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.