Dynamic optical probing of the magnetic anisotropy of nickel-ferrite nanoparticles

Field dependence of dynamic magneto-orientational birefringence in a ferrocolloid based on the nickel-ferrite nanoparticles is examined. The nanoparticles are electrostatically stabilized and suspended in glycerin at low-volume fractions Φ⩽0.75%. The colloids are tested under crossed magnetic fields: an alternating weak (probing) and a constant strong (bias) one. By comparison to a theoretical model of the birefringence relaxation, an evaluation of the mean nanoparticle magnetic anisotropy energy, Ea, is done. We get σ=Ea∕kT∼10, given that the particles are rather polydisperse: a log-normal distribution with exp[⟨lnd⟩]=6.5nm and the width s=0.55. With the parameter σ in such a range, the NiFe2O4 grains make a first example of a nanodisperse material (ferrofluid) whose anisotropy properties differ substantially from both magnetically soft (e.g., maghemite with σ<2) and magnetically hard (e.g., cobalt ferrite with σ>50) nanosubstances of the same grain size.