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Switching properties of a regular square lattice of 40x40x3 [[[micro]meter].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.

A rigorous mathematical model of nonlinear, parametric interactions of microwave magnetostatic dipole-dipole waves and electromagnetic waves in 3-D ferrite films in waveguiding structures is developed. The 3-D nonlinear diffraction boundary problem of the full nonlinear Maxwell's equations with electrodynamical boundary conditions, complemented by the Landau-Lifshitz equation (without the exchange term) is solved. The instability of parametric excitation process of magnetostatic waves in 3-D ferrite structures is simulated by the numerical method of bifurcation analysis of the nonlinear Maxwell's operator for the 3-D boundary problem. The results show discrete maxima in the magnitude of reflected electromagnetic waves for distinct ferrite thickness values in the 1 [micro]m to 1 mm range. The mode profiles were obtained. The instability regions, depending on the bifurcation parameters, were determined, and shown to be more sensitive to the pumping power than the parametric excitation of electromagnetic waves alone. Index Terms--Ferrite films, ferrite loaded waveguides, microwave magnetic devices, nonlinear equations, nonlinear media, numerical analysis.

A new approach based on the bifurcation theory to develop mathematical models of nonlinear electromagnetic waves and oscillations in waveguiding and resonator structures, containing a strongly nonlinear bounded gyromagnetic medium, is proposed. A new method for rigorous modeling of nonlinear phenomena due to the instability in the three-dimensional ferrite structures was developed based on the numerical analysis of the bifurcation points of the nonlinear Maxwell's operator. The bifurcation points were determined by our computational algorithm, using the eigenvalues of the matrix resulting from the linearized Maxwell's operator. The onset and the breakdown of self-oscillations in the ferrite resonator structure, caused by the instability, were modeled. The transition regime from regenerative parametric amplification to parametric generation, depending on the magnitude of the pumping wave and resonator detuning, was simulated infinitesimally close to the bifurcation points. Index Terms--Electromagnetic propagation in nonlinear media, ferrite loaded waveguides, microwave devices, nonlinear media, numerical analysis.

In-plane and out-of-plane ferromagnetic resonance measurements were performed on arrays of Permalloy rectangles, prepared by interference lithography. In the absence of crystalline anisotropy, the shape anisotropy dominates the angular dependence of the resonance and the linewidth. The 60-70-mT in-plane anisotropy follows the increase of the aspect ratio. The uniaxial out-of-plane anisotropy fields are on the order of 500 mT. The linewidth has a strong angular dependence, both in the in- and out-of-plane rotation, ranging from about 17 to 23 mT along the easy and hard axes and decreasing with increasing shape anisotropy. Two distinct resonance modes observed in the perpendicular orientation are associated with the main volume mode and shape mode of magnetostatic origin. Index Terms--Magnetic anisotropy, magnetic resonance, patterned thin films, Permalloy.

A numerical approach for the solution of three-dimensional nonlinear diffraction problems, based on nonlinear Maxwell's equations, complemented by the equation of motion of the magnetization vector in a ferromagnet, is developed. We have carried out the rigorous nonlinear analysis of propagation, diffraction, and nonlinear interactions of waves in waveguide structures, loaded with a strongly nonlinear ferrite insertion. The numerical analysis shows an efficient frequency multiplication mechanism. It is shown how the efficiency of frequency multiplication depends on the magnitude of the incident wave, the value and the orientation of bias magnetic field, and the length of the ferrite. A reciprocity effect for the second time harmonics wave is predicted. Index Terms--Ferrite-loaded waveguides, nonlinear wave propagation, numerical analysis.

Minor loop behavior have been investigated experimentally on magnetically uniaxial films of YIG based garnets and MnBi, all having the easy direction of the magnetization perpendicular to the film plane and unit squareness. In these films the magnetization changes by domain wall motion. A regular two-dimensional array of small, strongly uniaxial particles, etched from one of the films, is switching by rotation. The wiping out property is observed for each sample, however, the congruence of the minor loops is fulfilled only for the 2D array, for minor loops centered on the H=0 axis. Accommodation of minor loops is observed for MnBi. Depending on the start field of the switching process, the samples can be prepared to display minor loops of very different shape.

In-plane and out-of-plane angular dependence of ferromagnetic resonance (FMR) and magnetization measurements were performed on arrays of 20, 40, and 60 nm thick, 520 nm long, and 250 nm wide elliptical Permalloy elements. Besides the main FMR 'volume' mode resonance, a well-defined second FMR mode was observed, which exhibits a very strong angular dependence. This mode originates from localized regions where the magnetization has a strong component perpendicular to the bias field and to the volume magnetization. These regions of nonuniform magnetization may be associated with magnetization canting at the edges of the ellipses, due to the nonuniformity of the demagnetizing tensor elements, and with magnetization vortices, which are predicted by micromagnetic simulation. [ABSTRACT FROM AUTHOR]

The distribution of the magnetization in planar ferrite devices is nonuniform due to the non-uniformity of the demagnetizing field. A method for calculating the local demagnetizing tensor for arbitrary 3-dimensional shapes, using the Finite Difference Method (FDM), is developed. The sample is discretized into rectangular elements and the magnetostatic interaction between each pair of elements is computed. The local demagnetizing tensor is obtained as the superposition of all interactions at the given element. With proper boundary conditions the method can be applied to the computation of the local demagnetizing tensor for arbitrarily shaped bodies. The local demagnetizing tensor can be used to calculate the non-uniform distribution in the ferrite, and to optimize the shape and size of the ferrite in the CAD of microwave devices.

The energy barrier model was extended to calculate the coercivity of iridium inclusions in the substrate as a function of position and size. Coercivity was associated with local maxima in the anisotropy energy and local wall energy decrease. Coercivity did not vary linearly with inclusion depth and size of the substrate material.

The magnetic characteristics of reaction- and directly-milled copper-magnetite composite materials were investigated using x-ray powder diffraction analysis. The results showed that the most significant changes in phase composition, magnetization and coercivity occured during the initial hours of the ball milling process. Both direct and reaction milling produced small semihard particles after 1 h. Excessive milling resulted in the partial oxidation of magnetite and the formation of superparamagnetic magnetite particles.

The diagonal and off-diagonal components of the effective permeability tensor and the effective permittivity of three-dimensional (3D) periodic magnetic nanosystems are calculated by using the values of the propagation constants and the dispersion relations of clockwise and counterclockwise polarized electromagnetic waves (EMWs). The studies have shown that the magnetic and dielectric losses of weakly interacting systems are lower than that of yttrium iron garnet (YIG), but the losses have increased due to spin wave (SW) excitations upon reduced separation.

The stability of the magnetic state of individual elements within patterned nanoarrays has depended on the balance of interelement interactions and shape anisotropy. These effects are studied by ferromagnetic resonance and alternating gradient magnetometry to characterize large area hexagonal arrays of 20 nm thick 190 nm wide elliptical Permalloy elements.

Weak crystalline defects, revealed only by magnetic defect decoration, responsible for the local reduction and broad distribution of the switching fields are discussed. However, the region of reversed magnetization does not expand beyond the confines of the defect region if the applied field is smaller.

The mechanism of switching of uniaxial, single domain, single crystalline epitaxiai garnet particles on a 2D square array was investigated. The anisotropy field of the sample is 2 kOe, while the gaussian switching field distribution is 280 [+ or -] 85 Oe. The dominant contribution to the incoherent switching is from inhomogeneous internal fields and canted moments at the corners and edges, where the switching starts. The interaction field with the surrounding particles, reduced nucleation barriers at crystalline defects, thermal activation, and dynamical effects in FMR also contribute to the observed switching field and its standard deviation. Index terms - coercivity, magnetization switching, single domain particles

SQUID magnetization measurements and ferromagnetic resonance experiments have been performed at 9.77 GHz on reaction milled and subsequently annealed nanocomposites of iron particles in a ZnO matrix. The dominant contribution to the temperature and time dependence of the observed linewidth of 2.2 to 3.2 kOe arises from the spatial inhomogeneity in shape, size and interaction fields of the system. Further linewidth contributions from stress, magnetization changes, surface reconstruction are discussed. The Gilbert relaxation constant [Alpha] is decreasing with annealing temperature from 0.45 to 0.33 at 304 C. Index terms--nanocomposites, ferromagnetic resonance, magnetic relaxation.

Two series of self-organized hexagonal arrays of uniaxial Permalloy nanowires are grown by electroplating, filling of nanopores in anodic alumina films. Nanowires with periodicities of 65 and 105 nm and wire diameter of 25–60 nm were investigated by magnetization and ferromagnetic resonance (FMR) measurements. The length of the wires is 2.5 μm, the volume ratio 0.13<=v<=0.43. A crossover transition from a one-dimensional easy axis “wire” behavior of weakly interacting uniaxial nanowires to a two-dimensional behavior of strongly coupled “wire film” having an easy plane anisotropy is determined from FMR measurements. The crossover occurs at v=0.27 for 105 nm periodicity, and v=0.43 for 65 nm periodicity. The system of the thinnest nanowires corresponds to a statistical ensemble of weakly interacting uniaxial particles, characterized by the highest coercivity of 1276 Oe, the highest switching field distribution, and highest FMR linewidth, resulting from the broad distribution of individual wires. The coercivity monotonously decreases to 440 Oe with increasing nanowire radius and volume fraction, as the wire shape anisotropy is reduced and the array anisotropy takes over due to dipolar interwire interaction, smoothing out the individuality of the wires. The behavior of coercivity, switching field distribution, and FMR linewidth is in accordance with the increase of the coupling of the nanowires. [ABSTRACT FROM AUTHOR]

Aftereffect for magnetization processes by rotation and by domain wall motion was investigated on the same, single domain, two-state system of a square 2D (two-dimensional) array of garnet particles. Aftereffect measurements were performed magnetooptically. The particles are thermally stable, the particle energy is 10[sup -6] erg compared to the thermal energy of 10[sup -12] erg. No aftereffect of rotation switching of the system of “up” and “down” magnetized particles could be observed at room temperature. At increased temperatures thermally activated switching, very weakly depending on magnetic field, is observed. Each individual particle can be demagnetized into a metastable stripe domain structure. The barrier for DW (domain wall) motion is much lower than the barrier for rotation, and a significant aftereffect was measured on the same particles, demagnetized into a domain structure. The observed time dependence for DW aftereffect is exponential, M(t)/Ms=d[sup *] exp(-et), where d=0.04, and e=1/τ follows the increase of the magnetization with field, de/dH=0.021 /s/Oe. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents information on a study which measured the equilibrium optical absorption and the near steady state photoinduced absorption on charge-uncompensated calcium-doped yttrium iron garnet epitaxial thin films. Compensation by electron holes localized on the oxygen ions; Concept of the photoinduced absorption technique; Normal optical absorption spectra of the samples.

Presents information on a study which described the excitation effects of light and electric field on the electrical transport properties of CaGe:yttrium iron garnet epitaxial films. Room temperature-conductivity of the films; Method for increasing low-temperature conductivity of the films; Excitation of the high-conductivity state.

Presents a study which measured ferromagnetic resonance (FMR) spectra in reduced calcium-rich CaGe:YIG film. Analysis of ferromagnetic resonance measurements; Effect of reducing time on the room-temperature FMR spectra of an epitaxial CaGe:YIG crystal.

Presents an experimental verification of a vector moving Preisach model used to describe partially oriented particulate media by pseudoparticles. Description of the model considered in the study; Measurement of the angular dependence of the magnetic properties using a vibrating sample magnetometer; Comparison of the experimental results with the orientation dependence of the squareness and coercivity.

Presents a study that measured the magnetic full linewidth of yttrium iron garnet films. Methodology; Correlation between linewidth measurements and the temperature; Examination of the frequency dependence of the linewidth measurements.