摘要:

We have prepared superconducting thick films of the Bi‐based and the Tl‐based cuprates via the decomposition of aqueous‐glycerol solutions containing the salts of the elements. Preliminary results are presented in this work. The substrates are coated prior to heat treating, either by dipping or by spraying on various substrates heated at 200 °C. Short firing times are required in order to minimize the loss of the constituent Bi (Tl). We find that nitrates of the constituents dissolved in a water‐glycerol solution increase the reaction rate in comparison to pure nitrate aqueous solutions. They also help to produce the correct superconducting phase before some reaction with the substrate occurs or too much of the constituent Bi (Tl) is lost during heating to form the superconducting phase. However, the thallium phases cannot be obtained if the films are not fired in the presence of a high pressure of thallium in a sealed capsule. The films are composed of platelets, a few microns large, that are on average oriented parallel to the substrate with theircaxis normal. The Bi films show an onset temperature at 85 K and zero resistance at 75 K while the Tl films show an onset temperature of 105 K and zero resistance at 95 K. The critical currents obtained to date are quite low (∼50 A/cm2at 77 K for the thallium phase).

ISSN:0021-8979    

DOI:10.1063/1.342075

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

One goal of this work was to develop a reproducible method of preparing high quality Y‐Ba‐Cu‐O superconducting films and to study their properties versus thickness. This was accomplished by rf diode sputtering from a single target. Twenty‐seven depositions were made using a target containing 8.9‐at. % Y, 37.3‐at. % Ba, and 53.8‐at. % Cu. Film thicknesses ranged from 0.09 to 2.4 &mgr;m. The film compositions obtained were 15.6±1.0‐at. % Y, 35.8±1.0‐at. % Ba, and 48.7±1.7‐at. % Cu for the mean and standard deviation. The films were amorphous as‐deposited and crystallized by annealing in O2at 915 °C. X‐ray diffraction, transmission electron microscopy, and scanning electron microscopy indicated that, on (100)SrTiO3substrates, films with thickness less than ∼0.25 &mgr;m were epitaxially oriented with theircaxis perpendicular to the substrate. Films on (110)SrTiO3were oriented with theircaxis parallel to the substrate. On (100)SrTiO3, zero resistance was achieved in 30 samples from 27 runs at 85.6±1.4 K with a transition width (10%–90%) of 1.8±1.1 K independent of thickness. Results for deposition on a variety of other substrates were more variable. Diamagnetic shielding of up to 38% was calculated from the initial slope ofMvsH. This low value was attributed to the presence of second phases and a significant diffusion layer thickness, both observed by transmission electron microscopy. Critical currents, measured by transport using a four‐point probe, reached 8.1×105A/cm2at 77.35 K for a 0.2‐&mgr;m film deposited on (100)SrTiO3. Comparable values were obtained by calculation from theM‐Hhysteresis loop, from which we infer that there are either continuous epitaxial sheets ofcaxis oriented 123 or, if there are grain boundaries, the coupling across them is strong.

ISSN:0021-8979    

DOI:10.1063/1.342051

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Metallurgical studies and optimization of sintering condition to obtain the maximum volume fraction of the highTcphase (∼110 K) have been made for the Bi‐Sr‐Ca‐Cu‐O system. In the differential thermal analysis, double peaks of endothermic reaction at ∼870 and ∼910 °C were observed during the increase of temperature, and a weight loss accompanied each endothermic reaction. Optimum calcining and sintering conditions to obtain a large volume fraction of the highTcphase were 820 °C×14 h+840 °C×6 h and 875 °C×100 to 200 h in air, respectively. As for the variation of Cu concentration, nominal composition of Bi1Sr1Ca1Cu2Oxgave the maximum volume fraction of the highTcphase. (For these specimens, sintering time was varied up to 20 days.) Post annealing at 400 °C for 20 h in an oxygen atmosphere after the sintering at 875 °C for 100 to 470 h in air had little effect on the improvement of volume fraction of the highTcphase. As for the specimen to which Pb is added, however, the post annealing at 400 °C for 20 h increased the volume fraction of the highTcphase. Scanning electron microscopy showed that the Bi1Sr1Ca1Cu2Ox(nominal composition) specimens sintered at 875 °C for 100 to 470 h consist mostly of well‐grown thin layers, while the specimen sintered at 875 °C for 14 h consists mostly of fine pieces of thin layers piling up at random. Electron probe microanalysis revealed that microstructure of the specimen sintered at 875 °C consists of Bi‐poor regions and the matrix surrounding them, and that these Bi‐poor regions are classified into Cu‐rich regions and Ca‐rich regions. The superconducting phases of this material are considered to exist in the matrix region. We identified the Cu‐rich regions and Ca‐rich regions as (Sr0.45, Ca0.55)3Cu5Oxand (Sr0.08, Ca0.92)2CuOx, respectively. The average size of these regions increased with increasing time of sintering, although total volume fraction of these regions did not increase.

ISSN:0021-8979    

DOI:10.1063/1.342052

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

We report results obtained from several hundred Nb/Al2O3/Nb tunnel junctions fabricated by the ‘‘whole‐wafer’’ process. These results indicate that one of the factors determining resistance and quality variations in such devices is the microstructure of the underlying base layer of niobium. We have further developed the technique of anodization spectroscopy to provide a quantitative measure of the variation in the thickness of the thin aluminum layers which contain the tunnel barrier and have related this to the temperature at which the niobium base layer is deposited. We show that at least two mechanisms are responsible for normal‐state resistance variations in such devices and that device quality is closely related to the integrity of the aluminum layer underlying the tunnel barrier.

ISSN:0021-8979    

DOI:10.1063/1.342053

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

A statistical model, based upon earlier models of Brown [J. Appl. Phys.33, 3022 (1962)] and McIntyre [J. Phys. D3, 1430 (1970)] has been developed to examine the magnetization reversal of domain‐wall nucleation controlled permanent magnets such as sintered Fe‐Nd‐B and SmCo5. Using a Poisson distribution of the defects on the surface of the grains, a ‘‘weakest link statistics’’ type model has been developed. The model has been used to calculate hysteresis loops for sintered Fe‐Nd‐B‐type polycrystalline magnets. It is shown that the intrinsic coercivity measured for a bulk magnet should vary inversely as the logarithm of the surface area of the grain. The effect of demagnetizing field has been incorporated by a mean‐field‐type approximation, to calculate the overall nucleation field from the intrinsic coercivity. The hysteresis loops theoretically calculated are in excellent agreement with the overall form of those experimentally determined for similar nucleation controlled magnets. The model also predicts that for an inhomogeneous grain size distribution, such as a bimodal distribution, kinks will be observed in the second quadrant of the hysteresis loops.

ISSN:0021-8979    

DOI:10.1063/1.342054

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Results of experiments to study the effect of grain size and grain‐size distribution on the intrinsic coercivity and the hysteresis loop of sintered Fe‐Nd‐B magnets are presented. It is shown that the intrinsic coercivity decreases as the average grain size of the magnet is increased. It is also shown that the intrinsic coercivity decreases linearly with the logarithm of the square of the grain size. This is consistent with the predictions made based upon the statistical model developed in Part I. An increase in the sintering temperature leads to an increase in the average grain size, which consequently leads to a narrower hysteresis loop and lower intrinsic coercivity compared to magnets sintered at a lower temperature. It is also shown that a heterogeneous grain‐size distribution, such as a bimodal distribution, causes kinks to appear in the second quadrant of the hysteresis loop. By examining magnets with different fractions of large grains, the prediction that the magnitude of the kinks increases with the volume fraction of the large grains, has been verified experimentally.

ISSN:0021-8979    

DOI:10.1063/1.342055

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Cobalt‐noble metal (Co‐NM, NM; Pd and Pt) thin films deposited in an alternating multilayer structure by the magnetron dc‐sputtering method have an artificial superlattice structure, where Co and NM grow epitaxially on the closest planes of them and a compositionally sharp interface is fabricated. Films with a small periodicity have either a hexagonal closed pack (hcp) structure with thecaxis normal to the film plane or a face centered cubic structure (fcc) with the ⟨111⟩ axis normal to the film plane, depending on the thickness ratio of Co and NM layers. A long‐range crystalline order is hardly maintained due to generation of the crystalline phases of Co and NM in films with a large periodicity. The artificial superlattice structure of the films is thermally stable to temperatures up to 500 °C. Many films are ferromagnetic with an easy magnetization direction in the film plane. The perpendicular magnetization hysteresis loop with the coercivities smaller than 1 kOe was obtained in both the Co‐Pd films with a Co layer (less than 10 A˚) thinner than a Pd layer and the Co‐Pt films with a Co layer (less than 15 A˚) thinner than a Pt layer. Pd atoms at the interface in the Co‐Pd film show the ferromagnetism with an induced magnetization of about 300 G, 0.5 &mgr;Bper Pd atom.

ISSN:0021-8979    

DOI:10.1063/1.342056

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The whole eigenvalue spectrum is studied for the nucleation field in a sphere with a uniaxial surface anisotropy. Detailed calculation is given for the magnetization buckling mode in such a sphere, which is the only reversal mode for rather small radii before the onset of the magnetization curling mode.

ISSN:0021-8979    

DOI:10.1063/1.342057

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

An earlier dipolar treatment, including retardation and predicting an anisotropic behavior for a periodic cermet, has been applied to a suitable model. This has pointed out the need for carefully handling the concept of depolarizing field in the presence of retarded dipole‐dipole interactions. Calculations have been performed to carry out the effective permittivity tensor of the given model. The results show a certain amount of anisotropy but do not agree with a very recent experiment. Possible implications are briefly discussed.

ISSN:0021-8979    

DOI:10.1063/1.342058

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The ac field dependence of the polarization and strain responses of three different compositions of doped lead zirconate titanate ceramics were measured for samples in both the poled and the depoled states. The results indicate that a reversible domain wall excitation exists which contributes to the weak‐signal response of these materials. Irreversible domain wall motion can be excited within the frequency range measured by applying a field above a threshold field, which is much smaller than the coercive field. Therefore, the weak‐signal linear dielectric, piezoelectric, and elastic coefficients of these materials may not provide a suitable description of their behavior when the external applied field exceeds this threshold field. All the observed phenomena can be explained by thermally activated domain wall fluctuations and nucleations.

ISSN:0021-8979    

DOI:10.1063/1.342059

出版商:AIP    

年代:1988

数据来源: AIP