Amorphous FeF3: a non-crystalline magnet with antiferromagnetic interactions

Susceptibility and magnetisation curves for amorphous FeF 3 indicate the presence of strong antiferromagnetic exchange coupling which would normally lead to a Neel temperature of the order of 300K in a crystalline lattice. In fact the appearance of remanence and an irreversible susceptibility indicate that spin freezing sets in only below T f =29K, but there is no sharp phase transition. Mossbauer measurements indicate that the magnetic order is speromagnetic. A low value of the molar Curie constant measured in the range 50-300K (3.54) may be explained by assuming a spread in magnitude of the antiferromagnetic interactions. The results are discussed in terms of magnetic frustration.