Electrical properties and phase transition of CoFe[sub 2]O[sub 4] nanocrystals under pressure.

The electric resistance CoFe[SUB2]O[SUB4] 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 CoFe[SUB2]O[SUB4] is related to the high d-electron coupling between magnetic ions and to the small band gap in CoFe[SUB2]O[SUB4] 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 CoFe[SUB2]O[SUB4] nanocrystal decreases exponentially under pressure, according to R∝exp(-CP), where C = 0.7634 Gpa[SUP-1] for large nanocrystals (80 nm) and C = 0.5124 Gpa[SUP-1] for CoFe[SUB2]O[SUB4] of 6 nm. [ABSTRACT FROM AUTHOR]