Electrical transport properties of Fe 3- xCr xO 4 ferrite films on MgO (0 0 1) grown by molecular beam epitaxy
In this report, we fabricated a series of Fe 3- xCr xO 4(0≦x≦2) films by plasma-oxygen-assisted molecular beam epitaxy (MBE) and did structural and electrical characterizations of these films. These films show textured single phase quality and the lattice parameters are consistent with those of the bulk at low Cr composition ( x<0.9). However, the lattice parameters show severe deviation from the bulk value in the intermediate region of 0.9≦x≦1.5 and no diffraction can be resolved at x∼2. These discrepancies may be attributed to the cation distributions and the instability of spinal structure as Cr concentration becomes dominant. The resistivity presents a typical Arrhenius temperature dependence with ρ= ρ0 exp ( Ep/ kBT) indicating that the transport is due to a hopping mechanism. The prefactor ρ0 increases in Fe 3- xCr xO 4, at smaller x but tends to level out for x>1, suggesting that Cr 3+ ions may start to replace Fe 3+ ions at the A site in the high x region. The activation energy of electrical hopping gradually increases at low Cr concentration but abruptly rises to ∼110 meV at x>0.9, suggesting a crossover from electron-hopping mediated transport to a thermally activated band gap excitation.