Optical Creation of Skyrmions by Spin Reorientation Transition in Ferrimagnetic CoHo Alloys

Abstract

Manipulating magnetic skyrmions by means of a femtosecond (fs) laser pulse has attracted great interest due to their promising applications in efficient information-storage devices with ultralow energy consumption. However, the mechanism underlying the creation of skyrmions induced by an fs laser is still lacking. As a result, a key challenge is to reveal the pathway for the massive reorientation of magnetization from trivial to nontrivial topological states. Here, we studied a series of ferrimagnetic CoHo alloys and investigated the effect of a single laser pulse on the magnetic states. Thanks to the time-resolved magneto-optical Kerr effect and imaging techniques, we demonstrate that the laser-induced phase transitions from single domains into a topological skyrmion phase are mediated by the transient in-plane magnetization state, in real time and space domains, respectively. Combining experiments and micromagnetic simulations, we propose a two-step process for creating skyrmions through laser pulse irradiation: (i) the electron temperature enhancement induces a spin reorientation transition on a picosecond (ps) timescale due to the suppression of perpendicular magnetic anisotropy (PMA) and (ii) the PMA slowly restores, accompanied by out-of-plane magnetization recovery, leading to the generation of skyrmions with the help of spin fluctuations. This work provides a route to control skyrmion patterns using an fs laser, thereby establishing the foundation for further exploration of topological magnetism at ultrafast timescales.

Ultrafast switching to zero field topological spin textures in ferrimagnetic TbFeCo films

“…Under the assistance of a magnetic field, skyrmions can be obtained via the thermally activated motion of the domain walls on a ns timescale after laser excittion. [48][49][50] The Pt/Co/Ta multilayer with interfacial DMI is prepared to demonstrate the thermally activated nucleation mechanism of skyrmions. For the Pt/Co/ Ta multilayer with a saturated FM state at a perpendicular magnetic field of 600 Oe, the dominant contribution of the thermally activated effects is emphasized.…”

Gd Doping at the Co/Pt Interfaces to Induce Ultrafast All-Optical Switching

“…Such a typical state diagram has been reported recently by Wei et al 6 in GdFeCo and by Zhang et al 10 in Gd 1−x−y Tb y Co x alloys and other alloys. 21 In Figure 5, one can clearly notice that for a small amount of Gd (0.07 nm), perfect single-pulse switching is observed even for pulse duration larger than 1.5 ps. Surprisingly, the area in which perfect single-pulse switching is observed is smaller in the case of larger Gd thickness (0.21 nm).…”

“…Note that Beens et al 31 theoretically studied AO-HIS in Gd/Co bilayer systems and highlighted AO-HIS due to intermixing, but in their case of the intermix region, the Gd concentration is quite high and reach x Gd = 50%. In the light of the paper by Zhang et al 21 which discusses the criteria for deterministic all-optical switching by saying that F Multi needs to be higher than F switch , one can explain why AO-HIS can be observed in ultrathin Co/ Pt. Indeed, since Co/Pt is so thin that the dipolar field (or stray field or demagnetization field) which tends to break magnetization into domain is small, and consequently, F Multi is large and the optical switching will not be covered by the multidomain formation.…”

“…3 Moreover, the laser fluence needed to obtain AO-HIS is found to be minimum around compensation. 6,21 Theoretical studies and modeling have demonstrated that the exchange coupling between the two magnetization sublattices play a crucial role in the angular momentum transfer between the sublattices. 22 The two main approaches to describe the magnetization dynamic of the sublattices are the microscopic three-temperature model 23 (M3TM) and the atomistic Landau-Lifshitz-Gilbert equation.…”

From toggle to precessional single laser pulse switching

“…Since FSW depends on pulse duration, the underlying mechanism is likely governed by the electronic temperature which decreases with increasing the pulse duration as discussed in [16,17]. On the other hand, since FMD depends only slightly on pulse duration, a link to the Curie temperature, the energy transferred to the lattice and so the stabilisation of multi-domain structure is highly probable as suggested in [17,18,19]. As far as the state diagram of x = 5 % is concerned, the domain pattern is much more complex and multiple threshold fluence have to be defined TFn 11 : TF1 corresponds to the appearance of outer blue ring and can be referred as FSW for the ring structure, TF2 to the appearance of the red ring etc… The fact that the critical fluences (TF1, TF2,…) are mostly independent of pulse duration implies an underlying mechanism linked to the energy transferred to the lattice 11 .…”