Inter-particle interactions in systems of magnetic nanoparticles

Magnetic nanoparticles are often found to be superparamagnetic, i.e. the magnetization vector fluctuates among the easy directions of magnetization. We have studied strongly interacting monodisperse particles and have found that the interactions lead to a critical slowing down of the relaxation time and formation of a spin-glass phase below a critical temperature. These phenomena have been studied by Mossbauer spectroscopy and magnetization measurements. Further plans include to determine the critical exponents characterizing the phase transition and to study the dependence of the phase transition temperature on the interaction strength.