K. Moorjani

A novel microwave technique for detection of superconductivity

A new technique for detection of superconductivity which is based upon microwave resistivity is described. This method is similar in both its implementation and execution to the technique of electron‐spin resonance, and, as a consequence, exhibits high sensitivity. An additional benefit of this method is the fact that metal‐insulator phase transitions of materials are recorded only if they have a magnetic field dependence.

Mössbauer investigation of sputtered ferromagnetic amorphous FexB100−x films

Iron boride amorphous films were prepared by rf sputtering of polycrystalline FeB and Fe2B powders. The films were analyzed for purity, uniformity, and composition by secondary ion mass spectrometry and were analyzed for lack of long range order by x‐ray diffraction. As in the melt quenched alloys, Mossbauer spectra revealed a broad and nearly symmetric distribution of hyperfine fields in samples with a composition near a‐Fe2B. The hyperfine field distribution in samples with a composition near a‐FeB, however, is asymmetric, and more significantly, reveals a large number of Fe sites with zero magnetic hyperfine field at temperatures down to 10 K. Analysis of the data throws light on the chemical short range order in the alloys.

Microwave detection using granular Bi‐Sr‐Ca‐Cu‐O thin films

We report here results of our investigations of the nonbolometric mode of detection in granular Bi‐Sr‐Ca‐Cu‐O thin films. We find that both bolometric and nonbolometric modes appear in the same sample under mutually exclusive conditions: bolometric with light (633 nm) and nonbolometric with microwaves (9 GHz). Our measurements indicate that the nonbolometric mode of detection is more sensitive than the bolometric mode in our samples, a result which offers promise for application to microwave power measurement. Possible mechanisms for the nonbolometric mode are reviewed, and the concept of using a broadband spiral antenna directly coupled to the superconducting detector for microwave power sensing is briefly described.

Detection of weak link superconductivity by magnetically modulated electrical resistance

Abstract A new technique, based on observing the effect of magnetic field modulation on the DC resistance of superconductors, sensitively detects the presence of weak links in granular high temperature superconductors. The phase detected response to the magnetic modulation shows a peak at T c for both metallic and ceramic superconductors, analogous to that observed in magnetically modulated microwave absorption. A second peak, attributed to weak links, is observed below T c in the ceramic oxide superconductors when tailing is present in the resistance vs. temperature curve.

Hyperfine field distributions in ferromagnetic amorphous FexB1−x thin films

Sputtered films of amorphous FexB1−x (0.40?x?0.50) were studied by Mossbauer spectroscopy. The films were characterized for composition and homogeneity by secondary‐ion mass spectrometry (SIMS) and for lack of crystallinity by X‐ray diffraction. Below the magnetic ordering temperature Tc, the hyperfine spectra consist of a large amplitude quadrupole doublet superimposed on a broad magnetic hyperfine field distribution. The spectra of these alloys are remarkably different from that of polycrystalline FeB and from those of liquid‐quenched alloys which are restricted to compositions near the eutectic at x≊0.8. Comparison with the polycrystalline FeB spectrum clearly shows that for a significant fraction of the Fe atoms in the amorphous alloys, the local environment is substantially different from the Fe environment in FeB.

Correlation of electrical transport and magnetism in amorphous Mn‐B alloys

X‐ray scattering, magnetism, and electrical transport studies on amorphous thin films of MnxB100−x alloys with x=52 and 48 are reported. Each alloy exhibits a low‐field (5 G) static susceptibility peak (10 K, x=52; 16 K, x=48) associated with a spin‐glass transition. Isothermal magnetization data (6 K) are analyzed within the random anisotropy model of Chudnovsky, Saslow, and Serota. The magnetization isotherm for the x=52 alloy is dominated at high fields (>24 kG) by field‐induced moments, while for x=48 a term (αH−1/2) arising from a ferromagnet with a wandering axis prevails to the highest field strength (44 kG). Initially the electrical resistance for these Mn‐B alloys decreases monotonically with decreasing temperature, reaching a minimum (Tm) at 22 K (x=52) and 45 K (x=48). For T>Tm, a quadratic form can be effectively employed, with a negative T2 coefficient and a positive linear coefficient. The rise in resistivity for T

On distinguishing intrinsic and weak-link responses in magnetically-modulated dissipation in high Tc superconductors

It is noted that, contrary to a recent report, magnetic-field modulation of electromagnetic dissipation in superconductors yields both an intrinsic response indicative of the superconducting transition and extrinsic responses indicative of weak links. Ways of distinguishing between these difference responses are also noted.

Detection of weak link effects in superconducting YBa2Cu3O7−y epitaxial thin films by magnetically modulated resistance

Evidence of weak link effects in epitaxial thin films of YBa2Cu3O7−y with well‐aligned grains is obtained from magnetically modulated resistance measurements. These results are of interest because weak links could provide a mechanism for the residual microwave loss in these types of superconductor thin films which contain predominantly low angle grain boundaries.

Laser ablation deposition of superconducting Bi‐Sr‐Ca‐Cu‐O thin films on zirconia‐buffered crystalline quartz

Thin films of Bi‐Sr‐Ca‐Cu‐O have been deposited on crystalline quartz substrates by laser ablation. Without a buffer layer, superconducting regions exist in the films as detected by magnetically modulated microwave absorption. However, with a 1000‐A zirconia buffer layer, also deposited by laser ablation, continuous, superconducting thin films were obtained. It is shown that both annealing temperature and annealing time greatly affect the film quality.

Superconductivity and spin‐glass ordering in RBa2 (Cu1−xFex)3Oz: R=Y,Gd; 0≤x≤0.12

Samples of oxygen deficient perovskites, YBa2 (Cu1−x Fex )3Oz and GdBa2 (Cu1−x Fex )3 Oz (z≂7) were prepared with up to 12% Fe substituting for Cu. A novel variation of the microwave absorption technique was employed to establish the superconducting transition in these samples and to show that the reduction in the transition temperature with increasing Fe concentration is faster in the Gd samples than in the Y ones. In both series of samples, the room‐temperature Mossbauer spectra reveal two distinct Fe sites corresponding to two local oxygen coordinations, and more significantly the spectra at 4.2 K for specimens with x≥0.03 show that the Fe moments are magnetically ordered, leading to the coexistence of superconductivity and magnetism.