Influence of dipolar interactions on the formation of domains in layered Ni/Al$_{2}$O$_{3}$ nanocomposites

Pulsed laser deposition has been used to fabricate Ni/Al$_{2}$0$_{3}$ multilayer composites in which Ni nanoparticles of uniform size in the range of 3-60 nm are embedded as layers in an insulating Al$_{2}$0$_{3}$ host. At fixed temperatures, the coercive fields show well-defined peaks which define a critical size that delineates a crossover from single domain (SD) to multiple domain (MD) behavior. Most applications require that the particles be single domain with a uniform magnetization that remains stable with a sufficiently large anisotropy energy to overcome thermal fluctuations and beat the superparamagnetic limit, which establishes a temperature-dependent \textit{lower bound} to the particle size (superparamagnetic limit). These considerations must take into account the effect of interactions on magnetic properties as is evident for high-density recording media where particles are very close to each other. The effect of dipolar interactions on the establishment of an \textit{upper bound} to particle size ($d_{c})$, which defines the crossover from SD to MD behavior will be discussed. We show using coercivity measurements that, with increasing temperature, $d_{c}$ increases and then saturates due to attenuated dipolar interactions from thermally induced motions of neighboring randomly oriented particles.