Magnetic Properties of KMnF3. II. Weak Ferromagnetism

The static magnetic properties of single-crystal KMn${\mathrm{F}}_3$ have been studied by magnetic torsion measurements. These measurements are consistent with a transition to uniaxial antiferromagnetism below 88.3°K. Below 81.5°K the magnetic behavior is complex with the development of hysteresis and discontinuities in the torsion. Further, the torsion increases linearly with magnetic field in this range. These observations suggest the development of weak ferromagnetism in this crystal below 81.5°K. From a comparison of the direction of the weak moment and the known distortions in the crystal structure it is concluded that the weak moment results from a canting of the magnetic sublattices because of differences in the sublattice anisotropy. Between 81.5° and 88.3° a moment appears only in strong magnetic fields. It is shown that the moment is developed in a field because of the increased parallel susceptibility of a canted antiferromagnet. The canting transition is interpreted as a first order transition of the Jahn-Teller type. The antiferromagnetic transition itself is associated with a change in lattice parameter and is interpreted as an exchange-controlled first-order transition.