Terahertz Field-Induced Metastable Magnetization Near Criticality in FePS₃

Academic Background

In recent years, controlling the functional properties of quantum materials with light has emerged as a frontier in condensed matter physics, leading to the discovery of various light-induced phases of matter, such as superconductivity, ferroelectricity, magnetism, and charge density waves. However, in most cases, these photoinduced phases return to equilibrium on ultrafast timescales after the light is turned off, limiting their practical applications. Therefore, finding viable strategies to stabilize non-equilibrium states remains a complex and ongoing task. Terahertz (THz) pulses, due to their low photon energy, can selectively excite collective modes while keeping the orbital and electronic degrees of freedom in the ground state, making them a promising tool in this field.

In this study, the research team used intense THz pulses to induce a metastable magnetization in the van der Waals antiferromagnet FePS₃, with a remarkably long lifetime of more than 2.5 milliseconds. This discovery demonstrates the efficient manipulation of the magnetic ground state in layered magnets through non-thermal pathways using THz light and establishes regions near critical points with enhanced order parameter fluctuations as promising areas to search for metastable hidden quantum states.

Paper Source

This paper was co-authored by Batyr Ilyas, Tianchuang Luo, Alexander von Hoegen, Emil Viñas Boström, Zhuquan Zhang, Jaena Park, Junghyun Kim, Je-Geun Park, Keith A. Nelson, Angel Rubio, and Nuh Gedik, affiliated with the Massachusetts Institute of Technology, Max Planck Institute for the Structure and Dynamics of Matter, University of the Basque Country, Seoul National University, and the Flatiron Institute. The paper was published in Nature from December 19 to 26, 2024.

Research Process and Results

Experimental Design

FePS₃ is a van der Waals antiferromagnetic material with a honeycomb lattice, whose magnetic structure is determined by the spin-orbit coupling and single-ion anisotropy of Fe²⁺ ions. The research team resonantly drove the low-energy magnon and phonon modes in FePS₃ using intense THz pulses, thereby modifying the exchange parameters and driving the system into a state with finite magnetization. In the experiments, the team used different THz sources and monitored the dynamics using a weak near-infrared (800 nm) probe pulse.

THz-Induced Coherent Phonons

The experimental results showed that the transient changes induced by the THz pulse included fast oscillations and a strong positive signal near zero time. Through Fourier transform, the team identified four distinct phonon modes and one magnon mode. The frequencies of these modes were consistent with Raman and infrared spectroscopy results. As the temperature approached the antiferromagnetic transition point, a long-lived signal began to accumulate, indicating significant changes in the system’s dynamics near the critical point.

Metastable Magnetization

The team found that as the temperature approached the antiferromagnetic transition point, the long-lived signal induced by the THz pulse began to accumulate and peaked at 118 K. By analyzing the temperature dependence of the polarization rotation and ellipticity change signals, the team concluded that the ellipticity signal required a non-thermal mechanism. Further experiments showed that the THz-induced circular dichroism signal indicated that the new photoinduced state had a net out-of-plane magnetization.

Phonon-Induced Magnetization

Through a microscopic spin-lattice model and Monte Carlo simulations, the team found that a specific phonon mode (ω₂ = 3.27 THz) modulated the exchange parameters, leading to the generation of finite magnetization. The displacement of this phonon mode altered the Fe-Fe bond lengths, enhancing the exchange interactions within certain chains while weakening them in adjacent chains, resulting in a net magnetization.

Critical Slowing Down

The team observed that the lifetime of the THz-induced magnetization state significantly increased as the temperature approached the antiferromagnetic transition point, reaching 2.5 milliseconds. This phenomenon can be explained by critical fluctuation theory, where the system’s dynamics exhibit critical slowing down near the phase transition point, leading to a significant extension of the magnetization state’s lifetime.

Conclusion and Outlook

This study demonstrates the possibility of inducing metastable magnetization in FePS₃ using THz light and highlights the crucial role of critical fluctuations in stabilizing this non-equilibrium state. This discovery provides new insights into searching for metastable hidden quantum states near critical points and offers potential for future spintronic applications.

Research Highlights

1. Long Lifetime of Metastable Magnetization: The THz pulse-induced magnetization state has a lifetime exceeding 2.5 milliseconds, significantly longer than previously observed photoinduced phases.
2. Role of Critical Fluctuations: The study reveals the critical role of fluctuations in enhancing the magnitude and promoting the metastability of the non-equilibrium magnetization state.
3. Non-Thermal Magnetic Manipulation: The manipulation of the magnetic ground state in layered magnets through THz light provides a new tool for future quantum material research.

Research Significance

This study not only offers new perspectives on understanding non-equilibrium states in quantum materials but also provides a potential material platform for future spintronics and quantum computing applications. By manipulating the magnetic ground state with THz light, researchers can explore more hidden quantum states near critical points, advancing the field of quantum materials.