Oxygen vacancies effects on phase diagram of epitaxial La<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3</Subscript> thin films

We investigated the effects of oxygen vacancies on the structural, magnetic, and transport properties of La_1–x Sr _x MnO₃ (x=0.1, 0.2, 0.33, 0.4, and 0.5) grown around a critical point (without/with oxygen vacancies) under low oxygen pressure (10 Pa) and high oxygen pressure (40 Pa). We found that all films exhibit ferromagnetic behavior below the magnetic critical temperature, and that the films grown under low oxygen pressures have degraded magnetic properties with lower Curie temperatures and smaller magnetic moments. These results show that in epitaxial La_1–x Sr _x MnO₃ thin films, the magnetic and transport properties are very sensitive to doping concentration and oxygen vacancies. Phase diagrams of the films based on the doping concentration and oxygen vacancies were plotted and discussed.     

Synthesis of Ordered Ultra-long Manganite Nanowires via Electrospinning Method

We develop a new electrospinning method to prepare ultra-long ordered La_(1-x)Sr_xMnO_3(LSMO) nanowires.The length is up to several centimeters and is only limited by the size of the collector.The well-ordered straight-line structure ensures the transport measurement,which is impossible to be carried out for the random nanowires fabricated by the traditional electrospinning method.Magnetic and transport measurements indicate that the physical properties of the LSMO nanowires depend sensitively on the doping concentration.At the optimum doping,the LSMO wires are ferromagnetic at room temperature with a metal-insulator transition temperature close to room temperature.Magnetic force microscopy studies are also performed to provide a microscopic view of these ultra-long nanowires.     

Surface protonation and oxygen evolution activity of epitaxial La_(1-x)Sr_xCoO_3 thin films

As an alternative electrode material, transition metal oxides are promising candidates due to multivalent nature and oxygen vacancies present in the structure with facilitate redox reactions. The aim of this study is to explore the intrinsic mechanism of oxygen evolution reaction(OER) using two-dimensional thin film La_(1-x)Sr_xCoO_3 electrode as a model. Herein, we report a planar two-dimensional model La_(1-x)Sr_xCoO_3 electrode grown on a Nb-SrTiO_3 single-crystal substrate via pulsed laser deposition. The two-dimensional La_(1-x)Sr_xCoO_3 films offer different oxygen evolution activities at different pH electrolyte solutions. The mechanisms behind the variations of the oxygen evolution activity were discussed after comparing the oxygen evolution activity before and after treatments of the electrodes and measurements by various test methods. The results of this study offer a promising, low-cost electrode material for the efficient OER and a sustainable production of hydrogen fuel.     

The electrical and magnetic properties of epitaxial orthorhombic YMnO3 thin films grown under various oxygen pressures

Orthorhombic YMnO₃ thin films were epitaxially grown on bare and LaNiO₃ buffered (0 0 1)-SrTiO₃ substrates by pulsed laser deposition under various oxygen pressures from 5 to 30 Pa. The crystal structure and microstructure of these films have been characterized by both X-ray diffractions and transmission electron microscopy. The leakage current, modeled as the space charge limited current (SCLC) mechanism, decreased significantly with the increase of oxygen content. It is further found that the magnetic property of films is greatly enhanced in YMnO₃ films grown under high oxygen pressure, which can be explained decreased oxygen vacancies. In addition, bipolar switching behavior was obtained only in the films grown under 30 Pa oxygen pressure, which is attributed to the decrease of voltage-driven oxygen vacancy migration.     

Metal-insulator transition in SrTiO(3-x) thin films induced by frozen-out carriers

We report optical, electrical and magnetotransport properties of oxygen deficient SrTiO(3) (SrTiO(3-x)) thin films fabricated by pulsed laser deposition technique. The oxygen vacancies (O(vac)) in the thin film are expected to be uniform. By comparing its electrical properties to those of bulk SrTiO(3-x), it was found that O(vac) in bulk SrTiO(3-x) is far from uniform over the whole material. The metal-insulator transition (MIT) observed in the SrTiO(3-x) film was found to be induced by the carrier freeze-out effect. The low temperature frozen state can be reexcited by Joule heating, electric and intriguingly magnetic field.     

Effect of Terraces at the Interface on the Structural and Physical Properties of La_(0.8)Sr_(0.2)MnO_3 Thin Films

Employing atomic force microscopy,transmission electron microscopy and the second harmonic generation technique,we carefully explore the structural properties of 6-unit-cell-thick La_(0.8)Sr_(0.2)MnO_3 films grown on SrTiO_3 with atomically flat TiO_2-terminated terraces on the surface.The results clearly demonstrate that the terraces on the surface of TiO_2-terminated SrTiO_3 can improve the layer-by-layer epitaxial growth of the manganite films,which results in uniform film coverage at the beginning of growth and thus reduces the substrate-induced disorder at or near the interface.Comparing the magnetic and transport properties of La_(0.8)Sr_(0.2)MnO_3 films with the thicknesses varying from 6 unit cells to 80 unit cells grown respectively on as-received SrTiO_3 and TiO_2-terminated SrTiO_3,it is found that these atomically flat terraces on the surface of TiO_2-terminated SrTiO_3 can greatly enhance the Curie temperature and conductivities of the ultrathin La_(0.8)Sr_(0.2)MnO_3 films with thickness less than 50 unit cells,while no obvious difference is detected in the magnetic and transport properties of the 80unit-cell thick films.     

过渡金属元素X(X=Mn,Fe,Co,Ni)掺杂对ZnO基阻变存储器性能的影响

实验表明掺杂是一种改善阻变存储器性能的有效手段,但其物理机理鲜有研究.本文采用第一性原理方法系统研究了过渡金属元素X(X=Mn,Fe,Co,Ni)掺杂对ZnO基阻变存储器中氧空位迁移势垒和形成能的影响.计算结果表明Ni掺杂可同时有效降低+1和+2价氧空位在掺杂原子附近的迁移势垒,X掺杂均减小了氧空位的形成能,特别是掺杂Ni时氧空位的形成能减小最为显著(比未掺杂时减少了64%).基于该结果制备了未掺杂和Ni掺杂ZnO阻变存储器,研究表明通过掺杂控制体系中氧空位的迁移势垒和形成能,可以有效改善器件的初始化过程、操作电压、保持性等阻变性能.研究结果有助于理解探究影响阻变的微观机制,并可为掺杂提高阻变存储器性能提供一定的理论指导.     

生长条件对脉冲激光沉积制备ZnO：Al薄膜光电性能的影响

本文研究了脉冲激光沉积(PLD)生长过程中, 铝掺量、氧压及衬底温度等实验参数对ZnO:Al(AZO)薄膜生长的影响, 并利用扫描电子显微镜、原子力显微镜、X射线衍射、霍尔效应、光透射光谱等实验手段对其透明导电性能进行了探讨. 变温霍尔效应和光透射测量表明, 当靶材中铝掺量大于0.5 wt%时, 所制备AZO薄膜中铝施主在80 K时已完全电离, 因Bernstein-Moss (BM) 效应其带隙变大, 均为重掺杂简并半导体. 进一步系统研究了氧压和衬底温度对AZO薄膜透明导电性能的影响, 实验发现当氧压为1 Pa, 衬底温度为200 ℃时, AZO 导电性能最好, 其霍尔迁移率为28.8 cm²/V·s, 薄膜电阻率最小可达2.7×10^-4 Ω·cm, 且在可见光范围内光透过率超过了85%. 氧压和温度的增加, 都会导致薄膜电阻率变大. 关键词： 脉冲激光沉积法 ZnO：Al薄膜 透光性 导电性     

Effect of oxygen pressure on properties of Bi---Sr---Ca---Cu---O thin films grown by laser deposition

We present the results of a study of the effect of oxygen pressure on the properties of Bi---Sr---Ca---Cu---O (BSCCO) thin films grown by laser ablation. Films of the same phase, which were deposited at different oxygen pressures, show quite different properties. Here, we define the phases of BSCCO films from their c-axis lattice constants calculated from the 2θ X-ray diffraction angle of the 002 peak. In-situ films deposited at high oxygen pressure (≥ 100 mTorr) show higher critical temperatures and critical current densities. However, by post annealing, transport properties of films deposited at low oxygen pressure (several 10 mTorr) are improved significantly. With respect to this difference, the results of X-ray diffraction and energy dispersive X-ray spectroscopy studies, and Hall effect measurements are discussed.     

氧气分压对TiO2薄膜结构、磁性及输运性质的影响

采用脉冲激光沉积技术在LaAlO_3(100)基片上制备了TiO_2薄膜,研究了氧气分压对薄膜结构、磁性与输运性质的影响.结构测量表明,TiO_2薄膜的结构与沉积过程中的氧气分压有关,氧气分压的增大有利于薄膜向锐钛矿相转变.磁性测量表明,在较高的氧气分压下制备的TiO_2薄膜表现为顺磁性,在较低氧气分压下制备的TiO_2薄膜表现出明显的室温铁磁性,其铁磁性与氧空位有密切关系.输运测量进一步表明,TiO_2薄膜表现为半导体导电特性,在具有铁磁性的薄膜中还观察到了低温磁电阻效应.     

The coexistence of ferroelectricity and ferromagnetism in Mn-doped BaTiO3 thin films

Due to the fault of the first author, this article entitled “The coexistence of ferroelectricity and ferromagnetism in Mn-doped BaTiO3 thin films”, published in “Chinese Physics B”, 2011,Vol.20, Issue 12, Article No. 127701, has been found to copy from the article entitled“Decisive role of oxygen vacancy in ferroelectric versus ferromagnetic Mn-doped BaTiO3 thin films”, published in “Journal of Applied Physics”, 2011,Vol.109, Issue 8, article No. 084105. So the above article in “Chinese Physics B” has been withdrawn from the publication.<     

Phase separation and abnormal transport behaviours in La0.7-xGdxSr0.3MnO3 system

The magnetic and transport behaviours of the La_{0.7-x}Gd_xSr_{0.3}MnO_3 (0≤x≤0.70) system are investigated. The experimental results indicate that with increasing Gd doping content, the magnetism of the system changes from the long-range ferromagnetic order state to the cluster-spin glass state, then to the antiferromagnetic (AFM) state. It is interesting that the phase separation appears at x=0.30 and 0.40 and disappears for x≥0.50 where the AFM state occurs. At high doping content, the transport behaviours exhibit abnormality, e.g. there are two temperature ranges in which the ρ－T curves can be well fitted by a variable-range hopping (VRH) model. We suggest that the VRH does not come from the hopping of carriers between clusters, but from the different magnetic backgrounds in the clusters.     

Room-temperature ferromagnetism induced by Cu vacancies in Cu_x(Cu_2O)_(1-x) granular films

Cux(Cu2O)1-x(0.09 x 1.00) granular films with thickness about 280 nm have been fabricated by direct current reactive magnetron sputtering. The atomic ratio x can be controlled by the oxygen flow rate during Cux(Cu2O)1-x deposition. Room-temperature ferromagnetism(FM) is found in all of the samples. The saturated magnetization increases at first and then decreases with the decrease of x. The photoluminescence spectra show that the magnetization is closely correlated with the Cu vacancies in the Cux(Cu2O)1-x granular films. Fundamentally, the FM could be understood by the Stoner model based on the charge transfer mechanism. These results may provide solid evidence and physical insights on the origin of FM in the Cu2O-based oxides diluted magnetic semiconductors, especially for systems without intentional magnetic atom doping.     

显微拉曼研究外延生长的BaTiO_(3-x)薄膜的结构相变与结构动力学(英文)

我们用显微拉曼、卢瑟福背散射谱、X射线散射和非对称摇摆等实验手段研究了在1 0 - 2 到1 0 - 5帕氧气压下用激光分子束外延技术生长的BaTiO3-x薄膜的结构动力学特性。测量结果表明生长氧压越低,晶格常数c和c/a越大,晶格常数a稍微减小,晶胞体积变大。随着生长氧压的减小,薄膜中氧含量减小。在3 0×1 0 - 5帕氧气压下生长的薄膜中氧缺陷可达0 48,但是样品的四方相结构可以很好的维持。显微拉曼研究进一步确定了样品都是四方相结构。另外,在BaTiO2 52 薄膜的拉曼光谱中发现高频段有两个新峰,其可能是由于氧缺陷导致的二级拉曼散射引起的。随着生长氧压的减小,拉曼峰向低频移动,表明薄膜中的应力减小。同时,拉曼峰变宽,这可能是由于氧缺陷导致的结构畸变引起的。由于在薄膜中存在二维张应力,BaTiO3单晶样品中的结构相变特征在我们的样品中从78到5 5 0K的温度范围内不存在。     

Transport Behaviour of La0.8Sr0.2AlO3 Thin Film on Oxygen Deficient SrTiO3 Substrate

La_0.8Sr_0.2AlO₃ (LSAO) thin films are grown on SrTiO₃ (STO) and MgO substrates by laser molecular beam epitaxy. The LSAO thin film on oxygen deficient STO substrate exhibits metallic behaviour over the temperature range of 80--340K. The optical transmittance spectrum indicates that theLSAO thin films on MgO substrate are insulating at room temperature. The transport properties of LSAO thin films on STO substrates deposited in different oxygen pressure are compared. Our results indicate that oxygen vacancies in STO substrates should be mainly responsible for the transport behaviour of LSAO thin films.     

Optimization of Pr(0.9)Ca(0.1)MnO(3) thin films and observation of coexisting spin-glass and ferromagnetic phases at low temperature

Optimization of thin films of small bandwidth manganite, Pr(1-x)Ca(x)MnO3 (for x = 0.1), and their magnetic properties are investigated. Using different pulsed laser deposition (PLD) conditions, several films were deposited from the stoichiometric target material on SrTiO3 (001) substrate and their thorough structural and magnetic characterizations were carried out using x-ray diffraction, atomic force microscopy, x-ray photoelectron spectroscopy (XPS), SQUID magnetometry and ac susceptibility measurements. A systematic investigation shows that irrespective of the growth temperature (between 550 and 750?°C), all the as-deposited films have twin boundaries and magnetic double phases. Post-annealing in partial or full oxygen pressure removes the extra phase and the twin boundaries. Zero-field-cooled magnetization data show an antiferromagnetic to paramagnetic transition at around 100 K whereas the field-cooled magnetization data exhibit a paramagnetic to ferromagnetic transition close to 120 K. However, depending on the oxygen treatments, the saturation magnetization and Curie temperature of the films change significantly. Redistribution of oxygen vacancies due to annealing treatments leading to a change in ratio of Mn3+ and Mn4+ in the films is observed from XPS measurements. Low temperature (below 100 K) dc magnetization of these films shows metamagnetic transition, high coercivity and irreversibility magnetizations, indicating the presence of a spin-glass phase at low temperature. The frequency dependent shift in spin-glass freezing temperature from ac susceptibility measurement confirms the coexistence of spin-glass and ferromagnetic phases in these samples at low temperature.     

金属有机物化学气相法制备YBCO超导薄膜研究

运用金属有机物化学气相沉积法(MOCVD),在LaAlO3(LAO)单晶上沉积YBa2Cu3O7-x(YBCO)超导薄膜。通过使用优化的进液装置,使金属有机源连续、稳定地输送至蒸发皿进行闪蒸。通过优化总气压、氧分压等生长条件,获得高质量的YBCO薄膜。在固定的温度条件下,调节反应总气压和氧分压,在总压为380Pa,氧分压为180Pa获得YBCO薄膜临界电流密度Jc=0.6MA/cm2。随着氧分压增大,YBCO薄膜产生a轴取向,(005)峰向左偏移,且薄膜中的Cu/Ba由1.0变化至1.63。在Cu/Ba=1.48时,YBCO薄膜结构与性能较优。     

Structural and optical properties of Er, Yb co-doped Y2O3 thin films

Thin Er³⁺, Yb³⁺ co-doped Y₂O₃ films were grown on (1 0 0) YAG substrates by pulsed laser deposition. Ceramic targets having different active ion concentration were used for ablation. The influence of the rare-earth content and oxygen pressure applied during the deposition on the structural, morphological and optical properties of the films were investigated. The films deposited at the lower pressure, 1 Pa, and at 1/10 Er to Yb doping ratio are highly textured along the (1 1 1) direction of the Y₂O₃ cubic phase. In addition to the crystalline structure, these films possess smoother surface compared to those prepared at the higher pressure, 10 Pa. All other films are polycrystalline, consisting of cubic and monoclinic phases of Y₂O₃. The rougher surface of the films produced at the higher-pressure leads to higher scattering losses and different behavior of the reflectivity spectra. Optical anisotropy in the films of less than 0.004 was measured regardless of the monoclinic structure obtained. Waveguide losses of about 1 dB/cm at 633 nm were obtained for the films produced at the lower oxygen pressure.     

Sign reversal of anisotropic magnetoresistance and anomalous thickness-dependent resistivity in Sr2CrWO6/SrTiO3 films

High-quality Sr₂CrWO₆ (SCWO) films have been grown on SrTiO₃ (STO) substrate by pulsed laser deposition under low oxygen pressure. With decrease of the film thickness, a drastic conductivity increase is observed. The Hall measurements show that the thicker the film, the lower the carrier density. An extrinsic mechanism of charge doping due to the dominance of oxygen vacancies at SCWO/STO interfaces is proposed. The distribution and gradient of carrier concentration in SCWO films are considered to be related to this phenomenon. Resistivity behavior observed in these films is found to follow the variable range hopping model. It is revealed that with increase of the film thickness, the extent of disorder in the lattice increases, which gives a clear evidence of disorder-induced localization charge carriers in these films. Magnetoresistance measurements show that there is a negative magnetoresistance in SCWO films, which is considered to be caused by the magnetic scattering of magnetic elements Cr³⁺ and W⁵⁺. In addition, a sign reversal of anisotropic magnetoresistance (AMR) in SCWO film is observed for the first time, when the temperature varies across a characteristic value, T_M. Magnetization—temperature measurements demonstrate that this AMR sign reversal is caused by the direction transition of easy axis of magnetization from the in-plane ferromagnetic order at T > T_M to the out-of-plane at T < T_M.     

Effects of growing conditions on the electric and magnetic properties of strained La2/3Sr1/3MnO3 thin films

We investigate the growing condition dependences of magnetic and electric properties of the La_2/3Sr_1/3MnO₃ thin films grown on SrTiO₃（001） substrates.With reducing the film thickness and growth pressure,the Curie temperature（T_c）drops off,and the magnetism and metallicity are suppressed.At an appropriate deposition temperature,we can obtain the best texture and remarkably enhance the magnetic and electrical properties.However,the resistivity of film cannot be modulated by changing the dc current and green light intensity.This result may be induced by the coherent strains in the epitaxially grown film due to its lattice mismatching that of the SrTiO₃ substrate.Furthermore,we show that the relations between the magnetism and the resistivity for the typical films with different thickness values.For the 13.4-nm-thick film,the R-T curve presents two transition behaviors:insulator-to-metal and metal-to-insulator in the cooling process:the former corresponds to magnetic transition,and the later correlates with thermal excitation conduction.