Spintronics

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1.INTRODUCTION

As rapid progress in the miniaturization of semiconductor electronic devices leads toward chip features smaller than 100 nanometers in size, engineers and physicists are certainly faced with the alarming presence of quantum mechanics. One such peculiarity is a quantum property of the electron known as spin, which is closely related to magnetism. Devices that rely on an electron's spin to perform their functions form the foundation of Spintronics, also known as Magnetoelectronics. Information processing technology has thus far relied on purely charge-based devices -ranging from the now old-fashioned vacuum tube to today's million-transistor microchips. Those conventional electronic devices move electric charges around, ignoring the spin that tags along for the ride on each electron.

Spintronics is a new branch of electronics in which electron spin, in addition to charge, is manipulated to yield a desired outcome .All spintronic devices act according to the simple scheme: (1) information is stored (written) into spins as a particular spin orientation (up or down), (2) the spins, being attached to mobile electrons, carry the information along a wire, and (3) the information is read at a terminal. Spin orientation of conduction electrons survives for a relatively long time (nanoseconds, compared to tens of femto seconds during which electron momentum decays), which makes spintronic devices particularly attractive for memory storage and magnetic sensors applications, and, potentially for quantum computing where electron spin would represent a bit (called qubit) of information.

?Spintronics is a study that deals with spin of an electron, ("spin-based electronics") also known as magneto-electronics, is an emergent technology which exploits the quantum property of electrons to spin as well as making use of their charge. There are two spins (UP spin and DOWN Spin).This spintronic scanning technique is an efficient technique used in the medical field to detect cancer cells.
Cancer cells are easy to be identified only when they are large in number. These cells when matured results in formation of tumor, which has to be removed by surgery. After surgery there may be presence of even a single cancer cell, which would result in growth of tumor in effected part of the body. The spintronic scanning is an efficient technique to detect cancer cells even when they are less in number.
A Patient is exposed to a strong magnetic field so that his body cell gets magnetized.
A beam of electrons with polarized spin is introduced on the uneffected part of the body and the change in spin is detected by a polarimeter.
A beam of electrons with polarized spin is introduced on the part which had undergone surgery.
The difference in spin of electrons when introduced to normal area and abnormal area indicates whether cancer cells have been removed from the body. If not, it indicates the presence of traces of cancer cells and it has to be treated again for ensuring complete safety to the patient.???????????????????????????????

?Thus this technique efficiently identifies the presence of cancer cells in that part of the body that has undergone surgery to prevent any further development.

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INTRODUCTION:
An emerging research field in physics focused on spin-dependent phenomena applied to electronic devices is called spintronics. The promise of spintronics is based on manipulation not only of the charge of electrons, but also their spin, which enables them to perform new functions. Currently, the ability to manipulate electron spin is expected to lead to the development of remarkable improvements in electronic systems and devises used in photonics, data processing and communications technologies only.
Now this paper brings out an innovative idea of extending the hands of spintronics in MEDICAL FIELD, in the detection of cancer cells even when they are very few in number in the human body.
This approach is relied on two important aspects:

  • the behavior of electron spin in a magnetic field
  • the cancer cell?s abnormality over normal cells

SPINTRONICS:
Spintronics, or spin electronics, refers to the study of the role played by electron spin in solid state physics, and possible devices that specifically exploit spin properties instead of or in addition to charge degrees of freedom. In spintronics electron spin, in addition to charge, is manipulated to yield a desired outcome.
An electron is just like a spinning sphere of charge. It has a quantity of angular momentum (its "spin") and an associated magnetism, and in an ambient magnetic field its energy depends on how its spin vector is oriented. Every electron exists in one of two states, namely, spin-up and spin-down with its spin either +1/2 or ?1/2. In other words, an electron can rotate either clockwise or counterclockwise around its own axis with constant frequency. . Two spins can be "entangled" with each other, so that neither is distinctly up nor down, but a combination of the two possibilities.
In order to make a spintronic device, the primary requirement is to have a system that can generate a current of spin polarised electrons, and a system that is sensitive to the spin polarization of the electrons. Most devices also have a unit in between that changes the current of electrons depending on the spin states.
The simplest method of generating a spin polarised current is to inject the current through a ferromagnetic material. The most common application of this effect is a giant magnetoresistance (GMR) device. A typical GMR device consists of at least two layers of ferromagnetic materials separated by a spacer layer. When the two magnetization vectors of the ferromagnetic layers are aligned, then an electrical current will flow freely, whereas if the magnetization vectors are antiparrallel then the resistance of the system is higher. Two variants of GMR have been applied in devices, current-in-plane where the electric current flows parallel to the layers and current-perpendicular-to-the-plane where the electric current flows in a direction perpendicular to the layers.
Applications
Spintronic devices are used in the field of mass-storage devices; recently (in 2002) IBM scientists announced that they could compress massive amounts of data into a small area, at approximately one trillion bits per square inch (1.5 Gbit/mm?) or roughly 1 TB on a single sided 3.5" diameter disc. The storage density of hard drives is rapidly increasing along an exponential growth curve known as Kryder's Law. The doubling period for the areal density of information storage is twelve months, much shorter than Moore's Law, which observes that the number of transistors in an integrated circuit doubles every eighteen months. Also the hard disk drives use a spin effect to function, the Giant magnetoresistive effect (see below).
The most successful spintronic device to date is the spin valve. This device utilizes a layered structure of thin films of magnetic materials, which changes electrical resistance depending on applied magnetic field direction. In a spin valve, one of the ferromagnetic layers is "pinned" so its magnetization direction remains fixed and the other ferromagnetic layer is "free" to rotate with the application of a magnetic field. When the magnetic field aligns the free layer and the pinned layer magnetization vectors, the electrical resistance of the device is at its minimum. When the magnetic field causes the free layer magnetization vector to rotate in a direction antiparallel to the pinned layer magnetization vector, the electrical resistance of the device increases due to spin dependent scattering. The magnitude of the change, (Antiparallel Resistance - Parallel Resistance) / Parallel Resistance x 100% is called the GMR ratio. Devices have been demostrated with GMR ratios as high as 200% with typical values greater than 10%. This is a vast improvement over the anisotropic magnetoresistance effect in single layer materials which is usually less than 3%. Spin valves can be designed with magnetically soft free layers which have a sensitive response to very weak fields (such as those originating from tiny magnetic bits on a computer disk), and have replaced anisotropic magnetoresistance sensors in computer hard disk drive heads since the late 1990s.
Future applications may include a spin-based transistor which requires the development of magnetic semiconductors exhibiting room temperature ferromagnetism. The operation of MRAM or magnetic random access memory is also based on spintronic principles.

CANCER CELLS:
Cancer cells are the somatic cells which are grown into abnormal size. The cancer cells have different electromagnetic pattern when compared to normal cells.
For many types of cancer, it is easier to treat and cure the cancer if it is found early. There are many different types of cancer, but most cancers begin with abnormal cells growing out of control, forming a lump that's called a tumor. The tumor can continue to grow until the cancer begins to spread to other parts of the body. If the tumor is found when it is still very small, curing the cancer can be easy. However, the longer the tumor goes unnoticed, the greater the chance that the cancer has spread. This makes treatment more difficult.
Tumor developed in human body, is removed by performing a surgery. Even if a single cell is present after the surgery, it would again develop into a tumor. In order to prevent this, an efficient method for detecting the cancer cells is required.
Here, in this paper, we introduce a new method for detecting the cancer cells after a surgery. This accurate detection of the existence of cancer cells at the beginning stage itself ensures the prevention of further development of the tumor.

 

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PROPERTIES OF SPIN:

  • The spin of an electron has a great dependence over an external magnetic field. If a polarized electron is exposed in a magnetic field, its spin orientation gets varied. This behavior of the electron plays a crucial role in the presented approach in this paper.
  • Polarization of spin of electrons is possible so that the spin of all electrons orient in a particular direction.
  • The electron spin can be detected by using devices like polarimeters.
  • The electron spin can be controlled electrically just with the application of few volts.

DETECTION OF CANCER CELLS:
It is very important that the cancer cells should be diagnosed at the earlier stages itself failing which develop rapidly into acute tumors.
??????????????? AN INNOVATIVE APPROACH TO DETECT THE CANCER CELLS WITH THE HELP OF SPINTRONICS IS PRESENTED HERE.
The following setup is used for the detection of cancer cells in a human body:

  • Polarized electron source
  • Magnetic field
  • Spin detector

POLARISED ELECTRON SOURCE:
A beam of electrons is said to be ?polarized? if their spins point, on average, in a specific direction. There are several ways to employ spin on electrons and to control them. The requirement for this paper is an electron beam with all its electrons polarized in a specific direction.
The following are the ways to meet the above said requirement:

  • Photoemission from negative electron affinity GaAs
  • Chemi-ionization of optically pumped meta stable Helium
  • An optically pumped electron spin filter

A spin filter is more efficient electron polarizer which uses an ordinary electron source along with a gaseous layer of Rb. Free electrons diffuse under the action of an electric field through Rb vapour that has been spin polarized in optical pumping. Through spin exchange collisions with the Rb, the free electrons become polarized and are extracted to form a beam. To reduce the emission of depolarizing radiation, N2 is used to quench the excited Rb atoms during the optical pumping cycle.
SPIN DETECTORS:
There are many ways by which the spin of the electrons can be detected efficiently. The spin polarization of the electron beam can be analyzed by using:

  • Mott polarimeter
  • Compton polarimeter
  • Moller type polarimeter

Typical Mott polarimeters require electron energies of ~100 keV. But Mini Mott polarimeter uses energies of ~25 keV, requiring a smaller overall design. The Mini Mott polarimeter has three major sections: the electron transport system, the target chamber, and the detectors. The first section the electrons enter is the transport system. An Einsel lens configuration was used here. Two sets of four deflectors were used as the first and last lens.
The electrons next enter the target chamber. The chamber consists of a cylindrical target within a polished stainless steel hemisphere. A common material used for the high-Z nuclei target is gold. Low-Z nuclei help minimize unwanted scattering, so aluminum was chosen. Scattered electrons then exit the target chamber and are collected in the detectors.
Thus there are many methods for detecting the spin polarization of electrons.
EXTERNAL MAGNETIC FIELD:??????????
An external magnetic field is required during this experiment. The magnetic field is applied after the surgery has undergone. First, it is applied to an unaffected part of the body and then to the surgery undergone part of the body. It is already mentioned that the magnetic field could easily alter the polarization of electrons.

 

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