Electrical properties and phase transition of CoFe2O4 nanocrystals under pressure

The electric resistance ${\mathrm{CoFe}}_2{\mathrm{O}}_4$ nanocrystals of different sizes, obtained by the coprecipitation method was investigated under pressure up to 20 GPa in a diamond anvil cell at ambient temperature. The experimental results indicate that the phase transition (from the spinel to a tetragonal structure) takes place at 7.5 and 12.5 GPa for 6 and 80 nm, nanocrystals, respectively, in agreement with the prediction of scaling theory of the phase transition of nanocrystals. The relatively low transition pressure for ${\mathrm{CoFe}}_2{\mathrm{O}}_4$ is related to the high d-electron coupling between magnetic ions and to the small band gap in ${\mathrm{CoFe}}_2{\mathrm{O}}_4$ as compared to the other spinel ferrites. These results are discussed in terms of decreasing band gap with increasing pressure. The smaller the size of nanocrystals, the higher the phase transition pressure. The resistance of ${\mathrm{CoFe}}_2{\mathrm{O}}_4$ nanocrystal decreases exponentially under pressure, according to $\mathrm{R\propto }\exp \mathrm{(-CP),}$ where $\text{C=0.7634\hspace{0.05em}}{\mathrm{GPa}}^{\text{−1}}$ for large nanocrystals (80 nm) and $\text{C=0.5124\hspace{0.05em}}{\mathrm{GPa}}^{\text{−1}}$ for ${\mathrm{CoFe}}_2{\mathrm{O}}_4$ of 6 nm.