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

We have measured the irreversible magnetization $({\mathbf{M}}_{i})$ of an ${\mathrm{ErBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ 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 ${\mathbf{M}}_{L}(H)$ loops at fixed angles. The resulting ${\mathbf{M}}_{i}(\ensuremath{\Theta})$ 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 ${\ensuremath{\varphi}}_{L}$ follows the ${H}^{\ensuremath{-}1}$ theoretical prediction, while the temperature dependence is in agreement with entropic smearing effects corresponding to long range vortex-defects interactions.