Magnetic micro- and nano-order particles have been applied to magnetically controlled dampers and actuators in the conventional mechanical engineering field, as well as to medical engineering fields such as magnetic hyperthermia and drug delivery systems. In order to successfully develop applications in these fields, it is necessary to create high-order functional particles with desired properties and to develop static and dynamic physical properties to efficiently demonstrate their functionality in the application environment. In this study, we focused on a spheroidal particle dispersion system, and examined in detail the effects of the magnitude and direction of an external magnetic field on particle aggregation and phase transition phenomena on a two-dimensional plane using Monte Carlo simulations. The main results are as follows. As the magnetic particle-particle interaction strength increases, particles aligned along their short axes form raft-like clusters. In the absence of an external magnetic field, these raft-like clusters form with meandering shapes and random orientations. When an external magnetic field is applied parallel to the plane, the aggregates become longer and more linear along the magnetic field direction as the field strength increases. When a magnetic field is applied at an angle to the plane, independently moving particles begin to appear as the field direction approaches the normal to the plane. These results indicate that when the magnetic moments of the particles are tilted by about 55 degrees or more relative to the plane, the magnetic interaction between particles becomes repulsive, and the particles tend to disperse. However, when there is insufficient space for the particles to disperse, magnetic particle-particle interactions may locally dominate over the effect of the magnetic field, leading to the formation of aggregates.
Suzuki et al. (Thu,) studied this question.