Preparation and properties of ferrimagnets in the RbMgF 3-RbCoF 3 system

Abstract

Previously reported ABF 3-type ferrimagnets with the hexagonal BaTiO 3 structure all contain Ni 2+ as the predominant magnetic ion. This paper reports the discovery of new ferrimagnetic compositions in the RbMgF 3-RbCoF 3 system where Co 2+ is the only magnetic ion. Phase studies on this system show that at high temperatures the hexagonal phase is stable to about 70% RbCoF 3. Single crystals were grown over this compositional range and their magnetic and optical properties studied. Compositions containing in excess of about 35% RbCoF 3, are ferrimagnetic and their magnetic moments (as high as 24 e.m.u./g) and Curie temperatures (up to 36°K) vary with Co 2+ concentration. The magnetization data are consistent with a model of a 1/3 − 2/3 ferrimagnetic spin arrangement. The B-B interaction, which is the resultant of the 90°Co 2+-F-Co 2+ and the direct Co 2+-Co 2+ interactions appears to be positive. Anisotropy measurements at 4.2°K yielded values from 2.2 × 10 −6to8.9 × 10 −6ergs/cm 3 for K. Optical absorption data were used to determine the magnitude of the crystal field parameter Dq (870–880 cm −1) and the spin orbit interaction constant λ (190 cm −1). From the Dq values it is concluded that the strength of the crystalline field acting on the Co 2+ ion in the hexagonal structure, where 2/3 of the CoF 6 octahedra share faces, is not too different from that in cubic KCoF 3 and RbCoF 3, where they share corners.