Superparamagnetism and Exchange Anisotropy in Microparticles of Magnetite Embedded in an Inert Carbonaceous Matrix

Abstract

By the controlled thermal degradation of ferricinium nitrate a material composed of iron oxide particles embedded in an inert carbonaceous matrix was obtained. The iron oxide particles were found to be mostly magnetite, and their average diameter about 35 Å. This value was obtained from both x‐ray diffraction line broadening and from static magnetic measurements. The small particle size caused the material to be superparamagnetic down to and below liquid‐nitrogen temperature. At liquid‐helium temperature the material was ferromagnetic and exhibited exchange anisotropy in the form of a shifted hysteresis loop. A coupling energy of 2.28×10⁵ erg/cm³ was calculated from the shifted hysteresis loop. EPR and Mössbauer‐effect spectra showed electron hopping at room temperature within the Fe₃O₄. In the ferromagnetic state, local disorder around the magnetic ions manifests itself as broadening of the Mössbauer spectra and displacement of the hysteresis loop.