Influence of oxygen vacancies on magnetoresistance properties of bulk La0.67Ca0.33MnO3−δ

Bulk polycrystalline samples of La_0.67Ca_0.33MnO_3−δ were annealed in controlled oxygen atmospheres. Two types of conditions were chosen, firstly with the aim of taking oxygen from the grain boundaries and secondly with the aim of removing oxygen uniformly from the sample. Reducing conditions increase the overall resistivity and suppress the Curie–Weiss temperature (T_c). A second peak at temperatures below T_c appears in the resistivity and magnetoresistivity when the samples are deoxygenated. However, the presence of this peak does not strongly influence the low-temperature magnetoresistance.     

Synthesis,structural, electrical and magnetic studies of La0.5Ca0.45−xSrxBa0.05MnO3

Half doped mixed valence manganite system La_0.5Ca_0.45?xSr_xBa_0.05MnO₃ (with x=0.1, 0.2, and 0.3) synthesized through a low temperature nitrate route is systematically investigated in this paper. The electronic transport and magnetic properties are analyzed and compared apart from the study of unit cell structure and composition. The system is found to crystallize only in orthorhombic structure (Pnma) and the electronic phase transitions are observed to be of second order. The charge and orbital ordering have been observed to coexist with ferromagnetism in x=0.1 compound. Application of small polaron and variable range hopping models to resistivity data of the system corresponding to high temperature range shows increasing mobility of e_g electrons with x, with the later model describing the electronic transport very closely than the former. The temperature dependent magnetization of the compounds shows monotonic increase of paramagnetic to ferromagnetic transition (T_C) with x. Ferromagnetism is exhibited for the complete temperature range down from respective T_C in contrast to antiferromagnetism usually exhibited by half-doped compounds in the low temperature range. The plots of magnetization versus magnetic field reveal a transition from soft to hard magnetic character for all the compounds as the temperature is lowered.     

Crystal structure and magnetic state of the LaMn1−x VxO3 perovskites

The crystal structure and the magnetic state of polycrystalline LaMn_1?x V_xO₃ (0.1<x<0.9) compounds have been studied by x-ray and neutron diffraction methods, as well as by magnetization and ac susceptibility measurements. It is shown that substitution of vanadium for manganese ions leaves the orthorhombic crystal structure of the compounds (space group Pnma) unchanged. The magnetic structure is observed to change from a canted antiferromagnetic ordering (wavevector k=[0, 0, 0], with the antiferromagnetic moments aligned with the a axis and the ferromagnetic component of the magnetic moment parallel to the b axis) at vanadium concentrations x<0.4 to a collinear antiferromagnetic ordering (with the magnetic moments parallel to the b axis) at x>0.8; at this transition occurs through an intermediate state exhibiting spin-glass properties.     

Structural and magnetic characterisation of the perovskite oxides La0.7Ca0.3−x Na x MnO3

X-ray powder diffraction (XRD) and magnetic measurements were performed in order to investigate the effect of Na⁺ ion substitution for Ca²⁺ ions on the crystallographic structure, the character of magnetic ordering, and the effect of transition temperature in La_0.7Ca_0.3−x Na _x MnO₃ manganites series (0 ⩽ × ⩽ 0.2). All samples crystallise in an orthorhombic structure with the Pnma space group. We have found a strong dependence of structural and magnetic properties on the cation-size disorder parameter σ ². The temperature dependence of magnetization of all samples obeys the Bloch T ^3/2 law. The values of the spin wave constant at low temperature B increase with the increase of x and the Curie temperature decreases. It is concluded that the substitution of Ca by Na⁺ ions causes a decrease in total exchange integral Aof the samples.      

Influence of Bi3+ doping on electronic transport properties of La0.5−xBixCa0.5MnO3 manganites

Electronic transport properties of La_0.5?xBi_xCa_0.5MnO₃ (x=0, 1/16, 1/8, 1/4, 3/8 and 1/2) compounds have been studied systematically to investigate their charge ordering (CO) behaviors. The results show that the CO temperature increases with the substitution of Bi³⁺ ion for La³⁺ ion, suggesting that the charge ordering is enhanced. This is attributed to the special role of the 6s² lone pair of Bi³⁺. It is found that for all the samples the adiabatic small polaronic conduction mechanism is responsible for the transport behavior above CO transition, whereas Mott's variable range hopping mechanism dominates below the CO transition. In addition, the electronic transport behavior of La_0.5?xBi_xCa_0.5MnO₃ compounds is high sensitive to an external magnetic field, which could raise fresh opportunities for application in magnetic sensors.     

Effect of film thickness on the electromagnetic properties of La1−x Sr x MnO3 coatings

Coatings of La_1?x Sr _x MnO₃ (x = 0.225) with thicknesses of 6 and 1000 nm are deposited by magnetron sputtering on LaAlO₃ substrates and the electromagnetic properties of obtained films are studied in detail. It is found that the colossal magnetoresistance observed in the samples is considerably influenced by interface effects.     

Effects of A-site disorder on magnetic,electrical and thermal properties of La0.5−xLnxCa0.5−ySryMnO3manganites

The magnetic, electrical and thermal properties in the La_0.5?xLn_xCa_0.5?ySr_yMnO₃ (Ln=Pr, Nd, Sm) bulk system were investigated. Detailed dc magnetization and linear ac susceptibility measurements reveal that the samples first undergo phase transition from paramagnetic to ferromagnetic phase and then to an antiferromagnetic phase upon further cooling. It is found that both the Curie and Neel temperatures decrease systematically with increasing A-site disorder in these manganites. The electrical resistivity exhibits semiconducting behavior throughout the temperature range investigated and the electronic conduction mechanism can be conveniently described within the framework of the variable range hopping model above T=150 K. The Seebeck coefficient (S) in the magnetically ordered regime infers that the complicated temperature dependence of S is an indication of electron–magnon scattering. Specific heat measurements depict a broadened hump in the vicinity of T_C, indicating the existence of magnetic ordering and magnetic inhomogeneity in the samples. The temperature dependence of thermal conductivity, κ(T), reveals a positive dκ/dT in the paramagnetic region, which may be related to the local anharmonic lattice distortions associated with small polarons.     

Effect of oxygen deficiency on the magnetic field-dependent entropy in YBa2Cu3O7−δ

Roulin et al (1988), in one of their experimental papers, have presented a study of field-dependent entropy of high-purity YBa₂Cu₃O_7???δ (YBCO) as a function of oxygen deficiency. In order to explain their experimental results, we have used phenomenological GL-theory of anisotropic HTSCs in the London limit in line with of our earlier paper (Pattanaik et al, Physica B405, 3234 (2010)). Moreover, to account for the applicability of the theory at high field, we have incorporated the effect of vortex overlapping in the London theory done by Nanda (1995). Here, we have presented the variation of change in entropy (ΔS) with magnetic field for different oxygen deficiencies δ?=?0, 0.04, and 0.06. On comparison, we found that our results are in good agreement with the experimental data of Roulin et al (1988). The variation of penetration depth (λ) and anisotropic ratio of effective masses (γ) with concentration is also presented.     

Effect of High Resistance Phases on Metal—Insulator Transition of La0.67Ca0.33MnO3

The temperature dependence of resistivity ρ of YSZ doping composite of (1-x) La0.67Ca0.33MnO3 xYSZ and Y2O3 doping composite of (1-y) La0.67MnO3 yY2O3 is investigated,respectively,in a temperature range 77-300K,where the YSZ represents yttria-stabilized zirconia(8mol%Y2O3 92mol% ZrO2).Experimental results show that the YSZ doping level has important effects on both the metal-insulator(M-I) transition temperatures and zero field resistivity of the composites of (1-x) La0.67Ca0.33MnO3 xYSZ.However,the Y2O3 doping level has little effect on the M-I transition temperatures and the zero field resistivity of (1-y)La0.67Ca0.33MnO3 yY2O3 only increases slightly.The difference between the two types of composites may mainly results from the different distribution of high resistance phases at the grain boundaries and /or surfaces of La0.67Ca0.33MnO3 grains rather than the substitution of La^3 ions with Y^3 ions.     

Effect of Sr content on structure and electrical properties of La1−x Sr x MnO3 from ITSOFC cathode view point

The samples belonging to La_1−x Sr _x MnO₃ series, prepared by combustion route without using fuel, exhibit crystal structural phase transition with the change in Sr content. Less than 40 mol% Sr is partially substituted in the crystal structure of LaMnO₃. The structural phase transition from rhombohedral to cubic, cubic to tetragonal, and tetragonal to orthorhombic takes place on 40, 60, and 80 mol% addition of Sr. The highest electrical conductivity in La_0.4Sr_0.6MnO₃ is understood to be due to the maximum concentration of polaron. The polarons are formed due to the conversion of Mn³⁺ to Mn⁴⁺ so as to achieve electroneutrality after substitution of Sr²⁺ for La³⁺.     

Influence of processing conditions on the crystal structure and magnetic behavior of La0.7Ca0.3MnO3±δ samples

In this work we report crystallographic structure variations and the related modifications on the magnetic behavior of La_0.7Ca_0.3MnO_3±δ introduced by heat-treatments in different synthesis atmospheric conditions. We have prepared polycrystalline ceramic samples using a modified polymeric precursors method, which produces highly homogeneous specimens.The use of argon atmosphere enlarges the crystalline c-axis as detected by Rietveld refinements. As a consequence, an improvement in the magnetic transition temperature T_C of the samples was observed.Our results also indicate that different heat-treatment conditions change the magnetic interactions between the ferromagnetic (F) and antiferromagnetic (AF) structures of these systems. Our conclusions rely on the use of AC magnetic susceptibility measurements as the experimental tool for measuring these variations.     

Comment on ��The magnetic properties of nanosized La1???x Ca x MnO3 (0.5?��?x?��?0.8)��

Some data on the dc and ac magnetic properties of electron-doped La_1 ? x Ca _x MnO₃ (LCMO, 0.5 ?? x ?? 0.8) sol?Cgel prepared nanoparticles with mean particle size of ~50 nm have been reported in a recent article (Liu et al. J Nanopart Res 13:2305, 2011). In particular, it is claimed that the charge ordering (CO) transition, which is observed in the bulks, disappears completely, and the ferromagnetic (FM) one occurs in all the samples studied. The authors also attributed the enhanced FM state with relatively large values of spontaneous magnetization in their samples to the good connection among the adjacent particles. It is suggested in the comment through the present article that such claims seem to be doubtful, and a natural reason for the above noted effects may be the systematic shift of a real composition (Ca-doping level) from its nominal value due to, e.g., oxygen non-stoichiometry of the considered samples. The numerous uncertainties and obscurities are also characteristic of the article being commented upon.     

On the microstructure of the charge density wave observed in La1− x Ca x MnO3

We have used low temperature (90?K) transmission electron microscopy to investigate the ‘charge ordering’ modulation in the mixed valent manganite, La_{1? x} Ca _x MnO₃. It has been stated that Mn³⁺ and Mn⁴⁺ ions order at low temperature to produce a structural modulation composed of supercells whose size is an integer multiple of the unmodulated unit cell. Here, we use convergent beam electron diffraction to show that the periodicity of the modulation need not be an integer multiple of the undistorted cell, even on the smallest scales. We therefore suggest that this modulation is a charge density wave with a uniform periodicity. We show that the modulation wavevector lies close to the a* axis of the crystal but need not be exactly collinear. A typical grain of size 0.5?µm in La_0.48Ca_0.52MnO₃ had a wavevector which varied on a scale of tens of nanometres with an average of ?q??=?0.450a * and a standard deviation Δq?=?0.004?a* in its magnitude and Δθ?=?0.56° in its direction at 90?K. The magnitude of the wavevector in this composition fell by 20% as the temperature was increased from 90?K to room temperature. This change occurred by nucleation and growth. Although weak, the modulation was still present at room temperature, some 30?K above the ‘charge ordering temperature’.     

Effect of Ca doping level on the laser-induced voltages in tilted La1−x Ca x MnO3 (0.1 ≤ x ≤ 0.7) thin films

Tilted La_1?x Ca _x MnO₃ (0.1 ≤ x ≤ 0.7) thin films have been grown on vicinal cut LaAlO₃ (100) substrate by pulsed laser deposition. The laser-induced voltage effect was studied at room temperature with the KrF excimer laser using as the thermal source. The relationships between Ca doping level and voltage signal, response time and anisotropy Seebeck coefficient were established. The voltage signal and anisotropy Seebeck coefficient increase at first with increasing Ca doping level, reach a maximum at the same Ca content around x = 0.5, and then decrease. The respond time decreases with the Ca concentration increasing, and changes very little after x = 0.5. The figure of merit F _m was also the largest at this doping level, indicating a potential good performance of the photodetector devices. The variation of intrinsic structural and transport anisotropy induced by the change of Ca concentration has been proposed to account for the different LIV effects observed in LCMO thin films.     

Effect of cooling method on the electrochemical performance of 0.5Li2MnO3·0.5LiNi0.5Mn0.5O2 cathodes

0.5Li₂MnO₃·0.5LiNi_0.5Mn_0.5O₂ powders were synthesized by coprecipitation, and high temperature sintered with different cooled methods, such as cooled with furnace material, water material, and in liquid nitrogen (N-material). The effect of cooling methods on physical and electrochemical properties are discussed through the characterizations of X-ray diffraction (XRD), scanning electron microscopy, electrochemical impedance spectroscopy (EIS), and discharge, cyclic, and rate tests. XRD results show that all samples exhibit layered characteristics. The electrochemical performance results indicate that the N-material has the best electrochemical performance. The discharge capacity at 0.1 and 5 C are 279 and 99 mAhg^?1, respectively. The coulomb efficiency is highest, 78.4 %. The capacity retention after 50 cycles at 0.2 C is 97.1 %. EIS results show that the charge transfer resistance of N-materials is lowest, which is responsible for higher rate capacity.     

Effect of Nd doping on the magnetic properties of charge-ordered Bi0.6−x Nd x Ca0.4MnO3 (0.0≤x≤0.6) perovskite manganites

We have studied structure, magnetic and transport properties of polycrystalline Bi_0.6?x Nd _x Ca_0.4MnO₃ (x=0.0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6). Substitution of Nd at Bi sites induces a strong interplay between the magnetic and charge ordering. The charge-ordering temperature (T _CO) decreases with increasing x. Further, the antiferromagnetic ordering temperature (T _N) increases sharply at both extremes and remains nearly constant for x=0.2–0.4. At T<T _N a transition to a metamagnetic glass-like state is also seen. Nd doping also leads to enhancement in the magnetic moment and a concomitant decrease in resistivity up to x=0.3 and then an increase in resistivity up to x=0.5. Furthermore, Nd doping promotes an antiferromagnetic to ferromagnetic type fluctuation in the materials at room temperature, as evidenced by the change in the value of the paramagnetic Curie temperature. We find that the local lattice distortion induced by the size mismatch between the A-site cations and the 6s² character of Bi³⁺ lone pair electrons explains the observed peculiarity in magnetic and transport properties of Nd-doped Bi_0.6Ca_0.4MnO₃.     

Magnetotransport properties of La0.67Ca0.33MnO3//La0.67Sr0.33MnO3 bilayers

The perovskite bilayers La0.67Ca0.33MnO3 （LCMO） （100 nm） / La0.67Sr0.33MnO3（LSMO） （100 nm） and LSMO （100 nm） / LCMO （100 nm） are fabricated by a facing-target sputtering technique. Their transport and magnetic properties are investigated. It is found that the transport properties between them are different obviously due to distinguishable structures, and the different lattice strains in both films result in the difference of metal-to-insulator transition. Only single-step magnetization loop appears in our bilayers from 5K to 320K, and the coercive force of LSMO/LCMO varies irregularly with a minimum ～ 2387A/m which is lower than that of LCMO and LSMO single layer films. The behaviour is explained by some magnetic coupling.