SPINTRONICS A PART OF NANOELECTRONIC TECHNOLOGY

What is Nanotechnology?
The definition of nanotechnology is based on the prefix “nano”, which is from the Greek word meaning “dwarf”. Nanotechnology is the manipulation or self-assembly of individual atoms, molecules, or molecular clusters into structures to create materials and devices with new or vastly different properties. This can be achieved by reducing the size of the smallest structures to the nanoscale or by manipulating individual atoms and molecules into nanostructures. Changes made at the atomic level create significant changes in the physical, chemical, biological, mechanical, and/or electrical properties of individual elements.

Real-World Applications :
As an enabling technology, nanotechnology is expected to have an impact on a wide range of applications in industry including materials and manufacturing, nanoelectronics and computing, health and medicine (including biopharmaceuticals), environment and energy, automotive, national security, and aeronautics and space exploration.

Materials and Manufacturing
Nanotechnology has the potential to transform materials and manufacturing, and research is driven by the need to improve functionality of materials. For example, there has been a significant amount of research directed at developing “self-healing” materials. Biological structures have the ability to reconstruct naturally. However, all manmade materials experience some form of failure—glass can crack, rubber can break down, etc. By applying nanotechnology to materials manufacturing, the self-healing that occurs in biological structures can potentially be simulated in manmade structures.

Health and Medicine
The future of nanotech in the fields of health and medicine is far reaching. Anticipated advances include: effective and less expensive health care using remote and in-vivo devices; new formulations and routes for drug delivery; optimal drug usage; more durable, rejection-resistant artificial tissues and organs; and sensors for early detection and prevention. It is also anticipated that nanotechnology will allow for accelerated gene sequencing—from the Human Genome.

Automotive
Through the enabling technology of nanotech, future systems of automobiles may host the ability to electronically avoid collisions, as well as host some form of brake-by-wire, steer-by-wire systems (slowing the car and guiding electrically instead of manually), and allow for the development of sensory systems when new fuel sources become common.


Introduction about Spintronics:

What is Spintronics?

"Spintronics is a nanoscale technology in which information is carried not by the electron's charge, as it is in conventional microchips, but by the electron's intrinsic spin and if a reliable way can be found to control and manipulate the spins spintronic devices could offer higher data processing speeds, lower electric consumption, and many other advantages over conventional chips--including, perhaps, the ability to carry out radically new quantum computations.

Recent discoveries of Spintronics:

Spintronics has greater potential in the areas of information storage and quantum computing in the mere future. Two recent discoveries have rekindled interest in the utility of semiconductors as both sources and carriers of spin information. The first of these, by Awschalom and coworkers (Awschalom and Kikkawa 1999), demonstrated that optically injected spin-polarized carriers maintain their coherence over nanosecond time scales. This means that they can be transported over distances far in excess of tens of micrometers, making the transport of coherent spin information from device to device a practical reality. The second discovery, by Ohno and coworkers in Japan (Ohno et al. 1996), resulted in the fabrication of low concentration Mn substitution in GaAs epilayers with ferromagnetic ordering temperatures in excess of 100K. Other semiconducting materials with TC higher than room temperature are in the offing. Thus the natural integration of spin-sensitive and normal semiconductor functionalities will lead to new opportunities for integrating electronics, magnetics, and photonics into single technologies with multifunctional capabilities.

Materials for Spin Electronics:

The magnetic materials principally used in spin electronics are soft ferromagnetic alloys of the late 3d metals. These serve as sources and conduction channels for the spin-polarized electrons, as well as magnetic flux paths and shields. Most progress has been made with sensors, ranging from simple position sensors and elements for nondestructive testing of ferrous metals to sophisticated miniature sensor elements in read heads for digital tape and disc recording where requirements are very demanding; high permeability is required with a sharp low-field switching response that extends to frequencies in the GHz range. Magnetic memory and logic elements require square hysteresis loops. All AMR, GMR, TMR and magnetic random access memory (MRAM) devices developed so far are based on 3d ferromagnetic metals and alloys. So too are magnetic three-terminal devices such as spin transistors and spin injection switches, as well as the magnetic Schottky barriers for injecting spin-polarised hot electrons into semiconductors.

Antiferromagnets, which may be metals or insulators,.nd a use in exchange biasing of magnetic thin .lm structures. Hard magnetic materials in thin .lmform can be employed to generate a stray .eld to stabilize a particular domain structure in a contiguous soft layer. Ferromagnetic oxides are at the research stage, but it is hoped that in future their half-metallic character will be exploited in sources and analysers of completely spin-polarized electrons. Magnetic semiconductors are another class of potentially-interesting materials, but they su.er from the critical defect that their Curie temperatures are far below room temperature.

Spin up and Spin down:

“Spintronics,” which exploits an electron property known as spin. Roughly speaking, one can visualize spin as a bar magnet pierced through an electron. If the bar magnet is aligned with an outside magnetic field, it's considered to be spin-down and if it's pointing in the opposite direction, it's in a spin-up state. A device based on spintronics will use both the charge and spin of an electron to convey information.

Methods for Determining Spin Polarization:

Various methods exist for measuring material spin polarization. Our investigation frequently uses the following methods:

1. Spin-resolved photoemission measures the difference in spin up and spin down number density in relation to state electron density at the Fermi level. Taking advantage of the high-intensity synchrotron radiation available at the National SRRC light source, this study examines the PES and x-ray absorption spectra of the half-metallic ferromagnets and novel epitaxial transition-metal oxides in the working teams.

2. Transport measurements measure complicated average velocity projection or velocity square projection onto the net current direction. These experiments include ferromagnet
tunnel junctions, ferromagnet and superconductor tunnel junctions, and point contacts using special lithography techniques or contacts between a sharpened S(F) needle and F
(S) materials. Ballistic point contacts to probe the spin polarization

Spin Injection:

Spintronic devices require the controlled transfer of electrons from a strongly magnetic material (specifically, a ferromagnet) into a semiconductor. This process is called spin injection. For spin injection to be useful, it is important to maintain a relative imbalance in the spin states of the transferred electrons, so that the electrons are either mostly spin-up or spin-down.

Researchers from all over the world have attempted to produce efficient electrical spin injection into a semiconductor. There are two major challenges. One challenge is one must have an efficient spin injector (magnetic material) with high spin polarization. Spin polarization refers to the difference between the number of spin-up and spin-down electrons that are available for transport. If all electrons are spin-up or all are spin-down, then the spin polarization is 100%. Another challenge is to make the perfect interface between a spin injector and semiconductor.

Therefore, an ideal spin injector would have to meet several criteria: 1) the spin injecting material must be stable on the semiconductor and form a good interface with the
semiconductor; 2) the material must be magnetic near room temperature (have a high "Curie temperature," or Tc) to be useful for everyday devices; and 3) the material should possess a high degree of spin polarization (ideally 100%). Fortunately, Heusler alloys are the ideal candidates as they fulfill these three criteria.

The Heusler alloys are a unique class of materials which have the formula as X2YZ with three elements arranged in a specific three-dimensional atomic arrangement, where X and Y are transition elements (groups IB to VIIIB on the periodic table), and Z is a group III, IV, or V element. Some Heusler alloys are predicted to be "half-metals," in which the electrons are either all spin-up or all spin-down at the Fermi level, resulting in 100% spin polarization.

Demonstration of Electrical Spin Injection:
The below figure represents the demonstration of electrical spin injection from Heusler alloy Co2MnGe into Al0.1Ga0.9As/Ga As heterostructures. Recently spin polarization has been achieved upto 18% at 20K. Although spin injection from traditional metal Fe into GaAs has been reported to be ~ 34%, Co2MnGe only requires half of the applied magnetic field which was used to inject the spins from Fe into semiconductor.

Nano tools and fabrication techniques:

Microscopy:

Nanotechnology uses two main kinds of microscopy. The first involves a stationary sample in line with a high-speed electron gun. Both the scanning electron microscope (SEM) and transmission electron microscope (TEM) are based on this technique. The second class of microscopy involves a stationary scanner and a moving sample. The two microscopes in this class are the atomic force microscope (AFM) and the scanning tunnelling microscope (STM). Microscopy plays a paradoxical role in nanotechnology because, although it is the key to understanding materials and processes, on a nanoscale samples can be damaged by the high-energy electrons fired at them. This is not a problem with STM, but a further drawback is that most microscopes require very stringent sample preparation. The SEM, TEM, and STM need well prepared samples that are also electrically conductive. There are ways to get around this, but the fact remains that it can take hours to prepare and mount a sample correctly (and hours to actually synthesise the sample).

Top-down and bottom-up synthesis techniques:

There are two approaches to building nanostructures, both having their origins in the semiconductor industry13. In the traditional ”top-down” approach a larger material such as a silicon wafer is processed by removing matter until only the nanoscale features
remain. Unfortunately, these techniques require the use of lithography, which requires a mask that selectively protects portions of the wafer from light. The distance from the mask to the wafer, and the size of the slit define the minimum feature size possible for
a given frequency of light, e.g. extreme ultraviolet light yields feature sizes of 90 nm across, but this scale is near the fundamental limit of lithography. Nonetheless lithography can be used for patterning substrates used to produce nanomaterials, e.g. guiding the growth of quantum dots and nanowires. The ”bottom-up” approach starts with constituent materials (often gases or liquids) and uses chemical, electrical, or physical forces to build a nanomaterial atom-by-atom or molecule-by-molecule14. The simplest bottom up synthesis route is electroplating to create a material layer-by-layer, atom-by-atom. By inducing an electric field with an applied voltage, charged particles
are attracted to the surface of a substrate where bonding will occur. Most nanostructured metals with high hardness values are created with this approach. Chemical vapour deposition (CVD) uses a mix of volatile gases and takes advantage of thermodynamic principles to have the source material migrate to the substrate and then bond to the surface. This is the one proven method for creating nanowires and carbon nanotubes,
and is a method of choice for creating quantum dots. Molecular self-assembly promises to be a revolutionary new way of creating materials from the bottom up. One way to achieve self-assembly is to use attractive forces like static electricity, Van der Waals forces, and a variety of other short-range forces to orient constituent molecules in a regular array. This has proven very effective in creating large grids of quantum dots.

The bottom up approach promises an unheard-of level of customisability in materials synthesis, but controlling the process is not easy and can only produce simple
structures, in time-consuming processes with extremely low yields. It is not yet possible to produce integrated devices from the bottom up, and any overall order aside from repeating grids cannot be done without some sort of top-down influence like lithographic patterning. Nanotechnology synthesis is thus mainly academic, with only a few companies in the world that can claim to be nanotechnology manufacturers. And until
understanding of synthesis is complete, it will be impossible to reach a point of mass production.

SPINTRONICS: A SIGNIFICANT FIELD OF SCIENTIFIC INQUIRY?

It is possible to separate spin electronic phenomena into two major categories. The first of these is the so-called giant magnetoresistive effect (GMR). This effect was first observed in alternating multi-layers composed of magnetic and non-magnetic metallic metals and alloys, and depends on the effective single spin densities of states in a magnetically polarized medium. Resistivity is always larger if the transition is from a spin polarization in one direction to a density of states of spin polarization in the other.This is also true of scattering events, which, at least in the simplest case of Fermi.s golden rule, also involve the product of the two spin- dependent electronic densities of state.

Magnetic tunneling applied to memory:

Another closely related physical phenomenon is the tunneling magnetoresistive effect in which the two magnetic metals are separated by an insulating layer. Once again, the resistivity is strongly dependent on the sense of polarization of the two counter electrodes. Finally, there are more complex structures, which involve a metallic or degenerately semiconducting magnetic electrode. This acts as a source of spin-polarized electrons injected into a semiconductor where the spins may be modulated by a number of external processes such as electronic gating. Thereafter, the electrons are ejected into a detector capable of identifying the sense of polarization. The detector may be optical, another magnetic electrode or a more complex device.
magnetoresistance random access memories are nonvolatile, are read nondestructively, and show no signs of wearout to 1015 read or write cycles. The memory arrays in these memories use 2 mm photolithography, and for reasons to be discussed later, sense signals are relatively small (1mV) leading to a read access time of about 250 ns even though the number of squares (5–6) per cell is relatively large.

Spin dependent tunneling (SDT) materials offer yet another opportunity for enhanced random access nonvolatile memories. Cells with high impedance and low interlayer coupling could lead to much faster MRAM. This paper will explore how spin dependent tunneling devices might be used for random access nonvolatile memory, and how the resulting memory would compare with GMR memory concepts



SDT MEMORY DESIGN:

Three different design concepts are considered. First is a dense array design similar to AMR and GMR memory organizations. Second is a transistor per cell design similar to Semiconductor dynamic random access memory (DRAM). Third is a flip–flop concept that uses embedded SDT elements. This cell would operate similarly to semiconductor Static random access memory (SRAM).

Characteristics of Spin Dependent Tunneling Devices:

1) For low values of voltage across the device, the current changes relatively linearly with voltage, but the current changes faster than linearly for higher values of voltage.

2) The junction magnetoresistance (JMR) or the percentage change in tunneling current due to magnetic fields ranges from a few percent to about 20%. As the voltage across the device increases, JMR decreases, losing roughly one-half of the low-voltage values at several hundred mV.

3) The effective magnetoresistance increases to about twice the room-temperature values when the devices are cooled to 77 K but tunneling current increases only about 20%, indicating that the effective resistivity is relatively insensitive (1000 ppm/ °C) to temperature.

4) The observed resistances of the devices have ranged greatly in value from under 104 to over 109 V mm2.

5) The coupling fields between the magnetic layers in the SDT devices are relatively smaller (a few Oe) Compared with those between the magnetic layers in GMR magnetic
Sandwiches with equivalent interlayer thicknesses, where, for example, coupling fields in GMR sandwiches of 20 Oe are common for 20Å thick copper interlayer.

Advantages SDT devices over GMR devices:

1) High-density sense electronics has an inherent ‘‘offset’’ due to transistor characteristic mismatches and array feature imbalances. These are compensated out of AMR memories and proposed GMR memories with ‘‘auto zero’’ circuits, i.e., using circuits that initialize out all imbalances before sensing. The higher values of signal attainable with
SDT devices should make it possible to build sense circuits

2) Developing compatible metallurgies between the SDT and circuits.

3) Preparing the surface quality of the IC substrates suitable for SDT depositions. The smoothness of dielectrics used for ICs is not an issue for the circuits, but could be a
very large one for the SDT device operation.

4) Temperature durability of the SDT devices must allow for an annealing temperature of about 300 °C to anneal out radiation damage from transistors caused by plasma processes.

Thus GMR effect depends on spin-dependent transport and second category contains tunneling magnetoresistance and semiconductor spintronics, lumped together in a group called spin injection, detection, and manipulation.

What are the risks of Nanotechnology?

Keeping in mind the broad range of applications of nanotechnologies outlined it is self-evident, that the risks associated with nanotechnologies will also form a complex risk landscape rather than a homogenous set of risks. The emphasis on what kind of risks are key to consider will depend on the perspective of the particular organization involved in nanotechnologies. To name just a few:


• Business risks involved with marketing of nanotechnology enabled products,

• Risks related to the protection of intellectual property,

• Political risks regarding the impact on the economical development of countries and regions
• Risks regarding privacy when miniature sensors become ubiquitous,

• Environmental risks from the release of nanoparticles into the environment,

• Safety risks from nanoparticles for workers and consumers,

• Futuristic risks like human enhancement and self replications of nano machines.

The risk and safety discussion related to free nanoparticles will concern only a fraction of the applications of nanotechnologies. In most applications nanoparticles will be embedded in the final product and therefore not come into direct contact with
Workers, consumers or the environment. They are unlikely to raise concerns because of their immobilization. Exceptions are possible when the products or materials within which nanoparticles are enclosed are discarded, burned or otherwise destroyed
(e.g. in an accident).

Nanoparticles and the environment:

As nanotechnologies move into large-scale production in many industries, it is a just a matter of time before gradual as well as accidental releases of engineered nanoparticles into the environment occur. The possible routes for an exposure of the environment range over the whole lifecycle of products and applications that contain engineered nanoparticles:
• Discharge / leakage during production / transport and storage of intermediate and finished products,

• Discharge / leakage from waste,

• Release of particles during use of the products,

• Diffusion, transport and transformation in air, soil and water.


Conclusion:

To continue the rapid pace of discoveries, considerable advances in our basic understanding of spin interactions in the solid state along with developments in materials
science, lithography, minaturization of optoelectronic elements, and device fabrication are necessary. The progress toward understanding and implementing the spin degree of freedom in metallic multilayers and, more recently, in semiconductors is gaining momentum as more researchers begin to address the relevant challenges from markedly different viewpoints. Spintronic read head sensors are already impacting a multibillion dollar industry and magnetic random access memory using metallic elements will soon impact another multibillion dollar industry. With contributions from a diversity of countries and fields including biology, chemistry, physics, electrical engineering, computer science, and mathematical information theory, the rapidly emerging field of spintronics promises to provide fundamentally new advances in both pure and applied science as well as have a substantial impact on future technology.



With Reference To
IBM's magnetoresistive and giant magnetoresistive head technologies. By Jim Belleson, IBM Storage Systems Division, & Ed Grochowski, IBM Almaden Research Center.

J. M. Daughton Nonvolatile Electronics, Inc., 11409 Valley View Road, Eden Prairie, Minnesota 55344.

Spin dependent tunneling junctions with reduced Neel coupling Dexin Wang,a) James M. Daughton, Zhenghong Qian, Cathy Nordman, Mark Tondra, and Art Pohm.