Magnetic and electrical behavior of La1−xAxMnO3 (A=Li, Na, K and Rb) manganites

With a view to understand the magnetic and electrical behavior of monovalent substituted lanthanum manganites, a series of materials were prepared by sol-gel route by sintering at 1200 °C. The X-ray diffraction data were analyzed using Rietveld refinement technique and it has been found that all the samples were found to crystallize into rhombohedral structure with R3¯c space group. The values of ferro to paramagnetic (T_C) and metal-insulator transition (T_P) temperatures were obtained using ac susceptibility and electrical resistivity data, respectively. It has been found that sodium-, potassium- and rubidium-doped samples exhibit two peaks in the electrical resistivity vs. temperature plots. The observed behavior has been explained on the basis of oxygen deficiency present in the samples. The electrical resistivity data were analyzed using various theoretical models and it has been concluded that the electrical resistivity data in the low-temperature regime (T<T_P) can be explained using the equation ρ_(T)=ρ₀+ρ₂T²+ρ_4.5T^4.5, signifying the importance of the grain/domain boundary, electron-electron and two magnon scattering processes. On the other hand, the high-temperature resisitivity data (T>T_P) were explained using variable range and small polaron hopping mechanisms.     

Electrical behavior of some silver-doped lanthanum-based CMR materials

With a view to understand the structural, magnetic and electrical properties of La_1−xAg_xMnO₃ (x=0.05-0.3), a series of samples were prepared by polyvinyl alcohol (PVA) gel route. It has been found that both the metal-insulator and ferro- to paramagnetic transition temperatures after increasing up to the composition x=0.20, are found to remain constant thereafter. The electrical resistivity vs. temperature plot of the sample x=0.10 is found to exhibit an insulating behavior below 36 K, while the sample, x=0.20 exhibits two peaks, and the observed behavior is explained on the basis of the phase separation model. The low-temperature (T<T_P), electrical resistivity data were analyzed by a theoretical model, ρ=ρ₀+ρ₂T²+ρ_4.5T^4.5, indicating the importance of grain/domain boundary effects, electron-electron and two-magnon scattering processes. The low-temperature resistivity data (T<50 K) were fitted to an equation, which is based on the combined effect of weak localization, electron-electron and electron-phonon scattering.     

On the coexistence of spin and lattice polarons in the La0.67−xEuxCa0.33MnO3 CMR system

The electrical and magnetic transport properties of the La_0.67−xEu_xCa_0.33MnO₃ system exhibit lowering of insulator to metal and paramagnetic to ferromagnetic transition temperature (T_C) with the increase of Eu concentration in addition to possessing CMR property. The temperature variation of electrical resistivity and magnetic susceptibility for x=0.21 is found to have two distinct regions in the paramagnetic state for T>T_P; one with the localization of lattice polaron in the high-temperature region (T>1.5T_P) satisfying the dynamics of variable range hopping (VRH) model and the other being the combination of the spin and lattice polarons in the region T_P<T<1.5T_P. The resistivity variation with temperature and magnetic field, the cusp in the resistivity peak and CMR phenomenon are interpreted in terms of coexistence of spin and lattice small polarons in the intermediate region (T_P<T<1.5T_P). The spin polaron energy in the La_0.46Eu_0.21Ca_0.33MnO₃ system is estimated to be 106.73±0.90 meV and this energy decreases with the increase of external magnetic field. The MR ratio is maximal with a value of 99.99% around the transition temperature and this maximum persists till T→0 K, at the field of 8 T.     

Low temperature resistivity minimum and its correlation with magnetoresistance in La0.67Ba0.33MnO3 nanomanganites

A systematic investigation of structural, magnetic and electrical properties of nanocrystalline La_0.67Ba_0.33MnO₃ materials, prepared by citrate gel method has been undertaken. The temperature-dependant low-temperature resistivity in ferromagnetic metallic (∼50 K) phase shows upturn behavior and is suppressed with applied magnetic field. The experimental data (<75 K) can be best fitted in the frame work of Kondo-like spin-dependant scattering, electron-electron and electron-phonon interactions. It has been found that upturn behavior may be attributed to weak spin disorder scattering including both spin polarization and grain boundary tunneling effects, which are the characteristic features of extrinsic magnetoresistance behavior, generally found in nanocrystalline manganites. The variation of electrical resistivity with temperature in the high temperature ferromagnetic metallic part of electrical resistivity (75K<T<T_P) has been fitted with grain/domain boundary, electron-electron and magnon scattering mechanisms, while the insulating region (T>T_P) of resistivity data has been explained based on adiabatic small polaron hopping mechanism.     

Co-existence of giant magnetoresistance and large magnetocaloric effect near room temperature in nanocrystalline La0.7Te0.3MnO3

Sol-gel prepared nanocrystalline La_0.7Te_0.3MnO₃ has rhombohedral crystal structure (space group R3¯C) at room temperature and orders ferromagnetically at ∼280 K (T_C). A large magnetic entropy change of ∼12.5 J kg⁻¹ K⁻¹ is obtained near T_C for a field change of 50 kOe. This magnetocaloric effect could be explained in terms of Landau theory. The temperature dependence of electrical resistivity shows metal-insulator transition at T_C and a giant magnetoresistance of ∼52% in 50 kOe. The co-existence of giant magnetoresistance and large magnetocaloric effect near room temperature makes nanocrystalline La_0.7Te_0.3MnO₃ a promising material for magnetic refrigeration and spintronic device applications.     

Origin of the non-linear pressure effects in perovskite manganites: Buckling of Mn-O-Mn bonds and Jahn-Teller distortion of the MnO6 octahedra induced by pressure

High-pressure resistivity and X-ray diffraction measurements were conducted on La_0.85MnO_3−δ at ∼6 and ∼7 GPa, respectively. At low pressures the metal-insulator transition temperature (T_MI) increases linearly up to a critical pressure, P^* ∼3.4 GPa, followed by reduction in T_MI at higher pressure. Analysis of the bond distances and bond angles reveals that a bandwidth increase drives the increase in T_MI below P^*. The reduction in T_MI at higher pressures is found to result from Jahn-Teller distortions of the MnO₆ octahedra. The role of anharmonic interatomic potentials is discussed.     

Anomalous variation of magnetoresistance in Nd0.67−yEuySr0.33MnO3 manganites

A systematic investigation of Eu-doped Nd-based colossal magnetoresistive manganites with compositional formula Nd_0.67−yEu_ySr_0.33MnO₃(y=0-0.67) has been undertaken to understand their structural, magnetic as well as electrical behavior. These materials were prepared by the citrate gel route, and were later characterized by X-ray diffraction (XRD), AC susceptibility, electrical resistivity etc; measurements. A detailed structural characterization of the XRD data has also been undertaken, using Rietveld refinement method. From a systematic analysis of electrical resistivity versus temperature data, it has been found that the last two samples of the series (y=0.57 and 0.67) are found to exhibit charge order (CO) phenomenona, and that the CO state melts completely in the case of former sample (y=0.57) while it melts partially only in the case of later one (y=0.67), even in a magnetic field of 7 T. The observed behavior has been explained qualitatively. Some of the samples of the series are found to exhibit unusually large magnetoresistance over a wide temperature range in the low temperature region, and their behavior has been explained on the basis of phase segregation and an inter-grain spin polarized tunneling effect.     

Low temperature magnetoresistance in La1.32Sr1.68Mn2O7 layered manganite under hydrostatic pressure

The La_1.32Sr_1.68Mn₂O₇ layered manganite system has been studied by the low temperature electrical resistance and magnetoresistance under hydrostatic pressure up to 25 kbar. We have observe both, a Curie temperature (T_C) and a metal-insulator transition (T_MI) at 118 K in the ambient pressure. The applied pressure shifts the T_MI to higher temperature values and induces a second metal-insulator transition (T²_MI) at 90 K, in the temperature dependence of resistivity measurements. Also, the pressure suppresses the peak resistance abruptly at T_C. When an external field of 5 T is applied, we have observed a large negative magnetoresistance of 300% at the transition temperature and a 128% at 4.5 K. However, the increased pressure decreases the magnetoresistance ratio gradually. When the pressure reaches its maximum available value of 25 kbar, the magnetoresistance ratio decreases at a rate of 1.3%/kbar. From our experimental results, the decrease of magnetoresistance ratio with pressure is explained by the pressure induced canted spin state which is not favor for the spin polarized intergrain tunneling in layered manganites.     

Electrical conductivity and low field magnetoresistance in polycrystalline La1−xKxMnO3 pellets prepared by pyrophoric method

Electrical conductivity and magnetoresistance of a series of monovalent (K) doped La_1−xK_xMnO₃ polycrystalline pellets prepared by pyrophoric method have been reported. K doping increases the conductivity as well as the Curie temperature (T_C) of the system. Curie temperature increases from 260 to 309 K with increasing K content. Above the metal-insulator transition temperature (T>T_MI), the electrical resistivity is dominated by adiabatic polaronic model, while in the ferromagnetic region (50<T<T_MI), the resistivity is governed by several electron scattering processes. Based on a scenario that the doped manganites consist of phase separated ferromagnetic metallic and paramagnetic insulating regions, all the features of the temperature variation of the resistivity between ∼50 and 300 K are described very well by a single expression. All the K doped samples clearly display the existence of strongly field dependent resistivity minimum close to ∼30 K. Charge carrier tunneling between antiferromagnetically coupled grains explains fairly well the resistivity minimum in monovalent (K) doped lanthanum manganites. Field dependence of magnetoresistance at various temperatures below T_C is accounted fairly well by a phenomenological model based on spin polarized tunneling at the grain boundaries. The contributions from the intrinsic part arising from DE mechanism, as well as, the part originating from intergrannular spin polarized tunneling are also estimated.     

Effect of multielement doping on low-field magnetotransport in La0.7−xMmxCa0.3MnO3 (0.0?x?0.45) manganite

We report the synthesis, structure and low-field magnetotransport properties of Mischmetal (Mm)-doped La_0.7−xMm_xCa_0.3MnO₃ (0?x?0.45) manganite. Mischmetal—Mm—is a natural mixture of rare earth elements La, Ce, Pr and Nd with ∼28%, 50%, 6% and 16% composition, respectively. All the samples crystallize in orthorhombic structure. Increasing x (Mm), corresponding to decreasing the La-site average ionic radii (〈r_A〉) hence increasing the size mismatch (i.e. variance σ²), results in strong suppression of ferromagnetism (T_C) and the associated metallicity (T_IM). It may be pointed out that Mm (La, Ce, Pr and Nd) substitution has been done to create two effects. First, creation of multivalence of Mn (2+, 3+ and 4+) via Ce substitution and second to create higher degree of disorder due to size difference brought in not only by Ce but also by Pr and Nd. Evidences and arguments based on XPS analysis suggest that multivalent ions La, Mm and Ca, and the resulting presence of Mn²⁺, Mn³⁺ and Mn⁴⁺, causes the simultaneous operation of ferromagnetism-double exchange (Mn²⁺/Mn³⁺ and Mn³⁺/Mn⁴⁺) and antiferromagnetic-superexchange (Mn³⁺/Mn³⁺ and Mn²⁺/Mn²⁺) interaction. In addition, Mm doping also creates inhomogenities at La—as well as Mn—site due to size and valency difference. A curiously huge magnetoresistance as high as ∼63% for x=0.35, under a moderate magnetic field of ∼10 kOe has been observed and even at low magnetic field of ∼3 kOe MR is ∼30%. The competing double exchange and superexchange coupled with inhomogenities are the most likely cause for the occurrence of large ∼63% CMR in the Mm-doped LCMO.     

Growth of single-phase c-axis aligned La1.2Ca1.8Mn2O7 films on SrTiO3 (001)

We report the growth of single phase, c-axis aligned thin films of La_1.2Ca_1.8Mn₂O₇ on SrTiO₃ (001) substrates using a controlled pulsed laser deposition method. In this method, constraint of epitaxy is utilized to stabilize the Ruddlesdon-Popper (RP) phase of La_1.2Ca_1.8Mn₂O₇. Oxygen ambient pressure and the rate of deposition play a very important role in influencing the epitaxial growth as well as maintaining phase purity of the material. The oxygen pressure inside the deposition chamber was very precisely controlled and varied during the layer-by-layer growth of the film. Films, prepared by our method, show excellent electrical and magnetic characteristics with a sharp metal-insulator transition at T_M-I=90 K, closely followed by a magnetic transition at T_C=91 K.     

Structural, magnetotransport and morphological studies of Sb-doped La2/3Ba1/3MnO3 ceramic perovskites

We report here the structural, magnetotransport and morphological studies of Sb-doped La_2/3Ba_1/3Mn_1−xSb_xO₃ perovskite manganites. Pristine material La_2/3Ba_1/3MnO₃ (LBMO) shows two insulator-metal (I-M) transitions in the electrical resistivity-temperature (ρ-T) behavior. While the higher temperature transition (T_P1) at ∼340 K is reminiscent of the usual I-M transition in manganites, the lower temperature transition (T_P2) at ∼250 K has been ascribed to the grain boundary (GB) effects arising out of the ionic size mismatch between the ions present at the rare-earth site (La³⁺ and Ba²⁺). With Sb-doping T_P1 shifts to lower temperatures while T_P2 remains invariant up to 3% and shifts to lower temperature for 5%. Room temperature electrical resistivity and the peak values also increase successively with Sb-doping. Scanning electron micrographs of the samples exhibit a gradual increase in their grain sizes with Sb indicating a gradual decrease in the GB density. Shift of T_P1 with doping is explained on the basis of a competition between double-exchange and super-exchange mechanisms. The overall electrical resistivity increases and the shift in the electrical resistivity hump (T_P2) with Sb-doping is found related to be gradually decreasing GB density and the ensuing lattice strain increase at the GBs. The intrinsic magnetoresistance (MR) gets suppressed and extrinsic MR gets enhanced with Sb-doping. At T>T_P1, the electrical resistivity is found to follow the adiabatic polaron hopping model whereas the electron-magnon scattering is found to dominate in the metallic regime (T<T_P1).     

Effect of Cr doping in the bilayer manganite La1.4Sr1.6Mn2O7

The effect of Cr doping on magnetic and electrical properties in the bilayer manganites La_1.4Sr_1.6(Mn_1−yCr_y)₂O₇ (y=0-0.1) has been investigated. When y≤0.025, Cr doping enhances the three-dimensional magnetic transition temperature T_C and the insulator-metal transition temperature T_IM as well as decreases the peak resistivity at T_IM, and the saturated magnetization decreases slightly. When y≥0.035, T_IM decreases gradually accompanied by the increase of peak resistivity, but T_C remains nearly constant, and the saturated magnetization decreases heavily. In the whole doping region, the two-dimensional magnetic transition temperature T^? monotonously decreases with an increasing of Cr doping level. These results can be explained by considering different magnetic (including ferromagnetic and antiferromagnetic) interactions between Mn ions and Cr ions.     

Large negative magnetoresistance in thiospinel CuCrZrS4

We report on large negative magnetoresistance observed in ferromagnetic thiospinel compound CuCrZrS₄. The electrical resistivity increased with decreasing temperature according to the exp(T₀/T)^1/2, an expression derived from variable range hopping with strong electron-electron interaction. The resistivity under a magnetic field was expressed by the same form with the characteristic temperature T₀ decreasing with increasing magnetic field. Magnetoresistance ratio ρ(T,0)/ρ(T,H) is 1.5 for H=90 kOe at 100 K and increases divergently with decreasing temperature reaching 80 at 16 K. Results of magnetization measurements are also presented. A possible mechanism of the large magnetoresistance is discussed.     

Specific features of the behavior of optical properties upon the metal-insulator transition in EuBaCo<Subscript>2</Subscript>O<Subscript>5+δ</Subscript>

The temperature dependences of the optical properties and the electrical resistivity for EuBaCo₂O_5+δ single crystals are investigated. At temperatures below the metal-insulator transition (T _MI = 340 K), the electrical resistivity is well approximated by the relationship ρ = ρ₀exp(T/T ₀)^1/4. The optical band gap E _g = 0.05 eV for the insulating phase is underestimated as compared to the theoretical value. The specific features in the dispersion of the optical conductivity and the real part of the complex permittivity upon the metal-insulator transition are determined. It is demonstrated that the optical response from charge carriers on the metal side of the metal-insulator transition is caused by the redistribution of the spectral weight of the optical conductivity from the high-energy range to the low-energy range and exhibits a strongly incoherent character. The revealed features are associated with the manifestation of the strongly correlated metallic state.     

Low field magnetoresistance and conduction noise in layered manganite La1.4Ca1.6Mn2O7

Temperature dependence of conduction noise and low field magnetoresistance of layered manganite La_1.4Ca_1.6Mn₂O₇ (DLCMO) are reported and compared with the infinite layered manganite La_0.7Ca_0.3MnO₃ (LCMO). The double layered manganite was prepared using standard solid state reaction method and had a metal-insulator transition temperature (T_M-I) of 155 K. The temperature dependence of susceptibility showed evolution of ferromagnetic ordering at 168 K. The observed voltage noise spectral density (S_V) shows 1/f^α type of behaviour at all temperatures from 77 K to 300 K. In the ferromagnetic region (T<168 K), S_V/V² shows two peaks at 164 K and 114 K. The observed two peaks in normalised conduction noise of DLCMO is attributed to the excess noise generated due to setting up of short range 2D-ferromagnetic ordering and long range 3D-ferromagnetic ordering at two different temperatures T_C2 and T_C1. In temperature range between T_C1 and T_C2, the magnetoresistance (MR) showed a gradual increase with the magnetic field. The observed MR has been explained in the framework of the two phase model [ferromagnetic (FM) domains and paramagnetic (PM) regions].     

Study of structural, transport and magneto-resistive properties of La0.7Ca0.3−xCexMnO3 (0≤x≤0.2)

We have synthesized a series of La_0.7(Ca_0.3−xCe_x)MnO₃ (0≤x≤0.2) by standard solid-state reaction method. X-ray diffraction (XRD) measurement was carried out for structural studies and Rietveld refinement was done for structural analysis. The transport properties were studied using four probe technique. The temperature dependence of the resistivity was measured in the temperature range of 20 K to room temperature. It is found that all samples show a systematic variation in metal to insulator transition at transition temperature (T_P) and resistivity (ρ) with the relative concentration of hole and electron doping in the system. The samples showed varying amounts of colossal magnetoresistance depending upon temperature and applied magnetic field. The magnetoresistance values as high as 72% were observed in x=0 sample.     

Synthesis and characterization of K2NiF4-type phases Ln0.5Sr1.5Mn0.5Fe0.5O4 (Ln = La, Nd, Gd, and Dy)

A systematic investigation of layered perovskite oxides with general formula Ln_0.5Sr_1.5Mn_0.5Fe_0.5O₄ (Ln?=?La, Nd, Gd, and Dy) has been undertaken mainly to understand their structural, magnetic, as well as electrical behavior. The materials were prepared by the ceramic method. X-ray data have been analyzed by using program Checkcell and the variations of various parameters are explained. It has been concluded that not only A-site cation radius, <r _A>, but also the size variance factor (σ ²) influence electrical and magnetic properties. A systematic study of electrical resistivity of all the four materials was undertaken as a function of temperature to understand the conduction mechanism. On analyzing the electrical resistivity data, it has been concluded that variable range hopping model is found to fit well. The magnetic studies suggest that the phases are antiferromagnetic and this behavior could arise from Mn⁴⁺–O–Mn⁴⁺, and Fe³⁺–O–Fe³⁺ superexchange interaction.     

Study of the magnetization and transport properties of the La(2+x)/3Sr(1−x)/3Mn1−xCrxO3 (0≤x≤0.2) system

The samples with the Mn³⁺/Mn⁴⁺ ratio fixed at 2:1 La_(2+x)/3Sr_(1−x)/3Mn_1−xCr_xO₃ (0≤x≤0.20) have been prepared. The magnetic, electrical transport, and magnetoresistance properties have been investigated. Remarkable transport and colossal magnetoresistance (CMR) effect, as well as cluster glass (CG) behaviors have been clearly observed in the samples studied. It was found that the Curie temperature T_c and insulator−metal transition temperature T_p1 are strongly affected by Cr substitution. The experiment observations are discussed by taking into account the variety of tolerance factors t; the effects of A-site radius 〈r_A〉 and the A-site mismatch effect (σ²).     

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.