Measurement of the anisotropy constant in Ni-Fe and Fe3O4 magnetic fluids

The uniaxial anisotropy constant of two magnetic field systems has been measured using magnetization and torque magnetometry. A metallic, Ni-Fe system was frozen at 77 K to obtain torque curves while an Fe₃O₄ system was set in a polyester resin at room temperature in the presence of aligning magnetic fields. The induced texture gave rise to torque measurements which yielded values for the anisotropy constant of 2.9*10⁵ Jm^-3 for Ni-Fe and 0.58*10⁵ Jm^-3 for magnetite. The magnetization measurements resulted in lower values of 1.15*10⁵ Jm^-3 for Ni-Fe and 0.47*10⁵ Jm^-3 for Fe₃O₄. Even for the lowest value obtained, however, a particle elongation of 1.5 would be required for shape to be responsible for the anisotropy. No such elongations were observed in the electron microscope suggesting other factors are present.