The phonomagnet: Spontaneous order of phonon angular momentum
Magnetism is a fundamental property of matter, typically arising from collective ordering of electronic spin and orbital angular momentum. Other quasiparticles interact with magnetism. However, they have not been considered its primary source. Here we show that phonons can generate a distinct form of magnetism through spontaneous ordering of their angular momentum. This phonomagnetism emerges from spin-phonon angular-momentum coupling, producing an effective exchange interaction between local ionic angular momenta. We show that the interaction strength depends on spin susceptibility, with a phonomagnetic phase predicted near a ferromagnetic critical point. Our theory also reveals phononic analogues of established spin models, including phonon Heisenberg, phonon $Γ$-model, and phonon Dzyaloshinskii-Moriya interactions. Combining first-principles phonon calculations with recent data from Raman spectroscopy, we identify VI$_3$ as a candidate phonomagnet, providing a microscopic framework for interpreting the weak magnetic anomaly reported above its bulk ferromagnetic transition. These findings open avenues for new platforms for information storage and manipulation on picosecond timescales.