Eur. Phys. J. B 21, 19-29 (2001)
https://doi.org/10.1007/s100510170208

Alternating magneto-birefringence of ionic ferrofluids in crossed fields

¹ Laboratoire des Milieux Désordonnés et Hétérogènes, Université Pierre et Marie Curie (Paris 6), Tour 13, Case 78, 4 place Jussieu, 75252 Paris Cedex 05, France
² Laboratoire des Liquides Ioniques et Interfaces Chargées, Université Pierre et Marie Curie (Paris 6), bâtiment F, Case 63, 4 place Jussieu, 75252 Paris Cedex 05, France

Corresponding author: ^a rperz@ccr.jussieu.fr

Received: 5 June 2000
Revised: 8 January 2001
Published online: 15 May 2001

Abstract

A dynamic probing of magnetic liquids is performed experimentally, using a static magnetic field modulated by another smaller field, normal and alternating. The optical magneto-birefringence under these crossed magnetic fields is recorded as a function of the frequency for different field intensities and different sizes of the magnetic nanoparticles. A general reduced behavior is found for the in-phase and the out-of-phase optical response which is well-described by a simple mechanical model. Depending on the value H_ani of the anisotropy field of the nanoparticles, we can distinguish two different high magnetic field regimes: -a rigid dipole regime (large anisotropy energy with respect to ) for cobalt ferrite nanoparticles with a relaxation time inversely proportional to the field intensity , -a soft dipole regime (anisotropy energy of the order of ) for maghemite nanoparticles with a relaxation time independent of the field intensity .