Abstract
Lepidocrocite, a widespread environmentally relevant iron oxyhydroxide, has been investigated for decades using 57Fe Mossbauer spectroscopy and magnetic measurements. However, a coherent and comprehensive interpretation of all the data is still lacking due to seemingly contradictory interpretations. On one hand, temperature dependence of magnetic susceptibility and Mossbauer spectra resemble those of superparamagnetic nanoparticles with diameters less than 10 nm even though physically particles are lath-shaped with lengths on the order of 100-300 nm. On the other hand, in-field Mossbauer spectra show that lepidocrocite is an antiferromagnet and becomes paramagnetic above 50-70 K, a temperature close to the blocking temperature deduced from susceptibility data. The present study investigates a well-characterized synthetic sample of lepidocrocite, includes modelling of Mossbauer spectra and dc and ac magnetization data, and proposes a solution to this paradox. The new data are coherent with the presence of two entities in lepidocrocite: a bulk antiferromagnetic matrix and sparse ferrimagnetic nanosized inclusions (d = 3.4 nm), akin to maghemite, embedded within. The presence of nanosized ferrimagnetic inclusions is confirmed for the first time by Mossbauer spectroscopy.
Highlights
Iron oxides and oxyhydroxides are widespread magnetic minerals in geologic records
The Mössbauer spectra resemble those of an ensemble of superparamagnetic nanoparticles, it is clearly established, in particular through in-field Mössbauer spectra (De Grave et al, 1986), that lepidocrocite is paramagnetic above TN and that the shape of the Mössbauer spectra reflects an unusually broad distribution of Néel temperatures
Another puzzling feature of lepidocrocite is that the Field Cooled (FC) and Zero Field Cooled (ZFC) branches of the low field direct current susceptibility (Lee et al, 2004) are akin to those encountered in ensembles of superparamagnetic nanoparticles (Tronc et al, 1995), lepidocrocite laths or needles cannot be considered, from a magnetism point of view, as nanometric particles
Summary
Iron oxides and oxyhydroxides are widespread magnetic minerals in geologic records. Quantifying the magnetic assemblage of natural samples allows deciphering past geologic, environmental, climatic, pedogenic, or diagenetic conditions (e.g., Liu et al, 2012). The Mössbauer spectra resemble those of an ensemble of superparamagnetic nanoparticles, it is clearly established, in particular through in-field Mössbauer spectra (De Grave et al, 1986), that lepidocrocite is paramagnetic above TN and that the shape of the Mössbauer spectra reflects an unusually broad distribution of Néel temperatures Another puzzling feature of lepidocrocite is that the Field Cooled (FC) and Zero Field Cooled (ZFC) branches of the low field direct current (dc) susceptibility (Lee et al, 2004) are akin to those encountered in ensembles of superparamagnetic nanoparticles (Tronc et al, 1995), lepidocrocite laths or needles cannot be considered, from a magnetism point of view, as nanometric particles. This persistence of a remanent magnetization at temperatures higher than TN was observed in other samples (Till et al, 2014), suggesting that it is inherent to lepidocrocite. Till et al (2014) showed that nanoparticles of maghemite formed within the larger particles of lepidocrocite, during the early stage of lepidocrocite dehydroxylation achieved through moderate heating under oxidizing conditions
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