Observation of domains in antiferromagnetic fluorides by x-ray topography

Abstract

A brief introduction to the theory of susceptibility in antiferromagnets Is given and the experimental studies on antlferromagnetic domains are reviewed. X-ray topographic techniques are discussed. Low temperature domain studies in moderate fields have not been performed previously by X-ray topography due to the geometrical constraints. A new topographic technique using synchrotron radiation has been developed for domain wall motion studies in magnetic fields up to 14 KOe at temperatures down to 4.2K. Details of four successful cryostat designs are presented. The perfection of crystals KNiP(_3), KCoF(_3) and KFeF(_3) grown from the flux has been studied by X-ray topography. Very low dislocation densities and dynamical diffraction effects were observed. Dislocations directions in the flux, grown crystals of KNIF(_3) and KCoF(_3) prefer to run in such directions as to minimise their elastic energy per unit growth length in a similar manner to dislocations in crystals grown from aqueous solution. The topographic experiments on highly perfect crystals of KNIF(_3), and KCoF(_3), have revealed three types of domains with spin orientations parallel to each of the cube edges. Controlled domain wall motion in KNIF(_3) and KCoF(_3) has been studied using synchrotron radiation. Wall movement occurs to align spins perpendicular to the applied field and walls between domains already so oriented are found not to move. The general behaviour of antiferro magnetlc domains in KNIF^ and KCoF(_3) is in excellent agreement with the theoretical model proposed by Neel (1954). No catastrophic spin flop is observed, and some wall movements are reversible. Both direct observations of domain wall motion and the susceptibility measurements on single crystals of KNIF(_3) show that the spin flop occurs via domain wall motion. The effective anlsotropy is measured from the critical and threshold field data for both KCoF(_3) and KNIF(_3).