Constraining the Origins of the Magnetism of Lepidocrocite (γ-FeOOH): A Mössbauer and Magnetization Study

Abstract

Lepidocrocite, a widespread environmentally relevant iron oxyhydroxide, has been investigated for decades using 57 Fe Mössbauer 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 Mössbauer 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 Mössbauer 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 modeling of Mössbauer 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 Mössbauer spectroscopy.

The Low‐Temperature Besnus Magnetic Transition: Signals Due to Monoclinic and Hexagonal Pyrrhotite

“…The only one of these minerals that has been detected above the 0.1% level in our synchrotron XRD results is lepidocrocite (Figure 2), which become magnetically ordered below 50-70 K, and in which markedly higher SIRM values are observed below 30 K (Guyodo et al, 2016;Hirt et al, 2002;Lee et al, 2004). These minerals are paramagnetic at ambient temperatures, but they are magnetically ordered at low temperatures so they are worth considering in relation to the low-temperature magnetism of hexagonal (3T) pyrrhotite.…”

Unraveling the Mineralogical Complexity of Sediment Iron Speciation Using Sequential Extractions

“…However, data show that remanent magnetization has a sharp decrease upon warming in both FC and ZFC LTSIRM experiments between 33 and 80 K-interpreted to be related to the blocking temperature. Lepidocrocite also shows a similar sharp drop in remanence in FC and ZFC LTSIRM experiments below 30 to 75 K, interpreted to be the Néel temperature (Guyodo et al, 2016;Hirt et al, 2002;Till et al, 2014). Unfortunately, in natural samples with complex mixtures of minerals, it can be difficult to distinguish these phases from superparamagnetic minerals such as nanophase hematite or goethite (e.g., Guyodo et al, 2003).…”

Characterization of an active ingredient made of nanoscale iron(oxyhydr)oxide for the treatment of hyperphosphatemia

“…24,41 The mean hyperne eld obtained for sample 1 is 45.5(1) T. The most probable eld is, because of the asymmetric distribution of hyperne elds, 47.8(1) T. The reported hyper-ne eld at 4.2 K of most iron(oxyhydr)oxide is typically around 50 T or higher. 14,24,46,48,[51][52][53] For schwertmannite, 15,54 lepidocrocite, 15,24,53 goethit, 55 and akaganéite 15,24 as well as for ferrihydrite with a high organic matter content, 14,15 however, lower hyperne elds between 45 T and 48 T have been reported. However, the Mössbauer spectra are strongly inuenced by the particle size and the degree of crystallinity.…”