Showing papers on "Magnetic domain published in 2000"

Room Temperature Magnetic Quantum Cellular Automata

Abstract: All computers process information electronically. A processing method based on magnetism is reported here, in which networks of interacting submicrometer magnetic dots are used to perform logic operations and propagate information at room temperature. The logic states are signaled by the magnetization direction of the single-domain magnetic dots; the dots couple to their nearest neighbors through magnetostatic interactions. Magnetic solitons carry information through the networks, and an applied oscillating magnetic field feeds energy into the system and serves as a clock. These networks offer a several thousandfold increase in integration density and a hundredfold reduction in power dissipation over current microelectronic technology.

Property variation with shape in magnetic nanoelements

Abstract: Nanometre scale magnetic particles (`nanoelements' or `nanomagnets') form a rich and rapidly growing new area in condensed matter physics, with many potential applications in data storage technology and magnetic field sensing. This paper reviews an extensive study into the influence of shape on the properties of nanomagnets in the size range 35-500 nm. Elliptical, triangular, square, pentagonal and circular geometries have all been considered. It is shown that the size, thickness and geometric shape of nanomagnets all play a vital role in determining the magnetic properties. The shape, size and thickness of a nanomagnet are shown to be linked to its magnetic properties by two distinct phenomena. The first is called configurational anisotropy and describes the role played by small deviations from uniformity in the magnetization field within the nanostructures, which allow unexpected higher-order anisotropy terms to appear. These anisotropies can often dominate the magnetic properties. The second is the competition which exists between exchange energy and magnetostatic energy. This competition determines whether the nanomagnets exhibit single domain or incoherent magnetization and also controls the non-uniformities in magnetization which lead to configurational anisotropy. Understanding the influence of shape opens the way to designing new nanostructured magnetic materials where the magnetic properties can be tailored to a particular application with a very high degree of precision.

Direct observation of the alignment of ferromagnetic spins by antiferromagnetic spins

Abstract: The arrangement of spins at interfaces in a layered magnetic material often has an important effect on the properties of the material. One example of this is the directional coupling between the spins in an antiferromagnet and those in an adjacent ferromagnet, an effect first discovered1 in 1956 and referred to as exchange bias. Because of its technological importance for the development of advanced devices such as magnetic read heads2 and magnetic memory cells3, this phenomenon has received much attention4,5. Despite extensive studies, however, exchange bias is still poorly understood, largely due to the lack of techniques capable of providing detailed information about the arrangement of magnetic moments near interfaces. Here we present polarization-dependent X-ray magnetic dichroism spectro-microscopy that reveals the micromagnetic structure on both sides of a ferromagnetic–antiferromagnetic interface. Images of thin ferromagnetic Co films grown on antiferromagnetic LaFeO3 show a direct link between the arrangement of spins in each material. Remanent hysteresis loops, recorded for individual ferromagnetic domains, show a local exchange bias. Our results imply that the alignment of the ferromagnetic spins is determined, domain by domain, by the spin directions in the underlying antiferromagnetic layer.

Micromagnetic modelling—the current state of the art

Abstract: The increasing information density in magnetic recording, the miniaturization in magnetic sensor technology, the trend towards nanocrystalline magnetic materials and the improved availability of large-scale computer power are the main reasons why micromagnetic modelling has been developing extremely rapidly. Computational micromagnetism leads to a deeper understanding of hysteresis effects by visualization of the magnetization reversal process. Recent advances in numerical simulation techniques are reviewed. Higher order finite elements and adaptive meshing have been introduced, in order to reduce the discretization error. The use of a hybrid boundary/finite element method enables accurate stray field computation for arbitrary shaped particles and takes into account the granular microstructure of the material. A dynamic micromagnetic code based on the Gilbert equation of motion to study the time evolution of the magnetization has been developed. Finite element models for different materials and magnet shapes are obtained from a Voronoi construction and subsequent meshing of the polyhedral regions. Adaptive refinement and coarsening of the finite element mesh guarantees accurate solutions near magnetic inhomogeneities or domain walls, while keeping the number of elements small. The polycrystalline microstructure and assumed random magnetocrystalline anisotropy of elongated Co elements decreases the coercive field and the switching time compared to zero anisotropy elements, in which vortices form and move only after a certain waiting time after the application of a reversed field close to the coercive field. NiFe elements with flat, rounded and slanted ends show different hysteresis properties and switching dynamics. Micromagnetic simulations show that the magnetic properties of intergranular regions in nucleation-controlled Nd-Fe-B hard magnetic materials control the coercive field. Exchange interactions between neighbouring soft and hard grains lead to remanence enhancement of isotropically oriented grains in nanocrystalline composite magnets. Upper limits of the coercive field of pinning-controlled Sm-Co magnets for high-temperature applications are predicted from the micromagnetic calculations. Incorporating thermally activated magnetization reversal and micromagnetics we found complex magnetization reversal mechanisms for small spherical magnetic particles. The magnetocrystalline anisotropy and the external field strength determine the switching mechanism. Three different regimes have been identified. For fields, which are smaller than the anisotropy field, magnetization by coherent switching has been observed. Single droplet nucleation occurs, if the external field is comparable to the anisotropy field, and multi-droplet nucleation is the driving reversal process for higher fields.

Influence of Dipolar Interaction on Magnetic Properties of Ultrafine Ferromagnetic Particles

Abstract: We use Monte Carlo simulations to study the influence of dipolar interaction and polydispersity on the magnetic properties of single-domain ultrafine ferromagnetic particles. From the zero field cooling (ZFC)/field cooling (FC) simulations we observe that the blocking temperature ${T}_{\mathrm{B}}$ clearly increases with increasing strength of interaction, but it is almost not effected by a broadening of the distribution of particle sizes. While the dependence of the ZFC/FC curves on interaction and cooling rate are reminiscent of a spin glass transition at ${T}_{\mathrm{B}}$, the relaxational behavior of the magnetic moments below ${T}_{\mathrm{B}}$ is not in accordance with the picture of cooperative freezing.

Observation of antiferromagnetic domains in epitaxial thin films

Abstract: Antiferromagnetic domains in an epitaxial thin film, LaFeO3 on SrTiO3(100), were observed using a high-spatial-resolution photoelectron emission microscope with contrast generated by the large x-ray magnetic linear dichroism effect at the multiplet-split L edge of Fe. The antiferromagnetic domains are linked to 90i twinned crystallographic regions in the film. The Ne «el temperature of the thin film is reduced by 70 kelvin relative to the bulk material, and this reduction is attributed to epitaxial strain. These studies open the door for a microscopic understanding of the magnetic coupling across antiferromagneticferromagnetic interfaces. The current interest in magnetism is largely based on atomically engineered thin film structures, owing to the interesting physics of these materials and their technological use in the

Giant magnetostriction in an ordered Fe3Pt single crystal exhibiting a martensitic transformation

Abstract: Magnetostriction measurements have been made in an ordered Fe3Pt single crystal with degree of order of about 0.8, which exhibits a cubic-tetragonal martensitic transformation at 97 K. The specimen was cooled down to 4.2 K without magnetic field, and then a magnetic field of 4 T is applied to the specimen along 〈001〉 at 4.2 K and removed. As a result, a reversible giant magnetostriction of about 0.5% is observed. This reversible magnetostriction will be caused by the rearrangement of crystallographic domains, being three times as large as that of Terfenol-D (Fe2DyxTb1−x: typical magnetostrictive materials).

Thermally assisted magnetization reversal in submicron-sized magnetic thin films

Abstract: We have measured the rate of thermally assisted magnetization reversal of submicron-sized magnetic thin films. For fields H just less than the zero-temperature switching field H(C), the probability of reversal, P(exp)(s)(t), increases for short times t, achieves a maximum value, and then decreases exponentially. Micromagnetic simulations exhibit the same behavior and show that the reversal proceeds through the annihilation of two domain walls that move from opposite sides of the sample. The behavior of P(exp)(s)(t) can be understood through a simple "energy-ladder" model of thermal activation.

Magnetic properties and magnetization reversal of α-Fe nanowires deposited in alumina film

Abstract: Uniform arrays of Fe nanowires were prepared by electrochemical deposition of iron into nanoporous anodic aluminum oxide films. The microstructure and crystal structures of the nanowires were studied by transmission electron microscopy and electron diffraction. It was found that each nanowire looked like a chain of dots and each dot in the chain was supposed to be a single crystal of α-Fe. Each dot was shown to be a single magnetic domain. The magnetic properties of a uniform array of Fe nanowires and the magnetization reversal in a Fe nanowire were investigated by Mossbauer spectroscopy and vibrating sample magnetometry, which demonstrated that the film of Fe nanowires in alumina had superior perpendicular magnetic characteristics. The magnetic studies also revealed that the moments of each single domain dot were oriented along the chain. Experimental results could be interpreted by the reversal model of “chains of spheres” with the symmetric fanning mechanism.

Lorentz microscopy of circular ferromagnetic permalloy nanodisks

Abstract: Circular permalloy elements were fabricated by a combination of electron beam lithography, thermal evaporation and liftoff technique on electron transparent membrane substrates. The magnetic properties have been studied by Lorentz transmission electron microscopy. In situ magnetizing experiments have been carried out to obtain information about the nucleation and propagation of magnetic domains within the permalloy nanodisks and to determine the nucleation and saturation fields. The diameter of the patterned elements has been varied between 180 and 950 nm, the height was 15 nm. The experiments showed that the vortex configuration is the most favorable state in zero field conditions of all investigated permalloy nanodisks.

Magnetic pinning in superconductor-ferromagnet multilayers

Abstract: We argue that superconductor/ferromagnet multilayers of nanoscale period should exhibit strong pinning of vortices by the magnetic domain structure in magnetic fields below the coercive field when ferromagnetic layers exhibit strong perpendicular magnetic anisotropy. The estimated maximum magnetic pinning energy for single vortex in such a system is about 100 times larger than the pinning energy by columnar defects. This pinning energy may provide critical currents as high as 106−107 A/cm2 at high temperatures (but not very close to Tc) at least in magnetic fields below 0.1 T.

Low-Frequency Magnetic Noise in Micron-Scale Magnetic Tunnel Junctions

Abstract: We have observed low-frequency noise due to quasiequilibrium thermal magnetization fluctuations in micron-scale magnetic tunnel junctions (MTJs). This strongly field-dependent magnetic noise occurs within the magnetic hysteresis loops, either as $1/f$ or Lorentzian (random telegraph) noise. We attribute it to the thermally excited hopping of magnetic domain walls between pinning sites. Our results show that magnetic stability is a crucial factor in reducing the low-frequency noise in small MTJs.

Magnetic-field-induced twin boundary motion in magnetic shape-memory alloys

Abstract: This paper reports direct microscopic evidence of magnetoelastic coupling between ferroelastic twin domains and ferromagnetic Weiss domains in magnetic shape memory alloys, using the Ni-Mn-Ga Heusler alloys. In a martensitically transformed ${\mathrm{Ni}}_{2+x}{\mathrm{Mn}}_{1\ensuremath{-}x}\mathrm{Ga}$ single crystal, the magnetic domains were found to be superimposed upon the martensite twin domains. Simultaneous observation of magnetic domains and twin domains as a function of applied magnetic field shows that concomitant with the reconfiguration of magnetic domains in an applied magnetic field, ferroelastic twin domains also readjust their relative volume fraction in order to accommodate to the externally applied magnetic field. This readjustment is shown to occur by the displacement of twin domain walls. As a result, the volume fraction of one set of favorably oriented twin domains (with respect to magnetoelastic energy) increase at the expense of another set of unfavorably oriented twin domains. These studies would enable subsequent quantitative correlation between the magnetic-field-induced macroscopic strain and the microscopic self-adjustment of twin domains.

Size dependence of the exchange bias field in NiO/Ni nanostructures

Abstract: NiO/Ni wires have been investigated as a function of their width in order to investigate the size dependence of exchange bias. The samples have been prepared by e-beam lithography and ion milling of ion beam sputtered thin films. For NiO/Ni wires narrower than 3 μm, the exchange bias field significantly depends on the wire width. A NiO/Ni film shows an exchange bias field of −78 Oe whereas the exchange bias field of wires narrower than 200 nm is reduced to approximately −40 Oe. The coercive field of the NiO/Ni film is 28 Oe and increases to 210 Oe for the narrowest wires. The decrease of the exchange bias field for the narrowest wires is consistent with a recent microscopic model of exchange bias where the appearance of a unidirectional anisotropy in ferromagnet/antiferromagnet bilayers has been attributed to the presence of antiferromagnetic domains in the bulk of the antiferromagnet. A possible onset of a transition from a multidomain to a single-domain state of the antiferromagnet as a function of the NiO/Ni wire width seems to be the origin for the observed decrease of the exchange bias field for narrow wires.

Magnetoresistive effect device utilizing a magnetization-coupling layer which couples adjacent ferromagnetic layers perpendicularly

Abstract: A magnetoresistance effect element has two ferromagnetic films separated by an interlayer film coupling the magnetization of one ferromagnetic layer in a direction perpendicular to the magnetization direction of the other ferromagnetic film, with an antiferromagnetic layer disposed adjacent to one of the ferromagnetic layers, and a free magnetic layer disposed adjacent to an antiferromagnetic film. The heat treatment for producing in the free layer a simple magnetic domain and the heat treatment for fixing the magnetizations of the ferromagnetic layers are simultaneously carried out. Thereby, because maintaining a difference between the blocking temperature of the antiferromagnetic layer adjacent to the free layer and the blocking temperature of an antiferromagnetic layer adjacent to the pin layer becomes unnecessary, an antiferromagnetic layer having a high exchange coupling magnetic field and a high blocking temperature can be selected. Also, because the allowable range to the dispersion of the exchange coupling magnetic field is widen, thinning of the film of the antiferromagnetic layer can be realized and the magnetoresistance effect element can be suitably applied to a magnetic reproducing head requiring a narrow gap.

Magnetic polarons in a single diluted magnetic semiconductor quantum dot.

Abstract: Quasi-zero-dimensional magnetic polarons in single Cd0.93Mn0.07Te/Cd0.6Mg0.4Te quantum dots have been studied by photoluminescence spectroscopy. By comparing the experimental data with model calculations, the energy, the internal magnetic field, and the volume of the magnetic polarons are obtained. Moreover, the magnetic environment of the recombining electron hole pair causes a distinct broadening of the emission line (;4 meV) of one diluted magnetic single quantum dot. The alignment of the Mn-spins in high magnetic fields results in a linewidth narrowing of almost one order of magnitude and the linewidth becomes comparable to that of a nonmagnetic Cd0.93Mg0.07Te/Cd0.6Mg0.4Te reference sample. A variety of electronic or magnetic properties of a solid can be artificially tailored by including magnetic ions in a semiconductor crystal matrix. This has been shown, e.g., in diluted magnetic semiconductors ~DMSs!, where the strong sp -d exchange interaction between the charge carriers and the magnetic Mn 21 ions results in a giant Zeeman splitting of the valence- and conduction-band states, a large Faraday rotation, and the formation of magnetic polarons ~MPs!. 1‐3

Resistance spikes at transitions between quantum hall ferromagnets.

Abstract: We report a manifestation of first-order magnetic transitions in two-dimensional electron systems. This phenomenon occurs in aluminum arsenide quantum wells with sufficiently low carrier densities and appears as a set of hysteretic spikes in the resistance of a sample placed in crossed parallel and perpendicular magnetic fields, each spike occurring at the transition between states with different partial magnetizations. Our experiments thus indicate that the presence of magnetic domains at the transition starkly increases dissipation, an effect also suspected in other ferromagnetic materials. Analysis of the positions of the transition spikes allows us to deduce the change in exchange-correlation energy across the magnetic transition, which in turn will help improve our understanding of metallic ferromagnetism.

Magnetic ground state of a thin-film element

Abstract: By means of three-dimensional numerical calculations we studied possible micromagnetic configurations in a rectangular Permalloy-like thin-film element. The parameters were chosen to be compatible with the so-called micromagnetic standard problem 1. We demonstrate that for these parameters a diamond domain pattern is the lowest energy state that replaces cross-tie patterns favorable in larger elements. Only at smaller sizes does the originally envisaged Landau pattern form the ground state. The transition to high-remanence structures (or what would be comparable to a "single-domain" state) is found for lateral sizes that are an order of magnitude smaller than the benchmark parameters. The transitions among the different domain patterns become plausible in view of the energy of symmetric Neel walls in extended thin films. The features of the high-remanence structures can be derived from the principle of uniform charge distribution.

Antiferromagnetic thickness dependence of exchange biasing

Abstract: A theory for a ferromagnetic/antiferromagnetic (FM/AF) exchange coupled bilayer of finite thickness is presented. Calculations based on this theory describe the reversible and irreversible transitions of the magnetic moments in this FM/AF system. A description of the exchange bias effect is offered that explains the observed phenomena of enhanced coercivity and rotational hysteresis. The theory also explains the AF thickness dependence of the exchange field and the coercivity.

Colloidal quantum dots. From scaling laws to biological applications

Abstract: Over a twenty-year period, condensed matter physicists and physical chemists have elucidated a series of scaling laws which successfully describe the size dependence of solid state properties (1,2). Often the experiments were performed under somewhat exotic conditions, for instance on mass-selected clusters isolated in molecular beams or on quantum dots grown by molecular beam epitaxy and interrogated at low temperatures and in high magnetic fields. As a result, we now have an understanding of how thermodynamic, optical, electrical, and magnetic properties evolve from the atomic to the solid state limit. This area of research is presently undergoing a remarkable transformation. The scaling laws, previously the direct subject of research, now provide a tool for the design of advanced new materials. In the case of colloidal quantum dots, or semiconductor nanocrystals, these new insights are poised to have impact in disciplines remote from solid state physics (3). NANOCRYSTAL AS "SINGLE STRUCTURAL DOMAIN" Industrial processing of semiconductor materials is our most advanced technology, an amazing but also very complicated affair. Because even a few defects or impurities can alter the performance of a device, great effort is extended to ensure precise control of atomic composition at every stage. While very expensive, this is necessary for materials that are comprised of large (order microns) component domains. Using some scaling laws from magnetism and phase transitions as a guide, we can quickly see that it is possible, and indeed perhaps preferable, to prepare extremely high-quality nanometer-size components in more simple ways. One of the most famous size-dependent scaling laws concerns the variation of magnetic proper- ties as a function of the size. Over fifty years ago, Neel provided a theory that successfully describes the size dependence of magnetization reversal (4). In a small magnetic crystal the spins within each unit cell couple to each other, resulting in a single, giant magnetic moment (single magnetic domain). An exter- nal field can be used to align this magnetic moment. If this field is now reversed rapidly, it will take time for the magnetic moment of the crystal to realign. In the simple model of Neel, this time depends exponentially upon the volume of the small crystal. This can be understood if each unit cell of the crystal contributes equally to an energetic barrier (the crystalline anisotropy) that must be overcome thermally. The larger the crystal, the greater the barrier. Interestingly, recent studies of magnetization reversal in individual single domain magnets show that the Neel model provides a qualitatively correct picture, despite many simplifying assumptions (5) (chief among them neglect of the surface). Even more recent work suggests that in extremely small, molecular-size magnets, the magnetic moment changes in a discrete series of steps, corresponding to tunneling of the magnetization (6). How can this scaling law for magnetization reversal teach us anything about processing of na- nometer size materials? Consider what happens in ever-larger magnetic crystals. A sufficiently large

Geometry dependence of magnetization vortices in patterned submicron NiFe elements

Abstract: When the magnetization is switched in patterned submicron Ni80Fe20 thin-film elements, magnetization vortices, local micromagnetic structures with a core size of 10 nm, are often trapped. This lowers the magnetostatic self-energy of the elements. Trapped vortices can cause anomalous switching during spin reversal between stable magnetization states. To expel trapped vortices, a large external magnetic field, the vortex exit field, is required. The dependence of the vortex exit field on lateral dimensions of the patterned elements is measured. In the limit where the lateral dimensions of the patterned elements are much larger than the vortex core size, smaller structures are more prone to the formation of trapped vortices.

Suppression of exchange bias by ion irradiation

Abstract: The exchange bias effect in ferromagnetic/antiferromagnetic sandwich structures is generally believed to be sensitive on the interface exchange interaction, the magnetization, and the thickness of the ferromagnetic layer. Also the interface structure plays a crucial role. We show that, by irradiating samples with He ions, we can manipulate the exchange bias field in a controlled manner. Depending on the dose (1014–1017 ions/cm2) and the acceleration voltage (10–35 kV) of the ions, the shift of the hysteresis can be reduced or even fully suppressed. Potential applications of this effect for magnetic patterning on the nanoscale will be discussed.

Novel magnetostrictive memory device

Abstract: A stress-operated memory device consisting of an ellipsoidal magnetic particle array and an electrostrictive grid is proposed. In the device, the magnetic state of the particle can be controlled only by the magnetostriction effect. Each particle is located at the intersection of the grid and has an in-plane uniaxial anisotropy. A pair of electric contacts is connected to the end of each wire. In the writing process, the driving voltages are simultaneously applied to two pairs of the selected contacts. This allows to apply a local electric field whose direction and amplitude can be regulated by varying the voltage intensity and polarity. The exerting stress on the magnetic particle results in the linear magnetostriction and hence an additional anisotropy energy in the particle. The in-plane total energy minimum, corresponding to the magnetization direction, follows the local electric field. Consequently the magnetization of the single magnetic particle located at the intersection can therefore be selective...

Pulsed-current-induced domain wall propagation in Permalloy patterns observed using magnetic force microscope

Abstract: Pulsed-current controlled wall motion in 20 /spl mu/m wide/spl times/200 /spl mu/m long/spl times/160 nm thick patterned Permalloy strips was studied using magnetic force microscopy. By sequential imaging, the displacement of Bloch walls as far as 200 /spl mu/m along the strip was observed. The direction of motion was in the same direction as the carrier velocity, which reversed with current polarity. The displacement per pulse was dependent upon the sample thickness and current density, which suggests that the mechanism is a combination of s-d exchange and hydromagnetic domain drag forces.

Planar Magnetic Colloidal Crystals

Abstract: We report a novel form of planar magnetic colloidal crystals formed by coated magnetic microspheres floating on a liquid meniscus. Under an external magnetic field, the balance between the repulsive magnetic interaction and the ``attractive'' interaction, due to the weight of the particles projected along the surface tangent, yields not only the triangular lattice with a variable lattice constant, but also all the other planar crystal symmetries such as the oblique, centered-rectangular, rectangular, and square lattices. By using two different sized magnetic particles, local formations of 2D quasicrystallites with fivefold symmetry are also observed.

Magnetic and optical properties of ionic ferrofluids based on nickel ferrite nanoparticles

Abstract: New ionic ferrofluids containing NiFe2O4 nanoparticles of size ⩽10 nm are investigated. The crystalline structure of the particles is probed by transmission electron microscopy and x-ray scattering. Static magnetization and field-induced birefringence measurements are performed on three samples differing by particle volume fraction. Cross analyzing of the results of those two types of macroscopic tests completely rejects a simple single-domain particle model but readily supports the two-component scheme of a particle as consisting of a core with a uniform magnetization and a surface layer of comparable thickness stowed with a spin-glass-like arrangement.

Magnetic random access memory using current through MTJ write mechanism

Abstract: A non-volatile memory array includes a plurality of memory cells. Each memory cell includes a magnetic tunnel junction device having a first free ferromagnetic layer, a second free ferromagnetic layer and a highly conductive layer. The first ferromagnetic layer of each magnetic tunnel junction device extends in a direction that is substantially parallel to the second ferromagnetic layer of the magnetic tunnel junction device. The highly conductive layer of each magnetic tunnel junction device is formed between the first ferromagnetic layer and the second ferromagnetic layer of the magnetic tunnel junction device. A write current through each selected memory cell flows into the highly conductive layer and along at least a portion of the highly conductive layer. A self-field associated with the write current changes a first predetermined magnetization of the first and second ferromagnetic layers to a second predetermined magnetization. In a second embodiment, each memory cell includes a magnetic tunnel junction device having a first free ferromagnetic layer, a second pinned ferromagnetic layer, and a tunneling barrier layer formed between the first and second ferromagnetic layers. The first free ferromagnetic layer has a magnetization in a form of a vortex. The second pinned ferromagnetic layer has substantially the same shape as the shape of the first free ferromagnetic layer and a magnetization in a form of a vortex. A write current flows through the memory cell and producing a self-field that changes a magnetic vortex state of the first free ferromagnetic layer from a first predetermined handedness to a second predetermined handedness.

Magnetic structure of epitaxially grown MnAs on GaAs(001)

Abstract: We investigate in detail the occurrence of magnetic domains in epitaxially grown MnAs films on GaAs(001) by magnetic force microscopy (MFM). MnAs layers exhibit in their demagnetized state a very complex magnetic domain structure. High resolution MFM images reveal detailed information on the domain wall. Additionally, we imaged magnetic domains in the dependence on the applied magnetic field. This detailed investigation gives new insight into the correlation between film topography and magnetic domain structures. Systematic magnetization measurements in-plane and out-of-plane have shown high anisotropy in our films. The out-of-plane magnetization determined as a function of the applied field reveals that the direction of the magnetic moments in the domain walls are out-of-plane, thus the domain walls are determined as 180° Bloch type.

Nucleation and annihilation of magnetic vortices in submicron-scale Co dots

Abstract: We describe experiments on arrays of polycrystalline Co structures fabricated by interference lithography. The dots are thin (15–40 nm), submicron in size, and are patterned with a uniaxial, in-plane, shape anisotropy axis. We use magnetic force microscopy (MFM) in the presence of an applied field to directly observe magnetic reversal in the dots. These experiments reveal that reversal occurs predominantly through the nucleation and annihilation of a single magnetic vortex in each dot. Hysteresis loop measurements indicate that the vortices are stable over a wide range of applied fields and that the limits of this range depend on the size and thickness of the dots. Using the MFM data, we determine the statistical distribution of the single-vortex nucleation field for several different arrays. We attribute the observed variance to the random orientation of the polycrystalline grains. Finally, we show that the average vortex nucleation and annihilations fields are linearly correlated to the demagnetization ...