R. Seed

Pulsed laser deposition of epitaxial BaFe12O19 thin films

Epitaxial thin films of barium hexaferrite (BaFe12O19) have been fabricated by the pulsed laser deposition technique on basal plane sapphire. Structural studies reveal the films to be predominantly single phase and crystalline, with the c axis oriented perpendicular to the film plane. The magnetic parameters deduced from vibrating sample magnetometer and ferromagnetic resonance (FMR) measurements are close to the parameters associated with bulk materials. Post annealing of the films reduced the FMR linewidth by more than a factor of 3 so that it compares reasonably well with single‐crystal films. The derivative FMR linewidth was measured to be 66 Oe at 58 GHz and 54 Oe at 86 GHz. Spin‐wave‐like modes have been observed for the first time in barium ferrite films. The deduced exchange stiffness constant of 0.5×10−6 ergs/cm is in reasonable agreement with recent calculations.

Epitaxial yttrium iron garnet films grown by pulsed laser deposition

Epitaxial Y3Fe5O12 (YIG) films have been grown by the pulsed laser deposition (PLD) technique on (111) gadolinium gallium garnet substrates. The effect of substrate temperature and oxygen partial pressure on the structure, composition, and magnetic properties of the films was investigated and compared to liquid phase epitaxy YIG films. The results demonstrated that epitaxial YIG films could be prepared under a wide range of deposition conditions, but narrow linewidth (ΔH≂1 Oe) films were producible only at low oxygen partial pressures (O2<250 mTorr) and relatively high substrate temperatures (Ts≳800 °C). Since the linewidth of single‐crystal YIG is dominated by surface and volume defects and/or impurities, the narrow linewidth indicated that PLD is a viable technique for producing high‐quality ferrite films for microwave device applications. In addition, under all deposition conditions (50–1000 mTorr and 700–850 °C) there is a uniaxial axis perpendicular to the film plane. However, at low oxygen pressure ...

Oriented barium hexaferrite thin films prepared by pulsed laser deposition

Oriented thin films of barium hexaferrite, BaFe/sub 12/O/sub 19/, were grown in situ on (0001) sapphire substrates utilizing a pulsed laser deposition technique. X-ray diffraction, elastic backscattering spectrometry, ferrimagnetic resonance, and vibrating sample magnetometry confirm that the structure, composition, and magnetic parameters are consistent with films prepared by other techniques such as liquid phase epitaxy. >

Microwave characteristics of high T/sub c/ superconducting coplanar waveguide resonator

A theoretical formulation has been developed to calculate the coupling coefficient, London penetration depth, and surface resistance of a coplanar waveguide resonator fabricated from films of superconducting YBCO material. Experimental data of the reflection coefficient as a function of temperature and frequency agree reasonably well with calculations. The formulation is of sufficient generality to be applicable to other guided structures. >

EXCESS CURRENT IN SHUNTED JOSEPHSON WEAK LINKS

In a variety of Josephson weak links the resistively shunted junction model does not properly describe observed excess currents in the voltage current characteristics. A modification of the model is proposed which more adequately describes experimental data.

Magnetic and structural characteristics of pulsed laser deposited epitaxial Y/sub 3/Fe/sub 5/O/sub 12/ films

Epitaxial Y,FqO, films were grown on (111) gadolinium gallium garnet substrates using the pulsed laser deposition technique. The results demonstrated that epitrusial YIG fiims could be prepared under a wide range of deposition conditions, but narrow linewidth (AH ii 1 Oe) films were produced only at low oxygen partial pressuresand relatively hih substrate temperatures. In addition, the fh exhibited a uniaxiial magnetic anisotropy parameter which changed sign from negative to positive with increasing oxygen partial pressure. I. INTRODUCXION Epitaxial Y

Anomalous microwave absorption in high Tc YBCO crystals

esO,z films were grown in order to determine if pulsed laser deposition (PLD) can produce high quality ferrite thin films. Yttrium iron garnet (YIG) is an ideal system to deposit for calibration purposes, since an abundant amount of information is available in the literature including data on liquid phase epitaxy (LPE) films and bulk single crystals. In addition, the linewidth, AH, of YIG is dominated by impurities and surface and/or volume defects. Thus, linewidth measurements can be used as a practical means to determine the quality of PLD YIG films and consequently the potential of the PLD technique for preparing high quality ferrite films in general. 11. EXPERIMENTAL

YBCO microwave grain boundary mixer using a SrTiO/sub 3/ bicrystal substrate

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

A microwave mixer was patterned on a microstrip transmission line of superconducting YBCO (YBa/sub 2/Cu/sub 3/O/sub 7-x/). The YBCO film was epitaxially laser deposited on a SrTiO/sub 3/ bicrystal substrate. A weak link was constructed by patterning a microbridge in the microstrip at the bicrystal boundary. Microwave signals at 9.000 GHz and 9.941 GHz were applied at the input of the microstrip line. An output intermediate frequency signal was observed at 941 MHz and was detected as the transmitted signal. The microbridge junction, which behaved as a resistively shunted Josephson junction (RSJ), was current biased slightly above the critical current I/sub c/. The mixer conversion loss was measured at the input and output ports of the device package. The mixing efficiency was determined at the device junction. This efficiency was compared with the calculated efficiency determined by numerical solution of the Josephson equation for the weak link junction. >

Planar superconducting YBCO microwave mixer employing an engineered grain boundary weak link junction

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.