Magnetic field-induced transitions and phase diagram of aggregate structures in a suspension of polydisperse cubic haematite particles†

We investigated a polydisperse cubic haematite particle suspension in an external magnetic field and examined the dependence of magnetic field-induced transitions on the standard deviation of the particle size distribution using quasi-two dimensional Monte Carlo simulations. In the case of smaller polydispersity, stable clusters tend to form owing to stable face-to-face contact. In this case, however, larger magnetic particle–particle interaction strengths are necessary. Since the applied magnetic field enables the magnetic moment of each particle to incline in the field direction, it enhances the formation of chain-like clusters. In the case of larger polydispersity, compared to the smaller polydispersity cases, particle aggregates are formed even in the region of smaller magnetic particle–particle interactions. In this case, small particles combine with a growing cluster composed of large particles to form larger clusters. However, these small particles tend to disturb the internal structure of the particle aggregates, leading to chain-like clusters with narrower widths than those in the case of smaller polydispersity. These characteristics of the particle aggregates confirm that the broadness of polydispersity in a magnetic cubic particle suspension is applicable for controlling the internal structure and regime transition in the internal structure of particle aggregates. This may be an important feature in the development of surface modification techniques using magnetic cubic particle suspensions.