Dipolar interactions and their influence on the critical single domain grain size of Ni in layered Ni/Al2O3 composites

Pulsed laser deposition has been used to fabricate Ni/Al2O3 multilayer composites in which Ni nanoparticles with diameters in the range of 3-60 nm are embedded as layers in an insulating Al2O3 host. At fixed temperatures, the coercive fields plotted as a function of particle size show well-defined peaks, which define a critical size that delineates a crossover from coherently rotating single domain to multiple domain behavior. We observe a shift in peak position to higher grain size as temperature increases and describe this shift with theory that takes into account the decreasing influence of dipolar magnetic interactions from thermally induced random orientations of neighboring grains