Role of shell thickness and applied field on the magnetic anisotropy and temperature dependence of coercivity in Fe₃O₄/γ-Fe₂O₃ core/shell nanoparticles

We report on the effect of shell thickness and the agnetic field on the effective agnetic anisotropy, K_eff and on the teperature dependence of coercivity, H_C(T) inFe₃O₄/γ-Fe₂O₃ core/shell nanoparticles of fixed diaeter (8 n) and several shell thicknesses (1, 3 and 5 n). The particles are synthesized by the usual co-precipitation ethod. The phase and the orphology of the saple are characterized by X-ray diffraction and transission electron icroscopy. The orphology of the nanoparticles is confired by high resolution transission electron icroscopy (HRTEM) and selected area electron diffraction (SAED). The agnetic easureents are carried out using a SQUID in the teperature range of 2 K to 300 K both under zero field-cooling (ZFC) and field-cooling (FC) protocols with field-cooling values, H_FC of 0.5 T, 1 T, 2 T and 3 T. H_C(T) in the saples is found to significantly deviate fro the original Kneller’s law. These deviations are attributed to odifications of the interfacial agnetic anisotropy that resulted fro variations in the interfacial spin-glass regions with shell thickness and field-cooling. K_eff is calculated using two ethods based on the agnetization easureents. In the first ethod we used the H_C(T) curves obtained fro the ZFC and FC agnetization versus agnetic field M–H hysteresis loops. In the second ethod the ZFC agnetization versus teperature (M–T) easureents are used at several applied agnetic fields. We copared and discussed the different results of K_eff obtained by these two methods.