Dipolar Interactions between Iron-Oxide Nanoparticles in Frozen   Ferrofluids and Ferronematics
  We present a detailed study of the magnetic behavior of iron-oxide (gamma-Fe2O3 and Fe3O4) nanoparticles constituents of ferrofluids (FF's) with average particle sizes <d> = 2.5 and 10 nm. The particles were dispersed in the frozen liquid carrier (pure FF) and in a frozen lyotropic liquid crystalline matrix in the nematic phase or ferronematic (FN) (ferrolyomesophase). Both FF and FN phases displayed superparamagnetic (SPM) behaviour at room temperature, with blocking temperatures T_B ~ 10 and 100 K for <d> = 2.5 and 10 nm, respectively. Dynamic ac susceptibility measurements showed a thermally activated N\'eel-Brown dependence of the blocking temperature with applied frequency. Our results show that dipolar interactions are small, but non-negligible, as compared to the single-particle energy barriers from magnetic anisotropy. From the fit of ac susceptibility we calculated the effective magnetic anisotropy constant K_{eff} for 2.5 nm maghemite particles. Although interparticle interactions present in highly diluted samples do not appreciably modify the dynamic magnetic behavior of isolated particles, the calculated magnetic anisotropy were abut one order of magnitude larger that the bulk materials, suggesting the existence of large surface anisotropy. Using the thermally activated model to fit the dynamic data yielded effective energy barriers Ea = 3.5x10^{-21} J. From these data, we obtained K_{eff} = 422 kJ/m^3 for the single-particle effective magnetic anisotropy. 
G. F. Goya, S. L. G\'omez and S. M. Shibli
cond-mat.mtrl-sci