Showing papers on "Magnetic domain published in 1994"

Spin-Glass Behavior and Magnetic Phase Diagram of La1-xSrxCoO3 ( 0 ≤x ≤0.5) Studied by Magnetization Measurements

Abstract: Magnetic properties of La 1- x Sr x CoO 3 (0 ≤ x ≤0.5) have been studied by magnetization measurements in low magnetic fields. It has been found that there exist spin-glass (0< x ≤0.18) and cluster-glass (0.18 ≤ x ≤0.5) regions. Ferromagnetic long-range order previously reported does not take place. Spin-glass behavior is ascribed to the frustration of random competing exchange interactions, namely the ferromagnetic double-exchange interaction between Co 3+ and Co 4+ and the antiferromagnetic one between like spins.

Single‐domain magnetic pillar array of 35 nm diameter and 65 Gbits/in.2 density for ultrahigh density quantum magnetic storage

Abstract: Using electron beam nanolithography and electroplating, arrays of Ni pillars on silicon that have a uniform diameter of 35 nm, a height of 120 nm, and a period of 100 nm were fabricated. The density of the pillar arrays is 65 Gbits/in.2—over two orders of magnitude greater than the state‐of‐the‐art magnetic storage density. Because of their nanoscale size, shape anisotropy, and separation from each other, each Ni pillar is single domain with only two quantized perpendicular magnetization states: up and down. Each pillar can be used to store one bit of information, therefore such nanomagnetic pillar array storage offers a rather different paradigm than the conventional storage method. A quantum magnetic disk scheme that is based on uniformly embedding single‐domain magnetic structures in a nonmagnetic disk is proposed.

Ferromagnetic materials : structure and properties

Abstract: Soft magnetic alloys amorphous ferromagnetic alloys and ferrofluids ferrimagnetic materials permanent magnetic materials.

Two- and three-dimensional calculation of remanence enhancement of rare-earth based composite magnets (invited)

Abstract: Micromagnetic calculations using a finite element technique rigorously describe the magnetic properties of novel, isotropic rare‐earth‐based composite magnets. Numerical results obtained for a composite material of Nd2Fe14B, SmCo5 or Sm2(Fe0.8Co0.2)17N2.8 and α‐Fe particles show that remanence, coercivity, and coercive squareness sensitively depend on microstructural features. Interparticle exchange interactions enhance the remanence by about 60% with respect to noninteracting particles for a mean‐grain size approaching the exchange length of the soft magnetic phase and a significant percentage of α‐Fe. On the other hand, exchange interactions between the phases suppress the nucleation of reversed domains and thus preserve a high coercive field. Therefore, optimally structured, isotropic composite magnets show remarkably high energy products exceeding 400 kJ/m3.

Quenching of magnetic moments by ligand-metal interactions in nanosized magnetic metal clusters.

Abstract: Local density functional calculations and experimental magnetization studies on giant nickel carbonyl clusters are presented. The results show convincingly that the effect of the carbonyl ligation is to quench completely the magnetic moments of the nickel atoms at the surface of the clusters, leaving the inner-core metal atoms relatively unaffected. The relation with surface coating of magnetic particles for recording media is pointed out.

Superparamagnetism and Spin Glass Ordering in Magnetic Nanocomposites

Abstract: Mossbauer spectra of samples containing nano-sized magnetic particles exhibit a transition from a magnetically split spectrum to a doublet or singlet when the temperature is varied, and zero-field-cooled magnetization measurements show a maximum at a certain temperature. When the interactions are weak these features can be explained by superparamagnetic blocking, but in the case of strong interactions they can be explained by ordering of the magnetic moments. A phase diagram, which elucidates these possibilities, is presented.

Ultra high density, non-volatile ferromagnetic random access memory

Abstract: A random access memory element utilizes giant magnetoresistance. The element includes at least one pair of ferromagnetic layers sandwiching a nonmagnetic conductive layer. At least one of the two ferromagnetic layers has a magnetic moment oriented within its own plane. The magnetic moment of at least the first ferromagnetic layer of the pair has its magnetic moment oriented within its own plane and is typically fixed in direction during use. The second ferromagnetic layer of the pair has a magnetic moment which has at least two preferred directions of orientation. These preferred directions of orientation may or may not reside within the plane of the second ferromagnetic layer. The bit of the memory element may be set by applying to the element a magnetic field which orients the magnetic moment of the second ferromagnetic layer in one or the other of these preferred orientations. Once the bit is set, the value of the determined by the relative alignment of the magnetic moments of the first and second ferromagnetic layers. This value may be read by applying an interrogating current across the memory element, perpendicular to the plane within which the magnetic moment of the first ferromagnetic layer is oriented, and observing the variation in resistance. These ferromagnetic elements may be fabricated using conventional photolithography. Groups of these ferromagnetic element may be organized into word trees and other arrays.

Scanning near‐field optical microscope for the imaging of magnetic domains in optically opaque materials

Abstract: We describe a scanning near‐field optical microscope capable of magnetic imaging in optically opaque materials The near‐field probe is a ∼30 nm Ag particle, optically excited near the surface plasmon resonance frequency The separation of the probe and sample is regulated by a Newton ring interferometer The polarization rotation of the scattered light due to near‐field magneto‐optic interactions is measured A two‐dimensional image is presented of thermomagnetically recorded magnetic domains in a perpendicularly magnetized Co/Pt multilayer material with 05‐μm‐diam domains at one micrometer spacing Images suggest a resolution of <100 nm

Coherent magnetic imaging by TEM

Abstract: A novel transmission electron microscope optimised for the study of magnetic material is described. Using this instrument, a new method for revealing magnetic structures, coherent Foucault imaging, can be realised. Images appear directly in the form of magnetic interferograms and provide immediate access to a quantitative description of the induction distribution across the specimen. A simple analytical approach is given to the underlying theory and the main features are confirmed by computer modelling. Experimental images of small regular permalloy elements illustrate the power of the technique for studying domain structures. >

Study of nanoscale magnetic structures fabricated using electron‐beam lithography and quantum magnetic disk

Abstract: Two types of nanoscale single‐domain magnetic structures were fabricated using e‐beam nanolithography and were studied using magnetic force microscopy. The first structure is the isolated and interactive arrays of Ni bars on silicon that are 35 nm thick, 1 μm long, and have widths ranging from 15 to 200 nm and spacings ranging from 200 to 600 nm. The second structure is an array of Ni pillars on silicon that have a uniform diameter of 35 nm, a height of 120 nm, and a density of 65 Gbits/in2—over two orders of magnitude greater than the state‐of‐the‐art magnetic storage density. It was found that the magnetic properties of these structures can be controlled by engineering their size and spacing. When the bar width is smaller than 150 nm, the bars become single magnetic domain. As the width of the isolated bars decreased from 200 to 55 nm, the magnetic field needed to switch the magnetization of these bars increased monotonically from 100 to 740 Oe which is the highest field reported for Ni. However, further reduction of bar width led the switching field to decrease due to thermal effect. Furthermore, it was found that as the bar spacings become smaller, the interaction between the bars will reduce the switching field. Finally, based on the artificially patterned single‐domain magnetic structures, we propose a new paradigm for ultra‐high‐density magnetic recording media: quantum magnetic disk.

Fluctuations and instabilities of ferromagnetic domain-wall pairs in an external magnetic field.

Abstract: A classical continuum model of an effectively one-dimensional ferromagnet will exchange and anisotropies of hard and easy-axis type is considered. In the presence of an external magnetic field along the easy axis, the lowest-lying topological excitations are shown to be untwisted or twisted pairs of \ensuremath{\pi}-domain walls. The fluctuations around these structures are investigated. It is shown that the fluctuations around the twisted and untwisted domain-wall pair are governed by the same set of operators. The untwisted domain-wall pair has exactly one unstable mode and thus represents a critical nucleus for magnetization reversal in effectively one-dimensional systems. The twisted domain-wall pair is stable for small external fields but becomes unstable for large magnetic fields. The former effect is related to thermally induced coercivity reduction in elongated particles while the latter effect is related to ``chopping'' of twisted Bloch wall pairs in thin films. In view of a statistical mechanical theory of magnetization reversal which will be presented in a separate article, the scattering phase shifts of spin waves around these structures are calculated. The applicability of the present theory to magnetic thin films is discussed. Finally, it is noted that the static properties of the present model are equivalent to those of a nonlinear \ensuremath{\sigma} model with anisotropies and an external field.

Magnetic viscosity, fluctuation fields, and activation energies (invited)

Abstract: Time dependence of magnetization in ferromagnetic materials was first described towards the end of the 19th century. Subsequently, two types of mechanisms responsible for time dependent behavior were identified and became known as ‘‘diffusion’’ and ‘‘fluctuation’’ after‐effect or viscosity. The former depends on thermally induced motion of impurity atoms. The latter is a consequence of thermal activation of irreversible domain processes such as domain‐wall motion and the nucleation of domains of reverse magnetization. Fluctuation viscosity affects, to a greater or smaller extent, all magnetic materials subject to hysteresis. In the late 1940s descriptions of magnetic viscosity in terms of fluctuation fields (Neel) and activation energy distributions (Street and Woolley) were developed. The two approaches will be described. An analysis of the time dependent phenomena exhibited by magneto‐optical films will be presented as a simple example of the application of activation energy modeling.

New trends in magnetism, magnetic materials, and their applications

Abstract: Magnetic Clusters: Magnetic Properties of Small Transition Metal Clusters in a Molecular Beam (W.A. de Heer, I.M.L. Billas). Magnetic Properties of Transition Metal and Rareearth Metal Clusters (P.J. Jensen, K.H. Bennemann). One and Twodimensional Systems: The Onedimensional Hubbard Model with Attractive U as a Soluble Model for Exciton Bands and Electronhole Droplets (P. Schlottmann). Superlattices and Multilayers: Magnetization Patterns of Exchange Coupled Metallic Multilayers (D. Altbir, M. Kiwi). Surfaces and Interfaces: Spinpolarized Scanning Tunneling Spectroscopy on Fe and Ni (S.F. Alvarado). Alloys and Amorphous Materials: New Trends in Ferrimagnetism (T. Kaneyoshi). Special Techniques and Materials: Lessconventional Magnetic Domain Investigations (M. Schlenker). General Theoretical Developments: Slave Boson Approach to Local Moment Formation in the Hubbard Model (R.M. RibeiroTeixeira, M. Avignon). Concluding Remarks. 39 additional articles. Index.

High speed coherent switching below the Stoner-Wohlfarth limit

Abstract: Measurements of the switching characteristics of high coercivity magnetic tapes for field pulse widths /spl ges/0.6 ns have been reported previously. In an effort to develop a simple theoretical framework in which to develop a simple magnetization dynamics, the well established Gilbert form of the Landau-Lifshitz equation, applied to a uniaxial, single domain particle with uniform magnetization was chosen. The results which were obtained using straightforward numerical methods show that when the damping constant /spl alpha/ >

Low temperature magnetic force microscope utilizing a piezoresistive cantilever

Abstract: We have developed a low temperature magnetic force microscope capable of operation down to 6 K in vacuum by using piezoresistive cantilevers. We use the non‐contact frequency modulation technique to detect the magnetic force gradient between an iron‐coated tip and the sample. We demonstrate the operation of this new instrument by obtaining images of magnetic domains in VHS tape at room temperature, 77 and 6 K. This microscope is ideally suited for the characterization of thin films of high temperature superconductors.

Size effects on switching field of isolated and interactive arrays of nanoscale single‐domain Ni bars fabricated using electron‐beam nanolithography

Abstract: Isolated nanoscale Ni bars with a length of 1 μm, a width from 15 to 300 nm, and interactive bar arrays with a spacing from 200 to 600 nm were fabricated using electron‐beam lithography and were studied using magnetic force microscopy. The study showed that the virgin magnetic state of bars with a width smaller than 150 nm was single domain and otherwise multidomain. It also showed that the switching field of isolated bars initially increases with decreasing bar width, then reaches a maximum switching field of 740 Oe at a width of 55 nm, and afterwards decreases with further bar width reduction. Furthermore, it was found that the switching field of the interactive bars decreases almost linearly with reduction of the spacing between the bars.

Magnetic wire and box arrays (invited)

Abstract: Ferromagnetic wire and box arrays with widths and spacings in the range of 2–06 μm were successfully fabricated utilizing high‐resolution electron‐beam lithography and lift‐off techniques The box arrays possessed a unique magnetic switching mechanism As the magnetic field decreased, the magnetic coherency between the boxes which are in a row first disappeared Domain wall motion in each box occurred in the next step The demagnetizing fields observed in the wire arrays with short spacings were different from those calculated by a model, in which the wires are isolated from each other Ferromagnetic resonance measurement implied the appearance of interwire dipole‐dipole interaction Multilayered wire arrays were also prepared to study the magnetoresistive characteristics

Magnetic spin transistor

Abstract: A magnetic spin transistor, usable as a memory cell, magnetic pick-up head, or a current switch, that includes a trilayer planar structure of a conductive, nonferromagnetic layer (16) sandwiched between two ferromagnetic layers (12, 14) of different coercivities. A biasing current pumped between one of the ferromagnetic layers and the nonferromagnetic layers produces a voltage on the other ferromagnetic layer. The polarity of the voltage depends on the relative magnetic polarization of the two ferromagnetic layers. As a memory cell, current passing through adjacent lines magnetize the ferromagnetic layer of lower coercivity. As a magnetic pick-up head, an adjacent magnetic recording track supplies the magnetic field sufficient to switch the lower-coercivity ferromagnetic layer.

Surface-enhanced magnetic moment and ferromagnetic ordering of Mn ultrathin films on fcc Co(001)

Abstract: We have grown ultrathin films of Mn on fcc Co(001) substrates and have investigated their magnetic properties using the element-specific techniques of soft-x-ray absorption (SXA) and x-ray magnetic circular dichroism (XMCD). We find that a one monolayer film of Mn is ferromagnetically ordered at room temperature and magnetically aligned with the fcc Co (001) substrate. The absorption spectrum of monolayer Mn is in excellent agreement with calculations for an isolated high-spin atom. By comparing our XMCD and SXA results, we conclude that the surface layer of Mn is in a high-spin state. The ferromagnetic ordering of monolayer Mn results from this high-spin layer being in direct contact with the ferromagnetic Co substrate.

Three‐dimensional micromagnetic analysis of stability in fine magnetic grains

Abstract: We examine the stability of magnetic remanences in grains of magnetite near the critical single-domain grain size (d0). The magnetic structure of a grain is followed during a 180° magnetization reversal, and it is shown that noncoherent reversals can occur even in grains smaller than d0. The energy barrier to noncoherent revesal is much lower than that of coherent rotation, so that pseudo-single-domain grain behavior will be exhibited by grains significantly smaller than d0.

Exchange coupling and energy product in random two-phase aligned magnets

Abstract: Size effects in two-phase Boolean magnets composed of an aligned hard phase and a soft phase with high magnetization are investigated. In 'overcritical' magnets, where the size of the soft regions is larger than the Blochwall width /spl delta//sub h/ of the hard phase, the energy product is reduced compared to the corresponding 'undercritical' magnets with small soft regions, where theoretical energy products as high as 1 MJ/m/sup 3/ have been predicted. Nevertheless, the theoretical energy product only gradually drops with increasing size of the soft regions, and it may still exceed the theoretical energy product 516 kJ/m/sup 3/ of Nd/sub 2/Fe/sub 14/B. >

Theory of partial thermoremanent magnetization in multidomain grains: 2. Effect of microcoercivity distribution and comparison with experiment

Abstract: We extend the thermoremanent magnetization (TRM)and pTRM theories developed in paper 1 (Dunlop and Xu, this issue) to grains in which domain walls are pinned by microcoercivities of varying magnitudes. Assuming microcoercivities to be exponentially distributed, we find that the intensity of a total TRM is linearly proportional to the inducing field H0 for small H0, to a power of roughly 1–1/n for intermediate H0, and independent of H0 for large H0, similar to the results obtained in paper 1. Here n represents the temperature dependence of microcoercivity that goes as the n th power of the saturation magnetization Ms (T). The above three field dependent regions correspond to thermally blocked, field-blocked and reequilibrated walls, respectively. When being thermally demagnetized, a TRM induced in a high field has low unblocking temperatures, as observed. For a partial TRM acquired from T2 ( T0, an even higher T2 is required. In such cases, the room temperature intensity of pTRM is approximately proportional to H02 when H0 is small. The resulting thermal demagnetization curve, normalized to the intensity before heating, is independent of both H0 and the mean value of microcoercivities. Complete demagnetization will not occur at a demagnetizing temperature T2 but only at a temperature close to Tc. The theory is supported by experimental data of thermal demagnetizations of pTRMs measured for various multidomain magnetite samples.

Magnetic force microscopy of Co-Pd multilayers with perpendicular anisotropy

Abstract: The domain structure of sputtered Co‐Pd multilayer films of varying thickness has been investigated by magnetic force microscopy. The domains appear as stripe domains, typical of perpendicularly oriented films. The size of the domains was strongly influenced by the thickness of the film. The domain repetition lengths give an additional experimental parameter which has been used to provide a stronger test of a theoretical model developed for ferromagnetic multilayer films [H. J. G. Draaisma and W. J. M. de Jonge, J. Appl. Phys. 62, 3318 (1987)]. It is found that the experiment and theory are broadly in agreement provided that the increased magnetization of the multilayer caused by polarization of the Pd is accounted for. There is a noticeable difference between the variation of the measured and theoretical domain repetition lengths with film thickness. This is attributed to the effects of domain‐wall pinning which is not considered in the model. It is estimated that the characteristic length of the films i...

Effect of substitution of In(3+) ions on the electrical and magnetic properties and Mossbauer study of Mg(0.9)Mn(0.1)In(x)Fe(2-x)O(4) ferrites

Abstract: The effect of substitution of diamagnetic In3+ ions on the electrical and magnetic properties of Mg-Mn ferrites was studied in the ferrite series Mg0.9Mn0.1 In x Fe2−x O4 wherex varied from 0–0.8 in steps of 0.1. The incorporation of In3+ ions in place of Fe3+ ions resulted in an increase of lattice parameter owing to the larger size of the substituted ions, and an increase of d.c. resistivity owing to reduction of Verwey's hopping mechanism. It also resulted in improvement of saturation magnetization and produced a marked increase in the value of initial permeability, thus upgrading the bulk magnetic properties of these ferrites. These bulk magnetic properties improve due to substitution of diamagnetic In3+ ions,x, up to 0.5 only, whereas they deteriorate for a higher content of In3+ ions. The variations of saturation magnetization have been explained on the basis of modified cation distribution and their magnetic interactions. A large increase in the value of initial permeability has been attributed to its dependence onM s and magnetocrystalline anisotropy constant. A comparison of bulk magnetic properties with the inferences drawn from Mossbauer studies of these samples shows a similar trend. It is concluded that the substitution of In3+ ions,x, up to 0.5 in Mg-Mn ferrites results in the production of a hyperfine field at A as well as B sites, followed by ferromagnetic relaxation and paramagnetic transition for higher concentrations of In3+ ions. These variations have been explained on the basis of the effect produced by In3+ ions on the magnetic interactions, supertransferred hyperfine fields and domain-wall oscillations.

High density magnetic information storage medium

Abstract: Magnetic recording media comprising an ordered, ultra-high density array of 500 Å diameter circular magnetic thin film islands on a substrate. The magnetic material supports magnetization perpendicular to the film plan, and each circular island comprises a single magnetic domain and a single information storage bit. An areal bit density of 10 11 bits/in 2 can be achieved by such an array. The magnetic array is generated using a single level mask comprised of a self-ordering polymer array, either an array of polymer spheres or a regular array of polymeric blocks in a phase-separating polymer film.

High resolution magnetic force microscopy of domain wall fine structures (invited)

Abstract: Magnetic force microscope (MFM) images of various domain wall structures will be discussed. The systems studied in this work include single crystals of magnetite (Fe/sub 3/O/sub 4/) and a 0.5 /spl mu/m single crystal iron (Fe) film. In general, the observed structures can be explained in terms of a bulk Bloch wall terminated at the surface by a Neel cap. A model of the MFM response to such a structure has been fitted to the measured response, demonstrating the utility of MFM data for testing micromagnetic models. >