Effect of phase stratification into electro- and magnetotransport in heteroepitaxial La2/3Ca1/3MnO3 films

The La_2/3Ca_1/3MnO₃ films 25-nm thick biaxially compressed in the substrate plane, grown quasi-coherently on the (001) surface of the LaAlO₃ single crystal, were studied using laser vaporization. It was found that mechanical stresses acting during nucleation and growth promoted calcium enrichment of the cation sublattice of the manganite layer, which caused a decrease in its unit cell volume. Crystalline grains in manganite films were distinctly oriented along the normal to the substrate plane; the grain size in the substrate plane was within 20–40 nm, and the relative grain misorientation in the substrate plane did not exceed 0.2°. In zero magnetic field, the maximum in the temperature dependence of the resistivity ρ of La_2/3Ca_1/3MnO₃ films was observed at temperatures close to 210 K. At T < 100 K and μ₀ H = 2 T, the magnetoresistance of manganite films was negative, weakly depended on temperature, its value was about–0.45. The magnetic field caused transformation of nonferromagnetic phase inclusions to ferromagnetic ones, which resulted in a decrease in the La_2/3Ca_1/3MnO₃ film resistivity with increasing magnetic field. At low temperatures (T < 100 K), a hysteresis was observed in the dependences of the film resistivity on the magnetic field.     

Room temperature magnetoresistance in Ln2/3A1/3MnO3 manganites

The investigation of the manganites La_2/3−xPr_xSr_1/3MnO₃, La_2/3Sr_1/3−xCa_xMnO₃ and La_2/3+xCa_1/3−2xAg_xMnO₃, which all exhibit Mn³⁺:Mn⁴⁺=2, shows that it is possible to reach high magnetoresistance at room temperature, up to 21% under 1.2 T. These materials are compared to La_5/6Ag_1/6MnO₃ which corresponds to the same Mn³⁺:Mn⁴⁺ ratio and exhibits a magnetoresistance of 25% in this field. An interesting feature deals with the value of the insulator-metal transition temperature T_IM, often higher than T_C, especially for Ag-based compounds. It is suggested that the latter results either from a better oxygenation of the surface of the grains or from a migration of silver toward the surface.     

Magneto-transport and ferromagnetic resonance studies of polycrystalline La0.6Pb0.4MnO3 thin films

To understand the nature of grain boundaries in polycrystalline materials, magneto-transport and ferromagnetic resonance measurement have been performed in polycrystalline La_0.6Pb_0.4MnO₃ (LPMO) thin films prepared by pulsed laser deposition. Films are found to undergo a semiconductor to metal transition at 230 K and re-enter into the semiconducting state below 130 K. Microwave absorption measurements carried out as function of applied field show two components of resonant absorption signal. First component is in accordance with ferromagnetic transition of grains at Curie temperature and the second component shows antiferromagnetic transition of grain boundaries at 160 K. An additional non-resonant absorption signal centered at zero field has also been observed that supports transition from conducting to insulating grain boundaries at ∼160 K. Further, temperature dependence of resistance in semiconducting state at low temperatures is in accordance with coulomb blockade model indicating insulating nature of AFM grain boundaries.     

Search for high temperature coefficient of resistance La2/3Ca1/3MnO3 polycrystalline ceramics

La_2/3Ca_1/3MnO₃ polycrystalline ceramics were synthesized by sol–gel method. Sharp temperature coefficient of resistance (TCR) variation (with peak value up to 22 %) has been observed near the metal-insulator transition temperature T _MI (273 K) for the sample sintered at 1,450 °C. This TCR value is much higher than the previously reported values for the undoped and Ag-doped La_0.67Ca_0.33MnO₃ samples and is comparable to the optimized thin films. It was concluded that the improved physical properties of the La_0.67Ca_0.33MnO₃ material are due to its improved microstructure and homogeneity.     

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.     

La2/3Ca1/3MnO3/Eu2CuO4/La2/3Ca1/3MnO3磁性隧道结的制备与表征

采用磁控溅射，紫外线光刻和离子束刻蚀制备了La_2/3Ca_1/3MnO₃/Eu₂CuO₄/La_2/3Ca_1/3MnO₃磁性隧道结.通过对获得的磁性隧道结的I-V特性测量，发现非线性的I-V特性，显示结样品的隧穿特性.有趣的是发现在电极材料La_2/3Ca_1/3MnO₃的金属-绝缘体转变温度(Tp)以下，I-V曲线出现一个跳变.随着温度降低，开始出现跳变的临界电流增大，但是跳变都发生在同样的电压下～209mV.当电流增大或减小在跳变点附近出现回滞.这一跳变只发生在铁磁金属态，表明这是一个磁性相关联的效应，可能对应一种新的磁性开关过程.虽然，目前对这一现象背后的物理机理还不清楚，但是，这一现象有可能在未来自旋电子学器件方面具有潜在的应用价值. 关键词： 庞磁电阻 磁性隧道结 开关效应     

Epitaxial growth La0.67Ca0.33MnO3 thin film by radio frequency sputtering method

Perfect epitaxial growth of La_0.67Ca_0.33MnO₃ (LCMO) thin film has been achieved on (1 0 0) LaAlO₃ (LAO) single crystal substrate by radio frequency sputtering method. X-ray diffraction (XRD) and electron diffraction analysis indicates that La_0.67Ca_0.33MnO₃ film grows epitaxially on LaAlO₃ along [1 0 0] direction of the substrate. The resistivity variation with temperature of the film shows a sharp metal to semiconductor transition peak around 253 K, which is close to that of the target. The magnetoresistance (MR) also reveals high quality epitaxy film characteristic at low temperatures and near the metal to semiconductor transition temperature.     

La0.67Ca0.33MnO3薄膜皮秒光电效应

在1064 nm波长脉冲激光（脉宽25 ps）的照射下，钙钛矿氧化物薄膜La0.67Ca0.33MnO3/SrTiO3具有超快光电效应，对激光脉冲显示ps量级的响应时间，上升沿响应时间300 ps，半高宽700 ps，同时，对激光能量的响应灵敏度为500 mV/mJ。     

Thickness dependence of electroresistance in La0.67Ca0.33MnO3 epitaxial films

The electroresistance (ER) of La_0.67Ca_0.33MnO₃ (LCMO) epitaxial thin films with different thicknesses was studied. For the 110 nm thick LCMO film, its ER shows a maximum at T_p, where the resistance shows a peak, and decreases to zero at lower temperatures. While for the 30 nm thick LCMO film, its ER is remarkable in a wide temperature range. Another interesting observation in this work is that the electric current can tune the magnetoresistance of the ultrathin LCMO thin film. The results were discussed by considering the coexistence of ferromagnetic metallic phase with the charge ordered phase, and the variation of the phase separation with film thickness and electric current. This work also demonstrates that electric current can tune the magnetoresistance of the manganites, which is helpful for their applications.     

相分离La0.33Pr0.34Ca0.33MnO3薄膜体系中的交换偏置效应

在相分离La_0.33Pr_0.34Ca_0.33MnO₃薄膜体系中发现了大的交换偏置效应．在4 K时,交换偏置场的大小达到了约1 kOe．交换偏置效应可能源自薄膜内禀的电子相分离特性或薄膜的表面效应．交换偏置效应表现出强的温度、冷却磁场以及厚度依赖的关系．     

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.     

La0.7Ce0.3MnO3 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.     

Activation energy in La0.7Ca0.3MnO3/YBa2Cu3O7-δ / La0.7Ca0.3MnO3 superconducting trilayers

Resistivity vs. temperature measurements on La_0.7Ca_0.3MnO₃/YBa₂Cu₃O_7-δ /La_0.7Ca_0.3MnO₃ (LCMO/YBCO/LCMO) trilayers with different YBCO thickness, were performed in external magnetic field H up to 8 T. By evaluating the activation energy U from the slope of the resistivity Arrhenius plot, a strong depression of U has been observed when decreasing the YBCO layer thickness and the absolute U values appear to be reduced with respect to the values reported in literature in the case of YBCO thin films and YBCO/insulating multilayers. Moreover, a logarithmic U vs. H dependence is shown both in the case of thick and thin YBCO layers indicating the formation of a two dimensional vortex lattice. The experimental data are discussed considering the strong influence of the ferromagnetic LCMO on the superconducting YBCO properties which reduces the effective YBCO thickness more than predicted by the conventional theories.     

Reversal transient laser-induced voltages in La2/3Ca1/3MnO3 films

This paper reports that the transient laser-induced voltages have been observed in La2/3Ca1/3MnO3 thin films on MgO （001） in the absence of an applied current. A peak voltage of - 0.15 V was detected in response to 0.015J pulse of 308 nm laser. It is demonstrated that the signal polarity is reversed when the films are irradiated through the substrate rather than at the air/film interface. Off-diagonal thermoelectricity may support the inversion of the signal when the irradiation direction is reversed.     

La2/3Ca1/3MnO3薄膜的光致电阻率变化特性

射频磁控溅射法制备了La_2/3Ca_1/3MnO₃纳米薄膜(LCMO).该薄膜发生FM-PM相变的转变点温度为T_c≈308K（近似为电阻峰值温度T_p）；在不同温度下的光电导性质实验表明所制备的LCMO薄膜在连续激光作用时低温段（Tc）表现为光致电阻率增大效应，即ΔR/R＞0，并在R-T曲线拐点附近取得极大值，（ΔR/R）_max=43.5%；当T>T关键词： 钙钛矿薄膜 光响应 电子自旋 小极化子     

Coexistence of large positive and negative magnetoresistance in La0.7Ca0.3MnO3 films with microcracks

The coexistence of large positive and negative low-field magnetoresistance (LFMR) in the ferromagnetic La_0.7Ca_0.3MnO₃ thin films with ordered microcrack (MC) distributions is reported. For the films with the highest linear density of MC, the negative LFMR can be up to −60% and rapidly changes to the positive value of 25% at 200 Oe field with the increase of temperature. We discuss the effect based on the spin-polarized tunneling and inhomogeneous magnetic state induced by the natural formations of MC in the films.     

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     

Magnetoresistance and transport properties of different impurity doped La0.67Ca0.33MnO3 composite

An enhanced magnetoresistance and a two-fold effect result from impurity dopant were observed in composites of La_0.67Ca_0.33MnO₃/YSZ and La_0.67Ca_0.33MnO₃/Fe₃O₄. Where YSZ represents yttria-stabilized zirconia and the doping level of both YSZ and Fe₃O₄ is 1 mol%. Different electrical and magnetic transport properties, in particular a lower field magnetization behavior, were observed between pure La_0.67Ca_0.33MnO₃ and the impurity doped La_0.67Ca_0.33MnO₃ composites. Compared with pure La_0.67Ca_0.33MnO₃, a possible interpretation is presented by considering the influences of YSZ and Fe₃O₄ on the structure of grain boundaries and/or surfaces of La_0.67Ca_0.33MnO₃grains.     

Studies on the resistance switching properties of the La0.5Ca0.5MnO3/Nb : SrTiO3 heterojunction

In this study, we have investigated the resistance switching behavior of the La_0.5Ca_0.5MnO₃/Nb:SrTiO₃ heterojunction. The junction shows a negative resistance switching ratio (ER) below 140 K. When , the ER goes from negative to positive with increasing bias voltage. When T>220 K, the junction shows a positive ER. This variation from a negative to a positive value indicates that the ER is determined primarily by two phenomena: (a) the negative ER value can be attributed to a disruption of the charge-ordered insulating domains in La_0.5Ca_0.5MnO₃ under large electric fields, and (b) the positive ER value at high temperatures is due to the modulation of the interface barrier width driven by the electrochemical migration of oxygen vacancies.     

LaCaMnO/LaNdCaMnO/LaCaMnO三层膜中巨磁电阻增强效应

通过直流磁控溅射方法在SrTiO₃(001)衬底上制备了一系列La_0.67Ca_0.33MnO₃/(La_0.35Nd_0.65)_2/3Ca_1/3MnO₃/La_0.67Ca_0.33MnO₃(以下简写为LCMO/LNCMO/LCMO)外延三层膜，其中间层 关键词：