Microstructure, Magnetic And Transport Properties Of Polycrystalline Fe₃O₄ Films And Its Composite Structure

The effect of TMR is believed to origin from the electron spin-dependent tunneling in granular films and magnetic tunnel junctions and correlates with the spin polarization of electrons. Therefore, the materials with high spin polarization are expected to be explored. The half-metals with 100% spin polarization have been the promising spin injection materials for spintronic devices. Among the half-metals, the advantages of magnetite (Fe₃O₄) including the relatively high Curie temperature (Tc=858 K), low deposition temperature and ultra-thinness technique have made it a focus in the field of spintronics recently.In this thesis, polycrystalline Fe₃O₄ films, Fe₃O₄/Al (Mo) bilayers and Zn (Mn)-doped Fe₃O₄ films were prepared by facing-target reactive sputtering. Microstructure, magnetic properties and transport properties including magnetoresistance and Hall effect were studied systematically.The polycrystalline Fe₃O₄ film was prepared in O2 and Ar mixed atmosphere by facing-target reactive sputtering without substrate heating during the deposition. Structure analyses reveal that uniform Fe₃O₄ grains are well isolated by grain boundaries and grow with columnar structure. The room-temperature magnetization of the Fe₃O₄ films is 296 emu/cm3 under 50 kOe magnetic fields, which is much lower than that of the bulk value (471 emu/cm3) and was not saturated in such a high magnetic field. The negative magnetoresistance up to–11.7% at 80 K and–7.4% at 300 K under 90 kOe magnetic field perpendicular to the film plane were observed and discussed. The insaturation magnetization, loop shift at low temperatures and larger MR stem from the strong antiferromagnetic coupling due to the presence of lots of APBs. The transport mechanism is tunneling in the polycrystalline Fe₃O₄ films.The Fe₃O₄/Al (Mo) bilayers were fabricated with facing-target reactive sputtering system and the enhancement of magnetization in Fe₃O₄/Al (Mo) bilayers was observed. The magnetization under 50 kOe fields by inserting 30 nm Al-UL is 450 emu/cm3, nearly reaching the bulk value of magnetite (471 emu/cm3) and keeps the value with the increasing thickness of Al-UL. The magnetization of the bilayer with 30 nm Mo-UL is...