Surface spin-glass-like behavior of monodispersed superparamagnetic Mn 0.5Zn 0.5Fe 2O 4 magnetic fluid

Zero field cooled dc-magnetization measurements of monodispersed Mn <inf>0.5</inf>Zn <inf>0.5</inf>Fe <inf>2</inf>O <inf>4</inf> nanoparticles dispersed in kerosene exhibit two transitions at low temperatures. These transitions correspond to (i) the superparamagnetic to blocked superparamagnetic and (ii) the blocked superparamagnetic to surface spin-glass like/quantum superparamagnetic state upon lowering the temperature. The existence of a disorder surface is confirmed by recording small-angle neutron scattering data below and above the Curie temperature. Magnetic relaxation analysis shows a plateau at low temperature (below 5 K) with a slight minimum at 3 K, which is a characteristic of the surface spin-glass-like state. This is analyzed considering the energy distribution n(E)1/E. The existence of surface disorder dominates at low temperature and mimics the transition from superparamagnetic to quantum superparamagnetic states. © 2011 Springer-Verlag.