Single pulse all-optical magnetic switching in amorphous DyCox and TbCox

Abstract

Single pulse all-optical switching (SP-AOS) using 40 fs light pulses was discovered in films of the amorphous rare earth-transition metal (RE-TM) alloy a-Gd(FeCo)3 [1]. The switching takes less than 5 ps and relies on the different demagnetisation times of the Gd and Fe,Co subnetworks. Replacing Gd by another RE was expected to yield more materials that exhibit ultrafast switching, but it did not happened. Indeed, amorphous RE-TM alloys other than Gd(FeCo)3 have only been switched transiently by a single pulse [2] or slowly using many polarised pulses [3].Here, we report stable SP-AOS in amorphous Dy-Co and Tb-Co films with perpendicular anisotropy. Room-temperature coercivity is 30-50 mT, and compensation temperatures Tcomp are below 300 K. The example of a-DyCo3 is shown in Fig 1. The behavior is surprising. A single 200 fs pulse creates a non-uniform 40 µm dark spot on a uniformly-magnetized light film. Domain structure on a scale of ~ 3 µm is observed. After two pulses, the darker domains switch back, and the multidomain state will toggle for a few more pulses before leaving the sample in a nonuniform mid-gray state. Images are obtained on a strip of material irradiated with 1-5 pulses, using an off-site Kerr microscope.Other samples show a variety of behavior. The switching of a-TbCo4 shown in Fig. 2 is also granular, but the lighter and darker spots are aggregated into lighter and darker rings with unequal numbers of small dark and light domains. The contrast between rings again disappears after a few pulses, but the small domains persist.This is quite unlike the uniform toggling of large domains, comparable in size to the laser spot, that are seen in SP-AOS of ferrimagnetic a-Gd(FeCo)3 and crystalline Mn2RuxGa. The key difference is probably the random uniaxial anisotropy for the non S-state rare ions and temperature-dependence, which governs that of the coercivity. The influence of stray dipolar fields on a µm scale and possible of radial heat waves in the substrate will be considered in the context of ultra-fast SP-AOS in these a-RE-TM materials.  SP-AOS in a 10 nm a-DyCo3 alloy film with perpendicular magnetization. Scale bar 40 μm.  SP-AOS in 20 nm a-TbCo4 with perpendicular magnetic anisotropy.