INTRODUCTION TO SPINTRONICS

sces.phys.utk.edu/~dagotto/.../HW.../Introduction%20to%20Spintronics.ppt

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INTRODUCTION

Conventional electronic devices ignore the spin property and rely strictly on the transport of the electrical charge of electronsAdding the spin degree of freedom provides new effects, new capabilities and new functionalities

Spin does not replace charge current just provide extra control

Using suitable materials, many different bit states can be interpreted

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FUTURE DEMANDS

Moores Law states that the number of transistors on a silicon chip will roughly double every eighteen monthsBy 2008, it is projected that the width of the electrodes in a microprocessor will be 45nm acrossAs electronic devices become smaller, quantum properties of the wavelike nature of electrons are no longer negligibleSpintronic devices offer the possibility of enhanced functionality, higher speed, and reduced power consumption

New technology has been proposed which would involve a complete set of new materials, new handling and processing techniques, and altered circuit design. Such developments include single-electron transistors and molecular-electronic devices based on organic materials or carbon nanotubes.

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ADVANTAGES OF SPIN

Information is stored into spin as one of two possible orientationsSpin lifetime is relatively long, on the order of nanoseconds Spin currents can be manipulatedSpin devices may combine logic and storage functionality eliminating the need for separate componentsMagnetic storage is nonvolatileBinary spin polarization offers the possibility of applications as qubits in quantum computers

Charge state can be destroyed by interactions with impurities or other charges

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GMR

1988 France, GMR discovery is accepted as birth of spintronicsA Giant MagnetoResistive device is made of at least two ferromagnetic layers separated by a spacer layerWhen the magnetization of the two outside layers is aligned, lowest resistanceConversely when magnetization vectors are antiparallel, high R Small fields can produce big effectsparallel and perpendicular current

Think of optical polarizers

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PARALLEL CURRENT GMR

Current runs parallel between the ferromagnetic layersMost commonly used in magnetic read headsHas shown 200% resistance difference between zero point and antiparallel states

SPIN VALVE

Simplest and most successful spintronic deviceUsed in HDD to read information in the form of small magnetic fields above the disk surface

PERPENDICULAR CURRENT GMR

Easier to understand theoretically, think of one FM layer as spin polarizer and other as detectorHas shown 70% resistance difference between zero point and antiparallel statesBasis for Tunneling MagnetoResistance

TUNNEL MAGNETORESISTANCE

Tunnel Magnetoresistive effect combines the two spin channels in the ferromagnetic materials and the quantum tunnel effectTMR junctions have resistance ratio of about 70%MgO barrier junctions have produced 230% MR

230% with sputtering deposition

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MRAM

MRAM uses magnetic storage elements instead of electric used in conventional RAMTunnel junctions are used to read the information stored in Magnetoresistive Random Access Memory, typically a0 for zero point magnetization state and 1 for antiparallel state

Non volatile, instant-on computers

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MRAM

Attempts were made to control bit writing by using relatively large currents to produce fieldsThis proves unpractical at nanoscale level

SPIN TRANSFER

Current passed through a magnetic field becomes spin polarizedThis flipping of magnetic spins applies a relatively large torque to the magnetization within the external magnetThis torque will pump energy to the magnet causing its magnetic moment to precessIf damping force is too small, the current spin momentum will transfer to the nanomagnet, causing the magnetization will flipUnwanted effect in spin valvesPossible applications in memory writing

MRAM

The spin transfer mechanism can be used to write to the magnetic memory cellsCurrents are about the same as read currents, requiring much less energy

MRAM

MRAM promises:Density of DRAMSpeed of SRAMNon-volatility like flash

DRAM stores one bit in only one capacitor and transistor, very dense but very power hungry because capacitor loses charge, must be frequently refreshed

SRAM stores one bit in six transistors, faster than DRAM but less dense

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SPIN TRANSISTOR

Ideal use of MRAM would utilize control of the spin channels of the currentSpin transistors would allow control of the spin current in the same manner that conventional transistors can switch charge currentsUsing arrays of these spin transistors, MRAM will combine storage, detection, logic and communication capabilities on a single chipThis will remove the distinction between working memory and storage, combining functionality of many devices into one

DATTA DAS SPIN TRANSISTOR

The Datta Das Spin Transistor was first spin device proposed for metal-oxide geometry, 1989Emitter and collector are ferromagnetic with parallel magnetizationsThe gate provides magnetic fieldCurrent is modulated by the degree of precession in electron spin

Rashba effect consequence of spin orbit interaction, proportional to electric field in a structure with inversion asymmetry

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MAGNETIC SEMICONDUCTORS

Materials like magnetite are magnetic semiconductorsDevelopment of materials similar to conventional Research aimed at dilute magnetic semiconductorsManganese is commonly doped onto substrateHowever previous manganese-doped GaAs has transition temp at -88oCCurie temperatures above room must be produced

Ideally, each Mn dopant atom represents an acceptor that introduces a local spin and a hole carrier

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MAGNETIC SEMICONDUCTORS

F. Matsukura, H. Ohno, A. Shen, and Y. Sugawara, Transport Properties and Origin of Ferromagnetism in (Ga,Mn)As, Phys. Rev. B 57, R2037 (1998). A.M. Nazmul, T. Amemiya, Y. Shuto, S. Sugahara, and M. Tanaka, High Temperature Ferromagnetism in GaAs-Based Heterostructures with Mn Delta Doping; see http://arxiv.org/cond-mat/0503444 (2005). F. Matsukura, E. Abe, and H. Ohno, Magnetotransport Properties of (Ga, Mn)Sb, J. Appl. Phys. 87, 6442 (2000). X. Chen, M. Na, M. Cheon, S. Wang, H. Luo, B.D. McCombe, X. Liu, Y. Sasaki, T. Wojtowicz, J.K. Furdyna, S.J. Potashnik, and P. Schiffer, Above-Room-Temperature Ferromagnetism in GaSb/Mn Digital Alloys, Appl. Phys. Lett. 81, 511 (2002). Y.D. Park, A.T. Hanbicki, S.C. Erwin, C.S. Hellberg, J.M. Sullivan, J.E. Mattson, T.F. Ambrose, A. Wilson, G. Spanos, and B.T. Jonker, A Group-IV Ferromagnetic Semiconductor: MnxGe1x, Science 295, 651 (2002). Y. Matsumoto, M. Murakami, T. Shono, T. Hasegawa, T. Fukumura, M. Kawasaki, P. Ahmet, T. Chikyow, S. Koshihara, and H. Koinuma, Room-Temperature Ferromagnetism in Transport Transition Metal-Doped Titanium Dioxide, Science 291, 854 (2001). M.L. Reed, N.A. El-Masry, H.H. Stadelmaier, M.E. Ritums, N.J. Reed, C.A. Parker, J.C. Roberts, and S.M. Bedair, Room Temperature Ferromagnetic Properties of (Ga, Mn)N, Appl. Phys. Lett. 79, 3473 (2001). S. Cho, S. Choi, G.-B. Cha, S. Hong, Y. Kim, Y.-J. Zhao, A.J. Freeman, J.B. Ketterson, B. Kim, Y. Kim, and B.-C. Choi, Room-Temperature Ferromagnetism in (Zn1xMnx)GeP2 Semiconductors, Phys. Rev. Lett. 88, 257203 (2002). S.B. Ogale, R.J. Choudhary, J.P. Buban, S.E. Lofland, S.R. Shinde, S.N. Kale, V.N. Kulkarni, J. Higgins, C. Lanci, J.R. Simpson, N.D. Browning, S. DasSarma, H.D. Drew, R.L. Greene, and T. Venkatesan, High Temperature Ferromagnetism with a Giant Magnetic Moment in Transparent Co-Doped SnO2, Phys. Rev. Lett. 91, 077205 (2003). Y.G. Zhao, S.R. Shinde, S.B. Ogale, J. Higgins, R. Choudhary, V.N. Kulkarni, R.L. Greene, T. Venkatesan, S.E. Lofland, C. Lanci, J.P. Buban, N.D. Browning, S. DasSarma, and A.J. Millis, Co-Doped La0.5Sr0.5TiO3: Diluted Magnetic Oxide System with High Curie Temperature, Appl. Phys. Lett. 83, 21992201 (2003). H. Saito, V. Zayets, S. Yamagata, and K. Ando, Room-Temperature Ferromagnetism in a IIVI Diluted Magnetic Semiconductor Zn1xCrxTe, Phys. Rev. Lett. 90, 207202 (2003). P. Sharma, A. Gupta, K.V. Rao, F.J. Owens, R. Sharma, R. Ahuja, J.M. Osorio Guillen, B. Johansson, and G.A. Gehring, Ferromagnetism Above Room Temperature in Bulk and Transparent Thin Films of Mn-Doped ZnO, Nature Mater. 2, 673 (2003). J. Philip, N. Theodoropoulou, G. Berera, J.S. Moodera, and B. Satpati, High-Temperature Ferromagnetism in Manganese-Doped IndiumTin Oxide Films, Appl. Phys. Lett. 85, 777 (2004). H.X. Liu, S.Y. Wu, R.K. Singh, L. Gu, D.J. Smith, N.R. Dilley, L. Montes, M.B. Simmonds, and N. Newman, Observation of Ferromagnetism at over 900K in Cr-doped GaN and AlN, Appl. Phys. Lett. 85, 4076 (2004). S.Y. Wu, H.X. Liu, L. Gu, R.K. Singh, M. vanSchilfgaarde, D.J. Smith, N.R. Dilley, L. Montes, M.B. Simmonds, and N. Newman, Synthesis and Characterization of High Quality Ferromagnetic Cr-Doped GaN and AlN Thin Films with Curie Temperatures Above 900K (2003 Fall Materials Research Society Symposium Proceedings), Mater. Sci. Forum 798, B10.57.1 (2004).

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CURRENT RESEARCH

Weitering et al. have made numerous advancesFerromagnetic transition temperature in excess of 100 K in (Ga,Mn)As diluted magnetic semiconductors (DMS's).Spin injection from ferromagnetic to non-magnetic semiconductors and long spin-coherence times in semiconductors.Ferromagnetism in Mn doped group IV semiconductors.Room temperature ferromagnetism in (Ga,Mn)N, (Ga,Mn)P, and digital-doped (Ga,Mn)Sb.Large magnetoresistance in ferromagnetic semiconductor tunnel junctions.

The crystalline quality, surface topography, and thermal stability of the films indicate the possibility of growing epitaxial Ge on top of Mn5Ge3 so that epitaxial trilayers or spin valves and perhaps even multilayer structures can be fabricated for spintronics research and applications.

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CURRENT RESEARCH

Material scienceMany methods of magnetic dopingSpin transport in semiconductors

2006 Princeton, A. Yazdani et al. used STM to carefully place individual atoms on GaAs substrate

Swarms of bees, spin polarized currents

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CONCLUSION

Interest in spintronics arises, in part, from the looming problem of exhausting the fundamental physical limits of conventional electronics.

However, complete reconstruction of industry is unlikely and spintronics is a variation of current technology

The spin of the electron has attracted renewed interest because it promises a wide variety of new devices that combine logic, storage and sensor applications.

Moreover, these "spintronic" devices might lead to quantum computers and quantum communication based on electronic solid-state devices, thus changing the perspective of information technology in the 21st century.