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5. SPINTRONICS AND OTHER ASPECTS

ON SPIN-BASED ELECTRONICS

The following points are made by S.A. Wolf et al (Science 2001 294:1488):

1) Until recently, the spin of the electron was ignored in mainstream charge-based electronics. A technology has emerged called "spintronics" (spin transport electronics or spin-based electronics), where it is not the electron charge but the electron spin that carries information, and this offers opportunities for a new generation of devices combining standard microelectronics with spin-dependent effects that arise from the interaction between spin of the carrier and the magnetic properties of the material.

2) Traditional approaches to using spin are based on the alignment of a spin (either "up" or "down") relative to a reference (an applied magnetic field or magnetization orientation of the ferromagnetic film). Device operations then proceed with some quantity (electrical current) that depends in a predictable way on the degree of alignment. Adding the spin degree of freedom to conventional semiconductor charge-based electronics or using the spin degree of freedom alone will add substantially more capability and performance to electronic products. The advantages of these new devices would be nonvolatility, increased data processing speed, decreased electric power consumption, and increased integration densities compared with conventional semiconductor devices.

3) Major challenges in this field of spintronics that are addressed by experiment and theory include the optimization of electron spin lifetimes, the detection of spin coherence in nanoscale structures, transport of spin-polarized carriers across relevant length scales and heterointerfaces, and the manipulation of both electron and nuclear spins on sufficiently fast time scales. In response, recent experiments suggest that the storage time of quantum information encoded in electron spins may be extended through their strong interplay with nuclear spins in the solid state. Moreover, optical methods for spin injection, detection, and manipulation have been developed that exploit the ability to precisely engineer the coupling between electron spin and optical photons.

4) The author describes a new paradigm of electronics based on the spin degree of freedom of the electron. Either adding the spin degree of freedom to conventional charge-based electronic devices or using the spin alone has the potential advantages of nonvolatility, increased data processing speed, decreased electric power consumption, and increased integration densities compared with conventional semiconductor devices. To successfully incorporate spins into existing semiconductor technology, one has to resolve technical issues such as efficient injection, transport, control and manipulation, and detection of spin polarization as well as spin-polarized currents. Recent advances in new materials engineering hold the promise of realizing spintronic devices in the near future.(1-5)

References (abridged):

1. M. Baibich, et al., Phys. Rev. Lett. 61, 2472 (1988)

2. J. Barnas, A. Fuss, R. Camley. P. Grunberg and W. Zinn, Phys. Rev. B 42, 8110 (1990)

3. G. Prinz, Science 282, 1660 (1998)

4. B. Dieny, et al., J. Appl. Phys. 69, 4774 (1991)

5. S. Parkin, D. Mauri, Phys. Rev. B 44 7131 (1991)

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ON SPINTRONICS AND QUANTUM COMPUTING

The following points are made by J.H. Smet et al (Nature 2002 415:281):

1) Discussions of future information-processing technologies often assign a prominent role to the spin degree of freedom in addition to (or instead of) the charge degree of freedom(1), which is exploited in today's mainstream electronics. In the short term, this "spintronics" may deliver products with enhanced functionality or improved performance, such as high-speed, high-density non-volatile random access memories: whereas on a much longer timescale, contributions to the very challenging realm of quantum computation(2,3) have been anticipated.

2) Quantum computation attempts to benefit from correlations and dissipationless transformations of coupled quantum-mechanical systems. The main incentive is a certain degree of parallelism that computational schemes based on such principles bring with them. For example, such schemes offer algorithms for prime factorization(4) and for exhaustive search(5); unlike any apparatus based on classical physics, a quantum computer should be able to solve these problems in polynomial time -- provided that it can be implemented in a real machine, as energy dissipation is a fundamental source of concern.

3) There has been a wide variety of proposals for practical implementations of rudimentary logic gates in which quantum memory registers -- based on any of the abundant two-level systems in physics, like spin-1/2 electrons and nuclei -- can be externally manipulated. These proposals range from trap configurations in atom or ion physics, to techniques of nuclear magnetic resonance spectroscopy as used in organic chemistry, to a very bold all-electronic approach for nuclear spin solid-state devices9 that would marry the merits of electronics fabrication technology with the virtues of quantum computation.

4) Many of the ideas produced by workers in the spintronics and quantum computing communities may be deemed far out of reach. But they have sparked efforts to develop new ways to accomplish the more fundamental task of controlling and measuring the nuclear spin polarization in solid-state devices, in view of the dearth of existing techniques for locally manipulating nuclear spins. Particularly appealing is the use of mobile objects, like conduction electrons in semiconductors, as mediators to both probe and modify nuclear spins. Gating and optical techniques are able to tailor precisely the population and energy distribution of such electrons, especially when they are constrained to move in two dimensions -- as in quantum wells or field-effect transistors -- or even fewer dimensions. The creation of non-equilibrium populations of spin-polarized electrons using coherent polarized light pulses or gating techniques have, for example, already enabled dynamical control of nuclear spins or the electronic generation of net nuclear spin polarization. Progress in this area will rely on experiments specifically geared towards expanding limited knowledge of controlled spin interactions and the microscopic interaction processes that take place between spin systems in such low-dimensional structures.

5) In summary: The authors report procedures that carry out the controlled transfer of spin angular momentum between electrons --confined to two dimensions and subjected to a perpendicular magnetic field -- and the nuclei of the host semiconductor, using gate voltages only. The authors demonstrate that the spin transfer rate can be enhanced near a ferromagnetic ground state of the electron system, and that the induced nuclear spin polarization can be subsequently stored and "read out". These techniques can also be combined into a spectroscopic tool to detect the low-energy collective excitations in the electron system that promote the spin transfer. The existence of such excitations is contingent on appropriate electron–electron correlations, and these can be tuned by changing, for example, the electron density via a gate voltage.

References (abridged):

1. Prinz, G. A. Magnetoelectronics. Science 282, 1660-1663 (1998)

2. Bennett, C. H. & DiVincenzo, D. P. Quantum information and computation. Nature 404, 247-255 (2000)

3. Steane, A. Quantum computing. Rep. Prog. Phys. 61, 117-173 (1998)

4. Ekert, A. & Jozsa, R. Quantum computation and Shor's factoring algorithm. Rev. Mod. Phys. 68, 733-753 (1996)

5. Grover, L. K. Quantum mechanics helps in searching for a needle in a haystack. Phys. Rev. Lett. 79, 325-328 (1997)

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ON THE MEASUREMENT OF ELECTRONIC SPIN

The following points are made by Hari C. Manoharan (Nature 2002 416:24):

1) Magnetic resonance imaging (MRI) has revolutionized clinical medicine. Indeed, a manifold of resonant-imaging techniques now permeate physics, chemistry, biology, medicine, engineering and even computer science. For example, nuclear magnetic resonance, also known as MRI, can provide routine non-invasive imaging down to millimeter scales. In the laboratory, state-of-the-art MRI measurements can push this resolution down to about 10 microns. Meanwhile, the perhaps less familiar technique of electron spin resonance (ESR) has long been exploited to characterize a variety of materials.

2) Durkan and Welland(1) have reported the successful marriage of ESR techniques to a newer technique -- scanning tunneling microscopy -- to detect radio-frequency spin signals from clusters of a few organic molecules. In doing so, they combine the chemical sensitivity of spin resonance techniques with the unrivalled spatial resolution of the scanning tunneling microscope. This result, together with the pioneering efforts of others, opens the door to a new class of studies on, or below, the nanometer scale.

3) At the heart of the experimental tools is the unique interaction between an applied magnetic field and the electrons and nuclei of which all atoms are comprised. Most common high-resolution imaging techniques (for example, electron-beam microscopy) are sensitive to electronic charge, but newer, more advanced techniques rely on sensitivity to another quantum degree of freedom -- spin. Progress here has been driven by both science and technology. Spin leads to magnetism, which underpins the entire magnetic-storage industry; it takes quite sophisticated techniques to image a modern-day computer hard disk in order to map out and refine the stored magnetic patterns. Looking to the future, spin could provide the basis for new computing technologies based on quantum mechanics -- efforts along these lines have spawned the fields of "spintronics" and quantum computation. But currently the most common type of measurements that routinely access spin are spin resonance techniques.(2-5)

References (abridged):

1. Durkan, C. & Welland, M. E. Appl. Phys. Lett. 80, 458-460 (2002)

2. Roukes, M. L. Physics World 14, Feb., 25-31 (2001)

3. Bruland, K. J. et al. Appl. Phys. Lett. 73, 3159-3161 (1998)

4. Stipe, B. C. et al. Phys. Rev. Lett. 87, 277602 (2001)

5. Wulfhekel, W. & Kirschner, J. Appl. Phys. Lett. 75, 1944-1946 (1999)

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BOSE–EINSTEIN CONDENSATION OF THE TRIPLET STATES IN THE MAGNETIC INSULATOR TlCuCl3

The following points are made by C. Rueegg et al (Nature 2003 423:62):

1) One essential parameter for classification of particles is their spin or intrinsic angular momentum. Half-integer-spin fermions are constrained by the Pauli exclusion principle, whereas integer-spin bosons are not. As a result, the spin determines the nature of the energy distribution in a collection of the particles: upon cooling, fermions create a "Fermi sea" whereas bosons create a "condensate". In 1924, Bose and Einstein pointed out that bosons could condense in unlimited numbers into a single ground state, as they are not constrained by the Pauli exclusion principle(4,5). The collection into a single state is therefore called "Bose–Einstein condensation" (BEC).

2) Little notice was taken of this curious possibility until the anomalous behavior of liquid helium was studied carefully. When 4He is cooled to a critical temperature of 2.17 K, a remarkable discontinuity in heat capacity occurs, the liquid density drops, and a fraction of the liquid becomes a zero-viscosity superfluid. Superfluidity arises from the fraction of helium atoms that has condensed to the lowest possible energy. A condensation effect is also credited with producing superconductivity. In the Bardeen–Cooper–Schrieffer theory, pairs of electrons are coupled by lattice interactions: the pairs (called "Cooper pairs") act like bosons, and can condense into a state of zero electrical resistance. Finally, in 1995, alkali atoms were demonstrated to undergo BEC, and thereby to form a novel state of matter. Magnetically trapped, spin-polarized diluted gases of alkali atoms were cooled down to the nanokelvin temperature scale, where a fraction of the individual atoms condense into a "superatom" behaving as a single entity8.

3) Magnetic insulators provide an unrivalled experimental testing ground for the understanding of collective quantum phenomena. This relates both to the description of the magnetic degrees of freedom in terms of a well-defined spin hamiltonian, as well as the applicability of microscopic techniques like nuclear magnetic resonance or inelastic neutron scattering. BEC has so far escaped an experimental investigation within this framework. However, newly discovered materials, based on a crystalline network of dimers, provide high expectations of BEC behavior in these insulators(1).

4) In summary: Bose–Einstein condensation denotes the formation of a collective quantum ground state of identical particles with integer spin or intrinsic angular momentum. In magnetic insulators, the magnetic properties are due to the unpaired shell electrons that have half-integer spin. However, in some such compounds (KCuCl3 and TlCuCl3), two Cu2+ ions are antiferromagnetically coupled(1) to form a dimer in a crystalline network: the dimer ground state is a spin singlet (total spin zero), separated by an energy gap from the excited triplet state (total spin one). In these dimer compounds, Bose–Einstein condensation becomes theoretically possible(2). At a critical external magnetic field, the energy of one of the Zeeman split triplet components (a type of boson) intersects the ground-state singlet, resulting in long-range magnetic order; this transition represents a quantum critical point at which Bose–Einstein condensation occurs. The authors report an experimental investigation of the excitation spectrum in such a field-induced magnetically ordered state, using inelastic neutron scattering measurements of TlCuCl3 single crystals. The authors verify unambiguously the theoretically predicted(3) gapless Goldstone mode characteristic of the Bose–Einstein condensation of the triplet states.

References (abridged):

1. Rice, T. M. To condense or not to condense. Science 298, 760-761 (2002)

2. Nikuni, T., Oshikawa, M., Oosawa, A. & Tanaka, H. Bose-Einstein condensation of diluted magnons in TlCuCl3. Phys. Rev. Lett. 84, 5868-5871 (2000)

3. Matsumoto, M., Normand, B., Rice, T. M. & Sigrist, M. Magnon dispersion in the field-induced magnetically ordered phase of TlCuCl3. Phys. Rev. Lett. 89, 077203 (2002)

4. Bose, S. N. Plancks Gesetz und Lichtquantenhypothese. Z. Phys. 26, 178-181 (1924)

5. Einstein, A. Quantentheorie des einatomigen idealen Gases. Sitzungsber. Kgl. Preuss. Akad. Wiss. 261-267 (1924)

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ELECTRICAL DETECTION OF SPIN PRECESSION IN A METALLIC MESOSCOPIC SPIN VALVE

The following points are made by F.J. Jedema et al (Nature 2002 416:713):

1) To study and control the behavior of the spins of electrons that are moving through a metal or semiconductor is an outstanding challenge in the field of "spintronics", where possibilities for new electronic applications based on the spin degree of freedom are currently being explored(1-5). In semiconductors, electrical control of spin coherence and coherent spin precession during transport has been recently studied by optical techniques.

2) The authors report controlled spin precession of electrically injected and detected electrons in a diffusive metallic conductor, using tunnel barriers in combination with metallic ferromagnetic electrodes as spin injector and detector. The output voltage of the device is sensitive to the spin degree of freedom only, and its sign can be switched from positive to negative, depending on the relative magnetization of the ferromagnetic electrodes. The authors demonstrate that the spin direction can be controlled by inducing a coherent spin precession caused by an applied perpendicular magnetic field. By inducing an average precession angle of 180°, the authors are able to reverse the sign of the output voltage.

References (abridged):

1. Prinz, G. A. Magnetoelectronics. Science 282, 1660-1663 (1998)

2. Wolf, S. A. et al. Spintronics: A spin-based electronics vision for the future. Science 294, 1488-1495 (2001)

3. Johnson, M. & Silsbee, R. H. Interfacial charge-spin coupling: Injection and detection of spin magnetization in metals. Phys. Rev. Lett. 55, 1790-1793 (1985)

4. Jedema, F. J., Filip, A. T. & van Wees, B. J. Electrical spin injection and accumulation at room temperature in an all-metal mesoscopic spin valve. Nature 410, 345-348 (2001)

5. Hernando, D. H., Nazarov, Yu. V., Brataas, V. & Bauer, G. E. W. Conductance modulation by spin precession in noncolinear ferromagnet normal-metal ferromagnet systems. Phys. Rev. B 62, 5700-5712 (2000)

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ON MAGNETIC MICROSCOPY

The following points are made by M.R. Freeman and B.C. Choi (Science 2001 294:1484):

1) Our desire to observe the natural world beyond the limitations of our five senses has driven the development of many new tools. In the case of vision, when objects are too small, distant, or faint, or are moving too quickly or slowly to observe with the unaided eye, we have developed telescopes, microscopes, and cameras to render them visible. The story of imaging of magnetic systems is an interesting case in point, for here we are dealing with a physical phenomenon that is detectable only by our sense of touch with the magnetic force on strongly magnetized objects. The interaction of magnetism with light can be observed by the naked eye only under very special circumstances and only since Faraday's discovery of the magnetic influence on optical polarization. Nonetheless, a very impressive suite of tools has been developed over the intervening 150 years that renders magnetic phenomena and structure as images, thus making them "visible" to the naked eye. Some of these techniques are based on such modern instruments as the scanning tunneling microscope, whereas others harken back to Faraday or even to the direct detection of magnetic forces. Recent advances have given us the ability to directly image magnetic structure on surfaces with atomic resolution and to resolve element-specific contributions to magnetism in complex materials.

2) Magnetism in solids arises on a local scale through quantum mechanical exchange among electrons of neighboring atoms. In ferromagnets, the exchange favors parallel electron spins, and the spatial magnetic structure can range from simple -- a uniformly magnetized sample -- to complex. Except for special sample shapes, uniform magnetization carries a magnetostatic cost in terms of the energy associated with the long-range interaction between dipoles. The energy can be minimized if the dipoles are not all parallel, hence the formation of magnetic domains. Anisotropy effects that favor the orientation of magnetization along certain crystallographic directions further complicate the situation. The essence of this competition is summarized by so-called "exchange lengths", which dictate the minimum scale on which important variations in the direction of magnetization can occur and are often in the nanometer range. In the nonequilibrium regime, the presence of excess energy leads to additional complication including nucleation and growth of domains, propagation of spin-wave excitations on very short wavelengths, and generation of magnetostatic modes akin to the vibrations of a drumhead. The most successful model of this physics is classical (treating small volumes of material as big magnetic moments) and phenomenological: it is hand-built and constructed to follow reasonable guiding principles such as conserving the magnitude of the big moments, allowing only their directions to change. Only now are the tools becoming available to fully test this description against the complex behavior that can occur even in microscopic specimens and point the way toward improvements.(1-5)

References (abridged):

1. G. L. Verschuur, Hidden Attraction: The Mystery and History of Magnetism (Oxford Univ. Press, New York, 1993)

2. Y. Martin and H. K. Wickramasinghe, Appl. Phys. Lett. 50, 1455 (1987)

3. J. J. Sáenz, et al., J. Appl. Phys. 62, 4293 (1987)

4. L. Folks, et al., Appl. Phys. Lett. 76, 909 (2000)

5. H. J. Hug, et al., J. Appl. Phys. 83, 5609 (1998)

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SPIN ICE STATE IN FRUSTRATED MAGNETIC PYROCHLORE MATERIALS

The following points are made by S.T. Bramwell1 and M.J Gingras (Science 2001 294:1495):

1) Competing or frustrated interactions are a common feature of condensed matter systems. Broadly speaking, frustration arises when a system cannot, because of local geometric constraints, minimize all the pairwise interactions simultaneously (1). In some cases, the frustration can be so intense that it induces novel and complex phenomena. Frustration is at the origin of the intricate structure of molecular crystals, various phase transitions in liquid crystals, and the magnetic domain structures in ferromagnetic films. It has also been argued to be involved in the formation of the stripe-like structures observed in cuprate high-temperature superconductors. The concept of frustration is a broad one that extends beyond the field of condensed matter physics. For example, the ability of naturally occurring systems to "resolve" frustrated interactions has been argued to have bearings on life itself, exemplified by the folding of a protein to form a single and well-prescribed structure with biological functionality.

2) Historically, the first frustrated system identified was crystalline ice, which has frozen-in disorder remaining down to extremely low temperature, a property known as residual, or zero-point entropy. In 1933, Giauque and co-workers accurately measured this entropy (2,3), enabling Linus Pauling (1901-1994) to offer his now famous explanation in terms of the mismatch between the crystal symmetry and the local bonding requirements of the water molecule (4). He predicted a special type of proton disorder that obeys the so-called "ice rules". These rules, previously proposed by Bernal and Fowler (5), require that two protons are near to and two are further away from each oxide ion, such that the crystal structure consists of hydrogen-bonded water molecules, H2O. Pauling showed that the ice rules do not lead to order in the proton arrangement but rather the ice ground state is "macroscopically degenerate". That is to say, the number of degenerate, or energetically equivalent proton arrangements diverges exponentially with the size of the sample. Pauling estimated the degeneracy to be ~(3/2)^(N/2), where N is the number of water molecules, typically ~10^(24) in a macroscopic sample. This leads to a disordered ground state with a measurable zero-point entropy S(sub0) related to the degeneracy: S(sub0) ~= (R/2)ln(3/2), where R is the molar gas constant. Pauling's estimate of S(sub0) is very close to the most accurate modern estimate and consistent with experiment (2). The disordered ice-rules proton arrangement in water ice was eventually confirmed by neutron diffraction experiments.

3) In summary: A frustrated system is one whose symmetry precludes the possibility that every pairwise interaction ("bond") in the system can be satisfied at the same time. Such systems are common in all areas of physical and biological science. In the most extreme cases, they can have a disordered ground state with "macroscopic" degeneracy; that is, one that comprises a huge number of equivalent states of the same energy. Pauling's description of the low-temperature proton disorder in water ice was perhaps the first recognition of this phenomenon and remains the paradigm. In recent years, a new class of magnetic substance has been characterized, in which the disorder of the magnetic moments at low temperatures is precisely analogous to the proton disorder in water ice. These substances, known as "spin ice materials", are perhaps the "cleanest" examples of such highly frustrated systems yet discovered. They offer an unparalleled opportunity for the study of frustration in magnetic systems at both an experimental and a theoretical level.

References (abridged):

1. G. Toulouse, Commun. Phys. 2, 115 (1977)

2. W. F. Giauque and M. F. Ashley, Phys. Rev. 43, 81 (1933)

3. W. F. Giauque and J. W. Stout, J. Am. Chem. Soc. 58, 1144 (1936)

4. L. Pauling, J. Am. Chem. Soc. 57, 2680 (1935)

5. D. Bernal and R. H. Fowler, J. Chem. Phys. 1, 515 (1933)

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ON ULTRAFAST GENERATION OF LOCAL MAGNETIC FIELDS

The following points are made by Y. Acremann et al (Nature 2001 414:51):

1) For the development of future magnetic data storage technologies, the ultrafast generation of local magnetic fields is essential. Subnanosecond excitation of the magnetic state has so far been achieved by launching electric current pulses into micro-coils and micro-striplines, and by using high-energy electron beams. Local injection of a spin-polarized current through an all-metal junction has been proposed as an efficient method of switching magnetic elements, and experiments apparently confirm this. Spin injection has also been observed in hybrid ferromagnetic semiconductor structures.

2) The authors report a different scheme for the ultrafast generation of local magnetic fields in such a hybrid structure. The basis of the approach of the authors is to optically pump a Schottky diode with a focused approximately 150 femtosecond laser pulse. The laser pulse generates a current across the semiconductor-metal junction, which in turn gives rise to an in-plane magnetic field. This scheme combines the localization of current injection techniques with the speed of current generation at a Schottky barrier. The authors suggest specific advantages include the ability to rapidly create local fields along any in-plane direction anywhere on the sample, the ability to scan the field over many magnetic elements, and the ability to tune the magnitude of the field with the diode bias voltage.

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