  We have previously shown for powders that Mo substitution into the CuO chains
of YBa2Cu3O7 can create effective pinning centres which significantly increase
the critical current density (j_c) in 7 T field by a factor of 4 and 10 at 50
and 60 K, respectively. The present work reports on the influence of the Mo
substitution and high-pressure oxygen annealing on the pinning properties and
critical currents of YBa_2Cu_{3-x}Mo_xO_{7-d} by comparing pure (x = 0, d > 0)
and substituted (x = 0.03, d < 0) single crystals. Pinning properties have been
investigated by measurements of magnetization loops and calculations of j_c in
the ab-plane, in the temperature range from 2 to 90 K and in fields up to 14 T.
Depending on the Mo substitution and the oxygen treatment, several types of
pinning centres increasing j_c have been revealed and analysed in the frame of
Dew-Hughes' and Kramer's models.
