Tuning the Néel Temperature of Hexagonal Ferrites by Structural Distortion.

To tune the magnetic properties of hexagonal ferrites, a family of magnetoelectric multiferroic materials, by atomic-scale structural engineering, we studied the effect of structural distortion on the magnetic ordering temperature (T_{N}) in these materials. Using the symmetry analysis, we show that unlike most antiferromagnetic rare-earth transition-metal perovskites, a larger structural distortion leads to a higher T_{N} in hexagonal ferrites and manganites, because the K_{3} structural distortion induces the three-dimensional magnetic ordering, which is forbidden in the undistorted structure by symmetry. We also revealed a near-linear relation between T_{N} and the tolerance factor and a power-law relation between T_{N} and the K_{3} distortion amplitude. Following the analysis, a record-high T_{N} (185 K) among hexagonal ferrites was predicted in hexagonal ScFeO_{3} and experimentally verified in epitaxially stabilized films. These results add to the paradigm of spin-lattice coupling in antiferromagnetic oxides and suggests further tunability of hexagonal ferrites if more lattice distortion can be achieved.