Induced Magnetic Anisotropy of Ferrofluid Frozen in Magnetic Fields

The magnetization process of a ferrofluid whose carrier fluid is paraffin was investigated in the temperature range from 77 K to 300 K, as a function of the cooling field intensity and freezing rate. Phase transitions between the liquid and solid states can be simulated by using the ferrofluids as a magnetic probe. A uniaxial magnetic anisotropy was induced by field cooling in frozen ferrofluids. This induced anisotropy decreased with increasing temperature near the melting point, and disappeared at a temperature 30 K above the melting point. In this region rotational hysteresis was observed. The anisotropy constant K near the melting point is described by the equation K=A·exp(ΔE_K/kT). The value of the activation energy ΔE_K was almost the same as that of ΔE_η estimated from the viscosity vs. temperature relation, η ∞ exp(ΔE_η/kT).