Irreversible magnetization under rotating fields and lock-in effect on a ErBa2Cu3O7-delta single crystal with columnar defects

We have measured the irreversible magnetization (M-i) of an ErBa2Cu3O7-delta single crystal with columnar defects (CD), using a technique based on sample rotation under a fixed magnetic field H. This method is valid for samples whose magnetization vector remains perpendicular to the sample surface over a wide angle range-which is the case for platelets and thin films-and presents several advantages over measurements of M-L(H) loops at fixed angles. The resulting M-i(Theta) curves for several temperatures show a peak in the CD direction at high fields. At lower fields, a very well defined plateau indicative of the vortex lock-in to the CD develops. The H dependence of the lock-in angle phi (L) follows the H-1 theoretical prediction, while the temperature dependence is in agreement with entropic smearing effects corresponding to long range vortex-defects interactions.