Phase separation and anisotropy in the electrical properties of weakly doped lanthanum manganites

Variations in the thermopower, electrical resistivity, magnetoresistance, thermal expansion coefficients, and their anisotropy with temperature were detected near room temperature in single crystals of weakly doped lanthanum manganites La_1?xA_xMnO₃ (A = Ca, Sr; x = 0.07–0.125) with orthorhombic structure. The results obtained are discussed in terms of a model of phase separation related to polaron anisotropy. Due to a gain in exchange and elastic energies in the lattice, small-radius magnetic polarons can merge to form polarons of a larger size, which would contain now not one but rather a few electrons (equal in number to the polarons in the cluster). As a result, short-range order in a cluster and phase separation set in at a temperature T_ps ≈ 250–300 K, which is approximately equal to the Curie temperature T_C of conducting manganites with x ≈ 0.2–0.3.     

Room-temperature phase separation in weakly doped lanthanum manganites

The properties of single crystals of weakly doped lanthanum manganites La_1?xA_xMnO₃ (A = Ca, Ce, Sr; x = 0, 0.07?0.1) have been studied in the temperature range from 77 to 400 K. It is established that these lanthanum manganites exhibit (in addition to the well-known characteristic features observed in the region of the temperature of magnetic ordering) changes in the electrical and magnetic properties in the region of room temperature (T ≈ 270–300 K), which is about two times the Curie temperature (T ≈ 120–140 K) and is far from the temperature of structural transitions in the samples studied. The results are explained in terms of phase separation related to the formation of magnetic clusters in the nonconducting medium. The phase separation is caused by a gain in the exchange energy and by the development of elastic stresses in the crystal lattice and proceeds via combination of small-radius magnetic polarons into a large-size magnetic cluster containing several charge carriers. The short-range order in the cluster appears and the phase separation begins at a temperature T_ps, which is close to T_C ≈ 300 K, typical of doped conducting manganites. The results of magnetic measurements show that, as the temperature decreases from 300 to 190 K, the size of superparamagnetic droplets increases from about 8 to 15 Å.     

Efros-Shklovskii law and localized states in weakly doped lanthanum manganites

The existence of clusters in weakly doped lanthanum manganites at temperatures about twice as high as their Curie temperature T _C has been shown. Electrical resistance in weakly doped lanthanum manganites obeys the Efros-Shklovskii law. The temperature and magnetic-field dependences of a cluster size determined from the magnetotransport properties have been described using the model of phase separation into small metallic droplets within the dielectric paramagnetic and antiferromagnetic matrices. The results agree with the existence of the Griffiths phase.     

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.     

Conductivity in a disordered medium and carrier localization in weakly doped lanthanum manganites

The dc and 9.2-GHz electrical resistivities and magnetoresistance observed in La_1?xA_xMnO₃ crystals (A=Sr, Ce, x≤0.1) in the temperature interval 77–300 K are accounted for by the contributions due to carriers, both nonlocalized and localized in the valence-band tail and near the Fermi level. The localized-state tail extends to a depth of 0.15–0.25 eV inside the band gap, and the hopping activation energy varies from 0.06 to 0.15 eV, depending on the sample composition. Within the temperature region where magnetic ordering sets in, the variations of the electrical resistivity and magnetoresistance with temperature and magnetic field are caused by variations in the carrier mobility and concentration.     

Magnetic and high-pressure magnetotransport properties of cesium-substituted lanthanum calcium manganites

The nature of the double-exchange (DE) interaction in lanthanum manganites is studied through chemical substitutions, Cs for La, and high-pressure measurements. Static and high-frequency magnetic measurements and high-pressure electrical transport studies were carried out on bulk polycrystalline and radio-frequency sputtered thin films of La_0.7-xCs_xCa_0.3MnO₃ for x=0-0.1. The samples are found to be cubic. Curie temperature T_c measurements provide evidence for bond-length-related weakening of DE as x is increased from 0 to 0.03. For higher x, the bond-angle-related changes lead to an increase in the strength of DE. High-pressure mangetoresistance data indicate both bond length and bond-angle-related increase of 10–20 K/GPa in T_c with pressure, with the largest increase measured for x=0.03. The rate of increase in the Curie temperature with pressure decreases with increasing T_c. Anomalies are observed in the magnetic parameters for x=0.03. The Cs-concentration dependence of the low-temperature saturation magnetization shows a minimum close to x=0.03. Ferromagnetic resonance studies at x-band reveal a 5% decrease in the g-value for x=0.03 relative to the end members (x=0 and 0.1). The low-field magnetostriction for x=0.03 indicates a relatively strong electron–phonon spin coupling compared to neighboring compositions. Received: 15 May 2000 / Accepted: 24 July 2000 / Published online: 9 November 2000     

Magnetic properties of spin-orbital polarons in lightly doped cobaltates

We present a numerical treatment of a spin-orbital-polaron model for NaxCoO2 at small hole concentration (0.7 < x 1). We demonstrate how the polarons account for the peculiar magnetic properties of this layered compound: They explain the large susceptibility; their internal degrees of freedom lead both to a negative Curie-Weiss temperature and yet to a ferromagnetic intralayer interaction, thereby resolving a puzzling contradiction between these observations. We make specific predictions on the momentum and energy location of excitations resulting from the internal degrees of freedom of the polaron, and discuss their impact on spin-wave damping.     

Elastic properties of lanthanum manganites

We give a brief overview of the data on elastic properties of the CMR manganites published during last decade. The main emphasis is put on the results obtained for single crystals.     

Electrochemical promotion of propane deep oxidation on doped lanthanum manganites

Electrochemical catalysts based on LSM deposited on YSZ were used for propane deep oxidation. Electrochemical characterizations such as impedance spectroscopy and current–overpotential measurements were performed. Different thicknesses of LSM films were catalytically characterized and EPOC experiments were carried out on those electrochemical catalysts. It was found that the reaction rates can be in situ tuned by applying an anodic polarization but with weak promotional effects. The faradaic efficiency decreased as the applied potential, the temperature, and the thickness of the catalyst increased. Paper presented at the 11th EuroConference on the Science and Technology of Ionics, Batz-sur-Mer, Sept. 9–15, 2007.     

Magnetic inhomogeneity of lanthanum manganites single crystals

The magnetic inhomogeneity of the single crystals of La_1−xD_xMnO₃, where D=Ca, Sr, Ba, is studied. The Curie temperature distribution function is found. The form of this function is shown to depend on whether the mean value of the divalent element concentration is close to the “optimal” concentration (≈1/3) or not. The standard deviation for Curie temperature is evaluated. The correlation between the distribution coefficient for divalent elements and the standard deviation is established.     

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.     

Magnetic interaction in Mg, Ti, Nb doped manganites

An effect of Mn substitution with Me=Mg²⁺, Ti⁴⁺, Nb⁵⁺ in manganites has been investigated by preparing La_0.7Sr_0.3(Mn_1-xMe_x)O₃ and La_1-xSr_x(Mn_{1 - x/2}Nb_x/2)O₃ series. It was established that substitution of manganese with magnesium up to x = 0.16 leads to a collapse of a long-range ferromagnetic order whereas La_0.7Sr_0.3(Mn ^{3 +} _0.85Nb ^{5 +} _0.15)O₃ is ferromagnet with T _C = 123 K and exhibits a large magnetoresistance below Curie point despite an absence of four-valent manganese. Hypothetical magnetic phase diagrams are constructed for La_0.7Sr_0.3(Mn_1-xMe_x)O₃ and La_1-xSr_x(Mn_{1 - x/2}Nb_x/2)O₃. Our results show that Mn³⁺-O-Mn³⁺ exchange interaction is ferromagnetic in the orbitally disordered manganites as well as an increase of Mn⁴⁺ content above 50% from a total amount of manganese ions leads to formation of a spin glass state due to a competition between antiferromagnetic Mn⁴⁺-O-Mn⁴⁺ and ferromagnetic Mn³⁺-O-Mn⁴⁺(Mn³⁺) superexchange interactions. Received 24 January 2002 Published online 9 July 2002     

Influence of the Jahn-Teller effect on the structural, magnetic, and electrical properties of lightly doped manganites

Features of the structural, magnetic and electric properties in lightly doped La_1?x Sr _x MnO₃ (0.12 < x < 0.18) manganites are studied. Agreement between the types of orbital ordering and the local cooperative Jahn-Teller distortion octahedra is established. The interaction between the Jahn-Teller effect and magnetic ordering is described.     

Magnetic structure of the manganites heavily doped by Fe and Cr ions

Powder neutron diffraction and magnetic studies have been performed for NdMn_0.5Fe_0.5O₃ and NdMn_0.5Cr_0.5O₃ manganites. In NdMn_0.5Cr_0.5O₃, magnetic structure has been revealed consisting of ferromagnetic and G-type antiferromagnetic components as result of a 3d-ions magnetic moments ordering. Magnetic moments of Nd-ions are parallel to the ferromagnetic component. In NdMn_0.5Fe_0.5O₃ only the antiferromagnetic G-type structure has been found whereas Nd-sublattice was not ordered. In the both compounds, magnetic moments of 3d-ions are significantly less than one can expect, what is interpreted in terms of intrinsic chemical inhomogeneity. Magnetic phase diagrams have been constructed for the Nd(Mn_1−xM_x)O₃ (M=Fe, Cr) systems, interpreted assuming superexchange interactions Mn³⁺–O–Cr³⁺ to be positive, Mn³⁺–O–Fe³⁺ negative and taking into account a disordered arrangement of Mn and Cr ions in the crystal structure sublattice as well as interplay between Jahn–Teller effect and superexchange interactions.     

Griffiths phase and the magnetic and transport properties of doped manganites

A phenomenological model is proposed to describe the magnetic and magnetoresistance properties of ferromagnetic manganites. This model is based on the methods used to describe hysteretic systems, takes into account phase separation effects, and assumes the transition of ferromagnetic manganites into the Griffiths phase at above the Curie temperature. This formalism makes it possible to describe the conducting properties of the systems in the temperature range from low temperatures to the Griffiths temperature (T _G). This approach is used to qualitatively explain the experimental laws of the behavior of ferromagnetic manganites using the temperature and field dependences of the electrical resistivity and magnetization, the hysteretic properties, and the magnetoresistive effect (MRE) and to classify manganites in the magnitude of the MRE. The parameter that is responsible for the response of a system to thermal effects is the ratio of the maximum energy barrier separating various states of a system at zero temperature W _A (0) to thermal fluctuation energy W _Cfl at T _G. The W _A (0)/W _Cfl ratio is found to determine the temperature range of the Griffiths phase. The relation between the magnitudes of the MRE and parameter W _A (0)/W _Cfl for a certain system is revealed. The behavior of the magnetization and electrical resistivity of manganites in the Griffiths phase is discussed.     

Ordered states of Jahn-Teller distorted MnO6 octahedra in weakly doped lanthanum-strontium manganites

Physics of the Solid State - This paper presents the results of investigations of the ordering of Jahn-Teller distorted MnO6 octahedra in lightly doped single crystals of the composition La1...     

Magnetic and electric properties of transition-metal doped wurtzite CdSe from first-principles calculation

s The geometrical structures of Cd0.75TM0.25Se （TM = Ti, V, Cr and Mn） are optimized, and then their electric and magnetic properties are investigated by performing first-principles calculations within the generalized gradient approximation for the exchange-correlation function based on density functional theory. Cd0.75TM0.25Se （TM =Ti and V） are found to have high spin-polarization near 100% at the Fermi level. Cd0.75TM0.25Se （TM = Cr and Mn） are half-metallic ferromagnets whose spin-polarization at the Fermi level is absolutely ＋100%. The supercell magnetic moments of Cd0.75Cr0.25Se and Cdo.75Mno.25Se are 4.00 and 5.00 μB, which arise mainly from Cr-ions and Mnions, respectively. The half-metallicity of Cdo.75Cro.25Se is more stable than that of Cd0.75Mn0.25Se. The electronic structures of Cr-ions and Mn-ions are Cr eg2↑t22g↑ and Mn e2 3 ↑t23g↑, respectively.     

Magnetic and electric properties of transition-metal doped wurtzite CdSe from first-principles calculations

The geometrical structures of Cd_0.75TM_0.25Se (TM = Ti, V, Cr and Mn) are optimized, and then their electric and magnetic properties are investigated by performing first-principles calculations within the generalized gradient approximation for the exchange--correlation function based on density functional theory. Cd_0.75TM_0.25Se (TM = Ti and V) are found to have high spin-polarization near 100% at the Fermi level. Cd_0.75TM_0.25Se (TM = Cr and Mn) are half-metallic ferromagnets whose spin-polarization at the Fermi level is absolutely +100%. The supercell magnetic moments of Cd_0.75Cr_0.25Se and Cd_0.75Mn_0.25Se are 4.00 and 5.00~μ_B, which arise mainly from Cr-ions and Mn-ions, respectively. The half-metallicity of Cd_0.75Cr_0.25Se is more stable than that of Cd_0.75Mn_0.25Se. The electronic structures of Cr-ions and Mn-ions are Cr e_g²↑ t_2g²↑ and Mn e_g²↑ t_2g³↑, respectively.     

The effects of vacancies in oxygen sites on physical properties of the lanthanum manganites

Abstract

The non-stoichiometric solution Ln_1?xSr_xMnO_3-δ□_δ was prepared by the classic ceramic method. The physical properties as crystallographic, magnetic and electrical properties were studied. A structural phase transition from rhombohedral to orthorhombic was observed at a concentration of Mn⁴⁺ between 10% and 15% per Mn atom in the unit formula. The magnetic properties are very sensitive to the presence of vacancies at the oxygen sites. The non-stoichiometric, samples change from metallic to insulating behaviour depending on their vacancy concentration. In the semiconductor phase, the activation energy value changes with the structural phase, increasing in the rhombohedral phase and decreasing in the orthorhombic phase.