Showing papers on "Single domain published in 1996"

Nanoscale Magnetic Domains in Mesoscopic Magnets

Abstract: The basic magnetic properties of three-dimensional nanostructured materials can be drastically different from those of a continuous film. High-resolution magnetic force microscopy studies of magnetic submicrometer-sized cobalt dots with geometrical dimensions comparable to the width of magnetic domains reveal a variety of intricate domain patterns controlled by the details of the dot geometry. By changing the thickness of the dots, the width of the geometrically constrained magnetic domains can be tuned. Concentric rings and spirals with vortex configurations have been stabilized, with particular incidence in the magnetization reversal process as observed in the ensemble-averaged hysteresis loops.

Magnetic storage having discrete elements with quantized magnetic moments

Abstract: A magnetic storage includes a non-magnetic substrate. A plurality of discrete single magnetic domain elements formed of a magnetic material separated by nonmagnetic materials are carried on the non-magnetic substrate. Each single magnetic domain element has the same size, shape and has, without an external magnetic field, two quantized magnetization values. The two magnetization values are of substantially equal magnitude but of differing vector directions. The plurality of single domain elements are adapted for magnetic storage of information based upon direction of the magnetization vector. Each single magnetic domain element is used to store a bit of binary information. Writing each bit becomes to flip the quantified magnetic moment directions. Each bit can be tracked individually. The switching field of each bit can be controlled by controlling the size and shape anisotropy of each bit. Methods of fabricating the magnetic storage medium include obtaining the non-magnetic substrate and forming the plurality of single magnetic domain elements on the substrate.

Quantum Tunneling of Magnetization in Nanostructured Materials

Abstract: The study of very low temperature magnetic relaxation in nanostructured materials has provided the opportunity to observe the occurrence of quantum tunneling of magnetization (QTM) in many different systems. As the magnetization is a classical vector, this effect is also referred to as macroscopic quantum tunneling (MQT). In this paper, we review the key ideas underlying magnetic relaxation as well as the results obtained for different nanostructured systems, which, to date, include single domain particles, mesoscopic grains, and high-spin polynuclear molecules.

The fabrication and magnetic properties of acicular magnetic nano-elements

Abstract: Two different lithographic techniques have been developed for fabricating magnetic nano-elements on ultra-thin electron-transparent substrates thereby allowing transmission electron microscopy to be used to investigate their properties. Here we describe studies of elements of micropolycrystalline Permalloy and cobalt whose thicknesses lie in the range 20-30 nm. Bright field images show that a high degree of edge acuity is attained and elements with lateral dimensions /spl ap/25 nm have been produced. Lorentz microscopy has provided unique insight into the detailed magnetisation reversal mechanisms in acicular elements with widths in the range 100-300 nm. In cobalt elements a single domain occupies most of the volume with complex domain structures existing very close to the ends; switching fields increase markedly with decreasing element width but are substantially independent of element length. The reversal mechanism is found to involve the propagation of a small "reversing" domain structure along the element length. In Permalloy, extensive domain structures are observed in all but the narrowest elements in the remanent state. The response to applied fields is described.

Preparation and Quantitative Magnetic Studies of Single-Domain Nickel Cylinders

Abstract: For a complete experimental and theoretical explanation of the magnetic processes in an interacting collection of submicron magnetic particles, a fundamental understanding of the magnetic properties of individual single-domain particles must first be achieved. We have prepared elongated Ni columns ranging in diameter from 0.15 to 1.0 µm by electroplating into specially prepared Al2O3 and glass channeled pore membranes. We have also prepared controlled arrays of Ni columns using e-beam lithography, subsequently electroplating into the written patterns. Using transmission electron microscopy, we have characterized the shape, size, morphology, and crystal structure of the columns. Magnetic force microscopy has been used to determine the switching field Hs versus the applied field angle of the columns. Although the switching field data can be fit to the functional form for nucleation by curling in an infinite cylinder, the observed weak dependence of Hs on column diameter is inconsistent with that expected for curling, particularly for columns of diameter 0.3 µm.

Magnetization reversal dynamics in ultrathin magnetic layers

Abstract: We propose an analytical expression of the magnetization reversal in ultrathin magnetic layers, modeling the after effects, and taking into account both domain-wall motion and nucleation processes. We apply our modelization to quantify the dynamical properties of the magnetization of ${\mathrm{MoS}}_{2}$/Au/Co/Au sandwiches; we define the Barkhausen volume, the wall velocity, and the nucleation rate depending on the applied magnetic field and the radius and the density of nuclei sites on the surface of the sample. Finally, we investigate an original behavior of the switch of the magnetization in high dynamic regime. An attempt is made to explain the divergence of the coercive field in large field variation rates (dH/dt), by introducing both the thermally activated and the viscous processes driving the wall motion. \textcopyright{} 1996 The American Physical Society.

A magnetoelastic torque transducer utilizing a ring divided into two oppositely polarized circumferential regions

Abstract: Significant improvements in both transducer performance and constructional simplicity is achieved by dividing a single ring into two equal width regions having oppositely directed circumferential magnetizations. The square hysteresis loop associated with the uniaxial anisotropy in the ring enables this magnetization arrangement to be readily instilled by rotating the shaft/ring assembly in the dipole fields of a set of axially separated permanent magnets having opposing polarizations. Following the removal of the magnets, each such region effectively becomes a single domain separated from the other only by a centrally located 180° domain wall. Unchanging performance of the transducer throughout nearly 107 torque cycles, severe overloads, and a wide range of thermal environments attest to the stability of both the remanent magnetizations in each region and the position of the domain wall.

Gels with magnetic properties

Abstract: Materials producing strain in magnetic field are known as magnetoelastic or magneto strictive materials. A new type has been developed by preparing magnetic field sensitive gels, called ferrogels. Single domain, magnetic particles of colloidal size are incorporated into chemically cross-linked polyvinyl-alcohol hydrogels. The finely distributed colloidal particles having superparamgnetic behavior couple the shape of the gel to the nonuniform external magnetic field. We have shown that ferrogels undergo quick and reversible shape transformation by changes in external magnetic field. Elongation, contraction and bending can be realized by proper arrangement of external magnetic field. Unidirectional deformation measurements have been performed and an equation for the uniaxial magnetoelastic properties has been derived and compared with experimental results.

Imaging and magnetometry of switching in nanometer‐scale iron particles

Abstract: The reversal mechanisms in arrays of nanometer‐scale (<40 nm diameter) iron particles are studied by low‐temperature Hall magnetometry and room‐temperature magnetic force microscopy. Rotation of the net array magnetization at low temperatures (20 K) occurs by both reversible and irreversible modes, the latter revealed by Barkhausen jumps. Spatially resolved measurements at room temperature show the particles to be single domain with remanence and coercivity indicating they are not superparamagnetic. Individual particles are observed to switch irreversibly over a small field range (<10 Oe) between preferred magnetic directions parallel to the growth direction of the particles. Scaling of the arrays offers the possibility of magnetic storage at the 45 Gbit/in.2 level, nearly 50 times greater than current technology.

Model of thermally activated magnetization reversal in thin films of amorphous rare-earth-transition-metal alloys

Abstract: Monte Carlo simulations on a two-dimensional lattice of magnetic dipoles have been performed to investigate the magnetic reversal by thermal activation in rare-earth-transition-metal (RE-TM) alloys. Three mechanisms of magnetization reversal were observed: nucleation dominated growth, nucleation followed by the growth of magnetic domains containing no seeds of unreversed magnetization, and nucleation followed by dendritic domain growth by successive branching in the motion of the domain walls. The domain structures are not fractal; however, the fractal dimension of the domain wall was found to be a good measure of the jaggedness of the domain boundary surface during the growth process. The effects of the demagnetizing field on the hysteretic and time-dependent properties of the thin films were studied and some limitations in the application of the Fatuzzo model on magneto-optic media are identified.

Hysteresis models for the description of domain wall motion

Abstract: A physically based hysteresis model for materials where magnetic domain wall motion is the main magnetization mechanism is presented. The behavior of a single domain wall (DW) is described by a restriction of the Preisach model and the superposition of many DW contributions is then treated by introducing a multiplicative factor describing the dependence of the DW surface on magnetization. This model exhibits wiping-out property and a new kind of minor loop congruency.

Microstructure and Magnetic Properties of High-Coercive Fe-Pt Alloy Thin Films

Abstract: We investigated microstructures and magnetic domain structures of sputtered FePt alloy thin films in order to elucidate the origin of the high coercive force Hc. The FePt alloy thin films were prepared by RF sputtering method on water-cooled glass substrates. Transmission electron microscopy (TEM) shows that the as-deposited film consists of an fcc Y phase with a grain size of 10-20 nm. Rippled domains were observed in the Lorentz micrographs. After annealing at 823 K, a steep increase in Hc up to ∼800 kA/m and a decrease in resistivity p were observed. The TEM observation indicates that the annealed film consists of an fct γ 1 phase with a grain size of 20 - 80 nm. From random patterns of grain size scale in the Lorentz micrograph, it is suggested that the high Hc might be explained as being due to a rotation of magnetization for each grain with a single-domain state. The Hc of the annealed samples increases with film thickness up to a thickness of 100 nm. From the thermomagnetic analysis (TMA), it is implied that the order-disorder transformation occurs at T > 620 K ; the Tc of the disordered γ and ordered γ 1 phases were evaluated to be 599 K and 739 K, respectively.

Fine Metallic Particles for Magnetic Domain Observations

Abstract: Fine domain structures of a ferromagnetic specimen can be visualized by deposition of ultrafine ferromagnetic particles onto its surface. We have applied various ferromagnetic particles, such as Co, Fe, Co–Fe alloy and Permalloy, to observation of high-density recording states of thin-film recording media. These studies have revealed that single-domain particles mainly contribute to the domain pattern contrast, while very small superparamagnetic particles are so insensitive to the leakage field from the recording media that they deteriorate the image quality of the domain patterns. Therefore, suppressing the formation of the superparamagnetic particles is effective in improving the resolution and the image quality of the observed domain patterns. In order to suppress the formation of superparamagnetic particles, we have attempted to synthesize hcp-Co particles with large uniaxial anisotropy by alloying Co with some elements which increase the fcc hcp transformation temperature, but all the Co and Co-alloy particles exhibit a pure high-temperature fcc phase.

Magnetic force microscopy of single‐domain single‐crystal iron particles with uniaxial surface anisotropy

Abstract: Lithographic patterning techniques have been used to fabricate arrays of submicron particles from a [110] single‐crystal iron thin film with a strong uniaxial surface anisotropy and with an easy axis of magnetization lying in the [001] direction (in the plane of the film). Magnetic force microscopy images indicate that these islands are single domain over a wide range of island shapes and sizes. The uniaxial surface anisotropy is stronger than the shape anisotropy for the island geometries used, so the easy axes of the islands all lie roughly in the [001] direction, regardless of the island shape. Magnetic force images were also taken as both the magnetic tip and sample were subjected to a gradually increasing externally applied field. This technique allows us to monitor the magnetization reversal of individual islands and provides a direct measure of their switching fields.

Magnetic properties and dynamic domain behavior in grain-oriented 3% Si-Fe

Abstract: The magnetic properties of soft magnetic materials are based on dynamic behavior of magnetic domain walls. Many domain structure studies have contributed to the progress of magnetic materials. In this paper, the present state of new development in advanced grain-oriented silicon steel is introduced through the aid of dynamic domain observation of real materials with forsterite film. The basic magnetic phenomena of material, such as the dynamic behavior of surface closure domains, domain wall pinning, and domain structure under rotating magnetization, are explained mainly through the observation of static and dynamic domain wall movements under a high-voltage scanning electron microscope. A few techniques for manufacturing the latest grain-oriented silicon steel, such as improvement in alignment with (110) [001] orientation, increase in the number of mobile domain walls, and relaxation of domain wall pinning, are also described through the observation of dynamic domain patterns. Examples of core losses are given for ideal grain-oriented silicon steel as industrial material, and future developments are predicted.

Electron beam fabrication and characterization of high- resolution magnetic force microscopy tips

Abstract: The stray field, magnetic microstructure, and switching behavior of high‐resolution electron beam fabricated thin film tips for magnetic force microscopy (MFM) are investigated with different imaging modes in a transmission electron microscope (TEM). As the tiny smooth carbon needles covered with a thermally evaporated magnetic thin film are transparent to the electron energies used in these TEMs it is possible to observe both the external stray field emanating from the tips as well as their internal domain structure. The experiments confirm the basic features of electron beam fabricated thin film tips concluded from various MFM observations using these tips. Only a weak but highly concentrated stray field is observed emanating from the immediate apex region of the tip, consistent with their capability for high resolution. It also supports the negligible perturbation of the magnetization sample due to the tip stray field observed in MFM experiments. Investigation of the magnetization distributions within the tips, as well as preliminary magnetizing experiments, confirm a preferred single domain state of the high aspect ratio tips. To exclude artefacts of the observation techniques both nonmagnetic tips and those supporting different magnetization states are used for comparison.

Synthesis of single domain strontium ferrite powder by pulsed laser ablation

Abstract: Strontium hexaferrite nanoparticles have been successfully prepared from a sintered SrFe12O19 target, using pulsed excimer laser ablation and cold condensation technique. The as‐condensed powders do not exhibit the same magnetic properties as the target material. However, annealing at a high temperature of 850 °C for 3 h in air followed by fast cooling yields very high coercivity of 6665 Oe, exhibiting presence of single domain particles. Vibrating sample magnetometry (VSM), scanning electron microscopy (SEM), and x‐ray diffraction (XRD) techniques are used to study the magnetism, microstructure, and phase formation in the powders, respectively.

Magnetic properties of the stable fraction of remanence in large multidomain (MD) magnetite grains: Single-domain or MD?

Abstract: It has been recognized since the early work of Verhoogen (1959) that a considerahle proportion of remanence in multidomain (MD) magnetite grains is resistant to low-field a.f. or low-temperature demagnetization. The source of this high stability is still a matter of debate. A number of workers have suggested that MD grains of all sizes contain a remanence fraction with truly single domain (SD) character. We suggest that the critical diagnostic features which should be investigated to determine whether the high stability fraction is SD or MD in character are whether blocking (T b ) and unblocking (T ub ) temperatures are equivalent, and whether the intensity of remanence is affected by the thermal pre-history of the sample. We have carried out such experiments on samples containing crushed natural magnetites in 7 grain sizes from 5-10 μm to 100-150 μm. We show that T b and T ub are equivalent for pTRM 400 20 for grain sizes up to 15-20 μm, but that T ub extends up to the Curie temperature for larger grain sizes. We also show that the stable fraction of MD TRM and pTRM has the same dependence on pre-history as the total TRM. Out experiments demonstrate that the stable fraction has magnetic properties which are truly MD in character for magnetite grains larger than 20 μm.

Nanoscale magnetic particles: Synthesis, structure and dynamics

Abstract: In recent years, new synthetic routes, which include wet techniques and synthesis in confined geometries, have been developed for the preparation of nanoscale magnetic particles. The shape of the particles obtained is usually smooth and rounded all over, due to the influence of the surface energy. The final shape, however, depends on the preparation method and is much influenced by the substrate used, as this may stress and deform the particles in order that they adapt to it. Changes in lattice parameters, in comparison to the bulk, and surface modifications give way in many cases to changes in the magnetic and transport properties. For very small ferromagnetic metal particles (clusters), both enhanced magnetism, due to localization of the valence d-electrons, and giant magnetic moments for some nonmagnetic clusters are observed. Magnetic phenomena in real single-domain particles reveal that the statistics of the magnetization reversal cannot be described by a thermal activation over a single-energy barrier. Macroscopic quantum tunnelling of magnetization is observed at very low temperatures in several systems. Other interesting electrical and magnetic phenomena, such as finite size effects, coercivity enhancement due to surface effects, and giant magnetoresistance, are observed in fine particle systems.

Magnetic viscosity and switching volumes of annealed Fe/Pt multilayers

Abstract: Magnetic viscosity and the phenomenon of the sweep‐rate dependence of the coercivity are important for the characterization of magnetic recording media. The activation volume of magnetization reversal or switching volume is directly related to medium noise. The magnetic viscosity behavior of annealed Fe/Pt multilayers was studied and the switching volumes were obtained by both measurements of the sweep‐rate dependence of coercivity and time dependence of magnetization. It is found that samples with larger coercivities have smaller switching volumes, and an estimate of the magnetic grain diameter is about 9 nm. The coercivity mechanisms are also discussed.

Heat treatment of strontium hexaferrite powder in nitrogen, hydrogen and carbon atmospheres: a novel method of changing the magnetic properties

Abstract: Strontium hexaferrite powder has been treated in nitrogen, hydrogen and carbon atmospheres. The results show that the phase composition and morphology, and hence, the magnetic properties of the strontium hexaferrite are affected significantly by these gas/vapour treatments. Generally, the coercivity decreased to below 0.8 kOe (regardless of the initial coercivity) and the magnetization at 14 kOe increased significantly, when strontium hexaferrite powder had been treated in a nitrogen, hydrogen or carbon atmosphere. However, it was found that a post-gas treatment of calcination in air, under appropriate conditions, resulted in a recovery of the hexaferrite structure (i.e. it is a reversible reaction). However, the particle/grain sizes of the calcined samples were significantly smaller than those of the non-treated samples, and it is believed that they were single domain particles/grains. In some cases, the coercivity increased by about 400%. The magnetization at 14 kOe and the remanence were either not affected or sometimes increased; magnetic measurements indicated a preferred orientation of the grains.

Switching properties of a regular two-dimensional array of small uniaxial particles

Abstract: Switching properties of a regular square lattice of 40/spl times/40/spl times/3 /spl mu/m/sup 3/ highly uniaxial, single domain magnetic garnet pixels have been measured magnetooptically. "Up" and "down" switching fields, coercivities and interaction fields were measured on an assembly of hundreds of particles. Statistical analysis of the data shows the relationship between major loop parameters and mean values of the distributions. Local demagnetizing tensor elements were calculated for the non-ellipsoidal geometry and the interaction fields from different neighbors were computed. The importance of the effect of the geometry, shape and size of particles on the switching properties is demonstrated. The agreement with experimental values is very good. The simplicity of this model system is attractive for studying artificially structured 2-D arrays of small particles.

Analytical and computational study of magnetization switching in kinetic ising systems with demagnetizing fields

Abstract: An important aspect of real ferromagnetic particles is the demagnetizing field resulting from magnetostatic dipole-dipole interactions, which causes large particles to break up into equilibrium domains. Sufficiently small particles, however, remain single domain in equilibrium. This makes them particularly promising as materials for high-density magnetic recording media. In this paper we use analytic arguments and Monte Carlo simulations to quantitatively study the effects of the demagnetizing field on the dynamics of magnetization switching in two-dimensional, single-domain, kinetic Ising systems. For systems in the weak-field {open_quote}{open_quote}stochastic region,{close_quote}{close_quote} where magnetization switching is on average effected by the nucleation and growth of a single droplet, the simulation results can be explained by a simple model in which the free energy is a function only of magnetization. In the intermediate-field {open_quote}{open_quote}multidroplet region,{close_quote}{close_quote} a generalization of Avrami{close_quote}s law involving a magnetization-dependent effective magnetic field gives good agreement with the simulations. The effects of the demagnetizing field do not qualitatively change the droplet-theoretical picture of magnetization switching in highly anisotropic, single-domain ferromagnetic grains, which we recently proposed [J. Magn. Magn. Mater. {bold 150}, 37 (1995)]. {copyright} {ital 1996 The American Physical Society.}

Giant-magnetoresistance anomaly associated with a magnetization process in UFe4Al8.

Abstract: Magnetoresistance and magnetization measurements on a UFe4Al8 single crystal are reported. A strong anisotropic magnetoresistance is found in this compound and is used to prove the ferromagnetic order of the U lattice. A strong anomaly in the parallel and perpendicular magnetoresistance is interpreted as an unusual magnetization process in which the magnetization remains blocked perpendicularly to the applied magnetic field. This interpretation is confirmed by the magnetization measurements.

Micromagnetic properties and magnetization switching of single domain Co dots studied by magnetic force microscopy

Abstract: Magnetic force microscopy was applied to study the magnetic properties of Co dot microstructures. The high density magnetic dot arrays were fabricated using nanolithographic techniques on GaAs substrates. The ferromagnetic Co dots were found to be in a single domain state for Co film thicknesses of 7 nm and 17 nm. The magnetization of the as-prepared Co dot array was found to be in a non-uniform state. After applying a magnetic field the Co dots are in a uniform magnetization state. Induced switching of the magnetization of single Co dots by the stray field of the probing tip using an additionally applied in-situ magnetic field has been demonstrated.