F. J. Rachford

Ferromagnetic resonance studies of very thin epitaxial single crystals of iron

Ferromagnetic resonance (FMR) has been used to study a wide variety of very thin single crystals of Fe grown on (110) GaAs substrates by molecular beam epitaxy. Data were taken at room and liquid nitrogen temperatures for films with thicknesses L in the range 18–200 A. Due to surface anisotropy, the easy axis of the magnetization switches from [100] to [110] when L≤50 A, independent of whether the the film surface is passivated by an Al‐overcoat or has a thin Fe oxide surface layer. We suggest that this is an effective surface anisotropy arising in part from a depth dependent strain near the film‐substrate interface. The changes in the parameters describing the angular dependence of the FMR spectra upon cooling to 77 K can be explained as due to magnetostriction arising from thermally induced strains plus the temperature dependence of the cubic volume anisotropy. The FMR linewidth is shown to be a linear function of frequency in the range 5–40 GHz.

Interaction of metallized tubules with electromagnetic radiation

Several diacetylenic lecithins form tubular microstructures (tubules) when their liposomes are cooled through the chain‐melting transition. Recently, the tubules have been metal plated by an electroless technique. This paper reports on the interaction of permalloy coated tubules with electromagnetic radiation. At 10 vol % loading of tubules in an epoxy matrix has a real dielectric constant e’≊50 at a frequency of 9.5 GHz. Simple electrodynamics accounts well for the observed results. Far higher values of e’ may be achievable with longer tubules and with improved metal coatings.

Microwave resonance and propagation in nonsaturated ferromagnetic media. I. Magnetic resonance in single crystal ferrite platelets

We present calculations of the magnetic resonance frequencies of multidomain single crystal ferrite platelets. The theoretical results are compared to our microwave ferromagnetic resonance (FMR) experiments performed at frequencies between 8 and 40 GHz and magnetic fields between 0 and 20 kOe. Good agreement is found between theory and experiment with no adjustable parameters. In the calculation we have included the effect of a change in the periodicity of regularly patterned domain structures on the FMR frequencies. The variation in periodicity is seen to influence the measured FMR frequencies f for magnetic fields H applied normal to the platelet plane in agreement with our calculations. The predicted variation in periodicity agrees with our visual observation of domain structures in the sample. The in‐plane FMR data is also in reasonable agreement with calculations. Although the measured frequency and field FMR linewidths agree well with our calculations in magnetically saturating fields, the agreement...

Z type Ba hexagonal ferrites with tailored microwave properties

The microwave properties of Z type Ba hexagonal ferrites with uniaxial anisotropy can be controlled over a wide range by choice of an appropriate mixture of Zn, Ni, and Co as divalent species. Using unmagnetized powders of these ferrites, the microwave permeability spectra were taken in a swept frequency mode in the temperature range 200–550 K. From these data, the product γHa was derived. SQUID magnetometry was used to measure the magnetization and anisotropy separately. Pure Zn or Co Z type ferrites were found to have temperature dependences of γHa similar to those of other hexagonal ferrites containing Zn or Co, i.e., nearly flat for Zn and strongly increasing with T for Co. Ni–Z ferrite is notably different: the combination γHa, which determines the frequency of maximum absorption, is found to decrease rapidly with increasing temperature near 300 K. Magnetometry indicates that this temperature dependence γHa is caused mostly by variation of Ha with temperature. The temperature dependence of γHa for Ni...

Artificial dielectric properties of microscopic metallized filaments in composites

Measurements have been made of the dielectric response of distributions of metallized fibers aligned in epoxy matrices for different loading fractions and fiber lengths. These have been shown to be in reasonable agreement with a simple independent particle theory. Use of the theory allows classification of fiber dielectric behavior by fiber conductivity and a characteristic length. Some aggregation of the fibers has been observed, even at relatively low loading densities. Computer simulation suggests enhanced dielectric constants of the composites for a moderate range of separations where the fibers are offset about 75% of their lengths with respect to each other and are closer than about 10 radii. This effect may be significant for aligned fibers which tend to aggregate in the magnetic field with separations and offsets within this range. The resulting composites are rugged and easily machined, and with less than 5% fiber weight loading have large, highly anisotropic dielectric constants of 60 or more at...

Uniaxial dielectric anisotropy in Ba0.5Sr0.5TiO3 films studied by evanescent-probe microscopy

The dielectric permittivity, tunability (Δe/e), and loss tangent of Ba1−xSrxTiO3 (BST) films grown by pulsed-laser deposition are studied by near-field microwave microscopy. Based on theoretical simulations, a method is developed to measure the uniaxial dielectric anisotropy, e⊥/e∥, in BST films grown at different oxygen pressures. The measured e⊥/e∥ decreases with the film-growth oxygen pressure, consistent with the structural anisotropy. The films prepared at 50 mT, with e⊥≈e∥, have the highest permittivity, tunability, and figure of merit.

Simultaneous imaging of dielectric properties and topography in a PbTiO3 crystal by near-field scanning microwave microscopy

We use a near-field scanning microwave microscope to simultaneously image the dielectric constant, loss tangent, and topography in a PbTiO3 crystal. By this method, we study the effects of the local dielectric constant and loss tangent in the geometry of periodic domains on the measured resonant frequency, and quality factor. We also carry out theoretical calculations and the results agree well with the experimental data and reveal the anisotropic nature of the dielectric constant.

The microwave surface impedance of granular high TC superconductors in dc magnetic fields: Its relationship to frequency dependence

The surface impedance of bulk and thick film granular high temperature superconductors has been studied. The static magnetic field, temperature, and frequency dependence of the surface impedance have been analyzed in terms of the weakly coupled grain model. Accurate quantitative predictions of the frequency dependence of the surface resistance are made using this model. The model also quantitatively accounts for the negative magnetic field coefficient of the effective rf penetration depth. The rf penetration depth is a maximum when the ratio of penetration depth to skin depth is 0.930.

Effects of annealing on the microwave properties of spin-spray Ni-Zn ferrites

We have studied the properties of a series of spinel ferrite thin films prepared from aqueous solutions at temperatures slightly below 100/spl deg/C, for a range of compositions from pure Fe/sub 3/O/sub 4/ to about Ni/sub 0.34/Zn/sub 0.18/Fe/sub 2.48/O/sub 4/. As made, the films are semiconducting and have high dielectric losses, but have narrow ferromagnetic resonance linewidths and often display standing spinwave spectra. As a result of annealing in air at temperatures above 300/spl deg/C, these ferrites undergo a transformation to a state having much higher resistivity and a reduced magnetic moment compared to as-grown films. This annealing decreases the microwave dielectric losses, but increases the ferromagnetic resonance linewidth. The changes in these properties with annealing are consistent with a partial transformation from spinel to maghemite, /spl gamma/-Fe/sub 2/O/sub 3/. >

Chaos and chaotic transients in yttrium iron garnet (invited)

Long‐lived chaotic transients are a prominent feature of the spin‐wave behavior of spheres of yttrium iron garnet (YIG) being perpendicularly pumped in the region of the first‐order Suhl instability. These transients may appear after a sudden increase in rf pumping power or during transitions between quasiperiodic auto‐oscillations. The transients, which result from the collision of a chaotic attractor with the basins of attraction of multiple stable quasiperiodic attractors, vary in lifetime by more than six orders of magnitude, from milliseconds to hours, as a function of the rf driving field. The average lifetimes of these transients fit an extended Grebogi–Ott–Yorke scaling law. Roughening the surface of the YIG sphere drastically changes the behavior of these transients.