Flux jumps, second magnetization peak anomaly, and the peak effect phenomenon in single crystals of YNi2B2C and LuNi2B2C

We present magnetization measurements in single crystals of the tetragonal YNi2B2C compound, which exhibit the phenomenon of peak effect as well as the second magnetization peak anomaly for H>0.5 T (H parallel to c). At the lower field (50 mT < H < 200 mT), we have observed the presence of flux jumps, which seem to relate to a structural change in the local symmetry of the flux line lattice (a first-order reorientation transition across a local field in some parts of the sample, in the range of 100 to 150 mT). These flux jumps are also observed in a single crystal of LuNi2B2C for H parallel to c in the field region from 2 to 25 mT, which are compatible with the occurrence of a reorientation transition at a lower field in a cleaner crystal of this compound, as compared to those of YNi2B2C. Vortex phase diagrams drawn for H parallel to c in LuNi2B2C and YNi2B2C show that the ordered elastic glass phase spans a larger part of (H,T) space in the former as compared to latter, thereby, reaffirming the difference in the relative purity of the two samples.