We have performed superconducting quantum interference device and mu-Hall l
ocal magnetization measurements on Nb thin films of different thicknesses (
120 nm less than or equal to d less than or equal to 1200 nm) and widths (9
5 mu m less than or equal to w less than or equal to 900 mu m) in order to
characterize the effects due to thermomagnetic instabilities (TMI's) and th
eir geometry dependence. We show that the second magnetization peak which o
ccurs at fields H-SMP(T) H much less than H-c2(T) depends on the sample dim
ensions and discuss it in terms of the classical theory of TMI's. Local mag
netization measurements reveal a domelike flux profile at low fields which
is not due to edge barriers. The Bean-like Bur distribution is recovered at
applied fields H greater than or equal to H-SMP(T). [S0163-1829(99)04138-7
].