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ScienceWeek
APPLIED PHYSICS: ON MAGNETISM AND SEMICONDUCTORS
The following points are made by Gerrit E. W. Bauer (Science 2004 306:1898):
1) Since the Renaissance, physicians have used ferromagnets as a therapeutic tool, believing in the beneficial effects of their mysterious stray magnetic fields. Franz Anton Mesmer (1734-1815) [1] discovered that he could magnetize ("mesmerize") his patients even without using magnets. Recently, Kato et al[2,3] have performed a similar feat: They have magnetized nonmagnetic semiconductors without ferromagnets. They report[4] that this magnetization can flow. The observations confirm earlier predictions [5]. The observations do not require a completely new type of magnetism, but many details remain mysterious.
2) The electron has a charge and a magnetic moment, the spin. In a few materials (such as iron, cobalt, and nickel), the spins align below a certain temperature to create a macroscopic magnetic moment. Such ferromagnets are used, for example, in information storage devices. The young field of spintronics aims to also use the electron spin in semiconductor-based microelectronic devices. Unfortunately, semiconductors resist direct injection of spins through ferromagnetic contacts.
3) Theoreticians proposed possible alternative routes. It was predicted, for example, that in some nonmagnetic conductors, an electric current would excite a magnetization. The magic ingredient is the relativistic interaction between the spin and the motion of the electron (spin-orbit coupling). Electrons moving in a magnetic field experience an electric field. Similarly, electrons moving in an electric field experience a magnetic field. This internal magnetic field tends to align the spins, but they precess like a top, maintaining a constant projection along the direction of the magnetic field. As electrons scatter at defects and impurities, the magnetic field fluctuates. On average, only those spins accumulate that are aligned normal to the electric current. In piezoelectric materials such as III-V semiconductors, the electric field can be generated by a built-in strain [2-4]. The theory is complicated, but sometimes reduces to the simple picture sketched above.
4) In their recent studies, Kato et al [2,3] realized these theoretical ideas. The authors monitored the spin accumulation with a noninvasive optical spectroscopy technique that can resolve, both spatially and temporally, the spins that are aligned with the probing light beam. They investigated thin GaAs and InGaAs films that are lightly doped to maximize the lifetime of the spins. In [2], the authors confirmed the existence of the internal magnetic field in strained GaAs by observing the precession of an optically injected spin accumulation. In [3], they detected the current-induced spin accumulation in strained InGaAs. The observed magnetization is believed to differ fundamentally from the one that is injected into metals by ferromagnetic contacts. It is, for example, not accompanied by a spin current -- that is, a net flow of spin angular momentum --in the field direction.
References (abridged):
1. V. Buranelli, The Wizard from Vienna (Coward, McCann and Geoghegan, New York, 1975)
2. Y. Kato et al., Nature 427, 50 (2004)
3. Y. K. Kato, R. C. Myers, A. C. Gossard, D. D. Awschalom, Phys. Rev. Lett. 93, 176601 (2004)
4. Y. K. Kato, R. C. Myers, A. C. Gossard, D. D. Awschalom, Science 306, 1910 (2004)
5. M. I. Dyakonov, V. I. Perel, JETP Lett. USSR 13, 467 (1971)
Science http://www.sciencemag.org
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HISTORY OF PHYSICS: ELECTRICITY AND MAGNETISM
The following points are made by Dan Falk (citation below):
1) For two centuries following the publication of Newton's /Principia/, the mechanical world-view held sway. Scientists eagerly applied Newton's laws to problems in astronomy, physics, and engineering with spectacular success. Perhaps the best example is a prediction made by the English astronomer Edmond Halley (1656-1742). Observing a bright comet in 1682, Halley was able to use Newton's laws to work out its orbit, and predicted that it would return 76 years later. And sure enough, the space-faring chunk of rock and ice reappeared in the sky in 1758, right on cue. (Halley died in 1742, but is immortalized in the comet that now bears his name.)
2) Yet the mechanical picture, described by Newton's law of gravity and his laws of motion, did not seem to cover everything. In particular, the nature of electrical and magnetic forces was still a mystery. What little was known, in fact, had been known to the ancient Greeks two millennia earlier. For example, Thales of Miletus (c.625-547 BC) had noted that certain black rocks had the power to attract metals such as iron; the Greeks called these stones magnets, after the region of Magnesia, in Asia Minor, where they were commonly found. They also knew that rubbing certain materials, like amber, caused them to attract lightweight objects like cork, paper, or bits of hay.
3) In the Middle Ages, someone discovered that if a lightweight magnet were suspended so that it was free to rotate, it aligned itself in a north-south direction -- and the mariner's compass was born. (The discovery dates from around the 11th century, and may have originated in China.) And yet, in terms of the underlying theory, progress was painstakingly slow. In 1600, the English scientist William Gilbert (1544-1603), physician to Queen Elizabeth, summed up what was then known about electricity and magnetism in a grand treatise, /de Magnete/. Gilbert claimed --correctly -- that the Earth itself was a natural magnet; he also coined the word electricity from "elektron", the Greek word for amber.
4) However, Gilbert's attempts to link electricity and magnetism to other forces of nature failed. Like Johannes Kepler (1571-1630), Gilbert sought a link between magnetism and the motion of the planets around the Sun. It was, on the surface, plausible enough; he had already shown that the Earth had a magnetic field, and we know today that the Sun and many of the other planets do as well. With the work of Newton, however, it became clear that gravity, not magnetism, governed planetary motion.
5) After Gilbert, another two centuries slipped by with little progress in the realm of electricity and magnetism. The slow pace of discovery is perhaps not surprising. First, in the wake of Newton's great discoveries, both electricity and magnetism were seen as intriguing but not particularly important phenomena; secondly, until the dawn of the 19th century, the tools needed to study them in detail simply did not exist.
6) Some kind of link between electricity and magnetism had long been suspected, but the evidence was mostly anecdotal. In 1731, for example, lightning struck the kitchen of an English tradesman; when the dust had settled, he found that some of the knives and spoons had the power to pick up nails and other small bits of iron: they had become magnetized. In 1752, the American inventor and statesman Benjamin Franklin (1706-1790) flew a kite during a lightning storm -- and demonstrated the link between lightning and electricity.
7) But electricity is a slippery subject: you can't study what you can't store, and electric charges have a way of dissipating before they can be measured and analyzed. For many years, the only way to store electric charge was with a "Leyden jar" -- a sealed glass container lined with metal. The first scientists to examine charges quantitatively began the study of electrostatics -- the forces between stationary charges. They quickly realized there were two kinds of charge; we call them simply "positive" and "negative." Opposite charges were seen to attract one another, while similar charges repelled. Before long, an inverse-square law (analogous to that of gravity) was found to govern the strength of the force between charges. The rule is usually called Coulomb's Law after the French scientist Charles-Augustin de Coulomb (1736-1806); however, Joseph Priestley (1733-1804), a British clergyman and chemist known primarily for his work with gases, independently discovered the inverse-square law at about the same time. Another Englishman, Henry Cavendish (1731-1810), also made important contributions to electrostatics, though he's better known for isolating the element hydrogen and measuring the strength of gravity with great precision.
8) The next breakthrough came, as sometimes happens in science, by sheer accident. In 1786, the Italian physiologist Luigi Galvani (1737-98) touched the leg of a dissected frog with an electrical charge and observed a violent contraction. He thought the effect originated in the animal's organic tissue, but it was actually the salt within the tissue, in concert with Galvani's metal electrodes, that was responsible. His discovery led to the invention of the electrochemical battery. Another Italian, the physicist Alessandro Volta (1745-1827), took the next step. In 1800, he produced the "voltaic pile" -- a stack of alternating layers of silver, zinc, and cardboard which, when placed in an electrical circuit, produced a continuous stream of electricity. The quantitative study of electric current had begun.
9) The Emperor Napoleon, sensing the promise in this string of discoveries, called for more research, suggesting that prizes be awarded for advances in understanding electricity. "Galvanism, in my opinion, will lead to great discoveries," he declared. And indeed it did.
Adapted from: Dan Falk: Universe on a T-Shirt: The Quest for the Theory of Everything. Arcade Publishing 2004, p.72.
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CONTROL OF SEMICONDUCTOR MAGNETISM BY EXTERNAL ELECTRIC FIELDS
Notes by ScienceWeek:
In general, "ferromagnetism" is a property of certain materials subjected to a magnetic field, the magnetic field causing induced magnetism which combines with the applied field to increase the local field. Ferromagnetic materials are strongly attracted to a magnetic pole and have high effective magnetic permeabilities that are greatly dependent on the applied magnetizing field. Iron, cobalt, nickel, and certain alloys are typical examples of ferromagnetic materials.
During the past five decades, several ionically bound compounds have been discovered to be ferromagnetic. Some of these compounds are electrical insulators, but others have the conductivity of semiconductors.
Above its Curie point (Curie temperature), the spontaneous magnetization of a ferromagnetic material vanishes and the material becomes "paramagnetic", i.e., it remains only weakly magnetic. This evidently occurs because the thermal energy becomes sufficient to overcome the internal aligning forces of the material.
The term "spintronics" refers to a relatively new field that aims to combine ferromagnets with semiconductors to develop electronic devices that exploit the quantum mechanical "*spin" of electrons as well as their charge. One aim is to integrate information storage with information processing, but a broader goal is to develop new functionality that does not exist separately in a ferromagnet or in a semiconductor. To this end, investigators are searching for "emergent behavior" in combined ferromagnetic semiconductor structures.
The following points are made by H. Ohno et al (Nature 2000 408:944):
1) The authors point out that it is often assumed that it is not possible to alter the properties of magnetic materials once they have been prepared and put into use. For example, although magnetic materials are used in information technology to store trillions of bits in the form of magnetization directions established by applying external magnetic fields, the properties of the magnetic medium itself remain unchanged on magnetization reversal. The ability to externally control the properties of magnetic materials would be highly desirable from fundamental and technological perspectives, particularly in view of recent developments in *magnetoelectronics and spintronics. In semiconductors, the conductivity can be varied by applying an electric field, but the electrical manipulation of magnetism in such materials has proved elusive.
2) The authors report experiments that demonstrate electric-field control of ferromagnetism in a thin-film semiconduction alloy [(In,Mn)As], using an *insulating-gate field-effect transistor structure. By applying electric fields, the authors were able to vary isothermally and reversibly the transition temperature of *hole-induced ferromagnetism.
Nature http://www.nature.com/nature
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Notes by ScienceWeek:
spin: See related background material below.
magnetoelectronics: See related background material below.
insulating-gate field-effect transistor: The "field effect transistor" (FET) is a transistor consisting essentially of a channel of semiconductor material, the resistance of which can be controlled by the voltage applied to one or more input terminals (gates). It is a 3-terminal device in which current flow through one pair of terminals, the "source" and the "drain", is controlled or modulated by an electric field that penetrates the semiconductor, with this field introduced by the voltage applied at the third terminal, the "gate". The controlling field applied to the gate must be isolated somehow from the current flow in the channel, and there are two general methods of accomplishing this isolation: a) in the "junction field-effect transistor" (JFET), invented by Shockley, the isolation is provided by a special junction barrier across which current flow from gate to channel is very small; in the "insulated gate field-effect transistor" (IGFET), first proposed in the 1930s but not realized until 1960, an insulating layer is placed between the gate electrode and the conducting channel, preventing any current flow between them. The insulated-gate field-effect transistor is sometimes called a "surface field- effect transistor", since the effective conducting channel is the semiconductor surface. (In contrast, the JFET, in which the bulk of the semiconductor is the current carrier, is sometimes called a "bulk field-effect transistor".)
hole-induced ferromagnetism: In this context, a "hole" is an independently translocatable positively charged virtual particle produced by a translocated electron in a crystal semiconductor lattice, and the conductivity of the semiconductor is based on the mobility of both electrons and holes. In the alloy used in the Ohno et al experiments, manganese substitutes for indium at a number of loci in the alloy and simultaneously provides a localized magnetic moment and a hole, owing to its electron-acceptor nature. These holes apparently mediate magnetic interaction, resulting in so-called "hole-induced ferromagnetism".
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