We have performed magnetic force microscopy measurements on isolated 35 nm thick rectangular Co structures. The structures have a length L ranging between 0.25 and 10 μm and a width W ranging between 0.25 and 5.5 μm, covering aspect ratios m=L/W between 1 and 40. This enables us to map the transition from a magnetic single-domain state towards a magnetic multidomain state when increasing the size of the structures. This transition depends on the size as well as the aspect ratio of the structures. Our results can be interpreted in terms of the theoretical model developed by A. Aharoni [J. Appl. Phys. 63, 5879 (1988)]. © 2001 American Institute of Physics.