Spatial control of skyrmion stabilization energy by low-energy Ga+ ion implantation

Abstract

Magnetic skyrmions are candidates for information carriers in Brownian and stochastic computers. Developing a technique for fabricating a film with a suitable potential landscape, wherein the information carrier may diffuse freely, is essential for these probabilistic computers. In this study, to build the desired local potential into magnetic films, a 1.2 nm-thick Co-Fe-B film with a 5.2 nm-thick cap layer was irradiated by a focused ion beam (FIB) using Ga+ as the ion source under a low acceleration voltage of 5 keV. The fluences ranged from 0 to 25 × 1012 ions/cm2. Consequently, the critical temperature at which skyrmions appear or disappear is shifted by several 1–10 K depending on the ion fluence. The origin of this effect is discussed by observing the ion implantation profile and the surface sputtering depth using time-of-flight secondary ion mass spectrometry (TOF-SIMS) and atomic force microscopy (AFM). The results of TOF-SIMS measurements show that most of the Ga atoms exist in the Co–Fe–B layer. If all Ga atoms exist in the Co–Fe–B layer, the Ga concentration is 7 × 10−3 at. % after irradiation of 0.8 × 1012 ions/cm2. The AFM results show a sputtered pattern with 0.2 nm depth after irradiation of 16 × 1012 ions/cm2. Finally, the effect of irradiation on the diffusion coefficient was examined. It was determined that small fluences of 1.6 × 1012 and 0.8 × 1012 ions/cm2 can construct a potential barrier controlling skyrmions while maintaining diffusion coefficients as high as 10 μm2/s. The FIB process can be used to draw a circuit of probabilistic computers with skyrmions as information carriers.