What is meant by changing Magnetization by Neel rotationNot geometric rotation?Neel Rotation?
The magnetic fluids are colloidal suspensions of magnetic nanoparticles dispersed in a carrier liquid. The nanoparticles are magnetic monodomains and usually have irregular shape. The direction of the magnetic moment relative to the particle is given by the interaction with the easy magnetization axis which can have a crystallographic nature in the case of hard magnetic materials or a morphological nature in the case of soft magnetic materials [1] when the magnetic moment is coupled with the shape anisotropy axis of the particle. The strength of the coupling increases with the ellipticity and volume of the particle and is proportional with the squared cosine of the angle between the magnetic moment and the anisotropy axis [1]. Although free to rotate relative to the particle (Néel rotation), the rotation of the magnetic moment under the action of an external magnetic field induces a rotation of the particle bulk (Brown rotation) and the particles tend to align with their anisotropy axis parallel to the field, thus resulting in a macroscopic anisotropy of the magnetic fluid. This macroscopic anisotropy is the main cause of optical anisotropy of magnetic fluids: dichroism and birefringence.
The model developed by Hasmonay et al. (Eur. Phys. J. B 5 (1998) 859) for the static magnetic induced dichroism in magnetic fluids with particles whose magnetic moment is free to rotate inside the particle was used to fit experimental data. The model was found to describe the data more accurately than previous models for magnetic fluids with particles whose magnetic moment direction is frozen relative to the particle.
