ABSTRACT
The drastic reduction in the size of integrated circuits, known as Moore's law has been instrumental in the tremendous success of Very Large Scale Integrated circuits (VLSI) technology. However power consumption and interconnection delays are limiting the performance. Hence an emerging technology called spintronics comes into play. Spintronics is the field in which electron spin carries the information rather than the electron charge .The spin dependent effects can then be utilised and combined with standard electronics. Giant Magneto Resistive (GMR) materials and Magnetic Tunnel Junctions (MTJ) based on spin of electrons can be made use of in many applications. Spintronics devices have been developed based on these. MTJ offers non-volatile memory element capable of fast non-destructive read and write, scaling to nano meter dimensions, and high endurance. When used along with the VLSI technology, MTJ makes it possible not only to realize non-volatile, high density, and fast random access memories, but also to construct non-volatile VLSI logic circuits that have unprecedented low power capability and compactness that overcome the present limit of power and delay. The paper deals with basics of spintronics and mechanism of its working which can be made use of in vast applications including integrated circuits.
INTRODUCTION
Spintronics is arguably one of the largest applications of nanotechnology to date. The goal of spintronics is to understand the interaction between the particle spin and its solid-state environments and to make useful devices using the acquired knowledge. This field uses the phenomena that are dependent on the spin of the electrons. Spin-dependent scattering of conduction electrons had been observed for some time, but the advent of improved thin-film vapour 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. Important considerations for spintronics are effective ways to polarize the electrons, retaining time of the spin and detection of the spin. Some of the potential advantages are increased data processing speeds, larger integration densities and non- volatility.
Spin is the important consideration for this. Spin is a fundamental quantum-mechanical property. It is the intrinsic angular momentum of an elementary particle, such as the electron. Of course, any charged object possessing spin also possesses an intrinsic magnetic moment. It has been known for decades that in ferromagnetism the spins of electrons are preferentially aligned in one direction. Then, in 1988, it was demonstrated that currents flowing from a ferro magnet into an ordinary metal retain their spin alignment for distances longer than interatomic spaces, so that spin and its associated magnetic moment can be transported just as charge. This means that magnetization as well can be transferred from one place to another.
PHENOMENA USED
Giant Magneto Resistance(GMR) Effect
The first practical application of this phenomenon is in the giant magnetoresistive effect (GMR). The GMR is observed
in thin-film materials composed of alternate ferro magnetic and nonmagnetic layers The resistance of the material is lowest when the magnetic moments in ferromagnetic layers are aligned in the same direction, and highest when they are anti-aligned. This is because the spin-aligned currents from one layer are scattered strongly when they encounter a layer that is magnetically aligned in the opposite direction, creating additional resistance. But when the magnetic fields are oriented in the same direction, the spin-aligned currents pass through easily.
Current GMR materials operate at room temperature and exhibit significant changes in resistivity when subjected to relatively small external magnetic fields. Thus they can be used as magnetic field sensors .The imposed magnetic field changes the magnetic orientation of one of the two layers, disrupting their relative orientation and thus changing the
resistivity. The first GMR-based magnetic field sensor was created in 1994, and high-performance disk drives utilizing GMR-based read heads to detect magnetic fields were realized in 1997 and now are ubiquitous. These read heads are responsible for the very rapid growth in magnetic storage densities that has occurred in the last decade.
Fig 1.A Simple Trilayer GMR structure
The above figure shows a typical structure of GMR material used in device applications. It 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 of proper thickness, the same coupling mechanism can cause ferro magnetic coupling between the magnetic layers
resulting in no GMR effect.
Fig.2. Resistance Vs Applied Field for a GMR material
Fig 2 shows how the resistance changes with applied field for a 2um specimen of anti-ferro magnetically coupled multilayer GMR involving NiFeCo, CoFe, and Copper alloy.
Fig.3a.Spin alignment in normal state
Fig.3b. Spin alignment in same direction
Fig.3b. Shows decrease in resistance when the spin is aligned in same direction. Here the current direction is perpendicular to the common spin alignment direction. Hence the current can flow easily. Whereas the Fig.3a shows spin alignment in normal state . Upon application of the field alignment in a common direction takes place.
Spin Dependent Tunneling (SDT)
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 tunnelling. The size of the tunnelling 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.
A diagram of the layers in a pair of low-power shape-biased SDT junctions is shown in Fig.4. Junctions in sensors are usually designed in series connected pairs so that contact can be made to the top layer of the junction by subsequent metal layers. The magnetization in the lowest CoFe layer is pinned as part of a 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.
Fig.4.The layers and structure of a pair of SDT junctions.
Fig.5.
The response of an MTJ is shown in Fig.5.It shows the Resistance of a SDT junction as a function of applied field. The relative directions of the pinned and sense or free layers are shown.
GENERATION OF SPIN POLARIZATION
Spin polarization of not only of electrons, but also of holes, nuclei, and excitations can be defined as Px = Xs/X, ratio of the difference Xs = Xλ − X−λ and the sum X = Xλ + X−λ, of the spin-resolved λ components for a particular quantity X. Generation of Spin polarization can be done in two ways. They are optical spin orientation and electrical spin injection.
A. Optical Spin Orientation
Photo excitation with circularly polarized light creates spin-polarized electrons. In a semiconductor the photo excited spin-polarized electrons and holes exist for the time Ï„ before they recombine. If a fraction of the carriers' initial orientation survives longer than the recombination time, that is, if Ï„ < Ï„s, 26 where Ï„s is the spin relaxation time, the luminescence (recombination radiation) will be partially polarized. By measuring the circular polarization of the luminescence it is possible to study the spin dynamics of the non-equilibrium carriers in semiconductors and to extract such useful quantities as the spin orientation, the recombination time, or the spin relaxation time of the carriers.
While photo excitation with circularly polarized light
creates spin-polarized electrons, the non-equilibrium spin decays due to both carrier recombination and spin relaxation. The steady-state degree of spin polarization depends on the balance between the spin excitation and decay. Sometimes a distinction is made between the terms optical spin orientation and optical spin pumping. The former term is used in relation to the minority carriers (such as electrons in p-doped samples) and represents the orientation of the excited carriers. The latter term is reserved for the majority carriers (electrons in n-doped samples), representing spin polarization of the "ground" state.
B. Electrical Spin Injection
Spin injection utilizes the strong, short-range quantum mechanical exchange interaction of the injected spin polarized electrons with the atomic spins on the atoms in the FM sensor layer. In order to achieve a sufficient density of the injected spins one uses pillar diameters that are of the order of 100 nm. The current densities required for switching are of order 107 - 108 A/cm2.
Consider an example in which a charge current of random spin polarization is sent through a lithographically manufactured pillar as illustrated in Fig.6, containing two magnetic layers shown in blue. The "polarizer" (darker blue), whose magnetization direction is in practice fixed, serves to create a spin polarized current either in transmission or in reflection, depending on the direction of current flow through the pillar. The polarizer is separated from a second ferromagnetic layer, the "sensor" (light blue), by a non-magnetic spacer layer typically made of Cu.
Fig.9.Spin injection schematic
The above diagram (Fig.9.) shows a Schematic of a spin injection structure and the associated switching effects arising from current flow. The basic structure consists of a lithographically manufactured pillar composed of different thin layers. A spin polarizing ferromagnetic layer, the "polarizer", shown in dark blue, has a fixed magnetization direction, in practice accomplished through exchange biasing. When the current flows through it, towards a second ferromagnetic layer, the "sensor", shown in light blue, the spin polarized transmitted current can switch the sensor layer by spin torque, as shown in the middle. When the current direction is reversed, a spin accumulation builds up in front of the polarizer through reflection of spins on the polarizer. The reflected spins have the opposite direction from the transmitted spins and can switch the sensor back to the original state.
SPIN RELAXATION
Basically three mechanisms are there in spin relaxation namely Elliot-Yafet, D'yakonov-Perel ,Bir-Aronov-Pikus.
A. Elliot-Yafet Mechanism
Elliot was the first to realize that conduction electron spins can relax via ordinary momentum scattering (such as by phonons or impurities) if the lattice ions induce spin-orbit coupling in the electron wave function. This is the main spin relaxation method in metals and also important for small-gap semiconductors with large spin-orbit splitting
B. D'yakonov-Perel Mechanism
An efficient mechanism of spin relaxation due to spin orbit coupling in systems lacking inversion symmetry was found by D'yakonov and Perel. Without inversion symmetry the momentum states of the spin up and spin down electrons are not degenerate.
C. Bir-Aronov-Pikus Mechanism
Spin relaxation of conduction electrons in p-doped semiconductors can also proceed through scattering, accompanied by spin exchange, by holes, as was first shown
by Bir. This mechanism is basically an electron hole exchange scattering method dominant at lower temperatures.
APPLICATION IN INTEGRATED CIRCUITS
A. Zero Standby Power
Spintronics logic integrated circuits use two of the properties of electrons, namely negative charge and spin (the fact that electrons are similar to tiny magnets), to remember the results of calculations by flipping the polarity of these tiny magnets between "north" and "south" according to the direction of an electric current. This technology has become the focus of interest as a semiconductor technology because it has non-volatility, meaning that it can retain data even if the power supply is cut off because data is remembered using magnetic polarity. It is this non-volatility that has the potential to eliminate the power consumed by electronic devices while they are in standby mode.
B .Power Consumption Reduction
These technologies use vertical domain wall elements, which have vertical magnetization with respect to a magnetic body. Multiple vertical domain wall elements are loaded for each individual component of a logic integrated circuit, so that there is redundancy in the way that data is remembered. This enables a high level of reliability to be achieved because the data errors that occasionally occur with logic integrated circuits can be detected and corrected. With this technology, spin elements are connected in a series, which prevents power consumption from increasing and prevents the area of the circuit from becoming larger. These highly reliable circuit components also support automatic placement and wiring
C. Non-volatile logic integrated circuits with high reliability
Data redundancy is achieved by loading multiple "spin elements" (the elements that remember calculation results until the power is turned back on) for the components of logic integrated circuits. Although the probability of errors occurring with non-volatile logic integrated circuits is very low, errors where calculation result data is remembered incorrectly do occur, and having redundant spin elements enables these errors to be detected and corrected, so that the data can be read correctly. Normally, adding redundant elements would make the physical size of the circuit larger, but vertical domain wall elements can be placed on top of transistor elements and the elements are connected in a series so there is no need for a new transistor to branch the wires. This means high reliability can be achieved without increasing the circuit area.
Vertical domain wall elements also feature low electrical resistance on a current pathway, and so by connecting the elements in a series very little extra time or current is required to write data.
CONCLUSION
Spintronics has offered new areas of avenue in device applications and device development. This is a very interesting area of study. The advantages of having increased data processing speeds, larger integration densities and non- volatility make spintronics a very useful discipline in device applications but still there are some problems associated with this. Much remains to be understood about the behaviour of electron spins in materials for technological applications, but much has been accomplished. A number of novel spin-based microelectronic devices have been proposed, and the giant magnetoresistive sandwich structure is a proven commercial success, being a part of every computer coming off the production line. In addition, spintronic-based non-volatile memory elements may very well become available in the near future. But before we can move forward into broad application of spin-based multifunctional and novel technologies, we face some fundamental challenges of creating and measuring spin, understanding better the transport of spin at interfaces, particularly at ferromagnetic/semiconductor interfaces, and clarifying the types of errors in spin-based computational systems. Tackling these will require that we develop new experimental tools and broaden considerably our theoretical understanding of quantum spin, learning in the process how to actively control and manipulate spins in ultra-small structures. If we can do this, the payoff will be an entirely new world of spin technology with new capabilities and opportunities.
