Giant spin-induced electric polarization in absence of orbital order in (B i0.5 A g0.5) M n7 O12

Giant spin-induced ferroelectric polarization has been observed in the multiferroic CaMn7O12, the origin of which remains controversial. Various theoretical models have been proposed in this regard, some of which put the preformed orbital order as a prerequisite factor, while others attribute it to the combination of Dzyaloshinskii-Moriya interactions and exchange striction. In this paper, we resolve this issue by replacing Ca with disordered Bi3+/Ag+ ions through high-pressure and high-temperature methods, leading to the successful synthesis of single-phase isostructural (Bi0.5Ag0.5)Mn7O12. No orbital order is observed down to 30 K, while a giant electric polarization of ∼1900μCm-2 is realized below TN≈80 K when the system enters an antiferromagnetically ordered state. The low-temperature spin structure adopts the magnetic point group of 31′, the same as CaMn7O12. Our results strongly indicate that the giant ferroelectric polarization is primarily caused by exchange striction instead of orbital order.