R. Karim

Laser ablation deposition of YIG films on semiconductor and amorphous substrates

Yttrium Iron Garnet (YIG) films were grown on single crystal silicon, Corning glass, single crystal MgO and quartz substrates using pulsed laser ablation techniques. Films were grown over a range of temperature, deposition rate, and oxygen partial-pressure conditions. Sample microwave magnetic properties were deduced using ferromagnetic resonance (FMR) measurements, with the sample magnetic properties also being measured. All films were polycrystalline, with as-grown films deposited at temperatures below 800/spl deg/C having a weak magnetization. Good magnetic and microwave magnetic properties were obtained after annealing the samples in atmosphere at temperatures above 720/spl deg/C. FMR linewidths of 55 Oe were measured on the annealed films on glass substrates. We believe that growth of thick films with these properties will be sufficient for many polycrystalline-based YIG devices.

The engineering of ferrite films at the atomic scale

Abstract We have developed an artificial preparation scheme to produce ferrite films in which magnetic and non-magnetic layers can be alternated at the atomic scale. As a result of this growth process, we have prepared a ferrite film in which aluminum substitutions have minimal effects on the saturation magnetization in contrast to ferrite films grown by the natural process.

A study of hysteretic absorption in polycrystalline high-T/sub c/ superconductor YBCO

The low-field microwave absorption in polycrystalline specimens of the high-T/sub c/ superconductor YBCO is studied. The low-field absorption derivative signal as measured using EPR techniques in the X-band at low fields (H >

Low-Field microwave measurements of YBa2Cu3O7−x near the superconducting transition temperature

Novel microwave absorption and dispersion measurements have been performed on well-characterized single-crystal platelets of the high-Tc superconductor YBa2Cu3O7−x. The results are explained in terms of the rapid variation of the penetration depth near and belowTc. Since EPR measurements are very sensitive to small changes in absorption and dispersion, this technique should be very useful in the understanding of the transition temperature region in both “new” and “old” superconducting materials.

Magnetically modulated microwave-absorption measurement of the penetration depth in a polycrystalline YBa2Cu3O7-x thin-film

In previous work, a magnetically modulated microwave absorption (MMMA) technique to measure the Ginzburg–Landau parameter κ=(Λ/ξ) in high‐Tc superconductors (bulk samples of YBCO) was described. In this work, how this technique can be applied to the measurement of the London penetration depth Λ is described. Data are reported for the temperature variation of Λ in a well‐characterized c‐axis‐oriented polycrystalline thin film of YBa2Cu3O7−x, prepared by a pulsed laser deposition technique. For low temperatures, Λ(0)≂3640 A with the microwave electric field oriented parallel to the c axis. The temperature dependence of Λ(0)/Λ(T) can be well described by the two‐fluid model. The data are also reasonably fit by a BCS model, however the MMMA technique (in the work here reported) does not have the resolution at low temperatures to obtain strong quantitative evidence for an energy gap Δ (even if it exists).

Anomalous microwave absorption in high Tc YBCO crystals

Abstract Traditional EPR measurements have been useful in characterizing absorption peaks or maxima in magnetic materials. Recently EPR techniques have been modified to measure absorption “dips” or minima in high T c superconducting materials. We report here the observation at different temperatures of absorption maxima and minima in single crystals of YBCO. At temperatures above 52 K and below 39.5 K minimum absorption was observed at H = 0 as the applied magnetic field, H d.c. , was varied from −50 Oe to +50 Oe. We attribute the microwave absorption as due to the formation of fluxoids in the presence of H d.c. . For temperatures between 39.5 and 52 K maximum absorption was observed at H = 0 instead of the usual superconducting minimum. We attribute the EPR absorption peaks to the hopping motion of the oxygen vacancies in the basal plane of the crystal where fluxoids are pinned at oxygen vacancy sites.

Effects of dc transport current on low‐field microwave absorption in ceramic superconducting YBCO samples

The effects of dc transport current on low‐field microwave absorption have been investigated systematically on bulk ceramic YBa2Cu3O7−δ samples. At a fixed temperature T, the critical current Ic(T) at which the electron paramagnetic resonance (EPR) signal vanishes varies linearly with T. The EPR absorption characteristics obey a scaling rule in accordance with the flux creep model. A revised version of the flux creep model is also presented.

Field modulated microwave absorption in high T/sub c/ superconductors

Modified electron spin resonance (ESR) techniques were used to measure low-field (<200 Oe) microwave absorption of high-T/sub c/ cuprates YBa/sub 2/Cu/sub 3/O/sub 7-x/. A series of experiments on polycrystalline and single-crystal samples in which the angle between the DC and the modulating magnetic fields was varied showed that the microwave absorption obeys results predicted from thermodynamic considerations of fluxoids. The absorption takes place in the normal regions created by flux penetration at low fields in these samples. The change in magnetically modulated microwave absorption on passing through T/sub c/ is explained by the decrease in absorption that occurs when part of the sample becomes superconducting and by the rapid variation of the penetration depth near T/sub c/. The technique is an extremely sensitive method of detecting superconductivity in very small samples and for studying the superconducting state. Possible applications such as microwave attenuators, magnetic memories, and gaussmeters are discussed.

Measurement of kappa in high Tc superconductors

The experimental magnetic‐field derivative of the microwave absorption signal, obtained using electron paramagnetic resonance techniques, has a line shape that depends on the Ginzburg–Landau model parameter κ of the superconducting sample at a given temperature. The type‐II superconducting equation of state can be utilized to describe the drop in the output microwave power derivative, from its maximum at H ≂ Hc1 to its much lower values when H ≫ Hc1, in order to estimate the values of κ. The technique also allows for the experimental determination of hysteretic behavior in B as a function of H.

Electron-spin resonance in single crystals of YBa2Cu3O7-x and Bi4/3Pb2/3Sr2CaCu2O8+x

We report on electron‐spin resonance on single crystals of YBa2Cu3O7−x and Bi4/3Pb2/3Sr2CaCu2O8+x at 9 GHz. While in the YBCO there was no observable ESR signal in the normal state (above Tc) or in the superconducting state (below Tc), we did observe a large microwave nonresonant absorption below Tc at low field values (H<200 Oe). We attribute this form of absorption to fluxoid formation below Tc. In the BiPbSrCaCuO single crystals, a significantly different result was found in that an electron‐spin‐resonance signal was measured both above and below Tc, besides the low‐field absorption occurring only below Tc. The g value was found to be 2.055 and reasonably constant with temperature from 300 to 4 K with H parallel to the c axis. We attributed the ESR line to Cu++ ion excitation. The ESR signal increased as T decreased from 300 K to Tc and the variation with temperature fits a Boltzmann statistics law. Below Tc the ESR signal remained constant as T was decreased below Tc. We explained this result in terms...