2017 年 83 巻 856 号 p. 17-00378
We have investigated a regime change in the aggregate structures of a suspension composed of magnetic cubic particles in thermodynamic equilibrium, by means of Monte Carlo simulations. In concrete, we have addressed the dependence of the regime change on a variety of factors such as the magnetic field strength and the magnetic particle-particle interaction strength. The orientational distribution function and order parameters have been focused on for quantitatively discussing these characteristics. The main results obtained here are summarized as follows. If the magnetic interaction strength is sufficiently large for cluster formation, closely-packed clusters are formed under the combination and expansion of a cluster unit composed of 8 particles. A regime change in the internal structure of aggregates appears in a narrow range of the magnetic interaction strength, which is clearly exhibited by the order parameter employed here. A closely-packed configuration can be clearly characterized by the orientational distribution function; 8 high peaks appear in the orientational space in the case of the closely-packed structure. As the magnetic field is increased, the closely-packed clusters are collapsed and transformed into wall-like clusters along the magnetic field direction. This is because the magnetic moment of each particle has a strong tendency to incline in the magnetic field direction in the situation of a strong magnetic field.