Controlling the functional properties of quantum materials with light has emerged as a frontier of condensed-matter physics, leading to the discovery of various light-induced phases of matter, such as superconductivity¹, ferroelectricity^2,3, magnetism^4,5,6 and charge density waves⁷. However, in most cases, the photoinduced phases return to equilibrium on ultrafast timescales after the light is turned off, limiting their practical applications. Here we use intense terahertz pulses to induce a metastable magnetization with a remarkably long lifetime of more than 2.5 milliseconds in the van der Waals antiferromagnet FePS₃. The metastable state becomes increasingly robust as the temperature approaches the antiferromagnetic transition point, suggesting that critical order parameter fluctuations play an important part in facilitating the extended lifetime. 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 favours a ground state with finite magnetization near the Néel temperature. This analysis also clarifies how the critical fluctuations of the dominant antiferromagnetic order can amplify both 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 regions near critical points with enhanced order parameter fluctuations as promising areas to search for metastable hidden quantum states.

用光控制量子材料的功能特性已成为凝聚态物理学的前沿领域，导致了各种光诱导物质相的发现，例如超导^{性 1}、铁电性^2,3、磁力^4,5,6 和电荷密度波⁷。然而，在大多数情况下，光诱导相在关灯后在超快的时间尺度上恢复平衡，这限制了它们的实际应用。在这里，我们使用强太赫兹脉冲在范德华反铁磁体 FePS₃ 中感应出亚稳态磁化，其寿命非常长，超过 2.5 毫秒。随着温度接近反铁磁过渡点，亚稳态变得越来越稳健，这表明临界有序参数波动在促进延长使用寿命方面起着重要作用。通过将第一性原理计算与经典的蒙特卡洛和自旋动力学模拟相结合，我们发现特定声子模式的位移以有利于接近 Néel 温度的有限磁化基态的方式调制交换耦合。该分析还阐明了主要反铁磁级的临界涨落如何放大新磁态的大小和寿命。我们的发现证明了通过使用太赫兹光的非热路径有效地操纵层状磁体中的磁性基态，并在临界点附近建立了具有增强有序参数波动的区域，作为寻找亚稳态隐藏量子态的有希望的区域。