MnO3 System Research Articles

Low field magnetoresistance in La0.67Ca0.33MnO3 and Co3O4 combined system

We have studied the magneto-transport properties of the (1−x) La0.67Ca0.33MnO3/xCo3O4 (x∼0, 0.05, 0.10, 0.15 and 0.20) combined samples, prepared via sol–gel process. The powder X-ray diffraction patterns indicate no evidence of reaction between the La0.67Ca0.33MnO3 and Co3O4. A clear metal to insulator (M–I) transition is observed at 260K in virgin LCMO sample while that vanishes in the LCMO/Co3O4 combined samples in the measured temperature range (80–300K). Resistivity of the combined samples increases sharply as a consequence of Co3O4 addition, especially on the low temperatures, and magnetization decreases. An enhancement in low field magnetoresistance (LFMR) is observed at temperatures below 125K for the composites with lower Co3O4 content, namely, x=0.05 and 0.10. It is argued that this improved LFMR at low temperature (T<125K) arises due to the enhanced spin-polarized tunneling at the grain boundaries in the combined samples.

Abnormality of Magnetic Behavior and Resistivity of La0.7xDyxSr0.3MnO3 (0.00x0.30) System at Low Temperature

By measuring M-T curves, -T curves and M R-T curves of the samples under different temperatures, the influence of Dy doping (0:00 x 0:30) on the magnetic and electric properties of La0.7xDyxSr0.3MnO3 has been studied. The experimental results show that, with the increase of the Dy content, the system undergoes a transition from long range ferromagnetic order to the cluster-spin glass state and further to antiferromagnetic order. For the samples with x=0.20 and 0.30, their magnetic behaviors are abnormal at low temperature, and their resistivities at low temperature have a minimum value. These peculiar phenomena not only come from the lattice effect induced by doping, but also from extra magnetic coupling induced by doping.

Critical behavior studies in La1–xAgxMnO3 double‐exchange ferromagnet

AbstractA systematic low‐field ac susceptibility measurement was carried out to study the critical phenomena of La1–x Agx MnO3 (x = 0.10 and 0.15) system near its ferromagnetic to paramagnetic phase transition by determining the Curie temperature and asymptotic magnetic susceptibility critical exponent. The obtained values of critical exponent in the actual critical region (ACR) are in good agreement with the values predicted by the three‐dimensional Heisenberg Model. The width of the ACR decreases with increasing Ag concentration. The observed critical behavior is discussed based on the finite clusters plus ferromagnetic matrix model. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Structural and magnetic properties of Nd0.7Ce0.3MnO3 thin films

This paper reports on the structural and magnetic properties of Nd0.7Ce0.3MnO3 (NCeMO) epitaxial thin films lacking Ce-rich impurities with the intention being to investigate the effect of ionic radius of the A site cation on the microstructures and magnetic properties in comparison with (La,Ce)MnO3 (LCeMO) systems. The absence of Ce-rich impurities in NCeMO films was confirmed by high resolution transmission electron microscopy, which in fact differs from results of LCeMO films where such Ce-rich impurities are responsible for the emergence of extrinsic ferromagnetism. In addition, as the ambient atmosphere during film formation was reduced, not only the crystallinity tended to improve but also the ferromagnetic-like behavior was enhanced, unlike the case with LCeMO films.

Influences of W doping at Mn site on magnetic structure of La0.3Ca0.7MnO3 system

The influences of W doping at Mn site on the magnetic structure of the charge-ordering system La0.3Ca0.7MnO3 are studied by the measurements of M_T curves,M_H curves and ESR spectra of polycrystalline samples of La0.3Ca0.7Mn1-xWxO3(x=0.00,0.04,0.08,0.12,0.15) are studied. The results show that when the doping amount is 0.00≤x≤0.08, the charge-ordering(CO) phase exists in the system, the AFM/CO states coexist below the transition temperature, and the charge-ordering temperature Tco goes up with the increase of W doping content. For the sample of x=0.04, FM phase and AFM/CO phases coexist at low temperature, and FM is separated from PM both before and after the formation of CO phase; when x≥0.12, CO state melts, and the paramagnetism-ferromagnetism (PM-FM) transition exists at extremely low temperature.

Reentering spin glass behavior and charge ordering in phase separation Nd0.5Ca0.5MnO3 system

Half-doped manganese oxides are very important on clarifying physical mechanism of strong correlation electron system and colossal magnetoresistance (CMR) effec t due to their rich physical content. The structure, electrical and magnetic tra nsport properties of Nd0.5Ca0.5MnO3 manganite w as systemically studied. The results show that Nd0.5Ca0.5 MnO3 compound displays the O′-orthorhombic structure and indicate th e existence of the typical Jahn-Teller distortion under 300 K. The transport pro perties show that the system undergoes paramagnetic insulator-ferromagnetic meta l transition under a magnetic field of 8 T and is accompanied by CMR effect. Mea nwhile, it is found that the temperature of antiferromagnetic transition and cha rge ordering is around 150 and 240K, respectively. There appears a typical reent rant spin glass behavior around 41K and the negative magnetization is also obser ved in the compound. All these phenomena indicate that there exist several comp lex magnetic interactions in the ground state of this half-doped Nd0.5Ca0.5MnO3 compound at low temperature. The present result s provide experimental data for strongly electronic correlated system of manga nites.

Structural, magnetic and magnetotransport properties of La0.7−x Ce x Ba0.3MnO3 (x = 0.0–0.4)

Structural, magnetic and magnetotransport properties of La0.7−x Ce x Ba0.3MnO3 (x = 0–0.4) have been investigated although some unreacted secondary phases of CeO2 were present. The rhombohedral structure (R-3c) forx = 0 transforms to orthorhombic with the space groupImma forx = 0.3. All samples showed ferromagnetic transition above 300 K and a negative magnetoresistance. For x>0.1, magnetization data measured at 1 T showed a decrease at low temperatures (T < 50 K) due to antiferro-magnetic coupling between Ce-local moments and Mn-moments. Forx = 0.4, the resistivity showed a maximum around 200 K which corresponds to ordering temperature of cerium. Since these results are similar to that observed in the Sr-containing La0.5−x Ce x Sr0.5MnO3 (x = 0–0.4) system, we suggest that the cerium ions are in the trivalent state and the anomalous behaviour has been attributed to a Kondo-like effect.

Critical exponents of the La0.7Sr0.3MnO3, La0.7Ca0.3MnO3, and Pr0.7Ca0.3MnO3 systems showing correlation between transport and magnetic properties

From the low-temperature (down to 10K) dc-magnetization data of the La0.7Sr0.3MnO3 (LSM), La0.7Ca0.3MnO3 (LCM), and Pr0.7Ca0.3MnO3 (PCM) systems, we estimated the critical exponents β, γ, and hence δ from the analysis of the modified Arrot plots. The exponent β estimated for the LCM system is less than that predicted by Heisenberg model and resides within the zone predicted by Ising model while for the LSM sample, β is higher than that predicted from the Heisenberg model which is considered to be due to the presence of dipole-dipole interaction arising from the large spin moment in the LSM system. The magnetization data of the PCM system cannot be fitted to the modified Arrot plots, which suggest highly inhomogeneous ground state even under 5T magnetic field. Both LSM and LCM have almost equal values of γ. Seebeck coefficient data indicate a crossover from higher-temperature n-type to lower-temperature p-type conductivity behavior in both LSM and LCM systems, while the semiconducting PCM system shows p-type conductivity throughout the temperature range (300–80K). It is noticed that for LSM system TC (Curie temperature) and TMI (metal-insulator transition temperature) are almost equal (∼360K), whereas for the LCM system there exist a large difference between TC and TMI (TC∼245K and TMI∼265K), which may give some idea regarding the critical behavior of the respective samples.

Effects of cation disorder and size on metamagnetism in A-site substituted Pr0.5Ca0.5MnO3 system

The effects of A-site cation disorder and size on metamagnetism of ABO3 type charge and orbital ordered Pr0.5Ca0.5MnO3 system have been studied by substituting Ba+2 for Ca+2 or La+3 for Pr+3. Substitution of 5% Ba+2 or 5% La+3 drastically reduces the critical magnetic field (Hc) for metamagnetism and induces successive steplike metamagnetic transitions at low temperatures. Interestingly, with further increase in substitution, Hc rises. We find that there is a sharp decrease in electrical resistivity corresponding to the metamagnetic transitions, which is indicative of strongly correlated magnetic and electronic transitions in these manganites.

Magnetocaloric effect of the (Pr,Ca)MnO3 manganite at low temperatures

We have studied the magnetocaloric effect of the Pr1−xCaxMnO3 manganite, with 0.30⩽x⩽0.45, from 2.5 upto 50K. At this temperature range, the (Pr,Ca)MnO3 system present ferro/antiferromagnetic phase mixture which is responsible for the colossal magnetoresistance. Magnetocaloric studies in manganites, up to now, have neglected this magnetic phase mixture, mainly due to the antiferromagnetic phase which is not so well explored in magnetic refrigeration studies as ferromagnetic systems is. In our study, we explain the origin of the negative and positive values of the magnetic entropy change, their temperature dependence as well as the high values. The study of systems with antiferromagnetic phase, as shown in this work, may open an important field in the search of new magnetic materials for magnetic refrigeration.

Magnetic Characterization of La0.75-xGdxCa0.25MnO3 Manganites

Polycrystalline La0.75-xGdxCa0.25MnO3 samples (x=0, 0.10, 0.25, 0.33, 0.50, and 0.75) were prepared by the carbonate precipitation method. For a comparison the nanocrystalline samples with the same compositions were obtained by sol-gel method using urea as gelation agent. The XRD patterns were indexed in orthorhombically distorted perovskite-type unit cell. The unit cell volume steadily decreases with rising Gd content. Magnetic properties of La0.75-xGdxCa0.25MnO3 systems are discussed and compared to other magnetoresistive systems.

EPR study of the spin dynamics of the (La1−y Pry)0.7Ca0.3MnO3 system

The EPR of Mn ions in the (La1−y Pry)0.7Ca0.3MnO3 system has been studied within a broad range of temperatures (4<T<600 K) and Pr concentrations (0≤y≤1), as well as under isotope substitution of 18O for 16O. All compositions were shown to undergo transitions to a magnetically ordered state with decreasing temperature. Magnetic phase diagrams were constructed for systems with different oxygen isotopes. The diagrams include paramagnetic, ferromagnetic, and antiferromagnetic regions. In the paramagnetic region, at temperatures not too close to the phase transition points, the Mn ion linewidth ΔH pp (T) is related to the magnetic susceptibility χ(T) through the relation ΔH pp (T) = [χ0/χ(T)]ΔH pp (∞) + ΔH 0, where ΔH pp (∞) is the width of the exchange-narrowed line in the high-temperature approximation, χ0 ∝ 1/T is the susceptibility of noninteracting ions, and ΔH 0 is the residual width originating from the sample porosity and resonance-field scatter in unoriented grains of a powder sample. An analysis of the data on ΔH pp (∞), ΔH 0, and χ(T) made it possible to estimate the symmetric and antisymmetric exchange interaction of Mn ions and of the noncubic crystal-field component of the oxygen ions. These parameters were found to be independent of the oxygen isotope species to within experimental error.

Low temperature synthesis, and magnetic and magnetotransport properties of a(La1−xLux)0.67Ca0.33MnO3 (0≤ x ≤ 0.12) system

We have been able to synthesizeLu3+ substituted La0.67Ca0.33MnO3 (LCMO) by an auto-combustion method. Synthesis of this compound isnot successful by conventional ceramic or other chemical methods. Themagnetic and electrical transport properties of the Lu substituted LCMO[(La1−xLux)0.67Ca0.33MnO3 ()] system have been investigated and compared with those of theY3+,Pr3+,Dy3+ andTb3+ substitutedLCMO systems. All the compounds show a ferromagnetic metal to paramagnetic insulator transition atTC. The tolerance factorreduces from 0.917 for x = 0 to 0.909 for x = 0.12, and for this range all are ferromagnetic metals, indicating the dominance of the couplingbetween spins due to double exchange over the antiferromagnetic superexchange interaction.The transition temperatures and magnetization decrease as the Lu concentration increases.This is satisfactorily accounted for on the basis of a transition from ferromagnetic atx = 0 to cantedspin order for x>0. All the samples show a higher magnitude of MR compared to that in pure LCMO at80 kOe field in the temperature range 5–320 K. A fairly high value of low fieldmagnetoresistance (LFMR) of about 30% is obtained in all the samples at a field less than5 kOe.

Effect of progressive substitution of La3+ by Bi3+ on the structure, magnetic and transport properties of La0.67Sr0.33Mno3

Polycrystalline La0.67−xBixSr0.33MnO3 (x=0, 0.1, 0.2, 0.3 and 0.67) was prepared by solid-state reaction. The effects of La-substitution by Bi with a typical polarized lone pair electron character on the structure, magnetic and transport properties are presented. The results show that the polarized lone pair 6s2 electrons have a dramatic effect on these properties. Structure refinements by the Rietveld method show that the substitution elongates the a- and c-axes, and eventually changes the structure from rhombohedral to tetragonal. At the same time, the elongated Mn3+–O–Mn4+ chain weakens the double exchange between adjacent Mn3+ and Mn4+ ions via the O bridge. As a consequence, the ferromagnetic coupling temperature decreases from 370K down to 330K as x increases to 0.3 from 0, and down to 270K when La is totally replaced by Bi. With an increasing substitution level, the saturation moment (Ms) decreases from the near theory value of 3.67μB down to 2.5μB for x=0.3, and 0.08μB for the totally substituted sample. The sample with x=0.67 shows a very weak ferromagnetic property. However, Ms should not change if only the unchanged Mn3+/Mn4+ ratio is considered in the La0.67−xBixSr0.33MnO3 system. The decrease in Ms can only be explained by an enhanced anti-ferromagnetic coupling that is simultaneous with the weakening of the ferroelectric coupling by Bi. With increasing Bi content, the resistivity increases, and the temperature of the semiconductor to metallic transition also rises. Eventually the totally substituted sample becomes an insulator.

Phase separation and abnormal transport behaviours in La 0.7 x Gd x Sr 0.3 MnO 3 system

The magnetic and transport behaviours of the La0.7−xGdxSr0.3MnO3 (0≤x≤0.70) system are investigated. The experimental results indicate that with increasing Gd doping content, the magnetism of the system changes from the long-range ferromagnetic order state to the cluster-spin glass state, then to the antiferromagnetic (AFM) state. It is interesting that the phase separation appears at x=0.30 and 0.40 and disappears for x≥0.50 where the AFM state occurs. At high doping content, the transport behaviours exhibit abnormality, e.g. there are two temperature ranges in which the ρ–T curves can be well fitted by a variable-range hopping (VRH) model. We suggest that the VRH does not come from the hopping of carriers between clusters, but from the different magnetic backgrounds in the clusters.

Studies on the structure and transport properties for Y-doped La2/3Ca1/3MnO3 perovsksite manganate

The structure and transport properties of perovskite (La1-xYx)2/3Ca1/3 MnO3 (0≤x≤0.3) systems are systematically investigated. It is found that all the compounds show a single phase structure. With the increase of Y3+ doping content the metalinsulator transition temperature TMI(M—I) shifts to lower temperatures. While the relevant resistance peak ρp is sharply increased, for the sample of x=0.3, it has been enhanced by eight orders of magnitude larger than the non-doped sample(x=0.0). In these materials a large magnetoresistance effect has been observed under an external magnetic field. At the same time, it is also directly shown that the correlation between the transport properties and the variation of crystal structure from the experiment result. Based on the double exchange model and the variable-range-hopping(VRH) theory, the mechanism of the influence of Y doping for La in La2/3Ca1/3MnO3 systems is also discussed.

Influence of Gd Doping at A Site upon the Magnetic Structureof La0.7-xGdxSr0.3MnO3 Systems

We investigated the magnetism of systemsLa0.7-xGdxSr0.3MnO3 (x = 0.00, 0.10,0.15, 0.20, 0.30, 0.40, 0.50, 0.60, 0.70) by M-T curves, M-H curves,and electron-spin-resonance (ESR) spectra. The experimental resultsindicate that the system is in a ferromagnetic long-range order atlow-doping level; the samples of x = 0.30 and 0.40 show cluster-spinglass behaviour; and the antiferromagnetic characters are observed atx⩾0.50. For the samples of x = 0.30 and 0.40, the ESR spectrademonstrate that the phase separation occurs above the Curie temperature.

Insulator-metal transition shift related to magnetic polarons in La 0.67- xY xCa 0.33MnO 3

The magnetic transport properties have been measured for La0.67-xYxCa0.33MnO3 (0 <= x <= 0.14) system. It was found that the transition temperature Tp almost linearly moves to higher temperature as H increases. Electron spin resonance confirms that above Tp, there exist ferromagnetic clusters. From the magnetic polaron point of view, the shift of Tp vs. H was understood, and it was estimated that the size of the magnetic polaron is of 9.7~15.4 angstrom which is consistent with the magnetic correlation length revealed by the small-angle neutron-scattering technique. The transport properties at temperatures higher than Tp conform to the variable-range hopping mechanism.

Transport properties in La0.7-xGdxSr0.3MnO3 system

The influence of Gd doping at La-site on the electrical transport properties and the colossal magnetoresistance of La0.7−xGdxSr0.3MnO3 (x=0.00, 0.10, 0.15, 0.20, 0.30, 0.40, 0.50, 0.60, and 0.70) is studied. The experimental results indicate that the transport properties exhibit abnormal behavior under high doping condition. Forx=0.50, we find that a transition from metal to insulator occurs after the occurrence of insulator-metal transition nearTc, which seldom occurs in ABO3 structure. For samplesx=0.60 and 0.70, it exhibits insulator behavior far aboveTc. These abnormal behaviors are attributed to different magnetic background, i.e. the system undergoes a transition from long range ferromagnetic order to the cluster-spin glass state and further to antiferromagnetic order.

Yttrium-Substituted (La, Y, Sr)MnO3 as a SOFC Cathode Material.

A material consisting of (La1-xYx)0.7Sr0.3MnO3 was synthesized and evaluated to enhance the thermal expansion compatibility with an electrolyte material of Solid Oxide Fuel Cells (SOFC) compared with the standard system La0.7Sr0.3MnO3. In addition, its crystal structure, chemical stability, electrical conductivity and electrode performance were examined. For the substitution range x=0.0-0.2 in the (La1-xYx)0.7Sr0.3MnO3 system, a single perovskite phase was observed. The mixture of (La1-xYx)0.7Sr0.3MnO3 (x=0.0-0.6) and YSZ, which was fired at 1400°C after mixing, did not contain the reaction products, such as La2Zr2O7, Y4Zr3O12, and SrZrO3. The average thermal expansion coefficients between 50°C and 1000°C at x=0.0 and x=0.2 were 12.2×10-6K-1 and 11.5×10-6K-1, respectively. The electrical conductivity of the phases with x=0.0 and 0.2, at 1000°C in air, were 172 and 217S·cm-1, respectively. The electrode performance of (La0.8Y0.2)0.7Sr0.3MnO3 was almost equal to that of La0.7Sr0.3MnO3 at 1000°C. Substitution of Y at the content of x=0.2 proved to be effective in obtaining higher expansion compatibility and electrical conductivity than the standard system without any degradation of the other properties.