Magnetism of Rubidium Cobalt Hexacyanoferrate Nanoparticles.*

Although photoinduced magnetism in nanoparticles of Prussian blue analogs has been reported, these samples are superparamagnetic. We have generated and characterized nanoparticles of Rb$_{j}$Co$_{k}$[Fe(CN)$_{6}$]$_{l}\cdot $nH$_{2}$O, which exhibit photoinduced magnetism and, for the largest particles, long-range ferrimagnetism with finite coercive fields. The synthesis involves the variation of the concentration of the poly(vinylpyrrolidone), PVP, the encapsulating polymer, which controls the resulting particle sizes. From HR-TEM, the particle size distributions have been obtained for four batches of samples, with mean diameters ranging from nominally 3~nm to 13~nm. Upon irradiation with white light at 5~K, all samples exhibit photoinduced magnetism. Magnetization studies indicate that the smallest particles are superparamagnetic, while the largest ones are ferrimagnetic with long-range ordering temperatures ($T_{c}$ $\sim $ 17~K) and coercive fields ($H_{c}$ $\sim $ 250~G) varying with particle size in a manner consistent with the predictions of finite-size scaling.