Magnetic properties of Co3O4 nanoparticles

Magnetization and magnetic relaxation measurements of 200Angstrom Size CO3O4 particles are reported. Zero-field-cooled (ZFC) susceptibility exhibits a sharp peak characterizing a phase transition at T-t = similar to25 K. Above T-t, the reversal magnetizations vary linearly with H and the magnetizations scale with H/(T + theta) as would occur for an antiferromagnetic material above its Neel point. Below T, both ZFC and field-cooled hysteresis loops exhibit a large coercivity (similar to600 Oe at 5 K) and the irreversibility in loops are found up to H = 35 kOe. Exchange fields of several hundreds Oe are observed at low temperatures. Dependencies of the coercivity and the exchange field on both the temperature and the cooling magnetic field are studied. Magnetization relaxes according to the equation M(t) = M(t(0))[1 - S(T)ln((t + t(0))/tau(0))]. Temperature dependence of the magnetic viscosity S(T) is thus determined for 1.8 less than or equal to T less than or equal to 40 K. S is temperature independent for T < 10 K probably due to magnetic quantum tunneling, whereas it increases linearly with T in a limited range 8 less than or equal to T less than or equal to 20 K due to thermal effects. Around T, S exhibits a very sharp maximum which indicates a phase transition. (C) 2002 Elsevier Science B.V. All rights reserved.