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     

Optical evidence for lattice polarons in lightly doped lanthanum manganites in paramagnetic state

The reflection and absorption spectra and the resistivity data for La_0.93Sr_0.07MnO₃, La_0.9Sr_0.1MnO₃, La_0.85Sr_0.15MnO₃, La_0.92Ca_0.08MnO₃ and La_0.85Ba_0.15MnO₃ single crystals have been collected and analyzed by using small lattice polaron theory. The activation energies of the polaron hop are determined. The lattice polarons are found to be dominated in the electronic transport and optical properties in the paramagnetic state in La_0.93Sr_0.07MnO₃, La_0.92Ca_0.08MnO₃ and La_0.85Ba_0.15MnO₃ crystals. It is shown that lattice polaron involves more than one Mn site.     

The sound velocity, internal friction, and thermal expansion in a single crystal of La0.85Sr0.15MnO3

The results are given of the investigation of the temperature dependence of the sound velocity v, internal friction Q ^?1, and thermal expansion ΔL/L of a single crystal of La_0.85Sr_0.15MnO₃ in the temperature range from 5 to 400 K. Clearly defined singularities of the elastic properties at a temperature of charge ordering T _co≈200 K are revealed. The results of X-ray diffraction studies performed at room temperature are used to determine the orientations and estimate the sizes of twins. The correlation between magnetic and structure inhomogeneities is established. Based on the results of analysis of the temperature dependence of internal friction and thermal expansion, an assumption is made of the presence in a single crystal of La_0.85Sr_0.15MnO₃ of a structural transition in the temperature range from 15 to 60 K that has not been observed previously.     

Temperature dependence of infrared conductivity of manganites (x = 0, 0.05 and 0.2)

The temperature dependence of the reflectivity spectra of three manganites ceramics with compositions Pr_0.7Ca_0.3MnO₃, Pr_0.7Ca_0.25Sr_0.05MnO₃ and Pr_0.7Ca_0.1Sr_0.2MnO₃ has been investigated by infrared reflectivity spectroscopy in the wave number range 0.005-1.1 eV. The compound Pr_0.7Ca_0.25Sr_0.05MnO₃ which shows the largest conductivity jump at the ferromagnetic-paramagnetic phase transition has been studied in details. The optical conductivity of this compound is deduced from the best fit to reflectivity spectra of a “double-damping Drude” model, itself derived from the factorized form of the dielectric function. Excellent agreement with Kramers-Kronig transformation is reported. The model allows in particular to discriminate the contributions to the optical conductivity of trapped charges (polarons) and mobile charge carriers. Received 20 July 1999 and Received in final form 15 October 1999     

Cation disorder and size effects on the magnetic transition in Ba-containing ferromagnetic manganites

The results of ⁵⁵Mn NMR, dc magnetization, and ac susceptibility studies are presented for La_0.7Ca_0.15Ba_0.15MnO₃, La_0.7Sr_0.15Ba_0.15MnO₃, and La_0.7Ba_0.3MnO₃ ferromagnetic manganites. While is a function of the mean radius of the La and alkaline-earth ions and the cation disorder, the form of the temperature dependence of the magnetic moment may be expressed as function of only. The phase transition is continuous for all three compounds. Received 5 March 1999     

Orbital degrees of freedom and electron correlation in optical absorption of metallic LaSrMnO

The optical absorption in ferromagnetic metal La_1-xSr_xMnO₃ is anomalous; it has a wide-range absorption up to about 1 eV even at zero temperature. Since 3d electrons in La_1-xSr_xMnO₃ partially fill doubly degenerate e_g orbitals, the orbital degrees of freedom are crucial to understand this metallic system. We argue that the interband transition within e_g orbitals is important in the optical absorption. The optical spectrum is modified also by the inter-orbital Coulomb interaction. We have examined perturbatively the effect of the Coulomb interaction on the spectrum. Available experiments are discussed by comparing with the present results. Received: 13 February 1998 / Accepted: 17 March 1998     

Specific heat of the electronic superconductor Pr1.85Ce0.15CuO4

We have investigated the specific heat of the ceramic Pr_1.85Ce_0.15CuO₄ in the temperature range 2–800K in magnetic fields up to 8 T. We have determined the magnitude of the specific-heat discontinuity at the superconducting transition ΔC/T _c and estimate the coefficient of the electronic specific heat. In the temperature interval 5–800K we have separated out the phonon contribution to the specific heat, determined the temperature dependence of the characteristic Debye temperature Θ, and calculated the mean frequencies (moments) of the phonon spectrum. We compare the parameter values obtained in this way with data for the compound La_1.85Sr_0.15CuO₄ having similar crystal structure, but hole conductivity. The magnitude of the specific-heat discontinuity and consequently the effective electron mass in the “electronic” superconductor Pr_1.85Ce_0.15CuO₄ is several times smaller than in the “hole” superconductor La_1.85Sr_0.15CuO₄. The phonon spectrum in Pr_1.85Ce_0.15CuO₄ in the low-energy region is somewhat “more rigid,” and in the high-energy region somewhat “softer” than in La_1.85Sr_0.15CuO₄. Fiz. Tverd. Tela (St. Petersburg) 41, 1154–1159 (July 1999)     

Magnetoresistance of lanthanum manganites with activation-type conductivity

The temperature dependence of the resistivity and magnetic moment of La_0.85Ba_0.15MnO₃ and La_0.85Sr_0.15MnO₃ manganite single crystals in magnetic fields up to 90 kOe is investigated. Analysis of the experimental results shows that the magnetoresistance of lanthanum manganites far from the Curie temperature T _C can be described quantitatively by the s-d model normally used for ferromagnets and taking into account only the exchange interaction between the spins of charge carriers and magnetic moments. These data also show that the features of lanthanum manganites responsible for colossal magnetoresistance (CMR) are manifested in a narrow temperature interval δT ≈ 20 K near T _C. Our results suggest a CMR mechanism analogous to the mechanism of giant magnetoresistance (GMR) observed in Fe/Cr-type multilayers with nanometer layer thickness. The nanostratification observed in lanthanum manganites and required for GMR can be described taking into account the spread in T _C in the CMR range δT.     

Thermoelectric power of potassium doped lanthanum manganites at low temperatures

The temperature-dependent resistivity and thermoelectric power of monovalent (K) doped La_1−xK_xMnO₃ polycrystalline pellets (x=0.05, 0.10 and 0.15) between 50 and 300 K are reported. K substitution enhances the conductivity of this system. Curie temperature (T_C) also increases from 260 to 309 K with increasing K content. In the paramagnetic region (T>T_C), the electrical resistivity is well represented by adiabatic polaron hopping, while in the ferromagnetic region (T<T_C), the resistivity data show a nearly perfect fit for all the samples to an expression containing, the residual resistivity, spin-wave and two-magnon scattering and the term associated with small-polaron metallic conduction, which involves a relaxation time due to a soft optical phonon mode. Small polaron hopping mechanism is found to fit well to the thermoelectric power (S) data for T>T_C whereas at low temperatures (T<T_C) in ferromagnetic region (S_FM), S_FM is well explained with the spin-wave fluctuation and electron–magnon scattering. Both, resistivity and thermopower data over the entire temperature range (50–300 K) are also examined in light of a two-phase model based on an effective medium approximation.     

Polaron hopping conduction in La0.85Sr0.15MnO3 single crystal

Temperature dependences of electrical resistance and thermopower of a La_0.85Sr_0.15MnO₃ single crystal were measured in the paramagnetic and ferromagnetic dielectric phases. It was shown that charge transfer in both phases is due to the variable-range polaron hopping over the localized states. The activation energies in both phases linearly depend on T ^3/4 but differ from each other by a constant which is approximately equal to the exchange energy kT _c. The results obtained are in compliance with the concept of percolation character of metal-insulator transition in manganites.     

Connection between colossal magnetoresistance in La0.85Sr0.15MnO3 and magnetic ordering inhomogeneities of a sample

Resistive and magnetic measurements are made for La_0.85Sr_0.15MnO₃. The dependence of resistivity on the applied magnetic field (10, 20, 30, and 50 kOe) and temperature (200–310 K) is analyzed using the s-d model and the obtained experimental data. The physical features that should be contained in models proposed to explain the colossal magnetoresistance of manganites with activation-type conductivity are determined. It is shown that the proposed mechanism associating the colossal magnetoresistance effect with phase separation into ferromagnetic and paramagnetic microregions near the Curie temperature has the necessary features.     

Magnetic order in LaEuSrCuO studied [2pt] by Fe Mössbauer spectroscopy

⁵⁷Fe M?ssbauer effect studies of La_1.65Eu_0.20Sr_0.15CuO₄ doped with 0.5 at% ⁵⁷Fe performed in the temperature region 300 K > T > 4.2 K give an onset temperature for magnetic ordering of K. This temperature practically is the same as that found in Nd doped La_2-xSr_xCuO₄. It indicates that the magnetic ordering temperature in the LTT phase of rare earth (RE) doped La_2-xSr_xCuO₄ is independent of the RE moment. The direction of the ⁵⁷Fe magnetic moment in the magnetically ordered state is within the CuO₂ plane, while it has been found to be parallel to the c-axis in Nd doped La_2-xSr_xCuO₄. Received: 23 June 1998 / Accepted: 14 July 1998     

Thermal conductivity of silver doped lanthanum manganites between 10 and 300 K

Thermal conductivity (λ) of nanocrystalline La_1−xAg_xMnO₃ (x=0.05, 0.15, 0.25, 0.3) pellets prepared by pyrophoric method is reported between 10 and 300 K. Magnitude of thermal conductivity has been found to be strongly influenced by monovalent (Ag) substitution at the La site. Silver doping in LaMnO₃ enhances T_C of the system to ∼299 K. Qualitative nature of the temperature variation of thermal conductivity of the silver substituted lanthanum manganites remains closely similar to that for divalent doped systems. Our analysis demonstrates that in La_1−xAg_xMnO₃ also, the mechanism of heat conduction is predominantly by phonons. The contribution of the electronic part is only ∼1% of the total λ. The spin wave contribution is also estimated close to T_C, which for all the samples lies within ∼2%. At temperatures below ∼100 K, the measured data have been analyzed using phonon relaxation time method and the strengths of the various phonon scattering processes have been estimated. Our analysis further suggests strong influence of phonon scattering by 2D like defects in the thermal conductivity of monovalent doped lanthanum manganites at low temperatures (<70 K) in the ferromagnetic region.     

Pressure-induced modifications of crystal and magnetic structure of oxygen deficient La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3-d</Subscript> manganites

The crystal and magnetic structures of the oxygen deficient manganites La_0.7Sr_0.3MnO_3-d (d = 0.15, 0.20) have been studied by means of powder neutron diffraction over the 0–5.2 GPa pressure and 10–290 K temperature ranges. La_0.7Sr_0.3MnO_2.85 exhibits a coexistence of rhombohedral and tetragonal (I4/mcm) crystal structures and below T_g ~ 50 K a spin glass state is formed. La_0.7Sr_0.3MnO_2.80 exhibits a tetragonal (I4/mcm) crystal structure. Below T_g ~ 50 K a phase separated magnetic state is formed, involving coexistence of C-type AFM domains with spin glass domains. In both compounds the crystal structure and magnetic states remain stable upon compression. The factors leading to the formation of different magnetic states in La_0.7Sr_0.3MnO_3-d (d = 0.15, 0.20) and their specific high pressure behavior, contrasting with that of the stoichiometric A_0.5Ba_0.5MnO₃ (A = Nd, Sm) compounds showing pressure-induced suppression of the spin glass state and the appearance of the FM state, are analysed.     

Pressure effects in ferromagnetic manganites and carrier scattering by disordered magnetic rare earths

The influence of hydrostatic pressure on the transport properties of (La_1-xR_x)_0.67Ca_0.33MnO₃ (,Tb) ferromagnetic manganites is investigated. The enhancement of the Curie temperature T_C under pressure agrees with previous data. In the paramagnetic range, the resistivity can be represented by a Mott localisation law, with a characteristic temperature T₀ decreasing with pressure. The variation of T_C with pressure is compared to the effect induced by replacing La by a magnetic rare earth in (La_1-xR_x)_0.67Sr_0.33MnO₃ manganites (, ..., Tm). The main effect is not related to the decrease of the mean radius of the cation, but to an additional scattering by the magnetic moment of the rare earth. Received: 15 May 1998 / Revised: 6 July 1998 / Accepted: 16 July 1998     

Influence of the lanthanum deficit on electrical resistivity and heat capacity of silver-doped lanthanum manganites La<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ag<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>MnO<Subscript>3</Subscript>

The electrical resistivity and heat capacity of the silver-doped lanthanum manganites La_0.80Ag_0.15MnO₃ and La_0.85Ag_0.15MnO₃ have been investigated. Despite the nonstoichiometry of the composition, the La_0.80Ag_0.15MnO₃ manganite exhibits a bulk homogeneity and better physical properties from the applied point of view as compared to the La_0.85Ag_0.15MnO₃ manganite, viz., the former compound has a higher spontaneous magnetoresistance and a larger jump of the heat capacity with a small width of the phase transition, and the anomalies of the heat capacity and electrical resistivity in the vicinity of the Curie point of this compound agree with the fluctuation nature of the second-order phase transition. The behavior of the properties of lanthanum-deficient manganites under investigation in the region of the phase transition is consistent with the classical theory of indirect exchange interaction. The behavior of the temperature dependence of the electrical resistivity has been analyzed in terms of two models. One of these models is based on the tunneling of charge carriers between ferrons or polarons, and the other model is based on the polaron hopping conduction. Both approaches lead to consistent results, and their combination has made it possible to estimate the tunneling distance of charge carriers. The origin of the influence of technological parameters characterizing the synthesis of La_{1 − x} Ag _y MnO₃ ceramic materials on their physical properties has been elucidated.     

Structural, magnetic and magnetotransport behavior of La0.7SrxCa0.3−xMnO3 compounds

A systematic investigation of the structural, magnetic and electrical properties of a series of nanocrystalline La_0.7Sr_xCa_0.3−xMnO₃ materials, prepared by high energy ball milling method and then annealed at 900 °C has been undertaken. The analysis of the XRD data using the Win-metric software shows an increase in the unit cell volume with increasing Sr ion concentration. The La_0.7Sr_xCa_0.3−xMnO₃ compounds undergo a structural orthorhombic-to-monoclinic transition at x=0.15. Electric and magnetic measurements show that both the Curie temperature and the insulator-to-metal transition temperature increase from 259 K and 253 K correspondingly for La_0.7Ca_0.3MnO₃ (x=0) to 353 K and 282 K, respectively, for La_0.7Sr_0.3MnO₃ (x=0.3). It is argued that the larger radius of Sr²⁺ ion than that of Ca²⁺ is the reason to strengthen the double-exchange interaction and to give rise to the observed increase of transition temperatures. Using 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 resistivity versus temperature data measured in the range of 50-320 K and found that the activation barrier decreased with the raising Sr²⁺ ion concentration.     

Thermal conductivity of La0.67(CaxSr1−x)0.33MnO3 (x=0, 0.5, 1) and La0.6Y0.07Ca0.33MnO3 pellets between 10 and 300 K

Thermal conductivity (λ) of nanocrystalline La_0.67(Ca_xSr_1−x)_0.33MnO₃ (x=0, 0.5, 1) and La_0.6Y_0.07Ca_0.33MnO₃ pellets prepared by a novel ‘pyrophoric’ method have been studied between the temperature range 10 and 300 K. Our data show that the magnitude of thermal conductivity is strongly influenced by the ion substitutions at La-site. The analysis of the thermal conductivity data indicates that the thermal transport is governed largely by phonons scattering in these systems and the electronic contribution is as small as 0.2-1% of total thermal conductivity (λ_total). At low temperatures (<90 K) 2D like lattice defects contribute to the phonon scattering dominantly and its strength increases with increasing Sr content and also with partial substitution of La by Y. Depending upon the composition of the samples, the magnon thermal conductivity contributes 2-15% of λ_total close to T_C. In the paramagnetic regime the unusual increase in λ_total keeps signature of large dynamic lattice distortion.     

Temperature dependence of the activation energy of manganite conductivity in the paramagnetic phase

The temperature dependence of the magnetic component of the activation energy of hopping conductivity ?_m of manganites in the paramagnetic phase (?_m = ? _m ⁰ ± a/T, where the signs “+” and “?” correspond to the antiferromagnetic CaMnO_{3 ? δ} and ferromagnetic La_0.85Sr_0.15MnO₃ manganites, respectively) has been found and investigated. The results obtained have been qualitatively explained on the basis of the model proposed by Gor’kov and colleagues.     

Insulator-metal transition shift related to magnetic polarons in La<Subscript>0.67-x</Subscript>Y<Subscript>x</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript>

The magnetic transport properties have been measured for La_0.67-xY_xCa_0.33MnO₃ ( 0 ⩽ x ⩽ 0.14) system. It was found that the transition temperature T _p almost linearly moves to higher temperature as H increases. Electron spin resonance confirms that above T _p , there exist ferromagnetic clusters. From the magnetic polaron point of view, the shift of T _p vs. H was understood, and it was estimated that the size of the magnetic polaron is of 9.7 ∼ 15.4 ? which is consistent with the magnetic correlation length revealed by the small-angle neutron-scattering technique. The transport properties at temperatures higher than T _p conform to the variable-range hopping mechanism. Received 27 August 2002 / Received in final form 2 December 2002 Published online 14 March 2003