The isothermal remanent magnetization was measured on a twinned YBa2Cu
3O7-delta crystal as a function of magnetic field, temperature and ang
le between the applied magnetic field and the c axis of the crystal. T
he angular dependence of the field of first vortex penetration as well
as the orientation of the isothermal remanent magnetization vector, d
etermined from the data, are different at different temperatures. Our
results point to a crossover between two different pinning mechanisms
at a certain temperature.