Structure and magnetic properties in Ag-stabilized ferromagnetic sensor of CrO2 nanoparticles

In aqueous solution at 50–70 °C, AgNO3 induces a CrO3 → CrO2 reduction reaction, forming Ag-stabilized nanoparticles of CrO2. Well-resolved X-ray diffraction peaks of CrO2 of the D4h14:P42/mnm tetragonal crystal structure occurs after annealing the sample at 200–400 °C in air. During annealing, Ag+, if present, converts to metallic silver and other byproducts burn out as gases. Lattice parameters as large as a = 0.4413 nm and c = 0.2899 nm (density ρ = 4.954 g/cm3) appear after 2 h of annealing at 400 °C in comparison to a = 0.4417 nm and c = 0.2916 nm (ρ = 4.900 g/cm3) in pure CrO2. Small amounts of silver precipitate together with an incipient Cr2O3 growth in controlled oxidation of Ag-protected CrO2 particles. Over 200–400 °C, the Ag:CrO2 crystallites grow primarily along the c-axis as seen in gradually increasing intensity of the 0 0 2 X-ray reflection. Saturation magnetization as large as 80 A m2/kg occurs with 75.7 mT of coercivity after annealing at 300 °C. The results are analyzed in relation to the structure, which is changing sensitively by annealing at 200–400 °C in air.