Critical behaviour of magnetic transitions in KCoF3 and KNiF3 perovskites

• Thermal diffusivity and specific heat are measured at the magnetic transitions.
• A high resolution ac photopyroelectric calorimeter is used.
• KNiF3 and KCoF3 are not perfect 3D-Heisenberg systems.
• There is a slight deviation in KNiF3 due to a small uniaxial anisotropy.
• Spin–orbit coupling must be considered to describe the critical behaviour of KCoF3.

KAF3 (A = Mn, Co, Ni) are considered as perfect 3-D Heisenberg antiferromagnets though there are scarce experimental evidences of that behaviour in the case of the last two perovskites. In this work, a high resolution photopyroelectric technique has been used to study the critical behaviour of the antiferromagnetic transition in KCoF3 and KNiF3 measuring specific heat, thermal diffusivity and thermal conductivity. The critical behaviour of KNiF3 slightly deviates from the perfect Heisenberg universality class (the retrieved parameters are α = −0.110, A+/A− = 1.30), due to its small uniaxial anisotropy; this behaviour has been confirmed by the critical parameters of both the specific heat and the thermal diffusivity. In opposition to what is assumed in literature, the critical behaviour of KCoF3 cannot be simply described by the Heisenberg universality class (α = −0.081, A+/A− = 1.19) due to the spin–orbit interaction at the magnetic transition, which introduces a perturbation in the Hamiltonian and makes the pure Heisenberg model incomplete for this case. The results for both materials disagree with and correct previous works on them.