Low-temperature magnetic behavior of ferrihydrite

Two samples of six-line ferrihydrite (5Fe(2)O(3). 9H(2)O) were precipitated from an Fe(III) solution. The purity of the samples was verified by X-ray diffraction. Scanning electron microscope pictures indicate that the grains have good crystallinity and variable grain size (<0.5 to 100 mu m). Mossba uer spectra show a broad asymmetric doublet without split components betwee n 150 K and 250 K. Hyperfine magnetic splitting can be observed at 130 K, a nd a sextet spectrum is apparent below 100 K. The mass susceptibility of th e ferrihydrite was measured between 10 K and 300 K. The in-phase mass susce ptibility chi' has a value of 1.0 x 10(-6) m(3) kg(-1) at room temperature. At low temperatures, chi' depends on the frequency of the measurement fiel d, increasing with increasing temperature from 10 K to a maximum at similar to 90 K and then decreasing smoothly at higher temperatures. Over the same temperature range the quadrature mass susceptibility rises to a peak at 65 K then decreases to a low background value above 120 K. These characterist ics suggest the presence of superparamagnetic grain sizes below 120 K. Cubi c spline interpolations of the chi' versus T curves show that the temperatu re of the in-phase susceptibility peak increases with increasing frequency of the field from 86 K at 40 Hz to 94 K at 4000 Hz. The value of chi' was i ndependent of the field amplitude in the measurement range below 2.5 mT. Hy steresis loops, coercivity, and isothermal remanence at low temperature agr ee with the susceptibility measurements and suggest that ferrihydrite is an tiferromagnetic with a parasitic ferromagnetic moment and a Neel temperatur e around 120 K.