Crystalline-Dependent Magnon Torques in All-Sputtered Hf/Cr₂O₃/Ferromagnet Heterostructures

Electron motion in spin-orbit torque devices inevitably leads to the Joule heating issue. Magnon torques can potentially circumvent this issue, as it enables the transport of spin angular momentum in insulating magnetic materials. In this work, a sandwich structure composed of Hf/antiferromagnetic Cr₂O₃/ferromagnet is fabricated and demonstrates that the magnon torque is strongly dependent on the crystalline structure of Cr₂O₃. Magnon torques are stronger when the Néel vector of Cr₂O₃ aligns parallel to the spin polarization generated in Hf, while they are suppressed when the Néel vector is perpendicular to the spin polarization. The magnon torque efficiency is estimated to be -0.134 using in-plane second harmonic Hall measurements. Using magnon torques, perpendicular magnetization switching of CoFeB is achieved, with a critical switching current density of 4.09 × 10⁷ A cm^-2. Furthermore, the spin angular momentum loss due to the insertion of Cr₂O₃ is found to be lower than that of polycrystalline NiO. The work highlights the role of antiferromagnet crystalline structures in controlling magnon torques, broadening the potential applications of magnon torques.