Yuhan Liang, Xingyu Yan, Xiaoyu Jiang, Dingsong Jiang, Hao Bai, Zi-An Wang, Hetian Chen, Xiaofu Qiu, Daniel A. Pharis, Xiaoxi Huang, Rakshit Jain, Dingfu Shao, Wanjun Jiang, Di Yi, Daniel C. Ralph, Tianxiang Nan
Spin current provides an energy-efficient approach for manipulating magnetization, when its spin polarization aligns with the magnetization direction. However, conventional spin-source materials possess high crystalline symmetry, restricting spin polarization to be orthogonal to both spin and charge current directions. Here, we overcome this limitation by utilizing the concept of magnon-mediated spin-orbit torque through integration of the insulating multiferroic BiFeO3 with a conventional spin-source material. We observe that spin polarization generated by conventional spin-source material can excite unconventional magnon polarization due to the interplay between cycloidal antiferromagnetic order and the ferroelectric domain structure in BiFeO3. This produces an unconventional magnon torque that allows deterministic, field‑free switching of in‑plane magnetization collinear with the current direction, unattainable with conventional spin-source materials. Our results establish multiferroic-based heterostructure as a symmetry‑engineered magnon spin source, paving the way for low-power spintronic devices. Conventional spin-orbit torques are limited by symmetry. Here, the ferroelectric domain structure of multiferroic bismuth ferrite is engineered to manipulate the orientation of magnon spin current, enabling field-free type-x magnetic switching.