Showing papers on "Magnetization published in 1992"

Giant magnetoresistance in nonmultilayer magnetic systems.

Abstract: We have observed isotropic giant magnetoresistance (GMR) in nonmultilayer magnetic systems using granular magnetic solids. We show that GMR occurs in magnetically inhomogeneous media containing nonaligned ferromagnetic entities on a microscopic scale. The GMR is determined by the orientations of the magnetization axes, the density, and the size of the ferromagnetic entities.

Matrix-Mediated Synthesis of Nanocrystalline γ-Fe2O3: A New Optically Transparent Magnetic Material

Abstract: A new magnetic material with appreciable optical transmission in the visible region at room temperature has been isolated as a gamma-Fe(2)O(3)/polymer nanocomposite. The synthesis is carried out in an ion-exchange resin at 60 degrees C. Magnetization and susceptibility data demonstrate loading-dependent saturation moments as high as 46 electromagnetic units per gram and superparamagnetism for lower loadings where particle sizes are less than 100 angstroms. Optical absorption studies show that the small-particle form of gamma-Fe(2)O(3) is considerably more transparent to visible light than the single-crystal form. The difference in absorption ranges from nearly an order of magnitude in the "red" spectral region to a factor of 3 at 5400 angstroms. The magnetization of the nanocomposite is greater by more than an order of magnitude than those of the strongest room-temperature transparent magnets, FeBO(3) and FeF(3).

Magnetic field decay in isolated neutron stars

Abstract: We investigate three mechanisms that promote the loss of magnetic flux from an isolated neutron star. Ohmic decay produces a diffusion of the magnetic field with respect to the charged particles. It proceeds at a rate that is inversely proportional to the electric conductivity and independent of the magnetic field strength. Ohmic decay occurs in both the fluid core and solid crust of a neutron star, but it is too slow to directly affect magnetic fields of stellar scale. Ambipolar diffusion involves a drift of the magnetic field and charged particles relative to the neutrons. The drift speed is proportional to the second power of the magnetic field strength if the protons form a normal fluid. Variants of ambipolar diffusion include both the buoyant rise and the dragging by superfluid neutron vortices of magnetic flux tubes. Ambipolar diffusion operates in the outer part of the fluid core where the charged particle composition is homogeneous, protons and electrons being the only species. The charged particle flux associated with ambipolar diffusion decomposes into a solenoidal and an irrotational component. Both components are opposed by frictional drag. The irrotational component perturbs the chemical equilibrium between neutrons, protons, and electrons, thus generating pressure gradients that effectively choke it. The solenoidal component is capable of transporting magnetic flux from the outer core to the crust on a short time scale. Magnetic flux that threads the inner core, where the charged particle composition is inhomogeneous, would be permanently trapped unless particle interactions could rapidly smooth departures from chemical equilibrium. Magnetic fields undergo a Hall drift related to the Hall component of the electric field. The drift speed is proportional to the magnetic field strength. Hall drift occurs throughout a neutron star. Unlike ohmic decay and ambipolar diffusion which are dissipative, Hall drift conserves magnetic energy. Thus, it cannot by itself be responsible for magnetic field decay. However, it can enhance the rate of ohmic dissipation. In the solid crust, only the electrons are mobile and the tangent of the Hall angle is large. There, the evolution of the magnetic field resembles that of vorticity in an incompressible fluid at large Reynolds number. This leads us to speculate that the magnetic field undergoes a turbulent cascade terminated by ohmic dissipation at small scales. The small-scale components of the magnetic field are also transported by Hall drift waves from the inner crust where ohmic dissipation is slow to the outer crust where it is rapid. The diffusion of magnetic flux through the crust takes ~ 5 x 10^8/B_(12) yr, where B_(12) is the crustal magnetic field strength measured in units of 10^(12) G.

Magnetic live surface layers in Fe/Cu(100).

Abstract: The magnetic properties of Fe films epitaxially grown on Cu(100) have been correlated to their structure and morphology. Strained fcc iron films with thicknesses between 5 and 11 monolayers (ML) are ferromagnetically ordered at the surface with a perpendicular orientation of the magnetic moment, whereas the bulk of the films remains paramagnetic. The surface magnetism is related to an expanded interlayer distance at the surface. With the onset of dislocation formation at about 11 ML, the films become magnetic in the bulk and the magnetization switches to in-plane orientation.

Magnetization direction switching in Fe/Cu(100) epitaxial films: Temperature and thickness dependence.

Abstract: The magnetization direction switching is investigated in epitaxial Fe/Cu(100) films by spin-polarized scanning electron microscopy. We follow the transition from perpendicular to in-plane magnetization with both increasing film thickness and varying temperature. No variation of magnetic moment with magnetization direction change is found. A perpendicular stripe domain configuration is identified which evolves from the low-temperature single-domain state during the reorientation phase transition.

Prediction and confirmation of perpendicular magnetic anisotropy in Co/Ni multilayers.

Abstract: On the basis of first-principles (local-spin-density-approximation) calculations of the magnetocrystalline anisotropy energy, we have predicted a perpendicular orientation of the magnetization in a [111${]}_{\mathrm{fcc}}$ ${\mathrm{Co}}_{1}$/${\mathrm{Ni}}_{2}$ multilayer. This multilayer was then grown by vapor deposition and found to have the predicted properties. The saturation magnetization of 1 T is much larger than that of known magnetic multilayers with perpendicular anisotropy.

Low-temperature magnetic properties of the ferromagnetic organic radical, p-nitrophenyl nitronyl nitroxide.

Abstract: Low-temperature magnetic properties of the \ensuremath{\beta} and \ensuremath{\gamma} phases of p-nitrophenyl nitronyl nitroxide (p-NPNN) were determined by measuring the specific heat, the magnetic susceptibility, and the hysteresis curve of magnetization above $^{3}\mathrm{He}$ temperature in external magnetic fields. The \ensuremath{\beta}-phase crystal undergoes a bulk ferromagnetic transition at 0.60 K, which was confirmed by the magnetic entropy of ln2 due to one unpaired electron on the radical molecule and the hysteresis curve. The \ensuremath{\gamma} phase, on the other hand, revealed an antiferromagnetic transition at 0.65 K and one-dimensional ferromagnetic fluctuations above it. The specific-heat data of the \ensuremath{\gamma} phase in external fields were analyzed by a mean-field theory incorporated in the one-dimensional Heisenberg model. The details of sample characterization of each phase based on thermal analysis are also given.

Studies of HTSC crystal magnetization features using indicator magnetooptic films with in-plane anisotropy

Abstract: Indicator ferrimagnetic films with in-plane anisotropy are applied for studying the magnetic flux distribution in high-Tc YBaCuO (YBa2Cu3O7) single crystals. Induction gradients are measured to estimate the critical current density Jc. Temperature dependences of Jc are found to be different for different field directions. For the field along the orthorhombic c-axis Jc∥ab exhibits an exponential temperature decay. In the case of H∥ab, however, Jc∥ab is changed just a little up to 70 K and then drops sharply near Tc.

Effects of microgeometry and surface relaxation on NMR pulsed-field-gradient experiments: Simple pore geometries.

Abstract: We derive an expression for the magnetization M(k,\ensuremath{\Delta}) in a pulsed-field-gradient experiment for spins diffusing in a confined space with relaxation at the pore walls. Here k=\ensuremath{\gamma}\ensuremath{\delta}g, \ensuremath{\delta}= pulse width, g= gradient strength, \ensuremath{\gamma}= the gyromagnetic ratio, and \ensuremath{\Delta} is the time between gradient pulses. We show that the deviation of -ln[M(k,\ensuremath{\Delta})/M(0,\ensuremath{\Delta})] from quadratic behavior in k in experiments in porous media can be a more sensitive probe of the microgeometry (size, connectivity, size distribution, shape, etc.), than either the enhancement of 1/${\mathit{T}}_{1}$ over the bulk water value, or the macroscopic diffusion coefficient, which is derived from the slope of -ln[M(k,\ensuremath{\Delta})/M(0,\ensuremath{\Delta})] at small ${\mathit{k}}^{2}$, in the limit of large \ensuremath{\Delta}. We propose some simple models of randomly oriented tubes and sheets to interpret the k dependence of the amplitude beyond the leading small-k quadratic behavior. When the macroscopic diffusion coefficient is unobtainable, due to the decay, the present considerations should be useful in extracting geometrical information. The effective diffusion constant derived from NMR exactly equals that derived from electrical conductivity only when the surface relaxivity is zero, but can be close to each other in favorable circumstances even for finite surface relaxivity. Exact solutions with partially absorbing boundary conditions are obtained for a slab and a sphere to infer that the normalized amplitude M(k,\ensuremath{\Delta},\ensuremath{\rho})/M(0,\ensuremath{\Delta},\ensuremath{\rho}) depends only weakly on the surface relaxivity \ensuremath{\rho} for monodisperse convex-shaped pores in the parameter ranges of interest. We also obtain expressions for the mean lifetime of the amplitude in the geometries considered.

Suppression of tunneling by interference in half-integer-spin particles.

Abstract: Within a wide class of ferromagnetic and antiferromagnetic systems, quantum tunneling of magnetization direction is spin-parity dependent: it vanishes for magnetic particles with half-integer spin, but is allowed for integer spin. A coherent-state path-integral calculation shows that this topological effect results from interference between tunneling paths.

Magnetic and magneto‐optical properties of cobalt‐platinum alloys with perpendicular magnetic anisotropy

Abstract: Co1−xPtx alloys with Pt contents in the range 0.45≤x≤0.9 show sizable perpendicular magnetic anisotropy, 100% perpendicular remanence and coercivities in the range 160 kA/m. Thin films of this material are grown by electron beam evaporation onto fused silica or Si, at substrate temperatures between 150 and 350 °C. Spectroscopic investigations of the polar Kerr rotation show a significant enhancement of the Pt related UV peak. A comparison of the static signal levels R×(θk2+ek2)1/2 of Co/Pt multilayers and alloys shows an overall 50% enhancement in the case of alloys. Curie temperatures around 200 °C are observed for Co∼22Pt∼78 compositions. These properties, together with the potentially high chemical stability and ease of manufacturing make Co1−xPtx alloys very attractive materials for short wavelength magneto‐optic recording.

On the application of ultra‐fast rare experiments

Abstract: The ultra-fast application of the RARE experiment is described in detail, with special emphasis on its multifarious applications with preparation experiments that produce transverse magnetization. The factors affecting the temporal evolution of the magnetization during the experiment are described, and the implications for the slice profile when using a Gaussian refocusing pulse are experimentally examined. The choice of phase-encoding scheme for use with preparation experiments is discussed, as is the use of various phase-encoding schemes to reduce line broadening in the phase-encoding direction if a number of averages are acquired. An explanation for the decomposition of the echo are into two components if the read gradient is imbalanced is given, and the experimental conditions necessary for the coherent addition of these two echo groups are described. An alternative sequence that removes one of these groups from the acquisition window is proposed. The sensitivity of the sequence to flow and motion is investigated, and the drastic loss of signal in this situation explained. The in vivo and in vitro application of preparation experiments leading to the accurate measurement of T1, T2, diffusion constant, and magnetization transfer characteristics is presented. The implementation of zoom-imaging using spin- and stimulated-echo preparation is described, and 3D in vivo spin-echo zoom images are presented. Simple phantom experiments demonstrating the feasibility of chemical-shift selective and spectroscopic imaging are also given.

Dual spin valve magnetoresistive sensor

Abstract: A magnetoresistive read sensor based on the spin valve effect and having a multilayered, dual spin valve structure is described. The sensor read element includes first, second and third layers of ferromagnetic material separated from each other by layers of non-magnetic metallic material. The first and third layers of ferromagnetic material, i.e., the outer layers of the structure, have their magnetization orientation fixed, while the second, intermediate ferromagnetic layer is magnetically soft and has its magnetization oriented perpendicular to that of both the outer ferromagnetic layers in the absence of an applied magnetic field. In one preferred embodiment, the two outer ferromagnetic layers have their magnetizations fixed parallel to each other by exchange coupling with adjacent antiferromagnetic layers.

Formation and magnetic properties of nanocrystalline mechanically alloyed Fe‐Co

Abstract: Fe100−xCox powders were prepared by mechanical alloying of the elements in a planetary ball mill They were investigated with respect to phase formation and magnetic properties using x‐ray diffraction, differential scanning calorimetry, and measurements of the saturation magnetization and the coercivity The measurement of the saturation magnetization proved the true formation of the bcc (x≤80) and fcc (x=90) solid solutions by mechanical alloying A nonequilibrium microstructure originates from a grain‐size reduction to minimum 20–30 nm and the introduction of internal strain up to 1% (root‐mean‐square strain) An improvement in the soft magnetic properties by the nanocrystalline state, as hoped for, does not occur, because the high amount of internal strain together with the high saturation magnetostriction of the Fe‐Co alloys causes relatively high coercivities of 5–40 A/cm Grain growth and strain relaxation induced by controlled heat treatment of the as‐milled powders allowed the separation of the in

Magnetic field sensing device

Abstract: A self-contained magnetic field sensing device which has magnetic field sensing elements arranged in an electrical bridge network. An integral electrical conductor spaced from the sensing elements carries a current for setting and resetting the direction of magnetization of the sensing elements. A known magnetic field useful for test set up and calibration is provided at the sensing elements by a second integral electrical conductor.

Effect of electron-electron interactions on the magnetization of quantum dots.

Abstract: The low-temperature magnetization of parabolic quantum dots is calculated and is shown to be a sensitive probe of interaction effects. The interaction causes the ground state to occur at certain magic values of the total angular momentum, the strength of the magnetic field determining which of them is selected. Increasing the mangetic field causes the ground-state angular momentum to jump from one magic value to another and this causes the discontinuities in the magnetization. The effects of spin lead to extra discontinuities at low magnetic field. The magic angular momenta for the spin-polarized case are derived by a simple physical argument.

Equilibrium properties of ferrocolloids

Abstract: We consider a thermodynamic model of a stable macroscopically homogeneous ferrocolloid containing identical spherical particles with permanent moments suspended in a non-dissociated liquid on the basis of the hard sphere perturbation theory. Magnetostatic properties and the phase diagram of the ferrocolloid are shown to agree very well with experimental evidence. Dipole interparticle interactions result in an effective attraction between the particles which increase, as the strength of an externally applied magnetic field grows, favours the phase separation and cause a reduction in the coefficient of mutual Brownian diffusion of the particles.

Boltzmann-equation approach to the negative magnetoresistance of ferromagnetic-normal-metal multilayers.

Abstract: The Boltzmann equation is solved for a system consisting of a ferromagnetic--normal-metal--ferromagnetic metallic trilayer. The in-plane conductance of the film is calculated for two configurations: the ferromagnetic layers aligned (i) parallel and (ii) antiparallel to each other. The results explain the giant negative magnetoresistance encountered in these systems when an initial antiparallel arrangement is changed into a parallel configuration by application of an external magnetic field. The calculation depends on (a) geometric parameters (the thicknesses of the layers), (b) intrinsic metal parameters (number of conduction electrons, magnetization, and effective masses in the layers), (c) bulk sample properties (conductivity relaxation times), (d) interface scattering properties (diffuse scattering versus potential scattering at the interfaces), and (e) outer surface scattering properties (specular versus diffuse surface scattering). For perfect specular scattering at the surfaces the problem becomes identical to an infinite multilayer, periodic system. It is found that a large negative magnetoresistance requires, in general, considerable asymmetry in the interface scattering for the two spin orientations. All qualitative features of the experiments are reproduced. Quantitative agreement can be achieved with sensible values of the parameters. The effect can be conceptually explained based on considerations of phase-space availability for an electron of a given spinmore » orientation as it travels through the multilayer sample in the various configurations.« less

Quantum Tunneling of Magnetization in Solids

Abstract: Magnetic solids should, under certain circumstances, show macroscopic quantum behavior, in which coherence exists between completely distinct magnetization states, each involving a very large number of spins (~1012 spins). This article reviews the recent work in this field, concentrating particularly on macroscopic quantum tunneling (MQT) of magnetization. The two main phenomena discussed are (a) the tunneling of magnetization in singledomain particles or grains (in which some 103−104 spins rotate together through an energy barrier), and (b) the tunneling of domain walls in films or in bulk magnets; where walls containing ~ 1010 spins may tunnel off a pinning potential, or from one pinning centre to another. Some attention is also given to the quantum nucleation of magnetization reversal in a bulk magnet, and to the quantum motion of other magnetic solitons (such as vortices). After a thorough analysis of the basic grain and wall tunneling phenomena, we continue on to a discussion of the various dissipative or “decoherence” mechanisms, which destroy the phase correlations involved in tunneling. The coupling of grain magnetization to phonons, photons, and electrons is shown to have little consequence for weaklyconducting or insulating grains. Domain walls couple to these and also to magnons and impurities or defects; the 3rd order coupling to magnons can have serious effects, but if one uses pure insulators at low temperatures, these can also be ignored. As a result, theory indicates that MQT should be visible in both grains and bulk magnets at low temperatures (at least below ~1 K). The present experimental evidence for such behavior is inconclusive, partly because few experiments have been done. We discuss these experiments, and make some suggestions for future work. It is hoped this review will stimulate such work, not only because of the fundamental interest in macroscopic quantum phenomena, but also because of the considerable scope for technological innovation.

Polarized neutron reflection as a probe of magnetic films and multilayers.

Abstract: The application of polarized neutron reflection (PNR) to the study of the magnetic properties of thin films and multilayers is described. It is demonstrated that PNR provides a means of directly determining the magnetization-vector profile in multilayers of known layer thickness and layer density. Thus, the magnetization reversal process in these systems can be directly studied. A matrix method is presented which can be used to calculate the spin-dependent reflectivity from a multilayer with general in-plane orientation of the magnetic moment in each layer. In addition, we show that behavior of the spin asymmetry is dominated by multiple reflections and refraction just above the critical wave vector, but with increasing wave vector, such processes become progressively less important, and the response moves towards a ``diffraction limit'' in which a Fourier-transform approximation to the exact result can be used. The ideas in this paper are illustrated by a number of examples, including the exchange-biased structure Ag/Fe-Ni/Cu/Fe-Ni/Fe-Mn/Si.

Surface-step-induced magnetic anisotropy in thin epitaxial Fe films on W(001)

Abstract: Ultrathin epitaxial Fe films grown on W(001) surfaces prepared to yield uniform 25-\AA{}-wide terraces are shown to exhibit layer-dependent in-plane uniaxial magnetic anisotropy having an easy axis perpendicular to the steps. Hysteresis loops measured by magneto-optical techniques when steps are aligned parallel to the applied field H manifest the flip of magnetization that occurs near H=0. The absence of magneto-optic effects for films less than or equal to one monolayer thick is consistent with recent Iab initioR calculations that predict significant moment reduction resulting from strong film-substrate hybridization.

Magnetism in Cr thin films on Fe(100)

Abstract: The spin polarization of secondary electrons from a Cr film on Fe(100), measured with scanning electron microscopy with polarization analysis, oscillates as a function of Cr thickness with a period near two atomic layers, consistent with incommensurate spin-density-wave antiferromagnetism in the Cr. The position of a phase slip due to incommensurability varies reversibly by 14 layers over the temperature range of 310 to 550 K. The Cr surface magnetic moment persists well above the Neel temperature of bulk Cr

Flux-creep crossover and relaxation over surface barriers in Bi2Sr2CaCu2O8 crystals.

Abstract: Flux creep in Bi 2 Sr 2 CaCu 2 O 8 crystals exhibits two different regimes as a function of time, as well as of temperature and magnetic field. The short-time, low-temperature regime has a peak in current density versus field, which is enhanced by irradiation defects. The long-time, high-temperature regime has a monotonic and sharp falloff (step) in current density versus magnetic field, which is suppressed by irradiation defects. The former is identified with bulk pinning, and the latter with a surface barrier

Symmetry-induced uniaxial anisotropy in ultrathin epitaxial cobalt films grown on Cu(1 1 13).

Abstract: Uniaxial anisotropy has been found in ultrathin cobalt films grown on a Cu(1 1 13) surface. Our studies using scanning electron microscopy with polarization analysis clearly show that the easy axis of magnetization is parallel to the direction of the step edges of the Cu(1 1 13) substrate. In spite of the different anisotropy behavior, the domain structures in Co/Cu(001) and Co/Cu(1 1 13) are similar, which indicates that the domain pattern in ultrathin films is little affected by the anisotropy.

On the calculation of Jc from magnetization measurements on superconductors

Abstract: A simple but reliable procedure to evaluate J c ( B ) from magnetization measurements has been developed. It applies to the fully penetrated state and to situations where the flux density gradient has reversed completely. In an approach consistent with the Bean model, a uniform current density is assumed to flow throughout the entire sample being correlated to the corresponding average magnetic induction in the sample. In combination with the anisotropic Bean model, we present evaluations of J c ( B ) for a typical single crystalline high-temperature superconductor.

Design and implementation of magnetization transfer pulse sequences for clinical use.

Abstract: The transfer of magnetization between a free and a bound pool of spins is described in terms of the respective longitudinal relaxation times and the life times of spins in each pool. The effect of an off resonance radiofrequency (RF) pulse in producing saturation in the bound pool and a consequent decrease in both the available longitudinal magnetization and the T1 of spins in the free pool is described. The effects of increasing duration of the saturating RF pulse on image pixel signal intensity were used to determine values for the decrease in both T1 and the available magnetization in gray and white matter of the brain as well as in muscle, fat, and CSF. At 0.15 T the available magnetization of muscle was reduced by approximately 60% and its T1 was decreased from 350 to 150 ms. The available magnetization of white and gray matter was reduced by 40% and their values of T1 were reduced by 80-110 ms. The reduction in available magnetization was used to increase contrast on proton density weighted or T2-weighted SE pulse sequences. These changes were also used to design inversion recovery (IR) pulse sequences with particular contrast properties. A short inversion time (TI) magnetization transfer (MT) IR (MT-STIR) pulse sequence was used to reduce the signal from normal muscle to zero to produce an angiographic effect in the leg. Increased tissue contrast was observed with a T2-weighted (MT-SE) sequence in a patient with bilateral cerebral infarction and with an MT-IR pulse sequence in a patient who had an intracranial hematoma. Three patients with cerebral tumors showed high lesion contrast with MT-STIR sequences. Components within two tumors were changed to different degrees by MT and in one case change in the brain attributable to recent radiotherapy treatment was only identified with an MT-STIR sequence. Magnetization transfer can be used to manipulate both the available longitudinal magnetization and the T1 of normal and abnormal tissues. The changes in tissue contrast produced by this can be very substantial and are likely to be of importance in clinical imaging.

Surface magnetism in iron, cobalt, and nickel.

Abstract: We have calculated magnetic moments, work functions, and surface energies for several of the most closely packed surfaces of iron, cobalt, and nickel by means of a spin-polarised Green's-function technique based on the linear muffin-tin orbitals method within the tight-binding and atomic sphere approximations. We find enhanced spin moments at all the surfaces considered except for Ni fcc(111), where the moment at the surface reverts to its bulk value. This is in close agreement with earlier slab calculations

Enhanced current density Jc and extended irreversibility in single‐crystal Bi2Sr2Ca1Cu2O8 via linear defects from heavy ion irradiation

Abstract: Large enhancements in the critical current density Jc were produced in single crystals of the high‐temperature superconductor Bi2Sr2Ca1Cu2O8 by irradiation with high energy Sn ions. In addition, the irreversibility line was moved to considerably higher magnetic fields. In contrast with analogous studies on Y1Ba2Cu3O7, there was little, if any, selective pinning when the magnetizing field was applied parallel to the linear, ion‐damage‐produced tracks.