The Cation Distribution and Related Electrical Transport in $({\rm Fe}_{3-{x}}{\rm Mn}_{x}{\rm O}_{4})$ Ferrite Films on MgO Substrate Grown by Molecular Beam Epitaxy

We fabricated a series of Fe_3-xMn_xO₄ (0 ≤ x ≤ 1.5) films by plasma-oxygen-assisted molecular beam epitaxy and did magnetic and electrical characterizations of these films. The magnetization measurement shows that the saturation magnetization (Ms) is consistent with the curve of minimum possible Ms. Accordingly, the present result suggests that Mn³⁺ replace Fe³⁺ mainly in the B-site, which is basically different from the bulk distribution mainly in the A-site. Resistance as a function of temperature in the range of 80-300 K is carried out for all films. 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 with x in Fe_3-xMn_xO₄ at smaller x but tends to level out at x >; 0.6, suggesting that the structure varies from a inverse spinel to a normal spinel in that the Fe²⁺/Fe³⁺ ratio remains constant at x >; 0.6. The activation energy E_p of electrical hopping remains a constant value ~50 meV at x <; 0.9 but abruptly rises up at x >; 0.9 and reaches 300 meV at x=1.5 , suggesting a semiconductor-insulator transformation. This increase in the activation energy may indicate a percolation limit which is different from the previous transformation observed in other ferrites such as Fe_3-xMn_xO₄.