Scaling Laws for the Magnetocaloric Effect in Calcium Deficiency Manganites La0.8Ca0.2−x □ x MnO3 with a Second-Order Magnetic Phase Transition

The magnetic entropy change (ΔS _M ) of polycrystalline samples La_0.8Ca_0.2?x □ _x MnO₃ (x=0.00; x=0.10 and x=0.20) with a second-order phase transition has been investigated. The field dependence of the magnetic entropy change expressed as ΔS _M ∝H ⁿ follows the phenomenological curves and at the temperature of the peak corresponds to a large field independent exponent of n=0.581; 0.642 and 0.671 for x=0.00; x=0.10 and x=0.20 samples, respectively. Then, we have constructed the phenomenological universal curve by normalizing the magnetic entropy change curves with respect to their maximum values, $\Delta S_{M_{\mathrm{max}}}$ , and rescaling the temperature axis. These universal curves collapse onto a single curve for any applied magnetic field for all samples. Moreover, we note that the universal curve for x=0.10 and x=0.20 samples collapses with a small deviation compared with x=0.00 sample. This is consistent with our previous work where we demonstrated that the magnetic phase transition relative to the samples x=0.00 belongs to the 3D-Ising universal class and to the 3D-Heisinberg class for x=0.10 and x=0.20 samples. This universal curve can be used to predict the response of materials in different conditions not available in the laboratory by extrapolations in field or temperature.     

Magnetic and Magnetocaloric Properties of La0.67Pb0.33?x Ag x MnO3 Compounds

In this paper, we have studied the magnetic and magnetocaloric properties of the perovskite manganites La_0.67Pb_0.33?x Ag _x MnO₃ (0??x??0.15), which show a sharp paramagnetic-ferromagnetic phase transition over a wide temperature range (T=250?C401?K). The Curie temperature has been analyzed by two methods: using the numerical derivative dM/dT and the thermodynamic model. The experimental results indicate that T _C decreases from 368 to 288?K with increasing Ag substitution independently of the method used to obtain?T _C. Upon 10?KOe applied magnetic field, the large magnetic-entropy change (|??S _M|) reaches values of 2.75, 3, 3.25 and 3.5?J/kg?K for x=0, 0.05, 0.10 and 0.15 compositions, respectively, which are comparable to that of?Gd. The relative cooling power (RCP) increases with increasing Ag content from 68.75 (x=0) to 156.27?J/kg (x=0.15) for ??H=10?KOe. Through these results, La_0.67Pb_0.33?x Ag _x MnO₃ materials are strongly suggested for the use of active refrigerants for magnetic refrigeration technology near room temperature.     

Lanthanum Deficiency Effect on the Structural, Magnetic, and Transport Properties of the La0.8−x □ x Ca0.2MnO3 Manganites Oxides

The La_0.8?x □ _x Ca_0.2MnO₃ compounds were prepared by the solid-state reaction with 0.00≤x≤ 0.20. Morphologic, X-ray diffraction (XRD), magnetic, and electrical measurements were performed to examine the effect of the lanthanum deficiency on the physical properties of these materials. The scanning electron microscopy shown the existence of a secondary phase attributed to the Mn₃O₄ oxide. The XRD data have been analyzed by the Rietveld refinement technique. We have found that all samples crystallize in an orthorhombic symmetry with Pnma space group and the unit cell volume decreases with lanthanum deficiency. A magnetization versus temperature study has shown that all samples exhibit a magnetic transition from ferromagnetic (FM) to paramagnetic (PM) phase with increasing temperature. The Curie temperature T _C increases with the lanthanum deficiency from 236 K to 247 K for the samples x=0.00 and x=0.20, respectively. Moreover, the temperature dependence of electrical resistivity shows a metal–insulator transition at T _ρ for all samples which increase with vacancy rate. The electrical resistivity is fitted with the phenomenological percolation model, which is based on the phase segregation of ferromagnetic metallic clusters and paramagnetic insulating regions. Thus, show that the conduction mechanist was explained by the electron–electron and electron–phonon scattering process at low temperature and by small polaron hopping at high temperature. Furthermore, we found that the estimated results are in good agreement with experimental data.     

Magnetic and Charge-Ordering Properties of Bi0.2?x Pr x Ca0.8MnO3 (0.0=x=0.20) Perovskite Manganite

We have studied structure, magnetic and transport properties of polycrystalline samples synthesized with nominal compositions Bi_0.2?x Pr _x Ca_0.8MnO₃ (x=0.00, 0.02, 0.04, 0.08, 0.12, 0.16 and 0.20) via a solid-state reaction route. Magnetic measurements show that substitution of Pr at Bi site strongly affect the magnetic and charge-ordering properties. It has been found that the charge-ordering temperature (T _CO) decreases with increasing x up to x=0.12 and then after it increases with x up to x=0.20. A spin glass (SG) state has been observed at ??105 K in all the samples. Pr doping also leads to enhancement in the magnetic moment up to x=0.12 and then after a decrease in magnetic moment up to x=0.20. A concomitant increase in resistivity up to x=0.04 and then decrease in resistivity up to x=0.20 is also observed. We propose that the local lattice distortion induced by the size mismatch between the A-site cations and 6s² character of Bi³⁺ lone pair electron are responsible for the observed variation in the physical properties with doping content of Pr.     

Structural, magnetic, infrared and Raman studies of La0.8Sr x Ca0.2-x MnO3 (0 ≤ x ≤ 0.2)

We report the results of magnetic, X-ray powder diffraction, infrared and temperature dependent Raman spectra of the La_0.8Sr _x Ca_0.2-x MnO₃ (0 ≤ x ≤ 0.2) manganite. The structure refinement using the Rietveld method indicates that the partial substitution of strontium for calcium (for x ≥ 0.15) modifies the orthorhombic structure of the CaMnO₃ perovskite towards a rhombohedral phase. Magnetic measurement confirms the increase in the Curie temperature from 180 K for La_0.8Ca_0.2MnO₃ to 307 K for La_0.8Sr_0.2MnO₃, respectively. It is argued that the substitution with the larger Sr²⁺ ion strengthens the double-exchange interaction and gives rise to the observed increase of transition temperatures. All manganites show two IR active vibrational modes around 400 and 600 cm^?1. Moreover, when x ≤ 0.1, the absorption band around 400 cm^?1 splits into two peaks. In addition, we have analyzed the frequencies and widths of the observed Raman modes as a function of temperature for all samples with various Sr content. The mode splitting is attributed to both magnetic ordering and large orthorhombic distortion in doped rhombohedral manganites.     

Effect of Fe-doping on Magnetocaloric Properties of AMn1?x Fe x O3 Compounds (0≤x≤0.2)

We have investigated the effect of iron substitution on the magnetocaloric properties of manganites with La_0.75Ca_0.08Sr_0.17Mn_1?x Fe _x O₃ (x=0, 0.075, 0.15, 0.175, and 0.2) nominal composition. The magnetocaloric effect (MCE) was estimated, in terms of isothermal magnetic entropy change (???S _M ), using the M(T,?? ₀ H) data and employing the thermodynamic Maxwell equation. The large magnetic entropy in AMn_1?x Fe _x O₃ is attributed to the field-induced suppression of short-range charge-orbital ordering and antiferromagnetic fluctuations present above T _C . The increase of the Fe concentration x is accompanied by a decrease of (???S _M ), from 5.205 to 0.95?J/kg?K for x=0 and 0.20, respectively, with ?? ₀ H=5?T. For all samples, we find quite large values of (???S _M ), which are very close to that provided for Gd, the prototypical magnetocaloric material.     

Effect of annealing temperature on the electromagnetic properties of La0.8Sr0.2MnO3 prepared by sol–gel process

Journal of Materials Science: Materials in Electronics - La0.8Sr0.2MnO3 (LSMO) powder was prepared using the sol–gel method and treated at different annealing temperatures (AT), and the...     

Ultrasonic Anomaly Near the Charge Ordering Transition in Sr-Doped Nd0.3La0.2Ca0.5−x Sr x MnO3 Manganites

Sr doping in the charge-ordered compound Nd_0.3La_0.2Ca_0.5?x Sr _x MnO₃ has been systematically studied to examine its effect on ultrasonic velocity and elastic moduli as well as magnetic and electrical transport properties. DC electrical resistivity, ρ and AC susceptibility, χ′ measurements showed all samples exhibit metal-insulator (MI) behavior accompanied by ferromagnetic-paramagnetic (FM-PM) transition where the MI transition temperature, T _MI and FM-PM transition temperature, T _C increased with Sr content indicating the enhancement of double-exchange mechanism. Analysis of the resistivity change with respect to temperature, dlnρ/dT ^?1 versus T indicates onset of charge-ordering (CO) state where its CO transition temperature, T _CO decreased with Sr content indicating weakening of the CO state. On the other hand, both absolute longitudinal and shear velocities as well as elastic moduli measured at 80 K increased significantly with Sr doping indicating improvement in elastic properties, which is suggested to be due to the increase in formation of ferromagnetic domains. A longitudinal velocity anomaly characterized by a slope change around the vicinity of T _CO was observed for all samples. The longitudinal elastic anomaly is attributed to the Jahn–Teller (JT) effect of Mn³⁺ ions where analysis of the anomaly using the mean-field theory suggests involvement of the JT effect in the samples, which transforms from dynamic to static type with decreasing temperature. The elastic anomaly shifted down from 222 K (x=0) to 205 K (x=0.05) indicating that the static JT effect was weakened with Sr content.     

Fabrication of (La1−x Gd x )2/3Sr1/3MnO3 Manganite Perovskite Nanorods by Sonication-Assisted Coprecipation

(La_1?x Gd _x )_2/3Sr_1/3MnO₃ manganite perovskite nanorods were synthesized by sonication-assisted coprecipitation. Lower sintering temperatures were required for all the samples to crystallize. The fully crystallize samples of (La_0.5Gd_0.5)_2/3Sr_1/3MnO₃ and (La_0.4Gd_0.6)_2/3Sr_1/3MnO₃ show the formation of nanorods upon their synthesis with an average length and width of 160 nm and 20 nm, respectively. The structural, electrical, and magnetic transport properties were investigated.     

Photoconductivity in Oxygen Deficient La0.7 Pbx □0.3 ? x MnO3 ? δ Thin Films Prepared by Chemical Route

We report the influence of light irradiation on the electrical properties of oxygen deficient La_0.7Pb_{x 0.3 – x} MnO_{3 –} thin films with (x = 0.1) and without (x = 0.3) cation vacancies (noted by ). Transient photoconductivity and photoinduced insulator–metal transition occurs at low temperatures in these thin films. Introducing cation vacancies does not increase the Mn⁴⁺/Mn³⁺ ratio but creates a sheared structure that decreases the oxygen content and can explain our transport and photoconductivity results.     

Influence of Ce Doping on Structural and Transport Properties of Ca1?x Ce x MnO3 (x=0.2) Manganite

We have investigated structural, electric, magnetic and thermal transport properties of electron doped Ca_1?x Ce _x MnO₃ (x=0.2) manganites. The Cerium substitution for Ca²⁺causes electron doping into insulating CaMnO₃ without e _g electron. At room temperature the polycrystalline Ca_0.8Ce_0.2MnO₃ is in the crystallographic orthorhombic structure, with Pnma space group symmetry from the refinement of x-ray powder diffraction patterns. The electrical resistivity data infers that Ca_0.8Ce_0.2MnO₃ manganite is in the semiconducting phase. A smooth linear behavior of log plot values is obtained and is well fitted with adiabatic small polaron conduction model. Nearest-neighbor hopping of a small polaron leads to a mobility with a thermally activated form. The negative values of thermopower infer electron as carriers in Ca_0.8Ce_0.2MnO₃. From susceptibility measurements the Ce doped CaMnO₃ shows a transition from antiferromagnetic (AFM) to paramagnetic (PM) phase.     

Magnetocaloric study of monovalent-doped manganites Pr0.6Sr0.4−x Na x MnO3 (x = 0–0.2)

A systematic investigation of structural, magnetic, and magnetocaloric properties is reported for a series of monovalent sodium-doped manganites Pr_0.6Sr_0.4?x Na _x MnO₃ (x = 0, 0.05, 0.1, 0.15, and 0.2). Rietveld refinements of the X-ray diffraction patterns show that all powder samples are single-phased and crystallized in the orthorhombic structure with Pnma space group. Magnetic characterization and Arrott plot confirm the second-order phase transition at Curie temperature T _C decreasing from 310 K for x = 0 down to 272 K for x = 0.2. Magnetic entropy change is largest for x = 0 reaching 1.95 J kg^?1 K^?1 at 2 T field. This corresponds to a large relative cooling power of 102 J kg^?1 . Magnetic field sensitivity of magnetic entropy change and relative cooling power are analyzed and discussed.