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ScienceWeek
FERROMAGNETISM
2. FERROMAGNETISM: ON VORTEX CORES AND BLOCH POINTS
The following points are made by J. Miltat and A. Thiaville (Science 2002 298:555):
1) As a result of dipole-dipole interactions, the magnetization becomes circular in the vicinity of the center of an isolated Fe, Co, or Ni platelet. It cannot, however, remain circular down to the platelet center because of exchange interactions, which become dominant at short distances. The magnetization has to leave the plane (1-3), defining what is termed a "magnetic vortex".
2) Some 20 years ago, it was inferred from low-angle electron diffraction that the region with out-of-plane magnetization should not exceed ~15 nm (4). Wachowiak et al (5) have used sophisticated spin-polarized scanning tunneling microscopy (SP-STM) to show that the radius of a vortex within an Fe island (a supported nanoplatelet) amounts to only 4 to 5 nm. This is a striking result in at least three respects. First, the measured vortex width is comparable to the smallest flux line core sizes in high-temperature superconductors. Second, it is compatible with an almost analytical model of classical vortex lines in ferromagnets (1), as confirmed by numerical simulations (5). The pertinence of micromagnetics down to the nanometer scale is thus reinforced. Third, it confirms SP-STM as a low-noise spectroscopic imaging technique with unprecedented spatial resolution.
3) Consider now a vortex defined by its circulation and core magnetization. Construct a second vortex structure by reversing the sole core magnetization and combine the two structures such that their cores form a single straight line. The magnetization is everywhere continuous except in one point, called a "Bloch point". A Bloch point is a singularity of a three-dimensional magnetization vector field. Topology dictates that the reversal of the core magnetization of a vortex may occur either by sweeping the vortex out of the dot, followed by the nucleation of a fresh vortex configuration (3), or via Bloch point injection and punch-through. Indirect evidence for Bloch point injection exists. Real-time observations are, however, still lacking. A Bloch point naturally links classical and quantum magnetism.
References (abridged):
1. E. Feldtkeller, H. Thomas, Phys. Kondens. Materie 4, 8 (1965)
2. T. Shinjo et al., Science 289, 930 (2000).
3. M. Schneider et al., Appl. Phys. Lett. 79, 3113 (2001)
4. A. Tonomura et al., Phys. Rev. Lett. 44, 1430 (1980)
5. A. Wachowiak et al., Science 298, 577 (2002)
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ON FERROMAGNETIC SEMICONDUCTORS
The following points are made by Y.D. Park et al (Science 2002 295:651):
1) Ferromagnetic (FM) semiconductors are materials that simultaneously exhibit semiconducting properties and spontaneous long-range FM order. Classic examples include the europium chalcogenides and the chalcogenide spinels, both extensively studied several decades ago (1). The coexistence of these properties in a single material provides fertile ground for fundamental studies (2). However, device applications have languished because of low magnetic ordering (Curie) temperatures and the inability to incorporate these materials in thin film form with mainstream semiconductor device materials.
2) Interest in ferromagnetic semiconductors (FMSs) was rekindled with the discovery of spontaneous FM order in In1xMnxAs in 1989 (3) and in Ga1xMnxAs in 1996 (4-5), when FM properties were realized in semiconductor hosts already widely recognized for semiconductor device applications. These new FMS materials exhibit Curie temperatures up to 35 K and 110 K, respectively, for Mn concentrations of order 5% and sufficiently high hole densities and have been closely studied for their potential in future spin-dependent semiconductor device technologies. For example, Ga1xMnxAs has been used as a source of spin-polarized carriers in both light-emitting diodes and resonant tunneling diode heterostructures. Electric field control of FM order has recently been reported in In1xMnxAs heterostructures, demonstrating one of the unique properties of these materials and portending a host of new applications. Experimental evidence for Curie temperatures above 300 K has been reported in other materials, such as CdMnGeP2.
3) In summary: The authors report on the epitaxial growth of a group-IV ferromagnetic semiconductor, MnxGe1x, in which the Curie temperature is found to increase linearly with manganese (Mn) concentration from 25 to 116 kelvin. The p-type semiconducting character and hole-mediated exchange permit control of ferromagnetic order through application of a +-0.5-volt gate voltage, a value compatible with present microelectronic technology. Total-energy calculations within density-functional theory show that the magnetically ordered phase arises from a long-range ferromagnetic interaction that dominates a short-range antiferromagnetic interaction. Calculated spin interactions and percolation theory predict transition temperatures larger than measured, consistent with the observed suppression of magnetically active Mn atoms and hole concentration.
References (abridged):
1. C. Haas, CRC Crit. Rev. Solid State Sci. 1, 47 (1970)
2. E. L. Nagaev, Phys. Stat. Solidi B 145, 11 (1988)
3. H. Munekata, et al., Phys. Rev. Lett. 63, 1849 (1989)
4. J. De Boeck, et al., Appl. Phys. Lett. 68, 2744 (1996)
5. H. Ohno, et al., Appl. Phys. Lett. 69, 363 (1996)
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OBSERVATION OF COUPLED MAGNETIC AND ELECTRIC DOMAINS
The following points are made by M. Fiebig et al (Nature 2002 419:818):
1) Ferroelectromagnets are an interesting group of compounds that complement purely (anti-)ferroelectric or (anti-)ferromagnetic materials -- they display simultaneous electric and magnetic order(1-3). With this coexistence they supplement materials in which magnetization can be induced by an electric field and electrical polarization by a magnetic field, a property which is termed the "magnetoelectric effect"(4). Aside from its fundamental importance, the mutual control of electric and magnetic properties is of significant interest for applications in magnetic storage media and "spintronics"(2,3). The coupled electric and magnetic ordering in ferroelectromagnets is accompanied by the formation of domains and domain walls. However, such a cross-correlation between magnetic and electric domains has so far not been observed.
2) The authors report spatial maps of coupled antiferromagnetic and ferroelectric domains in YMnO3, obtained by imaging with optical second harmonic generation. The coupling originates from an interaction between magnetic and electric domain walls, which leads to a configuration that is dominated by the ferroelectromagnetic product of the order parameters.
3) The authors suggest that the ability to couple to the electric and/or the magnetic long-range order opens up an additional degree of freedom in device construction. This may for instance be used in the design of new data-storage media as an alternative to magneto-optical disks by replacing the slow magnetic writing process by a fast magnetization reversal through electric poling. Furthermore, in spin valves composed from one ferromagnetic and one FEM layer, one of the layers would no longer need to be pinned if electric writing and magnetic reading were used. FEM compounds with AFM spin ordering may permit us to control the degree of exchange-biasing by controlling the topology of the AFM order parameter with an electric field. These developments will be supported by the invention of new FEM compounds with high transition temperatures(1,5).
References (abridged):
1. Smolenskii, G. A. & Chupis, I. E. Ferroelectromagnets. Sov. Phys. Usp. 25, 475-493 (1982)
2. Schmid, H. Ferroelectrics 162, 317-338 (1994)
3. Hill, N. A. Why are there so few magnetic ferroelectrics? J. Phys. Chem. B 104, 6694-6709 (2000)
4. O'Dell, T. H. The Electrodynamics of Magneto-Electric Media (North-Holland, Amsterdam, 1970)
5. Aizu, K. Possible species of ferromagnetic, ferroelectric, and ferroelastic crystals. Phys. Rev. B 2, 754-772 (1970)
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ON THE SUPERPARAMAGNETIC LIMIT AND EXCHANGE BIAS
The following points are made by V. Skumryev et al (Nature 2003 423:850):
1) Interest in magnetic nanoparticles has increased in the past few years by virtue of their potential for applications in fields such as ultrahigh-density recording and medicine(1-4). Most applications rely on the magnetic order of the nanoparticles being stable with time. However, with decreasing particle size the magnetic anisotropy energy per particle responsible for holding the magnetic moment along certain directions becomes comparable to the thermal energy. When this happens, the thermal fluctuations induce random flipping of the magnetic moment with time, and the nanoparticles lose their stable magnetic order and become superparamagnetic5. Thus, the demand for further miniaturization comes into conflict with the superparamagnetism caused by the reduction of the anisotropy energy per particle: this constitutes the so-called "superparamagnetic limit" in recording media.
2) The authors demonstrate that magnetic exchange coupling induced at the interface between ferromagnetic and antiferromagnetic systems can provide an extra source of anisotropy, leading to magnetization stability. The authors demonstrate this principle for ferromagnetic cobalt nanoparticles of about 4 nm in diameter that are embedded in either a paramagnetic or an antiferromagnetic matrix. Whereas the cobalt cores lose their magnetic moment at 10 K in the first system, they remain ferromagnetic up to about 290 K in the second. This behaviour is ascribed to the specific way ferromagnetic nanoparticles couple to an antiferromagnetic matrix.
3) In summary: The authors suggest these results demonstrate that the magnetic coupling of FM nanoparticles with an AFM matrix is a source of a large effective additional anisotropy. This leads to a marked improvement in the thermal stability of the moments of the FM nanoparticles -- the authors observed an increase in the blocking temperature of almost two orders of magnitude. This mechanism provides a way to "beat" the "superparamagnetic limit" in isolated particles. Although it is clear that the system examined here is not suitable in itself for application, the approach developed should in principle apply to nanoparticles deposited on a single AFM layer, a structure suitable for use as a recording medium. With the right choice of FM and AFM components, exchange anisotropy coupling could ultimately allow magnetically stable dots only a few nanometres in size.(5)
References (abridged):
1. Kodama, R. H. Magnetic nanoparticles. J. Magn. Magn. Mater. 200, 359-372 (1999)
2. Martín, J. I. et al. Ordered magnetic nanostructures: Fabrication and properties. J. Magn. Magn. Mater. 256, 449-501 (2003)
3. Sun, S. H. et al. Monodisperse FePt nanoparticles and ferromagnetic FePt nanocrystal superlattices. Science 287, 1989-1992 (2000)
4. Häfeli, U., Schütt, W., Teller, J. & Zborowski, M. (eds) Scientific and Clinical Applications of Magnetic Materials (Plenum, New York, 1997)
5. Chikazumi, S. Physics of Ferromagnetism (Oxford Univ. Press, New York, 1997)
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ON MAGNETIC CARBON
The following points are made by T.L. Makarova et al (Nature 2001 413:716):
1) The discovery of nanostructured forms of molecular carbon has led to a renewed interest in the varied properties of this element. Pristine C(sub60) fullerene is a van der Waals crystal that can be converted to covalently bonded crystalline phases by compression. Depending on the treatment, the molecules interconnect to form 1-, 2-, or 3-dimensional polymers. Ferromagnetism has previously been observed in two C(sub60) compounds, in one case below 17 kelvins and in the other case below 19 kelvins.
2) Both graphite and C(sub60) can be electron-doped by alkali metals to become superconducting, and transition temperatures of up to 52 kelvins have been attained by field-induced hole-doping. Recent experiments and theoretical studies have suggested that electronic instabilities in pure graphite may give rise to superconducting and ferromagnetic properties even at room temperature.
3) The authors report the accidental discovery of strong magnetic signals in rhombohedral C(sub60). The intention of the authors was to search for superconductivity in polymerized C(sub60); however, it appears that the high-pressure, high-temperature polymerization process used by the authors results in a magnetically ordered state. The material exhibits features typical of ferromagnets: saturation magnetization, large hysteresis, and attachment to a magnet at room temperature. The authors suggest the temperature dependence of the saturation and remanent magnetization indicate a Curie temperature near 500 kelvins.
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FERROMAGNETISM IN 1-DIMENSIONAL MONATOMIC METAL CHAINS
In general, an "epitaxial film" is a crystalline film whose lattice structure is identical to the lattice structure of an underlying crystalline substrate.
The following points are made by P. Gambardella et al (Nature 2002 416:301):
1) Two-dimensional systems, such as ultrathin epitaxial films and superlattices, display magnetic properties distinct from bulk materials(1). A challenging aim of current research in magnetism is to explore structures of still lower dimensionality(2-5). As the dimensionality of a physical system is reduced, magnetic ordering tends to decrease as fluctuations become relatively more important. Spin lattice models predict that an infinite one-dimensional linear chain with short-range magnetic interactions spontaneously breaks up into segments with different orientation of the magnetization, thereby prohibiting long-range ferromagnetic order at a finite temperature. These models, however, do not take into account kinetic barriers to reaching equilibrium or interactions with the substrates that support the one-dimensional nanostructures.
2) Since the work of Ising (1925), magnetism in 1-dimensional systems has been the subject of continuous theoretical(4, 5) and experimental(2) research. Progress in atomic engineering makes it possible today to build 1-dimensional arrays of transition-metal chains by self-assembly epitaxial techniques on suitable substrates. Pioneering experiments in this direction investigated the magnetic behavior of Fe stripes 1 to 10 nanometers wide obtained by step-flow growth on W(110) and Cu(111) surfaces. Ideally, one would like to construct monatomic chains in very large numbers while maintaining a fine control of the dimensions, uniformity and spatial distribution of the individual chains.
3) The authors report a demonstration that the existence of both short- and long-range ferromagnetic order for 1-dimensional monatomic chains of Co constructed on a Pt substrate. The authors report evidence that the monatomic chains consist of thermally fluctuating segments of ferromagnetically coupled atoms which, below a threshold temperature, evolve into a ferromagnetic long-range-ordered state owing to the presence of anisotropy barriers. The Co chains are characterized by large localized orbital moments and correspondingly large magnetic anisotropy energies compared to 2-dimensional films and bulk Co.
References (abridged):
1. Schneider, C. M. & Kirschner, J. in Handbook of Surface Science (eds Horn, K. & Scheffler, M.) 511-668 (Elsevier, Amsterdam, 2000).
2. Himpsel, F. J., Ortega, J. E., Mankey, G. J. & Willis, R. F. Magnetic nanostructures. Adv. Phys. 47, 511-597 (1998).
3. Stamm, C. et al. Two-dimensional magnetic particles. Science 282, 449-451 (1998).
4. Weinert, M. & Freeman, A. J. Magnetism of linear chains. J. Mag. Magn. Mater. 38, 23-33 (1983).
5. Dorantes-Dávila, J. & Pastor, G. M. Magnetic anisotropy of one-dimensional nanostructures of transition metals. Phys. Rev. Lett. 81, 208-211 (1998).
CONTROL OF SEMICONDUCTOR MAGNETISM BY EXTERNAL ELECTRIC FIELDS
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.
In a commentary on this work (Nature 2000 408:923) D.D. Awschalom and R.K Kawakami state: "This experiment is a 'proof of concept' for the idea that the magnetic properties of ferromagnetic semiconductors can be controlled using standard electronic techniques. This finding, along with the discovery of new ways to control electronic spin... paves the way for practical spintronics."
Notes:
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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