Green magnetite (Fe3O4): Unusual optical Mie scattering and magnetic isotropy of submicron-size hollow spheres

Magnetite hollow spheres with a diameter of 650 nm and shell thickness of 40 nm exhibit an unexpected green color, in contrast with usual magnetite solids appearing black. This unusual green color is interpreted in terms of the Mie scattering on the inhomogeneous and low-density structure of the hollow spheres with a characteristic diameter that is comparable to the wavelength of the visible light. Further, thin films of the hollow spheres exhibit no two-dimensional shape anisotropy, reflecting the isotropic structure of individual particles. These results indicate the potential advantages in achieving unique physical properties through constructing hollow structures.