Magnetic properties of synthetic six-line ferrihydrite nanoparticles

• Published in: Physics of the earth and planetary interiors 2006-03, Vol.154 (3), p.222-233
• Main Authors: Guyodo, Yohan, Banerjee, Subir K., Lee Penn, R., Burleson, David, Berquo, Thelma S., Seda, Takele, Solheid, Peter
• Format: Article
• Language: English
• Subjects: Continental interfaces, environment; EXAFS; Ferrihydrite; Magnetic properties; Mössbauer effect; Ocean, Atmosphere; Rock magnetism; Sciences of the Universe
• ISSN: 0031-9201; 1872-7395; 0031-9201
• DOI: 10.1016/j.pepi.2005.05.009

Three synthetic samples of six-line ferrihydrite (Fh5.4, Fh4.1, and Fh3.0), with average particle sizes of 5.4, 4.1, and 3.0 nm (respectively), have been studied by low-temperature magnetic techniques, Mössbauer spectroscopy, transmission electron microscopy, and X-ray absorption spectroscopy. Magnetic data indicate that these nanoparticles are not only antiferromagnetic, but that they also possess a ferromagnetic-like moment due to the presence of uncompensated spins. Both magnetic and Mössbauer data indicate that nanoparticles of six-line ferrihydrite are superparamagnetic at room temperature, with a low-temperature transition between blocked and unblocked magnetic states dependent on average particle size. In particular, low-field magnetic susceptibility data display a peak in amplitude at 45 K (Fh3.0), 55 K (Fh4.1), and 80 K (Fh5.4). Low-temperature induced magnetization data, acquired in magnetic fields up to 5 T, also display clearly a superparamagnetic behavior. These data were tentatively modeled as the sum of two contributions: a linear term due to the antiferromagnetic susceptibility and a nonlinear term due to the uncompensated spins. Model estimates of the magnetization carried by the uncompensated spins ( M nc) show a decrease in M nc with increasing temperature. Extrapolation of M nc values down to zero provided an estimate of the Néel temperature for six-line ferrihydrite on the order of 500 K.