SPINTRONICS ( New era of Memory Storage Devices)

Abstract -

 In the future Spintronics could replace electronics, which in the race to produce increasingly rapid computer components, must at sometime reach its limits. Different from electronics, where whole electrons are moved (the digital "one" means "an electron is present on the component", zero means "no electron present"), here it is a matter of manipulating a certain property of the electron, its spin.

Spintronics, the technology of devices whose properties depend upon the spin of conduction electrons, often requires the fabrication of thin, multilayer structures with layers as thin as 1 nm. In this paper the principles of Giant Magneto Resistive (GMR) materials and Magnetic Tunnel Junctions (MTJ) are discussed as well as their application in commercial device such as Magnetic Random Access Memories (MRAM) are discussed.

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 femtoseconds during which electron momentum and energy decay), 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.

INTRODUCTION

          Spintronics is a new field of research that studies and applies phenomenon that are dependent upon the spin of electrons. Spin-dependent scattering of conduction electrons had been observed for some time, but the advent of improved thin-film vapor deposition systems resulted in the observations in 1988 of large magnetic field dependent changes in resistance of thin-film ferromagnetic/non-magnetic metallic multilayers. The change in resistance was much larger than previously observed changes in resistance due to magnetic field earning the phenomenon the name Giant Magnetoresistance (GMR). Applications using GMR materials include magnetic field sensors, high-speed data couplers or isolators, and magnetic random access memory (MRAM).This paper first describes the properties of GMR and Magnetic Tunnel Junctions (MTJ) materials. The application of these materials to  MRAM is discussed. It has been argued that the largest application of nanotechnology to date is the use of GMR materials in read heads for high-density magnetic data recording. The use of GMR read heads has allowed hard drives to reach recording densities of 100 GBy/in2 common today in laptop and desktop computers.

CONCEPTS OF SPINTRONICS.

Spintronics:

Exploiting the Spin Degree of Freedom in Electronics

The increasing interest in using the spin of electrons as an additional degree of freedom in electronic devices demands for new materials with a high degree of spin polarization P of the conduction electrons. In the ideal half-metallic case (P = 100%), only one single spin direction is present at the Fermi level. Such fully spin polarized materials are promising for the implementation of spintronic devices which make use of both the spin and the charge degree of freedom of the carriers.

Along with this, the injection of spin polarized electrons into semiconductors and the spin-dependent transport in semiconducting materials is of growing interest. Especially the development of spinelectronic devices based on magnetic metallic multi-layers (giant magnetoresistance in metallic heterostructures) was the decisive factor. In addition, magnetic tunnel junctions suitable for magnetic random access memories (MRAM) could be prepared successfully.

Currently, most spinelectronic devices are based on metallic systems. However, it is of great interest to transfer those to semiconductors in order to make use of both their specific physical advantages and the well-established semiconductor technology for spinelectronics. To reach this goal, it becomes necessary to inject spin-polarized currents into semiconductors. This can be done using ferromagnetic contact materials with high spin polarization at room temperature.

Efficient Electrical Spin Injection Into Silicon

             “Electron spin has a direction, like 'up' or 'down,' ” Appelbaum said. “In silicon, there are normally equal numbers of spin-up and -down electrons. The goal of spintronics is to use currents with most of the electron spins oriented, or polarized, in the same direction.”

 Scientists at the Naval Research Laboratory (NRL) have efficiently injected a current of spin-polarized electrons from a ferromagnetic metal contact into silicon, producing a large electron spin polarization in the silicon. Silicon is by far the most widely used semiconductor in the device industry, and is the basis for modern electronics.

Electrical injection of spin-polarized electrons into silicon has proven elusive, but is essential to enable the use of spin angular momentum as an alternative state variable in future silicon electronics. Efficient injection of spin-polarized electrons from the iron/aluminum oxide tunnel barrier contact is confirmed by the emission of circularly polarized electroluminescence from the silicon. The illustration shows the surface-emitting LEDs used in this study. (Credit: Naval Research Laboratory)
            This demonstration by NRL scientists is a key enabling step for developing devices which rely on electron spin rather than electron charge, a field known as semiconductor spintronics, and is expected to provide higher performance with lower power consumption and heat dissipation. The complete findings of this study titled, "Electrical spin injection into silicon from a ferromagnetic metal/tunnel barrier contact" are published in the August 2007 issue of Nature Physics.

The electronics industry to date has relied largely on the control of charge flow through size scaling (i.e. reducing the physical size of elements such as transistors) to increase the performance of existing electronics. This trend, predicted in a seminal paper published in 1965 by Gordon Moore (who later co-founded Intel) and widely known as "Moore's Law," has been remarkably successful, as evidenced by the powerful desktop computers and handheld devices available at modest cost to the consumer.

The International Technology Roadmap for Semiconductors (ITRS) has identified the use of the electron's spin as a new state variable which should be explored as an alternative to the electron's charge. This approach is known as "semiconductor spintronics."

By analyzing the weak electroluminescence generated in the silicon, the NRL research team determined a lower bound for the electron spin polarization of 30%. For comparison, the spin polarization of the electrons in common magnetic metals such as permalloy or iron is ~ 40- 45%. The realization of efficient electrical injection and significant spin polarization using a simple magnetic tunnel barrier compatible with "back-end" silicon processing should greatly facilitate development of silicon-based spintronicdevices.

Spin-polarized Electrons On Demand, With A Single Electron Pump

 

The goal of spintronics (also called spin electronics) is to systemically control and manipulate single spins in nanometer-sized semiconductor components in order to thus utilize them for information processing.

For this reason, components are needed in which electrons can be injected successively into the electron, and one must be able to manipulate the spin of the single electrons, e.g. with the aid of magnetic fields. Both are possible with a single electron pump, as scientists of the Physikalisch-Technische Bundesanstalt (PTB) in Germany have, together with colleagues from Latvia.

Electrons can do more than be merely responsible for current flow and digital information. If one succeeds in utilizing their spin, then many new possibilities would open up. The spin is an inner rotational direction, a quantum-mechanical property which is shown by a rotation around its own axis. An electron can rotate counterclockwise or clockwise. This generates a magnetic moment. One can regard the electron as a minute magnet in which either the magnetic North or South Pole "points upwards" (spin-up or spin-down condition). The electronic spins in a material determine its magnetic properties and are systematically controllable by an external magnetic field.

The advantages of this pump is given below:

            The components would be clearly faster than those that are based on the transport of charges. The process would require less energy than a comparable charge transfer with the same information content. And with the value and direction of the expected spin value, further degrees of freedom would come into play, which could be used additionally for information representation.

In order to be able to manipulate the spins for information processing, it is necessary to inject the electrons singly with predefined spin into a semiconductor structure using single electron pump. This semiconductor device allows the ejection of exactly one single electron per clock cycle into a semiconductor channel.

PRINCIPLES BEHIND THE SPINTRONICS

GIANT MAGNETORESISTIVE MATERIALS

       Resistance of metals depends on the mean free path of their conduction electrons -- the shorter the mean free path, the higher the resistance. The resistivity of thin films can be considerably larger than the bulk resistivity if the film thickness is less than the mean free path. In ferromagnetic materials conduction electrons can be either spin up if their spin is parallel to the magnetic moment of the ferromagnet or spin down if they are antiparallel. In nonmagnetic conductors there are equal numbers of spin up and spin down electrons in all energy bands. In ferromagnetic metals there is a difference between the number of spin up and spin down electrons in the conduction sub bands of ferromagnetic materials due to the ferromagnetic exchange interaction. Therefore, the probability of an electron being scattered when it passes into a ferromagnetic conductor depends upon the direction of its spin. If a thin nonmagnetic conducting layer separates two thin ferromagnetic layers, we can change the resistance by simply changing whether the moments of the ferromagnetic layers ar e parallel or antiparallel. In order for spin dependent scattering to be a significant part of the total resistance, the layers must be thinner than the mean free path of electrons in the bulk material. For many ferromagnets the mean free path is tens of nanometers, so the layers themselves must each be typically less than 10 nm (100 Å).

       A typical multilayer structure consists  of alternating thin layers of magnetic and non-magnetic metals. The thickness of the nonmagnetic layers is quite critical. At the proper thickness each magnetic layer is coupled antiparallel to the moments of the magnetic layers on each side – exactly the condition needed for maximum spin dependent scattering. An external field can overcome the coupling that causes this alignment and can align the moments in all the layers parallel reducing the resistance. If the conducting layer is not the proper thickness, the same coupling mechanism can cause ferromagnetic coupling between the magnetic layers resulting in no GMR effect.

MAGNETIC TUNNEL JUNCTION MATERIALS

       Magnetic Tunnel Junctions (MTJ) also known as Spin Dependent Tunneling (SDT) structures also can exhibit a large change in resistance with magnetic field. In contrast to GMR structures, MTJ structures utilize a thin insulating layer to separate two magnetic layers. This insulating layer is as thin as 1 nm (10 Å). The conduction between the conducting magnetic layers is by quantum tunneling. The size of the tunneling current between the two magnetic layers is affected by the angle between the magnetization vectors in the two layers. Changes of resistance with magnetic field of 10 to 70 % and even higher have been observed in MTJ structures. The field required for maximum change in resistance depends upon the composition of the magnetic layers and the method of achieving antiparallel alignment. Values of saturation field range from 0.1 to 10 kA/m (1.25 to 125 Oe) offering at the low end, the possibility of extremely sensitive magnetic sensors.

             The magnetization in the lowest CoFe layer is pinned as part of the structure consisting of a CrPtMn antiferromagnetic and the antiferromagnetically coupled CoFe/Ru/CoFe sandwich. The NiFeCo free layer on the bottom responds to the applied field. The shape factor biases the free layer along its long dimension.multilayer structure consisting of a CrPtMn antiferromagnetic and the antiferromagnetically coupled CoFe/Ru/CoFe sandwich. The NiFeCo free layer on the bottom responds to the applied field. The shape factor biases the free layer along its long dimension eliminating the necessity of orthogonal field coils. The measured field is applied parallel to the axis of the antiferromagnetic structure. Depending upon the direction of the field,the resistance increases as the moments become more anti-parallel or decreases as they become more parallel.

APPLICATION OF SPINTRONICS IN

MAGNETIC RANDOM ACCESS MEMORY

       Magnetic Random Access Memory is potentially an ideal memory because it has the properties of nonvolatile, high speed, unlimited write endurance and low cost. These memories use the hysteresis of magnetic materials for storing data and some form of magnetoresistance for reading out the data. Because of the difficulty of separately connecting a large array of memory cells with complex integrated support circuits, the memory cells and support circuits are connected together on chip.

   Work on MRAM started devices based on Anisotropic Magnetoresistance (AMR). The change in resistance of AMR materials, about 2 per cent, offered severe challenges in making high-density MRAM. The difference between reading a 1 and a 0 was about 1 mV in practical devices. The interest was mainly for military applications and small capacity nonvolatile memories and 16 kbit integrated MRAM chips were developed by Honeywell and qualified for military applications in the mid 1990s. The discovery of GMR with its much higher change in resistance, revitalized the interest in MRAM and offered the promise of higher density MRAM devices.

       A scaling indicates that a factor of 3 in magnetoresistance improves the read time by a factor of 9. A selected bit could be written by a combination of fields from currents in a matrix of word lines and sense current lines. This writing scheme is shown in Figure 9. Reading out the state of the bit is done by sensing the differential resistance of the cell when a sense current is passed through it. Ix y Ix y and IAlone Do Not Switch Cell and ITogether Switch Cell.

   Magnetic tunnel junctions(MTJs) with their very high magnetoresistance have opened the possibility of higher signals and faster read times for MRAM. The important characteristics of MTJs are their large magnetoresistance values, their high resistance, and their low operating voltage. The magnetoresistance decreases if the voltage across the junction is over a few tenths of a volt, and catastrophic breakdown occurring at 1 to 2 volts. Read and write circuits must be separate. One method used to select the cell to be read is to incorporate an isolation transistor in each cell. This scheme is shown in Figure 10.

       Several companies are currently working on programs to bring commercial MRAM to the market. Freescale Semiconductor (Motorola) sampled MRAM in 2003 with limited availability of 4 MB in 2004. Cypress Semiconductor has projected samples by year’s end 2004. Other companies with MRAM programs include. IBM, ST Microelectronics, Philips, Infineon, Toshiba, NEC, Taiwan Semiconductor and Samsung. There is little question that MRAM will be available. The question remains as to how much they will displace conventional semiconductor memory.

ADVANTAGES

Ø    Data can be stored permanently, and is nearly instantly available anytime, no lengthy “boot up” needed.

Ø    Spintronics technology has yet to be fully developed, it could result in computers that store more data in less space, process data faster, and consume less power.

Ø    "Nonvolatile" computer memory (memory that can retain stored information even when not powered).

Ø    Computers without "save" buttons or disk drives; they are the Instant- on, Instant- off computers.

Ø    Memory and storage functions could merge, eliminating disk drives entirely and creating Instant-on, Instant-off computers.

Ø    Controlling the spin of a single electron is essential if this spin is to be used as the building block of a future quantum computer.

Ø     The Spintronic field exploits electron spin rather than charge, thus offering nano-scale logic devices with enhanced functionality and lower power consumption.

Ø    Achieving spin polarization is the first step in converting the plastic into a device that could read and write spintronic data inside a working computer.

Ø    When groups of electrons spin in the same direction, ie spin up or spin down, the electro static and magnetic fields associated with all the electrons can add up to one large electric and magnetic moment 100 to 1,000 times greater than normal.

Ø    The unique concept of resonant absorption excitation (electrons have specific quantum energy requirements for valence to conduction band movement ) by UV/Blue laser light illumination causing molecular/electron dissociation and simultaneous electric field application (Pockels effect) can be used for writing 2D Area or 3D volume data.

Ø    Methods and functions for controlling electron spin up and spin down can be used to drastically increase data storage densities.

Ø    Non-Invasive Spintronic techniques using a non-contact mosfet transistor to retrieve the electron states of the multiferroic ferroelectric molecule will allow for the creation of very dense storage having non-volatility without causing decoherence of the data.

Why are researchers so interested in spintronics?

 Normal electronics encode computer data based on a binary code of ones and zeros, depending on whether an electron is present in a void within the material. But in principle, the direction of a spinning electron -- either “spin up” or “spin down” -- can be used as data, too. So spintronics would effectively let computers store and transfer twice as much data per electron.

Another bonus: once a magnetic field pushes an electron into a direction of spin, it will keep spinning the same way until another magnetic field causes the spin to change. This effect can be used to very quickly access magnetically stored information during computer operation -- even if the electrical power to a computer is switched off between uses. Data can be stored permanently, and is nearly instantly available anytime, no lengthy “boot up” needed.

CONCLUSIONS

       Spintronics is a technology with a fast track from the discovery of GMR and MTJ materials to the incorporation of these materials in commercial devices. Spintronics read heads dominate the hard-disk market. MRAM devices are on the horizon and offer the promise of laptop computers that do not need to boot up and cell phones with increased battery time and increased capabilities. Spintronics promises to provide fundamentally new advances in both pure and applied science as well as have a substantial impact on future technology.

REFERENCE

             www.wikipeadia.com

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

www.sciencedaily.com

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