There has been considerable interest in molecular magnets based on the Prussian blue class of transition-metal cyanide complexes.[1] Pioneering work on these materials has realized a broad range of ferro-and ferrimagnetic solids with Curie points ranging from cryogenic to above room temperature. [2,3] A parallel interest also exists in the development of nanocomposite structures containing nanoscale magnetic particles exhibiting single-domain magnetic behavior. [4][5][6] Investigations of magnetic nanocomposites is driven by their novel properties and their potential as new magnetic, optical, and electronic materials. With one exception, [7] however, the Prussian blue class of magnetic materials has not been produced as nanoparticles, and, to the best of our knowledge, there have been no reports of their incorporation into composite structures. Herein we report the fabrication of a new nanocomposite material containingferromagnetic analogue of Prussian blue, in a porous silica matrix. This material is made by a controlled multicomponent sol-gel synthesis in which the precipitation of the Prussian blue analogue is arrested at nanoscale dimensions by gelation of the silica network. The resulting materials are homogeneous, optically transparent, and exhibit superparamagnetic and tunable photomagnetic behavior. We believe that this study suggests a new approach for utilizing the Prussian blue class of magnetic materials in advanced optical and magnetic applications.Homogeneous silica xerogels containing cyanide-bridged Co II /Fe III centers were made by incorporation of both transition-metal components during the solution phase of the synthesis. To obtain homogeneous materials reproducibly, conditions were optimized for the amount of water and the concentration and molar ratio of Co II and ferricyanide by following previously developed procedures. [8,9] In the optimized preparation, Co II nitrate was dissolved in methanol and added to tetramethylorthosilicate. Aqueous potassium ferricyanide was added to this solution to give a 1:1 Co:Fe molar ratio. Upon mixing, the solution turns dark purple, which suggests the formation of the mixed-valence complex. At concentrations up to 0.03 mol % total-metal to silicon (Fe + Co/Si), the solution remained transparent through gelation, aging, and drying, and ultimately yielded a homogeneous, optically transparent xerogel (Figure 1, inset). The spectrum of this glass (Figure 1) is qualitatively similar to that of the bulk materials, with a broad intervalence charge-transfer band in the visible region between 450 and 650 nm and a sharp, higher energy peak around 400 nm. However, the maximum of the intervalence band lies at 452 nm (22 124 cm À1 ) which is blue-shifted by approximately 2900 cm À1 from that of the bulk materials. To determine whether the magnetic behavior was singular, the magnetic susceptibility of these materials was measured as a function of temperature and field strength.[10] Bulk M