Spintronics — Spin-Based Electronics · Spintronics — Spin-Based Electronics 8-3 I t has long been known that electrons in metals have two spin states and that,when electric ﬁeld

8Spintronics —

Spin-Based Electronics

CONTENTSAbstract8.1 Spin Transport Electronics in Metallic Systems

High-Density Data Storage and High-Sensitivity Read Heads • The Giant Magnetoresistance Effect • Spin Valves • Magnetic Tunnel Junctions • Device Applications for Spin Valves and Magnetic Tunnel Junctions • Magnetoresistive Random Access Memory

8.2 Issues in Spin ElectronicsSpin Injection • Research in New Materials for Spintronics • Optically and Electrically Controlled Ferromagnetism • Current-Induced Magnetization Switching and Spin Wave Generation • Optically Excited Spin States in Semiconductors

8.3 Potential Spintronics DevicesLight-Emitting Diode • Field-Effect Transistor • Resonant Tunneling Diode • New Spintronic Device Proposals

8.4 Quantum Computation and SpintronicsNuclear Spin Quantum Computer • Spin-Resonance Transistor • NMR Quantum Computer • Quantum Dots as Quantum Bits

8.5 ConclusionAcknowledgmentsReferences

Abstract

Spintronics is an acronym for a spin transport electronics and was originally used as a name for the Defense Advanced Research Projects Agency (DARPA) program. In 1994, DARPA started to develop magnetic field sensors and memory based on the giant magnetoresistance (GMR) effect and spin-dependent tunneling. The overall goal for the Spintronics program was to create a new generation of electronic devices where the spin of the carriers would play a crucial role in addition to or in place of the charge. Spintronic devices can be used as magnetic memories, magnetic field sensors, spin-based switches, modulators, isolators, transistors, diodes, and perhaps some novel devices without conventional ana-logues that can perform logic functions in new ways. Spintronics brings together specialists in semicon-ductors, magnetism, and optical electronics studying spin dynamics and transport in semiconductors, metals, superconductors, and heterostructures. As always, the key question is whether any potential benefit of such technology will be worth the production costs.

S.A. WolfDefense Advanced Research Projects Agency and Naval Research Laboratory

A.Y. ChtchelkanovaStrategic Analysis, Inc.

D.M. TregerStrategic Analysis, Inc.

© 2003 by CRC Press LLC

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This paper will provide an overview of the field and the reference material to the original papers for the future in-depth reading. In Section 8.1 we briefly describe spin transport electronics in metallic systems and commercially available devices utilizing magnetization and spin transport properties in metals.1,2,3 Section 8.2 addresses the issues in semiconductor spin electronics,4 which have to be resolved to create successful devices — efficient spin injection into semiconductors and heterostructures and the search for new spin-polarized materials. It also mentions effects potentially important for spintronics devices including optical and electrical manipulation of ferromagnetism, current-induced magnetization switching and precessing, long decoherence time for optically excited spins in semiconductors, etc. Section 8.3 briefly describes proposed devices utilizing spins, and Section 8.4 covers quantum computing schemes relying on spins.

Progress in spintronics would not be possible without the maturity of electron-beam and ion-beam fabrication, molecular beam epitaxy (MBE), and the ability to manufacture devices at the nanoscale with nanoimprint lithography. Advances in magnetic microscopy for direct, real-space imaging demonstrated for the past decade5 have also played a crucial role in new materials and device characterization.

8.1 Spin Transport Electronics in Metallic Systems

Conventional electronic devices are based on charge transport, and their performance is limited by the speed and dissipation of the energy of the carriers (electrons). Prospective spintronic devices utilize the direction and coupling of the spin of the electron in addition to or in place of the charge. Spin orientation along the quantization axis (magnetic field) is dubbed as up-spin and in the opposite direction as down-spin. Electron spin is a major source for magnetic fields in solids.

8.1.1 High-Density Data Storage and High-Sensitivity Read Heads

The spin has been an important part of magnetic high-density data storage technology for many years. Hard disk drives (HDD) store information as tiny magnetized regions along concentric tracks. Mag-netization pointing in one direction denotes a zero bit, and in the opposite direction a one bit. Areal density, expressed as billions of bits per square inch of disk surface area (Gbits/in2), is the product of linear density (bits of information per inch of track) multiplied by track density (tracks per inch) and varies with disk radius. The recent progress in the increasing storage areal density is due to high-sensitivity read heads made possible after the discovery of the GMR effect. The first commercially available disk drive using a GMR sensor head was the 1998 IBM Deskstar 16GP disk drive that had 2.69 Gbits/in2 areal density. The current commercially available density is up to 40 Gbits/in2 and 110 Gbits/in2 is under investigation.

8.1.2 The Giant Magnetoresistance Effect

The GMR effect was discovered in the late 1980s when two research groups performed magnetoresistivity studies6,7 of heterostructures comprised of alternating thin (10–100 Å) metallic layers of magnetic and nonmagnetic metals in the presence of high magnetic fields (2T) at low temperatures (4.2 K). They saw resistance changes up to 50% between the resistivities at zero field and in the saturated state. At room temperature the GMR effect was smaller but still significant. The resistance of the GMR structure is lower if the directions of the magnetization of the ferromagnetic (FM) layers are aligned than when they are anti-aligned. At zero field with the thickness of nonmagnetic metal in the range of 8–18 Å, indirect electron exchange provides a mechanism to induce an anti-parallel alignment of the magnetic layers.8 Application of an external magnetic field (100–1000 Oe) overcomes the anti-parallel coupling between the magnetic layers and, as a result, the resistance changes in the range of 10–60%. The GMR effect is measured as the change in resistivity divided by the resistivity at large fields, typically termed ∆R/R. GMR is observed when the current is in the plane of the layers (CIP) and perpendicular to the plane of the layers (CPP). GMR is attributed to spin-dependent scattering at the interfaces and spin-dependent conductivity.


Spintronics — Spin-Based Electronics

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It has long been known that electrons in metals have two spin states and that, when electric field is applied to a metal, two approximately independent currents flow. In nonmagnetic metals these two spin channels are equivalent because they have the same Fermi energy density of states (DOS) and the same electron velocities, but in ferromagnetic transition metals they are quite different. Spin polarization is defined as the ratio of the difference between the up- and down-spin channel population to the total number of carriers:

P = (n↑ – n↓)/(n↑ + n↓)

For most electronics applications, only the total conductivity resulting from these two parallel currents or spin channels is important. Recent technologically important effects such as GMR take advantage of the differences in electron transport between the two spin-channels. The physics of spin-dependent transport in magnetic multilayers is explained in Reference 9.

8.1.3 Spin Valves

A GMR structure widely used in HDD read heads is a spin valve (SV), originally proposed by IBM in 1994.10 An SV has two ferromagnetic layers (alloys of nickel, iron, and cobalt) sandwiching a thin nonmagnetic metal (usually copper) with one of the two magnetic layers being “pinned,” i.e., the magnetization in that layer is relatively insensitive to moderate magnetic fields.11 The other magnetic layer is called the free layer, and its magnetization can be changed by application of a relatively small magnetic field. As the alignment of the magnetizations in the two layers changes from parallel to anti-parallel, the resistance of the spin valve typically rises from 5% to 10%. Pinning is usually accomplished by using an antiferromagnetic layer that is in close contact with the pinned magnetic layer. In SV read heads for high-density recording, the magnetic moment of the pinned layer is fixed along the transverse direction by exchange coupling with an antiferromagnetic layer (FeMn), while the magnetic moment of the free layer rotates in response to signal fields. The resultant spin-valve response is given by ∆R ~ cos(θ1 – θ2), where θ1 and θ2 are the angles between the magnetization direction of the free layer and pinned layer and the direction parallel to the plane of the media magnetization, as seen in Figure 8.1. When a weak magnetic field, such as from a bit on a hard disk, passes beneath the read head, the magnetic orientation of the free layer changes its direction relative to the pinned layer, generating a change in electrical resistance due to the GMR effect. Because an SV is so important for industrial applications, there have been many improvements in recent years. The simple pinned layer is replaced with a synthetic antiferromagnet — two magnetic layers separated by a very thin (~10Å) nonmagnetic conductor, usually ruthenium.12 The magnetizations in the two magnetic layers are strongly anti-parallel coupled and are thus effectively immune to outside magnetic fields. The second innovation is the nano-oxide layer or NOL, which is formed at the outside surface of the soft magnetic film. This layer reduces resistance due to surface scattering,13 thus reducing background resistance and thereby increasing the percentage change in magnetoresistance of the structure. The magnetoresistance of spin valves has increased dramatically from about 5% in early heads to about 15 to 20% today, using synthetic antiferromagnets and NOLs.

8.1.4 Magnetic Tunnel Junctions

A magnetic tunnel junction (MTJ) is a device in which a pinned layer and a free layer are separated by a very thin insulating layer, commonly aluminum oxide. The tunneling resistance is modulated by the magnetic field in the same way as in a spin valve, and recently it has been shown to exhibit more than a 50% change14 in the magnetoresistance while requiring a saturating magnetic field equal to or somewhat less than that required for spin valves. Spin-dependent tunneling (STD) between FM materials separated by an insulator (I) was first studied in 1975.15 In 1995, FM-I-FM tunneling was measured in Fe/Al2O3/Fe,16 Fe/Al2O3/Ni1-xFex,17 CoFe/Al2O3/Co, and NiFe junctions.18 It was proposed that these junctions have potential use as low-power field sensors and memory elements. Because the


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tunneling current density is usually small, MTJ devices tend to have high resistances. The resistance of an MTJ exponentially depends on the thickness of the insulating layer. Uniformity over the MTJ insulating layer is crucial to device operation. Also, reproducibility from MTJ to MTJ is important for the proper working of arrays of such devices. MTJs currently have been made as small as 0.3 × 0.8 µm2, and the thickness of the insulating layer is typically 1 nm. Another potential issue is the repro-ducibility of the magnetic states of the ferromagnetic layers separated by 1 nm. Surprisingly, this issue has been resolved by careful device processing.

8.1.5 Device Applications for Spin Valves and Magnetic Tunnel Junctions

Spin valve and MTJ device applications are expanding and already include magnetic field sensors, read heads for hard drives, galvanic isolators, and magnetoresistive random access memory (MRAM). General-purpose GMR sensors have been introduced in the past 7 years19 (Figure 8.2), and several companies are producing GMR sensors for internal consumption. A new magnetic field sensor utilizing CPP geometry, a spin-valve transistor, was demonstrated in 1995,20 but it was not commercially produced. No commercial sensors using MTJ structures are available, but one is under development.21

FIGURE 8.1 Schematic of an IBM GMR spin valve sensor used in read head. I = the direction of the current, M = magnetization, θ1 and θ2 = angles from the longitudinal direction. The read head is moving perpendicular to the XY plane. (Adapted from C. Tsang et al., Design, fabrication and testing of spin-valve read heads for high density recording. IEEE Trans. Magn. 30, 3801, 1994. With permission.)

FIGURE 8.2 GMR sensor. (Courtesy of NVE Corporation, Eden Prairie, MN.)


Spintronics — Spin-Based Electronics 8-5

The GMR-based galvanic isolator is a combination of an integrated coil and a GMR sensor on an integrated circuit chip. GMR isolators introduced in 2000 eliminate ground noise in communications between electronics blocks, thus performing a function similar to that of opto-isolators — providing electrical isolation of grounds between electronic circuits. The GMR isolator is ideally suited for integra-tion with other communications circuits and the packaging of a large number of isolation channels on a single chip. Complex multichannel, bidirectional isolators are currently in production. The speed of the GMR isolator is currently 10 times faster than today’s opto-isolators and can eventually be 100 times faster, with the principal speed limitations identified as the switching speed of the magnetic materials and the speed of the associated electronics.

8.1.6 Magnetoresistive Random Access Memory

MRAM (Figure 8.3) uses magnetic hysteresis to store data and magnetoresistance to read data. GMR-based MTJ14,22 or pseudo-spin valve23 memory cells are integrated on a semiconductor chip and function like a static semiconductor RAM chip with the added feature that the data is retained with power off, i.e., nonvolatility. Potential advantages of the MRAM compared with silicon Electrically Erasable Pro-grammable Read-Only Memory (EEPROM) and flash memory are:

• One thousand times faster write times

• No wearout with write cycling (EEPROM and flash memories wear out with about one million write cycles)

• Lower energy for writing

MRAM data access times are about 1/10,000 that of hard disk drives. MRAM is not yet available commercially, but production of at least 4 MB MRAM is anticipated in 2–3 years. See Table 8.1 for a description of how MRAM tracks with the Semiconductor Industries Association’s roadmap for mem-ory technologies. Excellent reviews of the current status of MRAM with detailed descriptions of the working principles of the devices and progress in incorporating them into existing semiconductor technology are given in References 24, 25, and 26. In just a few years we have seen devices develop very rapidly. Theory suggests that, with new materials or structures and with switching controlled by magnetism or current, further improvements in the magnetoresistance effect27–29 can be achieved from the 15% to 40% available today in GMR and MTJ structures to hundreds of percentage point changes at room temperature.

FIGURE 8.3 256 Kb Motorola MRAM. (From S.A. Wolf et al., Spintronics: a spin-based electronics vision for the future. Science. 294, 1488, 2001. With permission.)



8.2 Issues in Spin Electronics

8.2.1 Spin Injection

To be commercially useful, a spintronic device has to work at room temperature and be compatible with existing semiconductor-based electronics. Almost every imaginable spintronics device should have means of spin injection, manipulation, and detection. Well-known sources of spin-polarized electrons are ferromagnetic materials (FM). The magnetic field of the FM interacts with the spins of electrons; as a result, the majority of electrons are in the states such that their spins are aligned with the local magnetization. Spin-polarized current injection was achieved from FM into superconductors,30 from FM into normal metals,31 between two FM separated by a thin insulating film,15 from a normal metal using magnetic semiconductors as an aligner into nonmagnetic semiconductors32–34 and hole injection from p-type FM into nonmagnetic semiconductor.35

8.2.1.1 Electrical Injection

The best electrical injection (forming an ohmic contact) from FM into semiconductors reported to date36–38 reached 4.5% at T < 10 K. To date the best room temperature electric injection from an FM into a semiconductor was reported by Hammar et al.,39–42 but it is not clear if experimental data establishes conclusively electrical injection into the semiconductor.43–45

The main obstacle to effective electrical injection is the conductivity mismatch between the FM electrode and the semiconductor.46 The effectiveness of the spin injection depends on the ratio of the (spin-dependent) conductivities of the FM and nonferromagnet (NFM) electrodes, σF and σN, respec-tively. When σF = σN as in the case of a typical metal, then efficient and substantial spin injection can occur; but when the NFM electrode is a semiconductor, σF >> σN and the spin-injection efficiency will be very low. Only for a ferromagnet where the conduction electrons are 100% spin-polarized can efficient spin injection be expected in diffusive transport.

8.2.1.2 Tunnel Injection

Insertion of a tunnel contact (T) at a FM-to-normal conductor interface can be a solution of the conductivity mismatch problem.47 Two possible configurations considered were FM-T-semiconductor and Shottky barrier diode. A 2% room temperature tunneling spin injection was achieved from Fe into GaAs in the Shottky diode configuration.48 Measurements of spin polarization of the current transmission across an FM-insulator-2DEG junction yield 40% with little dependence over the range 4 K to 295 K.49 A 30% injection efficiency was achieved from an Fe contact into a semiconductor light-emitting diode structure (T = 4.5 K) and persisted to almost room temperature (4% at T = 240 K).50

TABLE 8.1 Comparison of Memory Technologies for the Year 2011

CMOS

MRAMTechnology DRAM Flash SRAM

Reference SIA 1999 SIA 1999 SIA 1999Generation at Introduction 64 GB 64 GB 180 MB/cm2 64 GBCircuit Speed 150 MHz 150 MHz 913 MHz >500 MHzFeature Size 50 nm 50 nm 35 nm <50 nmAccess Time 10ns 10 ns 1.1 ns <2 nsWrite Time 10 ns 10 µs 1.1 ns <10 nsErase Time <1 ns 10 µs 1.1 ns N/ARetention Time 2–4 s 10 years N/A InfiniteEndurance Cycles Infinite 105 Infinite InfiniteOperating Voltage (V) 0.5–0.6 V 5 V 0.5–0.6 V <1 VVoltage to Switch State 0.2 V 5 V 0.5–0.6 V <50 mVCell Size 2.5 F2*/bit

0.0005 µm2

2F2/bit 12F2/bit 2F2/bit

* F = minimal lithographic feature size.



The Shottky barrier formed at the Fe/GaAs interface provides a natural tunnel barrier for injecting spin-polarized electrons under reverse bias.

Tunnel injection of “hot” (energy much greater than EF) electrons into a ferromagnet can be used to create spin-polarized currents. Because inelastic mean free paths for majority and minority electrons differ significantly, hot electron passage through a 3-nm Co layer can result in 90% spin-polarized currents.20,51,52 The highly polarized current then can be used for further injection into the semiconductor. The disadvantage of hot electron injection is that the overall efficiency is low.

8.2.2 Research in New Materials for Spintronics

Progress in new materials engineering and research is very important because, as was mentioned above, one can expect efficient spin injection if the source of spin-polarized electrons is 100% spin polarized. Measurements in a variety of metals, half-metals, metallic binary oxides, Heusler alloys, and other compounds have shown a 35 to 90% range of spin polarization.53–56 The measurement methods employed were spin-polarized photoemission, spin-polarized tunneling, and superconducting point of contact (Andreev reflection). This section describes materials as sources of spin-polarized carriers.

8.2.2.1 Magnetic Semiconductors

Materials combining ferromagnetic and semiconducting properties, magnetic semiconductors, can be a very attractive option as a source of spin-polarized carriers because there are no interface problems or conductivity mismatch. To achieve large spin polarization in semiconductors, the Zeeman splitting of the conduction (valence) band must be greater than the Fermi energy (EF) of the electrons (holes). In concentrated materials, this occurs easily because the net magnetization upon ordering is proportional to the concentration of magnetic species. If the concentration of magnetic species is low, ~ 5% and below, large externally applied magnetic fields and low temperatures are required to produce large polarization.

8.2.2.1.1 Mixed-Valence Manganese Perovskites with the General FormulaA1-xBxMnO3

(where A = La, Nd, or Pr, and B = Ca, Ba, or Sr) have been extensively studied57 for their magnetic and transport properties resulting in colossal magnetoresistance (CMR) observed at the vicinity of the Curie Temperature, TC. Spin ordering in these materials can be obtained by applying external magnetic fields and/or lowering the temperature. Magnetoresistance values of more than 400% at low temperatures were measured in all-oxide spin valves in which La0.7Sr0.3MnO3 electrodes were separated by a thin insulating layer, and spin polarization was estimated to be at least 83% at Fermi level.58 For many of the mixed-valence manganese perovskites the TC is above room temperature, but photoemission data show that the spin polarization decreases to almost zero at room temperature.

8.2.2.1.2 Europium Chalcogenides Europium chalcogenides with the general formula EuX (X = O, S, Se, Te), in which the magnetic ion Eu2+ resides on every lattice site, have low TC (~80 K) with little hope of improvement.59

8.2.2.1.3 Diluted Magnetic Semiconductors (DMS) DMS are alloys in which atoms of a group-II element of a II-VI compound (CdTe or ZnS) semiconductor are randomly replaced by magnetic atoms (e.g., Mn). However, II-VI-based DMS have been very difficult to dope to create p- and n-type semiconductors used in electronic applications. At high magnetic atom concentration and low temperature, antiferromagnetic interaction between magnetic ions creates a mag-netically ordered phase. Below the TC, DMS exhibit ferromagnetic behavior. So far for II-VI semicon-ductors (e.g., CdMnTe), the ferromagnetic phase has been observed below 2 K.60

The equilibrium solubility of transitional atoms in III-V compound semiconductors is low, and only sophisticated nonequilibrium growth conditions of low-temperature molecular beam epitaxy allowed the successful preparation of (In,Mn)As61 for which the hole-induced ferromagnetic ordering was detected below 35 K.62 A GaAs-based DMS was fabricated in 1996,63 and TC around 110 K was reported for some samples in 1998.64 For a detailed review of the properties of ferromagnetic III-V semiconductors,



see Ohno.65 GaMnAs heterostructures can be epitaxially grown with abrupt interfaces and with atomically controlled layered thickness.64–66 Tanaka and Higo reported large tunneling magnetoresistance in GaM-nAs/AlAs/GaMnAs ferromagnetic semiconductor tunneling junctions.67 Although the total change in the magnetoresistance was 44% at 4.2 K, the TMR ratio due to SV effect was estimated to be only 15 to 19%.68 From a technological point of view, the use of semiconducting magnetic elements may reduce the large currents required in all-metal spin valves.

So far Mn is the only successfully used ion for doping DMS. Some groups have started to use Cr ions to dope GaAs and grow new DMS Ga1-xCrxAs by MBE.69

A thin-film magnetic system consisting of nanoscale Mn11Ge8 ferromagnetic clusters (TC ~ room temperature) embedded into a MnxGe1-x DMS semiconductor matrix exhibits magnetoresistance at low fields (2 kOe) and at low temperatures (22 K).70 A group IV DMS MnxGe1-x was successfully grown using MBE in which the Curie temperature was found linearly dependent on the Mn concentration from 25 to 116 K.71 Theoretical calculations predict even higher TC than was measured.

TC calculations72 predicting above room temperature ferromagnetism prompted active interest in DMS GaMnN.73 Ferromagnetic behavior was recently confirmed at 300 K,74,75 and TC is estimated to be 940 K.76

8.2.2.2 Half-Metallic Ferromagnets

Half-metallic ferromagnets (HMF) are another class of potential sources of spin-polarized electrons. The Fermi level of HMF intersects the majority spin electron band while the minority band has an energy gap near the Fermi level. The HMFs have simultaneously both metallic and semiconducting character-istics, and theory predicts that the conduction electrons are 100% spin polarized.77 As a result the magnetoresistance in magnetic multilayers or tunneling junctions is expected to be significantly higher than with conventional ferromagnetic materials.

8.2.2.2.1 Heusler Alloys Spin-polarized tunneling from NiMnSb was studied in a trilayer NiMnSb/Al2O3/Al TMJ.78 A 58% spin-polarization for Ni2MnGa was measured via Andreev reflection82 and TC ~340 K was measured by Dong et al.79 There are TC measurements available for other HMF79 such as Ni2MnGe (~320 K) and Ni2MnIn (~290 K), but no spin-polarization data is mentioned.

8.2.2.2.2 Half-Metallic Oxides Half-metallic oxides were predicted80 to be potential sources of fully spin-polarized electrons. Spin-resolved photoemission from polycrystalline CrO2 films have shown81 a spin polarization of nearly +100% with confirmation by point contact Andreev reflection measurements.82,83 Large TMR effects were observed for magnetite Fe3O4

84 (TC ~860 K) at helium temperatures, but at room temperature the effect is less than 1%.

8.2.2.2.3 Transition Metal Pnictides Transition metal pnictides MnAs, MnSb, CrAs, CrSb can be easily incorporated into existing semicon-ductor technology if proven to be useful as spin-polarized sources. For magneto-optical device applica-tions, such as an optical isolator, high optical transmission and large magneto-optical effects must be realized at room temperature. The GaAs:MnAs nanocluster system was fabricated and characterized by Shimizu and Tanaka.85 Zinc blende structure CrAs thin films were synthesized on GaAs(001) substrates by MBE and show a ferromagnetic behavior at room temperature. Calculations predict a highly spin-polarized electronic band structure.86

Thin films of CrSb grown by MBE on GaAs, (Al,Ga)Sb, and GaSb are found to have a zinc blende structure and TC > 400.87

A thousand-fold magnetoresistance effect was discovered in granular MnSb films88 at room tempera-ture with low magnetic fields (less than 0.5 T). A 20% positive, photoinduced magnetoresistance effect was observed in GaAs with inclusions of MnSb nanomagnets when irradiated with photons with energies above the band gap of GaAs. This effect is presumably due to an enhancement of the tunneling probability between MnSb islands by photogenerated carriers in the GaAs matrix.89



8.2.2.2.4 Diluted Magnetic Semiconductor OxideEpitaxial thin films of diluted magnetic semiconductor oxide, Mn-doped ZnO, fabricated by pulse-laser deposition, showed considerable magnetoresistance at low temperature.90 Following the prediction that ZnO has a TC above room temperature72 and would become ferromagnetic by doping with 3d transition elements, intensive combinatorial work began in Japan on dilute magnetic oxides. Room-temperature ferromagnetism was reported in transparent Co-doped (up to 8%) Ti2O atanase films.91 This material is transparent to visible light and might be of great importance to optoelectronics.

8.2.2.3 Borides

Electronic structure calculations for La-doped CaB692

(TC ~900 K) showed that it has a semiconductor band structure and can be considered as a new semiconducting material for spin electronics.

8.2.3 Optically and Electrically Controlled Ferromagnetism

Optically and electrically controlled ferromagnetism is now a low-temperature effect, but extension to higher temperatures may have important implications in areas ranging from optical storage to photon-ically and electrically driven micromechanical elements.

Ferromagnetism was induced by photogenerated carriers in an MBE-grown p-(In,Mn)As/GaSb film at temperatures below 35 K.93 The order was preserved even after the light was switched off and recovered to the original paramagnetic condition above 35 K. The results were explained in terms of hole transfer from GaSb to InMaAs, which then enhanced the ferromagnetic spin exchange between Mn ions in the heterostructures.

Electric-field control of hole-induced ferromagnetism was demonstrated in (In,Mn)As using an insu-lating-gate field-effect transistor (FET) at temperatures below 20 K94 in 2000. Manganese substitutes indium and provides a localized magnetic moment and a hole. These holes mediate magnetic interactions resulting in ferromagnetism. Changing the hole concentration by applying a gate voltage modifies the ferromagnetic properties in the DMS below the transition temperature. It was also found that the new group IV DMS, MnxGe1-x, allows control over ferromagnetic order by applying a 0.5 V gate voltage.71

8.2.4 Current-Induced Magnetization Switching and Spin Wave Generation

Current-induced magnetization switching and spin wave generation are recently discovered effects allow-ing manipulation of magnetization of the FM layers in heterostructures. The change in scattering of the electrons traversing alternating ferromagnetic and nonmagnetic multilayers depends on the relative orientation of the magnetization. The scattering of the electrons within the alternating layers of FM and regular metals can affect the moments in the magnets. Theoretical calculations indicate95–98 that spin-polarized currents perpendicular to the layers can transfer angular momentum between layers, causing torque on the magnetic moments and, as a result, causing rotation and possibly even precession of the layer magnetization and high-frequency switching. A few independent experiments have demonstrated current-induced magnetization rotation.99–105 Current-induced magnetization switching has potential applications to high-speed, high-density GMR-based MRAM as a convenient writing process.106 Other possible applications might include spin-filter devices and spinwave-emitting diodes.107 In the presence of a large magnetic field, the spin rotation can become a high-frequency coherent precession of the moments. This effect, labeled spin amplification by simulated emission of radiation (SASER), was predicted by Berger,95 and the feasibility of it was studied by Tsoi et al.105

8.2.5 Optically Excited Spin States in Semiconductors

Optically excited spin states in semiconductors have been studied by optoelectronic manipulation of spins allowing spatial selectivity and temporal resolution. This is a way to study spin transport in bulk semiconductors and across junctions in heterostructures. For a very detailed description of electron spin and optical coherence in semiconductors see Awschalom and Kikkawa in Reference 108.



8.2.5.1 The Measurement of Long-Spin Lifetimes

Time-resolved optical experiments109–112 have revealed a remarkable persistence of coherent electron spin states. These insensitivities to environmental sources of decoherence in a wide variety of direct bandgap semiconductors and long-spin lifetimes in bulk semiconductors have been shown to exceed a hundred nanoseconds. Optical pulses are used both to create a superposition of the basis spin states defined by an applied magnetic field and to follow the phase, amplitude, and location of the resulting electronic spin precession (coherence) in bulk semiconductors, heterostructures, and quantum dots. In heterostruc-tures and quantum dots, nanosecond dynamics persist to room temperature, providing pathways toward practical coherent quantum electronics.

8.2.5.2 Transfer of Spin Coherence across Heterojunctions

Data show that spin coherence across heterojunctions113 can be preserved when a “pool” of coherent spins is crossing the interface between two semiconductors. A phase shift of spins on opposite sides of the interface can be set by the difference in electron g-factors between the two materials and can be controlled by utilizing epitaxial growth techniques. More recent measurements have established an increase in spin injection efficiency with bias-driven transport: relative increases of up to 500% in electrically biased structures, and 4000% in p-n junctions with intrinsic bias have been observed114 relative to the unbiased interfaces.

8.2.5.3 Defects and Spin Coherence

Another important aspect for the development of spin-based electronics is the effect of defects on spin coherence. In this context the III-V semiconductor GaN is intriguing because it combines a high density of charged threading dislocations with high optical quality, allowing optical investigation of the effects of momentum scattering on coherent electronic spin states. Despite an increase of eight orders of magnitude in the density of charged threading dislocations, studies reveal electron spin coherence times in GaN epilayers reach ~20 ns at T = 5 K, with observable coherent precession at room temperature.115

Detailed investigations reveal a dependence on both magnetic field and temperature qualitatively similar to previous studies in GaAs, suggesting a common origin for spin relaxation in these systems.

8.3 Potential Spintronics Devices

The devices described below are highly experimental and can be considered as potential proof of concept that integrated spintronic devices can be built.

8.3.1 Light-Emitting Diode

A spin-polarized light-emitting diode (spin-LED), schematically depicted in Figure 8.4, is a very impor-tant device used primarily to measure the effectiveness of the injection of spin-polarized currents into semiconductor heterostructures. In a conventional LED, electrons and holes recombine in the vicinity of a p-n junction under forward bias, producing light. The resulting light is unpolarized because all spin-state carriers are equally populated and allow transitions to occur with equal probability. In a spin-LED, recombination of spin-polarized carriers results in the emission of right (σ –) or left (σ +) circularly polarized light in the direction normal to the surface according to selection rules.116 Polarization analysis of the resulting electroluminescence (EL) provides quantitative measurement of the injection efficiency. A spin-LED was used to measure the electrical spin injection of electrons into a GaAs quantum well at low temperatures (4.2K) and with an external magnetic field up to 8 T. It was demonstrated using a diluted magnetic semiconductor spin-aligner concept.32 Unpolarized electrons are injected from a metal into a DMS incorporating Mn magnetic ions. Even a small external magnetic field produces Zeeman splitting of the conduction band states, and the injected electrons became highly polarized because of the sp-d exchange interaction with the Mn ions. The resulting spin-polarized electrons diffuse into thenonmagnetic semiconductor. Two groups33,34 used Mn-doped ZnSe quaternary DMS alloys as spin



aligners and achieved 90% and 50% injection efficiency, respectively. Another group electrically injected holes35 into III-V heterostructures based on GaAs at T < 52 K. Injection into nonmagnetic semiconductors is achieved at zero field using a p-type ferromagnetic semiconductor (Ga,Mn)As as the spin polarizer. It was measured that hole spins can be injected and transported across the interfaces over 200 nm, which confirms the idea that spin-polarized transport can survive the length of the device. At this time, however, there are no plans for mass production of a spin-LED, but they are very useful in the measurement of spin injection despite low temperatures and high magnetic field requirements. If progress in new materials development brings to fruition an n-doped high Curie temperature FS, then the role of a mass-produced spin-LED can be reconsidered.

8.3.2 Field-Effect Transistor

The spin-polarized field-effect transistor (spin-FET), schematically depicted in Figure 8.5, was proposed by Datta and Das in 1990.117 The device was proposed as an electronic analog of the electro-optic modulator. We will briefly describe how the electro-optic modulator works. As light passes through the electro-optic material, the two components of polarization (z and y) undergo a phase shift because dielectric constants, εzz and εyy , differ. The gate voltage can control the phase shift between polarization components by changing the dielectric constants. As a result, the output power of the light coming out of the analyzer can be modulated by changing the gate voltage. In a proposed spin-FET, the spin-polarized electrons are injected and collected by the FM electrodes. Two physical phenomena are important in understanding the principle of the spin-FET. First, at the Fermi level in FM materials, the density of states for electrons with one spin exceeds that for the other; and as a result, the electrode preferentially injects and detects electrons with a particular spin. Second, in a 2DEG in narrow gap semiconductors, there is an energy splitting between electrons with up-spin and down-spin even in the absence of an

FIGURE 8.4 Schematic cross-sections of the samples in spin LED. (Adapted from B.T. Jonker et al., Robust electrical spin injection into a semiconductor heterostructure. Phys. Rev. B 62, 8180, 2000. With permission.)

FIGURE 8.5 Schematic of spin-FET. The current modulation is controlled by a gate voltage which affects the spin precession due to spin orbit coupling in 2DEG semiconductor. FM source and drain are used to inject and detect specific spin orientation. The magnetization direction can be considered as 4th terminal.



external magnetic field. The main effect responsible for zero-field splitting is the Rashba term in the effective mass Hamiltonian.118 The electric field perpendicular to the 2DEG at the heterojunction interface yields an effective magnetic field for moving electrons and lifts the spin degeneracy. The spin–orbit interaction is proportional to the value of the electric field at the interface and can be controlled by the applied gate voltage. A spin–orbit interaction causes the spins of the carriers to precess. The modulation of the current can be controlled by changing the alignment of the carrier spins with the magnetization vector of the collector electrode. By changing the interface electric field, the gate electrode on the top of the device can be used to control spin–orbit interaction. In 1997, gate control of the spin–orbit interaction in an inverted In0.53Ga0.47As/In0.52Al0.48As heterostructures was reported.119 Another spintronics semicon-ductor field-effect transistor operating at low temperatures was designed in 1999.37 In this device, resis-tance modulation was achieved through the spin-valve effect; but it was done by having ferromagnetic contacts with different coercivities and varying an applied magnetic field. At this time both devices operate at low temperatures and achieve very small changes in resistance.

8.3.3 Resonant Tunneling Diode

The resonant tunneling diode, RTD, is normally a vertical tunneling diode with the vertical dimensions produced by growth and the lateral dimensions produced by lithography. If a quantum well is placed between two thin barriers, the tunneling probability is greatly enhanced when the energy level in the quantum well coincides with the Fermi energy (resonant tunneling). The typical dimensions in the tunneling direction are a few atomic layers thick and determine the current and power dissipation. To produce lower power devices, smaller dimensions are required, and issues of control of the uniformity of the tunneling layer become very important because the tunneling current depends exponentially on the thickness of the tunnel barrier.

Introduction of ferromagnetic materials into RTDs can greatly enhance their functionality. Magnetiza-tion-controlled resonance tunneling in GaAs/ErAs120 RTDs shows splitting and enhancement of the reso-nant channels, which depend on the orientation of the external magnetic field with respect to the interface. Theoretical calculations predict that polarization of the transmitted beam can achieve 50%121 or even higher.122 Spin RTDs can be used both as spin filters and energy filters. Current–voltage characteristics of AlAs/GaAs/AlAs double-barrier resonant tunneling diodes with ferromagnetic p-type (Ga, Mn)As on one side and p-type GaAs on the other, have been studied.64,123 A series of resonant peaks have been observed in both polarities, i.e., injecting holes from p-type GaAs and from (Ga, Mn)As. When holes are injected from the (Ga, Mn)As side, spontaneous resonant peak splitting has been observed below the ferromagnetic transition temperature of (Ga, Mn)As without magnetic field. The temperature dependence of the splitting is explained by the spontaneous spin splitting in the valence band of ferromagnetic (Ga, Mn)As.

Introducing a ferromagnetic quantum well in a ferromagnetic junction is shown to greatly enhance the tunneling magnetoresistance effect124 due to spin filtering as well as energy filtering.

8.3.4 New Spintronic Device Proposals

New spintronic devices are highly speculative and have not been implemented yet. One proposal125 is considering the possibility of constructing unipolar electronic devices by utilizing ferromagnetic semi-conductor materials with variable magnetization directions. Such devices should behave very similarly to p-n diodes and bipolar transistors and could be applicable for magnetic sensing, memory, and logic.

Another theoretical device model126 is a spin-polarized p-n junction with spin polarization induced either optically (in which case it is a solar cell) or electronically to majority or minority carriers. It is demonstrated that spin polarization can be injected through the depletion layer by both minority and majority carriers, making semiconductor devices such as spin-polarized solar cells and bipolar transistors feasible. Spin-polarized p-n junctions allow for spin-polarized current generation, spin amplification, voltage control of spin polarization, and a significant extension of spin diffusion range.

Spin filters can possibly be constructed by carefully engineering127 ordered interfaces between FM and S, and a ballistic spin-filter transistor128 can be added to the growing list of possible spintronic devices.



8.4 Quantum Computation and Spintronics

The introduction of coherent spins into ferromagnetic structures could lead to a new class of quantum spintronics. As an example, some proposed quantum computation schemes rely on the controllable interaction of coherent spins with ferromagnetic materials to produce quantum logic operations.129 There is also experimental evidence that ferromagnetic materials can be used to imprint nuclear spins in semiconductors, offering a way of manipulating and storing information at the atomic scale.130

Semiconductor-based quantum spin electronics is focused on developing solid-state quantum information processing devices. Nuclear spins have been proposed as candidates for storing both classical and quantum information due to spin lifetimes that exceed those of electrons by at least several orders of magnitude.

8.4.1 Nuclear Spin Quantum Computer

In a nuclear spin quantum computer proposal, individual phosphorous nuclei 31P embedded in silicon are treated as quantum bits (qubits),131 as shown in Figure 8.6. By placing a gate electrode (A) over a qubit and applying a bias, one can control the overlap of the bound electron with the nucleus and thus the hyperfine interaction between nuclear spin and electron spin (controlled one-bit rotations). Another gate (J) controls the potential barrier between neighboring nuclear spins, allowing them to interact via electron-spin exchange (entanglement).

8.4.2 Spin-Resonance Transistor

Spin-resonance transistor (SRT) from Si-Ge compounds,132 seen in Figure 8.7, was proposed to sense and control a single donor (31P) electron spin. The choice of group-IV semiconductors has the advantage

FIGURE 8.6 Schematic representation of two cells in one-dimensional array containing 31P donors and electrons in Si host separated by metal gates. A-gates control the resonant frequency of the nuclear spin qubits, and J-gates control the electron-mediated coupling between adjacent nuclear spins. The ledge over which the gates cross localizes the gate electric field in the vicinity of the donors. (Adapted from B. Kane, Nature, 393, 133, 1998. With permission.)



of reduced spin–orbit coupling that could lead to longer spin coherence times. One- and two-qubit operations are performed by applying a gate bias. The electric field bias pulls the electron wave function away from the dopant ion into layers of different alloy compositions. Because different layers have different g-factors, this displacement changes the spin Zeeman energy, allowing single-bit operations. By displacing the electron even further, the overlap with neighboring qubits is achieved, allowing two-qubit operations.

8.4.3 NMR Quantum Computer

An NMR quantum computer was proposed to be constructed from isolated chains of 29Si (spin 1/2) embedded in 28Si or 30Si (spin 0) and attached to a thin bridge structure. The nuclei within each chain are distinguished by a large magnetic field gradient created by a micromagnet. Magnetic moment of the ensemble of nuclei causes mechanical force on the flexible bridge, which can be detected by Magnetic Resonance Force Microscope. Issues of initialization, manipulation, computation, and readout are addressed in Reference 133.

8.4.4 Quantum Dots as Quantum Bits

It has been proposed that the spin of an electron confined to quantum dots is a promising candidate for quantum bits and that arrays of quantum dots can be used in principle to implement a large-scale quantum computer.134,135 Quantum operations in these proposals are provided by the coupling of electron spins in neighboring quantum dots by an exchange interaction between them. This interaction can be switched by applying controlled gate voltage pulses, thus allowing realization of fundamental quantum gates such as the exclusive OR. The read-out of such a spin-qubit can be performed efficiently as a spin-polarized electric current passing through the dot136 or optically through integration in solid-state microcavities.137 Alternatively, qubit rotations can be implemented by local electrostatic shifting of the electron into a region with a different effective magnetic field, such as that which occurs at heterointerfaces and in magnetic semiconductor structures.

Direct optical manipulation of charge-based coherent wave packets has been achieved in individual quantum dots.138 Many proposals exist for a hybrid technique of spin-to-charge conversion that may be desirable for combining the longer spin-coherent lifetimes with the sensitivity of charge detection. Recent experiments have revealed that the transverse and longitudinal relaxation times for electron spins in insulating quantum dots are in the nanosecond regime and offer promise for their utilization as com-puting elements in quantum electronics.111,139 The challenge of performing a suitably large number of

FIGURE 8.7 Spin resonance transistor. The left transistor gate is biased V > 0, producing single-qubit unitary transformations in the left SRT. The right gate is unbiased. The n-Si0.4Ge0.6 ground plane is a counterelectrode to the gate. It is used for sensing the spin. (Adapted from R. Vrijen et al., Electron spin-resonance transistors for quantum computing in silicon-germanium heterostructures. Phys. Rev. A 62, 012306, 2000. With permission.)



qubit rotations within the spin-coherence time has been addressed by a new technique developed in quantum wells that produces rotations of electron spins on 100 femtosecond time scales.140 In these experiments, intense laser pulses energetically tuned below the semiconductor bandgap generate a light-induced effective magnetic field via the optical Stark effect and successfully operate on quantum-confined electron spin states.

All proposals briefly described above are very far from being implemented, and it is very early to predict which are going to be the winners.

8.5 Conclusion

Spintronics is a rapidly evolving field that will have increasing impact as it matures in the decades to come. GMR devices have revolutionized magnetic disk storage, and spin-dependent tunneling devices will soon have a similar impact on random access memory. Using the spin degree of freedom in semi-conductor heterostructures, we are reaching the stage of development in which devices are appearing, and many more are on the horizon. The real dream of spintronics is the use of spin-phase coherence to develop totally new devices and methods for computation and communication because the coupling of spin coherence and coherent light is remarkable. This concept, taken to the extreme of coherence between single spins, may lead to the next revolution in computing and communication — namely, quantum information science and the quantum internet.

Acknowledgments

This paper was inspired and based on the review article.1 We would like to express our gratitude to David Awschalom, Robert Buhrman, Jim Daughton, and Stephan von Molnár for their contributions to the original paper. We would like also to thank Berend Jonker for providing help with references and figures. We are grateful to Jim Daughton, Saied Tehrani, Bruce Kane, Eli Yablonovitch, and Ching Tsang for providing original figures.

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Spintronics — Spin-Based Electronics · Spintronics — Spin-Based Electronics 8-3 I t has long been known that electrons in metals have two spin states and that,when electric ﬁeld