Showing papers on "Magnetic domain published in 1999"

Modern Magnetic Materials: Principles and Applications

Abstract: Introduction and Overview. Magnetostatics. Classical and Quantum Phenomenology of Magnetism. Quantum Mechanics, Magnetism, and Exchange in Atoms and Oxides. Quantum Mechanics, Magnetism, and Bonding in Metals. Magnetic Anisotropy. Magnetoelastic Effects. Magnetic Domain Walls and Domains. Magnetization Process. Soft Magnetic Materials. Amorphous Materials: Magnetism and Disorder. Magnetism in Small Structures: Exchange Coupling and Nanocrystals. Hard Magnetic Materials. Magnetic Annealing and Directional Order. Electronic Transport in Magnetic Materials. Surface and Thin-Film Magnetism. Magnetic Recording. Appendices. Index.

Percolative phase separation underlies colossal magnetoresistance in mixed-valent manganites

Abstract: Colossal magnetoresistance1—an unusually large change of resistivity observed in certain materials following application of magnetic field—has been extensively researched in ferromagnetic perovskite manganites. But it remains unclear why the magnetoresistive response increases dramatically when the Curie temperature (T C) is reduced. In these materials, T C varies sensitively with changing chemical pressure; this can be achieved by introducing trivalent rare-earth ions of differing size into the perovskite structure2,3,4, without affecting the valency of the Mn ions. The chemical pressure modifies local structural parameters such as the Mn–O bond distance and Mn–O–Mn bond angle, which directly influence the case of electron hopping between Mn ions (that is, the electronic bandwidth). But these effects cannot satisfactorily explain the dependence of magnetoresistance on T C. Here we demonstrate, using electron microscopy data, that the prototypical (La,Pr,Ca)MnO3 system is electronically phase-separated into a sub-micrometre-scale mixture of insulating regions (with a particular type of charge-ordering) and metallic, ferromagnetic domains. We find that the colossal magnetoresistive effect in low-T C systems can be explained by percolative transport through the ferromagnetic domains; this depends sensitively on the relative spin orientation of adjacent ferromagnetic domains which can be controlled by applied magnetic fields.

Current-Induced Switching of Domains in Magnetic Multilayer Devices

Abstract: Current-induced switching in the orientation of magnetic moments is observed in cobalt/copper/cobalt sandwich structures, for currents flowing perpendicularly through the layers. Magnetic domains in adjacent cobalt layers can be manipulated controllably between stable parallel and antiparallel configurations by applying current pulses of the appropriate sign. The observations are in accord with predictions that a spin-polarized current exerts a torque at the interface between a magnetic and nonmagnetic metal, due to local exchange interactions between conduction electrons and the magnetic moments.

Spatially Inhomogeneous Metal-Insulator Transition in Doped Manganites

Abstract: Scanning tunneling spectroscopy was used to investigate single crystals and thin films of La1– x Ca x MnO3(with x of about 0.3), which exhibit colossal magnetoresistance. The different spectroscopic signatures of the insulating (paramagnetic) and metallic (ferromagnetic) phases enable their spatial extent to be imaged down to a lateral scale of the order of 10 nanometers. Above the bulk transition temperature T c, the images show mostly insulating behavior. Below T c, a phase separation is observed where inhomogeneous structures of metallic and more insulating areas coexist and are strongly field dependent in their size and structure. Insulating areas are found to persist far below T c. These results suggest that the transition and the associated magnetoresistance behavior should be viewed as a percolation of metallic ferromagnetic domains.

Propagation of a magnetic domain wall in a submicrometer magnetic wire

Abstract: The motion of a magnetic domain wall in a submicrometer magnetic wire was detected by use of the giant magnetoresistance effect. Magnetization reversal in a submicrometer magnetic wire takes place by the propagation of a magnetic domain wall, which can be treated as a “particle.” The propagation velocity of the magnetic domain wall was determined as a function of the applied magnetic field.

Magnetic Domains. The Analysis of Magnetic Microstructures

Abstract: A. Hubert and R. Schafer, Magnetic Domains. The Analysis of Magnetic Microstructures, Springer-Verlag, Berlin, Heidelberg, New York ~1998!, 720 pp., 400 figs., ISBN 3-540-64108-4 An overwhelming majority of magnetic materials used in science and engineering exist in the polydomain state. Hence the physical properties of magnetic domains and their evolution in external magnetic fields determine most of the functional characteristics of devices containing magnetic components. Decades of intensive studies have yielded numerous methods for observing and analyzing magnetic domains and a large body of experimental results, as well as different theoretical methods which were developed for describing the polydomain states. Recent advances in the field of nanotechnology have led to the production of basically new magnetic materials, which has further stimulated fundamental and applied research of magnetic domains. This is mainly due to the fact that at the current level of miniaturization of magnetoelectronic instruments, the behavior of individual domains affects significantly the functioning of such devices. In spite of the large number of monographs and reviews devoted to individual aspects of the physics of magnetic domains, the present book is the first attempt in the world literature to present this comprehensive branch of magnetism in a single volume. The book is authored by Prof. Alex Hubert, a leading authority in the field of magnetic materials science, and his pupil Dr. R. Schafer, a specialist in magneto-optical research. It should be recalled that Hubert’s monograph entitled ‘‘Theorie der Domanenwande in Geordneten Medien’’ ~Theory of Domain Walls in Ordered Media! ~Springer-Verlag, Berlin, 1974!, which was also translated into Russian in 1977 by Mir Publishers, Moscow, continues to be one of the most veritable publications on the basic principles of the physics of domain walls in condensed media. The authors of the project realized it at the expense of enormous efforts and time. The book was written over a very long time ~beginning in 1984!. During this period, the authors gathered, analyzed and systematized a very large number of publications devoted to the experimental, theoretical and applied aspects of the investigations of domain structure. The monograph has an elegant and carefully planned structure. The historical review presented in Chapter 1 is followed by a detailed description of the methods of

Magnetic Force Microscopy

Abstract: ▪ Abstract This review on magnetic force microscopy does not provide an exhaustive overview of the past accomplishments of the method but rather discusses the present state of the art. Magnetic force microscopy is a special mode of noncontact operation of the scanning force microscope. This mode is realized by employing suitable probes and utilizing their specific dynamic properties. The particular material composition of the probes and the dynamic mode of their operation are discussed in detail. The interpretation of images acquired by magnetic force microscopy requires some basic knowledge about the specific near-field magnetostatic interaction between probe and sample. The general magnetostatics as well as convenient simplifications of the general theory, which often can be used in practice, are summarized. Applications of magnetic force microscopy in the magnetic recording industry and in the fundamental research on magnetic materials are discussed in terms of representative examples. An important asp...

Magnetic Dipole Microwave Emission from Dust Grains

Abstract: Thermal fluctuations in the magnetization of interstellar grains will produce magnetic dipole emission at ν100 GHz. We show how to calculate absorption and emission from small particles composed of material with magnetic, as well as dielectric, properties. The Kramers-Kronig relations for a dusty medium are generalized to include the possibility of magnetic grains. The magnetic permeability as a function of frequency is discussed for several candidate grain materials. Iron grains, or grains containing iron inclusions, are likely to have the magnetic analog of a Frohlich resonance in the vicinity of ~50-100 GHz, which results in a large magnetic dipole absorption cross section. We calculate the emission spectra for various interstellar grain candidates. Although "ordinary" paramagnetic grains or even magnetite grains cannot account for the observed "anomalous" emission from dust in the 14-90 GHz range, stronger magnetic dipole emission will result if a fraction of the grain material is ferromagnetic, as could be the case given the high Fe content of interstellar dust. The observed emission from dust near 90 GHz implies that not more than ~5% of interstellar Fe is in the form of metallic iron grains or inclusions (e.g., in "GEMS"). However, we show that if most interstellar Fe is in a moderately ferromagnetic material, with the magnetic properties suitably adjusted, it could contribute a substantial fraction of the observed 14-90 GHz emission, perhaps comparable to the contribution from spinning ultrasmall dust grains. The two emission mechanisms can be distinguished by measuring the emission from dark clouds. If ferromagnetic grains consist of a single magnetic domain and are aligned, the magnetic dipole emission will be linearly polarized, with the polarization depending strongly on frequency.

Magnetic memory array with magnetic tunnel junction memory cells having flux-closed free layers

Abstract: An improved magnetic tunnel junction (MTJ) memory cell for use in a nonvolatile magnetic random access memory (MRAM) array has a free layer formed as two ferromagnetic films that are magnetostatically coupled antiparallel to one another by their respective dipole fields. The magnetostatic or dipolar coupling of the two ferromagnetic films occurs across a nonferromagnetic spacer layer that is selected to prevent exchange coupling between the two ferromagnetic films. The magnetic moments of the two ferromagnetic films are antiparallel to another so that the multilayer free layer structure has a reduced net magnetic moment. In the presence of an applied magnetic field, such as during writing to the cell, the moments of the two ferromagnetic films switch directions substantially simultaneously, so that the net magnetic moment of the multilayer free layer structure can have two possible orientations relative to the orientation of the fixed or pinned layer of the MTJ cell, thus resulting in the two stable magnetic states of the MTJ cell. The reduced net magnetic moment of the multilayer free layer structure reduces the magnetostatic coupling between the multilayer free layer and the pinned ferromagnetic layer in the MTJ cell, as well as the magnetostatic coupling between adjacent MTJ cells in the array. As a result, the cells, and thus the MRAM array, can be made smaller.

Effect of finite magnetic film thickness on Néel coupling in spin valves

Abstract: Spin valves are widely studied due to their application as magnetoresistive material in magnetic recording heads and other magnetic field sensors. An important film property is the interlayer coupling field (called offset field Ho or ferromagnetic coupling field Hf). It has been shown that the Neel model for orange-peel coupling can be applied successfully to describe this interlayer coupling. The waviness associated with the developing granular structure is thereby taken as the relevant waviness. The original Neel model describes the ferromagnetic magnetostatic interaction between two ferromagnetic layers, of infinite thickness, separated by a nonmagnetic spacer with a correlated interface waviness. In this article, this physical picture is refined to account for the effect of the finite thickness of the magnetic films in a spin valve. Magnetic poles created at the outer surfaces of the magnetic layers result in an antiferromagnetic interaction with the poles at the inner surface of the opposite layer. A...

Chiral Magnetic Domain Structures in Ultrathin FePd Films

Abstract: The magnetization profile of magnetically ordered patterns in ultrathin films was determined by circular dichroism in x-ray resonant magnetic scattering (CDXRMS). When this technique was applied to single crystalline iron palladium alloy layers, magnetic flux closure domains were found whose thickness can constitute a large fraction (∼25 percent) of the total film.

Magnetoresistance of a two-dimensional electron gas due to a single magnetic barrier and its use for nanomagnetometry

Abstract: We investigate the longitudinal resistance of a semiconductor near-surface two-dimensional electron gas (2DEG) subjected to a magnetic barrier induced by the stray field from a single sub-micron ferromagnetic line on the surface of the device. The amplitude of the magnetic barrier is controlled by the application of an external magnetic field in the plane of the 2DEG. We show that this type of magnetoresistance can be used to deduce properties of the ferromagnetic line, so that our hybrid ferromagnet-semiconductor structure acts as a nanomagnetometer.

Magnetic Dipole Microwave Emission from Dust Grains

Abstract: Thermal fluctuations in the magnetization of interstellar grains will produce magnetic dipole emission at ν100 GHz. We show how to calculate absorption and emission from small particles composed of material with magnetic, as well as dielectric, properties. The Kramers-Kronig relations for a dusty medium are generalized to include the possibility of magnetic grains. The magnetic permeability as a function of frequency is discussed for several candidate grain materials. Iron grains, or grains containing iron inclusions, are likely to have the magnetic analog of a Frohlich resonance in the vicinity of ~50-100 GHz, which results in a large magnetic dipole absorption cross section. We calculate the emission spectra for various interstellar grain candidates. Although "ordinary" paramagnetic grains or even magnetite grains cannot account for the observed "anomalous" emission from dust in the 14-90 GHz range, stronger magnetic dipole emission will result if a fraction of the grain material is ferromagnetic, as could be the case given the high Fe content of interstellar dust. The observed emission from dust near 90 GHz implies that not more than ~5% of interstellar Fe is in the form of metallic iron grains or inclusions (e.g., in "GEMS"). However, we show that if most interstellar Fe is in a moderately ferromagnetic material, with the magnetic properties suitably adjusted, it could contribute a substantial fraction of the observed 14-90 GHz emission, perhaps comparable to the contribution from spinning ultrasmall dust grains. The two emission mechanisms can be distinguished by measuring the emission from dark clouds. If ferromagnetic grains consist of a single magnetic domain and are aligned, the magnetic dipole emission will be linearly polarized, with the polarization depending strongly on frequency.

Magnetic properties of submicron co islands and their use as artificial pinning centers

Abstract: We report on the magnetic properties of elongated submicron magnetic islands and their influence on a superconducting film. The magnetic properties were studied by magnetization hysteresis loop measurements and scanning-force microscopy. In the as-grown state, the islands have a magnetic structure consisting of two antiparallel domains. This stable domain configuration has been directly visualized as a $2\ifmmode\times\else\texttimes\fi{}2$-checkerboard pattern by magnetic-force microscopy. In the remanent state, after magnetic saturation along the easy axis, all islands have a single-domain structure with the magnetic moment oriented along the magnetizing field direction. Periodic lattices of these Co islands act as efficient artificial pinning arrays for the flux lines in a superconducting Pb film deposited on top of the Co islands. The influence of the magnetic state of the dots on their pinning efficiency is investigated in these films, before and after the Co dots are magnetized.

Microscopic tips having stable magnetic moments and disposed on cantilevers for sensing magnetic characteristics of adjacent structures

Abstract: A magnetic force microscope (MFM) needle has a magnetic material with a magnetic moment that is pinned in a preferred direction. The magnetic moment can be of lower than conventional magnitude without risking an undesirable change in the direction of magnetization. The magnetic needle can have a ferromagnetic layer (or layers) that is stabilized by an antiferromagnetic layer (or layers). The needle can be employed as a magnetoresistance sensitivity microscope (MSM) to map the sensitivity of a magnetic sensor, such as a magnetoresistive (MR) or giant magnetoresistive (GMR) sensor. Alternatively, the needle can be employed in measuring magnetic fields, such as with a high frequency magnetic force microscope (HFMFM).

Spin-valve magnetic transducing element and magnetic head having free layer with negative magnetostriction

Abstract: A spin-valve magnetic transducing element. In one embodiment, a spin-valve magnetic transducing element is disclosed in which a ferromagnetic tunneling junction film, including first and second ferromagnetic layers and an insulating layer is enclosed between the ferromagnetic layers. A nonmagnetic metal thin film is inserted between the second ferromagnetic layer and the insulating layer, all of which are formed on a substrate.

Domain configurations of nanostructured Permalloy elements

Abstract: The magnetization distributions of an array of small NiFe elements were studied using Lorentz transmission electron microscopy (LTEM) and magnetic force microscopy (MFM). The dependence of the domain configurations at zero field as a function of the aspect ratio was observed using MFM, and confirms the earlier observations using LTEM. Comparison of the images of similar islands using both techniques elucidate the complementarity between the LTEM and MFM measurements which individually show different facets of the magnetization distributions on soft magnetic thin films.

Large field-induced strains in ferromagnetic shape memory materials

Abstract: We study magnetostriction in ferromagnetic shape memory materials caused by a redistribution of twin bands in response to an applied magnetic field. Recent measurements of strain versus magnetic field in Ni-24 at% Mn-24.7 at% Ga are presented. We report strains of about 0.5% under cyclic application of a field, and field-induced strains of about 4% on specimens that have been previously cooled through the martensitic transformation under stress. A comparison between these measurements and theoretical calculations reveals a discrepancy. We hypothesize that this is due to magnetization rotation, accompanied by the formation of fine magnetic domains within each band of martensite.

Writing and reading of single magnetic domain per bit perpendicular patterned media

Abstract: By fabricating patterned media with a large number of nanoscale single domain magnetic particles embedded in a nonmagnetic substrate, and by writing the magnetization for each of these particles in a desired direction, nonvolatile magnetic storage of information could reach densities much higher than what is currently thought possible for longitudinal continuous media. We have fabricated high aspect ratio perpendicular nickel columnar nanoparticles embedded in a hard Al2O3/GaAs substrate. We show that the magnetization states of the individual magnets can be controlled by demonstrating that prototype patterned "single magnetic domain per bit" data tracks can be written and read back using current magnetic information storage technology.

Domain Branching in Uniaxial Ferromagnets: A Scaling Law for the Minimum Energy

Abstract: We address the branching of magnetic domains in a uniaxial ferromagnet. Our thesis is that branching is required by energy minimization. To show this, we consider the nonlocal, nonconvex variational problem of micromagnetics. We identify the scaling law of the minimum energy by proving a rigorous lower bound which matches the already-known upper bound. We further show that any domain pattern achieving this scaling law must have average width of order L 2/3, where L is the length of the magnet in the easy direction. Finally we argue that branching is required, by considering the constrained variational problem in which branching is prohibited and the domain structure is invariant in the easy direction. Its scaling law is different.

Tuning of colossal magnetoresistance via grain size change in La0.67Ca0.33MnO3

Abstract: In this article, we show how colossal magnetoresistance effect (CMR) can be tuned in polycrystalline mixed valence manganite La0.67Ca0.33MnO3 via changing grain size by means of a sol-gel method. Below a critical diameter (150 nm), CMR disappears, but large intergrain MR remains even well above Tc (1.2Tc for ≈95 nm particles). Possible explanation for this effect involves single magnetic domain behavior in samples annealed at low temperature.

Powder magnetoresistance (invited)

Abstract: Magnetoresistance observed in pressed powder compacts of half-metallic ferromagnetic oxides is reviewed. The main, isotropic negative magnetoresistance, which exceeds 50% in CrO2 at low temperature, is due to alignment of the the ferromagnetic moments of contiguous ferromagnetic grains. The effect is related to interparticle tunelling and shows hysteresis similar to that of the bulk magnetization. Spin-dependent Coulomb blocade observed in the smallest particles. There is also, an anisotropic magnetoresistance of up to 1% and a high-field increase in conductivity of order 1%/T in the powder compacts.

Magnetotransport and magnetic domain structure in compressively strained colossal magnetoresistance films

Abstract: We have studied the magnetoresistance (MR) of compressively strained La0.7Sr0.3MnO3 (LSMO) films in various magnetic states in order to understand the role of magnetic domain structure on magnetotransport. In thin films of LSMO on (100) LaAlO3, the perpendicular magnetic anisotropy results in perpendicularly magnetized domains with fine scale ∼200 nm domain subdivision, which we image directly at room temperature using magnetic force microscopy. The main MR effects can be understood in terms of bulk colossal MR and anisotropic MR. We also find evidence for a small domain wall contribution to the MR, which is an order of magnitude larger than expected from a double exchange model.

A ferromagnet having no net magnetic moment

Abstract: Coupling between the spins (magnetic moments) of assemblies of ions can lead to ordered magnetic systems—classified as ferromagnetic, antiferromagnetic, ferrimagnetic and so forth, depending on the nature of the ordering1. A simple picture of the coupling and ordering is usually adequate for describing properties of these systems, such as the magnitude and thermal variation of the magnetization. Here we describe a system whose magnetic behaviour appears counterintuitive on the basis of this picture. The materials in question are based on the ferromagnetic compound, SmAl2, in which some of the magnetic samarium ions, Sm3+, have been substituted with other rare-earth elements. The resulting system exhibits the seemingly incompatible properties of large spin polarization but no bulk magnetization: this state occurs at a specific temperature, when the two components of the magnetization (the electron's spin and its orbital motion) exactly compensate for one another. This property should be generic to ferromagnets containing trivalent samarium ions, and may find potential application in, for example, spin-resolving devices for charged particles.

Injection of a magnetic domain wall into a submicron magnetic wire

Abstract: Two types of magnetic wires (150 nm width) with trilayer structure consisting of NiFe (20 nm)/Cu (20 nm)/Co (20 nm) were prepared. One was connected to a square pad (0.5×0.5 μm2) at one end, while the other has a symmetrical shape with two flat ends. Magnetization reversal was detected sensitively by magnetoresistance measurement. Switching field of the Co layer for the wire with a pad was much smaller than that for the wire without a pad. This indicates that a domain wall nucleates initially in the pad and is injected into the wire at the switching field. This model for the magnetization reversal process is supported by the angular dependence of the switching field.

2e2/h to e2/h switching of quantum conductance associated with a change in nanoscale ferromagnetic domain structure

Abstract: We demonstrate the electrical conductance quantization in a Ni nanowire formed in a break junction between a ferromagnetic Ni wire and a Ni plate in applied magnetic fields. The conductance of the nanowire is clearly quantized in units of 2e2/h in a zero magnetic field, but it is switched to e2/h by applying magnetic fields above 60 Oe. This switching behavior seems closely related to a ferromagnetic domain formation in the vicinity of a nanowire, suggesting that nanoscale magnetic domain walls play an important role in determining nanoscale spin-dependent transport. The effect offers the possibility of a new device, a nanoscale colossal magnetoresistive sensor.

Spin-polarized scanning tunneling microscopy on ferromagnets

Abstract: A straightforward approach to spin-polarized scanning tunneling microscopy based on the magnetotunneleffect between a ferromagnetic tip and a ferromagnetic sample is demonstrated. By periodically changing the magnetization of the tip in combination with a lock-in technique, topographic and spin-dependent parts of the tunnel current are separated and the topography and the magnetic structure of the sample are recorded simultaneously. Results are given for polycrystallineNi and single crystalline Co(0001) surfaces, revealing a high spin contrast, low data acquisition times, and a resolution down to 10 nm. Potentials and limitations of this technique are discussed.

Tunneling via individual electronic states in ferromagnetic nanoparticles

Abstract: We measure electron tunneling via discrete energy levels in ferromagnetic cobalt particles that are less than 4 nm in diameter, using nonmagnetic electrodes. Because of magnetic anisotropy, the energy of each tunneling resonance shifts as an applied magnetic field rotates the particle's magnetic moment. We see both spin-increasing and decreasing tunneling transitions, but do not observe the spin degeneracy at small magnetic fields seen previously in nonmagnetic materials. The tunneling spectrum is denser than predicted for independent electrons, possibly due to spin-wave excitations.