Magnetic irreversibility and relaxation in CuFe2O4 nanoparticles

In this work, we present a study on the magnetic behavior of CuFe2O4 nanopa rticles 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 an d zero-field-cooled magnetization data showed a blocking temperature T-B si milar to 225 K, independent of particle size, which is associated to interp article interactions. It was observed that T-B and the irreversibility temp erature T-irr, shift to lower temperature with increasing applied fields, b oth with an H-1 dependence. Strong training effects were observed from rela xation data, assigned to the irreversible behavior of the spin-disordered p article surface. The magnetic viscosity at T = 4.2 K was analyzed for sampl e with D-mean = 7.7 nm, revealing strong irreversibility after each major h ysteresis loop. These results are discussed in terms of multiple spin-disor dered configurations, at particle surface, with quasidegenerate states. (C) 2000 Elsevier Science B.V. All rights reserved.