Abstract Mn Zn ferrite films deposited on (100) MgO substrates by rf sputtering technique with different thicknesses ξ in the range of 30‐450 nm were studied. AFM images show grain size increase as film thickness increases. Grains with diameters between L ∼ 70 and 700 nm were observed. The mono and multidomain regime in MnZn ferrite films and their effect on the pinning energy of domain walls are observed via Magneto‐optical Kerr Effect, MOKE. At ξ ∼ 300 nm, the coercive field, H c , reaches a maximum value of 80 Oe. This result indicates the existence of a multidomain regime associated to a critical grain size, L c . We used the Jiles‐Atherton model (JAM) to discuss the experimental hysteresis loops. The k pinning parameter obtained from JAM shows a maximum value of k /μ o = 67 Am 2 for grains with L c ∼ 529 nm. The total energy per unit area E was correlated with k and D . We found a simple phenomenological relationship given by E α kD; where D is the magnetic domain width. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)