F. M. de Aguiar

Studies of coupled metallic magnetic thin-film trilayers

Results are reported of a detailed study of static and dynamic responses in symmetric systems consisting of two ferromagnetic films separated by a nonferromagnetic spacer layer. A comparison is made with experimental results for two systems grown by sputter deposition in an UHV chamber, namely, NiFe/Cu/NiFe and Fe/Cr/Fe. First, we present model calculations where the coupling between the magnetic films through magnetic dipolar, bilinear, and biquadratic exchange interactions are fully taken into account, together with surface, in-plane uniaxial, and cubic anisotropies. An analytical expression is given that can readily be used to consistently interpret magnetoresistance, magneto-optical Kerr effect, ferromagnetic resonance, and Brillouin light scattering (BLS) data in such trilayers. Application of the results to BLS data in Ni81Fe19(d)/Cu(25 A)Ni81Fe19(d), with d=200 and 300 A, shows that it is essential to treat the dipolar interaction adequately in moderately thick systems. The results are also applied...

Photo-induced polymerization of polyaniline

A straightforward route to prepare polyaniline is presented in which photons and metallic ions replace conventional oxidants to promote polymerization of aniline monomer. The photopolymerization methods yield a composite material that has been characterized by its UV, visible and Raman spectroscopic analysis and by scanning electron microscopy and X-ray microanalysis as well. Intriguing forms of silver wires embedded in polyaniline are observed, typically of 1 μm wide and up to 100 μm long. It is shown that the morphology of the resulting conducting polymer strongly depends on the excitation wavelength, while a globular morphology is found for the UV synthesis and a fibrillar one is found for visible light excitation.

Ferromagnetic resonance linewidth and anisotropy dispersions in thin Fe films

In-plane ferromagnetic resonance (FMR) has been used to study the room-temperature linewidth ΔH of single crystal Fe films grown by dc magnetron sputtering onto MgO(100) substrates. Several samples were grown with the film thickness in the range 70 A<tFe<250 A. The measurements were carried out in the frequency range from 7.0 to 12.3 GHz. A phenomenological model for the FMR linewidth was developed that includes simultaneous effects due to intrinsic damping and angular dispersions of the cubic and uniaxial axes of anisotropy. These angular dispersions are found to be responsible for the relatively larger linewidths observed as a function of the in-plane field direction with fixed frequency, and as a function of frequency for the easy and hard directions as well. The behavior of the linewidth with the film thickness can be described by a sum of a constant volume term plus a term proportional to 1/tFe, representing the relaxation due to the misfit dislocations.

Measurements of exchange anisotropy in NiFe/NiO films with different techniques

One of the puzzles of the recent investigations on the exchange anisotropy in ferromagnetic (FM)/antiferromagnetic (AF) bilayers is the fact that different techniques yield different values for the exchange field (HE) between the layers. We report an investigation on sputtered NiFe/NiO carried out with three different techniques, namely, magneto-optical Kerr effect magnetometry (MOKE), Brillouin light scattering (BLS), and ferromagnetic resonance (FMR). In an attempt to reconcile the measurements obtained with the various techniques, we interpret the data with a model that includes the formation of a planar domain wall in the AF layer, giving rise to a torque on the FM moment represented by an effective domain wall field (HW). We find out that while the same pair of values of HE and HW provide equally good fits to the reversible FMR and BLS measurements, different pairs are necessary to fit the irreversible magnetometry data.

Order and chaos in ferromagnetic spin wave instabilities

Abstract Spin wave instabilities driven by microwave fields display chaotic phenomena similar to other nonlinear dynamical systems. We present observations of period-doubling and odd-period bifurcation routes to chaos in microwave parallel pumping experiments with Yttrium Iron Garnet (YIG). The results are interpreted on the basis of numerical solutions of a symmetric nonlinear two-mode model. An asymmetric two-mode model is also proposed to explain recent similar experiments with the perpendicular second-order Suhl instability in YIG.

Biquadratic coupling in sputtered Fe/Cr/Fe still in need of a new mechanism

The bilinear (J1) and biquadratic (J2) exchange coupling constants were measured in sputtered trilayers of (100) Fe(40 A)/Cr(s)/Fe(40 A) for several Cr spacer layer thicknesses in the range s=8–35 A and as a function of temperature T, using magneto-optical Kerr effect magnetometry, Brillouin light scattering, and ferromagnetic resonance. In the samples in the range s=8–13 A, corresponding to the first antiferromagnetic peak of J1, J2 follows J1 with a room temperature ratio J2/J1≅0.1, while in the range 25–35 A, corresponding to the second antiferromagnetic peak, J2 also follows J1 but with a much larger ratio J2/J1≅1. This result, as well as the temperature dependence of J2 in all samples but the one with s=15 A, cannot be explained by any of the intrinsic or extrinsic mechanisms that have been proposed for the origin of the biquadratic exchange coupling in Fe/Cr/Fe.

Numerical experiments on quantum chaotic billiards

A recently proposed numerical technique for generation of high-quality unstructured meshes is combined with a finite-element method to solve the Helmholtz equation that describes the quantum mechanics of a particle confined in two-dimensional cavities. Different shapes are treated on equal footing, including Sinai, stadium, annular, threefold symmetric, mushroom, cardioid, triangle, and coupled billiards. The results are shown to be in excellent agreement with available measurements in flat microwave resonator counterparts with nonintegrable geometries.

Magnetic properties of Fe films epitaxially grown on Cr/GaAs(100) by dc magnetron sputtering

We report on the growth of single-crystal Fe films by magnetron sputtering onto GaAs substrates. In order to establish the epitaxial orientation with the substrate a 100 A Cr buffer layer was rf sputtered. The crystalline and magnetic properties were studied by x-ray diffraction, ferromagnetic resonance, and Kerr effect magnetometry techniques. The θ-2θ x-ray measurements show that only the Fe (200) peak is present, and the rocking curve shows a full width half maximum of 2°. Ferromagnetic resonance lines exhibit a line width of about 30 Oe, and the in-plane resonance field as a function of the azimuth angle presents a fourfold symmetry with no induced anisotropy. The in-plane hard- and easy-axis magneto-optical hysteresis loop traces are consistent with the ferromagnetic resonance results.

Exchange anisotropy and spin-wave damping in CoFe/IrMn bilayers

The magnetic properties and the spin-wave damping in FM/AF bilayers of CoFe/IrMn have been investigated with magneto-optic Kerr effect magnetometry, ferromagnetic resonance (FMR), and Brillouin light scattering (BLS). As observed in other systems, the values of the interlayer exchange field obtained with different techniques are discrepant, but they all exhibit the expected t−1 dependence with the FM film thickness t. On the other hand the spin-wave relaxation rates measured by BLS and FMR are fit with a t−2 dependence plus a constant term. This is interpreted as the sum of two independent contributions, an intrinsic mechanism dominated by Gilbert damping and an extrinsic mechanism dominated by two-magnon scattering due to fluctuations of the interlayer exchange coupling caused by interface roughness.

Spin‐wave auto‐oscillations still in need of a good model

The onset of spin‐wave auto‐oscillations has been studied in spheres of yttrium iron garnet pumped by a microwave field parallel or perpendicular to the dc field. Data were taken for the auto‐oscillation threshold field, amplitude, and frequency as a function of sample dimension, shape, and the spin‐wave relaxation rate and wave vector. The behavior near the onset confirms the theoretical prediction that the auto‐oscillation results from a Hopf bifurcation. However, data for the auto‐oscillation frequency and threshold field are in clear disagreement with predictions. These results indicate that although the area of nonlinear spin‐wave phenomena has been under investigation for over 30 years, a good model is still needed for the auto‐oscillations.