Dipolar interactions and their influence on the critical single domain grain size of Ni in layered Ni/Al(2)O(3) 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.