Surface and interparticle interactions effects on nano-cobalt ferrites

• CoxFe3−xO4 (x = 1, 1.2, 1.4, 1.6 and 1.8) were synthesized with solvothermal route.
• Nanoparticle sizes are less than 10 nm.
• Surface effects and interparticle interactions were found for x = 1.6 and 1.8.
• Surface effects have been deduced by Bloch's law fitting.
• Interparticle interactions have been deduced by Kneller's law fitting.

Cobalt ferrites nanoparticles CoxFe3-xO4 (1≤ x < 1.8) were synthesized by a solvothermal route. X-ray diffraction, Transmission Electron Microscopy and magnetic measurements show a strong correlation between the structural and the magnetic properties. The highest cobalt amount substituted samples (x = 1.6 and 1.8) show a jump at low temperature in the hysteresis loops in the range of low applied fields (<10 kOe). This behavior has been explained by surface effects and interparticle interactions. A difference has been found using two different estimation of the magnetic anisotropy: from the law of approach to saturation and using the Neel-Brown approach. The last approach leads to an increase of the anisotropy for x = 1.6 and 1.8 coming from finite size effects. Surface effects have been pointed out using the Bloch law fitting. Zero Filed Cooling (ZFC) measurements exhibit a blocking temperature (TB) range from 294 to 240 K. Kneller's law fitting of the coercive field dependence on the temperature exhibits a non-neglected interparticle interaction and a wide size distribution according to the difference found between TB and the Kneller transition temperature TBK.

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