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

TL;DR

This study demonstrates how focused Ga+ ion irradiation can precisely control domain-wall nucleation in Ta/CoFeB/MgO nanomagnets by modulating magnetic anisotropy, enabling bidirectional manipulation and defect engineering.

Contribution

It introduces a novel method using Ga+ ion irradiation to control magnetic anisotropy and nucleation sites in nanomagnets, expanding the toolkit for spintronic device engineering.

Findings

Irradiation modulates magnetic anisotropy in a dose-dependent manner.

Focused irradiation creates localized defects controlling nucleation points.

Nucleation mechanisms shift from coherent rotation to depinning from anisotropy gradients.

Abstract

Comprehensive control of the domain wall nucleation process is crucial for spin-based emerging technologies ranging from random-access and storage-class memories over 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/CoFeB/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 new, bidirectional manipulation schemes. First, the irradiation effects are assessed on film level, sketching an overview of the dose-dependent changes in the magnetic energy landscape. Subsequent 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 pre-irradiated magnets, with the diameter of the introduced circular defect controlling the coercivity. Special attention is given to the nucleation mechanisms, changing from a Stoner-Wohlfarth particle's coherent rotation to depinning from an anisotropy gradient. Dynamic micromagnetic simulations and related measurements are used in addition to model and analyze this depinning-dominated magnetization reversal.