Anisotropic Dynamics Of Magnetic Colloidal Cubes Studied By X-Ray Photon Correlation Spectroscopy

Herein we present our results on the investigation of the influence of external magnetic fields on the anisotropic collective dynamics of core/shell colloidal cubes having a hematite core and silica shell. Owing to the hematite cores, these micrometer-sized particles possess permanent dipole moments, which are at an angle with respect to the long diagonal of the cubes. As a result, they self-assemble into chains, which subsequently sediment to form higher-order structures. Using multispeckle ultrasmall-angle x-ray photon correlation spectroscopy, the anisotropic dynamics within these structures at the nearest-neighbor length scale was probed. The relaxation of the intermediate scattering function follows a compressed exponential behavior along all the different directions with respect to the external field-parallel, perpendicular, and at an angle approximate to 45 degrees-indicating hyperdiffusive behavior. We believe that the inhomogeneous distribution of stress points originating from the interplay of external field-induced (both gravitational and magnetic) alignment of the chains are responsible for the anomalous dynamics. The effective diffusion coefficients along and at approximate to 45 degrees angle exhibit mild de Gennes narrowing, which is not very common for hyperdiffusive dynamics. We rationalize our observations by considering a superposition of diffusive and stress-induced ballistic processes and argue that depending on the azimuthal direction the relative contribution from these two processes changes.