Two types of additional maxima in magnetization curves of layered superconductors (abstract)

Abstract

We report on magnetic measurements of LaSrCuO and YBaCuO single crystals in order to clear up their anomalous behavior and to attribute it to the crystals’ anisotropic properties. Magnetization curves were obtained by means of a vibrating sample magnetometer at temperatures from 4.2 to 90 K. The magnetic field up to 8 kOe was applied at various angles with respect to the crystal c axis. Our main findings are that there are two physically different types of additional maxima, as ruled out from the evolution of magnetization curves with varying the tilt angle of the external field. One type of the maxima is attributed to anisotropic flux penetration into a sample and flux-line-lattice rotation with increasing the field. We provide an experimental justification of this explanation. The second type of maxima is related to an anomalous increase of the critical current in magnetic field which is often referred to as the ‘‘fishtail’’ phenomenon. It was found that the existence of the fishtail is closely related to the anisotropy of flux pinning. On this basis a qualitative model was developed to describe this phenomenon. The model takes into account vortex interaction with stretched pinning sites and the vortex bending due to stray fields in the sample. The formation of staircase flux lines is also assumed, as specific for layered superconductors. The qualitative explanation of the fishtail is as follows. At low fields the flux lines are strongly curved. When increasing the field, they straighten and fit the linear pinning sites better, which gives rise to more effective flux line fixing. The proposed model allows to describe the angular and temperature dependencies of the additional maximum position.