Magnetization processes and reorientation transition for small magnetic dots

A theory for magnetization processes in interacting arrays of small magnetic structures at finite temperatures is presented. Hysteresis and magnetic ordering of weakly coupled arrays of single-domain ferromagnetic particles are examined. The dots are arranged on a planar lattice and the effects of lattice geometry are examined via long-ranged dipolar coupling between magnetic dots. Small clusters of dots arranged in finite arrays are shown to have complicated hysteresis determined by the shape, size, and orientation of the cluster in externally applied fields. One result is an array induced ``shape'' anisotropy that controls how reversal occurs in the array itself. Finite temperature effects are examined and the strength of the dipolar coupling, though weak, can be significant for closely packed particles at low temperatures. A reorientation transition from in plane to perpendicular is shown to occur as the temperature is increased for dots with perpendicular anisotropy.