Anisotropic Magnetotransport Properties Of La_1-xSr_xCoO₃ Epitaxial Films And Fe₃O₄/NiO Epitaxial Bilayers

Multifunctional oxides attract more and more attentions due to the complicated magnetotransport properties resulting from the strong coupling among spin, orbit, and lattice. The research on the multifunctional oxides was mainly based on the monolayer films and the artificial heterostructures. The perovskites cobaltites films possess special magnetotransport properties, and the anisotropy stemming from the interface of ferromagnet?FM? and antiferromagnet?AFM? has been intensively explored recently for the spintronic applications of spin valves and magnetic tunneling junctions, etc. We fabricated the spontaneous magnetic phase-separated La_1-xSr_xCoO₃ epitaxial films and the epitaxial bilayers with ferromagnetic Fe₃O₄ and antiferromagnetic Ni O layers by rf-magnetron sputtering and dc reactive magnetron sputtering, respectively. Their structure, magnetic and anisotropic transport properties were investigated systematically.In phase-separated ferromagnetic/spin-glass La_1-xSr_xCoO₃ epitaxial films?x=0.07, 0.17, 0.26, 0.30, 0.40, 0.60?, the configuration with high-spin?HS? state Co3+ and low-spin?LS? state Co4+ was determined to be the most stable spin-state configuration at ground state, which was demonstrated by measuring the magnetic moments of La_1-xSr_xCoO₃, as well as the first principle calculations. The stretched Co-O bond by Sr doping causes the decrease of crystal field splitting, resulting in the HS state Co3+ and LS state Co4+. The spin dynamics in the La_1-xSr_xCoO₃ thin films was found to be faster than that in classic spin-glass compounds, which is ascribed to the rather small ferromagnetic cluster size?².16 to ²1.5 nm? in the epitaxial films compared to polycrystalline compounds?³5 to ²40 nm?.In the magnetic phase separated La_1-xSr_xCoO₃?0.07?x?0.60? epitaxial films, the crystalline anisotropy and spin-orbital coupling are the main contributions to the unexpected fourfold symmetric planar Hall effect?PHE?. The uniaxial anisotropy field and cubic anisotropic field were calculated to be Huni=70 Oe and Hcub=143 Oe, respectively. The magnetic anisotropy was weakened by Sr doping, which corresponds with the disappearance of the fourfold symmetry in PHE with the increasing Sr concentration. The first principle calculations proved that the contribution of Co-d orbitals to the magnetic anisotropy strongly depends on the Sr concentration. The eg-dx2-y2 and eg-d3z2-r2 orbitals play a dominant role in the magnetic anisotropy of the samples with x=0.125, 0.25, while the t2g-dxy,dyz,dxz orbitals contribute mainly to the magnetic anisotropy of the samples with x=0.375, 0.5, 0.625.An unexpected fourfold symmetric PHE was observed in the Fe₃O₄/Ni O epitaxial bilayers. As the thickness of the antiferromagnetic layer exceeds ³7 nm, the PHE of the bilayer shifts to twofold symmetry, which is considered to originate from the dying interfacial coupling effect with increasing antiferromagnetic layer thickness. According to the fitting based on Stoner-Wohlfarth model, it was found that an extra cubic anisotropic filed in the bilayers was obviously amplified by attenuating the thickness of the antiferromagnetic layer. First principle calculations revealed that the amplified cubic anisotropic filed comes from the interfacial bonding structure and charge transfer.