Magnetic dynamics of weakly and strongly interacting hematite nanoparticles

The magnetic dynamics of two differently treated samples of hematite nanoparticles from the same batch with a particle size of about 20 nm have been studied by Mössbauer spectroscopy. The dynamics of the first sample, in which the particles are coated and dispersed in water, is in accordance with the Néel expression for the superparamagnetic relaxation time of noninteracting particles. From a simultaneous analysis of a series of Mössbauer spectra, measured as a function of temperature, we obtain the median energy barrier $K_{\mathrm{Bu}}{V}_{\mathrm{m}}/k=570\mathrm{}\pm 100$ K and the preexponential factor ${\tau }_0{=1.3\mathrm{}}_{-0.8}^{+1.9\mathrm{}}\times {10}^{-10}$ s for a rotation of the sublattice magnetization directions in the rhombohedral (111) plane. The corresponding median superparamagnetic blocking temperature is about 150 K. The dynamics of the second, dry sample, in which the particles are uncoated and thus allowed to aggregate, is slowed down by interparticle interactions and a magnetically split spectrum is retained at room temperature. The temperature variation of the magnetic hyperfine field, corresponding to different quantiles in the hyperfine field distribution, can be consistently described by a mean field model for “super-ferromagnetism” in which the magnetic anisotropy is included. The coupling between the particles is due to exchange interactions and the interaction strength can be accounted for by just a few exchange bridges between surface atoms in neighboring crystallites.