Coherent Terahertz Control Enables Femtosecond Manipulation of Metastable Magnetization in FePS₃

Controlling magnetic materials with light offers exciting possibilities for next-generation computing, and recent research focuses on manipulating materials at the atomic level to achieve this goal. Batyr Ilyas, Tianchuang Luo, and Honglie Ning, alongside Emil Vinas Bostrom, Alexander von Hoegen, and Jaena Park, report a significant advance in this field, demonstrating coherent control of a newly discovered metastable magnetization within the van der Waals antiferromagnet FePS3. The team utilises terahertz pulses to modulate the material’s magnetic state, revealing that specific vibrational modes, or phonons, drive and amplify this effect. This achievement not only clarifies the underlying mechanism responsible for the metastable magnetic state in FePS3, but also establishes vibrational coherences as a powerful method for manipulating material properties and controlling phases far from equilibrium, paving the way for ultrafast optoelectronic computation.

Coherent Antiferromagnetic Spin Control with THz Pulses

This research demonstrates the ability to coherently control antiferromagnetic spins in FePS3 using precisely timed terahertz pulses. This control arises from coupling the spins to the crystal lattice vibrations, known as phonons, and specifically through resonant enhancement at certain phonon frequencies. This achievement opens possibilities for manipulating magnetic materials with light, potentially leading to new technologies in spintronics and information storage. Scientists synthesized FePS3 single crystals and used a double-THz pump-probe technique to investigate the material’s response to light.

They generated two terahertz pulses with controlled timing and used a weaker beam to measure changes in the polarization of the transmitted terahertz radiation. First-principles calculations, employing density functional theory, were used to verify experimental findings and understand the underlying mechanisms. The experiments revealed resonant enhancement of the terahertz-induced polarization change at specific phonon frequencies, including 3. 27 terahertz, 4. 34 terahertz, and 4. 80 terahertz. These calculations confirmed the zig-zag antiferromagnetic order in the ground state of FePS3, suggesting that the terahertz pulses coherently drive the phonon modes, which then couple to and control the antiferromagnetic spins.

Terahertz Control of Magnetization via Phonons

This work demonstrates coherent phononic control of a newly discovered metastable magnetization in the van der Waals antiferromagnet FePS3, opening new avenues for ultrafast computation. Scientists achieved coherent modulation of the magnetization amplitude by precisely controlling lattice vibrations with terahertz pulses. Experiments revealed that a sequence of terahertz pulses modulates the magnetization amplitude at frequencies corresponding to coherent phonon vibrations, a phenomenon confirmed through polarization- and field-strength-dependent measurements. The team employed two-dimensional terahertz spectroscopy, alongside first-principles simulations, to show that these phonons nonlinearly displace a Raman-active phonon, which then induces the metastable net magnetization.

Specifically, the researchers discovered that the material’s response exhibits coherent oscillations when the time separation between two terahertz pulses is varied, with amplitudes increasing as the material approaches its magnetic ordering temperature of 118 Kelvin. A Fourier transform of these oscillations revealed a broad peak near 4. 5 terahertz, distinct from the previously known Raman-active phonon frequency of 3. 27 terahertz. Further analysis demonstrated that the oscillation amplitude scales linearly with the electric field strength of a single terahertz pulse, indicating the involvement of an infrared-active phonon mediating the displacement of the Raman mode.

Polarimetry measurements revealed two infrared-active modes with perpendicular dipole moments, carrying electric dipole moments along the crystallographic a and b axes, and following Bu and Aus symmetries. These findings are supported by two-dimensional terahertz spectroscopy, which directly probes the nonlinear interactions between low-energy excitations and rectification processes, confirming the involvement of the 4. 5 terahertz phonons in displacing the lattice and controlling the magnetization.

Terahertz Control of Magnetization and Phonons

This research demonstrates coherent control of a newly discovered metastable magnetized phase in the van der Waals antiferromagnet FePS3 using terahertz pulses. By carefully timed sequences of these pulses, scientists modulate the magnetization amplitude at specific frequencies corresponding to vibrational modes within the material, revealing a direct link between phonon coherences and magnetic properties. Two-dimensional spectroscopy, combined with theoretical calculations, clarifies that these phonons displace a Raman-active phonon, ultimately inducing the metastable magnetization. These findings extend the principles of coherent control, previously established in molecular systems, to crystalline materials, opening new avenues for manipulating material properties far from equilibrium.

Experiments confirm that terahertz radiation selectively interacts with these vibrational and magnetic degrees of freedom, offering a unique advantage over other approaches. The ability to switch magnetization with picosecond-scale response time represents a significant advancement, potentially enabling ultrafast quantum gates that operate much faster than current technologies. The authors acknowledge that the duration of phonon coherence currently limits the potential for long-term data storage. Future research will focus on identifying materials with extended coherence times, with the goal of developing functional quantum memory systems that combine laser-based switching with magnetic readout. This work establishes a fundamental framework for manipulating quantum materials with terahertz radiation, promising advancements in information processing through precise control of emergent phases.