Magnetic irreversibility and relaxation in CuFe 2O 4 nanoparticles

Abstract

In this work, we present a study on the magnetic behavior of CuFe 2O 4 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 ∼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.