Giant Magnetoelectric Effect in Antiferromagnetic BaMnO$_{3-\delta}$ and Its Derivatives

Hexagonal perovskite 15R-BaMnO$_{2.99}$ with a ratio of cubic to hexagonal layers of 1/5 in the unit cell is an antiferromagnetic insulator that orders at a N\'eel temperature T$_{N }$= 220 K. Here we report structural, magnetic, dielectric and thermal properties of single crystal BaMnO$_{2.99}$ and its derivatives BaMn$_{0.97}$Li$_{0.03}$O$_{3}$ and Ba$_{0.97}$K$_{0.03}$MnO$_{3}$. The central findings of this work are: \textbf{(1)} these materials possess a usually large, high-temperature magnetoelectric effect that amplifies the dielectric constant by more than an order of magnitude near their respective N\'eel temperature; \textbf{(2)} Li and K doping can readily vary the ratio of cubic to hexagonal layers and cause drastic changes in dielectric and magnetic properties. These findings provide a new paradigm for developing \textit{novel}, \textit{high-temperature magnetoelectric} \textit{materials} that may eventually contribute to technology.