Nonuniform Magnetization Research Articles

Three dimensional finite element vector potential formulation of magnetostatic field with non-uniform permanent magnet source distribution

A three dimensional magnetic vector potential (m.v.p.) formulation of magnetostatic fields with non-uniform permanent magnet source distribution is given. A right-hand term expression describing the nonuniform permanent magnet source for a first order tetrahedral finite element is derived. The three dimensional finite element m.v.p. formulation presented is rather straightforward and easy to implement. Moreover, it is applicable both to uniformly and non-uniformly magnetized permanent magnet fields and hence has high generality. The field solution for a rectangular parallelepiped magnet stabilised in air is computed by the presented method, and is found to be in good agreement with measured results.

Magnetostatic wave soliton in nonuniformly magnetized YIG film

In this paper, experiments on the Magnetostatic Forward Volume Wave (MSFVW) solitons in a nonuniformly magnetized YIG film are reported. It is observed that compared to solitons in uniformly magnetized YIG films, solitons of narrower durations that show stronger nonlinearity with input power can be excited in nonuniformly magnetized film. Two explanations of the above mentioned behavior are given that are based on the numerical simulation of solitons satisfying nonlinear Schrodinger equation (NLSE). One reason is the reduction in propagation loss in the YIG film due to nonuniform magnetization. The other reason may be the adiabatic excitation of soliton due to the varying dispersion along the length of the film.

Magneto-optical indicator film study of the magnetization of a symmetric spin valve

A magneto-optical indicator film (MOIF) technique is used for direct experimental study of the magnetization reversal process in a symmetric NiO/Co/Cu/NiFe/Cu/Co/NiO spin valve. It is shown for the first time that the reversal of the free center layer proceeds by nonuniform magnetization rotation. The observed switching mechanism is presumed to be associated with the influence of the nonuniform magnetostatic field that follows from surface roughness and the polycrystalline structure of the magnetic layers.

Switching of Single Hexagonal Particles with Nonuniform Magnetic Properties

Switching fields of hexagonal particles with nonuniform magnetic properties are investigated using computer simulation. The switching field of a hexagonal particle with a low anisotropy energy constant (K u) region at a corner of all three thickness layers is lower than that of the uniform particle when the field is applied almost perpen-dicular to the hexagonal plane. However, when the field is applied almost parallel to the hexagonal plane, the switching field of the nonuniform hexagonal particle is similar to that of the uniform particle. A hexagonal particle with a low K u region at the center also switches in a lower field than the uniform particle does when the field is applied perpendicular to the hexagonal plane. The switching field of a hexagonal particle with low K u region in the surface layer is almost the same as that of a uniform particle.

Nonuniform magnetization of a ferromagnet by the magnetic field of a superconducting vortex (as a problem of micromagnetism)

This paper considers the magnetization of a ferromagnetic plate in the field of a superconducting vortex. The problem is solved as a problem of micromagnetism taking into account the induction of Meissner currents in a thin ( d ⪡ δ L) superconducting plate where the vortex exists. It is assumed that the magnetization deviates slightly from uniformity.

Physical and magnetic properties of submicron lithographically patterned magnetic islands

We have developed a procedure for high-resolution patterning of magnetic thin films using direct-write electron beam lithography and a multistep sputter etching process [New et al., J. Vac. Sci. Technol. B 12, 3196 (1994)]. Using this patterning procedure we have defined large arrays of 0.2 by 0.4 μm magnetic islands out of a 200-Å-thick polycrystalline film of cobalt. The physical and magnetic structure of these islands has been examined using atomic and magnetic force microscopy, as well as transmission electron microscopy. Hysteresis loop measurements have been taken from large arrays of weakly interacting islands. The particles are not single domain and show considerable nonuniformity of magnetization when they are completely demagnetized. However, after application and removal of a large field along the long axes of the islands, most of the particles do relax into an almost uniform magnetization state.

Statistical mechanics of nonuniform magnetization reversal.

The magnetization reversal rate via thermal creation of soliton pairs in quasi-1D ferromagnetic systems is calculated. Such a model describes e.g. the time dependent coercivity of elongated particles as used in magnetic recording media. The energy barrier that has to be overcome by thermal fluctuations corresponds to a soliton-antisoliton pair whose size depends on the external field. In contrast to other models of first order phase transitions such as the phi^4 model, an analytical expression for this energy barrier is found for all values of the external field. The magnetization reversal rate is calculated using a functional Fokker-Planck description of the stochastic magnetization dynamics. Analytical results are obtained in the limits of small fields and fields close to the anisotropy field. In the former case the hard-axis anisotropy becomes effectively strong and the magnetization reversal rate is shown to reduce to the nucleation rate of soliton-antisoliton pairs in the overdamped double sine-Gordon model. The present theory therefore includes the nucleation rate of soliton-antisoliton pairs in the double sine-Gordon chain as a special case. These results demonstrate that for elongated particles, the experimentally observed coercivity is significantly lower than the value predicted by the standard theories of N\'eel and Brown.

Magnetic properties of amorphous nanocolumns created by heavy ion irradiation of paramagnetic YCo2 thin films (invited)

Recent studies have demonstrated the possibility of inducing amorphous latent tracks in metallic materials by GeV heavy ion irradiation. In the present work, 0.880 GeV 238U beams have been used to induce ferromagnetic amorphous nanocolumns in nonmagnetic crystalline YCo2 films. The loss of crystallinity deduced by x-ray analysis is in good agreement with the one determined by magnetic measurements. The Co magnetic moment in the columns is approximately equal to 1 μB, as in bulk amorphous YCo2, but the ordering temperature (150 K) is strongly reduced with respect to the bulk. Magnetization measurements reveal the nanocolumns to be single domain. A perpendicular anisotropy is observed in samples irradiated at the smallest fluencies, which is interpreted to be due to shape anisotropy. A progressive decrease of the anisotropy with increasing fluence is observed and qualitatively described in terms of dipolar interactions between columns. At low temperature, the coercive field reaches 650 Oe. A simple nonuniform magnetization reversal process is suggested.

Magnetic properties of fine maghemite particles in an electroconducting polymer matrix

The magnetic properties of γ- Fe 2 O 3 grains of a few nanometers in size, included in films of conducting polypyrrole by electrochemical synthesis, are investigated. A theoretical treatment of the combined effects of the possible non-uniform spontaneous magnetization of the grains and of their crystalline anisotropy energies is presented. The magnetization of samples of this composite material has been measured at several temperatures as a function of the field up to 30 kOe. The interpretation of these data shows that the grains in the polymer behave as independent monodomains, coexist with the polymer without any notable interference, and have the same size distribution as in the ferrofluid solution used for the electrochemical inclusion.

Estimation of electronic and structural influence on the thermal magnetic properties of clusters.

Using an effective Heisenberg model for clusters the spin-wave spectrum is calculated by direct diagonalization. The inclusion of a nonuniform magnetization profile at zero temperature, nonuniform exchange interactions as a result of structural relaxations, longer-range interactions, and of magnetic surface anisotropy have been studied. Very small effects are found on the thermal magnetic properties relative to those predicted for the simple nearest-neighbor Heisenberg model by Hendriksen, Linderoth, and Lindg\aa{}rd [J. Phys. C 31, 5675 (1993); Phys. Rev. B 48, 7259 (1993)].

High resolution magnetic Barkhausen noise measurements

High resolution magnetic Barkhausen noise (MBN) measurements of slits in steel have been made using a small magnetic disk read head mounted in a U-shaped laminated sweep field core. Detection occurred for sweep fields oriented both parallel and perpindicular to a slit or through-cut. For sweep fields oriented parallel to slits with widths from 0.25 mm to 0.7 mm, a maximum depression of 62% to 95% of the MBN signal was found. The corresponding width of the signal at 50% depreciation ranged from 2 to 4 mm. The range of depression was a factor of 2 to 5 times larger for sweep fields applied perpendicular to the slit. Through-cuts demonstrated similar depression of the MBN signal but with the distance of the suppression at 50% ranging from 4 to 11 mm. An observed increase in the MBN signal at the edges of slits cut with a blade was associated with increased residual stress induced by the blade at the edges. Results are interpreted in terms of the ability of the small magnetic disk read head to resolve small regions of non-uniform magnetization that produced MBN.

Thermally activated magnetization reversal in elongated ferromagnetic particles.

Elongated ferromagnetic single-domain particles are believed to exhibit a high stability of their remanent magnetization. However, here it is shown that for elongated particles thermal fluctuations lower the coercivity much more drastically than predicted by current theories. The rate for magnetization reversal is calculated for a classical model of a ferromagnet that allows for a spatially nonuniform magnetization distribution along the sample. The prefactor of the Arrhenius factor is explicitly evaluated and analytical results are obtained in the experimentally important limit of external magnetic fields close to the anisotropy field and for moderate damping.

Model for nonuniform magnetization processes in particles with enhanced surface anisotropy

A one-dimensional model for a nonuniform magnetization process is investigated to show how a small change from Stoner-Wohlfarth uniform rotation results in qualitatively different behavior in particles that are smaller than the conventional domain wall thickness. The main effects are initiated when the axes for the surface anisotropy and the bulk anisotropy are not collinear. >

Magnetic and tectonic studies of the dueling propagating spreading centers at 20°40′S on the East Pacific Rise: Evidence for crustal rotations

We present the results of a magnetic study of a 225 km by 240 km area centered on the dueling propagating spreading centers located at 20°40′S on the East Pacific Rise. A majority of the data used were collected during a cruise aboard the R/V Moana Wave during which continuous SeaMARC II coverage was obtained. These data were combined with additional data to produce an anomaly map which extends to anomaly‐2‐aged crust. A three‐dimensional inversion in the presence of bathymetry was carried out for the area. The resulting magnetization distribution was interpreted and compared to side scan sonar and bathymetry data sets in order to determine the recent history of the discontinuity. The results indicate consistent asymmetric spreading faster to the east, discontinuous high magnetizations in the discordant zone associated with the discontinuity, and southward migration of the feature at a rate of 90–100 mm/yr between Jaramillo and Brunhes time (0.95 to 0.73 Ma) with slowing during the Brunhes to less than 10 mm/yr. There is also an overlapping Jaramillo isochron on the west flank and a gap in that isochron on the east flank indicating a transfer of crust during this time period from the Nazca to the Pacific plate. In addition, areas of oblique lineations possibly representing rotated crust were modelled using an inverse method which enables the specification of a nonuniform magnetization unit vector. Results from this second model support the presence of highly rotated pre‐Brunhes Nazca crust within Brunhes Pacific crust which has been deformed by bookshelf faulting. This indicates at least two episodes of crustal transfer from the Nazca plate to the Pacific plate. The discontinuity appears to mark the boundary between rigid plate tectonics to the north and deformation within the Nazca plate between the discontinuity and the Easter microplate to the south. The detailed history of the discontinuity involves dueling propagation with a great deal of variation in the amount of overlap of the two ridges as well as inward and outward cutting and abandonment of the tips of both ridges.

A generalization of the demagnetizing tensor for nonuniform magnetization

The demagnetizing tensor for ferromagnets is generalized to include interactions between uniformly magnetized bodies. This “mutual” demagnetizing tensor is symmetric, has a trace of zero, and has other simple geometric properties. The tensor is then used to develop an expression for the macroscopic magnetic field in non‐uniformly magnetized bodies of arbitrary shape. Finally, the theory is applied to a block model of magnetization and explicit formulae for the tensor components are given.

Simulations of the magnetization reversal of clustered Co-Cr particles

Micromagnetic simulations of the magnetization reversal of clusters of 9 and 25 magnetostatically coupled Co-Cr particles are presented. The magnetostatic interaction between neighbouring particles facilitates reversal. The non-uniform magnetization configuration plays an important role in this effect.

Increasing the information content of magnetic prospecting by invariant measurement of magnetic induction nonuniformities

Increasing the information content of magnetic prospecting by invariant measurement of magnetic induction nonuniformities

Physics Modeling of Tokamak Transport, a Grand Challenge for Controlled Fusion

A challenge facing fusion is accurate prediction and understanding of plasma and heat confinement. Com puters and modeling techniques have advanced to where we can envision simulating real Tokamaks start ing from fundamental physics models. Charged parti cles in magnetic fields execute rapidly gyrating mo tions about the field. These orbits gradually drift where electric fields and magnetic nonuniformities exist. The gyrokinetic model averages out the rapid gyration and treats the plasma as a large number of drifting rings of charge and current; large numbers (approx. 106) of such rings are followed in their self-consistent electric and magnetic fields. Three-dimensional simulations of plasma and heat confinement for situations relevant to fusion give turbulence spectra and transport coeffi cients similar to experiments. Simpler particle models of plasma run on hypercubes achieves a high degree of parallelism (95%). With new parallel machines sub stantial gains in speed and memory can be expected; modeling of real Tokamaks should be possible.

A strategy of tectonomagnetic observation for monitoring possible precursors to earthquakes in the western part of the North Anatolian Fault Zone, Turkey

In order to investigate changes in the geomagnetic field associated with seismic activity in the western part of the North Anatolian Fault Zone (NAFZ), seven continuous measurement stations and eighteen repeated-survey sites were set up in the İnik-Geyve region, taking into account the fault trace and its features. Simple differences in the total intensity between these continuous stations have been derived for detecting possible anomalous phenomena associated with earthquake occurrences. Theoretical studies imply that changes in the total intensity associated with fault activity are larger in the case of non-uniform distribution of rock magnetization than those in the uniform case. Meanwhile, inversion analyses of magnetic anomalies at the North Anatolian Fault Zone (NAFZ) suggest a dyke-like structure extending parallel to the fault strike. This structure enables us to test our strategy, based on non-uniform magnetization, for detecting pre- and/or co-seismic changes in the geomagnetic field. We made a theoretical estimation of geomagnetic changes for the case of a magnetic dyke structure intruding a non-magnetic body parallel to the strike of a fault. It turns out that, unlike the uniform case, the magnetic field arising from stress-induced magnetization is enhanced. A tectonomagnetic observation system talcing into account such a non-uniform magnetization structure in the İnik-Geyve region of the NAFZ is also described.

Micromagnetic computation for wall coercivity caused by magnetic nonuniformity

The wall coercivity caused by the magnetic nonuniformity has been studied numerically. The Landau-Lifshits-Gilbert (LLG) equation is integrated by an explicit scheme of the modified Dufort-Frankel method. The computation was carried out for a two-dimensional grid system representing the magnetization direction in a magnetic film plane. Typical magnetic parameters for magneto-optic recording media were assumed. Spatial variations of uniaxial anisotropy were treated as the magnetic nonuniformity. The validity of our numerical approach was demonstrated with preliminary one-dimensional computations, compared to analytical solutions. A wall coercive field of 2.5 kOe was observed for the wall coupling with a pinning site (30-Å width and 360-Å spacing along the wall) of 10 times larger anisotropy compared to the ordinary region. A two-dimensional anisotropy variation (K=106–107 erg/cm3) with a wavelength larger than 60 Å also caused a wall coercivity on the order of 1 kOe, compared to those in magneto-optic recording media. It was also found that a fine pinning site on the order of 100 Å caused a notable coercivity for the bubble domain wall surrounding it.