A-site magnetic ordering in quadruple perovskite oxides

Magnetic exchange interaction in A-site ordered quadruple perovskites AA'$_3$B$_4$O$_{12}$ is comprehensively investigated by using first-principles calculations. The ideal crystal structure ($Im{\bar 3}$) is characterized by the square-planer oxygen coordination around A' cations as well as the $a^+a^+a^+$ type tilting of BO$_6$ octahedra. Owing to these structural features, the compounds can include transition-metal ions both at the A' and B sites. Consequently, there are two magnetic sublattices with different oxygen coordination. Unlike the B-site magnetism that has been investigated for decades, detailed mechanisms for the A'-site magnetism and A'-B intersublattice magnetism are still unclear. In insulating compounds such as CaCu$_3$Ge$_4$O$_{12}$ and YMn$_3$Al$_4$O$_{12}$, it is found that the nearest-neighbor superexchange interaction between the A' sites determines the ground-state magnetic ordering. Furthermore, in our simulation, it is shown that magnetic phase transition from antiferromagnetism to ferromagnetism will occur along with insulator-to-metal transition in YMn$_3$Al$_4$O$_{12}$ by modulating the tilting of AlO$_6$ octahedra. Possible strategies are suggested to realize such a modulation, for example, by imposing chemical or physical pressure.