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 (Ca 2+ ). The morphology of the resulting nanoparticles can be controlled with the Ca 2+ 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.