Magnetic irreversibility and relaxation in CuFe2O4 nanoparticles

In this work, we present a study on the magnetic behavior of CuFe2O4 nanoparticles with different particle sizes. Magnetic particle size distribution obtained from M(H) curves in the superparamagnetic (SPM) state showed good agreement with data obtained from X-ray powder diffraction. Field-cooled and zero-field-cooled magnetization data showed a blocking temperature T-B similar to 225 K, independent of particle size, which is associated to interparticle interactions. It was observed that T-B and the irreversibility temperature T-irr, shift to lower temperature with increasing applied fields, both with an H-1 dependence. Strong training effects were observed from relaxation data, assigned to the irreversible behavior of the spin-disordered particle surface. The magnetic viscosity at T = 4.2 K was analyzed for sample with D-mean = 7.7 nm, revealing strong irreversibility after each major hysteresis loop. These results are discussed in terms of multiple spin-disordered configurations, at particle surface, with quasidegenerate states. (C) 2000 Elsevier Science B.V. All rights reserved.