstructed when the two ferromagnetic layers have
opposite orientations and is allowed when their orientations are the same. MRAM stores bits
as magnetic polarities rather than electric charges. When its polarity points in one direction, it
holds1 and when its polarity points in other direction it holds 0. These bits need electricity to
change the direction but not to maintain them. MRAM is non-volatile so, even after shutting
down the computer, all the bits retain the 1`s and 0`s.

5. SPINTRONICS
QUANTUM COMPUTER
In a quantum computer, the fundamental unit of information (called a quantum bit or qubit),
is not binary but rather more quaternary in name. This qubit property arises as a direct
consequence of its adherence to the laws of quantum mechanics. A qubit can exist not only in
a state corresponding to the logical state 0 or 1 as in a classical bit, but also in states
corresponding to a blend or superposition of these classical states. In other words, a qubit can
exist as a zero, a one or simultaneously as both 0 and 1, with a numerical coefficient
representing the probability for each state. Each electron spin can represent a bit; for instance,
a 1 for spin up and 0 for spin down. With conventional computers, Engineers go to great
lengths to ensure that bits remain in stable, well-defined states. A quantum computer, in
contrast, lies on encoding information within quantum bits, or qubits, each of which can exist
in a superposition of 0 and 1. By having a large number of qubits in superposition of
alternative states, a quantum computer intrinsically contains a massive parallelism.
Unfortunately, in most physical systems, interactions with the surrounding environment
rapidly disrupt these superposition states. A typical disruption would effectively change a
superposition of 0 and 1 randomly into either a 0 or a 1, as process called decoherence. State-
of-the-art qubits based on the charge of electrons in a semiconductor remain coherent for a
few picoseconds at best and only at temperatures too low for practical applications. The rapid
decoherence occurs because the electric force between charges is strong and long range. In
traditional semiconductor devices, this strong interaction is beneficial, permitting delicate
control of current flow with small electronic fields. To quantum coherent devices, however, it
is a disadvantage. As a result, an experiment was conducted on the qubits, which are based on
the electron-spin. Electron-spin qubits interact only weakly with the environment surrounding
them, principally through magnetic fields that are non-uniform in space or changing in time.
Such fields can be effectively shielded.
The goal of the experiment was to create some of these coherent spin states in a
semiconductor to see how long they could survive. Much to the surprise, the optically excited
spin states in ZnSe remained coherent for several nanoseconds at low temperatures—1,000
times as long as charge-based qubits. The states even survived for a few nanoseconds at room
temperature. Subsequent studies of electrons in gallium arsenide (GaAs) have shown that,
under optimal conditions, spin coherence in a semiconductor is possible.

6. SPINTRONICS
SPINTRONIC QUBITS
1. In a conventional computer every bit has a definite value of 0 or 1. A series of eight bits
can represent any number from 0 to 255, but only one number at a time.
2. Electron spins restricted to spin up and spin down could be used as bits.
3. Quantum bits, or qubits, can also exist as superposition of 0 and 1, in effect being both
numbers at once. Eight qubits can represent every number from 0 to 255 simultaneously.
4. Electron spins are natural qubits; a tilted electron is a coherent superposition of spin up and
spin down and is less fragile than other quantum electronic states.
5. Qubits are extremely delicate: stray interactions with their surroundings degrade the
superposition extremely quickly, typically converting them in to random ordinary bits.

7. SPINTRONICS
FIGURE 1-CONSTRUCTION OF GMR
FIGURE 2-THE HARD DISK

8. SPINTRONICS
FIGURE 3-MRAM: A THREE LAYERED DEVICE HAVING A THIN INSULATED LAYER BETWEEN
TWO FERRO MAGNETS

9. SPINTRONICS
FIGURE 4-BLOCH SPHERE: A GEOMETRICAL REPRESENTATION OF THE PURE STATE SPACE
OF A TWO-LEVEL QUANTUM MECHANICAL SYSTEM (QUBIT)
FIGURE 5-EXCITATON FROM
GROUND STATE TO THE FIRST
EXCITED STATE