Terahertz field induced metastable magnetization in a van der Waals antiferromagnet*

Controlling the functional properties of quantum materials with light has emerged as a frontier of condensed matter physics, leading to discoveries of various light-induced phases of matter, such as superconductivity, ferroelectricity, magnetism, and charge density waves. However, in most cases, the photoinduced phases return to equilibrium on ultrafast timescales after the light is turned off, limiting their practical applications. In this study, we use intense terahertz pulses to induce a metastable magnetization with a remarkably long lifetime of over 2.5 milliseconds in a van der Waals antiferromagnet, FePS₃. The metastable state becomes increasingly robust as the temperature approaches the transition point, suggesting a significant role played by critical fluctuations in facilitating extended lifetimes. By combining first principles calculations with classical Monte Carlo and spin dynamics simulations, we find that the displacement of a specific phonon mode modulates the exchange couplings in a manner that favors a ground state with finite magnetization close to the Néel temperature. This analysis also clarifies how critical fluctuations amplify the magnitude and the lifetime of the new magnetic state. Our discovery demonstrates the efficient manipulation of the magnetic ground state in layered magnets through non-thermal pathways using terahertz light, and establishes the regions near critical points with enhanced fluctuations as promising areas to search for metastable hidden quantum states.