Exchange Bias, Memory and Freezing Effects in NiFe2O4 Nanoparticles

Abstract

Single-phase NiFe2O4 nanoparticles embedded in SiO2 matrix have been synthesized by sol-gel method. Average particle size lies in the range 8–12 nm. Magnetic measurements are taken by SQUID-magnetometer with a maximum applied field of ±7 T and temperature down to 4.2 K. An exchange bias effect in nanoparticles is due to the existence of strong core-shell interactions and it vanishes as the particle size decreases (<4 nm). Spin disorder and frustration appear at the core-shell interface due to broken bonds on the surface. We have observed the exchange bias effect via hysteresis loop shift, when the sample is cooled in an applied field of 5 T. In both AC and DC fields, our system exhibit memory effects at the halted temperatures. Furthermore, a sharp increase of coercivity at low temperatures (<50 K) is observed, which is attributed to increased surface anisotropy at low temperatures. For saturation magnetization vs. temperature data, Bloch’s T 3/2 law (M(T)=M(0)⋅(1−BT b )) is fitted well and yields: B=4×10−05 K−3/2 and b=1.53. All these measurements prove the presence of exchange bias (core-shell interactions), memory effects, validation of Bloch’s T 3/2 law and freezing effects in nickel ferrite nanoparticles dispersed in SiO2 matrix.