Size effect in (Pr,Sr,Ca)MnO3 manganites

It is well known that the radius of A-site ion is an important parameter to determine the magnetoresistance (MR) ratio of (R,A)MnO₃ manganites. However, the size effect of A-ion is inevitably mingled with the valence effect of Mn-ion when an R-ion of 3+ valence is replaced by an A-ion of 2+. In order to unravel the mystery of the unusually large MR in these materials, it is essential to distinguish the contribution of size effect from that of the valence effect. In this paper, size effect on various physical properties of two compound systems, Pr_0.7Sr_0.3−xCa_xMnO₃ and Pr_0.6Sr_0.4−yCa_yMnO₃ are reviewed extensively. Based on the transport data, the electronic state of Pr_0.7-system is quite different from that of Pr_0.6-system. However, the size effects in these two systems are very similar from the viewpoint of changing their magnetic state and MR ratio. Adopting the mechanism of composite polaron to explain our observations, CMR arises from the depression of spin–wave excitation and the size effect activates its influence on MR ratio via the changing of the effective mass of composite polaron.     

Magnetic, electronic, dielectric and optical properties of Pr(Ca:Sr)MnO 3

The charge-ordered perovskite Pr_0.65Ca_0.28Sr_0.07MnO₃ was investigated by means of magnetic susceptibility, specific heat, dielectric and optical spectroscopy and electron-spin resonance techniques. Under moderate magnetic fields, the charge order melts yielding colossal magnetoresistance effects with changes of the resistivity over eleven orders of magnitude. The optical conductivity is studied from audio frequencies far into the visible spectral regime. Below the phonon modes hopping conductivity is detected. Beyond the phonon modes the optical conductivity is explained by polaronic excitations out of a bound state. ESR techniques yield detailed informations on the (H,T ) phase diagram and reveal a broadening of the linewidth which can be modeled in terms of activated polaron hopping. Received 9 August 2000     

Ordering of polarons in Pr0.5Ca0.5MnO3

We report detailed dielectric investigations of Pr_1?xCa_xMnO₃ (PCMO, x=0.5) ceramics, a member of the perovskite manganite family that exhibits a colossal magnetoresistance. Analysis of the electrical conductivity and dielectric constant data revealed that in the low temperature phase, below 42 K, small polarons are responsible for the charge transport in the system. This provides a new understanding of the phase diagrams in PCMO systems     

相分离Nd0.5Ca0.5MnO3体系的再入型自旋玻璃行为和电荷有序

K左右出现典型再入型自旋玻璃行为,同时观察到了负的磁化率异常. 结果表明,Nd0.5Ca0.5MnO3体系在低温下存在着典型的相分离和多种复杂的磁相互作用竞争机制,这为强关联锰氧化物体系物理机制的理解提供了重要的实验资料.     

Charge ordering fluctuation and optical pseudogap in La(1-x)Ca(x)MnO(3)

Optical spectroscopy was used to investigate the optical gap ( 2 Delta) due to charge ordering (CO) and related pseudogap developments with x and temperature ( T) in La(1-x)Ca(x)MnO(3) ( 0.48< or =x< or =0.67). Surprisingly, we found 2 Delta/k(B)T(CO) is as large as 30 for x approximately 0.5, and decreases rapidly with increasing x. Simultaneously, the optical pseudogap, possibly starting from T* far above T(CO) becomes drastically enhanced near x = 0.5, producing non-BCS T-dependence of 2 Delta with the large magnitude far above T(CO), and systematic increase of T* for x approximately 0.5. These results unequivocally indicate systematically enhanced CO correlation when x approaches 0.5 even though T(CO) decreases.     

Charge/orbital ordering structure of Pr(1-x)Ca(x)MnO(3)(x = 3/8) examined by low-temperature transmission electron microscopy

The structural phase transition of Pr(1-x)Ca(x)MnO(3)(x = 3/8) was investigated by means of low-temperature transmission electron microscopy. Superlattice reflection spots with a modulation wave vector q(1) = (0,1/2,0) appeared below 230 K, indicating formation of the d(3x(2-r(2))/d(3y(2)-r(2)) type of charge/orbital ordering. Below 150 K, a new series of superlattice reflection spots with a modulation wave vector q(2) = (1/4,1/4,1/2) appeared, suggesting an additional ordering of excess 1/8 Mn(3+), necessary due to the deviation of x from 1/2, with the occupation of the d(3z(2-r(2)) type of e(g) orbital.     

Structural and magnetic ordering in La0.6Ca0.4MnO3

The structural and magnetic ordering in La_0.6Ca_0.4MnO₃ has been studied by neutron powder diffraction as a function of temperature between 15 and 300 K. The para-ferromagnetic transition at T∼250 K is accompanied by significant structural distortions in the form of octahedral Mn–O₆ rotations. At 15 K, the total refined ferromagnetic moment on the Mn site was obtained as 3.1 μ_B, in reasonable agreement with the total expected average moment of mixed Mn³⁺/Mn⁴⁺ matrix.     

Nonlinear optical spectroscopy of (La0.6Pr0.4)0.7Ca0.3MnO3 manganite

The electronic structure of doped orthorhombic (La_0.6Pr_0.4)_0.7Ca_0.3MnO₃ manganite is studied using the second optical harmonic generation method. The analysis of the frequency and polarization dependence of the intensity related to the second harmonic demonstrates that the applied magnetic field stimulates a transition of spin-polarized electrons from the d orbitals of Mn ions to the p orbitals of O₂ ligands. The manifestation of such transition in the orthorhombic RMnO₃ manganite with the distorted crystal lattice is attributed to the dipole-forbidden electronic transitions arising in the applied magnetic field. The mechanism of magnetooptical activity induced by the charge transfer between Mn and O ions is discussed.     

Symmetry of the charge-ordered phases in Pr0.5Ca0.5MnO3

The crystallographic symmetry of the charge-ordered Pr_0.5Ca_0.5MnO₃ manganite is studied at ～98?K using convergent beam electron diffraction technique. These studies have revealed appearance of two different type of charge-ordered phases in different domains of the same grain. These correspond to be monoclinic and triclinic phases. These studies reveal that the symmetry of the charge-ordered phase is highly sensitive to the local strain conditions.     

Effect of charge ordering and disordering on terahertz radiation characteristics of magnetoresistive Pr0.7Ca0.3MnO3 thin films

Terahertz (THz) radiation from voltage-biased photoswitching devices made on magnetoresistive manganite Pr0.7Ca0.3MnO3 have been investigated for thin films grown under different conditions. We find that the temperature dependence of THz radiation near the charge-ordering temperature strongly depends on the growth condition of Pr0.7Ca0.3MnO3, whereas THz radiation shows no such temperature dependence near the spin-ordering temperature.     

晶粒尺寸对Pr0.5(Ca0.47Ba0.03)MnO3中台阶效应的影响

采用溶胶-凝胶法制备了不同颗粒尺寸的Pr0.5(Ca0.47Ba0.03)MnO3多晶样品.对样品进行电阻测量发现,随颗粒边界的减小,其电阻率随磁场的变化即ρ(H)曲线上台阶数目和位置发生变化;实验同时发现,热循环对样品低温下的基态也有很大的影响.实验结果表明,颗粒尺寸大小,可以有效地调节反铁磁相(AFM)和铁磁相(FM)之间的相互作用,从而对台阶效应产生影响.进一步证明了台阶效应与相应力有关.     

Microphase separation in Pr0.67Ca0.33MnO3 by small-angle neutron scattering

We have evidenced by small-angle neutron scattering at low temperature the coexistence of ferromagnetism (F) and antiferromagnetism (AF) in Pr0.67Ca0.33MnO3. The results are compared to those obtained in Pr0.80Ca0.20MnO3 and Pr0.63Ca0.37MnO3, which are F and AF, respectively. Quantitative analysis shows that the small-angle scattering is not due to a mesoscopic mixing but to a nanoscopic electronic and magnetic "red cabbage" structure, in which the ferromagnetic phase exists in the form of thin layers in the AF matrix (stripes or 2D "sheets").     

Strain effects in charge-ordered Pr0.5Ca0.5MnO3 manganite thin films

Thin films of the charge-ordered (CO) compound Pr_0.5Ca_0.5MnO₃ have been grown by utilizing the Pulsed Laser Deposition technique. Films are deposited onto LaAlO₃ and SrTiO₃ substrates in order to check the effect of strains (compression and tensile). Using various techniques of characterization, we show that the strains of substrate influence the lattice parameters, the orientation of the orthorhombic structure, the transport properties and the stability of the CO state.     

Manganite Pr0.7Ca0.3MnO3 films obtained by extraction pyrolysis

The magnetic properties of manganite Pr_0.7Ca_0.3MnO₃ films, obtained for the first time by the extraction pyrolysis method, have been investigated. The effect of the synthesis temperature of the magnetic properties of the manganite films is demonstrated.     

La0.33Pr0.34Ca0.33MnO3薄膜的应变效应

采用脉冲激光沉积法分别在(100)LaAlO3和(100)SrTiO3基片上生长了La0.33Pr0.34Ca0.33MnO3 薄膜,并通过磁测量和电输运测量对生长在不同基片上的La0.33Pr0.34Ca0.33MnO3薄膜的物性进行了研究.结果表明,基片和薄膜之间的压应力导致La0.33Pr0.34Ca0.33MnO3薄膜容易出现铁磁相,而电荷有序相则被抑制.基片和薄膜之间的张应力则导致La0.33Pr0.34Ca0.33MnO3薄膜容易出现电荷有序相,铁磁相被抑制.导致该现象的可能原因之一是应力使得晶格中Mn-O键角发生改变而引起双交换作用的改变.     

晶粒尺寸对Bi0.2Ca0.8MnO3电荷有序的影响

利用溶胶-凝胶法制备了平均晶粒尺寸在38—1500 nm的系列Bi_0.2Ca_0.8MnO₃多晶样品,研究了它们的结构、电输运性质和磁性.研究结果表明,随着晶粒尺寸的减小,电荷有序逐渐消失、最后被完全压制,导致低温下出现自旋玻璃态.利用马氏体相变图像和表面效应解释了相关现象. 关键词： 钙钛矿锰氧化物 电荷有序 马氏体相变 自旋玻璃     

Thermal conductivity of (La0.25Pr0.75)0.7Ca0.3MnO3 under giant isotope effect conditions

The thermal conductivity of (La_0.25Pr_0.75)_0.7Ca_0.3MnO₃ manganite has been studied. The isotope substitution of ¹⁸O for ¹⁶O in this compound leads to a ferromagnetic-antiferromagnetic phase transition at low temperatures. It has been found that the thermal conductivity in the ferromagnetic state is approximately two times higher than in the antiferromagnetic state. It has been shown that the small value of thermal conductivity and its temperature dependence can be due to strong phonon scattering from crystal lattice defects, which are thought of as Jahn-Teller distortions. The parameters of this scattering can be determined within the Debye model of thermal conductivity from a comparison of samples differing in their isotope composition.     

Transport properties of Pr0.7Ca0.3MnO3/Nb:SrTiO3 heterojunctions

Current–voltage (J–V) characteristics of epitaxial hetero-junctions composed of Pr_0.7Ca_0.3MnO₃ and Nb:SrTiO₃ were studied under forward and reversed bias conditions. Detailed analysis showed that the J–V characteristics of these heterojunctions can be well-fitted by the thermally-assisted tunnelling model. While the dielectric constant of Nb:SrTiO₃ extracted under the forward bias was about one order of magnitude smaller than that of bulk SrTiO₃, the value obtained under reverse bias was very close to that of the bulk SrTiO₃. The result can be explained by the existence of an interface layer on the Nb:SrTiO₃ substrate with a smaller effective dielectric constant. The current finding suggested that the properties of interface layer should be taken into account in order to accurately simulate the J–V characteristics of such heterojunctions.     

Thermal and electronic transport properties and two-phase mixtures in La(5/8-x)Pr(x)Ca(3/8)MnO3

We measured thermal conductivity kappa, thermoelectric power S, and electric conductivity sigma of La(5/8-x)Pr(x)Ca(3/8)MnO3, showing an intricate interplay between metallic ferromagnetism (FM) and charge ordering (CO) instability. The change of kappa, S, and sigma with temperature (T) and x agrees well with the effective medium theories for binary metal-insulator mixtures. This agreement clearly demonstrates that with the variation of T as well as x, the relative volumes of FM and CO phases drastically change and percolative metal-insulator transition occurs in the mixture of FM and CO domains.