La0.67Ca0.33MnO3/NdGaO3(001)薄膜中外延应变诱导的各向异性磁阻效应

本文使用脉冲激光沉积方法在正交的NdGaO3(001)衬底上外延生长了La0.67Cao.33MnO3薄膜.退火后,由于外延应变引起薄膜正交畸变增强,薄膜在低于250K的整个温度区间都出现了铁磁金属态和反铁磁绝缘态的相分离和相竞争.这两相的竞争强烈依赖于磁场相对晶轴的方向,由此导致存在于很宽温度范围内的低场各向异性磁阻...     

Antiferromagnetic interlayer coupling of (111)-oriented La_(0.67)Sr_(0.33)MnO_3/SrRuO_3 superlattices

We report a strong antiferromagnetic(AFM) interlayer coupling in ferromagnetic La_(0.67)Sr_(0.33)MnO_3/SrRuO_3(LSMO/SRO) superlattices grown on(111)-oriented SrTiO_3 substrate. Unlike the(001) superlattices for which the spin alignment between LSMO and SRO is antiparallel in the in-plane direction and parallel in the out-of-plane direction, the antiparallel alignment is observed along both the in-plane and out-of-plane directions in the present sample. The low temperature hysteresis loop demonstrates two-step magnetic processes, indicating the coexistence of magnetically soft and hard components. Moreover, an inverted hysteresis loop was observed. Exchange bias tuned by the temperature and cooling field was also investigated, and positive as well as negative exchange bias was observed at the same temperature with the variation of the cooling field. A very large exchange field(H_(EB)) was observed and both magnitude and sign of the H_(EB)depend on the cooling field, which can be attributed to an interplay of Zeeman energy and AFM coupling energy at the interfaces. The present work shows the great potential of tuning a spin texture through interfacial engineering for the complex oxides whose spin state is jointly determined by strongly competing mechanisms.     

Effects of Si surficial structure on transport properties of La_(2/3)Sr_(1/3)MnO_3 films

La_(2/3)Sr_(1/3)MnO_3 films are deposited on(001) silicon substrates,in which the silicon surfaces have artificially been treated into the scallops-like,pyramid-like,and smooth polishing structure,by pulsed laser deposition.The magnetoresistances of the films on etched substrates under low applied field are very sensitive to the applied field,and much larger(14.3% for acid-etched,and 42.9% for alkali-etched) than that on the polished Si at 5 K.Zero-field-cooled and field-cooled magnetization behaviors are measured and analyzed.Remarkable upturn behaviors in temperature-dependent resistivity for all samples are observed at low temperature,which follows the Efros-Shkloskii variable range hopping law and the Arrhenius law.We believe that the rough surface may be useful in device design.     

La_(2/3/)Sr_(1/3)MnO_3单晶的低场磁热效应

本文对La2/3Sr1/3MnO3单晶中b和a(c)轴方向的磁热效应进行了研究,由于晶体的磁晶各向异性,磁热效应表现出一定的各向异性.通过不同温度下的等温起始磁化曲线的实验分析表明,在外加场为10kOe的情况下,b和a(c)轴方向的磁熵变-ΔSM(H)在370K达到最大值,分别为1.942Jkg-1K-1和1.873Jkg-1K-1.低场下较大的磁熵变是由于磁化强度随外场迅速变化以及自旋-晶格的耦合造成的.La2/3Sr1/3MnO3单晶在低场下表现出相对较大的磁热效应,表明其在磁制冷方面有一定的潜在应用价值.     

Abundant photoelectronic behaviors of La_(0.67)Sr_(0.33)MnO_3/Nb:SrTiO_3 junctions

Temperature dependence on rectifying and photoelectronic properties of La_(0.67)Sr_(0.33)MnO_3/Nb:SrTiO_3(LSMO/STON) junctions with the thickness values of LSMO film varying from 1 nm to 54 nm are systematically studied. As shown experimentally, the junctions exhibit good rectifying properties. The significant differences in photoemission property among the LSMO/STON junctions are observed. For the junction in a thicker LSMO film, the photocurrent shows a monotonic growth when temperature decreases from 300 K to 13 K. While for the junction in an ultrathin LSMO film, the behaviors of photocurrent are more complicated. The photocurrent increases rapidly to a maximum and then smoothly decreases with the decrease of temperature. The unusual phenomenon can be elucidated by the diffusion and recombination model of the photocarrier.     

Anisotropic transport properties in the phase-separated La_(0.67)Ca_(0.33)MnO_3/NdGaO_3 (001) films

The anisotropic transport property was investigated in a phase separation La_(0.67)Ca_(0.33)MnO_3(LCMO) film grown on(001)-oriented Nd GaO_3(NGO) substrate. It was found that the resistivity along the b-axis is much higher than that along the a-axis. Two resistivity peaks were observed in the temperature dependent measurement along the b-axis, one located at 91 K and the other centered at 165 K. Moreover, we also studied the response of the resistivities along the two axes to various electric currents, magnetic fields, and light illuminations. The resistivities along the two axes are sensitive to the magnetic field. However, the electric current and light illumination can influence the resistivity along the b-axis obviously, but have little effect on the resistivity along the a-axis. Based on these results, we believe that an anisotropicstrain-controlled MnO_6 octahedra shear-mode deformation may provide a mechanism of conduction filaments paths along the a-axis, which leads to the anisotropic transport property.     

Anisotropic transport properties of charge-ordered La_(5/8-y)Pr_yCa_(3/8)MnO_3(y= 0.43) film

The anisotropic resistances along [001] and [1-10] axes are investigated for an La5/8-yPryCa3/8MnO3(y = 0.43)(LPCMO) film grown on(110)-oriented La AlO3 substrate. It is found that the charge order(CO) transition is much stronger and the resistance is larger along the [001] direction than that along the [1-10] direction. Special attention has been paid to the different effects of a magnetic field on the resistances of the two axes. The resistance is more susceptible to the magnetic field along the [001] direction compared with that along the [1-10] direction. Our results demonstrate that the anisotropic transport properties can be ascribed to the intrinsic anisotropic strain field in the film, which changes the shape of metallic domains for the phase separation manganite film. We also provide a feasible method to rule out the Joule heat effect from the electric current effect. This could be useful for future construction and application of materials and devices.     

(La_(1-x)Ce_x)_(2/3)Ca_(1/3)MnO_3体系的结构和输运性质研究

对Ce掺杂锰氧化物 (La1 -xCex) 2 3Ca1 3MnO3(x =0— 1.0 )多晶样品的结构和输运性质系统研究的结果 .实验表明 ,在低掺杂浓度下 ,样品呈现完整的正交钙钛矿结构 ;随掺杂浓度的增加 ,有少量CeO2 杂相出现 ,同时伴随有Ce3 和Ce4 离子两种价态的涨落和Mn2 Mn3 Mn4 混合价态的共存 ,Ce掺杂导致的体系无序度增加 ,使得绝缘体 -金属 (I M)和顺磁 -铁磁 (PM FM)转变温度向低温方向移动 .有趣的是 ,Ce掺杂样品的电阻率测量I M转变峰值温度TIM 较PM FM转变温度Tc 为高 ,其差值ΔT(=TIM -Tc)随Ce掺杂含量的增加而增加 ,最大差值ΔT达 5 0K .各样品的磁电阻则随温度的降低而增加 ,在Tc 附近MR达到最大值 ,且随Ce掺杂含量增加Tc 附近的MR最大值迅速增加 ,达到 10 4 %以上 ,表现出很强的庞磁电阻效应 .在x≥ 0 .7时 ,磁电阻效应则表现出反常减小 .整体上而言 ,各样品的磁特性与输运行为间有较强的关联 ,电输运特性可用双交换作用模型进行很好的解释 ,在高温区满足热激活模型 .最后 ,在Ce掺杂对Tc 和MR的影响机理方面进行了初步讨论 .     

Transport and magnetoresistance effect in an oxygen-deficient SrTiO_3/La_(0.67)Sr_(0.33)MnO_3 heterojunction

An oxygen-deficient SrTiO3/La0.67Sr0.33MnO3 heterojunction is fabricated on an SrTiO3 （001） substrate by a pulsed laser deposition method. The electrical characteristics of the heterojunction are studied systematically in a temperature range from 80 K to 300 K. The transport mechanism follows I ∝ exp （eV/nkT） under small forward bias, while it becomes space charge limited and follows I ∝ Vm（T） with 1.49〈 m 〈1.99 under high bias. Such a heterojunction also exhibits magnetoresistance （MR） effect. The absolute value of negative MR monotonically increases with temperature decreasing and reaches 26.7% at 80 K under H=0.7 T. Various factors, such as strain and oxygen deficiency play dominant roles in the characteristics.     

La_(0.5)Sm_(0.2)Sr_(0.3)MnO_3/Ag_x中的室温磁电阻增强

用固相反应法制备La0.5Sm0.2Sr0.3MnO3/(Ag2O)x/2(x=0.00,0.04,0.08,0.25,0.30)样品,通过M～T曲线,ρ～T曲线,ρ～T拟合曲线,研究样品的磁性质、输运行为、输运机制及磁电阻效应.结果表明:少量掺杂时Ag可能参与反应.掺杂量较多时,Ag主要以金属态分离到母体颗粒的界面处,使体系形成两相复合体.少量的Ag掺杂可以明显提高自旋相关散射产生的晶界磁电阻.掺Ag为30%摩尔比时,样品的电阻率较低掺杂样品的电阻率降低一个数量级,在300K、0.5T磁场下,磁电阻明显增强,达到9.4%,这与颗粒母体界面结构的改善有关,也与材料电阻率的降低有关.     

晶界效应对钙钛矿La_(0.7)Ba_(0.3)MnO_3多晶材料的磁输运特性的影响(英文)

本文通过比较La0.7Ba0.3MnO3多晶和外延薄膜样品磁输运行为之间的差异,讨论了晶界对多晶样品输运特性的影响,并计算了晶界电阻率随温度和磁场的变化关系.与晶粒的本征电阻率相比,晶界的电阻率要大一个量级,并且其金属-绝缘转变温度低约50K.此外,在晶界中观察到了新奇的正磁阻效应.我们认为,晶界在其转变温度附近出现了相分离现象,并形成铁磁金属渗流通道.由于晶界的特殊构型,Ba掺杂锰氧化物材料中大的磁致伸缩效应对其输运行为影响很大,导致了晶界中正磁阻现象的出现.     

Field-induced insulator-metal-insulator transitions in low-energy H_2~+ ion-irradiated epitaxial La_(2/3)Ca_(1/3)MnO_3 thin films

Epitaxial La 2/3 Ca 1/3 MnO 3 thin films grown on LaAlO 3 (001) substrates were irradiated with low-energy 120-keV H 2 + ions over doses ranging from 10 12 ions/cm 2 to 10 17 ions/cm 2 . The irradiation suppresses the intrinsic insulator-metal (I-M) transition temperature and increases the resistance by reducing the crystallographic symmetry of the films. No irradiation-induced columnar defects were observed in any of the samples. The specific film irradiated at a critical dose around 8 × 10 15 ions/cm 2 is in a threshold state of the electric insulator where the I-M transition is absent. In an external field of 4 T or higher, the I-M transition is restored and thus an enormous magnetoresistance is observed, while a negative temperature coefficient resumes as the temperature is reduced further. Magnetic relaxation behavior is confirmed in this and other heavily irradiated samples. The results are interpreted in terms of the displacement of oxygen atoms provoked by ion irradiation and the resulting magnetic glassy state, which can be driven into a phase coexistence of metallic ferromagnetic droplets and the insulating glass matrix in a magnetic field.     

First-principle investigation on perovskite La_(1-x)Eu_xGaO_3

The pseudopotential method has been used to investigate the structural, electronic and magnetic properties of La_(1-x)Eu_xGaO_3(x = 0, 0.25, 0.5, 0.75, and 1) within the scheme of generalized gradient approximation. The spin-polarized calculations demonstrate that the ground state is an antiferromagnetic insulator for x ≤ 0.5, while it is ferromagnetic halfmetal at x 0.5. The substitutions of magnetic Eu ions for non-magnetic La ions produce and strength spin polarization,which forcefully urges the system from the insulator to the half metal. Meanwhile, Eu doping strengthens a stoner mechanism for ferromagnetism of La_(1-x)Eu_xGaO_3(x = 0.75 and 1), which may lead to a rapid increasing in the total magnetic moment and therefore, antiferromagnetic–ferromagnetic transition happens.     

High resistance modulation by the electric field based on La_(0.9)Sr_(0.1)MnO_3/SrTiO_3/Si structure

A field-effect configuration based on La0.9Sr0.1MnO3/SrTiO3/Si structure is fabricated on Si substrate by laser molecular-beam epitaxy. The resistance modulation by electric field of the La0.9Sr0.1MnO3/SrTiO3/Si structure is investigated in detail. An evident resistance modulation effect is observed at 80 K. The channel resistance modulation by field effect reaches 1.4×107% and 2.6×106% when VDS are -2 and -6.5 V, respectively. The ON/OFF ratio of approximately 4000 is obtained. The present results are wort...     

La_(1-x)Sr_xMnO_3/TiO_2(x=0.2,0.15,0.04)异质pn结的整流特性

采用磁控溅射法制备的La1-xSrxMnO3(LSMO)/TiO2异质pn结表现出很好的整流特性.室温电流电压特性曲线显示随着Sr掺杂的增加,扩散电压增大,这可能由于Sr掺杂的增加导致载流子浓度增大所致.电流电压变温特性曲线显示随着测量温度的降低,扩散电压增大,这可能由于随着测量温度的变化导致界面电子结构的变化所致.值得提出的,异质pn结电阻随温度变化曲线表现出单层LSMO的金属绝缘相变特性,并且在低测量温度时表现出随着测量温度的降低结电阻增大,这可能是由于宽带隙的TiO2的引入导致.     

钙钛矿氧化物异质结的研究进展

在过去几年中,为了研究钙钛矿氧化物异质结的物性和潜在的应用前景,我们进行了许多探索。在本文中,我们总结了对钙钛矿氧化物异质结一些物理性质的理论描述,揭示了La0.9Sr0.1MnO3/SrNb0.01Ti0.99O3异质结中发现的正磁电阻效应和横向光电效应的物理起源。我们提出的理论模型将有助于理解界面对钙钛矿氧化物异质结各种性质的影响。     

Strain Induced Nanopillars and Variation of Magnetic Properties in La_(0.825)Sr_(0.175)MnO_3/LaAlO_3 Films

To investigate the process of strain relaxation and resultant variation of microstructure and magnetic properties,low-doped La_(0.825)Sr_(0.175)MnO_3 epitaxial films with different thicknesses are deposited on LaAlO_3 substrates and strain induced nanopillars are discovered inside the La_(0.825) Sr_(0.175)MnO_3 film. Perpendicular oriented nanopillars mainly exist below 30 nm and tend to disappear above 30 nm. The distribution of nanopillars not only induce the variation of lattice parameters and local structural distortion but also lead to the deviation of easy magnetization axis from the perpendicular direction. Specifically, the out-of-plane lattice parameters of the film decrease quickly with the increase of the thickness but tend to be constant when the thickness is above 30 nm. Meanwhile, the variations of magnetic properties along in-plane and out-of-plane directions would also decline at first and they then remain nearly unchanged. Our work constructs the relationship between nanopillars and magnetic properties inside films. We are able to clearly reveal the effects of inhomogeneous strain relaxation.     

La2/3(Ca0.65Ba0.35)1/3MnO3/0.035TiO2/xCuO复合体系电输运和磁电阻效应研究

用甘氨酸-硝酸盐(GNP)方法合成出La2/3(Ca0.65 Ba0 35)1/3MnO30.035/TiO2/xCuO纳米复合材料(x=0;0.23;0.25;0.27;0.30).通过X射线衍射、Raman光谱、扫描电子显微镜和磁电阻效应测试,对合成产物的结构及性能进行表征.结果表明,样品的对称性由立方向正交结构畸...     

氧缺陷导致的结构无序对La_(2/3)Sr_(1/3)MnO_3薄膜电输运特性的影响(英文)

为了澄清结构无序对电输运特性,特别是低温下电阻极小的影响,采用脉冲激光沉积方法在LaAlO3(100)基片上制备了两类La2/3Sr1/3MnO3薄膜.一块是在通40Pa氧气的气氛下沉积,沉积后不再进行通氧气的原位退火.另一块在同样通40Pa氧气的气氛下沉积,但沉积后再在6×104Pa的氧气气氛下进行原位退火.对两种薄膜的结构,磁特性和电输运特性进行了对比研究.结果表明:氧缺陷导致了薄膜结构的无序,更导致了类似自旋玻璃行为的磁无序.电输运特性结果表明原位退火使得薄膜的金属-绝缘体转变温度Tc从约195K提高到335K.对由于结构无序的存在对低温电阻较小行为的影响作了系统对比研究,曲线拟合的结果表明对于缺氧薄膜和进行了原位退火的薄膜的低温电阻较小行为的主要影响因素分别是类近藤散射作用和电子-电子相互作用,这一结果对于澄清庞磁电阻锰氧化物体系的低温电阻极小现象有一定积极意义.     

Dielectric loss of half-doped manganite La_(0.5)Ca_(0.5)MnO_3

The dielectric loss tanδ of half-doped manganite La 0.5 Ca 0.5 MnO 3 is investigated using Green's function technique. The La 0.5 Ca 0.5 MnO 3 is described by the Kondo-lattice model in the double exchange limit, taking into account the Jahn-Teller distortion and the super-exchange interaction between the localized electrons. It is found that the intensity of tanδ decreases with increasing | ε JT |, V, and U. It is also observed that the transition temperature T P rises as | ε JT | and U increase. It is worth noting that T P remains unchanged and the strength of tanδ increases with increasing g. The calculated dielectric loss results are explained theoretically, and these behaviors are in qualitative agreement with the experimental results.