Ageing effect in carbon-coated cobalt nanoparticles embedded in a cement matrix studied by FMR

Samples containing agglomerated cobalt magnetic nanoparticles coated with carbon and dispersed in a cement matrix have been prepared. The samples with (25/75 wt.%) carbon/cobalt content dispersed at low concentration in cement (about 0.5%) have been studied using the ferromagnetic resonance (FMR) method at room temperature (RT). Very intense and broad FMR lines have been recorded in the fresh samples, more intense than those obtained from the same agglomerates dispersed in a paraffin matrix. The FMR spectrum of the cement matrix sample consisted of three different magnetic resonance components: (a) a very broad FMR line attributed to iron oxide, (b) an EPR line arising from trivalent iron ions in a crystal field of low symmetry, and (c) a line from divalent manganese ions. After two months the FMR spectra of cobalt nanoparticles dispersed in cement has changed drastically, while the same agglomerates dispersed in paraffin has shown only weak time dependence. Magnetic interaction between magnetic nanofillers could be more intense in the case of a cement matrix for two reasons: one is due to the porous state of this material and the other is owing to the chemical and physical reactions of agglomerates with various cement constituents. Applications of the obtained results could lead to the production of cement with much better mechanical properties.