Magnetization reversal in arrays of perpendicularly magnetized ultrathin dots coupled by dipolar interaction

Arrays of micron size perpendicularly magnetized ultrathin Co dots with 20 nm separation were obtained using ion irradiation by a focused ion beam and studied by polar magneto-optical microscopy. Because irradiation induces easy nucleation regions along dot borders, magnetization reversal inside the dots under a perpendicular field is due only to domain wall propagation, driven by applied field and dipolar interactions. Frustrated checkerboard patterns are observed in the demagnetized state, in agreement with numerical simulations. This opens the way to experimental studies on model arrays of interacting Ising dots.