Incoherent magnetization reversal in 30-nm Ni particles

The magnetic properties of a 100-nm-period large-area array of regular, 30- nm polycrystalline nickel particles have been studied. The particles are fo und to reverse incoherently, and their hysteresis behavior has been compare d with a computational model over a range of temperatures. Excellent agreem ent with the model is obtained, indicating that switching of the particles is dominated by the reversal of approximately 10-nm-diameter volumes within each particle. These switching volumes are identified with the columnar gr ains in the polycrystalline nickel, showing that the microstructure determi nes the magnetic behavior of the particles. This explains the anisotropy di stribution and the onset of superparamagnetism in the sample. Incoherent re versal occurs even though the particles are only 1.5 times the exchange len gth in nickel, a size at which nearly uniform rotation is expected to occur if the particles were homogeneous.