Role of the inter-sublattice exchange coupling in short-laser-pulse-induced demagnetization dynamics of GdCo and GdCoFe alloys

Abstract

The dynamics of the demagnetization induced by an ultrashort laser pulse in GdCo and GdCoFe alloys is shown to be substantially different from the expected trend. We find that the intersublattice exchange interaction between Gd and Co(Fe) is used as an additional route of heat transfer during the demagnetization process and leads to a temporary cooling of the Co(Fe) spin subsystem. The observed results are described by a four-temperature model (4T model) in which electrons, lattice, and 4f and 3d spins represent four reservoirs of heat energy. In this model, the coupling between the electron and spin reservoirs is responsible for an ultrafast demagnetization, while the intersublattice exchange coupling gives rise to an immediate remagnetization and serves as a thermal link between the Co(Fe) and Gd spins.

Ultrafast dynamics of 4f electron spins in TbFeCo film driven by inter-atomic 3d–5d–4f exchange coupling

“…Such two-step demagnetization dynamics, which includes a strong ultrafast component and a weak slower one within tens of picoseconds, is different from the typical one-step demagnetization dynamics of 3d spins in pure TMs like Fe and Co [1,20]. Similar two-step demagnetization dynamics of FeCo(Co) component were also reported in GdFeCo(GdCo) alloy films [26], where the second-step slower demagnetization process occurring within hundreds of picoseconds accounted for a large proportion in total demagnetization amplitude, and was attributed to lattice-spin(3d) coupling. However, here the amplitude of the first-step ultrafast demagnetization process seems absolutely dominant, and the timescale of the second-step demagnetization process is much shorter than the corresponding one in GdFeCo or GdCo.…”

“…Recently, indirect evidence about the coupling between RE-4f and TM-3d demagnetization dynamics was reported. Mekonnen et al observed an anomalous two-step demagnetization dynamics of the TM component in GdCo and GdCoFe alloy films, and performed a calculation based on a four-temperature (4T) model, indirectly revealing the role of inter-atomic 3d-5d-4f exchange coupling on the demagnetization dynamics [26]. For further verification and clearer insight of the 3d-5d-4f coupling dynamics, direct observations of the 4f spin dynamics are very necessary.…”

Interface-induced phenomena in magnetism

“…This switching was explained within a general theoretical framework [Mentink et al ., 2012], and reproduced by both atomistic LLG [Ostler et al ., 2012] and the microscopic three-temperature model with two coupled spin baths [Schellekens and Koopmans, 2013b]. Further measurements on a -Gd-Fe-Co alloys support this interpretation [Mekonnen et al ., 2013; Hashimoto et al ., 2014; Medapalli et al ., 2014; Le Guyader et al ., 2015]. …”

Watching ultrafast responses of structure and magnetism in condensed matter with momentum-resolved probes

“…In this context, it is also relevant to take note of the recently studied phenomenon of femtosecond-laser driven switching of magnetism in ferrimagnetic FeGdCo alloys that appears to rely on an ultrafast angular momentum transfer between nanoscale inhomogeneities 23–25 . Here, a key element of the physics behind this is the interaction of localized f moments and itinerant d electrons which show a different behavior in the transient state 26 . Recent x-ray diffraction measurements on elemental Ho have further explored this interaction in a system with magnetic order in both the 5d and 4f states but no net magnetic moment 27 .…”