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.     

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.     

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.     

Room temperature magnetocaloric effect of La-deficient bulk perovskite manganite La0.7MnO3−δ

Room temperature magnetocaloric effect in La-deficient bulk perovskite manganite La_0.7MnO_3−δ prepared by conventional solid-state reaction has been reported. The maximum value of the magnetic entropy change (about−1.32 J/kg K) and the refrigerant capacity (approximately close to 37 J/kg) had been obtained at 290 K corresponding to a magnetic field variation of 1 T for La_0.7MnO_3−δ. It is the strong Jahn-Teller coupling that changes Mn-O bond length and Mn-O-Mn bond angles and then the canted spin arrangement and induces the strong double-exchange coupling to a comparatively high magnetic transition temperature. This Curie temperature near room temperature with easy fabrication and higher chemical stability makes La_0.7MnO_3−δ a potential candidate as a working substance in magnetic refrigeration technology.     

Improvement of refrigerant capacity of La0.7Ca0.3MnO3 material with a few percent Co doping

The influence of first and second order magnetic phase transitions on the magnetocaloric effect (MCE) and refrigerant capacity or relative cooling power (RCP) of La_0.7Ca_0.3MnO₃ and La_0.7Ca_0.3Mn_0.95Co_0.05O₃ materials has been investigated. Large low-field-induced magnetic entropy changes are observed in La_0.7Ca_0.3MnO₃ and La_0.7Ca_0.3Mn_0.95Co_0.05O₃ materials. The La_0.7Ca_0.3MnO₃ material experiences a large entropy change with a first-order magnetic phase transition at the Curie temperature, T_C. On the other hand, La_0.7Ca_0.3Mn_0.95Co_0.05O₃ displays a smaller entropy change with a second order phase transition. While a first-order magnetic transition material induces a larger MCE (7.528 J/kg K at 5 T) at T_C, this is limited to a narrow temperature range, resulting in a relatively small RCP (218 J/kg), while the Co-doped second-order magnetic transition material induces a smaller MCE (7.14 J/kg K for 5 T), but it is spread over a broader temperature range, resulting in a larger RCP (308 J/kg). The maximum magnetoresistance (MR, defined as ρ(0)/ρ(H)-1) under a field of 5 T is about 206% and 333% for La_0.7Ca_0.3MnO₃ and La_0.7Ca_0.3Mn_0.95Co_0.05O₃, respectively. The refrigeration capacity (RCP) is enhanced in La_0.7Ca_0.3Mn_0.95Co_0.05O₃ (by about 41%) due to small changes from Co doping. The magnetocaloric features of these materials at lower magnetic fields (MCE=3.163 for La_0.7Ca_0.3Mn_0.95Co_0.05O₃ and 4.63 J/kg K for La_0.7Ca_0.3MnO₃ at 1 T), and the high RCP and MR can provide some ideas for exploring novel magnetic refrigerants that can operate with permanent magnets rather than superconducting ones as the magnetic field source.     

Magnetic resonances spectroscopy of nanosize particles La0.7Sr0.3MnO3

Using a co-precipitation method, perovskite-type manganese oxide La_0.7Sr_0.3MnO₃ nanoparticles (NPs) with particle size 12 nm were prepared. Detailed studies of both ⁵⁵Mn nuclear magnetic resonance and superparamagnetic resonance spectrum, completed by magnetic measurements, have been performed to obtain microscopic information on the local magnetic structure of the NP. Our results on nuclear dynamics provide direct evidence of formation of a magnetically dead layer, of the thickness ≈2 nm, at the particle surface. Temperature dependences of the magnetic resonance spectra have been measured to obtain information about complex magnetic properties of La_0.7Sr_0.3MnO₃ fine-particle ensembles. In particular, electron paramagnetic resonance spectrum at 300 K shows a relatively narrow sharp line, but as the temperature decreases to 5 K, the apparent resonance field decreases and the line width considerably increases. The low-temperature blocking of the NPs magnetic moments has been clearly observed in the electron paramagnetic resonances. The blocking temperature depends on the measuring frequency and for the ensemble of 12 nm NPs at 9.244 GHz has been evaluated as 110 K.     

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.     

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.     

Electrical and magnetic behavior of La0.7Ca0.3MnO3/La0.7Sr0.2Ca0.1MnO3 composites

The electrical transport properties and the magnetoresistance of La_0.7Ca_0.3MnO₃/La_0.7Sr_0.2Ca_0.1MnO₃ composites are investigated as a function of sintering temperature. On the basis of an analysis by X-ray powder diffraction and scanning electron microscopy we suggest that raising the sintering temperature enhanced the interfacial reaction and creates interfacial phases at the boundaries of the La_0.7Ca_0.3MnO₃ and La_0.7Sr_0.2Ca_0.1MnO₃. Results also show that in 3 kOe, and at the Curie temperature, the magnetoresistance value of 14% was observed for the composite sintered at 1300 °C. Based on 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 experimental resistivity—temperature data from 50-300 K and find that the activation barrier decreases as temperature is increased.     

Effect of Ag substitution on the electromagnetic property and microwave absorption of LaMnO3

La_1−xAg_xMnO₃ perovskites with different doping Ag-content were prepared by the sol–gel method. The electromagnetic characteristics and microwave loss behavior of these ion-doped rare-earth manganites were studied in the 2–18 GHz frequency range. The microstructure and morphology of the samples were characterized by X-ray diffraction (XRD), and scanning electron microscopy (SEM) techniques. The complex permittivity spectra, the complex permeability spectra and microwave reflection loss were measured by a microwave vector network analyzer system. The XRD patterns show that the crystalline perovskite main phase ABO₃ is formed and impurity phases disappear when calcined at 1100 °C, and Ag metal as an impurity phase appears when excessive Ag⁺ is doped. The SEM image indicates that many of the La_0.85Ag_0.15MnO₃ particles are fiber-like or ellipsoidal. Magnetic loss and dielectric loss coexist and cooperate in microwave attenuation by moderate substitution of Ag⁺ for La³⁺. The microwave absorption property of the La_0.85Ag_0.15MnO₃ sample is enhanced with the bandwidth below −10 dB at about 6 GHz and the peak value of reflection loss is near −25.0 dB at the layer thickness of 2 mm.     

Hydrothermal synthesis and Magnetocaloric effect of La0.5Ca0.3Sr0.2MnO3

The hydrothermal synthesis and magnetic entropy change for the perovskite manganite La_0.5Ca_0.3Sr_0.2MnO₃ have been studied. The La_0.5Ca_0.3Sr_0.2MnO₃ can be produced as phase-pure, crystalline powders in one step from solutions of metal salts in aqueous potassium hydroxide solution at a temperature of 513 K in 72 h. Scanning electron microscopy shows that the materials are made up of cuboid-shaped particles in typical dimension of 4.0×2.5×1.6 μm. Heat treatment can improve the magnetocaloric effect for the hydrothermal sample. The maximum magnetic entropy change ΔS_M for the as-prepared sample is 0.88 J kg⁻¹ K⁻¹ at 315 K for a magnetic field change of 2.0 T. It increases to 1.52 J kg⁻¹ K⁻¹, near its Curie temperature (317 K) by annealing the sample at 1473 K for 6 h. The hydrothermal synthesis method is a feasible route to prepare high-quality perovskite material for magnetic refrigeration application.     

Thickness and microstructure dependent transport and MR in La0.7Pb0.3MnO3 manganite films

Thickness dependent modifications in the structure and microstructure and their effect on the transport and magnetotransport in chemical solution deposition (CSD) grown La_0.7Pb_0.3MnO₃ (LPMO) manganite thin films grown on single crystalline LaAlO₃ (LAO) (1 0 0) substrates have been studied. X-ray diffraction (XRD) measurements show the reduction in the microstrain at the film-substrate interface with increasing thickness. Increase in grain size and island like grain growth with the reduction in surface roughness as a function of film thickness has been observed from the microstructural studies using atomic force microscopy (AFM) and lateral force microscopy (LFM) measurements. Improvement in the surface to volume ratio (D⁻¹) resulting in the modifications in transport and magnetotransport properties of the LPMO films has been discussed in detail in the light of thickness dependent microstructural effects on the resistivity and magnetoresistance (MR) behavior. The variations in intrinsic and extrinsic MR with D⁻¹ show an interesting interplay between them, which can be explained on the basis of thickness dependent grain size, grain boundary density and grain boundary nature.     

Synthesis and performance of Y-doped La0.7Sr0.3CrO3−δ as a potential anode material for solid oxygen fuel cells

Y-doped La_0.7Sr_0.3CrO_3−δ is a promising anode catalyst for solid oxygen fuel cell (SOFC). The performances of chemical and physical are measured by SEM, XRD and FT-IR. The conductivities of catalyst are measured by DC four-probe method in 20% H₂S-N₂, 3% H₂-N₂ and air from 573 K to 1173 K, respectively. The results show that Y-doped La_0.7Sr_0.3CrO_3−δ powders have perfect perovskite phase structure with no extra peaks and exhibit good chemical compatibility with Ce_0.8Sm_0.2O_1.9 (as electrolyte) in air. Through XRD and FT-IR analysis no sulfur-containing species is detected after exposure to the 20% H₂S at 1173 K for 5 h. Meanwhile, Y-doped La_0.7Sr_0.3CrO_3−δ shows that the highest conductivity is 0.21 S/cm at 1173 K in H₂S. The open circuit voltages are 0.85 V at 1173 K in H₂S and 1.04 V at 823 K in H₂. The maximal power densities are 12.4 mW/cm² in H₂S and 1.59 W/cm² in H₂ for cells comprising Y-doped La_0.7Sr_0.3CrO_3−δ-Sm_0.2Ce_0.8O_1.9/Sm_0.2Ce_0.8O_1.9/Ag.     

Particle size effects on La0.7Ca0.3MnO3: size-induced changes of magnetic phase transition order and magnetocaloric study

The structure, magnetic properties, and magnetocaloric effect of La_0.7Ca_0.3MnO₃ ceramics with different particle sizes have been investigated. It is found that the Curie temperature increases first, and then decreases as particle size decreases and the type of magnetic phase transition changes from first-order to second-order, which may be attributed to surface pressure effects. The maximum magnetic entropy change and relative cooling power (RCP) show non-monotonic behaviors with decreasing the particle size. However, for the 3400 nm sample, the magnetic entropy change −ΔS_M reaches the maximum values of 6.41 and 8.63 J/kg K for the field changes of 2.0 and 4.5 T, respectively. Furthermore, the estimated large RCP values under lower magnetic fields in La_0.7Ca_0.3MnO₃ are comparable with those of typical magnetic refrigerant materials in the corresponding temperature range, suggesting those compounds might be promising candidates for magnetic refrigeration.     

Oxygen pressure dependent interfacial polarization of perovskite oxide multilayers on Si substrates

The SrTiO₃/La_0.9Sr_0.1MnO₃ (STO/LSMO) multilayers were fabricated on n-Si (1 0 0) substrates using a computer-controlled laser molecular-beam epitaxy (MBE) technique at 1 × 10⁻² Pa and 2 × 10⁻⁴ Pa oxygen partial pressures, respectively. The dielectric properties of the multilayers (MLs) were investigated. The differences of the dielectric properties of the two samples were explained by an impedance analysis technique, which indicates that the interfacial polarization has a close relationship with the oxygen pressure.     

Defect structure and oxide ion transport in Sr- and Cr-doped LaCoO3 − δ

The results of oxygen nonstoichiometry, δ, measured by means of coulometric technique as a function of oxygen partial pressure, p_o2, in temperature range 1223 ≤ T, K ≤ 1323 are presented for the perovskite-type doped with chromium solely LaCo_0.7Cr_0.3O_3 − δ and simultaneously doped both with strontium and chromium La_0.7Sr_0.3Co_0.7Cr_0.3O_3 − δ cobaltites. The limit stability of the latter was found to exceed that of undoped cobaltite LaCoO_3 − δ on six orders of magnitude of p_o2 at a given temperature. The modeling of the defect structure of these perovskites was carried out and its adequate model was found. Chemical and self-diffusion coefficients of oxygen vacancies and oxygen ionic conductivity and ionic transport numbers were measured for the first time for La_0.7Sr_0.3Co_0.7Cr_0.3O_3 − δ as a function of oxygen partial pressure p_o2and temperature in the ranges − 4 ≤ log(p_o2, atm) ≤ 0 and 1223 ≤ T, K ≤ 1323, respectively. The additional substitution of Sr for La in LaCo_0.7Cr_0.3O_3 − δ was shown to lead to noticeable increase of ionic conductivity and oxygen chemical diffusion coefficient at given values of oxygen partial pressure and temperature as compared to lanthanum cobaltite doped with chromium solely. Self-diffusion coefficient of oxygen vacancies and their mobility in La_0.7Sr_0.3Co_0.7Cr_0.3O_3 − δ were found to be dependent on oxygen partial pressure and nonstoichiometry unlike undoped and doped with chromium lanthanum cobaltites.     

Laser patterning and magnetic properties of perovskite-type La0.7Sr0.3MnO3 crystals on the glass surface

The laser-induced crystallization technique has been applied to 10La₂O₃-35SrO-25MnO₂-30B₂O₃ glass (mol%) in order to examine the possibility of the formation and morphology control of perovskite-type La_1?xSr_xMnO₃ crystals on the glass surface. It is found from X-ray diffraction analyses that La_0.7Sr_0.3MnO₃ crystals are formed in the sample obtained by continuous wave Y b:Y V O₄ fiber laser (wavelength: 1080 nm) irradiations (power: 350 mW, scanning speed: 10 μm/s). La_0.7Sr_0.3MnO₃ manganites in the laser-irradiated samples are proposed to be ferromagnetic crystals having a Curie temperature of ～315 K from magnetization measurements and the Arrott plot. The surface morphology of laser-irradiated parts is not smooth, and La_0.7Sr_0.3MnO₃ crystals are present randomly without any orientations. It is clarified that Mn⁴⁺ ions in the glass act as suitable transition metal ions for the laser-induced crystallization.     

Effects of crystallographic orientation on the oxygen exchange rate of La0.7Sr0.3MnO3 thin films

The oxygen surface exchange of La_0.7Sr_0.3MnO₃ (LSM) thin films was investigated using the electrical conductivity relaxation (ECR) method. Epitaxial (100)-, (110)-, and (111)-oriented LSM films were fabricated on corresponding SrTiO₃ (STO) substrates using pulsed laser deposition. The LSM films had well-controlled surface qualities, exhibited bulk-like steady-state electrical properties, and exhibited surface dominated responses in ECR. The chemical surface exchange coefficients (k_chem) were determined and varied from ≈ 1 × 10^− 6 to 65 × 10^− 6 cm/s, depending on temperature and orientation, with activation energies of between 0.8 and 1.2 eV. At 800 °C, a four fold variation is observed in the k_chem values, with (110)/(100) being the highest/lowest, explained well by the high activation energy for (110), ≈ 1.16 eV, and the low energy for (111) and (100), ≈0.83 eV.     

Effect of Fe substitution on magnetocaloric effect in La0.7Sr0.3Mn1−xFexO3 (0.05≤x≤0.20)

We have studied the effect of Fe substitution on magnetic and magnetocaloric properties in La_0.7Sr_0.3Mn_1−xFe_xO₃ (x=0.05, 0.07, 0.10, 0.15, and 0.20) over a wide temperature range (T=10-400 K). It is shown that substitution by Fe gradually decreases the ferromagnetic Curie temperature (T_C) and saturation magnetization up to x=0.15 but a dramatic change occurs for x=0.2. The x=0.2 sample can be considered as a phase separated compound in which both short-range ordered ferromagnetic and antiferromagnetic phases coexist. The magnetic entropy change (−ΔS_m) was estimated from isothermal magnetization curves and it decreases with increase of Fe content from 4.4 J kg⁻¹ K⁻¹ at 343 K (x=0.05) to 1.3 J kg⁻¹ K⁻¹ at 105 K (x=0.2), under ΔH=5 T. The La_0.7Sr_0.3Mn_0.93Fe_0.07O₃ sample shows negligible hysteresis loss, operating temperature range over 60 K around room temperature with refrigerant capacity of 225 J kg⁻¹, and magnetic entropy of 4 J kg⁻¹ K⁻¹ which will be an interesting compound for application in room temperature refrigeration.     

Thin porous Ni-YSZ films as anodes for a solid oxide fuel cell

Porous Ni-YSZ (YSZ—yttria-stabilized zirconia) films were fabricated by reactive co-sputtering of a Ni and a Zr-Y target, followed by sequentially annealing in air at 900 °C and in vacuum at 800 °C. The Ni-YSZ films comprised small grains and pores that were tens of nanometers in size. The porous Ni-YSZ films were used as an anode on one side of a YSZ electrolyte disc and a La_0.7Sr_0.3MnO₃ thick film was used as a cathode on the other side of the disc to form solid oxide fuel cells (SOFCs). The voltage-current curves of the SOFCs with single- and a triple-layered porous anodes were measured in a single-chamber configuration, in a mixture of CH₄ and air (CH₄:O₂ volume ratio=2:1). The maximum power density of the SOFC using the single-layered porous Ni-YSZ thin films as the anode was 0.38 mW cm⁻², which was lower than that of 0.76 mW cm⁻², obtained using a screen-printed Ni-YSZ thick anode. The maximum power density of the SOFC with a thin anode was increased, but varied between 0.6 and 1.14 mW cm⁻² when a triple-layered porous Ni-YSZ anode was used.