We consider experimentally and theoretically the effect of the thickness on the critical current density J(c) of superconducting alms. In order to eliminate possible contributions from intrinsic pinning, our measurements of J(c)(phi) as a function of the amplitude and orientation of the magnetic field H with respect to the film plane were performed on isotropic Nb-Ti films having thicknesses d ranging from lambda/4 to 4 lambda, where lambda is the London penetration depth, and H perpendicular to J. The angular dependent J(c)(phi) has a sharp peak for H parallel to the film surface, similar to that observed for high-T-c firms. The amplitude of the peak increases as d decreases and reaches 20-30 % of the depairing current density (J(d)) for the lambda/2 him. The ratio of J(c) values for parallel (J(c parallel to)) and perpendicular (J(c perpendicular to) ) film orientation increases as d decreases, so that J(c parallel to) << J(c perpendicular to) for the 4 lambda, film and J(c parallel to) >> J(c perpendicular to) for the lambda/4 film, the crossover occurring at d approximate to 2 lambda. A proposed interpretation of these results is based on our calculations of the vortex behavior in thin (d << lambda) films, which give analytical formulas for the field distribution around a fluxon, the lower critical field, H-c1, the surface barrier, and the vortex-vortex interaction potential. The film geometry gives rise to a significantly enhanced surface barrier and H-c1, a marked decrease of the range of the intervortex repulsion (to d instead of lambda), and noncentral, position-dependent forces between vortices. These results are employed to evaluate the bulk and surface contributions to J(c parallel to)(d), both being shown to increase as d decreases. The bulk component of J(c parallel to) exhibits a l/d(2) dependence at d > d(c) due to the decrease of the tilt elastic modulus C-44(d) of the vortex structure, a crossover from the collective to a single-vortex regime of pinning occurring below a critical thickness at d < d(c). The surface magnetic pinning gives the main contribution to J(c) for our lambda/2 and lambda/4 films, leading to J(c parallel to), which increases as l/d and becomes of order J(d) at H approximate to H-cl. These calculations show that the ratio J(c parallel to)(d)/J(c perpendicular to) (d) increases as d decreases, with the J(c parallel to)(d)/J(c perpendicular to) (d) value being much less than unity at d >> lambda and much larger than unity at d << lambda. The results obtained indicate that the effect of the him geometry can be very important when interpreting the angular dependences critical currents of thin films.
