Terahertz-induced metastable magnetization and dynamical criticality in FePS₃

A major theme in contemporary condensed matter research has been to leverage light-matter interactions to control and stabilize new phases of matter. Dramatic effects can be achieved by driving selected phonon modes far out of equilibrium, in a scheme known as non-linear phononics. For systems where the physics is mainly driven by phonon fluctuations, such as ferroelectric and superconducting materials, this can lead to drastic changes in the transition temperature. Similarly, since magnetic interactions depend on a delicate interplay of local correlations and virtual hopping processes, which in turn depend strongly on the bond angles and bond lengths of the crystal, magnetic systems are sensitive to the ionic positions and dynamics. These effects are expected to be amplified near critical points, where divergent fluctuations enhance the magnetic system's susceptibility to phonon displacements.

Here we show that driving the van der Waals antiferromagnet FePS₃ with intense terahertz pulses induces a metastable magnetization with a remarkably long lifetime of over 2.5 milliseconds. The magnetic state becomes increasingly robust as the temperature approaches the transition point, suggesting that dynamical critical fluctuations play a significant role in facilitating the extended lifetime. By combining first principles calculations with an effective Ginzburg-Landau theory applicable in the long wavelength limit, we identify a specific phonon mode whose displacement modulates the exchange couplings in a manner that favors a finite magnetization state close to the Néel temperature. Specifically, we find that the phonon mode mediates a coupling between the induced magnetization and the fluctuations of the dominant antiferromagnetic order, leading to a dynamical critical response of the magnitude and lifetime of the magnetization close to the Néel temperature. Our discovery demonstrates an efficient non-thermal pathway to manipulate the magnetic ground state in layered materials using terahertz light, and establishes the regions near critical points as promising areas to search for metastable hidden quantum states.