Tuning of perpendicular magnetic anisotropy in Bi-substituted yttrium iron garnet films by He$^+$ ion irradiation

TL;DR

This study demonstrates how He$^+$ ion irradiation can precisely tune the magnetic anisotropy of Bi-YIG films, enabling control over magnetization orientation and domain structures with minimal impact on damping, and reversible effects via annealing.

Contribution

It introduces a method to continuously tune magnetic anisotropy in Bi-YIG films using He$^+$ ion irradiation, with insights into strain relaxation and domain formation.

Findings

Magnetization switches from out-of-plane to in-plane after irradiation.

Magnetic bubble domains form at specific ion fluences.

Effects of irradiation are reversible through thermal annealing.

Abstract

We report the continuous tuning of magnetic anisotropy in perpendicularly magnetized bismuth-substituted yttrium iron garnet (Bi-YIG) films via He$^+$ ion irradiation. Our findings indicate that the magnetization direction of epitaxial Bi-YIG films on sGGG substrates transitions from out-of-plane in the as-grown state to in-plane after He+ ion irradiation at a fluence exceeding $2\times 10^{14}$ ions/cm$^2$. The reorientation is attributed to the relaxation of tensile film strain, which reduces the perpendicular magnetic anisotropy without affecting the saturation magnetization. The Gilbert damping parameter and the inhomogeneous broadening of the ferromagnetic resonance linewidth show only minimal increases with ion irradiation. Additionally, at a fluence of $5\times 10^{13}$ ions/cm$^2$, we observe the formation of magnetic bubble domains in the Bi-YIG films. Micromagnetic simulations estimate a Dzyaloshinskii-Moriya interaction of 0.006 mJ/m$^2$, which is insufficient for stabilizing N\'eel-type skyrmions. Finally, we demonstrate that the effects of He$^+$ ion irradiation can be largely reversed through thermal annealing in an oxygen atmosphere.