360∘ domain walls in magnetic thin films with uniaxial and random anisotropy

Abstract

X-ray photoemission electron microscopy (XPEEM) and magneto-optic Kerr effect (MOKE) microscopy have been performed on a metal-insulator multilayer of $[{\mathrm{Co}}_{80}{\mathrm{Fe}}_{20}(t=1.8\phantom{\rule{4pt}{0ex}}\mathrm{nm}$)/${\mathrm{Al}}_{2}{\mathrm{O}}_{3}{(3\phantom{\rule{4pt}{0ex}}\mathrm{nm})]}_{9}$ to image ${360}^{\ensuremath{\circ}}$ domain walls (DWs) along easy and hard axes, respectively. Their creation and annihilation can be directly visualized under application of a magnetic field. XPEEM experiments and micromagnetic simulations show that ${360}^{\ensuremath{\circ}}$ DWs occur through the merger of ${180}^{\ensuremath{\circ}}$ DWs of opposite chiralities along the easy axis. They are stable even under application of large magnetic fields. Formation of ${360}^{\ensuremath{\circ}}$ DWs observed along the hard axis is attributed to symmetry breaking of the coherent spin rotation. Their formation in metal-insulator multilayers is explained as being due to the presence of an orientational dispersion of anisotropy axes in the film grains that is comparable to an overall uniaxial anisotropy term. Our results are confirmed numerically using micromagnetic simulations.