Vortex structure in BSCCO (2212) single crystals as a function of the magnetic flux penetration temperature L. Ya. Vinnikov and M. V. Dugaev Institute of Solid-State Physics, Russian Academy of Sciences, 1424342 Chernogolovka, Moscow Region, Russia I ˘ . Markl Physical Institute III of the University Erlangen—Nuerenberg, D-91058 Erlangen, Germany Submitted 22 December 1995; resubmitted 1 February 1996 Pis’ma Zh. E ´ ksp. Teor. Fiz. 63, No. 5, 358–362 10 March 1996 With the aid of the decoration technique, a method is developed for observing the vortex structure forming at different temperatures. Be- havior due to the effect of volume pinning and the geometric barrier in thin BSCCO 2212 single crystals are found. © 1996 American In- stitute of Physics. S0021-36409601205-4 1. The discovery of high-T c superconductors has aroused interest in the investigation of vortex structure. The decoration of the surface of a type-II superconductor with dis- persed ferromagnetic particles is an effective method for studying the mixed state. The decoration method has been used to measure fundamental quantities, such as the magnetic-flux quantum, which is found to be  0 =hc /2e , 1,2 and the anisotropy of the effective masses of carriers in YBCO. 3,4 However, the features of the vortex structure which are due to the wide temperature interval in which the mixed state in high-T c superconductors exists could not be investigated, since the formation temperature of the vortex structure remained unknown. The point is that the overwhelming majority of the experiments performed with the decoration technique were successful only in the frozen- flux regime—cooling of the sample in a magnetic field from the normal state ‘‘field cooling’’—and the decoration temperature T was limited to the region of liquid-helium temperatures. It is assumed that the vortex structure observed as a result of decoration corresponds to a temperature close to T irr —the onset temperature of the irreversible behavior of the magnetization curve. 5 The main obstacle for decoration at high tempera- tures is that the following condition must be satisfied: The magnetic interaction energy must be higher than the thermal energy of the particles p•  H3 kT , 6 where p is the magnetic moment of a particle, H is the magnetic field, and k is Boltzmann’s constant. Rough estimates show that the threshold lies in the region of several tens of degrees Kelvin. It is nonetheless very tempting to use the decoration technique to study directly vortex structures at high temperatures, since most experimental information about the ( H , T ) phase diagram of the vortex state is indirect. At the same time, the theory presup- poses a large diversity of both thermodynamically equilibrium and nonequilibrium states of vortex structures in a high-T c superconductor. 7 The method described below can compete with the recently developed methods for direct observation of vortex structures, such as Lorentzian microscopy 8 or scanning tunneling microscopy. 9 370 370 0021-3640/96/050370-06$10.00 © 1996 American Institute of Physics