Study of the Brownian Broadening in the Mössbauer Spectra of Magnetic Nanoparticles in Colloids with Different Viscosities

The line broadening in the Mössbauer spectra of nanoparticles in model colloids with known viscosity has been studied. Magnetite nanoparticles with average hydrodynamic sizes of 140 and 40 nm and a set of stable aqueous colloids based on them have been synthesized. The nanoparticles have been enriched in the 57Fe isotope to compensate for the significant decrease in the Mössbauer effect probability in liquid media. Two series of temperature experiments have been carried out: in an aqueous colloid of nanoparticles with a 90-% glycerol content and on dehydrated samples of nanoparticles in the absence of Brownian motion. It is established that the Brownian motion of nanoparticles causes an additional broadening of Mössbauer lines, which is inversely proportional to the viscosity and nanoparticle size. It is demonstrated that Mössbauer spectroscopy allows one to separate the contributions of the Brownian motion and the Néel relaxation of magnetic nanoparticles. It is confirmed that the shape of Mössbauer spectrum and the probability of Mössbauer effect depend strongly on both the size of particles suspended in a liquid and the dynamic viscosity coefficient of the liquid.