Showing papers on "Single domain published in 1998"

Hysteresis in magnetism : for physicists, materials scientists, and engineers

Abstract: Introduction: Magnetic Hysteresis. Types of Hysteresis. Maxwells Equations and Thermodynamics: Maxwells Equations in Magnetic Media. Magnetic Work and Thermodynamics. Magnetic Free Energy: Exchange and Anisotropy. Micromagnetics. Magnetic Domains and Domain Walls. The Magnetization Process: Coherent Rotation. Domain Wall Motion. Magnetization Curves. Coercivity Mechanisms. Eddy Currents. Preisach Systems: Collections of Bistable Units. Hysteresis in Preisach Systems. Appendixes: Systems of Units. Vector Relations. Reciprocity Theorems. Micromagnetic Parameters. Stochastic Processes. Bibliography. Index.

Magnetic properties of dipolar interacting single-domain particles

Abstract: The remanence and coercivity of an assembly of single-domain ferromagnetic particles are studied using the Monte Carlo simulation technique. The particles have random locations, possess random uniaxial anisotropy, and are coupled through dipolar interactions. The dependence of the magnetic properties on the packing density, the size of the particles, and the temperature are examined systematically. The role of the packing geometry ~sc, fcc! and the sample boundaries are discussed. Dipolar interactions are shown to reduce the coercivity with respect to values for the noninteracting assembly in all cases except for strongly dipolar systems below the percolation threshold. An enhancement of the remanence is found in weakly dipolar systems and is attributed to the macroscopic Lorentz field. The fcc packing of the particles leads to more pronounced ferromagnetic behavior than the sc packing. The sample free boundaries and the corresponding demagnetizing field have a strong effect on the remanence of the assembly while they produce a minor reduction to the coercivity. The results from the simulations are compared with magnetic measurements on frozen ferrofluids and granular metal solids. @S0163-1829~98!03241-X#

Temperature-induced magnetization reversal in a YVO3 single crystal

Abstract: The total energy of a magnet in a magnetic field is lowest when the magnetic moment is aligned parallel to the magnetic field. Once aligned, the magnetic moment can be reversed by applying a sufficiently large field in the opposite direction. These properties form the basis of most magnetic recording and storage devices. But the phenomenon of magnetization reversal in response to a change in temperature (in a small magnetic field) is rarer. This effect occurs in some ferrimagnetic materials consisting of two or more types of antiferromagnetically ordered magnetic ions1, and forms the operational basis of ferrimagnetic insulators. Here we report the observation of multiple temperature-induced magnetization reversals in YVO3. The net magnetic moment is caused by a tilting of the antiferromagnetically aligned moments of (crystallographically identical) V3+ ions, due to orthorhombic distortion in the crystal structure. We observe an abrupt switching at 77 K associated with a first-order structural phase transition, and a gradual reversal at ∼95 K without an accompanying structural change. The magnetization always reverses if the crystal is cooled or warmed through these two temperatures in modest fields. We propose a possible mechanism involving a change in orbital ordering which may be generic to a broad class of transition metal oxides.

Magnetization Reversal in Micron-Sized Magnetic Thin Films

Abstract: We have measured and simulated the dynamics of magnetization reversal in 5 nm by 0.8 by 1.6 $\ensuremath{\mu}\mathrm{m}$ ${\mathrm{Ni}}_{60}{\mathrm{Fe}}_{40}$ thin films. The films measured form the upper electrode of a spin-polarized tunnel junction so that the magnetization direction of the film can be probed by measuring the tunneling resistance of the junction. When a magnetic field pulse is applied, the time to switch the film magnetization changes from greater than 10 ns to less than 500 ps as the pulse amplitude is increased from the coercive field to 10 mT and beyond. We have simulated these transitions using micromagnetic modeling of the exact experimental conditions. The simulations agree well with the experimental measurements.

Thermally Activated Relaxation Time of a Single Domain Ferromagnetic Particle Subjected to a Uniform Field at an Oblique Angle to the Easy Axis: Comparison with Experimental Observations

Abstract: New asymptotes of the relaxation time of the magnetic moment of a single domain particle with a uniform magnetic field applied at an oblique angle to the easy axis (in excellent agreement with exact numerical results from the Fokker-Planck equation for the N\'eel-Brown model) are used to model the experimental angular variation of the switching field for individual Co and BaFeCoTiO particles. Good agreement is obtained, justifying the N\'eel-Brown (in effect, the Kramers) conception of the superparamagnetic relaxation process and allowing one to deduce the value of the damping constant (hitherto almost unknown).

Effect of annealing on the structure and magnetic properties of graphite encapsulated nickel and cobalt nanocrystals

Abstract: We report the structure and magnetic studies of carbon coated nanocrystals of nickel and cobalt synthesized in a special low carbon to metal ratio arc chamber. Powder x-ray diffraction profiles show peaks associated with single phase of fcc nickel or cobalt and major peaks of graphite with no evidence of carbides or solid solutions of carbon in the metal. Measured lattice spacing of crystalline particles and that of graphite coating from high-resolution transmission electron microscope images also confirm such findings. Magnetization measurements as a function of temperature in the range 20–900 °C give a Curie temperature equal to that of bulk metal within the experimental error. Upon heating and recooling of the particles a larger magnetization as high as 57% of bulk Co and 53% of bulk Ni was measured. Also M–H hysteresis loop of the particles have been measured at room temperature after annealing in the temperature range 20–650 °C for Ni, and 20–900 °C for Co. The dependence of room temperature saturation magnetization, remanent magnetization, and coercive field of the particles on annealing temperature is reported. These data are described by transition of particles form single domain to multidomain as a result of particle growth due to annealing. We also present the particle size distribution measurements that show log-normal behavior, and indicate substantial particle size growth due to annealing.

Origin of the cluster-glass-like magnetic properties of the ferromagnetic system

Abstract: The magnetic behaviour of at low magnetic fields has been studied by ac susceptibility, and field cooled (FC) and zero field cooled (ZFC) magnetization measurements. The cluster-glass-like magnetic behaviour of the compound is found to originate from its magnetocrystalline anisotropy as similar properties are observed for ferromagnetic systems also. The cluster glass freezing temperature and its magnetic field dependence, the irreversibility between the FC and ZFC magnetization curves, the shape of the low-field susceptibility curves, etc are related to the magnitude and temperature variation of the coercivity which is a measure of the anisotropy, and the ratio where is the applied magnetic field.

Soft to hard magnetic anisotropy in nanostructured magnets

Abstract: The effective anisotropy of hard-soft magnetic nanostructures is analyzed using the concept of the exchange correlation length of both phases. The dependence of coercivity on volume fraction, fluctuation length, temperature, and magnetic properties of the components is derived from the degree of magnetic coupling, defined through an effective interphase exchange constant. Coercivity and remanence measurements carried out on devitrified FeZrBCu amorphous alloys point out the transition from an uncoupled to a coupled regime by increasing the temperature in a very diluted system, according to the predictions of the analysis.

Phase transitions in planar magnetic nanostructures

Abstract: Using numerical micromagnetics we have studied the ground state magnetization distribution of square planar ferromagnetic elements (“nanostructures”). As the element size is reduced from 250 to 2 nm at constant thickness (2–35 nm), we find that the magnetization distribution undergoes up to three phase transitions involving as many as three different near single domain states. One of these phase transitions is analogous to the reorientation phase transition observed in continuous ultrathin magnetic films.

End domain states and magnetization reversal in submicron magnetic structures

Abstract: Patterned submicron magnetic thin films of various geometries have been systematically studied. We have observed end domain states in rectangular elements, which is in excellent agreement with micromagnetic simulation results. Significant deviation from single domain behavior has been found in low aspect ratio elements. We will show that this deviation is attributed to behavior of the end domains.

Superconducting properties of natural and artificial grain boundaries in bulk melt-textured YBCO

Abstract: A range of experimental techniques have been used to characterize melt-processed YBa 2 Cu 3 O 7− δ samples containing single-grain boundaries. Both natural high-angle boundaries, which sometimes appear during the grain growth process, and artificial low-angle boundaries, obtained by joining two single domains, have been investigated. Electrical resistivity, current–voltage characteristics, magnetic moment measurements and Hall probe mapping techniques have been employed to investigate the boundaries. Results are compared with the properties of single domain material (i.e., containing no grain boundary) for which T c ≈89 K and J c ‖ ab (77 K, 1 T)>10 4 A/cm 2 . Resistance measurements across all the grain boundaries show a stronger dependence on current and magnetic field than that measured within the grains and exhibit a pronounced resistive `tail'. The I – V curves obtained for the high-angle natural grain boundary are sharp and differ from the rounded I – V curves which are characteristic of single-grains. Field mapping measurements used to evaluate the critical current anisotropy are in agreement with magnetisation measurements. The limitations of this technique for investigating boundaries are discussed. It was found that current anisotropy can conceal a weak link between two grains, leading to a false indication of single-grain behaviour. Artificially engineered boundaries are shown to have significant potential for applications in high fields at 77 K.

Magnetically textured γ-Fe2O3 nanoparticles in a silica gel matrix: Structural and magnetic properties

Abstract: This paper is devoted to magnetic and structural properties of anisotropic γ-Fe2O3 superparamagnetic particles dispersed in a transparent xerogel matrix. The effect of frozen anisotropy axes and magnetic texture, induced by a magnetic field applied during the solidification of the matrix on the in-field magnetization process, is studied by alternating gradient force magnetometry and first and second order magneto-optical effects. The changes of magnetization curves with respect to the ferrofluid solution at the same particle concentration are interpreted on the basis of an existing statistical approach extended to systems with particle size distribution, which has to be taken into account for real samples. A very good agreement between the experiment and theory was achieved for a log-normal distribution of diameters which well resembles that deduced from electron microscopy observations in different imaging modes. This structural analysis states the parameter values used in calculations and confirms the relevance of basic assumptions of the model for the specimens studied. The experimental results and the related theoretical discussion should be of use to understand magnetic properties of other magnetically textured superparamagnetic systems.

Negative remanent magnetization of fine particles with competing cubic and uniaxial anisotropies

Abstract: The magnetic properties of noninteracting single-domain particles whose anisotropy is made up of a cubic magnetocrystalline and a uniaxial components were investigated. Various directions of the uniaxial anisotropy were considered and the dependencies of the reduced remanence as a function of the ratio between the two anisotropies were obtained. It was found that for sufficiently strong uniaxial anisotropy and random arrangement of the particle orientations, reduced remanence lower than 0.5 is, in general, an intrinsic property of the system due to the negative remanent magnetization of some of the particles.

High-efficiency miniature magnetic integrated circuit structures

Abstract: The present invention, generally speaking, provides a magnetic memory element that is single domain in nature and has a geometry that mitigates the effects of half-select noise. In a preferred embodiment, the magnetic memory element takes the form of a magnetic post or tube having an aspect ratio in the range of 2:1 (more preferably 4:1). The outside diameter of the magnetic tube or post is preferably less than 0.8 microns, more preferably 0.6 microns or less. The magnetic post or tube then functions as a single magnetic domain. In the case of a magnetic tube, the skin of the tube is formed of a magnetic material and the interior of the tube is formed of a non-magnetic material. Suitable non-magnetic materials include copper, gold and silicon. The coercivity of the magnetic tube structure may be adjusted by adjusting the thickness of the magnetic skin. As a result, the magnetic memory element is readily scalable to smaller geometries as lithographic techniques improve. The combination of very small, single-domain size and a relatively large aspect ratio results in uniquely desirable properties. Current levels within any reasonable expectation operate to switch the state of the magnetic tube only when the magnetic tube is destabilized by running current through it. With current flowing through the magnetic tube, its state may be readily changed by running modest currents in opposite directions through two parallel conductors, one on each side of the magnetic tube. When the magnetic tube is switched, the single domain nature of the magnetic tube produces a signal that is typically 10-15 times stronger than signals produced by conventional magnetic memory elements. The magnetic tube functions as a vertical magnetic field generator and may be formed in intimate proximity to a magnetic field sensor such as above the gate of a magFET.

Reversal mechanism of submicron patterned CoNi/Pt multilayers

Abstract: With laser interference lithography Co/sub 50/Ni/sub 50//Pt multilayers with perpendicular magnetic anisotropy have been patterned into regular matrices of submicron sized dots. Their magnetic properties have been studied with Vibrating Sample Magnetometry. Compared to continuous multilayers (H/sub c/=15 kA/m) the coercivity for the patterned multilayers is much larger (H/sub c/=115-270 kA/m). Though the hysteresis curves of 180 nm dots and 60 nm dots have identical shapes, virgin curves seem to indicate that the 180 nm dots are multidomain while the 60 nm dots are single domain. The latter has been confirmed with Magnetic Force Microscopy observations.

Magneto-resistance effect element having a magnetic biasing film

Abstract: A spin valve GMR element comprises a spin valve GMR film stacked in turn a pinned layer, a non-magnetic layer, and a free layer of which magnetization direction varies according to an external magnetic field, and a magnetic biasing film providing a bias magnetic field to the free layer. A spin valve GMR film can be a dual element type. A magnetic biasing film has a stacked film of a high saturation magnetization magnetic layer and a hard magnetic layer. The high saturation magnetization magnetic layer has saturation magnetization Mshigh which, when saturation magnetization of the free layer is Msfree and saturation magnetization of the hard magnetic layer is Mshard, satisfies at least one of Mshigh ≧Msfree or Mshigh ≧Mshard. In a spin valve GMR head of a reversed structure or a dual element type, even when a track width is narrowed, occurrence of Barkhausen noise can be effectively suppressed.

Vertical polarization of quantum magnets in high density arrays of nickel dots with small height-to-diameter ratio

Abstract: Using interferometric lithography and postexposure processing we have fabricated high density (1.3×1010/in.2) magnetic nickel dot arrays on silicon substrates. The dots have the shape of truncated cones and are 75 nm in height and about 120 nm in diameter. The arrays are characterized using magnetic force microscopy (MFM). We demonstrate that such shallow dots with a small height-to-diameter ratio of only 0.63, show single-domain behavior with vertical, out-of-plane magnetization, i.e., along their short axis. The coercive field of these dots is drastically enlarged due to shape anisotropy and is exceeding the dot interaction strength by about one order of magnitude. Local manipulation of the magnetization state using an additional external field and the stray field of the MFM tip is demonstrated.

Two step resetting of magnetization of spin valve read head at the row level

Abstract: A method is provided for resetting the magnetization of the pinned and hard biasing layers of a spin valve read head at the row level. In a first embodiment of the invention a first magnetic field is applied substantially perpendicular to the air bearing surface (ABS) at room temperature for setting the magnetic moment of the pinned layer substantially perpendicular to the ABS followed by applying a second magnetic field substantially parallel to the ABS for setting the magnetic moments of the hard biasing layers substantially parallel to the ABS. In a second embodiment of the invention the antiferromagnetic pinning layer is also reset. This is done by heating the pinning layer with a current pulse conducted through the leads to the conductive layers of the spin valve head so that localized heating takes place adjacent the pinning layer as contrasted to ambient heating of the spin valve head. Simultaneous with the localized heating the first magnetic field is applied for orienting the magnetic spins of the pinning layer perpendicular to the ABS and resetting the magnetic moment of the pinned layer perpendicular to the ABS in a single domain state. Subsequently, a second magnetic field is applied for resetting the magnetic moment of the hard biasing layer parallel to the ABS in a single domain state.

Magnetoresistance due to domain walls in an epitaxial microfabricated fe wire

Abstract: The domain wall (DW) contribution to magnetoresistance has been investigated using an epitaxial microfabricated bcc (110) Fe wires of 2 μm linewidth. A strong in-plane uniaxial component to the magnetic anisotropy perpendicular to the wire axis causes a regular stripe domain pattern with closure domains. The stripe domain width in zero-applied magnetic field is strongly affected by the magnetic history and can be continuously varied from 0.45 to 1.8 μm. This enables a measurement of the resistivity as a function of DW density in a single wire. Clear evidence is presented that the resistivity is reduced in the presence of DWs at low temperatures.

Thin film magnetic head having hard films for magnetizing a shield layer in a single domain state

Abstract: Hard films consisting of a permanent magnet material are formed on both the sides of shield layers to orient the magnetization of the shield layers in an easy axis of magnetization (X direction), thereby putting the shield layers into a single domain state. Therefore, anisotropic dispersion does not occur in the shield layers, the hysteresis of a thin film magnetic head is eliminated as a whole, a coercive force becomes very small, and the magnetic reversibilities of the shield layers in a direction perpendicular to the drawing paper surface becomes preferable. For this reason, the reliability of a reproduced output signal from a magnetoresistance element layer is improved.

Non-Arrhenius behavior in single domain particles

Abstract: Thermally assisted reversal in noninteracting single domain particles is studied through the use of a micromagnetic model with a stochastic fluctuation field. The magnetization decay of these particles at high and low anisotropy fields is studied and compared to Arrhenius-Neel decay. Estimates of the attempt frequency are made by fitting to exponential decay. The attempt frequency is also examined while varying the applied field, the anisotropy field, and the temperature. Special attention is given to combinations of these variables which give a constant height of the energy barrier to reversal. It is shown that the attempt frequency can vary dramatically even for a constant height of the energy barrier. The basic scaling laws of the system are discussed, and the scaled attempt frequencies are plotted to test their universality.

On corner singularities in micromagnetics

Abstract: The magnetization distribution in the vicinity of a sharp square edge is investigated analytically, in two and three dimensions. The aim of this calculation is to investigate the consequences of the logarithmic divergence of some components of the demagnetizing field at this point, in order to understand better some recent numerical calculations by Rave, Ramstock and Hubert. A definite solution is found, that depends solely on the magnetization direction at the corner point, and displays a mild r 2 In (r) singularity. Possible applications of this solution range from a plug-in box in numerical simulations to analytical micromagnetics of square single domain particles.