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.     

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.     

Magnetic and magnetotransport properties of Co-doped manganites with perovskite structure

A systematic study of the doping of the Mn-sites by cobalt in three series of manganites — La_0.76Ba_0.24(Mn_1−xCo_x)O₃ single crystals, La_2/3Ba_1/3(Mn_1−xCo_x)O₃ and La(Mn_1−xCo_x)O₃ ceramics has been performed. It was found that La(Mn_1−xCo_x)O₃ annealed at 800°C in the range 0.4x0.9 is a mixture of ferromagnetic domains with ordered Mn and Co ions and ionically disordered spin-glass domains. In the quenched samples the fraction of spin-glass-type component increases strongly. The La_2/3Ba_1/3(Mn_1−xCo_x)O₃ solid solutions exhibit also an evidence for phase separation in the range 0.5x0.8. All the La(Mn_1−xCo_x)O₃ samples show an insulating behavior, however, magnetoresistance reduces strongly when the cobalt content rises to x=0.5. The La_0.76Ba_0.24(Mn_1−xCo_x)O₃ single crystals show first-order phase transition below their Curie points associated with a change of ground state of the Co²⁺ ions. The magnetic phase diagrams are depicted. The results are discussed in terms of positive Mn³⁺–O–Mn⁴⁺, Mn³⁺–O–Mn³⁺, Mn⁴⁺–O–Co²⁺ and negative Mn⁴⁺–O–Mn⁴⁺, Co²⁺–O–Co²⁺, Co²⁺–O–Mn³⁺ superexchange interactions as well as Co²⁺ and Mn⁴⁺ ionic ordering.     

Effect of La doping on magnetotransport and magnetic properties of double perovskite Sr2FeMoO6 system

We have investigated the magnetotransport and magnetic properties on polycrystalline samples of Sr_2−xLa_xFeMoO₆ (x=0$x=0$, 0.2, 0.4, 0.6, 0.8 and 1.0). The magnitude of intergrain tunneling magnetoresistance with low magnetic field of 0.88 T for x=0.2$x=0.2$ and 0.4$0.4$ samples are as large as 5% and 7% at room temperature and 13% and 10% at 10 K, respectively. The increase of coercivity (_Hc$H_{\mathrm{c}}$), ratio of remanent magnetization with respect to saturation magnetization (_Mr/_Ms$M_{\mathrm{r}}/M_{\mathrm{s}}$), high saturation fields, and reduction of the saturation magnetization indicate that random disorder of spin orientation is mainly responsible for enhancement of the low-field magnetoresistance for samples with x?0.4$x?0.4$. Whereas rapid drop of _Hc$H_{\mathrm{c}}$, _Mr/_Ms$M_{\mathrm{r}}/M_{\mathrm{s}}$, _Mr$M_{\mathrm{r}}$, and saturation fields for samples with x>0.4$x>0.4$ signifies the growth of antiphase boundary, which gives rise to lower values of low-field MR.     

Spin glass-like behavior,giant magnetocaloric and giant magnetoresistance effect in PrPb manganites

The Pr_1−xPb_xMnO₃ (x=0.1–0.5) perovskites have been fabricated by solid-state reaction. The X-ray diffraction patterns show that the samples are of single phase with orthorhombic structure. The field-cooled (FC) and zero-field-cooled (ZFC) thermomagnetic curves measured at low field and low temperatures exhibit the spin glass-like state. The Curie temperature of samples increased with increase in Pb content. The maximum magnetic entropy change |ΔS_m|_max reaches the giant values of 3.91 and 3.68 J/kg K for quite low magnetic field change of 1.35 T for the samples x=0.1 and 0.4, respectively. The resistance measurements show that there is insulator–metal phase transition on the R(T) curves for samples with x?0.3. The giant magnetoresistance effect is also observed for all samples studied.     

Local structure and magnetic properties of Mn substituted manganites studied by EXAFS and Dc magnetic measurements

We report extended X-ray absorption fine structure (EXAFS) measurements at the Mn K edge and magnetic measurements performed on (La_1−xCa_x)(Mn_1−yM_y)O₃ samples (M=Cr or Ni; x=0.37 and 0.75 and y=0.03 or 0.08). The Mn substitution produces important effects on both the sides of the LaMnO₃-CaMnO₃ phase diagram. For x<0.5 the ferromagnetic-metallic phase maintains its main character even after Mn substitution, but both the doping species (Ni or Cr) lower T_C and broaden the magnetic transition, and the EXAFS study evidences two Mn-O distances, suggesting the presence of zones of distorted insulating phase. For x>0.5, after the doping with Cr, the charge ordered phase persists but on a shorter scale, whereas the Jahn-Teller distortion is weakened as indicated by EXAFS measurements, and the formation of ferromagnetic clusters is evidenced by magnetic measurements.EXAFS and magnetic measurements are in mutual agreement, thus confirming the correlation between the local disorder determined by charge localization and magnetic degrees of freedom.     

Effect of pressure and magnetic field on bilayer La1.25Sr1.75Mn2O7 single crystal

We report the temperature dependence of susceptibility for various pressures, magnetic fields and constant magnetic field of 5 T with various pressures on La_2−2xSr_1+2xMn₂O₇ single crystal to understand the effectiveness of pressure and magnetic field in altering the magnetic properties. We find that the Curie temperature, T_c, increases under pressure (dT_c/dP=10.9 K/GPa) and it indicates the enhancement of ferromagnetic phase under pressure up to 2 GPa. The magnetic field dependence of T_c is about 26 K for 3 T. The combined effect of pressure and constant magnetic field (5 T) shows dT_c/dP=11.3 K/GPa and the peak structure is suppressed and broadened. The application of magnetic field of 5 T realizes 3D spin ordered state below T_c at atmospheric pressure. Both peak structure in χ_c and 3D spin ordered state are suppressed, and changes to 2D-like spin ordered state by increase of pressure. These results reveal that the pressure and the magnetic field are more competitive in altering the magnetic properties of bilayer manganite La_1.25Sr_1.75Mn₂O₇ single crystal.     

Electrical transport and magnetic properties of perovskite-type electron-doped La–Pr–Mn–O epitaxial films

La_1−xPr_xMnO₃ (LPrMO) thin films have been epitaxially grown on (1 0 0)SrTiO₃ single-crystal substrates by pulsed-laser deposition. The films have a perovskite structure and give rise to the colossal magnetoresistance effect with the maximum magnetoresistance ratio of 10³% (at 240 K and 5 T). The electrical transport and magnetic properties have been investigated for the La_0.8Pr_0.2MnO₃ film with thickness 3000 Å. The results indicate that the films have quite a distinctive magnetotransport behavior compared to the bulk. The analysis of X-ray photoemission spectroscopy suggests that the valence state of Pr is 4+ in LPrMO film. Therefore, the epitaxial film is most likely an electron-doped colossal magnetoresistance system.     

Electron-doped Sm1−xSrxMnO3 perovskite manganites: Crystal and magnetic structures and physical properties

We present the results of a study of electron-doped Sm_1−xSr_xMnO₃ (x>0.5) perovskite manganites by combining high-resolution neutron powder diffraction with measurements of resistivity, magnetization and magnetic susceptibility. Although investigated Sm_0.45Sr_0.55MnO₃ and Sm_0.37Sr_0.63MnO₃ compounds belonging to the same phase diagram area differ significantly in the strontium content, they are homogeneous antiferromagnetic (AF) insulators and do not exhibit CMR. They have different crystallographic symmetries (orthorhombic Pbnm and tetragonal I4/mcm, respectively) in the entire temperature range under study (1.5-288 K), differ in the type of spin ordering at low temperatures (AF-A and AF-C), are characterized by different orbital polarizations (d_x²_−y² and d_3z²_−r²), and possess two- and one-dimensional magnetic properties, respectively. The lack of magnetoresistance for these compositions is explained by the lack of coexisting magnetic phases involving double exchange ferromagnetism, in contrast to what is observed for the magnetoresistive Sm_1−xSr_xMnO₃ compounds, that is with x?0.52.     

Magneto-transport and ferromagnetic resonance studies of polycrystalline La0.6Pb0.4MnO3 thin films

To understand the nature of grain boundaries in polycrystalline materials, magneto-transport and ferromagnetic resonance measurement have been performed in polycrystalline La_0.6Pb_0.4MnO₃ (LPMO) thin films prepared by pulsed laser deposition. Films are found to undergo a semiconductor to metal transition at 230 K and re-enter into the semiconducting state below 130 K. Microwave absorption measurements carried out as function of applied field show two components of resonant absorption signal. First component is in accordance with ferromagnetic transition of grains at Curie temperature and the second component shows antiferromagnetic transition of grain boundaries at 160 K. An additional non-resonant absorption signal centered at zero field has also been observed that supports transition from conducting to insulating grain boundaries at ∼160 K. Further, temperature dependence of resistance in semiconducting state at low temperatures is in accordance with coulomb blockade model indicating insulating nature of AFM grain boundaries.     

Critical behavior studies in Ti-substituted lanthanum bismuth perovskite manganites

Perovskite manganite La_0.4Bi_0.6Mn_1−xTi_xO₃ (x = 0.05 and 0.1) synthesized using conventional solid state route method give rise to critical phenomenon in their magnetic interactions due to the substitution of non magnetic Ti ions. The critical behavior is observed near paramagnetic–ferromagnetic transition and is studied by magnetization measurements. Various techniques like Modified Arrott plot, Kouvel–Fisher method, scaling equation of state analysis and the critical magnetization isotherm were used to analyze the magnetization data on magnetic phase transition. The values of critical exponents β and γ obtained using different techniques are in good agreement. The obtained critical exponents are found to follow scaling equation with the magnetization data scaled into two different curves below and above the transition temperature, T_C. This confirms that the critical exponents and T_C are reasonably accurate. The obtained critical exponents for both the samples deviates from mean-field model and do not completely follow the static long range ferromagnetic ordering. This behavior is consistent with non magnetic nature of Ti substituted at Mn site and can be associated with Griffiths phase like phenomenon.     

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.     

Correlation between extrinsic magnetoresistance and electroresistance in La0.6Pb0.4MnO3 thin films as revealed from current-voltage and ferromagnetic resonance studies

We have investigated magnetoresistance (MR) and electroresistance (ER) of well characterized La_0.6Pb_0.4MnO₃ (LPMO) films having two different crystallinity (i) (00l) oriented single-crystalline, SC, i.e. without any grain boundaries, and (ii) nanocrystalline, NC, with an average grain size of 17 nm. Both MR and ER were remarkably different for the two films, that is, (i) NC films exhibited a highest MR of ∼100% near the metal-insulator transition temperature; while for SC films the MR was in the range of 40-60%, (ii) NC films exhibited a finite ER, which increased monotonically with decreasing temperature; while in SC films ER was completely absent. Using current-voltage characteristic and ferromagnetic resonance studies we demonstrate that both enhanced MR and ER in NC films are extrinsic in nature and originate due to the presence of spin glassy grain boundaries.     

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.     

Role of structure imperfection in the formation of the magnetotransport properties of rare-earth manganites with a perovskite structure

The structure, the structure imperfection, and the magnetoresistance, magnetotransport, and microstructure properties of rare-earth perovskite La_0.3Ln_0.3Sr_0.3Mn_1.1O_3–δ manganites are studied by X-ray diffraction, thermogravimetry, electrical resistivity measurement, magnetic, ⁵⁵Mn NMR, magnetoresistance measurement, and scanning electron microscopy. It is found that the structure imperfection increases, and the symmetry of a rhombohedrally distorted R3?c perovskite structure changes into its pseudocubic type during isovalent substitution for Ln = La³⁺, Pr³⁺, Nd³⁺, Sm³⁺, or Eu³⁺ when the ionic radius of an A cation decreases. Defect molar formulas are determined for a real perovskite structure, which contains anion and cation vacancies. The decrease in the temperatures of the metal–semiconductor (T _ms) and ferromagnet–paramagnet (T _C) phase transitions and the increase in electrical resistivity ρ and activation energy E _a with increasing serial number of Ln are caused by an increase in the concentration of vacancy point defects, which weaken the double exchange 3d ⁴(Mn³⁺)–2p ⁶(O^2–)–3d ³(Mn⁴⁺)–V ^(a)–3d ⁴(Mn³⁺). The crystal structure of the compositions with Ln = La contains nanostructured planar clusters, which induce an anomalous magnetic hysteresis at T = 77 K. Broad and asymmetric ⁵⁵Mn NMR spectra support the high-frequency electronic double exchange Mn³⁺(3d ⁴) ? O^2–(2p ⁶) ? Mn⁴⁺(3d ³) and indicate a heterogeneous surrounding of manganese by other ions and vacancies. A correlation is revealed between the tunneling magnetoresistance effect and the crystallite size. A composition–structure imperfection–property experimental phase diagram is plotted. This diagram supports the conclusion about a strong influence of structure imperfection on the formation of the magnetic, magnetotransport, and magnetoresistance properties of rare-earth perovskite manganites.     

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.     

Structural, transport, and magnetic properties in the Ti-doped manganites LaMn1−xTixO3 (0≤x≤0.2)

The magnetism and transport properties of the samples LaMn_1−xTi_xO₃ (0≤x≤0.2) were investigated. All samples show a rhombohedral structure () at room temperature. The sample with x=0 undergoes the paramagnetic-ferromagnetic (PM-FM) transition accompanied by an insulator-metal (I-M) transition due to the oxygen excess. The doped samples show ferromagnetism and cluster behavior at low temperatures. Though no I-M transition associated with the PM-FM transition appears, the magnetoresistance (MR) effect was observed especially at low temperatures under the applied fields of 0.5 T. Due to the fact that the oxygen content in the Ti-doped samples is nearly stochiometry (3.01) and the Hall resistivity at room temperature is negative, the ferromagnetism in LaMn_1−xTi_xO₃ (0.05≤x≤0.2) is believed to be consistent with the Mn²⁺-O-Mn³⁺ double exchange (DE) mechanism. These results suggest that DE can be obtained by direct Mn-site doping.     

Structural, magnetic, magnetocaloric and magnetotransport properties in Ge doped Ni-Mn-Sb Heusler alloys

The effect of Ge substitution on the magnetic, magnetocaloric and transport properties of Ni₄₅Co₅Mn₃₈Sb_12−xGe_x (x=0-3) has been investigated. The decrease in the exchange interaction brought by Ge substitution can be seen from the reduction in the magnetization of austenite phase and the increase in the martensitic transition temperature. The magnetocaloric effect and the magnetoresistance values are found to be quite sensitive to small changes in Sb/Ge ratio. Taking into account various properties, the present series seems to be a promising multifunctional system.     

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