Eddy De Grave

Mössbauer Effect Studies of Oxidic Spinels

Since the determination of the crystal structure of the mineral spinel, MgAl2O4, by Bragg,1 and Nishikawa2 in 1915, many metal oxides and mixed metal oxides have been found to possess the same structure,3 and these are usually referred to as spinel compounds or spinels. Such materials, and in particular those containing iron, exhibit suitable combined electrical and magnetic properties and therefore soon became extremely important for many technological applications, such as high-frequency devices, memory cores, and magnetic recording. The rapid expansion of these technologies after World War II was a great stimulus for many researchers to focus their efforts on this kind of ferrite materials. Consequently, the fifties and the early sixties were characterized by intensive investigations of spinel systems with a large variety of compositions. Examples in this respect are the works of Gorter,4,5 followed by that of Blasse,6 reporting structural and magnetic properties of hundreds of new synthetic spinel compounds. The ability of the spinel structure to contain different kinds of cations, which can, moreover, gradually be substituted, made these materials also very suitable for experiments on certain magnetic and structural concepts, new at that time, such as Neel’s ferrimagnetism, ligand field stabilization, and the cooperative Jahn-Teller effect.

The magnetic structure of bernalite, Fe(OH)3

Abstract Mossbauer spectra of bernalite, a recently described mineral with a distorted perovskite-type structure and chemical formula Fe(OH) 3 , were recorded at 4 K in external magnetic fields up to 6 T to obtain information about the magnetic structure. Spectra show a single hyperfine magnetic component, where the angle between the net magnetic moment and the direction of the external field is close to 90°. The most likely spin arrangement is a slight deviation from a collinear antiferromagnet, resulting in a weak ferromagnetic moment.