Field-dependent phase diagram of the structural pattern in a ferrofluid film under perpendicular magnetic field

Influences of both the magnetic field strength H and the sweep rate dH/dt on the structural pattern evolution in a magnetic fluid film under perpendicular fields are systematically investigated. When the magnetic field is increased at a given rate, the structure evolves from a monodispersed state to a disordered one, then to the first-level ordered hexagonal structure, and finally to the second-level ordered structure through a transition state. With a higher dH/dt, the range of the applied magnetic field corresponding to the first-level ordered structure is widened and the field H_s, at which the transition occurs becomes higher. An empirical power law H_s ∞ (dH/dt) ^0.4 was found. A phase diagram for the magnetic fluid film in the H-dH/dt plane is presented.