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...

Laser-induced ultrafast demagnetization in TbFeCo has been studied with a dual-pumping system. Five different laser fluence combinations were applied at three different time intervals between two pump pulses. The experimental results are also compared with computational simulations using the atomistic model. Importantly, this demagnetization can be controllably manipulated in both its magnitude and temporal response.

Using angle-dependent X-ray magnetic circular dichroism we have measured magnetic hysteresis loops at the CoL 2,3 edges of oxide-doped Co 80 Pt 20 thin films. The magnetocrystalline anisotropy energy (MAE) of the Co atoms, which is the main source of the magnetocrystalline anisotropy of the CoPt magnetic grains, has beendetermined directly from these element-specific hysteresis loops. When the oxide volume fraction (OVF) is increased from 16.6% to 20.7%, the Co MAE has been found to decrease from 0.117 meV/atom to 0.076 meV/atom.While a larger OVF helps to achieve a smaller grain size, it reduces the magnetocrystalline anisotropyas demonstrated unambiguously from the direct Co MAE measurements.Our results suggest that thoseCo 80 Pt 20 :oxide films with OVF between 19.1% and20.7%aresuitable candidates for high-density magnetic recording.

Using a time-resolved pump-probe system, we experimentally demonstrated that all-optical magnetization switching in TbFeCo can be observed at both room temperature and a low temperature of 110 K. This switching can only be triggered by a circularly polarized pump beam within a very narrow range of pulse energies. We believe these findings will lead to a better understanding of optically induced magnetization reversal on rare-earth transition metal alloys and provide an ultrafast and efficient way for future magnetic recording technique.

Laser-induced ultrafast demagnetization in TbFeCo has been studied with a dual-pumping system. Five different laser fluence combinations were applied at three different time intervals between two pump pulses. The experimental results are also compared with computational simulations using the atomistic model. Importantly, this demagnetization can be controllably manipulated in both its magnitude and temporal response.