image: (a, c, e) Phase dia- grams of temperature-magnetic field effects on τs , d λ_ /d H and T d M /d T , respectively. (b, d, f) Temperature dependence of _τs , _d λ_ /d H and _( T d M /d T ), respectively. The magnetic-field-induced change _R ( R = τs , d λ_ /d H and T d M /d T ) is calculated as _R = R |7T −R |1T . Twill shadow is the area of demagnetization anomalies.
Credit: ©Science China Press
The discovery of femtosecond laser-induced demagnetization in ferromagnetic nickel in 1996 marked the birth of ultrafast spin dynamics research. This phenomenon not only provides new insights into the energy transfer mechanisms among electron spins, lattice, and charge carriers but also lays the foundation for developing next-generation high-speed magnetic storage and logic devices. By precisely manipulating magnetic states with femtosecond lasers, data storage speeds could potentially reach the femtosecond regime, revolutionizing information processing paradigms. Consequently, understanding ultrafast demagnetization mechanisms remains a central challenge in spintronics.
Ultrafast demagnetization involves complex coupling between electron spins, lattice vibrations, and charge carriers, arising from competing energy transfer pathways. Studying the influence of the external field, such as magnetic field, electric field, and temperature, is crucial for unraveling this many-body process. Among these, magnetic field offer the most direct means of spin control by acting on spin angular momentum. However, most ultrafast demagnetization studies have been conducted in a weak magnetic field (<1 T), leaving the role of magnetic field modulation largely unexplored.
Recently, a collaborative team led by Prof. Zhigao Sheng from the High Magnetic Field Laboratory of the Chinese Academy of Sciences and Prof. A.V. Kimel from Radboud University Nijmegen systematically investigated magnetic field effects on laser-induced ultrafast demagnetization. Using time-resolved magneto-optical Kerr spectroscopy under high magnetic field, they demonstrated for the first time in the 2D van der Waals ferromagnet Fe3GeTe2 that an external magnetic field can simultaneously accelerate demagnetization speed and suppress its efficiency. Near the Curie temperature (TC = 210 K), applying a 7 T magnetic field reduced the demagnetization time from 22.2 ps to 9.9 ps, while decreasing the efficiency from 79% to 52%. By combining temperature-dependent studies, the team proposed a thermodynamic explanation within the three-temperature model framework, attributing the acceleration effect to magnetic field-modulated spin entropy. This work establishes a universal approach to controlling ultrafast spin dynamics, which not only advances fundamental understanding of spin-lattice-charge interactions but also paves the way for designing multifunctional spintronic devices with field-programmable ultrafast operations.
This work is published in National Science Review with the title "Acceleration of ultrafast demagnetization in van der Waals ferromagnet Fe3GeTe2 in high magnetic field".