Magnetic force microscopy (MFM) with in situ magnetic fields was used to classify and determine the distributions of magnetic domain structures in 190-nm-thick Ni 81Fe 19 particles with diameters from 850 to 250 nm. A deposition induced perpendicular anisotropy energy resulted in a simple stripe domain pattern observed in MFM images on unpatterned witness films. The domain structures in the particles are not stripes but consist of a small number of distinct magnetization states for each diameter of particle. Repeated in-plane magnetic field saturations were applied to effectively anneal the magnetic states of an array of same diameter particles. For some of the states Boltzmann statistics were used to calculate an "energy ratio" of the particles magnetization states based on the observed distribution for each diameter.