OBSERVATION OF A NEW PHASE-TRANSITION OF THE VORTEX LATTICE IN A SINGLE-DOMAIN TMBA2CU3O7-X SINGLE-CRYSTAL AS IT IS ROTATED IN AN EXTERNAL MAGNETIC-FIELD

The angular variation of the magnetic moment of a single-domain TmBa2C u3O7-x single crystal in a magnetic field has been studied. There is a sharp phase transition of the vortex lattice from an essentially unpi nned state with vortex ''chains,'' at small values of the angle (theta ) between the external magnetic field and the ab plane of the crystal, to a mixed state, which contains both vortex chains and ordinary Abri kosov vortices and which is characterized by a strong vortex pinning, at theta> theta(cr) = (2 degrees-9 degrees). The field dependence thet a(cr)(H) is very nonmonotonic. (C) 1995 American Institute of Physics.