Magnetic properties of synthetic six-line ferrihydrite nanoparticles

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, Mِssbauer 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 Mِssbauer 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 (Mnc) show a decrease in Mnc with increasing temperature. Extrapolation of Mnc values down to zero provided an estimate of the Néel temperature for six-line ferrihydrite on the order of 500 K.