Structural Tuning of Ferromagnetism in a 3D Cuprate Perovskite

The perovskite ${\mathrm{SeCuO}}_3$ is unusual for possessing (i) a very small $A$-site $\left({\mathrm{Se}}^{4+}\right)$ radius, (ii) highly distorted Cu-O-Cu bond angles $\left(\alpha \right)$, and (iii) a ferromagnetic (FM) ground state. Negative chemical pressure is applied by replacing Se with Te in solid solution, causing a continuous transformation between the FM state and an antiferromagnetic (AFM) state. This is the first FM-AFM transformation to be controlled by a single microscopic parameter, the in-plane $\alpha$, allowing a precise determination of the crossover angle $({\alpha }_c=127\mathrm{}°\pm 0.5°)$ between FM and AFM superexchange.