(BiFeO3)25/(La0.7Sr0.3MnO3)25 多层膜的光学和电学性质

采用射频磁控溅射方法在(001)SrTiO₃衬底上制备(001)取向的(BiFeO₃)₂₅/(La_0.7Sr_0.3MnO₃)₂₅多层膜.光学测试结果表明,1.3-2.1 eV范围内,相对于衬底而言多层膜光吸收增强; BiFeO₃的带隙为2.7 eV. 另外,结合绝缘介质导电模型分析了所测得的电流-电压数据,在所测试的温度及电压下,所制备的(BiFeO₃)₂₅/(La_0.7Sr_0.3MnO₃)₂₅多层膜的导电机理由空间电荷限制电导主导. 关键词： 多层膜 吸光度 空间电荷限制电导     

(La0.7Sr0.3MnO3)m(BiFeO3)n超晶格间隔的La0.7Sr0.3MnO3三明治结构制备及表征

采用射频磁控溅射的方法在SrTiO₃（001） 基片上制备了（La_0.7Sr_0.3MnO₃）_m（BiFeO₃）_n超晶格间隔的La_0.7Sr_0.3MnO₃三明治结构.X射线衍射分析证明（La_0.7Sr_0.3MnO_{关键词： 超晶格薄膜 电诱导效应 隧道效应}     

La0.7Ce0.3MnO3 epitaxial films fabricated by a pulsed laser deposition method

La_0.7Ce_0.3MnO₃ epitaxial films were successfully fabricated via a pulsed laser deposition method by controlling the experimental conditions. A series of experiments with varying the oxygen pressure and the substrate temperature demonstrated that the use of appropriate conditions is crucial for fabricating the epitaxial thin films. The existence of such suitable conditions was thermodynamically interpreted in terms of the stability of Mn²⁺ ion. Both XRD and EPMA measurements indicated that La_0.7Ce_0.3MnO₃ thin films fabricated herein form single phases, although it was difficult to present the direct experimental evidence to prove that Ce ion can really exist within the perovskite structure. The resultant films with oxygen annealing showed a metal-insulator transition and ferromagnetic property with Curie temperature of 275 K.     

Ti/Pr0.7Ca0.3MnO3/La0.67Sr0.33MnO3/Pt异质结电致电阻效应的改善

采用脉冲激光沉积技术制备了Ti/Pr_0.7Ca_0.3MnO₃/Pt和Ti/Pr_0.7Ca_0.3MnO₃/La_0.67Sr_0.33MnO₃/Pt异质结并研究了La_0.67Sr_0.33MnO₃功能插层对异质结电致电阻特性的影响. 实验结果表明La_0.67Sr_0.33MnO₃功能层的引入有效提高了器件的电阻转变特性,尤其是电阻转变率和疲劳性得到了极大的改善. 对La_0.67Sr_0.33MnO₃插层改善电致电阻转变特性的机理进行了定性的分析. 关键词： 电致电阻效应 电阻转变比率 疲劳特性     

Electrical and magnetic behavior of La0.7Ca0.3MnO3/La0.7Sr0.2Ca0.1MnO3 composites

The electrical transport properties and the magnetoresistance of La_0.7Ca_0.3MnO₃/La_0.7Sr_0.2Ca_0.1MnO₃ composites are investigated as a function of sintering temperature. On the basis of an analysis by X-ray powder diffraction and scanning electron microscopy we suggest that raising the sintering temperature enhanced the interfacial reaction and creates interfacial phases at the boundaries of the La_0.7Ca_0.3MnO₃ and La_0.7Sr_0.2Ca_0.1MnO₃. Results also show that in 3 kOe, and at the Curie temperature, the magnetoresistance value of 14% was observed for the composite sintered at 1300 °C. Based on the phenomenological equation for conductivity under a percolation approach, which depends on the phase segregation of ferromagnetic metallic clusters and paramagnetic insulating regions, we fitted the experimental resistivity—temperature data from 50-300 K and find that the activation barrier decreases as temperature is increased.     

Influence of microstructures on exchange bias behaviors of La0.7Sr0.3MnO3/La0.33Ca0.67MnO3 bilayers

Ferromagnetic La_0.7Sr_0.3MnO₃ (LSMO) and antiferromagnetic La_0.33Ca_0.67MnO₃ (LCMO) layers were grown on SrTiO₃ (STO) substrates by the pulsed laser deposition technique. LSMO films had rougher surfaces and larger grain sizes than LCMO films. Fully strained bilayers, in which each layer was as thin as 10 nm, were prepared by changing their stacking sequences, i.e. LSMO/LCMO/STO and LCMO/LSMO/STO. The former had higher T_C (350 K) than the latter (300 K), and exchange bias effects were only observed in the former bilayers. This revealed that microstructures could play an important role in the transport and magnetic properties of manganese oxide thin films.     

Inelastic magnetic X-ray scattering from highly correlated electron systems: La1.2Sr1.8Mn2O7, La0.7Sr0.3MnO3 and Fe3O4

Magnetic Compton profiles have been measured for the colossal magnetoresistance manganites La_1.2Sr_1.8Mn₂O₇ and La_0.7Sr_0.3MnO₃, and for magnetite Fe₃O₄, along various crystallographic directions, over a wide range of temperatures and magnetic fields. The experimental results are interpreted via first-principles computations for the double layer manganite, La_1.2Sr_1.8Mn₂O₇, and by using a simple model involving atomic d-orbitals and free electrons for the other two compounds. For all three materials a preference for the occupation of e_g orbitals is found, particularly, for orbitals of dx²−y² symmetry. An itinerant electron contribution is adduced at all temperatures in magnetite; such a contribution also appears in La_1.2Sr_1.8Mn₂O₇, but it is present only at low temperatures in La_0.7Sr_0.3MnO₃.     

Spin polarized La0.7Sr0.3MnO3 thin films on silicon

La_0.7Sr_0.3MnO₃ polycrystalline manganite thin films were grown on silicon (Si) substrates covered by SiO_x amorphous native oxide. Curie temperatures of about 325 K were achieved for 70-nm-thick films. Strong room temperature XMCD signal was detected indicating high spin polarization at the surface. Cross-sectional TEM images show sharp interface between SiO_x and manganite without signature of chemical reaction at the interface. Unusual sharp splitting of the manganite film was observed: on the top of a transition layer characterized by low crystalline order, a magnetically robust layer is formed.     

(La0.7Sr0.3MnO3 )m(BiFeO3)n 超晶格结构的导电机理

利用射频磁控溅射的方法在SrTiO₃(001) 基片上制备了(La_0.7Sr_0.3MnO₃)_m(BiFeO₃)_n超晶格结构.对所制备的超晶格结构进行了50—150℃温度范围内的电流-电压测试分析.结果表明,随着BiFeO₃薄膜的厚度减小,温度的升高,(La_0.7Sr_0.3MnO₃)_m(BiFeO₃)_n超晶格结构的电流变大.进一步根据介质导电模型对(La_0.7Sr_0.3MnO₃)_m(BiFeO₃)_n超晶格结构的导电特性做了分析.在温度较低或者电场较弱时,所制备的(La_0.7Sr_0.3MnO₃)_m(BiFeO₃)_n超晶格结构表现为欧姆导电,而在高温,高电场的情况下,其导电行为由空间电荷限制电流机理主导. 关键词： 超晶格薄膜 多铁 空间电荷限制电流     

Electroresistive and magnetoresistive properties of Nd0.7Sr0.3MnO3 after quenching under pressure of 9 GPa

Polycrystalline Nd_0.7Sr_0.3MnO₃ was quenched from 1300 K to 300 K and 80 K after it had been subjected to a high quasihydrostatic pressure of 9 GPa. Such high pressure and high temperature treatment (HPT) results in significant changes of the crystallochemical parameters—Mn-O lengths and Mn-O-Mn angles within unchanged lattice symmetry of the Pnma-type. A strong increase of the resistivity and a large decrease of the FM-PI transition temperature were detected for the Nd_0.7Sr_0.3MnO₃ HPT treated samples. The intrinsic characteristic T_MI(T_C) (T_MI is the metal-insulator and T_C is the ferromagnetic-paramagnetic transition temperature) correlates with the change of the Mn-O(1)-Mn angle, which is consistent with the double exchange model of the ferromagnetic metallic state in manganites. Remarkable electroresistive (ER) and magnetoresistive (MR) effects appear after HPT treatment, which were not present in the starting Nd_0.7Sr_0.3MnO₃ sample. The structure sensitive properties such as resistivity, MR and ER effects correlate with the change of the nanograin sizes after HPT treatment. Nonlinear current-voltage characteristics showing a hysteresis appear for HPT treated samples at low temperatures. The transport in granular Nd_0.7Sr_0.3MnO₃ samples is likely defined by spin-dependent scattering of charge carriers inside the ferromagnetic metallic grains with embedded small charged isolating islands and by jumping over charged insulating barriers at the intergrain boundaries, which can be strongly affected by the external electric and magnetic fields.     

Structure and magnetic order in La0.7Ca0.3Mn0.5Co0.5O3 and La0.8Sr0.2Mn0.5Co0.5O3 perovskites

In this paper we report the study of the perovskites La_0.7Ca_0.3Mn_0.5Co_0.5O₃ and La_0.8Sr_0.2Mn_0.5Co_0.5O₃ by neutron powder diffraction at various temperatures and magnetization measurements in zero applied field and at low cooling regimes. The replacement of half Mn by Co in La_0.7Ca_0.3MnO₃ and La_0.8Sr_0.2MnO₃ destroys their long-range ferromagnetism exhibiting a cluster glass ferromagnetic order similar to the one observed in many cobaltites.     

Structural, magnetic and magnetotransport behavior of La0.7SrxCa0.3−xMnO3 compounds

A systematic investigation of the structural, magnetic and electrical properties of a series of nanocrystalline La_0.7Sr_xCa_0.3−xMnO₃ materials, prepared by high energy ball milling method and then annealed at 900 °C has been undertaken. The analysis of the XRD data using the Win-metric software shows an increase in the unit cell volume with increasing Sr ion concentration. The La_0.7Sr_xCa_0.3−xMnO₃ compounds undergo a structural orthorhombic-to-monoclinic transition at x=0.15. Electric and magnetic measurements show that both the Curie temperature and the insulator-to-metal transition temperature increase from 259 K and 253 K correspondingly for La_0.7Ca_0.3MnO₃ (x=0) to 353 K and 282 K, respectively, for La_0.7Sr_0.3MnO₃ (x=0.3). It is argued that the larger radius of Sr²⁺ ion than that of Ca²⁺ is the reason to strengthen the double-exchange interaction and to give rise to the observed increase of transition temperatures. Using the phenomenological equation for conductivity under a percolation approach, which depends on the phase segregation of ferromagnetic metallic clusters and paramagnetic insulating regions, we fitted the resistivity versus temperature data measured in the range of 50-320 K and found that the activation barrier decreased with the raising Sr²⁺ ion concentration.     

Low-temperature transport properties of chemical solution deposited polycrystalline La0.7Sr0.3MnO3 ferromagnetic films under a magnetic field

Polycrystalline La_0.7Sr_0.3MnO₃ (LSMO) films were prepared on SiO₂/Si (001) substrates by chemical solution deposition technique. Electrical and magnetic properties of LSMO were investigated. A minimum phenomenon in resistivity is found at the low temperature (<50 K) under magnetic fields from 0 T to 3 T. Kondo-like spin dependent scattering, which includes both spin polarization and grain boundary tunneling, was observed in the low-temperature electrical transport for the LSMO polycrystalline films. The temperature-dependent resistivity at low temperatures can be well fitted in the framework of elastic scattering, electron-electron (e-e) interaction, Kondo-like spin dependent scattering, and electron-phonon (e-ph) interaction.     

Oxygen pressure dependent electroresistance in La0.9Sr0.1MnO3 thin films grown by laser molecular beam epitaxy

We report here studies on the influence of oxygen pressure on the electroresistance behavior of La_0.9Sr_0.1MnO₃ thin films fabricated by laser molecular beam epitaxy. It was found that the film deposited at lower oxygen pressure shows larger c-axis parameter, higher resistance, and more distinct electroresistance. These results reveal that the electroresistance of manganite thin films can be tuned by the conditions of film fabrication. Supported by the National Natural Science Foundation of China (Grant No. 10334070) and the National Key Basic Research Program of China (Grant No. 2004CB619004)     

Pressure-induced modifications of crystal and magnetic structure of oxygen deficient La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3-d</Subscript> manganites

The crystal and magnetic structures of the oxygen deficient manganites La_0.7Sr_0.3MnO_3-d (d = 0.15, 0.20) have been studied by means of powder neutron diffraction over the 0–5.2 GPa pressure and 10–290 K temperature ranges. La_0.7Sr_0.3MnO_2.85 exhibits a coexistence of rhombohedral and tetragonal (I4/mcm) crystal structures and below T_g ~ 50 K a spin glass state is formed. La_0.7Sr_0.3MnO_2.80 exhibits a tetragonal (I4/mcm) crystal structure. Below T_g ~ 50 K a phase separated magnetic state is formed, involving coexistence of C-type AFM domains with spin glass domains. In both compounds the crystal structure and magnetic states remain stable upon compression. The factors leading to the formation of different magnetic states in La_0.7Sr_0.3MnO_3-d (d = 0.15, 0.20) and their specific high pressure behavior, contrasting with that of the stoichiometric A_0.5Ba_0.5MnO₃ (A = Nd, Sm) compounds showing pressure-induced suppression of the spin glass state and the appearance of the FM state, are analysed.     

Effect of preferred orientation on magnetoelectric properties of multiferroic La0.7Sr0.3MnO3/BaTiO3 heterostructure

The epitaxial La_0.7Sr_0.3MnO₃/BaTiO₃ bilayer heterostructures were deposited on LaAlO₃ (001) and (110) substrates by pulsed laser deposition. The inherent ferromagnetic, ferroelectric properties and strong magnetoelectric (ME) effect at room temperature were approved, which correlated to the preferred orientation of the films. Both heterostructures showed similar frequency-dependent ME behavior in 0.1 kHz-100 kHz, the ME voltage coefficients were around 140 mV/cm Oe and 104.8 mV/cm Oe at 1 kHz for (001) and (110) oriented bilayers, respectively. This was at least one order of magnitude higher than previously reported results of the related heterostructures, which is mainly ascribed to the lower dielectric constant of BTO film.     

Percolative transport in the vicinity of charge-order ferromagnetic transition in a hole-doped manganite

We report measurements of non-linear charge transport in epitaxial (La_1−x Pr _x )_0.7Ca_0.3MnO₃ thin films fabricated on (100) oriented SrTiO₃ single crystals by pulsed laser deposition. The end members of this series, namely Pr_0.7Ca_0.3MnO₃ and La_0.7Ca_0.3MnO₃ are canonical charge-ordered (CO) and ferromagnetic manganites, respectively. The onset of the CO state in Pr_0.7Ca_0.3MnO₃ is manifested by a pronounced insulating behavior below ∼ 200 K. The CO state remains stable even when a large (∼ 2×10⁵ V/cm) electric field is applied across the thin film samples. However, on substitution of Pr with La, a crossover from the highly resistive CO state to a state of metallic character is observed at relatively low electric fields. The current-voltage characteristics of the samples at low temperatures show hysteretic and history dependent effects. The electric field driven charge transport in the system is modelled on the basis of an inhomogeneous medium consisting of ferromagnetic metallic clusters dispersed in a CO background.     

Synthesis of magnetoresistive La0.7Sr0.3MnO3 nanoparticles by an improved chemical coprecipitation method

La_0.7Sr_0.3MnO₃ nanoparticles were prepared by a simple chemical coprecipitation route. Structural, magnetoresistance (MR), and magnetic properties were investigated. Rietveld refinement of X-ray powder diffraction result shows that the sample is single-phase with the space group of R3¯C. The result of field-emission scanning electronic microscopy shows that most of the grain sizes are distributed from 50 to 200 nm. The composition determined by energy-dispersive spectroscopy is the stoichiometry of La_0.7Sr_0.3MnO₃. The ferromagnetic to paramagnetic transition is sharp with Curie temperature T_C=367 K, which further confirms that the sample is single-phase. The steep change in MR at low fields is attributed to the alignment of the magnetization, while the high-field MR is due to the grain boundary effect.     

钙钛矿锰氧化物La0.7Sr0.3MxMn1-xO3(M=Cr,Fe)的巨磁电阻效应与磁性

研究了溶胶-凝胶法制备氧化物巨磁电阻材料的工艺,制备了La_0.7Sr_{0.3< /sub>CrxMn1-xO3(x=0,0.10,0.15)和La0.7Sr0.3FexMn1-xO3(x=0.05,0.10,0.16)两 系列的单相钙钛矿锰氧化物多晶样品,并研究了Cr,Fe替代La0.7Sr0.3MnO3中部分Mn后对其结构、磁性和巨磁电阻性质的影响.观察到La0.7 Sr0.3Cr0.15Mn0.85O3和La0. 7Sr0.3Fe0.05Mn0.95O3两个样品的 电阻-温度曲线都出现了双峰.定性讨论了可能产生双峰的机制.随Cr(或Fe)替代量的增加, 材料的居里温度很快下降,铁磁性减弱,导电性降低,巨磁电阻效应增强.但与Fe掺杂相比 ,相同数量的Cr掺杂对材料的影响要小. 关键词： 巨磁电阻效应 溶胶-凝胶工艺 电阻-温度曲线 金属绝缘体转变}     

巨磁电阻材料La0.9Sr0.1MnO3与半导体Si组成的二极管的整流特性

掺杂锰氧化物La_0.9Sr_0.1MnO₃薄膜被直接沉积在n型 硅基片上，构成p-n结.这种p-n结在很宽的温度范围内都有很好的整流特性.研究结果表明， 这种p-n结的结电阻对低磁场敏感，在3×10^-2T的磁场下，磁电阻可达70％.磁 电阻的正负依赖于温度.磁电阻的大小可通过加在p-n结上的电压调节.