Magnetic symmetry breaking driven by photoinduced piezomagnetism*

Symmetries govern the macroscopic behavior of solids and dictate how their properties can be controlled by external fields. Resonantly driving optical phonons offers the possibility to coherently manipulate symmetry and induce new functional properties away from equilibrium. Here, we demonstrate magnetic symmetry breaking by light leading to an induced ferrimagnetic phase in the classical antiferromagnet CoF₂. We utilize high-intensity terahertz pulses to simultaneously excite degenerate in-plane phonon modes and observe the resulting magnetization dynamics by optical Faraday rotation and circular dichroism. We find that the excitation generates a net c-axis magnetization on the picosecond time scale, evidencing an ultrafast magnetic phase transition. First-principles calculations show that the effect is driven by a novel photoinduced piezomagnetic effect, in which anharmonic phonon dynamics uncompensate the equilibrium antiferromagnetic order via a site-selective modulation of the crystal field. This rare example of photoinduced symmetry breaking provides a new mechanism for magnetic control, which could also be used for manipulating spins at heterointerfaces and engineering magnetoelectric phenomena by light.