Turning point identification via exploring the thickness dependence of magnetoresistance in polycrystalline Fe3O4 thin films

Stoichiometric polycrystalline Fe3O4 films with thicknesses ranging from 25 to 165 nm were fabricated on Si (100) substrates with a native SiO2 layer via pulsed laser deposition, followed by vacuum annealing at 780 °C. Negative magnetoresistance was observed, and the absolute magnetoresistance under an applied magnetic field of 0.9 T (|MR|0.9 T) was systematically investigated. In contrast to previous studies, the maximum |MR|0.9 T of 2.68% was achieved at a thickness of 50 nm. Films with thicknesses in the range of 40–88 nm retained relatively high |MR|0.9 T values. However, when the thickness increased to 100 nm, |MR|0.9 T dropped sharply to 1.70% and then decreased gradually with further increases in thickness. Raman characterization and magnetoresistance curve fitting revealed that the largest |MR|0.9 T value corresponded to the lowest density of antiphase boundaries. Additionally, the Verwey transition temperature (TV) derived from temperature-dependent resistance curves was closest to that of bulk Fe3O4 when the film thickness was in the range of 45–55 nm.