Finite size effects on the magnetocaloric properties around blocking temperature in γ-Fe2O3 nanoparticles

We investigate the magnetocaloric properties of gamma-Fe2O3 nanoparticles with an emphasis on size effects. The magnetic measurements have demonstrated that the properties are relevant for spin glasses at low temperature. The blocking temperature (T-B) was found to decrease from 208 to 94 K with decreasing average size <D> from 8.5 to 2.7 nm due to surface effects. The temperature dependence of the magnetic entropy change (-Delta S-M) was calculated from the isothermal magnetization. The caret shape of the (-Delta(S)) curves shows that the magnetic transition near T-B is a phase transition in line with Arrott plots. Under a magnetic field change of 10 kOe, the (-Delta S-M) and the relative cooling power increase from 1.07 to 1.4.J/kg.K and 26 to 94 J/kg with increasing <D> from 2.7 to 8.5 nm, respectively. The increase of the magnetocaloric performance with increasing size was attributed to the increase in magnetization. (C) 2019 Elsevier B.V. All rights reserved.