Flux jumps, second magnetization peak anomaly, and the peak effect phenomenon in single crystals of YNi<SUB>2</SUB>B<SUB>2</SUB>C and LuNi<SUB>2</SUB>B<SUB>2</SUB>C

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&gt;0.5 T (H||c). At the lower field (50 mT&lt;H&lt;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||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||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