摘要:

The critical current densityJcof the epitaxialYBa2Cu3O7−&dgr;thin film was measured as a function of applied magnetic fieldHup to 6 T for three special configurations: (1)H∥caxis andH⊥J;(2)H⊥caxis andH⊥J;(3)H⊥caxis andH∥J.We observed a large anisotropy ofJc(H)betweenH∥caxis andH⊥caxis withH⊥J,and a very small anisotropy ofJc(H)betweenH∥JandH⊥JwithH⊥caxis. The field dependence of the critical current density can be well described by the modified Kim–Anderson model taking into account the Kramer scaling law. The weak dependence of the transport critical current density on its orientation relative to the applied magnetic field is discussed. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365279

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Based on previously developed melt processing methods,Y2BaCuO5(211) precipitates with various size were obtained inYBa2Cu3Ox(123). In particular, a wide distribution of 211 size from 100 Å to 10 &mgr;m has been found to exist in the matrix of 123, and strongly effected the critical current density,Jc.An enhancement ofJcwas always found to be associated with the presence of extremely fine 211 precipitates. The flux pinning by these precipitates can be understood by considering the interaction of flux lines with cavities. It was found that although the flux lines are strongly pinned by cavities, the effectiveness decreases with the increasing number of flux lines trapped by them. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365280

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We present numerical simulations of the dynamics of two-dimensional Josephson junction arrays to study the mechanism of mutual phase locking. We show that in the presence of an external magnetic field two mechanisms are playing a role in phase locking: feedback through the external load and internal coupling between rows due to microwave currents induced by the field. We have found the parameter values (junction capacitance, cell loop inductance, impedance of the external load) for which the interplay of both these mechanisms leads to the in-phase solution. The case of unloaded arrays is discussed as well. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365439

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Superconducting tunnel junctions are promising detector elements in next generation x-ray, UV, and optical detectors. Quasiparticles which approach the neighborhood of the normal core may scatter and emit a phonon, thus being trapped in the Abrikosov vortex (AV). In this way the existence of AVs reduces the lifetime of quasiparticles. In this experiment excess quasiparticles in aNb/AlOx/Nbjunction are created by means of a low temperature scanning electron microscope. The trapping rate of quasiparticles into AVs is determined by measuring the quasiparticle lifetime as a function of the AV concentration. This results in an effective trapping radius of the AV of14.8±1.4 nm,which is slightly less than theoretical expectations. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365281

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Measurements of temperature dependence of the return current in high qualityNb/AlOx/NbJosephson junctions are presented. From the experimental data, we obtain the effective resistance, i.e., the effective dissipation, for the retrapping process, according to the generalized resistively shunted junction model proposed by Chen, Fisher, and Leggett. We present a careful analysis, based on a comparison between the measured temperature dependencies of both the return and the quasiparticle tunneling current. We find that the junction subgap conductance, which includes the quasiparticle and the quasiparticle-pair interference terms, is responsible for the return process. The measurements have been performed on various samples, in a wide range of critical current densities from 50 to2250 A/cm2,covering different damping regimes and spanning over the high and low temperature limits. Junctions with low critical current density show ideal dissipation which makes the return current scale with temperature according to the BCS exponential behavior without flattening out effects. This result may be relevant for the possible use ofNb/AlOx/Nbjunctions in macroscopic quantum coherence experiments, which strongly require a very low dissipation. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365282

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The time dependence of the magnetization in samples having a different degree of interparticle interactions is analyzed. The selected compound was a nanocrystalline powder of the compositionBaFe10.4Co0.8Ti0.8O19with and without an admixture of nanosizedSiO2particles. Two different approaches were considered for the analysis of the thermally activated demagnetization: theT⋅ln(t/&tgr;0) scaling of the thermomagnetic relaxation in zero applied field, and the classical fluctuation field and activation volume analysis. The energy barrier distribution obtained from the former approach shows that the occurrence of larger demagnetizing interactions leads to a relative enhancement of the lowest energy barriers with respect to the largest barriers. Activation volumes are found to increase with demagnetizing interactions and the leading demagnetizing mechanism appears to shift from an individual particle mode (coherent or nucleation) to a collective one. Both analyses are suggested to be taken into account in order to ascertain the influence of interactions on the magnetic properties of nanocrystalline particles.©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365442

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We have studied the film thickness dependence of the remanent magnetization (deltaMplots), the magnetic viscosity, and the activation volumes of Fe thin films sputtered on commercial nanochannel alumina of 20 nm average pore diameter. The films have the form of a contiguous interconnected network. A critical film thickness of&fgr;∼20nm defines two regions with very different magnetic behavior. For this critical value of&fgr;we have found a maximum in the remanent coercivity, a minimum in the interparticle interaction parameter&agr;,and a rapid increase in the magnetic viscosity, the irreversible susceptibility and the activation volume. Our results are interpreted in terms of the pinning effects caused by nonmagnetic inclusions. The functional relationship between the remanent coercivity and the activation volume is similar to the one found in long cylinders that reverse magnetization by a curling mechanism. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365283

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The magnetic properties and recording characteristics of (Dy,Tb)FeCo films are dependent upon the anisotropy dispersion of rare earth constituents and the exchange integral between rare earth and transition metal sub networks. The exchange coupling coefficient &lgr; and the domain wall mobility&mgr;effare found to be interrelated by both measurements and calculations. The higher the &lgr; or the lower the&mgr;eff,the higher bias field that is required for magneto-optical recording. The temperature independent &lgr; is demonstrated to be an important parameter in the assessment of magnetic properties and recording characteristics of sperimagnetic films. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365284

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Longitudinal recording media made with chromium and with nickel-aluminum underlayers are compared. NiAl films have smaller grains and random crystallographic orientation, while Cr films have larger grains and either a {110} or a {200} preferred orientation depending on the deposition conditions. Cobalt alloy films grown on NiAl have nearly random crystallographic orientation and significantc-axis out-of-plane component, and lower hysteresis loop squareness and coercivity than films made on a heated Cr underlayer. NiAl-underlayer media do not exhibit in-plane magnetic anisotropy induced by the substrate texture lines. CoCrTaPt/Cr/NiAl media had higher overwrite than CoCrTaPt/Cr media due to higher coercive squareness and a sharper hysteresis loop closure. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365285

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Magnetoresistance of theLa0.67Ca0.33MnO3−&dgr;thin films was investigated. The magnetoresistance depends on ablation conditions such as the substrate temperature and oxygen pressure and has a relation with the lattice constants of the thin films. A colossal magnetoresistance ratio with4.79×106&percent;was obtained at 70 K and a magnetic field of 5.5 T. The resistivity of the thin films has a close relation with the substrate temperature and the oxygen pressure. When the substrate temperature is fixed, the resistivity shows double peaks in the oxygen pressure range of1×10−7to 1 Torr. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365286

出版商:AIP    

年代:1997

数据来源: AIP