P. E. Wigen

Spin-wave spectra of perpendicularly magnetized circular submicron dot arrays

Dynamic microwave properties of arrays of circular Ni and Ni81Fe19 dots were studied by X-band ferromagnetic resonance (FMR) technique. All of the dots had the same radius 0.5μm, thickness 50–70nm, and were arranged into rectangular or square array with different interdot separations. In the case of perpendicular magnetization multiple (up to 8) sharp resonance peaks were observed below the main FMR peak, and the relative positions of these peaks were independent of the interdot separations. Quantitative description of the observed multiresonance FMR spectra is given using the dipole-exchange spin wave dispersion equation for a perpendicularly magnetized film where in-plane wave vector is quantized due to the finite dot radius, and the inhomogenetiy of the intradot static demagnetization field in the nonellipsoidal dot is taken into account.

Pt layer thickness dependence of magnetic properties in Co/Pt multilayers

The magnetic properties of Co/Pt multilayer films prepared by dc sputtering have been investigated using ferromagnetic resonance and a vibrating sample magnetometer. The thickness of the Co layer is constant at 25 A while the thickness of the Pt layer varies from 0.9 to 18 A. The saturation magnetization per unit volume of Co is nearly constant for all the films and close to that of bulk Co. In the ferromagnetic resonance spectra, the presence of two resonance modes having an amplitude ratio that varies substantially with the Pt thickness is strong evidence that a significant coupling between cobalt layers exists even at 18 A of Pt. For Pt thicknesses greater than 3.6 A, a uniaxial perpendicular magnetic anisotropy field component increases with Pt thickness and saturates at about 9.0 A. If the induced anisotropy energy is attributed to a surface anisotropy energy at the Co/Pt interface, it has a value Ks of approximately 0.57 erg/cm2. The temperature dependence of the magnetic hysteresis loop and the fer...

Magnetic and structural properties of spin-reorientation transitions in orthoferrites

Magnetic and structural characteristics of ErFeO3, TmFeO3, and YbFeO3 single crystals were studied over a wide temperature range. Magnetic measurements found that the spin-rotation transitions in all crystals are well described by the earlier proposed theory with no fitting parameters. Additionally, they have shown the absence of the magnetic compensation point in TmFeO3 and a noticeable growth of the c-axis magnetization at low temperatures in TmFeO3 and ErFeO3. The x-ray measurements found no symmetry-lowering lattice distortions during the reorientation. Overall, the measurements cover a wide range of material parameters and demonstrate the generality of the modified mean field theory of the Γ4→Γ24→Γ2 orientation phase transitions in orthoferrites.

Observation of an exchange dominated surface spin wave in yttrium iron garnet films

Abstract Ferromagnetic resonance techniques have been used to investigate the spin-wave spectra of thin single crystal yttrium iron garnet films. The investigation has disclosed a spin-wave with a high degree of localization at the surface in the parallel resonance configuration. This surface mode appears to be located on the film-substrate interface with a penetration depth of approximately 500 A.

Magnetic properties of sputtered Bi/sub 3/Fe/sub 5/O/sub 12/

A series of Bi-substituted yttrium-iron-garnet films were grown including its end members, YIG and BiIG, by an RF-diode sputtering method. The low-temperature magnetization of highly Bi-substituted garnet is much lower than expected from the linear expansion of the lattice. The temperature dependence of the magnetization of these materials is described by a molecular field model in which some of the moment on the tetrahedral site Fe/sup 3+/ ion is assumed to be transferred to the Bi/sup 3+/ ion on the dodecahedral site. The proposed model contains three sublattices: Fe/sup 3+/ ion at the octahedral site, Fe/sup 3+/ with reduced spin at the tetrahedral site, and partially spin polarized Bi/sup 3+/ ions at the dodecahedral sites. Reasonable agreement between the model and data is obtained for BiIG if it is assumed that the spin on the tetrahedral iron sublattice is reduced by 0.5 mu /sub B/ and the Bi ion has a moment of 0.2 mu /sub B/ which is strongly coupled to the iron sublattice. The large values of the uniaxial anisotropy energy, the crystalline anisotropy energy, and the linewidths measured by ferromagnetic resonance are complementary to this result. >

In-plane and out-of-plane uniaxial anisotropies in rectangular arrays of circular dots studied by ferromagnetic resonance

Ferromagnetic resonance at 9.2 GHz (X band) was used to characterize the uniaxial magnetic anisotropies in rectangular arrays of submicron circular Ni dots. The in-plane anisotropy, originated from interdot interactions in the rectangular lattice, and the perpendicular anisotropy, due to individual dot shape and magnetostriction, were explored. For in-plane dependencies of the resonance field (Hr), the main resonance mode angular dependence was well described by the standard Kittel formula. As the interdot distances decreased from 800 to 50 nm, the in-plane uniaxial anisotropy field changed from 5 to 130 Oe, in reasonable agreement with calculations. Simultaneously, the position of perpendicular Hr increased from 6.38 to 6.83 kOe, also following Kittel’s formula.

Picosecond recombination of charged carriers in GaAs

The recombination kinetics of charged carriers in GaAs have been investigated on a picosecond time scale. A pump‐probe technique was used to measure reflectivity changes as a function of time up to 900 ps. Initially, (1.2±0.2)×1020 carriers/cm3 were excited into the conduction band. The decay curve indicated dominance of a three‐body (Auger) recombination process up to about 120 ps, with two‐body recombination processes dominating after 120 ps, the switch occurring at a carrier concentration of (5±2)×1019 carriers/cm3. Values for the Auger recombination coefficient and the two‐body recombination coefficient were determined to be (7±4)×10−31 cm6 s−1 and (3.4±1.7)×10−11 cm3 s−1, respectively. The change in reflectance was observed to have essentially returned to zero within 900 ps.

Origin of fourfold anisotropy in square lattices of circular ferromagnetic dots

We discuss the fourfold anisotropy of the in-plane ferromagnetic resonance field Hr, found in a square lattice of circular Permalloy dots when the interdot distance a becomes comparable to the dot diameter d. The minimum Hr along the lattice 11 axes and the maximum along the 10 axes differ by 50 Oe at a/d=1.1. This anisotropy, not expected in uniformly magnetized dots, is explained by a mechanism of nonuniform magnetization mr in a dot in response to dipolar forces in the patterned magnetic structure under strong enough applied field. It is well described by an iterative solution of a continuous variational procedure.

Domain wall oscillations in magnetic garnet films

Abstract Domain wall oscillations in magnetic garnet films have been observed in the stripe lattice and in the bubble lattice for applied in-plane magnetic fields approaching the saturation limit of the film. Observations are reported in (111) and (001) oriented films. An effective domain wall mass model is developed which allows the variation of the azimuthal angle of the spins in a moving wall when an in-plane field is applied in the plane of the wall. The new model gives results which are in much better agreement with the experimental results than previous models. Reasonable agreement is also observed between theory and experiment when the cubic anisotropy is included. Experimental evidence of the Hubert wall structure and its change for in-plane fields less than 8 M is also reported.

Controlling chaos in thin YIG films with a time-delayed method

Abstract We report the first experimental and numerical results of a new time delayed method to control chaos. Time delayed control in this experiment involves perturbing a system parameter by an amount proportional to the samples chaotic absorption signal from a previous moment in time. By increasing the control gain, the chaotic absorption is controlled through a reverse bifurcation series to a state where the absorption is constant in time (the quiescent state).