On the interpretation of Mössbauer spectra of magnetic nanoparticles

Highlights: • Expressions for the superparamagnetic relaxation time of magnetic nanoparticles are reviewed. • Consequences of using the simple Néel and Brown expressions with constant pre-factor are evaluated. • The interpretation of Mössbauer spectra of iron oxide nanoparticles with asymmetrically broadened lines is discussed. • Relaxation times of weakly/non-interacting maghemite particles reported in the literature are compared. • We argue that spectra with broadened lines in some cases are due to particle interactions. Mössbauer spectra of magnetic nanoparticles are usually influenced by fluctuations of the direction of the magnetic hyperfine field. In samples of non-interacting particles, the superparamagnetic relaxation usually results in spectra consisting of a sum of a sextet and a doublet with a temperature dependent area ratio. This is in accordance with the exponential dependence of the superparamagnetic relaxation time on particle size and temperature in combination with the particle size distribution. An alternative interpretation of these features is a first order magnetic transition from a magnetically ordered state to a paramagnetic state. We point out that this interpretation seems not to be correct, because the doublet component has been found to transform to a magnetically split component when relatively small magnetic fields are applied, and therefore it cannot be due to a paramagnetic state. In other cases, spectra of magnetic nanoparticles consist of sextets with asymmetrically broadened lines without the presence of doublets. It has been suggested that such spectra can be explained by a multilevel model, according to which relaxation takes place between a large number of states. We point out that spectra with asymmetrically broadened lines at least in some cases rather should be explained by the influence of magnetic inter-particle interactions on the magnetic fluctuations.