Volume 96, Issue 3, 01 August 2004
Index of content:
- MAGNETISM AND SUPERCONDUCTIVITY (PACS 74-76)
96(2004); http://dx.doi.org/10.1063/1.1763237View Description Hide Description
-doped thin films were deposited on substrate via a pulsed laser deposition. The deposited composite thin films were characterized by x-ray diffraction, scanning electron microscopy, and electro- and magneto-transport measurements. The main phase in the composite films was the perovskite phase. Texturelike microstructure was observed in the composite films while the average grain size remained almost unchanged compared to the pure samples. The metal-insulator transition temperature decreased as a result of the addition of and further reduced with increasing content. A maximum low field mangetoresistance of was achieved in the thin films, which could be well explained in terms of the grain boundary tunneling effect. The composition of the composite thin films can be easily tuned by adjusting the target composition. This method is believed to be applicable to exploring the combinations of other manganites and insulators.
96(2004); http://dx.doi.org/10.1063/1.1763996View Description Hide Description
The high power ferromagnetic resonance response, as well as butterfly curves of the spin wave instability threshold microwave field amplitude versus in-plane static field profiles, have been measured for Permalloyfilms with thicknesses of 104, 128, and at a nominal pumping frequency of . The values range from about 1 to . Both the resonance saturation response at the field and the subsidiary absorption (SA) response for static fields below the field are similar in appearance to those for bulk ferrites. Butterfly curves over the response region, while similar to those for ferrites, exhibit a film thickness dependent band edge cutoff effect not found in bulk ferrites. The butterfly curve data were analyzed on the basis of a spin wave instability theory adapted to thin films. The observed shift in the band edge cutoff with thickness agrees with calculations based on the thin film dispersion response and the assumption of first order instability processes with critical modes at one half the pumping frequency. The fitted spin wavelinewidths give values consistent with metallic relaxation processes, but indicate critical modes with wave vectors that always make relatively small angles with the static field, very different from the critical modes for bulk ferrites. Three key conclusions from this work are (1) the nonlinear microwave response in Permalloyfilms is a threshold effect related to well established spin wave instability processes, (2) the details of the response are controlled largely by the thin filmspin wave dispersion, and (3) these nonlinear processes occur for very small precession angles.
Quantitative determination of the magnetization of proton irradiated spots in graphite with magnetic force microscopy96(2004); http://dx.doi.org/10.1063/1.1759396View Description Hide Description
Using the point probe approximation of magnetic force microscopy(MFM) and measurements of the MFM signal as a function of the tip-to-sample distance, we have determined quantitatively the magnetization of proton irradiated spots in highly oriented pyrolytic graphite. From different spots produced with ion fluences ranging from 0.05 to 75 we obtained magnetization values of the order of These values are in the same range of those from soft magnetic materials.
High frequency magnetization rotation induced by a dc spin-polarized current in magnetic nanostructures96(2004); http://dx.doi.org/10.1063/1.1766408View Description Hide Description
While most recent studies of the spin-transfer torque effect in nanoscale magnetic structures mainly concern with spin-wave excitation and the magnetization hysteretic switching induced by spin-polarized current, we theoretically investigate the large angle magnetization precessions triggered and sustained by the spin current in great detail using the Landau-Lifshitz-Gilbert equation. The study is conducted particularly on the nanostructures comprising square magnets with an easy-plane anisotropy. A simple nanoscale magnetoelectronic device is proposed based on the structure without mechanical components to function as a motor to convert a dc current into an ac voltage of microwave frequency. The device is unique because the output amplitude and frequency can be continuously tuned by the electrical current in a wide range. Quantitative analysis of the device structure, function, and realization is provided.
Effect of low-temperature baking on the radio-frequency properties of niobium superconducting cavities for particle accelerators96(2004); http://dx.doi.org/10.1063/1.1767295View Description Hide Description
Radio-frequency superconducting (SRF) cavities are widely used to accelerate a charged particle beam in particle accelerators. The performance of SRF cavities made of bulk niobium has significantly improved over the last ten years and is approaching the theoretical limit for niobium. Nevertheless, RF tests of niobiumcavities are still showing some “anomalous” losses that require a better understanding in order to reliably obtain better performance. These losses are characterized by a marked dependence of the surface resistance on the surface electromagnetic field and can be detected by measuring the quality factor of the resonator as a function of the peak surface field. A low-temperature “in situ” bake under ultrahigh vacuum has been successfully applied as final preparation of niobium RF cavities by several laboratories over the last few years. The benefits reported consist mainly of an improvement of the cavity quality factor at low field and a recovery from “anomalous” losses (so-called “ drop”) without field emission at higher field. A series of experiments with a CEBAF single-cell cavity have been carried out at Jefferson Lab to carefully investigate the effect of baking at progressively higher temperatures for a fixed time on all the relevant material parameters. Measurements of the cavity quality factor in the temperature range and resonant frequency shift between provide information about the surface resistance, energy gap, penetration depth, and mean free path. The experimental data have been analyzed with the complete BCS theory of superconductivity. The hydrogen content of small niobium samples inserted in the cavity during its surface preparation was analyzed with nuclear reaction analysis. The single-cell cavity has been tested at three different temperatures before and after baking to gain some insight on thermal conductivity and Kapitza resistance and the data are compared with different models. This paper describes the results of these experiments and comments on existing models to explain the effect of baking on the performance of niobium RF cavities.