Magnetic properties of the micro-silica/cement matrix with carbon-coated cobalt nanoparticles and free radical DPPH

Abstract Samples of micro-silica/cement containing iron oxide, Fe2O3, and doped with carbon-coated cobalt nanoparticles and free radical DPPH were prepared and studied by the magnetic resonance method. The concrete’s main components (silica and cement) produced very complicated FMR/EPR (ferromagnetic and electron paramagnetic resonance) spectra. The temperature dependence of the FMR/EPR spectra was recorded in the 90–300 K temperature range. The cement/micro-silica matrix produced a very broad FMR line originating from iron oxide particles and two EPR lines originating from iron(III) ions in the crystal field of low-symmetry (centered at geff ∼ 4.3) and from manganese(II) ions (geff ∼ 2) of hyperfine structure. Additionally, a very narrow line and a very broad EPR/FMR line were registered and, respectively, attributed to DPPH and cobalt nanoparticles. The isolated paramagnetic iron(III) and manganese(II) centers displayed increasing intensity of the EPR spectra with decreasing temperature, while no influence of the magnetic nanoparticles was observed. The intensity of the FMR spectrum of iron oxide decreased strongly and the resonance field was effectively shifted toward low magnetic fields with decreasing temperature. The observed FMR behavior is similar to what was registered for iron oxide magnetic nanoparticles. The introduction of magnetic nanoparticles influenced the EPR spectrum of the free radical DPPH significantly: its intensity decreased above 260 K and increased slightly below this temperature, while the resonance field changed with decreasing temperature. This behavior may be associated with the porous state of cement and/or the reaction of the multi-component magnetic system. The FMR/EPR method could be very useful for the characterization of matrices containing small amounts of magnetic nanoparticles.