Roles of heating and helicity in ultrafast all-optical magnetization switching in TbFeCo

Abstract

Using the time-resolved magneto-optical Kerr effect (TR-MOKE) method, helicity-dependent all-optical magnetization switching (HD-AOS) is observed in ferrimagnetic TbFeCo films. Our results reveal the individual roles of the thermal and nonthermal effects after single circularly polarized laser pulse. The evolution of this ultrafast switching occurs over different time scales and a defined magnetization reversal time of 460 fs is shown -the fastest ever observed. Micromagnetic simulations based on a single macro-spin model, taking into account both heating and the inverse Faraday effect, are performed that reproduce HD-AOS demonstrating a linear path for magnetization reversal.Since the demonstration of magnetization reversal by a single femtosecond laser pulse in 2007 1 , the field of all-optical switching (AOS) has been extensively studied both theoretically and experimentally. AOS in the ferrimagnetic alloy, GdFeCo (the initially investigated material for AOS), has been shown reverse through a purely thermal effect 2-5 where the dynamics proceed via a transient ferromagnetic-like state 6,7 . Very recently, ultrafast electronic heat currents have been shown experimentally to be sufficient to switch the magnetization in this same material 8,9 , which provides further evidence of the thermal origins of AOS in GdFeCo 10 . Consequently, AOS in GdFeCo is almost independent of the laser helicity of the laser pulse, which is named helicity-independent AOS (HI-AOS).On the other hand, there are many examples of AOS observed in other materials, that are strongly helicity dependent, e.g. ferromagnetic Co/Pt multilayers 11 , FePt nanoparticles 12 , synthetic ferrimagnetic heterostructures 13 and Tb-based ferrimagnets [14][15][16] . For these materials, there is a one-to-one correspondence of the helicity of the laser light controls and the magnetization orientation, deemed helicity dependent AOS (HD-AOS). A dependence on helicity was observed in GdFeCo for single pulses applied to the alloy for a narrow range of fluence 17 , which was quantitatively explained as arising from magnetic circular dichroism (MCD) 18 . Besides a) the purely thermal effect and MCD 19 , other mechanisms have been proposed to explain the observed AOS, e.g. inverse Faraday effect (IFE) 1,20-22 , stimulated Raman scattering 23,24 , sublattice exchange relaxation 25 , ultrafast exchange scattering 26 , and optical spin pumping 27 . However, the underlying physics of HD-AOS in a larger variety of materials is still unclear, especially of the roles of the helicity and thermal effects of the laser pulse. Several experimental criteria and models have been proposed to interpret HD-AOS. A so-called low-remanence criterion was reported whereby HD-AOS is only obtained below a magnetization remanence threshold of 220 emu/cm 3 for several materials 15 . Recently, a domain size criterion for the observation of HD-AOS has been proposed, whereby the laser spot size should be smaller than the equilibrium size of magnetic domains forming during the cooling process a...

Role of element-specific damping in ultrafast, helicity-independent, all-optical switching dynamics in amorphous (Gd,Tb)Co thin films

“…The simulations were only able to reproduce the experimental data when the element-specific Gilbert damping of the Gd, Tb, and Co sites were assigned separately in the LLG equation, rather than when using a net damping of the system as is typically done when simulating such RE-TM systems [28,35,36] (see Ref. [25] for comparison).…”

Strain‐Controlled Spin Wave Excitation and Gilbert Damping in Flexible Co2FeSi Films Activated by Femtosecond Laser Pulse

“…Related to the above is the idea of ultrafast optical manipulation of magnetic order inspired by the discovery of ultrafast demagnetization [ 11 ] and all‐optical helicity dependent switching (AO‐HDS), [ 12–15 ] which has led to further investigation of ultrafast magnetization dynamics, [ 16 ] exchange interactions, [ 17,18 ] subpicosecond demagnetization, [ 11 ] etc. In those studies, femtosecond pump‐probe technique has played an essential role with several advantageous features.…”

“…New paradigms enabling high frequency and low consumption data transmission are now in great demand in the wireless communication industry. [1] Compared with existing radio-frequency/microwave demagnetization [11] and all-optical helicity dependent switching (AO-HDS), [12][13][14][15] which has led to further investigation of ultrafast magnetization dynamics, [16] exchange interactions, [17,18] subpicosecond demagnetization, [11] etc. In those studies, femtosecond pump-probe technique has played an essential role with several advantageous features.…”

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

“…However, they could not observe the coupling dynamics between the 3d and 4f spins because 400 nm pumping light was used to simultaneously excite 3d and 4f electrons, led to the 3d-4f coupling dynamics submerged by directly excited 4f spin dynamics. Moreover, pump-probe magneto-optical Kerr (MOKE) experiments with two wavelengths were also used on RE-TM alloys in different cases [28][29][30], but the 3d-4f coupling dynamics still has not been demonstrated directly.…”