Nanoscale multiphase separation at La(2/3)Ca(1/3)MnO3/SrTiO3 interfaces

55Mn nuclear magnetic resonance experiments are reported on a series of fully strained epitaxial La(2/3)Ca(1/3)MnO3 thin films on SrTiO3. We have found evidence of multiple phase segregation into ferromagnetic metallic and nonmetallic regions as well as regions that are nonferromagnetic and insulating. These insulating regions are mainly located close to interfaces and may have a significant impact on the performance of spin-tunnel devices. As a result of phase segregation, the ferromagnetic coupling within the metallic regions is depressed. This accounts for the reduction of the Curie temperature and conductivity in nanometric thin films.     

Noise probe of the dynamic phase separation in La(2/3)Ca(1/3)MnO3

Giant random telegraph noise (RTN) in the resistance fluctuation of a macroscopic film of perovskite-type manganese oxide La(2/3)Ca(1/3)MnO3 has been observed at various temperatures ranging from 4 to 170 K, well below the Curie temperature ( T(C) approximately 210 K). The amplitudes of the two-level fluctuations vary from 0.01% to 0.2%. We discuss the origin of the RTN to be a dynamic mixed-phase percolative conduction process, where manganese clusters switch back and forth between two phases that differ in their conductivity and magnetization.     

(La1-xYx)2/3Ca1/3MnO3(x=0.195)系统中的磁电阻行为

本文实验上探讨了 (La1-xYx)2/3Ca1/3MnO3(x=0.195)窄带系统不同磁场下的磁电阻行为.实验表明,最大磁电阻效应随磁场增加而实质性提高,例如,7T磁场下最大磁电阻效应高达1.8×105%.同时,我们在远远低于半导体-金属转变温度下也观察到明显的磁电阻效应.文中就磁电阻的起因进行了讨论,特别是基于最近提出的自旋-极化子模型,对实验数据进行了定量的比较性分析,结果表明,该模型虽能预言实验的主要特征,但理论预言随磁场增加而明显偏离实验.     

Growth and characterization of La2/3Ca1/3MnO3 thin films on ‘silicon on insulator’ substrates

La_2/3Ca_1/3MnO₃ thin films have been grown on SrTiO₃, LaAlO₃, and yttria-stabilized zirconia buffered silicon-on-insulator (SOI) substrates by the pulsed laser deposition technique. While full cube-on-cube epitaxy was achieved on the SrTiO₃ and LaAlO₃ substrates, a coexistence of the cube-on-cube and cube-on-diagonal epitaxy was observed in the the manganite films on SOI substrates. Besides the intrinsic four-fold magnetocrystalline anisotropy, a uniaxial anisotropy also exists in the films, which is determined by the demagnetization field and the mismatch-induced strain. A tensile strain leads to an easy plane, while a compressive strain favors an easy axis. The different magnetization configurations in the films on different substrates are the reason for their varied transport and magnetic properties. Due to a combined effect of these magnetic anisotropy, the magnetization in the two crystallography domains in the film on SOI tends to lie in the film plane but align in their respective easy axes. There are always large spin angles across the domain boundaries. As a result, a quite large low-field magnetoresistance (LFMR) based on spin-dependent tunnelling was observed. It shows a resistance change of ∼20% at 50 K in a magnetic field ∼700 Oe, which is promising for real applications. PACS 75.47.Lx; 72.25.Mk     

对靶溅射技术制备的La2/3Ca1/3MnO3/YBa2Cu4O8/La2/3Ca1/3MnO3薄膜中Tc的降低

本文用对靶溅射技术制备了La2/3Ca1/3MnO3/YBa2Cu4O8/La2/3Ca1/3MnO3薄膜.与YBCO单层薄膜相比,由于超导/铁磁系统中的磁性邻近效应,三层薄膜表现出较低的超导转变温度.薄膜的R～T测量曲线显示出超磁阻(CMR)效应和超导转变,预示着超导和铁磁特性共存于LCMO/YBCO/LCMO三文治结构.     

La2/3Ca1/3MnO3的巨磁熵效应

本文对超大磁阻材料Ｌａ２／３Ｃａ１／３ＭｎＯ３的巨磁熵效应进行了研究,通过测量不同磁场下的磁化－温度曲线,发现伴随铁磁－顺磁相变有一个大的磁熵变化,这个结果表明钙钛矿锰氧化物可以作为磁致冷技术中的工作物质。     

(La1-xCex)2/3Ca1/3MnO3体系的输运性质和反常磁特性研究

本文报告了对Ce掺杂锰氧化物(La1-xCex)2/3Ca1/3MnO3 (x=0～1.0)系列样品的输运特性和反常磁特性的研究结果.实验表明,Ce掺杂对Tc有明显的抑制作用,整体上电阻率随Ce掺杂含量增加而上升,在外加磁场时表现出极大的磁电阻效应.磁化强度随温度变化的曲线出现了两个转变,高温处对应于Mn离子磁矩的铁磁金属转变,低温处的转变则对应于Ce离子磁矩自旋有序排列的形成.表明Ce掺杂引起样品中铁磁双交换作用和反铁磁超交换作用之间的竞争,Ce离子与Mn离子有很强的相互作用.随Ce掺杂含量的增加,铁磁有序转变温度下降,而反铁磁有序转变温度则向高温处移动,铁磁区域明显减小.     

(La1-xCex)2/3Ca1/3MnO3体系的结构和输运性质研究

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Ferromagnetic domain structures and nanoclusters in Nd(1/2)Sr(1/2)MnO3

Magnetic domain structures of Nd(1/2)Sr(1/2)MnO3 were investigated by means of low-temperature Lorentz electron microscopy. On cooling, magnetic domain walls started to appear at 250 K, and they were oriented straight along the [100] and [110] directions. With a further decrease in temperature, the volume of each magnetic domain increased with discontinuous domain-wall jumps. A characteristic granular image was observed at around 140 K, near the charge-ordering transition temperature. We consider that this originated from ferromagnetic nanoclusters that appeared in the antiferromagnetic matrix.     

La2/3Ca1/3MnO3薄膜自旋输运特性研究

本文通过射频磁控溅射法制备了La2/3Ca1/3MnO3(LCMO)薄膜.该薄膜在接近峰值电阻温度Tp(Tp=308K)发生铁磁金属相-顺磁绝缘体相相变.磁场的作用下,电阻温度曲线较无磁场时发生右移,而且Tp点也向高温移动,(H=0.5T时,Tp≈314K;H=1.0T时,Tp≈318K);1T磁场所产生最大的磁电阻值为34%.激光作用下,电阻温度曲线向左移动,Tp变为300K,其最大的光致电阻变化相对值为43.5%.能带理论定性分析表明产生这一不同现象主要是由于eg电子的不同状态引起的.     

Orbital-order-induced metal-insulator transition in La(1-x)Ca(x)MnO3

We present evidence that the insulator-to-metal transition in La(1-x)Ca(x)MnO3 near x approximately 0.2 is driven by the suppression of coherent Jahn-Teller distortions, originating from d-type orbital ordering. The orbital-ordered state is characterized by large long-range Q2 distortions below T(O'- O*). Above T(O'- O*) we find evidence for coexistence between an orbital-ordered and an orbital-disordered state. This behavior is discussed in terms of electronic phase separation in an orbital-ordered insulating and an orbital-disordered metallic state.     

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La2/3Ca1/3MnO3lEu2CuO4/La2/3Ca1/3MnO3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La2/3Ca1/3MnO3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.     

Electron spin resonance analysis of magnetic structures in La2/3Ca1/3MnO3

Measurements of electron spin resonance (ESR) of La_2/3Ca_1/3MnO₃ (LCMO) in the ferromagnetic and paramagnetic phases were carried out. Phase transition and temperature dependence of the peak-to-peak ESR linewidth were determined. The transition temperature between ferromagnetic and paramagnetic phases was observed at 265 K. A prominent increase of the peak-to-peak linewidth with decreasing temperature below T_c was observed. Using the dynamic scale theory and block spin transformation in critical phenomenon, the quantitative calculation of peak-to-peak linewidth at near T_c was made, which was in good agreement with the experimental data. It was believed that the long interactions between the ferromagnetic microregions for LCMO played a key role in determining the ESR linewidth.     

多自旋态离子Co替代诱导La2/3Ca1/3MnO3体系的输运反常

利用具有多自旋态的Co离子进行Mn位替代.制备了La2/3Ca1/3Mn1-xCoxO31(0≤x≤0.15)系列样品并研究了体系的结构和输运特性.结果表明,在替代范围内,样品呈现很好的单相结构,各晶格参数随替代量的增大而减小;Co替代导致体系出现电输运反常,具体表现为在居里温度Tc以下电阻-温度曲线的二次金属-绝缘转变(M-I)行为(双峰效应),且随Co替代量的增大,无论是高温峰还是低温峰,其峰值温度均向低温区移动;所不同的是,对Co替代样品而言,随外加磁场增加,高温峰值温度TPH向高温区移动,而低温峰值温度TPL.则保持不变,表现出磁场无关的特征;相应的峰值电阻率对Co替代和外加磁场表现出很强的依赖关系,随Co替代含量的增加,各峰值电阻率增加.而低温峰值电阻对Co替代更为敏感;对照样品磁特性测量结果,证明高温峰对应于未替代体系的M-I转变,低温峰对应的反常变化则与Co3 离子替代Mn4 后在体系中引入氧缺位和高的自旋态相关联.     

Colossal pressure-induced negative resistance change in La2/3Ca1/3MnO3 thin films

2/3 Ca_1/3MnO₃ thin films as a function of temperature from 4 to 300 K are studied. The application of external pressure increases the temperature of the metal–insulator transition (T_MI). For a film showing T_MI at about 177 K, a colossal change in resisitivity (R(0)-R(p))/R(p) qualitatively comparable to the magnetoresistance (R(0)-R(B))/R(B) around the transition temperature, is observed. However, this change for the film with high T_MI (267 K) is smaller by a factor of about 100. The increase of T_MI with pressure is intimately associated with the pressure-induced contraction and alignment of Mn-O-Mn bonds and the possible enhancement of the double-exchange interaction with pressure. Received: 11 September 1998/Accepted: 12 September 1998     

La2/3 Ca1/3 MnO3中绝缘体-金属相变与CMR效应

我们研究了超大磁阻材料La2/3Ca1/3MnO3的磁电特性,发现由电阻测量得到的绝缘体－金属相变与由磁化曲线得到的顺磁－铁磁相变温区完全一致,均随磁场的增强推向高温区。结果证明绝缘体－金属相变起因于磁序的变化,并伴随着越大磁电阻效应的CMR的发生。     

Electrical properties of La0.67Sr0.16Ca0.17MnO3 perovskite

La_0.67Sr_0.16Ca_0.17MnO₃ perovskite was prepared by the solid-state reaction technique. Preliminary room-temperature structural studies confirm the formation of the sample in the rhombohedral structure. The electrical response was studied using the impedance spectroscopy technique over a broad frequency range (250 Hz–3 MHz). The results indicate that the electrical properties of the material are strongly dependent on temperature and frequency. Impedance spectrum analysis shows that the material can be described as a grain and grain boundary medium and permits to estimate the grain boundary contribution. Electronic conduction is found to be dominated by a thermally activated hopping of small polaron at high temperatures.