Magnetization of thin films in simultaneous dc and ac fields

physica status solidi (b), 1992

The magnetic flux density distribution in thin epitaxial YBaCuO films is detected using the magneto‐optical Faraday effect. The external magnetic field is applied perpendicular to the film plane after zero field cooling. The results for the magnetic polarization, J, the critical current density, jc, and the magnetic field, μ0H*, of complete flux penetration are compared with SQUID magnetometer measurements. The flux density gradients as derived from the detected flux density distribution reveal the importance of the radial field in the investigated thin films with respect to the critical current density jc and the magnetic polarization J.

Supercond. Sci. Technol. 19, 385-391 (2006).

We report measurements of the hysteretic magnetization of YBCO under superimposed transport current cycles, together with numerical simulations of magnetization and current density profiles in the corresponding parallel configuration. Field cooled (FC) and zero-field-cooled (ZFC) experiments were carried out on polycrystalline YBa 2 Cu 3 O 7−x cylinders, with both the applied magnetic field and transport current in the axial direction, and the current cycled several times, around and above the dissipative threshold. As in previously reported multicomponent field configuration experiments, the magnetization is seen to collapse to a more stable state both in FC and ZFC, because of the interplay between the shielding and transport currents. The results of our numerical simulations are in good qualitative agreement with the measurements, and the competition between shielding and transport due to vortex-pinning interactions and equilibrium magnetization effects are shown to play an important role in the range of our experiments.

Physica C: Superconductivity, 1994

By means of high-sensitivity capacitance torque magnetometers we have measured the superconducting current Js and the dynamic magnetic-moment relaxation of YBazCu307 and YBazCu4Os films of typically 100 nm thickness at temperatures between 2 K and Tc in magnetic fields up to 6 T. For the measurements of the dynamic relaxation rate Q==-d lnjs/d ln(dBc/dt) magnetic-field sweep rates were varied between 0.5 and 40 mT/s. At low fields (typically 0.5 T) the dynamical relaxation rate exhibits a plateau at Q~ 0.06 in YBa2Cu307 and 0.04 in YBa2Cu4Os. At high fields (Be=/toHc-~ 6 T) the plateaus have completely disappeared and Q increases almost linearly with increasing temperature. At all fields a sharp increase up to Q~ 1 is observed when the irreversibility line is approached.

Revista Mexicana De Fisica, 1998

Hemos investigado la magnetizacion dc de peliculas delgadas superconductoras de (Y;Gd)Ba2Cu307_o) en la escala temporal de lOOlIS. El momento magnetico en funcion del campo aplicado fue medido con una tecnica no convencional que combina el uso de un campo magnetico pulsado y un laser pulsado sincroni7,ado. Se examinaron tanto mucstras virgenes cOl11omagneticamcnte cicladas, y los resultados se describen muy satisfactoriamcnte en el marco del modelo de estado critico de Bean para un disco en un campo magnetico transversal.

Physica C: Superconductivity, 2003

The complex ac susceptibility of YBCO films was measured as a function of temperature (32-83 K) and magnetic field amplitude (0-450 G), by using a screening method with two coils. In addition this ac susceptibility was computed by a first principle continuum method. Comparison of this theory with the experimental data yielded the activation energy, U c ð0Þ, for flux creep. The activation energy values U c ð0Þ were 480 and 930 K for samples I and II respectively.

Journal of the Less-Common Metals, 1990

have been measured on epitaxial YBaaCus07 thin films deposited by laser ablation on (100) SrTiOa substrates. Hysteresis loops are reported at different temperatures.

2006

We report measurements of the hysteretic magnetization of YBCO under superimposed transport current cycles, together with numerical simulations of magnetization and current density profiles in the corresponding parallel configuration. Field cooled (FC) and zero-field-cooled (ZFC) experiments were carried out on polycrystalline YBa 2 Cu 3 O 7−x cylinders, with both the applied magnetic field and transport current in the axial direction, and the current cycled several times, around and above the dissipative threshold. As in previously reported multicomponent field configuration experiments, the magnetization is seen to collapse to a more stable state both in FC and ZFC, because of the interplay between the shielding and transport currents. The results of our numerical simulations are in good qualitative agreement with the measurements, and the competition between shielding and transport due to vortex-pinning interactions and equilibrium magnetization effects are shown to play an important role in the range of our experiments.

Journal of Applied Physics, 1990

We have investigated the hysteresis properties of Au/Co/Au films with ultralow CO thicknesses and perpendicular easy axis. At low temperature we observe a very strong thickness dependence of the coercivity, whereas striking dynamical effects are present at room temperature. We propose a model of wall motion which provides a consistent explanation of the overall observations. This interpretation emphasizes the crucial role played by the roughness with respect to the hysteresis properties of ferromagnetic ultrathin films.

Journal of the Magnetics Society of Japan, 1989

The magnetic properties of high Tc superconducting thin films were discussed as the perpendicularly magnetizing media. In a weak field region below Hcl, every specimen film is very soft diamagnetic along the perpendicular direction to the film plane. This effect is caused by the shape anisotropy of thin films. The magnetic linear response is so excellent that an inductance curve is flat up to 70 MHz. Once the magnetic fluxoids are trapped in the films, it is unstable with showing the strong magnetic relaxation. Considering such properties, possibilities of magnetic application of the films at 77 K would be helpful.

Physical Review B, 1991

We propose a mechanism that may account for the temperature-insensitive relaxation of the magnetic-shielding current in epitaxial thin films of YBa2Cu307 q. We show that such relaxation is related to the shape of the current-voltage (J-E) characteristic of the superconductor in its critical state. The weak temperature dependence of the relaxation implies a temperature-insensitive J-E characteristic that resembles that of conventional type-II superconductors when a spatial variation of critical current density (J,) is present. We suggest such a distribution of J, as an explanation for the apparently large and temperature-insensitive relaxation observed in YBa2Cu307-$. YBa2Cu307 & samples exhibit a relatively slow onset of dissipation when the critical current density (J,) is approached, even for high-quality samples. ' Such dissipation presents itself in two different but related forms. It causes the decay of the otherwise persisting magneticshielding current, ' and it leads to a finite voltage rise when a transport current below the critical current density is driven through the superconductor by external electromagnetic forces.