In 1996, the discovery of femtosecond laser-induced demagnetization in ferromagnetic nickel marked a paradigm shift in the field of ultrafast spin dynamics, inaugurating decades of research dedicated to understanding how spins behave on sub-picosecond timescales. This fundamental advance revealed the possibility of dramatically accelerating magnetic transitions, challenging the traditional limits imposed by spin relaxation mechanisms. By using ultrashort laser pulses to perturb electron spins, researchers uncovered a wealth of information on how the spin, lattice, and charge subsystems exchange energy and momentum. The implications stretch far beyond academic curiosity, with potential applications in developing magnetic storage and logic devices whose operation speeds could ultimately approach the femtosecond regime, heralding a new era in information technology.
Understanding ultrafast demagnetization hinges on unraveling the complex interplay among electron spins, lattice vibrations, and charge carriers. The coupling between these subsystems involves competing energy transfer pathways, each with unique characteristic timescales and efficiencies. External stimuli such as temperature, electric field, and magnetic field alter these interactions by changing energy landscapes and spin scattering probabilities. Among these parameters, magnetic fields serve as the most direct and controllable means of manipulating spin angular momentum, thereby exerting significant influence on the ultrafast magnetization dynamics. However, while many studies have examined ultrafast demagnetization under weak magnetic fields, typically below 1 Tesla, the effects of strong magnetic fields on spin relaxation processes remain largely unexplored due to experimental challenges.
Recently, a groundbreaking collaboration led by Prof. Zhigao Sheng at the High Magnetic Field Laboratory of the Chinese Academy of Sciences, together with Prof. A.V. Kimel from Radboud University Nijmegen, has made significant strides in this area by investigating the influence of high magnetic fields on laser-induced ultrafast demagnetization. Utilizing time-resolved magneto-optical Kerr effect (TR-MOKE) spectroscopy under fields as high as 7 Tesla, the team explored the 2D van der Waals ferromagnet Fe₃GeTe₂ (FGT), a layered material exhibiting strong spin-orbit coupling and robust magnetism. Their experimental results revealed a surprising dual effect: high magnetic fields simultaneously accelerated the demagnetization process while reducing its overall efficiency, challenging existing assumptions about spin relaxation behavior.
In the vicinity of Fe₃GeTe₂’s Curie temperature (~210 K), application of a 7 T magnetic field shortened the characteristic demagnetization time (τ_s) from approximately 22.2 picoseconds at 1 T to just 9.9 ps. Meanwhile, the demagnetization efficiency, quantified as the relative reduction in magnetization per laser pulse, decreased markedly from 79% down to 52%. This counterintuitive finding implies that while the system’s spins respond more quickly under strong fields, the total magnetic moment quenching is less pronounced. Such behavior highlights the nuanced role external magnetic fields play in modulating ultrafast spin dynamics, pointing toward subtle thermodynamic mechanisms at work.
To unravel the underlying physics, the researchers analyzed their data within the framework of the three-temperature model, which treats electrons, spins, and the lattice as interacting thermal reservoirs. They proposed that magnetic field-induced modifications of spin entropy are crucial to explaining their observations. Specifically, strong magnetic fields suppress spin fluctuations by aligning magnetic moments, thereby reducing spin entropy and facilitating energy transfer. This leads to a more rapid demagnetization process because the spin system attains thermal equilibrium with the electron and lattice baths faster. However, the same alignment also constrains the maximum possible reduction in magnetization, accounting for the diminished demagnetization efficiency.
This pioneering study demonstrates a universal approach for controlling ultrafast spin dynamics using high magnetic fields, offering unprecedented access to regimes where spin-lattice-charge interactions can be precisely manipulated. By tuning magnetic field strength and temperature, it becomes possible to tailor the temporal profile and magnitude of demagnetization, providing a new dimension for engineering spintronic devices. Such devices could exploit field-programmable ultrafast operations that exceed current speed and energy efficiency limits, potentially transforming computing architectures reliant on magnetic information storage and logic.
The choice of Fe₃GeTe₂ as the experimental platform is notable given its distinct two-dimensional layered structure and van der Waals bonding, which afford enhanced spin-orbit interaction and magnetic anisotropy compared to conventional bulk ferromagnets. These properties make FGT an ideal candidate for studying fundamental magneto-optical phenomena and their response to extreme conditions, such as strong magnetic fields and rapid laser excitation. The insights gained are thus not only relevant to this specific material but also have broader applicability across the emerging family of 2D magnetic materials.
Moreover, the study employed time-resolved magneto-optical Kerr spectroscopy, a sensitive technique that detects changes in spin polarization by measuring rotation of the polarization plane of reflected light. This method enables direct observation of femtosecond to picosecond spin dynamics with high temporal resolution. The incorporation of high magnetic fields into TR-MOKE experiments presents considerable technical challenges, including maintaining optical access and signal sensitivity, highlighting the significance of this work’s experimental achievements.
Understanding how external fields influence ultrafast demagnetization is critical, as it informs the design of future spintronic devices that operate at the intersection of speed, stability, and energetic efficiency. Precise magnetic field control could allow engineers to balance fast switching times against retention of magnetic states, optimizing performance for applications in ultrafast memory or logic components. Additionally, the interplay between temperature and field effects revealed by this research may guide operating conditions for such devices under varying environmental factors.
The work also sheds light on spin entropy as a key parameter in ultrafast magnetism, a concept often overshadowed by focus on energy dissipation alone. By emphasizing changes in spin disorder and alignment, the researchers provide a more complete thermodynamic picture of how magnetic systems move toward equilibrium after intense laser excitation. This enhanced understanding paves the way for theoretical modeling that integrates entropic contributions, potentially unveiling novel mechanisms to harness or mitigate spin relaxation.
Altogether, this study represents a significant advance in the field of ultrafast magnetism and spintronics. By experimentally demonstrating how magnetic fields modulate both the speed and efficiency of laser-induced demagnetization in a layered van der Waals ferromagnet, it opens new avenues for controlling spin dynamics at ultrashort timescales. The combination of high-field spectroscopy and temperature-dependent measurements creates a versatile platform for investigating complex spin-lattice-charge interactions, propelling the quest for faster, energy-efficient spin-based technologies.
This research has been published in the prestigious National Science Review under the title “Acceleration of ultrafast demagnetization in van der Waals ferromagnet Fe₃GeTe₂ in high magnetic field,” providing the scientific community with detailed experimental data and theoretical interpretations. The collaborative effort underscores the importance of interdisciplinary partnerships that combine advanced material synthesis, cutting-edge spectroscopy, and high magnetic field facilities to explore frontier physics phenomena.
Looking ahead, future investigations could extend these concepts to a broader range of two-dimensional magnets and explore the interplay of other external perturbations, such as strain or electric fields, in conjunction with magnetic field control. The ultimate goal is to establish comprehensive methodologies for dynamically tuning spintronic device functionalities on demand, driving innovations that exploit the quantum mechanical nature of electron spins.
Subject of Research: Experimental investigation of magnetic field effects on laser-induced ultrafast demagnetization dynamics in two-dimensional van der Waals ferromagnet Fe₃GeTe₂.
Article Title: Acceleration of ultrafast demagnetization in van der Waals ferromagnet Fe₃GeTe₂ in high magnetic field.
Web References:
- DOI: 10.1093/nsr/nwaf185
Image Credits: ©Science China Press
Keywords
Ultrafast demagnetization, van der Waals ferromagnets, Fe₃GeTe₂, high magnetic field, spin dynamics, magneto-optical Kerr effect, three-temperature model, spin entropy, femtosecond laser, spintronics, spin-lattice-charge coupling, time-resolved spectroscopy
