The role of the magnetically induced anisotropy of the pair correlation function in the dichroism of magnetic fluids

Experimental evidence is presented that the susceptibility for magnetically induced dichroism in concentrated magnetic fluids decays with further increasing of the magnetic nanoparticle concentration. The analysis of the predictions derived from our dichroism model, based on the pair correlation function developed by Elfimova and Ivanov (2010), showed that the observed phenomenon can be explained by the manner in which the symmetry of the pair correlation function changes with increasing magnetic field intensity and sample concentration. The increase of the dichroism with increasing concentration in diluted samples (0.2-1%) is mainly caused by the pair correlations in the first coordination layer, predominantly along the magnetic field direction. In the 1-3% concentration range, the first coordination layer becomes increasingly isotropic while pair correlations in the second coordination layer grow transversally to the magnetic field direction. Thus, with the increase of the concentration above 2%, the dichroism decreases as a combined effect of the changes in the symmetry of the pair correlations in the first and the second coordination layers. The proposed dichroism model predicts the possibility of negative magnetically induced dichroism in highly concentrated perfect colloidal dispersions of small spherical magnetic nanoparticles.