Stress induced linear dichroism of antiferromagnetic perovskites : KNif3, KCoF3 and RbCoF3. I. — Magnetic origin and evidence for domain twin-walls movements

We report here on the stress induced linear dichroism (LD) of KNiF 3, KCoF3 and RbCoF3 above and below their Néel temperature. Above TN we obtain a linear variation of the dichroism with a < 001 > applied stress S, but below TN we find two different parts for this curve when S increases. Under a small stress (S < Scr) the LD increases rapidly and is reversible with S. From these experiments, we infer that there is an easy reversible movement of the domain twin-walls. This is confirmed by the small energy required to obtain a single domain in comparison to the k = 0 magnon energy deduced from Raman experiments. The value of this energy agrees with our recent magnetic circular dichroism studies. For all samples above Scr (S // < 001 >) we obtain an untwinned crystal where the spins align along the < 001 > direction. In agreement with our calculations, no effect is observed for KCoF3 and RbCoF3 under a < 011 > stress since they have a small λ111 magnetostrictive coefficient. Under a high stress (S > Scr), the LD increases rapidly with decreasing temperature just below TN. We attribute this dichroism to a pure magnetic origin which is related to the exchange splitting in the ground state, the stress being only used to align the spins along a definite direction.