Charge ordering characteristics in Y0.5Ca0.5MnO3 manganite

The charge ordering characteristics in Y_0.5Ca_0.5MnO₃ manganite, prepared by sol–gel process, have been investigated experimentally. It is found that the superlattice diffractions appear in the electron diffraction patterns recorded at low temperatures, while only basic Brag diffraction spots can be observed when temperature is higher than 300 K. This provides direct evidence for the existence of charge ordering in Y_0.5Ca_0.5MnO₃. The magnetization and specific heat measurements indicate the charge ordering temperature of Y_0.5Ca_0.5MnO₃ is 290 K, around which both the magnetization and specific heat reveal anomalous behaviors. We also observed that the MnO bond length changed remarkably and the effective number of carriers reduced prominently with decreasing temperature around charge ordering temperature through transform infrared spectra measurements.     

Suppression of charge ordering phenomenon in nanocrystalline Nd0.67Ca0.33MnO3 manganite

A systematic investigation of electrical, magnetic and elastic properties was undertaken in nano and microcrystalline Nd_0.67Ca_0.33MnO₃ manganite, mainly to understand the charge ordering phenomenon. There is a clear and distinct behaviour in the electrical and magnetic properties of nano and microcrystalline samples and the observed behaviour is explained.     

Charge ordering in the electron doped Ca1−xYxMnO3 manganites

Structural studies on the electron doped Ca_1−xY_xMnO₃ are presented. At 300 K, orthorhombic O-phase was observed in all cases, associated to low electric resistivity and high Curie–Weiss temperature. For samples with x>0.07, structural phase transitions to more distorted orthorhombic and monoclinic phases were found at T<170K. In these phases only weak ferromagnetic interactions were observed.     

Magnetotransport properties of epitaxial Pr0.5Ca0.5MnO3 films grown by a solution technique

Epitaxial Pr_0.5Ca_0.5MnO₃ films have been synthesized on (0 0 1) SrTiO₃ substrate using a chemical solution deposition technique and two-step post-annealing process. The zero field resistivity of the films shows semiconducting behavior and a characteristic of charge ordering is observed at 230 K. The resistivity of the 10 nm film did not show any effect with the magnetic field. However, melting of charge ordering was observed for the 120 nm film at an applied magnetic field of 4 T. Large decrease in the resistivity of the 120 nm film (<100 K) resulted in magnetoresistance of nearly −100% at 75 K.     

Charge ordering modulations in Bi0.4Ca0.6MnO3 film with a thickness of 110 nm

Low temperature sample stage in transmission electron microscope is used to investigate the charge ordering behaviours in Bi_0.4Ca_0.6MnO₃ film with a thickness of 110 nm at 103 K. Six different types of superlattice structures are observed using selected-area electron diffraction (SAED) technique, while three of them match well with the modulation stripes in high-resolution transmission electron microscopy (HRTEM) images. It is found that the modulation periodicity and direction are completely different in the region close to the Bi_0.4Ca_0.6MnO₃/SrTiO₃ interface from those in the region a little far from the Bi_0.4Ca_0.6MnO₃/SrTiO₃ interface, and the possible reasons are discussed. Based on the experimental results, structural models are proposed for these localized modulated structures.     

Asymmetric phase diagram and phase separation in Pr0.5−δCa0.2+δSr0.3MnO3 (−0.04δ0.04)

We present electric and magnetic properties of polycrystalline Pr_0.5−δCa_0.2+δSr_0.3MnO₃, for δ between −0.04 and 0.04, where the hole concentration is n=0.5+δ. In this series, we study the effects of moving n away from 0.5 on both, the phase diagram and phase separated state. We found that the low temperature ferromagnetic fraction X_FM continuously decreases when n increases. As a result, the samples with n<0.5 (large X_FM) exhibit metallic resistivity at low T while for n>0.5 the insulating state predominates. We construct a detailed T–n phase diagram showing the asymmetries around half-doping.     

Heisenberg-like ferromagnetism and percolative conductivity in the half-doped manganite Nd0.5Ca0.25Sr0.25MnO3

The magnetic behavior and electronic transport in the half-doped manganite Nd_0.5Ca_0.25Sr_0.25MnO₃ have been investigated. The critical exponents are studied by using isothermal magnetization methods. The results show that the paramagnetic-ferromagnetic transition is second order and the magnetic interaction is satisfied with the prediction of three-dimensional Heisenberg model. The electronic transport belongs to the percolation mechanism. These findings demonstrate that the critical behavior of the magnetic transition and conductivity for manganites are related to Mn-site ordering degree.     

Size-induced ferromagnetism and metallicity in Nd0.5Sr0.5MnO3 nanoparticles: A neutron diffraction study

The phenomenon of destabilization of antiferromagnetic insulating state into a ferromagnetic metallic one in Nd_0.5Sr_0.5MnO₃ with the variation of particle/grain size is critically investigated. Based on our neutron diffraction study, magnetic and transport experiments, we observe ferromagnetism and metallic behavior in Nd_0.5Sr_0.5MnO₃ (∼40 nm grain size) as against A-type antiferromagnetic order in the sample with the largest grain size (∼800 nm). The latter shows a systematic change in the lattice parameters with temperature, and an antiferromagnetic ground state similar to that of a bulk system. Interestingly, the sample with the smallest grain sizes exhibits insignificant structural changes (compared to the largest grain size sample) but a complete change in the magnetic state (ferromagnetic behavior) as revealed from the neutron diffraction study. Magnetic measurements also confirm a ferromagnetic state in the small-grained sample. Electronic transport measurements exhibit a metal-insulator transition in this sample. The effects are primarily attributed to enhanced surface disorder.     

Correlation between magnetic and transport properties of phase-separated La0.5Ca0.5MnO3

The effect of low-magnetic fields on the magnetic and electrical transport properties of polycrystalline samples of the phase-separated compound La_0.5Ca_0.5MnO₃ is studied. The results are interpreted in the framework of the field-induced ferromagnetic fraction enlargement mechanism. A fraction expansion coefficient _f, which relates the ferromagnetic fraction f with the applied field H, was obtained. A phenomenological model to understand the enlargement mechanism is worked out.     

相分离Nd0.5Ca0.5MnO3体系的再入型自旋玻璃行为和电荷有序

研究了Nd_0.5Ca_0.5MnO₃体系的结构和输运特性. 结构 分析表明，在300K下，体系表现为O′型正交结构并存在典型的Jahn-Teller畸变.在8 T磁场 下，体系出现顺磁绝缘-铁磁金属的转变，庞磁电阻效应发生. 磁测量发现，样品的奈尔温 度T_N和电荷有序转变温度T_CO分别在150和240K左右，在41K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，Nd关键词： 庞磁电阻 自旋玻璃态 负磁化强度 电荷有序     

半掺杂Sm0.5Ca0.5MnO3体系的电荷有序和再入型自旋玻璃行为

研究了半掺杂锰氧化物Sm_0.5Ca_0.5MnO₃体系的结构、输运和磁特性，结果表明，在半掺杂情况下，该体系呈现O′类正交结构，表明体系存在典型的Jahn-Teller效应畸变；输运结果在整个测量温区均呈现半导体导电行为，没有出现金属－绝缘体(M-I)转变和CMR效应；电荷有序转变发生在T=270K左右，反铁磁转变温度出现在200K附近，且表现出典型的再入型自旋玻璃(spin-glass)行为，自旋玻璃转变温度T_SG在4 关键词： 自旋玻璃 电荷有序 负磁化现象 多相竞争     

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.     

Strain induced phase separation on La0.5Ca0.5MnO3 thin films

The effect of epitaxial strain on La_0.5Ca_0.5MnO₃ films of various thicknesses grown on SrTiO₃, SrLaAlO₄, and SrLaGaO₄ substrates is studied by Raman spectroscopy, magnetic, and resistivity measurements. The transport and magnetic properties as well as Raman spectra of the films are affected by epitaxial strains. The energy of the A_g(2) mode and the tilting angle of the MnO₆ octahedra is affected by the strain imposed by the substrate. In the spectra of the films deposited on the (1 0 0) SrTiO₃ substrate strong Jahn-Teller (JT) modes appear, which couple with charge-ordering. In all other films these modes are suppressed and no additional Raman lines are present at low temperatures contrary to the bulk compound. The low frequency continuum scattering decreases at low temperatures indicating a coupling with both the charge and orbital transitions. Comparison of the Raman spectra with the magneto-transport properties suggests an interpretation in terms of a strain induced phase separation between ferromagnetic metallic and antiferromagnetic insulating states.     

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.     

Evidence for instability in charge ordering Nd0.5Sr0.5MnO3

The transport properties and magnetic phase transitions of charge ordering manganites Nd_0.5Sr_0.5MnO₃ have been investigated. From resistivity measurements, a continuous increase of resistivity upon the thermal cycling occurs at , and shows an instable behavior in the system. The experimental results of magnetization and electron-spin-resonance spectra indicate that the ferromagnetic phase and antiferromagnetic phase coexist in a broad temperature region. We think that the origin of the instability stems from an inhomogeneous strain yielded in the ferromagnetic interface, due to the competition among different phases.     

Research on charge-ordering state in La0.5Sr0.5MnO2.88 and La0.5Sr0.5Mn0.5Ti0.5O3 systems

The structural, magnetic and transport properties of La_0.5Sr_0.5MnO_2.88 and La_0.5Sr_0.5Mn_0.5Ti_0.5O₃ samples have been investigated systematically. Indeed, this series has been considered to understand the influence of physical parameters such as oxygen deficiency and titanium doping effect in undoped La_0.5Sr_0.5MnO₃ sample. Ceramic material based on La_0.5Sr_0.5MnO_2.88 exhibits interesting behaviours of charge-ordering (CO), ferromagnetic (FM) states and a good conductivity down to the lowest temperatures. The substitution of Ti for Mn destroyed drastically the CO, damaged the motion of itinerant e_g electrons and changed the local parameters of perovskite cell. A change of the structure from tetragonal to rhombohedral symmetry is observed causing a weakening of double-exchange interaction. The experiment results show that the suppression of the CO is sensitive to the variety of Mn³⁺/Mn⁴⁺ ratio. In a field of 8 T at 10 K, FM and CO phase can be evaluated to be ∼20:80 according to the μ_exp/μ_cal ratio for La_0.5Sr_0.5MnO_2.88, whereas the CO state is suppressed for La_0.5Sr_0.5Mn_0.5Ti_0.5O₃ sample, FM and anti-ferromagnetic (AFM) phase are coexisted and evaluated to be ∼54:46, respectively.     

Influence of epitaxial growth on phase competition in Pr0.5Sr0.5MnO3 films

A series of Pr_0.5Sr_0.5MnO₃ (PSMO) films with various thickness were epitaxially grown on substrates of (0 0 1)-oriented (LaAlO₃)_0.3(SrAl_0.5Ta_0.5O₃)_0.7 (LSAT), LaAlO₃ (LAO) and SrTiO₃ (STO), and (0 1 1)-oriented STO using pulse laser deposition. Influence of epitaxial growth on phase competition was investigated. A ferromagnetic metal to antiferromagnetic insulator (FMM-AFI) transition upon cooling is present in both largely compressed situations deposited on LAO (0 0 1) and tensile cases deposited on STO (0 0 1) but absent in little strained films grown on LSAT (0 0 1), indicating that the antiferromagnetic insulating state is favored by strains. On the other hand, the 400 nm films deposited on (0 1 1)-oriented STO as well as LAO substrates show FMM-AFI transition. These results reveal that both the orientation of epitaxial growth and substrate-induced strain affect the FMM-AFI transition.     

Electronic, structural and magnetic phase diagram of Sm1-xSrxMnO3 manganites

The electronic, structural and magnetic phase diagram of the colossal magnetoresistive Sm_1−xSr_xMnO₃ (0.16≤х≤0.67) manganites is constructed on the basis of their systematic studies by high-resolution neutron powder diffraction. It is shown the tendency of researched system to formation of the phase-separated states on crystallographic as well as, in the even greater extent, on magnetic level. A clear correlation between fine specific features and temperature evolution of crystal structures, spin ordering of the manganese ions, and the physical properties of the samarium-strontium series of manganites obtained from temperature magnetic and transport measurements is demonstrated. It was found that ground state of Sm-Sr manganites shows very various physical properties depending first of all on the doping level х and, for given х, determined by two distortion parameters, namely, the average A-cation size <r_A> and the local A-cation size mismatch σ².     

Magnetic and charge ordering properties of Bi0.6−xEuxCa0.4MnO3 (0.0≤x≤0.6)

We have studied structure, magnetic and transport properties of polycrystalline Bi_0.6−xEu_xCa_0.4MnO₃ (x=0.0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6) perovskite manganites. Magnetic measurements show that the charge-ordering temperature (T_CO) decreases with increasing x up to x=0.4 and then slightly increases with further increasing x up to x=0.6. Further, the antiferromagnetic (AFM) ordering temperature (T_N) decreases with increasing x. At T<T_N a transition to metamagnetic glass like state is also seen. Eu doping also leads to enhancement in the magnetic moment and a concomitant decrease in resistivity up to x=0.2 and then an increase in resistivity up to x=0.5. We propose that the local lattice distortion induced by the size mismatch between the A-site cations and 6s² character of Bi³⁺ lone pair electron are responsible for the observed variation in physical properties.