Number Of Domain Walls Research Articles

Multiturn Counter Using the Movement and Storage of 180$^circ$Magnetic Domain Walls

We describe a novel giant magnetoresistive (GMR) stack geometry which enables the counting of the number of turns of a rotating magnetic field. A race-track-like spiral of N turns acts as a 180deg domain-wall memory. At one end of the spiral, an enlarged film area works as a domain-wall generator. Due to the small width of the spirals themselves in the order of 100-200 nm, there is only domain-wall movement within the spiral. By rotating the magnetic field with/against the sense of the spiral, every half turn increases/decreases the number of domains by one. The number of domain walls have a one-to-one correspondence with the resistance of the spiral due to the GMR effect

Defect junctions and domain wall dynamics

We study a number of domain wall forming models where various types of defect junctions can exist. These illustrate some of the mechanisms that will determine the evolution of defect networks with junctions. Understanding these mechanisms is vital for a proper assessment of a number of cosmological scenarios: we will focus on the issue of whether or not cosmological frustrated domain wall networks can exist at all, but our results are also relevant for the dynamics of cosmic (super)strings, where junctions are expected to be ubiquitous. We also define and discuss the properties that would make up the ideal model in terms of hypothetical frustrated wall networks, and provide an explicit construction for such a model. We carry out a number of numerical simulations of the evolution of these networks, analyze and contrast their results, and discuss their implications for our no-frustration conjecture.

Dynamic losses in FeSi filled polymer bonded soft magnetic composites

The dynamic magnetization behavior of polypropylene filled with iron silicon particles in various proportions ( 20.5 – 40.1 vol % ) in the frequency range from 0.1 Hz ⩽ f ⩽ 100 Hz was measured with a computer controlled hysteresis recorder. The frequency dependence of the anomaly factor η and the effective number of active domain walls n eff in relation to the excess field were estimated. The anomaly factor follows the relation η = η 0 f x with - 1.20 ⩽ x ⩽ - 1.41 , whereas x changes with the amount of FeSi in the composite. The effective number of active domain walls is relatively low at excess fields below 20 A/m and increases rapidly for higher excess fields.

Domain statistics in a finite Ising chain

We present a comprehensive study for the statistical properties of random variables that describe the domain structure of a finite Ising chain with nearest-neighbor exchange interactions and free boundary conditions. By use of extensive combinatorics we succeed in obtaining the one-variable probability functions for (i) the number of domain walls, (ii) the number of up domains, and (iii) the number of spins in an up domain. The corresponding averages and variances of these probability distributions are calculated and the limiting case of an infinite chain is considered. Analyzing the averages and the transition time between differing chain states at low temperatures, we also introduce a criterion of the ferromagnetic-like behavior of a finite Ising chain. The results can be used to characterize magnetism in monatomic metal wires and atomic-scale memory devices.

Growth and magnetism of Fe on Cr(001): a spin-polarized scanning tunneling spectroscopy and magnetic force microscopy study

The growth and magnetic domain structure of Fe nanoislands and films on Cr(001) are investigated by spin-polarized scanning tunneling microscopy and magnetic force microscopy (MFM). Topographic images of films grown at different substrate temperatures reveal that the highest film quality is obtained by evaporation at room temperature and subsequent annealing at 500 K for 4 min. Spin-resolved studies of the magnetic structure of submonolayer Fe films (coverage θ≤0.2 ML) show the expected antiferromagnetic Fe–Cr coupling, i.e. any Fe island is magnetized antiparallel with respect to the underlying Cr(001) terrace. As the Fe coverage exceeds 0.2 ML the magnetic contrast decreases and completely vanishes for θ≥0.4 ML. Only for θ≥3 ML does a weak magnetic contrast reappear, which is interpreted in terms of a small spatial variation of the 90° coupling between the Cr substrate and the Fe overlayer. MFM reveals that the number of visible domain walls decreases with increasing film thickness and completely vanishes at 12 ML.

Field sweep rate dependence of domain-wall speed distributions in a thin Ni80Fe20 film

The magnetization reversal in a thin (100nm) Ni80Fe20∕GaAs(001) film prepared by molecular-beam epitaxy has been investigated using a focused spot Kerr magnetometer. The field was sinusoidal and applied along an in-plane easy axis, with sweep rate at the coercive point in the range ∼1kOe∕s–1MOe∕s. Real time measurements of the Kerr response enabled a study of the probability distributions P(V) of the domain-wall speed as a function of sweep rate. The shape of P(V) changes from being characteristic of stochastic domain-wall motion at low sweep rate to being characteristic of quasicontinuous domain-wall motion at high sweep rate. Increasing the diameter of the focused spot from 2to80μm increases the number of domain walls participating in the reversal within the probed area, which leads to an averaging effect and dulls the transition from stochastic to quasicontinuous wall motion seen in the probability distributions.

Quantitative study of magnetoresistance in patterned Ni80Fe20 wires

AbstractThe domain wall resistivity of the patterned Ni80Fe20 zigzag wires has been studied as functions of the number of corners and temperature in zigzag wires. The quantitative ratio of domain wall magnetoresistance (MR) is estimated by two methods. One is the discontinuous jump that represent the domain state sweep between two states in MR curve. The other is the angular dependence on the resistance at remanent state. The ratio of domain wall MR increased when domain wall number density over total wire length (DWs/L) changed from 12/73 to 34/73 with film thickness 40 nm at 10–300 K. Especially, the ratio of domain wall MR decreased slightly as the temperature ranged from 10 to 300 K. The anisotropic magnetoresistance (AMR) has been subtracted in the calculation. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Generalized integro-differential equation of magnetization reversal dynamics in polycrystals and its application to processes in manganite-based CMR materials

We report on the development of the self-consistent model of magnetization reversal dynamics in polycrystalline materials taking into account magnetodipole interaction, number of domain walls in grains, influence of microinhomogeneities, nucleation and collision of walls. The evolution of angular distribution of magnetic moments and the shape of output voltage produced by impulsive reversal of magnetic core are calculated for the parameters of model corresponding to the bulk composite La 0,7Sr 0,3MnO 3/SrTiO 3 with distortions acting as pinning centers for domain walls.

Variation of domain formation in a 15 nm NiFe layer exchange coupled with NiO layers of different thicknesses

The correlation between ferromagnetic domain formation and exchange bias in a series of NiFe/NiO samples with varying NiO thicknesses has been investigated using the magneto-optic Kerr effect and magnetic force microscopy. Below a critical thickness (15 nm) of NiO, the exchange bias HE is zero and ripple domains exist in the NiFe layer. Above this critical thickness, cross-tie type domain walls appear concurrently with the appearance of exchange bias. Both the number of cross-tie domain walls and the exchange bias increase with an increase in NiO thickness, reaching a maximum at 35 nm NiO, after which both show a gradual decrease. This variation of domain wall formation in the NiFe layer with the NiO thickness possibly reflects the variation of the domain structure in the NiO layer through interfacial exchange coupling.

Fabrication and physical properties of permalloy nano-size wires

Nano-size NiFe wires with patterned shapes in half-ring-in-series, octagon-in-series, and zigzag-in-series configurations were fabricated. Their magnetoresistance was studied below room temperature and their magnetic domain images were investigated at room temperature by a magnetic force microscope. In general, we have experimentally demonstrated that the variation of the magnetoresistance of our patterned nano-size wires can be related to different domain configurations and explained by the domain switching effect. The number of magnetic domain walls in our patterned wires can be controlled by the shape anisotropy and the size of each section of patterns that form the wires.

Effect of the slight surface oxidation of ribbons on the dynamic magnetization of nanocrystalline soft magnetic ribbons

The effect of slight surface oxidation of ribbons of the optimal-annealed Nanoperm alloys on the dynamic properties of domain wall motion is studied. It is shown that the surface oxidation of ribbons may strengthen the domain wall pinning, make the number of domain walls unpinned from the initial sites decrease and the effective permeability measured smaller than the real value. Simultaneously, the existence of the surface oxidation layer may reduce the distance between the pinning sites of domain walls, which results in the relaxation frequency moving toward higher frequency. The condition of the ribbon surface is very important to the dynamic behavior of nanocrystalline soft magnetic ribbons.

Counting domain walls inN=1super Yang-Mills theory

We study the multiplicity of BPS domain walls in N=1 super Yang-Mills theory, by passing to a weakly coupled Higgs phase through the addition of fundamental matter. The number of domain walls connecting two specified vacuum states is then determined via the Witten index of the induced worldvolume theory, which is invariant under the deformation to the Higgs phase. The worldvolume theory is a sigma model with a Grassmanian target space which arises as the coset associated with the global symmetries broken by the wall solution. Imposing a suitable infrared regulator, the result is found to agree with recent work of Acharya and Vafa in which the walls were realized as wrapped D4-branes in IIA string theory.

Structure and Properties of TGS Crystal Grown from the Seed Irradiated with Alpha-Particles

Investigations of the structure and properties of TGS crystals grown from the seed irradiated with f -particles have been performed. Analysis of habitus of the crystal shows asymmetry of the growth rates for the facets relative to the seed along polar axis Y (V 0.10 > V 0.10 ). Diffraction patterns obtained using powder technique and the calculation of parameters for elementary cell unit show an increase of parameter "b". An increase of microhardness at the surface of X, Y and Z cut-offs was observed independent on the distance from the sample to a seed. The obtained distribution of microhardness in the crystal grown from the irradiated seed is related with the character of dislocations distribution along the different directions and specific features of domain structure, and also with the number of domain walls emerging to the surface of the crystal.

Tunable quantum tunnelling of magnetic domain walls.

Perhaps the most anticipated, yet experimentally elusive, macroscopic quantum phenomenon is spin tunnelling in a ferromagnet, which may be formulated in terms of domain wall tunnelling. One approach to identifying such a process is to focus on mesoscopic systems where the number of domain walls is finite and the motion of a single wall has measurable consequences. Research of this type includes magnetotransport measurements on thin ferromagnetic wires, and magnetization experiments on single particles, nanomagnet ensembles and rare-earth multilayers. A second method is to investigate macroscopic disordered ferromagnets, whose dynamics are dominated by domain wall motion, and search the associated relaxation-time distribution functions for the signature of quantum effects. But whereas the classical, thermal processes that operate in these experiments are easily regulated via temperature, the quantum processes have so far not been tunable, making difficult a definitive interpretation of the results in terms of tunnelling. Here we describe a disordered magnetic system for which it is possible to adjust the quantum tunnelling probabilities. For this material, we can model both the classical, thermally activated response at high temperatures and the athermal, tunnelling behaviour at low temperatures within a unified framework, where the domain wall is described as a particle with a fixed mass. We show that it is possible to tune the quantum tunnelling processes by adjusting the 'mass' of this particle with an external magnetic field.

Exchange-coupled CoZrNb/CoSm underlayer for perpendicular recording media

The characteristics and effects of an exchange-coupled CoZrNb/CoSm underlayer for perpendicular recording media are reviewed. A CoSm layer pins CoZrNb domains via exchange coupling resulting from large induced radial anisotropy of the CoSm layer in the disk medium as well as in the CoZrNb layer. The number of CoZrNb domain walls is considerably reduced by the CoSm pinning effect which is effective in both signal stabilization to stray fields and noise reduction. Furthermore, the CoZrNb/CoSm underlayers exhibit high-frequency responses of over 1 GHz in permeability, which also result from their large induced anisotropy. The CoZrNb/CoSm underlayer is expected to enable the practical realization of perpendicular recording in data storage applications requiring high densities and high-speed data rates.

Mn-Zn フェライト複合材料における透磁率の磁場効果

Complex permeability spectra (μ*=μ'-iμ) of Mn-Zn ferrite composite materials were studied in the frequency range from 10 kHz to 10 GHz under magnetic fields up to 1 kOe. In sintered ferrite, μ' at low frequency decreases with increasing applied magnetic field, and the dispersion character changes from resonance type to relaxation type. This can be attributed to the decrease in the number of domain walls. The maximum of μ in sintered ferrite and its composite materials shifts to higher frequencies with increasing magnetic field. The contribution of domain walls and spin rotational resonance to the permeability under a dc magnetic field was estimated by numerical fitting of a simple formula for sintered ferrites to the actual measurement data.

Mass hierarchy from many domain walls

We construct a new model with exponential mass hierarchy by starting with the Einstein–Hilbert action with the cosmological constant in five dimensions plus an action describing many domain walls in four dimensions. The model includes many hidden sectors and one visible sector, and each four-dimensional domain wall, that is, 3-brane, interacts with one another through only a gravitational interaction and realizes many universe cosmology inspired by D-brane perspective. It is shown that in the present model only even numbers of domain walls are allowed to locate in five dimensional space-time and the validity of Randall–Sundrum scenario, which explains mass hierarchy between the Planck mass and the electro-weak scale in our world, depends on a relative relation between our world and hidden worlds.

Resistance of a domain wall in La0.7Ca0.3MnO3

Although colossal magnetoresistance (CMR) materials exhibit large changes in electrical resistance (up to 106%), large magnetic fields (several tesla) must be applied. To obtain a sizeable low-field effect (<102% in several millitesla), it is necessary to incorporate structural discontinuities such as grain boundaries, or other types of interfaces. The potential for applications, however, remains limited because structural discontinuities increase electrical resistance by several orders of magnitude and hence create noise. Moreover, it has proven to be difficult to fabricate structural discontinuities reproducibly. We have attempted to investigate discontinuities that are purely magnetic via transport measurements through a precisely controlled number of magnetic domain walls of known area in thin film devices of the ferromagnetic CMR perovskite La0.7Ca0.3MnO3. A sharp low-field switching seen below ∼110 K is ascribed to the formation of a precise number of magnetic domain walls, each with resistance-area product 8×10−14 Ω m2 at 77 K. This is four orders of magnitude larger than expected, suggesting that the domain walls contain an additional structure. Our findings demonstrate that CMR devices are capable of low-noise low-field switching, and suggest the possibility of exploiting a hitherto unexpected intrinsic effect reproducibly and therefore commercially.

Three-dimensional atom probe field-ion microscopy observation of Cu/Co multilayer film structures

Focused ion-beam milling has been used to fabricate field-ion specimens from a multilayer film structure containing 100 repetitions of a (Cu2 nm/Co2 nm) bilayer deposited directly onto a planar substrate. The as-deposited films showed a magnetoresistance ratio of ∼5% over a 250 Oe range at room temperature, and a coercivity of ∼60 Oe. The magnetic data suggest that the films are coupled ferromagnetically. Successful field-ion specimen preparation has allowed the observation of these layers by field-ion imaging and three-dimensional atom probe compositional analysis. Examination of the multilayer images reveals that, in some regions, the layers are nonparallel, but the interfaces are chemically quite sharp, with a diffuse interface region of ∼3 atomic layers. In addition, in some areas adjacent cobalt layers appear to be in contact. The fact that the layers are wavy suggests that the ferromagnetic coupling may be a result of Néel “orange peel” type magnetostatic coupling between adjacent cobalt layers. The relatively high coercivity may be a result of the poor layer planarity leading to a high number of domain wall pinning sites.

Angle-resolved magnetic and transport properties of Pr0.7Sr0.3MnO3 thin films

In this communication, the variation of the resistance and of the magnetization with respect to the angle between the applied magnetic field and the substrate normal is discussed for a Pr0.7Sr0.3MnO3 thin film at 30 K. An angular hysteresis is observed for these properties and this behavior is correlated with the magnetic domain wall motion. It is shown that the number of domain walls is controlled through the rotation of the applied magnetic field, and that these domain walls play an important part in the colossal magnetoresistance behavior of these films. Furthermore, a model describing the reversal of the magnetization with the rotation of the magnetic field has been developed. This model enables one to extract the dependence of the resistance upon the direction of the magnetization with respect to the crystalline axis.