Journal ArticleDOI

# Studies of transport and magnetic properties of Ce-doped LaMnO3

, B. Roy1
25 Feb 2004-Journal of Alloys and Compounds (Elsevier)-Vol. 365, Iss: 1, pp 94-101

AbstractThe X-ray diffraction, electrical resistivity (ρ), piezoresistance, thermoelectric power (TEP, S) and magnetisation experiments have been performed on polycrystalline La1−xCexMnO3 (x=0–0.6) samples over a temperature range (20–320 K). All the compounds except x=0.1 display a metal–insulator (MI) transition at TMI. From the analysis of the resistivity data we find that the double peaks observed in the ρ(T) data of ceramic samples are originating from the grain boundary effects. Moreover, the charge conduction at higher temperature occurs by means of a thermally activated polaron hopping mechanism. The application of pressure suppresses the resistivity and enhances the resistive transition temperature (TMI) and hence the Curie temperature (TC). The thermopower (TEP, S) of all the samples is positive and above TC, both the ρ(T) and S(T) curves show that charge conduction at high temperatures takes place according to Emin and Holstein’s theory of adiabatic polaron hopping. At low temperatures, thermal variation of the magnetization (M) can be explained considering the conventional spin wave theory containing T3/2 and T2 terms. In the paramagnetic region, the M(T) data fits well with the Curie–Weiss law.

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Abstract: With a view to investigate the influence of sintering temperature and oxygen stoichiometry on electrical and magnetic properties of sodium-doped lanthanum manganite sintered at different temperatures, a series of samples were prepared by the sol–gel route. The samples were characterized by the XRD studies and the data were analyzed using the Rietveld refinement technique and it has been observed that the materials are having rhombohedral structure with R 3 ¯ c space group. The electrical resistivity and thermoelectric power studies were investigated both as a function of crystallite size and oxygen content. To understand the conduction mechanism, the electrical resistivity data have been analyzed and it has been concluded that the variation of electrical resistivity in the ferromagnetic region can be explained by electron–electron scattering process (∼ T 2 ) and two magnon scattering processes, while that in the paramagnetic region is explained by the small polaron hopping mechanism. Similarly, the variation of thermopower in the ferromagnetic region is explained on the basis of electron–magnon scattering.

43 citations

Journal ArticleDOI
Dinesh Varshney
Abstract: A B S T R A C T The effect of Fe and Co doping on structural, electrical and thermal properties of half doped La0.5Ce0.5Mn1� x(Fe, Co)xO3 is investigated. The structure of these crystallizes in to orthorhombically distorted perovskite structure. The electrical resistivity of La0.5Ce0.5MnO3 exhibits metal-semiconductor transition (TMS at � 225 K). However, La0.5Ce0.5Mn1� xTMxO3 (TM = Fe, Co; 0.0 � x � 0.1) manganites show semiconducting behavior. The thermopower measurements infer hole as charge carriers and electron–magnon as well spin wave fluctuation mechanism are effective at low temperature domain and SPC model fits the observed data at high temperature. The magnetic susceptibility measurement confirms a transition from paramagnetic to ferromagnetic phase. The observed peaks in the specific heat measurements, shifts to lower temperatures and becomes progressively broader with doping of transition metals on Mn-site. The thermal conductivity is measured in the temperature range of 10– 350 K with a magnitude in between 10 and 80 mW/cm K.

27 citations

Journal ArticleDOI

Abstract: Influence of rare earth doping on electrical, magnetic and thermopower studies of La 0.34 Re 0.33 Ba 0.33 MnO 3 compound was investigated. Ferro to paramagnetic transition and metal to insulator transition temperatures decrease with decreasing ionic radius of the dopant ion. Electrical resistivity in the entire temperature range is explained by phase separation model. The magnitude of Seebeck coefficient increases with increasing dopant ionic radius. A cross over from negative to positive sign has also been observed in thermopower data with decreasing A site ionic radius (〈 r A 〉). The low temperature thermopower data has been explained using a qualitative model containing diffusion; magnon drag and phonon drag effects while the paramagnetic insulating part has been analyzed using small polaron hopping mechanism.

23 citations

Journal ArticleDOI
Dinesh Varshney
Abstract: The effect of Ce-doping on structural, magnetic, electrical and thermal transport properties in hole-doped manganites La0.7−xCexCa0.3MnO3 (0.0≤x≤0.7) is investigated. The structure of the compounds was found to be crystallized into orthorhombically distorted perovskite structure. dc Susceptibility versus temperature curves reveal various magnetic transitions. For x≤0.3, ferromagnetic regions (FM) were identified and the magnetic transition temperature (TC) was found to be decreasing systematically with increasing Ce concentration. The electrical resistivity ρ(T) separates the well-define metal-semiconducting transition (TMS) for low Ce doping concentrations (0.0≤x≤0.3) consistent with magnetic transitions. For the samples with 0.4≤x≤0.7, ρ(T) curves display a semiconducting behavior in both the high temperature paramagnetic (PM) phase and low temperature FM or antiferromagnetic phase. The electron–phonon and electron–electron scattering processes govern the low temperature metallic behavior, whereas small polaron hopping model is found to be operative in PM phases for all samples. These results were broadly corroborated by thermal transport measurements for metallic samples (x≤0.3) in entire temperature range we investigated. The complicated temperature dependence of Seebeck coefficient (S) is an indication of electron–magnon scattering in the low temperature magnetically ordered regime. Specific heat measurements depict a broadened hump in the vicinity of TC, indicating the existence of magnetic ordering and magnetic inhomogeneity in the samples. The observation of a significant difference between ρ(T) and S(T) activation energies and a positive slope in thermal conductivity κ(T) implying that the conduction of charge carriers were dominated by small polaron in PM state of these manganites.

22 citations

Journal ArticleDOI
Abstract: With a view to explaining the structural and electrical transport behaviour of some rare earth manganites, having the general formula Pr067D033MnO3 (D = Ca, Sr, Pb and Ba), a systematic investigation of the electrical resistivity, thermopower and magnetic properties has been undertaken These materials were prepared using the citrate–gel route by sintering at 900 ◦ C All the materials were characterized by x-ray diffraction, scanning electron microscopy, etc measurements The x-ray data were analysed using the Rietveld method and the variation of various parameters involved in the process as explained The average crystallite size of the materials has been estimated using the peak broadening method, while TC and TP values were determined from ac magnetic susceptibility and electrical resistivity measurements, respectively Finally, the magnetoresistance (MR) measurements were also carried out over a magnetic field of 1–7 T and the data clearly exhibit the extrinsic MR in ferromagnetic metallic region

20 citations

##### References
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Abstract: Metal-insulator transitions are accompanied by huge resistivity changes, even over tens of orders of magnitude, and are widely observed in condensed-matter systems. This article presents the observations and current understanding of the metal-insulator transition with a pedagogical introduction to the subject. Especially important are the transitions driven by correlation effects associated with the electron-electron interaction. The insulating phase caused by the correlation effects is categorized as the Mott Insulator. Near the transition point the metallic state shows fluctuations and orderings in the spin, charge, and orbital degrees of freedom. The properties of these metals are frequently quite different from those of ordinary metals, as measured by transport, optical, and magnetic probes. The review first describes theoretical approaches to the unusual metallic states and to the metal-insulator transition. The Fermi-liquid theory treats the correlations that can be adiabatically connected with the noninteracting picture. Strong-coupling models that do not require Fermi-liquid behavior have also been developed. Much work has also been done on the scaling theory of the transition. A central issue for this review is the evaluation of these approaches in simple theoretical systems such as the Hubbard model and $t\ensuremath{-}J$ models. Another key issue is strong competition among various orderings as in the interplay of spin and orbital fluctuations. Experimentally, the unusual properties of the metallic state near the insulating transition have been most extensively studied in $d$-electron systems. In particular, there is revived interest in transition-metal oxides, motivated by the epoch-making findings of high-temperature superconductivity in cuprates and colossal magnetoresistance in manganites. The article reviews the rich phenomena of anomalous metallicity, taking as examples Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Ru compounds. The diverse phenomena include strong spin and orbital fluctuations, mass renormalization effects, incoherence of charge dynamics, and phase transitions under control of key parameters such as band filling, bandwidth, and dimensionality. These parameters are experimentally varied by doping, pressure, chemical composition, and magnetic fields. Much of the observed behavior can be described by the current theory. Open questions and future problems are also extracted from comparison between experimental results and theoretical achievements.

5,274 citations

Journal ArticleDOI
Abstract: Recently, Jonker and Van Santen have found an empirical correlation between electrical conduction and ferromagnetism in certain compounds of manganese with perovskite structure. This observed correlation is herein interpreted in terms of those principles governing the interaction of the $d$-shells of the transition metals which were enunciated in the first paper of this series. Both electrical conduction and ferromagnetic coupling in these compounds are found to arise from a double exchange process, and a quantitative relation is developed between electrical conductivity and the ferromagnetic Curie temperature.

4,843 citations

Journal ArticleDOI

15 Apr 1994-Science
TL;DR: A negative isotropic magnetoresistance effect has been observed in thin oxide films of perovskite-like La0.67Ca0.33MnOx, which could be useful for various magnetic and electric device applications if the observed effects of material processing are optimized.
Abstract: A negative isotropic magnetoresistance effect more than three orders of magnitude larger than the typical giant magnetoresistance of some superlattice films has been observed in thin oxide films of perovskite-like La0.67Ca0.33MnOx. Epitaxial films that are grown on LaAIO3 substrates by laser ablation and suitably heat treated exhibit magnetoresistance values as high as 127,000 percent near 77 kelvin and ∼1300 percent near room temperature. Such a phenomenon could be useful for various magnetic and electric device applications if the observed effects of material processing are optimized. Possible mechanisms for the observed effect are discussed.

3,944 citations

Journal ArticleDOI
TL;DR: It is proposed that in addition to double-exchange physics a strong electron-phonon interaction arising from the Jahn-Teller splitting of the outer Mn $d$ level plays a crucial role.
Abstract: The ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ system with $02\ensuremath{\lesssim}x\ensuremath{\lesssim}04$ has traditionally been modeled with a double-exchange'' Hamiltonian in which it is assumed that the only relevant physics is the tendency of carrier hopping to line up neighboring spins We present a solution of the double-exchange model, show it is incompatible with many aspects of the data, and propose that in addition to double-exchange physics a strong electron-phonon interaction arising from the Jahn-Teller splitting of the outer Mn $d$ level plays a crucial role

2,272 citations

Journal ArticleDOI
Abstract: Zener has suggested a type of interaction between the spins of magnetic ions which he named "double exchange." This occurs indirectly by means of spin coupling to mobile electrons which travel from one ion to the next. We have calculated this interaction for a pair of ions with general spin $S$ and with general transfer integral, $b$, and internal exchange integral $J$.One result is that while the states of large total spin have both the highest and lowest energies, their average energy is the same as for the states of low total spin. This should be applicable in the high-temperature expansion of the susceptibility, and if it is, indicates that the high-temperature Curie-Weiss constant $\ensuremath{\theta}$ should be zero, and $\frac{1}{\ensuremath{\chi}}$ vs $T$ a curved line. This is surprising in view of the fact that the manganites, in which double exchange has been presumed to be the interaction mechanism, obey a fairly good Curie-Weiss law.The results can be approximated rather well by a simple semiclassical model in which the spins of the ion cores are treated classically. This model is capable of rather easy extension to the problem of the whole crystal, but the resulting mathematical problem is not easily solved except in special circumstances, e.g., periodic disturbances (spin waves).

2,000 citations