Domain wall displacement in Co/Pt thin films induced by not only fs-but also ps-laser pulses is demonstrated using time-resolved magneto-optical Faraday imaging. We evidence multi-pulse helicity-dependent laser-induced domain wall motion in all-optical switchable Co/Pt multilayers with a laser energy below the switching threshold. Domain wall displacement of ∼ 2 nm per 2ps pulse is achieved. By investigating separately the effect of linear and circular polarization, we reveal that laser-induced domain wall motion results from a complex interplay between pinning, temperature gradient and helicity effect. Then, we explore the microscopic origin of the helicity effect acting on the domain wall. These experimental results enhance the understanding of the mechanism of all-optical switching in ultra-thin ferromagnetic films.

We present an experimental study of all-optical helicity-dependent switching (AO-HDS) of ferromagnetic Pt/Co/Pt heterostructures with perpendicular magnetic anisotropy. The sample is patterned into a Hall cross and the AO-HDS is measured via the anomalous Hall effect. This all-electrical probing of the magnetization during AO-HDS enables a statistical quantification of the switching ratio for different laser parameters, such as the threshold power to achieve AO-HDS and the exposure time needed to reach complete switching at a given laser power. We find that the AO-HDS is a cumulative process, a certain number of optical pulses is needed to obtain a full and reproducible helicity-dependent switching. The deterministic switching of the ferromagnetic Pt/Co/Pt Hall cross provides a full “opto-spintronic device,” where the remanent magnetization can be all-optically and reproducibly written and erased without the need of an external magnetic field.

Deterministic control of magnetization by light, often referred to as all-optical switching (AOS), is an attractive recording method for magnetic nanotechnologies because magnetization control becomes possible without the need of an external magnetic field 7-11 and therefore incorporates the potential for ultra-fast magnetization switching up to 1000 times faster than that by magnetic fields while using lower energies 12 . The first demonstration of the magnetization switching by light was in ferrimangnetic GdFeCo film which is a magneto-optical material 7 where the Gd and FeCo spin sub-lattices are antiferromagnetically exchange coupled. While several mechanisms for the ultrafast magnetization switching of GdFeCo have been explored [14][15][16] , the current understanding for AOS in GdFeCo is that the ultrafast laser excitation demagnetizes the two sublattices at different Hall cross region to measure the evolution of the magnetization to a series of ultrafast laser pulses.We initially used 40 optical pulses for the first two exposure steps, and then, increased to 80 pulses for the next six exposure steps (see methods and given by (N -N)/(N + N). We assume that with each optical pulse there is a switching probability from the spin-up to spin-down state given by P1 and from spin-down to spin-up by P2. The number of FePt grains with spin-up and spin-down states after the n pulses can be expressed by the following:The fitted lines to Eq. What is the origin for different switching probability of P1 and P2 for circularly polarized light pulses? The fact that linear light leads to demagnetization of the sample suggest that heating of the FePt grains by the femto-second laser exposure is sufficient to cause thermal activated reversal.The circularly polarized light then breaks the symmetry of the system favoring one magnetic state 6 over the other and leading to an imbalance in the P1 and P2. This symmetry breaking could result from a direct interaction between the light and the magnetic systems such as the inverse Faraday field that prefers one direction over the other 22 . The difference in P1 and P2 could also arise from differential absorption for RCP and LCP (i.e. magnetic circular dichroism) that will result in a slight difference in temperature for one set of grains compared to the other. 17 We have roughly estimated the difference in temperature needed during the optical excitation to explain the difference in switching probability using a simple Arrhenius-Néel model for single domain particles (see supplementary section for details). A temperature difference of 1-2 K would be sufficient to explain the difference in P1 and P2 observed in Fig. 1, which is consistent with typical dichroism differences in magnetic metals. 17However, independent of the mechanism we find that magnetic switching for granular FePt films is statistical in nature in contrast to the reports on GdFeCo films. Because of this we don't achieve full deterministic switching, which would be needed for magnetic recording applications. To ac...

Time-resolved magneto-optical imaging reveals that the dynamics of the helicity-dependent alloptical switching (HD-AOS) of Co/Pt ferromagnetic multilayers occurs on the timescales from nanoseconds to seconds. We find HD-AOS proceeds by two stages. First, for an optimized laser fluence, the ultrashort laser pulse demagnetizes the film to 25% of the initial magnetization. Subsequent laser pulses aids nucleation of small reversed domains. The observed nucleation is stochastic and independent of the helicity of laser light. At the second stage circularly polarized light breaks the degeneracy between the magnetic domains promoting a preferred direction of domain wall motion. One circular polarization results in a collapse of the reversed magnetic domains. The other polarization causes the growth of reversed magnetic domain from the nucleation sites, via deterministic displacement of the domain wall resulting in magnetization reversal. This mechanism is supported by further imaging studies of deterministic laser-induced displacement of the domain walls when excited by circular polarized optical pulses.

Employing magnetization-sensitive microscopy techniques, we address the light-induced magnetization dynamics of patterned samples of rare-earth transition metal alloys. We experimentally find that smaller structures require a lower energy density to undergo all-optical switching of the magnetization. With the aid of simulations we explain this reduction in terms of enhanced light absorption by interference of the light within the structure. This results in a decrease of about 60% in the energy densities required for all-optical switching compared with those in continuous thin films of the same alloy. Moreover, we envisage that an energy lower than 10 fJ should be sufficient to switch a 20 × 20 nm 2 structure.