Anisotropic Properties Of Epitaxial Iron Oxide Films And Heterostructures

Ferrites, including half-metallic Fe₃O₄ and spin-filtering NiFe₂O₄, CoFe₂O₄ andγ–Fe₂O₃, are of vital importance in spintronics due to the high Curie temperature and simple structures. Polycrystalline Fe₃O₄ films, epitaxial Fe₃O₄ films, epitaxial ?–Fe₂O₃ films and fully epitaxial Fe₃O₄/ZnO and Fe₃O₄/Nb:SrTiO₃ heterostructures were fabricated by using facing-target reactive sputtering. The structures and optical, magnetic and transport properties were analyzed systematically.It was found that the epitaxial growth of spinel Fe₃O₄ andγ–Fe₂O₃ can be realized by optimizing the deposition conditions such as single crystal substrates with small mismatch, smaller deposition rate and higher substrate temperatures. The magnetic and magnetotransport properties of the Fe₃O₄ films are governed by the antiferromagnetic coupling strength across the grain boundaries. Butterfly-shaped magnetoresistance (MR) in the epitaxial Fe₃O₄ films are closely related to the antiphase boundaries (APBs), and the low-field MR peaks at the coercivity of hysteresis loops and the high-field MR is linear to the magnetic field. The features of the low- and high-field MR were considered to be caused by the fact that the moments far away from APBs orient towards the direction of low magnetic fields and then those near the APBs align gradually at high fields. The magnetization of the epitaxialγ–Fe₂O₃ films with different thicknesses keeps the value of bulk (³90 emu/cm3), and their coercivity increases with the film thickness.The anomalous four-fold symmetric anisotropic magnetoresistance (AMR) that increases with the magnetic field was observed in the epitaxial Fe₃O₄ films, which can not be explained by traditional AMR theory. It is supposed that the four-fold AMR in the epitaxial Fe₃O₄ films is related to the magnetocrystalline anisotropy and native growth defect APBs. At low fields, the moments primarily orient towards the applied magnetic field and the scattering is mainly caused by the small-polaron hopping between the uncoupled grains (spins) far away from APBs. While the magnetic field rotates in film plane, the moments far away from APBs always keep the same direction with the magnetic field. Therefore, the scattering far away from APBs is not influenced by the magnetocrystalline anisotropy, leading to the traditional two-fold symmetric AMR. At high fields, the scattering mainly focuses among the moments near the APBs. The magnetocrystalline anisotropy field modifies the alignment of the spins near APBs further when the magnetic field rotates in film plane, leading to the oscillating scattering possibility of electrons along the hard or easy axes, i.e. four-fold symmetric AMR.It was demonstrated that the transport mechanism of the Fe₃O₄ (111)/ZnO (0001) heterostructure is thermal diffusion/emission for T<30 K. The Schottky barrier of 0.51 eV (30 K) was found to form at the interface between Fe₃O₄ and ZnO. The spin polarization of the transport electrons from Fe₃O₄ to ZnO was determined to be 28.5%. AMR of ⁸0% across the Schottky interface between the ferromagnetic Fe₃O₄ film and semiconductor Nb:SrTiO₃ was observed while the applied bias is close to the height of Schottky barrier. It can be understood that the magnetocrystalline anisotropy of the epitaxial Fe₃O₄ film significantly modifies the transport of electrons across the interface and leads to the sinusoidal four-fold and six-fold symmetric AMR, where the symmetry is consistent with that of the magnetocrystalline anisotropy of epitaxial Fe₃O₄ films.