Transcript Introduction to Spintronics

INTRODUCTION TO SPINTRONICS
Josh Schaefferkoetter
February 27, 2007
INTRODUCTION


Conventional electronic devices
ignore the spin property and rely
strictly on the transport of the
electrical charge of electrons
Adding the spin degree of
freedom provides new effects,
new capabilities and new
functionalities
FUTURE DEMANDS
Moore’s Law states that the number of transistors
on a silicon chip will roughly double every
eighteen months
 By 2008, it is projected that the width of the
electrodes in a microprocessor will be 45nm across
 As electronic devices become smaller, quantum
properties of the wavelike nature of electrons are
no longer negligible
 Spintronic devices offer the possibility of enhanced
functionality, higher speed, and reduced power
consumption

ADVANTAGES OF SPIN
Information is stored into spin as one of two
possible orientations
 Spin lifetime is relatively long, on the order of
nanoseconds
 Spin currents can be manipulated
 Spin devices may combine logic and storage
functionality eliminating the need for separate
components
 Magnetic storage is nonvolatile
 Binary spin polarization offers the possibility of
applications as qubits in quantum computers
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GMR
1988 France, GMR discovery is
accepted as birth of spintronics
 A Giant MagnetoResistive device is
made of at least two ferromagnetic
layers separated by a spacer layer
 When the magnetization of the two
outside layers is aligned, lowest
resistance
 Conversely when magnetization
vectors are antiparallel, high R
 Small fields can produce big effects
 parallel and perpendicular current
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PARALLEL CURRENT GMR
Current runs parallel between the ferromagnetic
layers
 Most commonly used in magnetic read heads
 Has shown 200% resistance difference between
zero point and antiparallel states
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SPIN VALVE
Simplest and most successful spintronic device
 Used in HDD to read information in the form of
small magnetic fields above the disk surface
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PERPENDICULAR CURRENT GMR
Easier to understand theoretically, think of one
FM layer as spin polarizer and other as detector
 Has shown 70% resistance difference between
zero point and antiparallel states
 Basis for Tunneling MagnetoResistance
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TUNNEL MAGNETORESISTANCE
Tunnel Magnetoresistive effect combines the two
spin channels in the ferromagnetic materials and
the quantum tunnel effect
 TMR junctions have resistance ratio of about 70%
 MgO barrier junctions have produced 230% MR
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MRAM
MRAM uses magnetic storage elements instead of
electric used in conventional RAM
 Tunnel junctions are used to read the information
stored in Magnetoresistive Random Access
Memory, typically a”0” for zero point
magnetization state and “1” for antiparallel state
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MRAM
Attempts were made to control bit writing by
using relatively large currents to produce fields
 This proves unpractical at nanoscale level
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SPIN TRANSFER
Current passed through a magnetic field becomes
spin polarized
 This flipping of magnetic spins applies a
relatively large torque to the magnetization
within the external magnet
 This torque will pump energy to the magnet
causing its magnetic moment to precess
 If damping force is too small, the current spin
momentum will transfer to the nanomagnet,
causing the magnetization will flip
 Unwanted effect in spin valves
 Possible applications in memory writing
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MRAM
The spin transfer mechanism can be used to
write to the magnetic memory cells
 Currents are about the same as read currents,
requiring much less energy

MRAM
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MRAM promises:
Density of DRAM
 Speed of SRAM
 Non-volatility like flash
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SPIN TRANSISTOR
Ideal use of MRAM would utilize control of the
spin channels of the current
 Spin transistors would allow control of the spin
current in the same manner that conventional
transistors can switch charge currents
 Using arrays of these spin transistors, MRAM will
combine storage, detection, logic and
communication capabilities on a single chip
 This will remove the distinction between working
memory and storage, combining functionality of
many devices into one
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DATTA DAS SPIN TRANSISTOR
The Datta Das Spin
Transistor was first spin
device proposed for metaloxide geometry, 1989
 Emitter and collector are
ferromagnetic with
parallel magnetizations
 The gate provides
magnetic field
 Current is modulated by
the degree of precession in
electron spin
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MAGNETIC SEMICONDUCTORS
Materials like magnetite are magnetic semiconductors
 Development of materials similar to conventional
 Research aimed at dilute magnetic semiconductors
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Manganese is commonly doped onto substrate
However previous manganese-doped GaAs has transition
temp at -88oC
Curie temperatures above room must be produced
MAGNETIC SEMICONDUCTORS
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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).
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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).
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F. Matsukura, E. Abe, and H. Ohno, “Magnetotransport Properties of (Ga, Mn)Sb,” J. Appl. Phys. 87, 6442 (2000).
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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: MnxGe1−x,” 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 (Zn1−xMnx)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. Das Sarma, H. D. Drew, R. L. Greene, and T. Venkatesan, “High
Temperature Ferromagnetism with a Giant Magnetic Moment in Transparent Co-Doped SnO2−δ,” 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. Das Sarma, and A. J. Millis, “Co-Doped La0.5Sr0.5TiO3−δ:
Diluted Magnetic Oxide System with High Curie Temperature,” Appl. Phys. Lett. 83, 2199–2201 (2003).
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J. Philip, N. Theodoropoulou, G. Berera, J. S. Moodera, and B. Satpati, “High-Temperature Ferromagnetism in Manganese-Doped Indium–Tin Oxide Films,” Appl. Phys. Lett. 85, 777 (2004).
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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 900 K in Cr-doped GaN and AlN,” Appl. Phys. Lett. 85, 4076 (2004).
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H. Saito, V. Zayets, S. Yamagata, and K. Ando, “Room-Temperature Ferromagnetism in a II–VI Diluted Magnetic Semiconductor Zn1−xCrxTe,” 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).
S. Y. Wu, H. X. Liu, L. Gu, R. K. Singh, M. van Schilfgaarde, 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 900 K” (2003 Fall Materials Research Society Symposium Proceedings), Mater. Sci. Forum 798, B10.57.1 (2004).
CURRENT RESEARCH
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Weitering et al. have made numerous advances
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Ferromagnetic 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.
CURRENT RESEARCH
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Material science
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Many methods of
magnetic doping
Spin transport in
semiconductors
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.