Controlling Domain-Wall Nucleation in Ta/Co-Fe-B/MgO Nanomagnets via Local Ga+ Ion Irradiation

Comprehensive control of the domain-wall nucleation process is crucial for spin-based emerging tech- nologies ranging from random-access and storage-class memories through domain-wall logic concepts to nanomagnetic logic. In this work, focused Ga+ion irradiation is investigated as an effective means to control domain-wall nucleation in Ta/Co-Fe-B/MgO nanostructures. We show that, analogously to He+irradiation, it is not only possible to reduce the perpendicular magnetic anisotropy but also to increase it significantly, enabling bidirectional manipulation schemes. First, the irradiation effects are assessed at the film level, sketching an overview of the dose-dependent changes in the magnetic energy landscape. Sub- sequent time-domain nucleation characteristics of irradiated nanostructures reveal substantial increases in the anisotropy fields but surprisingly small effects on the measured energy barriers, indicating shrinking nucleation volumes. Spatial control of the domain-wall nucleation point is achieved by employing focused irradiation of preirradiated magnets, with the diameter of the introduced circular defect controlling the coercivity. Special attention is given to the nucleation mechanisms, changing from the coherent radiation of a Stoner-Wohlfarth particle to depinning from an anisotropy gradient. Dynamic micromagnetic sim- ulations and related measurements are used in addition to model and analyze this depinning-dominated magnetization reversal.