Ferromagnetic enhancement in the clusters of Co-Pd magnetic nanoparticles induced by the formation of cross-linkage

Co-Pd alloy nanoparticles with the surface modified with carboxylic acid functional groups are prepared to connect the nanoparticles with a cross-linkage of calcium ion (Ca²⁺). The morphology of the resulting nanoparticles can be controlled with the Ca²⁺ ion concentration. At diluted concentrations, a dimer structure is formed, in which constituent nanoparticles are coupled ferromagnetically with the aid of dipole-dipole interaction. At high concentrations, many nanoparticles are multiple-cross-linked, resulting in forming highly packed three-dimensional structures. The magnetization is strongly enhanced 8 times as large as that of pristine sample owing to dipolar ferromagnetism in the 3D structures.