We report transport measurements in the mixed state of single crystals of Bi2Sr&CaCu20"using a modified version of the dc flux transformer geometry pioneered by Giaever in low-T, materials. We find that over a wide range of the phase diagram the vortices are effectively two dimensional in character.Our results suggest that vortex cutting is an efficient process and that it is unlikely that the vortices are strongly entangled in this region of the phase diagram.The magnetic phase diagram of the mixed state of the oxide superconductors has proven to be a rich system with many physical phenomena playing a role. It has been found that thermal fluctuations, ' pinning disorder, and dimensionality ' are all important in the statics and dynamics of magnetic vortices. The observations of novel dynamics have been variously described in terms of melting, ' vortex glass behavior, giant flux creep, and flux entanglement.At the moment there is strong evidence to support the vortex glass model as a good description of the low-temperature ordered phase in these materials in the disordered limit with the transition becoming first order in the clean limit. However, the role that vortex entanglement plays in the high-temperature dynamics is still very much an open question.The key issue with regard to entanglement is the extent to which the vortices can be considered as elastic rods piercing the entire thickness of the sample. It is clear, if the vortices can easily break and reconnect, that the importance of entanglement will be minimal. Likewise, if the vortices are really two-dimensional pancakes existing as separate entities in the individual superconducting layers, then the importance of entanglement will also be minimized. In this limit of extreme anisotropy, with pancake vortices, one would also expect that the importance of point pins would be very different. This extreme anisotropy would also affect the relevant dimensionality within the vortex glass and melting' pictures. In this work we will present data using a modified dc flux transformer effect to show that over a wide range of the phase diagram the vortices are effectively two dimensional, suggesting that it is unlikely that the vortices are strongly entangled.In the Bi2Sr2CaCu20"system, for currents flowing in the a-b plane and the magnetic field parallel to the c axis, it is well known that the resistive transition broadens with increasing field without noticeable changes in the onset temperature. ' This behavior has been interpreted as the dissipation arising as three-dimensional vortices hop between nearby pinning sites. " However, this contradicts the evidence for a low-temperature phase transition in the vortex structure. ' Analysis of the I-V curves provides support for the existence of a threedimensional vortex glass transition. However, the role that the layered structure plays as well as the importance of thermal fluctuations at high temperatures in overwhelming the interlayer coupling is unclear. In this regime the vortices may very well be pancakes lying...

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