Magnetic Anisotropy of KCoF3 and KNiF3

Magnetic anisotropy of single crystals KCoF 3 and KNiF 3 was investigated the torque magneto-meter in the temperature range from 78° to 300°K. In the antiferromagnetic region, torque curves of these crystals when suspended along one of the cube edges can be expressed the formula {aligned} T{=}C₁H²sin 2θ-C₂H⁴sin 4θ, {aligned} where H is the magnetic field intensity, θ the angle of rotation of H relative to one of the cubic axes, and C 's are constants. In the case of KCoF 3 the 4θ-term was not observed, while in KNiF 3 , the 4θ-term was rather dominant in all specimens when no external stress was applied. The amplitude of the 2θ-term is sensitive to the stress in both cases. The experimental results on KCoF 3 and KNiF 3 are well interpreted taking the antiferromagnetic spin axis to be parallel to the c -axis below their Néel temperatures. The origin of the 4θ-term which was observed in KNiF 3 is discussed, and it is concluded that this term arises from reversible movements of the antiferromagnetic domain walls existing in this compound.