Dynamic magnetic phenomena in fine-particle goethite

Abstract

The distinctive Mossbauer spectra of fine-particle goethite (a-FeOOH) have been attributed by many authors to superparamagnetism.However, measurements of the magnetic anisotropy energy and particle volume show that the superparamagnetic blocking temperature for most samples is much greater than the Neel temperature. A model involving magnetic ordering of clusters created by high concentrations of vacancy defects is proposed, in which the cluster moments slowly relax, thus producing a Boltzmann distribution in the z component of the magnetization. The model provides excellent fits to the temperature-dependent hyperfine-field distributions observed in the Mossbauer spectra, and may have wider applicability to other diamagnetically substituted iron oxide materials. A linear relation is observed between Tn and the inverse mean crystallite dimension in the [111]direction of -1060 (130) K nm, and it is shown that goethite particles must have a volume of less than 1000 nm' in order to show superparamagnetic Mossbauer spectra at room temperature.

Magnetic dynamics of weakly and strongly interacting hematite nanoparticles

“…In the present study we have directly measured the superparamagnetic blocking temperature of the non-interacting hematite particles, and we have found that essentially all particles have blocking temperatures below 250 K. Thus, the assumption used in the ''superferromagnetism'' model is clearly fulfilled above this temperature, and there is no reason to introduce new parameters as in the model by Bocquet et al 42 The fact that the application of a magnetic field of 1.5 T at 295 K results in sharper lines in the Mössbauer spectra and an increase in the average hyperfine field shows that the line broadening is due to fluctuations of the sublattice magnetization vectors in the particles, which to some extent can be suppressed by the applied field. If the reduced values of the hyperfine field were due to a lowering of the Néel temperature, as it has been suggested for goethite, 41 one would not expect such a dependence on the applied field.…”

“…Studies of goethite particles with different interparticle interactions seem to support the ''superferromagnetism'' model. 14,16,40 The model was, however, later criticized by Bocquet et al, 41,42 who performed neutron diffraction, Mössbauer spectroscopy and magnetization measurements as a function of temperature on similar samples. Based on the disappearance of the magnetic peaks in neutron diffraction spectra, the collapse of the sextet in the Möss-bauer spectra and the occurrence of a cusp of the zero field cooled magnetic susceptibility at approximately the same temperature, they concluded that the observed phenomena were due to a lower Néel temperature of the fine particles compared to bulk goethite rather than due to interactions between superparamagnetic particles.…”

“…Bocquet et al 42 also estimated the magnetic anisotropy energy constant of goethite from the field dependence of Mössbauer spectra at low temperatures. Using the value estimated in this way they calculated a superparamagnetic blocking temperature, which was far above the temperatures at which the broadened spectra were measured, indicating that the assumptions behind the model for ''superferromagnetism'' were not fulfilled.…”

“…Bocquet et al 42 proposed that the asymmetric broadening in the Mössbauer spectra can be explained by a model in which it is assumed that the particles consist of clusters created by a high concentration of vacancies. Furthermore, they assumed that the magnetization vectors in the clusters precess rapidly with a precession angle .…”

Critical review of analytical applications of Mössbauer spectroscopy illustrated by mineralogical and geological examples (IUPAC Technical Report)

“…Within a small grain, several clusters are created by high local concentrations of vacancies or other diamagnetic defects, and the cluster size varies with the temperature [52]. Magnetic relaxation effects have also been observed with olivines, (Mg 2-x Fe x )SiO 4 , as a function of x [54].…”