Electrical transport properties of Fe3−xCrxO4 ferrite films on MgO (0 0 1) grown by molecular beam epitaxy

In this report, we fabricated a series of Fe_3−xCr_xO₄(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 (E_p/k_BT) indicating that the transport is due to a hopping mechanism. The prefactor ρ₀ increases in Fe_3−xCr_xO₄, at smaller x but tends to level out for x>1, suggesting that Cr³⁺ ions may start to replace Fe³⁺ 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.