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 μm). Mössbauer spectra show a broad asymmetric doublet without split components between 150 K and 250 K. Hyperfme magnetic splitting can be observed at 130 K, and a sextet spectrum is apparent below 100 K. The mass susceptibility of the ferrihydrite was measured between 10 K and 300 K. The in‐phase mass susceptibility χ′ has a value of 1.0×10 −6 m 3 kg −1 at room temperature. At low temperatures, χ′ depends on the frequency of the measurement field, increasing with increasing temperature from 10 K to a maximum at ∼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 characteristics suggest the presence of superparamagnetic grain sizes below 120 K. Cubic spline interpolations of the χ′ versus T curves show that the temperature 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 χ′ was independent of the field amplitude in the measurement range below 2.5 mT. Hysteresis loops, coercivity, and isothermal remanence at low temperature agree with the susceptibility measurements and suggest that ferrihydrite is antiferromagnetic with a parasitic ferromagnetic moment and a Néel temperature around 120 K.