【作者】 王平；

【导师】 金朝；

【作者基本信息】 天津大学 ， 材料物理与化学， 2017， 硕士

【摘要】 尖晶石铁氧体是强关联体系过渡金属氧化物的典型代表之一,在自旋电子学器件中具有重要的应用,其基态的自旋、电荷、晶格和轨道自由度相互耦合,对应力十分敏感。因此,利用应力对其物理性质进行调控,实现其性质的优化,可以促进自旋电子学器件的应用与发展。利用直流磁控反应溅射技术在SrTiO₃和MgO基片上生长了不同Lu含量的Lu_xFe_3–xO₄（0?x?0.26）外延薄膜,研究了静态应力对尖晶石铁氧体薄膜结构、磁性和输运特性的调控。SrTiO₃和MgO基片上生长的Lu_xFe_3–xO₄薄膜分别受到面内压应力和张应力作用。相比于MgO基片上生长的Lu_xFe_3–xO₄薄膜,SrTiO₃基片上的样品表现出较大的饱和磁化强度,较小的交换偏置和矫顽力,其各向异性磁电阻也出现了相位移。随Lu含量的增加,SrTi O₃基片上生长的Lu_xFe_3–xO₄薄膜的c轴晶格常数呈现减小趋势,而在MgO基片上呈现增加的趋势。Lu含量的增加引起薄膜内的自旋倾斜增强,使Lu_xFe_3–xO₄薄膜的交换偏置和矫顽力增加,同时也增强了各向异性磁电阻的四重对称性和平面霍尔效应的二重对称性。进一步研究了动态应力调控尖晶石铁氧体薄膜的磁性质。利用直流磁控反应溅射技术在（011）-Pb(Mg_1/3Nb_2/3)_0.7Ti_0.3O₃单晶基片上生长具有不同氧空位密度的CoFe₂O₄薄膜。在电场作用下,由于基片的应力效应,CoFe₂O₄薄膜的磁化强度在[01?1]方向减小而在[100]方向增加,这源于应力诱导了尖晶石铁氧体中的八面体畸变。此外,较高氧流量（10 sccm）条件下生长的CoFe₂O₄薄膜中仅存在两种不同的磁化状态,而在较低氧气流量（4 sccm）条件下生长的CoFe₂O₄薄膜中却出现了四种不同的磁化状态,额外的两种磁化状态来源于铁电场调控效应。铁电场效应改变了CoFe₂O₄薄膜中的氧空位分布,影响了薄膜的超交换作用,使得移除正负电场之后样品的磁化强度不同。应力和铁电场效应的共同作用实现了CoFe₂O₄薄膜中磁化强度的四重态调控。更多还原

【Abstract】 Spinel ferrite,as a typical representative of transition metal oxides in the strong correlation system,has important applications in spintronic devices.Spinel ferrite exhibits large sensitivity to strain due to the coupled spin,charge,lattice and orbital degrees of freedom.As a consequence,mediating and optimizing the physical properties of spinel ferrite by strain will promote the application and development of spintronic devices.We fabricated the epitaxial Lu_xFe_3–xO₄（0?x?0.26）films on SrTiO₃ and MgO substrates by DC magnetron reactive sputtering,and investigated the static strain mediated structure,magnetic and transport properties of spinel ferrite films.The epitaxial Lu_xFe_3–xO₄ films are grown on SrTiO₃ and MgO substrates with in-plane compressive and tensile strains,respectively.The Lu_xFe_3–xO₄ films on SrTiO₃substrates exhibit larger saturation magnetization,smaller exchange bias and coercivity.Phase shift of anisotropic magnetoresistance is also observed in the Lu_xFe_3–xO₄ films on SrTiO₃ substrates.In addition,with the increasing Lu content,the out-of-plane lattice constant of the Lu_xFe_3–xO₄ films on SrTiO₃ substrates decreases while that on MgO substrates increases.The increase of Lu content enhances the spin canting,which further increases the exchange bias and coercivity,strengthens the four-fold symmetry of anisotropic magnetoresistance and the two-fold symmetry of planar Hall effect.Dynamic strain mediated magnetic property was further studied in spinel ferrite films with different densities of oxygen vacancies,grown on the（011）-Pb(Mg_1/3Nb_2/3)_0.7Ti_0.3O₃ single crystal substrates by DC magnetron reactive cosputtering.Owing to the volatile strain effect of substrate,the magnetization of the CoFe₂O₄ films decreases along[01?1]direction whereas it increases along[100]direction under the electric field,which attributes to the octahedron distortion in the spinel ferrite.Moreover,only two different magnetization states exist in the CoFe₂O₄film under high oxygen flow rate（10 sccm）while four different magnetization states exist in the CoFe₂O₄ film under low oxygen flow rate（4 sccm）.The additional two different magnetization states derive from the ferroelectric-field mediation effect.It changes the oxygen vacancy distributions and influences the superexchange interactions in the CoFe₂O₄ film.The magnetization of the sample is different after removing the positive and negative fields.The cooperation of strain and ferroelectric-field effects achieves mediation of four different magnetization states in the CoFe₂O₄ film.更多还原