Discussion again of the magnetic and transport property of Ag-doped LaMnO3

Samples La_1−aAg_aMnO₃ (0.05?a?0.50) were sol–gel fabricated. A part of Ag was found to dissociate and run off the samples in sintering process when sintering temperature exceeds 700 °C, resulting in a composite of La_1−xAg_xMnO₃ and MnO₂/Mn₂O₃. The magnetic and transport properties of the composite have been studied. The sample with the nominal composition La_0.7Ag_0.3MnO₃ was found to show the greatest magnetoresistance in the sample group. Detailed analysis on average Mn valence reveals a composite of (La_0.985Ag_0.015MnO₃)_0.776[(MnO₂)_0.590(Mn₂O₃)_0.410]_0.224. Its MR ratio at room temperature exceeds 24% under a field of 1.8 T. A conductivity leap has been observed around a=0.30. It suggests a kind of field-induced fluctuation in percolation in the samples investigated.     

Structural, magnetic and magnetocaloric properties of La0.7−xEuxBa0.3MnO3 perovskites

Polycrystalline perovskite manganites La_0.7−xEu_xBa_0.3MnO₃(x=0.05, 0.1 and 0.15) were prepared by sol-gel method. The prepared samples remain single phase with a perovskite structure, revealed by X-ray diffraction. The structure refinement of La_0.7−xEu_xBa_0.3MnO₃(x=0.05, 0.1 and 0.15) samples was performed in the hexagonal setting of the R3¯c space group. The dependence of magnetization M on applied magnetic field H and temperature T was measured carefully near the Curie temperature T_C for all the samples. With the increasing Eu content, both the unit cell volume and Curie temperature T_C of 298 K has been detected with a maximum of magnetic entropy |ΔS_M^max| for the La_0.7−xEu_xBa_0.3MnO₃ with x=0.15, reaching a value of 2.3 J/kg K when a magnetic field of 10 kOe was applied and the relative cooling power (RCP) is 46 J/kg. These results suggest that the material may be a suitable candidate as working substance in magnetic refrigeration near room temperature.     

Magnetic and transport properties in Sr1−xLaxFe1−xMnxO3

The magnetic and transport properties in the perovskite Sr_1−xLa_xFe_1−xMn_xO₃ have been explored. As x rises, the systemic ferromagnetism increases gradually and cluster-spin-glass state occurs in the low-temperature region. For 0.3?x?0.7, the ferromagnetic phase separation from the paramagnetic phase was observed from the results of electron-spin-resonance measurement. Although all samples show a semiconducting behavior, their transport properties are dominated by two different mechanisms, namely, the electronic transport of x?0.5 samples is realized by thermal activation but the variable-range hopping is applied in x?0.7 ones. The different transport mechanism can be understood from the Mn/Fe ions interaction.     

Synthesis and enhanced low-field magnetoresistance in La0.67Ca0.33MnO3 /CuO composites

The (1−x)La_0.67Ca_0.33MnO₃+xCuO composites have been synthesized by a new liquid phase method. The XRD and SEM measurements reveal that little CuO is soluble in the structure of La_0.67Ca_0.33MnO₃ and is mainly distributed at the grain boundary of La_0.67Ca_0.33MnO₃. As CuO content x increases, the magnetization M values increase until x=0.05 and M values decrease when x further increases at low temperature. For x=0.10, 0.20 and 0.30 composites, double metal-insulator transitions accompanying a single ferromagnetic transition are observed. Large low-field magnetoresistance is achieved for the composites and the largest magnetoresistance appeared when x=0.20.     

Magnetocaloric effect at room temperature in manganese perovskite La0.65Nd0.05Pb0.3MnO3 with double resistivity peaks

Series of polycrystalline manganese perovskite oxides La_0.7−xNd_xPb_0.3MnO₃ (x=0, 0.05, and 0.1) are prepared by the sol-gel technique, La_0.65Nd_0.05Pb_0.3MnO₃ were representatively investigated because the peculiar double resistivity peaks were found; the maximum magnetic entropy change ΔS_H=−2.03 J/kg K and its good refrigerant capacity 71.05 J/kg around room temperature were obtained under 9 kOe magnetic field variation. The expected double peaks of magnetocaloric effect had not occurred since magnetic entropy change originated from the differential coefficient of magnetic moment to temperature; the relatively well refrigerant capacity possibly results from the faint magnetic inhomogeneity mixed in the double exchange strong magnetic signal.     

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.     

Magnetic entropy change in perovskite manganites La0.6Pr0.1Pb0.3MnO3 with double metal-insulator peaks

Following the double metal-insulator peaks found in series of perovskite manganites La_0.7−xPr_xPb_0.3MnO₃ (x=0, 0.05, 0.1), the magnetic entropy change of La_0.6Pr_0.1Pb_0.3MnO₃ was carefully investigated as a representative. The maximum magnetic entropy change (ΔS_H=−1.7 J/kg K at 300 K) and the expanded refrigerant capacity (about 123.8 J/kg) had been obtained under 10 kOe magnetic field variation, though the double peak of maximum magnetic entropy change had not occurred since the comparative faint magnetic signal from the Pr ions inhomogeneity existed in the octahedral frame submerged in the strong magnetic signal originated from the dominating octahedral frame both in the double exchange mechanism, but the width at half maximum in the magnetic entropy change comparatively broadened.     

Effects of nonmagnetic silver Ag doping on the structural, magnetic and electric properties in La0.67Pb0.33MnO3 manganese oxides

The influence of the silver Ag-substitution for Pb ions in the mixed valence perovskites La_0.67Pb_0.33−xAg_xMnO₃ (0≤x≤0.15) was investigated by X-ray magnetic and electric transport measurements. All compositions were synthesized using the sol-gel technique. X-ray diffraction and structure refinement show that they crystallize in the rhombohedral structure with the R3?c space group. Upon Ag doping on Pb sites, the lattice parameters, unit cell volume, and the Mn-O-Mn bond angle are reduced. All the samples exhibit a ferromagnetic-paramagnetic transition and metallic-semi-conductor one with increasing temperature. The substitution of Pb by Ag has great influence on the magnetic and electrical transport properties of this family of compounds, decreasing continuously both the Curie temperature (from 361 to 290 K) and the resistivity transition temperature T_p.     

The effect of Gd-doping on the charge ordering state of Bi0.3−xGdxCa0.7MnO3 (0≤x≤0.30)

The effect of Gd-doping on the charge ordering (CO) state in perovskite-type manganates Bi_0.3−xGd_xCa_0.7MnO₃ with x=0, 0.02, 0.05, 0.1, 0.3 has been investigated by transport and magnetic property measurements. It is found that CO temperature (T_CO) and antiferromagnetic (AFM) ordering temperature T_N occurring below T_CO decrease obviously with increasing Gd-doping level. Accompanying the variation of T_CO, the increased magnetization and the decreased resistivity are observed. In addition, the increased magnetic inhomogeneity has been also observed in the samples based on the difference between the zero-field-cooling (ZFC) magnetization M_ZFC and field-cooling (FC) magnetization M_FC, which is ascribed to the competition between ferromagnetic (FM) phase induced by Gd-doping and CO AFM phase. The experimental results indicate that the Bi³⁺ lone pair electron with 6s² character plays a dominating role on the CO state of Bi_0.3Ca_0.7MnO₃.     

The influence of the sintering temperature on the structural and the magnetic properties of doped manganites: La0.95Ag0.05MnO3 and La0.75Ag0.25MnO3

La_1−xAg_xMnO₃ samples were synthesized by standard sol-gel method with Ag concentrations of x=0.05 and 0.25. The samples from each concentration were pressed and sintered at 1000, 1200 and 1400 °C for 24 h in air for a systematic study. They were examined structurally by Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) with Energy Dispersive Spectroscopy (EDS) and X-ray Diffraction (XRD) and magnetically by Magnetic Properties Measurements System (MPMS). AFM and SEM analyses show that surface morphology changes with Ag concentration and sintering temperature (T_S). It was observed that high temperature sintering leads Ag to leave material as determined from EDS analyses. XRD spectra exhibited that the crystal structure changes with Ag concentration while showing pronounced change with the sintering temperature. From the magnetic measurements, the Curie temperatures (T_C) and the isothermal magnetic entropy changes (−ΔSM) were calculated. It was observed that T_C increases with Ag concentration and decreases with T_S. The maximum −ΔSM was calculated to be 7.2 J/kg K under the field change of 5 T for the sample sintered at 1000 °C with x=0.25.     

Effect of illumination on magnetization and microwave absorption of La1−xCaxMnO3 (x<0.2) films

Light-induced changes of the hysteresis loops of magnetization and microwave absorption are investigated in low-doped La_1−xCa_xMnO₃ (x<0.2) thin films. The width of the hysteresis loops decreases clearly under illumination with visible or near-infrared light at temperatures below 50 K. The microwave conductivity has a minimum value at magnetic fields corresponding to the magnetization reversal and is shifted towards weaker fields under illumination. These effects show complex nonexponential time evolution and dependence on strength of the magnetic field. The results can be explained by assuming that small ferromagnetic metallic regions exist within the insulating ferromagnetic phase of the sample, and that these regions are expanded by optically induced charge transfer between Jahn–Teller split e_g states of neighboring Mn³⁺ ions. Decrease of the Mn³⁺ XPS core level spectrum is observed in the samples under illumination with a HeNe laser.     

A magnetic, magnetostrictive and Mössbauer study of Tb0.3Dy0.7−xPrx(Fe0.9Al0.1)1.95 alloys

The effect of Pr substitution for Dy on the magnetization, magnetostriction, anisotropy and spin reorientation of a series of Tb_0.3Dy_0.7−xPr_x(Fe_0.9Al_0.1)_1.95 alloys (x=0, 0.1, 0.20, 0.25, 0.30, 0.35) at room temperature has been investigated. It was found that the magnetization and magnetostriction of the homogenized Tb_0.3Dy_0.7−xPr_x(Fe_0.9Al_0.1)_1.95 alloys decreases drastically with increasing x and the magnetostrictive effect disappears for x>0.2, but the spontaneous magnetostriction λ₁₁₁ increases approximately linearly with increasing x. Moreover, the magnetostriction exhibits slightly bigger value at x=0.1 than the free alloys and is saturated more easily with the magnetic field H, showing that a small amount of Pr substitution is beneficial to a decrease in the magnetocrystalline anisotropy. The analysis of the Mössbauer spectra indicated that the easy magnetization direction in the {1 1 0} plane deviates slightly from the main axis of symmetry with Pr concentration x, namely spin reorientation. Comparing with the Al substitution, the effect of Pr substitution for Dy on the spin reorientation is smaller.     

The effect of annealing process on the physical properties of La1−xNaxMnOy

The effect of Na doping and annealing time on the structure, electrical properties, magnetoresistance and thermopower properties has been investigated in perovskite La_1−xNa_xMnO_y (x=0.025, 0.075 and 0.1) systems. La_1−xNa_xMnO_y crystallizes in a single-phase rhombohedral structure. It is observed a simultaneous occurrence of the ferromagnetic to paramagnetic state and metallic to insulating state. In the meanwhile, a large negative magnetoresistance with low applied magnetic field is observed. In addition, ρ(T) curves for Na-doped samples exhibit another broad transition T_ms2 below T_ms. Such double peak behavior in the ρ(T) curve interpreted by the electronic inhomogeneity in the samples. The sign of S changes from positive to negative depending on composition. The values of Seebeck coefficient are small (in the microvolt range).     

Enhanced low-field magnetoresistance in LSMO/SFO composite system

The granular composites of (1−x)La_0.7Sr_0.3MnO₃/xSrFe₁₂O₁₉ [(1−x)LSMO/xSFO] were prepared. The magnetic, electrical, and magnetoresistive properties of the composites were investigated systematically. Two magnetic transitions originating from LSMO and SFO are observed for x=0.05, 0.10, and 0.20. The addition of hard-magnetic SFO ferrite leads to the increased substantially resistivity of the composites and the shift of insulating-metallic transition temperature T_IM correlated with LSMO. Enhanced low-field magnetoresistance (LFMR) in the composites can be mainly attributed to the enhanced spin disorder and spin-dependent tunneling at LSMO grain boundaries induced by the interaction between LSMO and SFO ferrite. The transport mechanisms in detail are analyzed in LSMO/SFO composite system.     

Low-field transport properties of (1−x)La0.7Ca0.2 Sr0.1MnO3+x(ZnO) composites

Composite samples (1−x)La_0.7Ca_0.2Sr_0.1MnO₃(LCSMO)+x(ZnO) with different ZnO doping levels x have been investigated systematically. The structure and morphology of the composites have been studied by the X-ray diffraction (XRD) and scanning electronic microscopy (SEM). The XRD and SEM results indicate that no reaction occurs between LCSMO and ZnO grains, and that ZnO segregates mostly at the grain boundaries of LCSMO. The magnetic properties reveal that the ferromagnetic order of LCSMO is weakened by addition of ZnO. The results also show that ZnO has a direct effect on the resistance of LCSMO/ZnO composites, especially on the low-temperature resistance. With increase of the ZnO doping level, T_P shifts to a lower temperature and the resistance increases. It is interesting to note that an enhanced magnetoresisitance (MR) effect for the composites is found over a wide temperature range from low temperature to room temperature in an applied magnetic field of 3 kOe. The maximum MR appears at x=0.1. The low field magnetoresistance (LFMR) results from spin-polarized tunneling. However, around room temperature, the enhanced MR of the composites is caused by magnetic disorder.     

Magneto-transport and magnetic properties of (1-x)La0.7Ca0.3MnO3+xAl2O3 composites

We report magneto-transport and magnetic properties of (1-x)La_0.7Ca_0.3MnO₃+xAl₂O₃ composites synthesized through a solid-state reaction method combined with a high energy milling method. Most interestingly, the effective magnetic anisotropy is found to decrease with increase in the non-magnetic insulating Al₂O₃ phase fraction in the composites. In addition, we observed that the magnitude of low-field magnetoresistance arising from spin-polarized tunneling of conduction electrons, as well as that of high-field magnetoresistance, displays a Curie-Weiss law-like behavior. Finally, we found that the temperature dependence of low and high-field magnetoresistance is controlled predominantly by the nature of temperature response of surface magnetization of the particles.     

The correlation between structure and magnetic properties in the manganites La0.7Ca0.3−xTexMnO3 (0≤x≤0.15)

The effect of Te-doping at La-site on structural, magnetic and transport properties in the manganites La_0.7Ca_0.3−xTe_xMnO₃ (0≤x≤0.15) has been investigated. All samples show an orthorhombic structure (O′-Pbnm) at room temperature. It shows that the Mn-O-Mn bond angle decreases and the Mn-O bond length increases with the increase in the Te content. All samples exhibit an insulator-metal (I-M) transition and the resistivity increases with the increase in the Te-doping level. Additionally, the Curie temperature T_c decreases and the transition becomes broader with increasing Te-doping level, in contrast, the magnetization of Te-doping samples at low temperatures decrease with increasing x as x≤0.10 and then increase with further increasing x to 0.15. The results are discussed in terms of Jahn-Teller (JT) vibrational anisotropy Q₃/Q₂ and the opening of the new DE channel between Mn²⁺-O-Mn³⁺ due to the introduction of Mn²⁺ ions because of the substitution of Te⁴⁺ ions for Ca²⁺ ions.     

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.     

Structural, magnetic and transport properties in the manganites La0.7Sr0.3−xTexMnO3 (0≤x≤0.15)

The effect of Te-doping at La-site on structural, magnetic and transport properties in the manganites La_0.7Sr_0.3−xTe_xMnO₃ (0≤x≤0.15) has been investigated. All samples show a rhombohedral structure with the space group . It shows that the Mn-O-Mn bond angle decreases and the Mn-O bond length increases with the increase of Te content. The Curie temperature T_C decreases with increasing Te-doping level, in contrast, the magnetization magnitude of Te-doping samples at low temperatures increase with increasing x as x≤0.05 and then decrease with further increasing x to 0.15. The results are discussed in terms of the combined effects of the opening of the new double exchange (DE) channel between Mn²⁺-O-Mn³⁺ due to the introduction of Mn²⁺ ions because of the substitution of Te⁴⁺ for Sr²⁺ and the reduction of the transfer integral b due to the decrease of the Mn-O-Mn bond angle.     

Enhanced low-field-magnetoresistance and electro-magnetic behavior of La0.7Sr0.3MnO3/BaTiO3 composites

We report the structural, magentoresistance and electro-magnetic properties of ferromagnet–ferroelectric–type (1−x)La_0.7Sr_0.3MnO₃/xBaTiO₃ (with x=0.0%, 3.0%, 6.0%, 12%, 15.0% and 18.0%, in wt%) composites fabricated through a solid-state reaction method combined with a high energy milling method. The insulator–metal transition temperature shifts to a lower temperature and resistivity increases while the feromagnetic–paramagnetic transition temperature remains almost unchanged with the increase of BaTiO₃ content. Magnetoresistance of the composites at an applied magnetic field H=3 kOe is enhanced in the wide temperature ranges with the introduction of BaTiO₃, which could be explained by the enhanced spin polarized tunneling effect induced by the introduction of BaTiO₃. The low-field magnetoresistance of the composite is analyzed in the light of a phenomenological model based on the spin polarized tunneling at the grain boundaries. Furthermore, the temperature dependence of resistivity for this series has been best-fitted by using the adiabatic small polaron and variable range hopping models. These models may be used to explain effect of BTO on the electronic transport properties on high temperature paramagnetic insulating region.