Photo-excitation from dia- to ferri-magnetism in a Rb-Co-hexacyanoferrate Prussian blue analogue

We studied the photo-excitation process, the relaxation of the photo-excited state towards the stable state, and the photo-induced magnetic properties of the Prussian blue analogue Rb0.52Co [Fe(CN)(6)](0.84), 2.3 H2O. Magnetic, Mossbauer and reflectivity measurements have been performed during and after illumination. The efficiency of the photo-excitation device is maximum at lambda congruent to 700-750 nm. The process, however, is severely hindered by bulk absorption of the light; it is rapidly completed at the surface of the sample and then proceeds slowly in the bulk. Under the effect of photo-excitation the system turns from a dia- to a ferri- magnet, with a T-C value greater than or equal to 21 K, indicative of the transformation of the material due to the following optical electron transfer: Fe-II(LS) + Co-III(LS) --> Fe-III(LS) + Co-II(HS) Thermal relaxation towards the stable electronic state is observed in the 95-110 K temperature interval, obeying a self-accelerated kinetics. At low temperature, a weak, non-exponential, relaxation is detected. These features are discussed in terms of co-operative effects iii the frontal process of photo- excitation. The peculiarities of a photo-excited state created below the magnetic ordering temperature are discussed. A metastable magnetic state has been observed in low fields, denoted "Raw Photo-Induced State" (RPI), with a magnetization curve in-between the field-cooled and zero-field-cooled curves.