Magnetic moment M of c-axis oriented YBa2Cu3O7-delta films of thickness d similar to 4 x 10(-5) cm in a perpendicular field was measured at elevated temperatures close to T-c to clarify the field dependence j(c)(B) while the magnetic flux vortices were widely separated. A similarity of hysteretic magnetization curves at different temperatures was observed showing that the critical current at zero field j(c0) determined also the scale B-c of the magnetic field dependence M(B)/M(0) = F(B/B-c), while the zero field value M(0) proportional to j(c0) varied over two orders of magnitude, and the intervortex distance a at B = B, reached the values a similar or equal to 10 d. This result was related to the long-range vortex interaction suggested by the Pearl solution for thin films. The deviations from the dependence Bc proportional to j(c0)d in the region (T-c - T) < 1 K were observed and discussed in comparison with edge effects, where at large penetration depths X > d the thickness of the sample should be replaced by the Pearl's two-dimensional screening length lambda(eff) = 2lambda(2)/d.

The temperature dependence of the penetration depth lambda in the vicinity of the critical temperature T-c was determined from the diamagnetic response of YBa2Cu3O7-delta thin films. At temperatures 0.1 K < T-c - T < 1 K the experimental data demonstrated the scaling lambda proportional to (T-c - T)(-1/3) in agreement with the 3D XY model.

Resistivity and magnetisation measurements were done with bulk and thin-film samples of La0.5Ca0.5MnO3. Bulk samples showed an antiferromagnetic insulating ground state as expected for this composition, whereas thin films demonstrated a ferromagnetic metallic ground state. This effect is attributed to the film adhesion to the substrate which prevents the lattice distortions necessary for the charge and antiferromagnetic ordering that occur in the bulk samples.

The remanent magnetic flux creep in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\mathrm{\ensuremath{\delta}}\mathrm{}}$ thin films was investigated in the vicinity of the critical temperature ${T}_{c}.$ The trapped magnetic flux was generated in the specimen by ramping the applied field to a finite value and then back to zero. The remanent magnetic field of persistent currents circulating in the sample was measured as a function of temperature and time. Data on the critical current density ${j}_{c}$ and the creep activation energy U were extracted. A sharp decrease of U at temperatures ${T}_{c}\ensuremath{-}T\ensuremath{\lesssim}1 \mathrm{K}$ was related to critical fluctuations. In this critical region a power-law time decay of the currents $j\ensuremath{\propto}{t}^{\ensuremath{-}p}$ was observed and explained by the logarithmic dependence ${U=U}_{0}\mathrm{ln}{(j}_{c}/j)$ for the current-assisted thermal unbinding of vortex-antivortex pairs generated by the critical fluctuations. In the critical region, the temperature dependence ${j}_{c}\ensuremath{\propto}{(T}_{c}\ensuremath{-}{T)}^{2}$ was observed. A possibility to relate this result to the critical behavior of the three-dimensional $\mathrm{XY}$ model is discussed.

The remanent magnetic flux creep in YBa2Cu3O7-delta thin films was investigated in the vicinity of the critical temperature T-c. The trapped magnetic flux was generated in the specimen by ramping the applied field to a finite value and then back to zero. The remanent magnetic field of persistent currents circulating in the sample was measured asa function of temperature and time. Data on the critical current density j(c) and the creep activation energy U were extracted. A sharp decrease of U at temperatures T-c - T less than or similar to 1 K was related to critical fluctuations. In this critical region a power-law time decay of the currents j proportional to t(-p) was observed and explained by the logarithmic dependence U = U(0)l(j(c)/j) for the current-assisted thermal unbinding of vortex-antivortex pairs generated by the critical fluctuations. In the critical region, the temperature dependence j(c)proportional to(T-c - T)(2) was observed. A possibility to relate this result to the critical behavior of the three-dimensional XY model is discussed. [S0163-1829(98)07545-6].

A new type of low-frequency mechanical oscillator was developed to study the vortex lattice melting transition in untwinned single crystals. The dissipation associated with the oscillatory motion of the sample was measured as a function of temperature down to 77 K in magnetic fields from 2 up to 10 T. A sharp changeover in the dissipation was identified as a transition signal. The data were analysed with a linear ohmic response model. The transition signal at low fields was interpreted as a peak effect of the critical current. The peak effect vanished at high magnetic fields.

The vortex lattice melting transition in untwinned crystals of YBa2Cu3O7−δ has been probed with a low-frequency mechanical oscillator in the field range from 2 to 10 T. A dramatic change in the transition signal occurs in the field range 3 < B < 6 T which could be related to a dimensional crossover.

A new type of radio-frequency size effect was observed experimentally, due to ''cutoff'' of the trajectories of the effective electrons on the boundary of a layer of impurity atoms that have diffused into the interior of a metallic single crystal. The coefficient of diffusion of lead in single-crystal indium was measured on the basis of this effect.