Cation distribution and microwave absorptive behavior of gadolinium substituted cobalt ferrite ceramics

In the first step, gadolinium substituted cobalt ferrite nanoparticles with composition of CoFe2-xGdxO4 (x = 0-0.05 in a step of 0.01) were synthesized by a hydrothermal technique and without subsequent annealing. X-ray diffraction and field-emission scanning electron microscopy evaluations exhibited that single phase spinel ferrites with narrow size distribution were achieved. The EDS analysis confirmed that the applied process for fabrication of CoFe2-xGdxO4 nanoparticles is a proper process for the synthesis of spinel cobalt ferrites with homogeneity in composition. Mössbauer spectroscopy was utilized to distinguish the site preference of constitutive elements. A well-resolved doublets pattern of ferrite nanoparticles is mainly attributed to the superparamagnetic behavior. The results of magnetic hysteresis revealed that the gadolinium substituted cobalt ferrite nanoparticles exhibit superparamagnetic trend at a room temperature. The zero field cooled magnetization curves of gadolinium substituted cobalt ferrite nanoparticles showed that with an increase in gadolinium content, the blocking temperature increases. Fitting the experimental data of susceptibility with the Neél-Brown model expresses that there are powerful interactions between nanoparticles of gadolinium substituted cobalt ferrite and the Neél-Brown model was not operate for such ferrites. In the second step, the gadolinium substituted cobalt ferrite nanoparticles were annealed and the structural characteristics, magnetic properties and microwave absorptive behavior were evaluated. The Mössbauer spectras show a well-resolved six-line pattern of ferrite nanoparticles which is mostly imputed to the ferrimagnetic treatment. The magnetic hysteresis loops at a room temperature confirmed that the coercive field increased from 802 Oe for x = 0 to 965 Oe for x = 0.02 and then at higher contents, decreased. The magnetic hysteresis curves of CoFe1.98Gd0.02O4 for fields applied parallel and perpendicular to the sample axis, expresses high value of magnetocrystalline anisotropy field of 13 kOe which exhibits that the resonance frequency can be shifted to higher frequency. Consequently the microwave absorber can be covered a wide-band of electromagnetic waves in the X-band.