Magnetic vortex structure for hollow Fe3O4 spherical submicron particles

Magnetic particles with a hollow structure have arisen as an important class of nanomagnets because of a large pore volume and higher surface-to-volume ratio compared with the same-sized solid particles. The hollow structure results in unique magnetic features such as enhanced surface exchange bias, spin freezing, and preferential stability of a magnetic vortex. Despite a recent growing understanding of sub-100 nm hollow spherical magnetic nanoparticles, magnetic properties of larger-sized hollow particles were not currently understood in detail. Here, we report results of observations of magnetic microstructures for 420 nm-sized hollow Fe3O4 spherical particles with an electron holography imaging technique, where a magnetic-vortex formation is inferred from bulk measurements. We directly observe a magnetic vortex in a remanence state with magnetization circularly oriented within the shell and the reduced stray field. Micromagnetic simulations demonstrate an increasing stability of a vortex for a hollow sphere and the formation of a field-induced curling double vortex with a pair of clockwise and counterclockwise vortices. This double vortex structure is not confirmed for the solid counterpart, and its stability enhances with decreasing the shell thickness. The present work provides useful knowledge in designing magnetic particles, where a hollow structure and a magnetic vortex are key factors for high-performance biomedical applications.