Magnetic ordering transition in amorphous Fe(III) hydroxide

Susceptibility measurements were made, in the temperature range 4.2–120 K, on iron (III) hydroxide samples prepared by oxidation of Fe(CO) 5 in ethanolic solution. One sample was constituted by amorphous particles, while two other samples were partially crystallized, either as a result of solvent change or of spontaneous aging. Data for the amorphous particles show a kink at T = 85 K, indicating a change of behavior associated with magnetic ordering. The crystallized samples, in contrast, exhibit superparamagnetism of antiferromagnetic microcrystals in the whole measuring temperature range. Second-order susceptibility coefficients have also been measured in small dc fields, from which average particle magnetic moments could be deduced.