International Journal of Engineering Research & Technology (IJERT) ISSN: 2278-0181 Vol. 2 Issue 6, June - 2013 Overview Of Spintronics Mukesh D. Patil Jitendra S. Pingale Umar I. Masumdar Ph.D. IIT Mumbai. M.E. Electronics, M.E. Electronics and Ramrao Adik Institute of Ramrao Adik Institute of Telecommunication, Technology, Navi Mumbai, Technology, Navi Mumbai, Terna Engineering college, India. India. Navi Mumbai, India. Abstract equilibrium and non-equilibrium spin populations, as Spintronics refers commonly to phenomena in which well as spin dynamics. the spin of electrons in a solid state environment plays the determining role. Spintronics devices are 1.1 History based on a spin control of electronics, or on an In the information era, a new promising science has electrical and optical control of spin or magnetism. been strongly addressed called Spintronics, the This review provides a new promising science which contracted form of spin based electronics. The 2007 has been strongly addressed as Spintronics, the Nobel Prize for physics, with whom A. Fert and P. A. contracted form of spin based electronics and Grunberg have been awarded, is another clear signal presents selected themes of semiconductor that the importance of Spintronics for society is Spintronics, introducing important concepts in spin worldwide understood. In the far 1933 the physicist transport, spin injection, Silsbee-Johnson spin- F. Mott published his innovative concept of spin charge coupling, and spin dependent tunneling. Most dependent conduction. Only forty years later semiconductor device systems are still theoretical experimental evidence of current spin polarisation concepts, waiting for experimental demonstrations. was reported by P. Tedrow and R. Meservey, IJERTIJERTcarrying out experiments of tunneling between Keywords- Giant Magnetoresistance, Magnetism, ferromagnetic metals and superconductors. In 1975 Magnetoresistance, Spintronics, Tunneling experiments on a Fe/GeO/Co junction led to the Magnetoresistance. discovery of tunneling magnetoresistance (TMR) by M. Julliere, only verified in 1995 by T. Miyazaki and N. Tezuka and J. S. Moodera. In1988 experiments on 1. Introduction layered thin films of FMs alternated to a non- In a narrow sense Spintronics refers to spin magnetic metal (NM) led to the simultaneous and electronics, the phenomena of spin-polarized independent discovery of the giant magnetoresistance transport in metals and semiconductors. The goal of (GMR) by A. Fert and P. A. Grunberg. Nowadays this applied Spintronicsis to find effective ways of theprincipal application of Spintronics devices is the controlling electronic properties, such as the current magnetic data storage with an information density or accumulated charge, by spin or magnetic field, as growth rate faster than the corresponding Moore law. well as of controlling spin or magnetic properties by electric currents or gate voltages. The ultimate goal is 2. Spin Injection to make practical device schemes that would enhance functionalities of the current charge based 2.1 Spin Drift electronics. An example is a spin field effect Electrons which can be labeled as spin up and spin transistor, which would change its logic state from down. The total number of electrons is assumed to be ON to OFF by flipping the orientation of a magnetic preserved. If the electron densities are n↑and n↓for field [1]. In a broad sense Spintronics is a study of the spin up and spin down states, the total particle spin phenomena in solids, in particular metals and density is, semiconductors and semiconductor hetero-structures. n = n↓+ n↓. (1) Such studies characterize electrical, optical, and while the spin density is, magnetic properties of solids due to the presence of IJERTV2IS60046 www.ijert.org 27 International Journal of Engineering Research & Technology (IJERT) ISSN: 2278-0181 Vol. 2 Issue 6, June - 2013 s = n↓-n↓. (2) Let probability of w thata spin is flipped in the time of τ, so that the spin flip rate is w/ τ. We will assume that w <<1.The actual spin flip probability during therelaxation time τis typically 10-3 to 10-6; so that electrons need to experience thousandsscatterings before spin flips. Therefore the density spin polarization as well as the current spin polarization is given by, 푛 −푛 푠 푃 = ↑ ↓ = , (3) Figure 2: The equivalent circuit of the standard 푛 푛 푛 푗 −푗 푗 model of spin injection in F/N junctions. 푃 = ↑ ↓ = 푠. (4) 푗 푛푗 푗 this will be useful in our model of spin injection. i. Ferromagnet Current spin polarization at x = 0 in the ferromagnet 2.2 Spin Injection Standard Model is, 1 휎푓↑휎푓↓ 푝푗퐹 0 = 푝휎퐹 + 4 , (5) 푗 휎푓 Where effective resistance of the ferromagnet, 휎푓 푅퐹 = 퐿푠퐹 . (6) 4휎푓↑휎푓↓ This is not the electrical resistance of the region, only an effective resistance that appears in the spin- polarized transport and is roughly equal to the actual resistance of the region of size LsF. ii. Nonmagnetic Conductor Since in the nonmagnetic conductor Pσ= 0, and σN↓= Figure 1: Scheme of our spin-injection geometry. σN↑, the current spin polarization in the nonmagnetic conductor then becomes, The standard model of spin injection has its roots in 1 1 푝푗푁 0 = − , (7) the original proposal of Aronov (1976). The 푗 휎푁 thermodynamics of spin injection has been developed where, effective resistance of the nonmagnetic by Johnson and Silsbee, who also formulated IJERTa IJERTregion, 퐿푠퐹 Boltzmann-like transport model for spin transport 푅퐹 = . (8) 휎푁 across ferromagnet nonmagnet (F/N) interfaces. Our iii. Contact goal is to find the current spin polarization, Pj(0) in The conductance spin polarization is, the normal conductor. We will assume that the lengths of the ferromagnet and the nonmagnetic ∑ −∑ 푃 0 = ↑ ↓ (9) regions are greater than the corresponding spin ∑ ∑ diffusion lengths. The spin injection scheme is Where, ∑ = ∑↑ + ∑↓is the conductance, while spin illustrated in Fig. 1. The ferromagnetic conductor (F) conductance is∑ = ∑↑ − ∑↓ and the effective forms a junction with the nonmagnetic conductor resistance of the contact is, ∑ (N). The contact region (C) is assumed to be 푅푐 = , (10) infinitely narrow, forming the discontinuity at x = 0. 4∑↓∑↑ Current spin polarization, at the contact given by, It is assumed that the physical widths of the 1 4∑↓∑↑ 푝 0 = 푃 + . (11) conductors are greater than the corresponding spin 푗푐 ∑ 푗 ∑ diffusion lengths. We assume that at the far ends of the junction, the non-equilibrium spinvanishes. We Let assume spin current continuity at the contact: now look at the three regions separately. The Pj= Pjf= Pjn= Pjc. (12) ferromagnet, contact, and normal conductor regions The above equalities are justified if spin-flip are identified. The electric current splits into the spin scattering can be neglected in the contact. For up and spin down components, each passing through contacts with paramagnetic impurities, we would the corresponding spin-resolved resistors[3]. need to take into account contact spin relaxation which would lead to spin current discontinuity. This assumption of the low rate of spin flip scattering at the interface should also be carefully reconsidered IJERTV2IS60046 www.ijert.org 28 International Journal of Engineering Research & Technology (IJERT) ISSN: 2278-0181 Vol. 2 Issue 6, June - 2013 when analyzing room temperature spin injection 휇푠퐹 −∞ = 0. (15) experiments. Using the spin current continuity Induced electromotive force, defined by, equations, we can solve our algebraic system and 푒푚푓 = 휇푠푁 ∞ − 휇푠퐹 −∞ . (16) readily obtain for the spin injectionefficiency, The emf can be detected as a voltage drop. The drop 푅퐹 푝휎퐹 +푅푐푃∑ of thequasi-chemical potential across the contact is 푝푗 = . (13) 푅퐹+푅푐 +푅푁 due to the spinfiltering effect of the contact. If the The standard model of spin injection can be contact conductance were spin-independent, the summarized by the equivalent electrical circuit chemical potential would be continuous.The shown in Fig. 2. Spin up and spin down electrons electrostatic potential drop across the contact is due form parallel channels for electric current. Each tothe spin polarization of the ferromagnet as well as region of the junction is characterized by its own due tothe spin filtering effects of the contact. There is effective resistance, determined by the spin diffusion an emf developed if equilibriumspins in electrical lengths in the bulk regions, or by the spin- contact with a nonequilibrium spin. This effect dependentconductance in the contact. allows detection of non-equilibrium spin, by putting a ferromagnetic electrode over the region of spin 3. Spin Detection accumulation. By measuring the emf across this junction, we obtain information about the spin in the 3.1. Silsbee-Johnson Spin-charge Coupling nonmagnetic conductor. In electrical spin injection we drive spin-polarized electronsfrom a ferromagnet into a nonmagnetic 3.2. Giant Magneto Resistance (GMR) conductor. As aresult, non-equilibrium spin GMR or the MR is the percent difference in accumulates in the nonmagneticconductor. The resistance for parallel and antiparallel orientations of opposite is also true: If a spin accumulation is the two ferromagnetic regions in the spin valve. Spin generated in a nonmagnetic conductor that is in valves are nanostructures that consist of stacked proximity of a ferromagnet, a current flows in a layers of magnetic and nonmagnetic material. They closed circuit, or an electromotive force (emf) are built of two small Ferro-magnets (Co or an alloy appears in an open circuit. This inverse effect is of Ni and Fe called permalloy), separated by a called the Silsbee-Johnson spin-charge coupling. This nonmagnetic spacer layer (such as Cu) (Fig. 3). The coupling was first proposed by Silsbee (1980) and typical thicknesses of the layers are in the order of 10 experimentally demonstrated by Johnson and Silsbee - 100 nm, and may be even smaller. A current can be (1985) in the first electrical spin injection applied to the spin valve in two directions: experiment. IJERTIJERTa) CIP: Current In Plane/Parallel to the planes. b) CPP: Current Perpendicular to planes.