Surface spin-glass-like behavior of monodispersed superparamagnetic MnZnFeO magnetic fluid.

Zero field cooled dc-magnetization measurements of monodispersed MnZnFeO 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. [ABSTRACT FROM AUTHOR]