The relation between magnetoresistance and magnetocaloric effect in La0.85Ag0.15MnO3 manganite

We present the temperature dependence of La_0.85Ag_0.15MnO₃ resistivity in the temperature interval between 77 and 340 K and magnetic fields up to 26 kOe. We offer a method of separating tunnel magnetoresistance from total magnetoresistance. A change in both the magnetic entropy, which is caused by the magnetocaloric effect (MCE), and the magnetoresistance are shown to be connected through a simple relationship to La_0.85Ag_0.15MnO₃.     

Correlation of electrical and thermalphysical properties of La0.85Ag0.15MnO3 manganite

The relationship of electrical and thermal properties of manganites has been traced through the analysis of temperature dependence of the La_0.85Ag_0.15MnO₃ resistivity. The discussion of results has been held on the basis of percolation theory in the framework of the phase-layered manganite. A behavior of the heat capacity, as well as a change in magnetic entropy can be predicted from the analysis of temperature and magnetic-field dependences of resitivity.     

Effect of inhomogeneity on magnetic, magnetocaloric, and magnetotransport properties of La0.6Pr0.1Ca0.3MnO3 single crystal

The effect of magnetic inhomogeneity on magnetic, magnetocaloric, and transport properties of the colossal magnetoresistance manganites with first order ferromagnetic-to-paramagnetic phase transition is studied. The experiments were performed on the single-crystalline samples of La_0.6Pr_0.1Ca_0.3MnO₃. The inhomogeneity is described by the Curie temperature distribution function, which is found from the magnetization data. The temperature dependence of the magnetic field induced change in the entropy is shown to be determined by the distribution function and the shift of the transition temperature in a magnetic field. Similarly, magnetoresistance in the transition region is determined by the resistivity at H=0 and the shift of the transition temperature. The maximum entropy change as well as maximum magnetoresistance can be achieved in the magnetic field of order δT_C/B_M where δT_C is the transition width and B_M is the rate of change of the Curie temperature with magnetic field.Our approach to analysis of the effects of inhomogeneity is general and therefore can be used for all compounds with the first order magnetic phase transition.     

Effects of ZnO film thickness on electrical and magnetoresistance characteristics of La0.8Sr0.2MnO3/ZnO heterostructures

The La_0.8Sr_0.2MnO₃/ZnO heterostructures with different thicknesses of ZnO films are fabricated by using RF magnetron sputtering technique. The heterojunctions exhibit excellent rectifying properties at 300 K. At low temperatures the temperature dependent junction resistance exhibits a metal-insulator transition like behavior. A magnetic field strongly impacts on electrical characteristics of La_0.8Sr_0.2MnO₃/ZnO p-n junctions, i.e., depressing the junction resistance greatly and driving the metal-insulator transition temperature (T_MI) towards higher temperatures. Large magnetoresistance is observed below T_MI, and it increases with increasing magnetic field and almost saturates at 5 T, i.e., above −90% at 100 K and 5 T.     

Critical behaviour of the specific heat of La0.9Ag0.1MnO3 manganite

 For the first time the specific heat of polycrystalline La_0.9Ag_0.1MnO₃ manganite near the Curie temperature was precisely measured by AC calorimetry and its critical behaviour was studied. The critical behaviour of the specific heat was approximated by the relationship taking into account corrections to the scaling. The regularities of variations in the universal critical parameters near the critical point are determined, and their values are calculated. Detailed analysis and comparison of the received results with the theoretical predictions and other experimental results was carried out.     

Synthesis and Sm/Sm doping effects on photoluminescence properties of Sr4Al14O25

Complete and partial samarium reduction was achieved under strong reducing atmosphere by solid-state and combustion synthesis of Sr_3.96Sm_0.04Al₁₄O₂₅. Dependence of different fluxing agents on the formation of various strontium aluminates was examined. The samples were investigated by X-ray powder diffraction, temperature dependent luminescence decay and photoluminescence measurements. Excitation with UV radiation resulted in sharp and well resolved emission lines of samarium ions. Distinct temperature behavior for Sm²⁺ and Sm³⁺ were detected in the range of 100-500 K. Estimated emission thermal quenching values (TQ_1/2) for divalent samarium were approximately 270 K while for trivalent state around 660 K. Measured luminescence decay values of Sm²⁺ are substantially lower than for Sm³⁺,≈1.7 and ≈2.7 ms, respectively. The spectral feature of Sm²⁺ emission spectrum indicates that dopant occupies low symmetry site in Sr₄Al₁₄O₂₅ compound.     

Magnetotransport and magnetoelastic effects in Co-doped La0.7Sr0.3MnO3 nanocrystalline perovskites

La_0.7Sr_0.3Mn_1−xCo_xO₃ (x=0, 0.05, 0.1) nanoparticles, prepared by sol-gel method, were studied by means of X-ray diffraction, transmission electron microscopy, resistivity, magnetoresistance, thermal expansion and magnetostriction measurements. Results show that partial substitution of Mn by Co leads to a reduction in lattice parameters, enhancement of resistivity and room temperature magnetoresistance MR, decrease of metal-insulator transition temperature T_MI and T_C, an increase in thermal expansion coefficient, volume magnetostriction and anisotropic magnetostriction. The latter increases about one order of magnitude with 10% Co substitution. In comparison with Mn ions, the Co ions possess higher anisotropy energy, larger magnetostriction effect, smaller ionic size and spin state transitions with increase in temperature and magnetic field; this suggests that Co substitution leads to double-exchange interaction weakening, resulting in suppression of ferromagnetic long-range order and metallic state and increase of magnetic anisotropy. Furthermore, our samples have a relatively lower T_MI and T_C, higher resistivity and MR, compared with the reported values for similar compounds with larger particle sizes. This is attributed to the nanometric grain size and spin-polarized tunneling between neighboring grains.     

Giant magnetoimpedance and permeability change in La2/3Sr1/3MnO3 manganite under low fields

We demonstrated that La_2/3Sr_1/3MnO₃ sintered manganite could exhibit a magnetoreactance ΔX/X₀ of −25.5% at 100 kHz, a giant magnetoimpedance ΔZ/Z₀ of −20% at 1-2 MHz and a giant AC magnetoresistance ΔR/R₀ of −39.3% at 5 MHz under a very low field of 300 Oe at room temperature, whereas the DC magnetoresistance Δρ/ρ₀ was −3.95% under H=10 kOe and only about −0.18% under H=300 Oe. Large field-induced change of real and imaginary circular permeabilities (Δμ′_?/μ′_?(0) and Δμ″_?/μ″_?(0)) were obtained for La_2/3Sr_1/3MnO₃ sintered manganite. The giant magnetoreactance (giant magneto-inductive effect) at very low frequencies originates from the field induced change of transverse permeability. At 100 kHz under H=300 Oe, La_2/3Sr_1/3MnO₃ sintered manganite has Δμ′_?/μ′_?(0)=−25.8% and Δμ″_?/μ″_?(0)=−10.9%. The values of ΔR/R₀ and ΔZ/Z₀ are very small under 300 Oe at 100 kHz. The giant magnetoimpedance at high frequencies mainly originates from the large transverse permeability change induced by DC magnetic fields, via the penetration depth. Under H=300 Oe, La_2/3Sr_1/3MnO₃ sintered manganite presents values of Δμ′_?/μ′_?(0)=−24.9%, Δμ″_?/μ″_?(0)=−49.8% at 1 MHz, and Δμ′_?/μ′_?(0)=−21.2%, Δμ″_?/μ″_?(0)=−58.2% at 5 MHz.     

Synthesis and magnetic properties of Sm–Y3Fe5O12 nanoparticles

Garnet nanoparticles Y_3−xSm_xFe₅O₁₂ were fabricated by a sol–gel method. The XRD patterns of all the samples have only peaks of the garnet structure and the sizes range from 34 to 67 nm. Results of VSM show that the saturation magnetization of Y_3−xSm_xFe₅O₁₂ (0<x3) decreases on increasing the Sm concentration x evidently. Meanwhile, it is observed that with the enhancement of the surface spin effects, the saturation magnetization rises as the nanoparticle size is increased.     

Energy transfer between Sm and Er in orthophosphate YPO4

The energy transfer between Sm³⁺ and Er³⁺ ions in yttrium orthophosphate is studied. This choice of ions is based on the possibility of quantum cutting processes and the host material is selected according to the position of the 5d bands of the Sm³⁺ ion. The Sm³⁺ and Er³⁺ doped and Sm³⁺, Er³⁺ co-doped YPO₄ have been synthesized. Spectroscopic studies were done in the ultraviolet and vacuum ultraviolet ranges. The energy transfer between Sm³⁺ and Er³⁺ is very efficient but it does not lead to Er³⁺ visible emission. Whatever the excitation wavelength, the emission of co-doped samples mainly occurs in the infrared range.     

DC current effect on the magnetic phase and transport properties of polycrystalline La0.7Ca0.3MnO3

The influence of DC current on the resistivity and phase transition of polycrystalline La_0.7Ca_0.3MnO₃ has been investigated. The specific heat measurement found that charge carriers and ferromagnetic spin-wave contributions were changed after applied DC current. Applying high electric fields leads to the formation of ferromagnetic regions. The resistivity drops abruptly once the percolating current path is established. As current through the sample disappears, the larger ferromagnetic (FM) clusters, however, remain and are frozen in giving a measurable contribution to the specific heat of the system. The larger clusters should give rise to the value of spin-wave stiffness constant (D), as it is expected to increase the strength of the ferromagnetic coupling. The metallic ferromagnetic regions would make the charge carrier delocalization and attribute to specific heat linear term γ.     

Sol-gel synthesis, structural and optical properties of rare earth ions (Sm or Dy) activated Ca3Ga2Si3O12 powder phosphors

Structural, morphological and optical properties of rare earth ions (RE³⁺=Sm³⁺ or Dy³⁺) activated Ca₃Ga₂Si₃O₁₂ (CaGaSi) phosphors synthesized by the sol-gel method are reported. XRD results confirmed the cubic phase structure of RE³⁺:CaGaSi phosphors. From the SEM images of RE³⁺:CaGaSi phosphors, it is observed that the particles are agglomerated. Photoluminescence spectra of Sm³⁺:CaGaSi phosphors have shown bright orange red emission at 598 nm (⁴G_5/2→⁶H_7/2) with an excitation wavelength of λ_exci=401 nm. In the case of Dy³⁺:CaGaSi phosphors bright yellow emission has been observed at 574 nm (⁴F_9/2→ ⁶H_13/2) with λ_exci=451 nm. From the PL spectral results, the rare earth ion concentration of CaGaSi phosphors is optimized.     

Structure,magnetic and magnetoresistance properties of Pr0.67Sr0.33MnO3 manganite oxide prepared by ball milling method

A sample of Pr_0.67Sr_0.33MnO₃ nanoparticles was synthesized by the ball milling method. X-ray diffraction pattern of the sample showed orthorhombic system with Pnma space group. The average crystallite size of 110 nm was obtained by both Scanning Electron Microscopy and X-ray diffraction. Magnetic measurements showed para-to-ferromagnetic transition with a Curie temperature of T_C=269 K. Electrical investigations showed that all our samples exhibit a semi-conducting behavior above T_C and a metallic-like one at lower temperatures. The sample exhibited a large magnetoresistance of 30% at room temperature in an applied magnetic field of 2 T. The transport and the magnetic properties were interpreted in terms of the existence of magnetic polarons in the sample.     

Correlation of electrical, magnetic, and thermal properties of the La0.85Ag0.15MnO3 manganite near the phase transition temperature

The correlation between electrical, magnetic, and lattice properties of manganites has been studied by analyzing the temperature dependence of the electrical resistivity of the La_0.85Ag_0.15MnO₃ manganite. The results have been discussed within the model of a phase-separated manganite in terms of the percolation theory. It has been shown that, from analyzing the temperature and magnetic field dependences of the electrical resistivity, it is possible to predict the behavior of the magnetization and specific heat, as well as the change of magnetic entropy near the phase transition temperature, and that the obtained agreement with experimental data for this analysis is quite reasonable.     

Energy transfers between Sm and Eu in YPO4, LaP5O14 and LaP3O9 phosphates. Potential quantum cutters for red emitting phosphors

This work concerns the studies of energy transfers between Sm³⁺ and Eu³⁺ ions in some phosphates as new luminescent materials emitting in the orange-red color. The choose of ions is based on the possibility of quantum cutting process and the matrices are selected according to the 5d bands position of Sm³⁺ ion. The Sm³⁺ and Eu³⁺ doped YPO₄, LaP₅O₁₄ and LaP₃O₉ are synthesized and spectroscopic studies in ultraviolet and vacuum ultraviolet ranges have been achieved.     

Investigation of magnetocaloric effect in La0.45Pr0.25Ca0.3MnO3 by magnetic,differential scanning calorimetry and thermal analysis

We investigated magnetocaloric effect in La_0.45Pr_0.25Ca_0.3MnO₃ by direct methods (changes in temperature and latent heat) and indirect method (magnetization isotherms). This compound undergoes a first-order paramagnetic to ferromagnetic transition with T_C=200 K upon cooling. The paramagnetic phase becomes unstable and it transforms into a ferromagnetic phase under the application of magnetic field, which results in a field-induced metamagnetic transition (FIMMT). The FIMMT is accompanied by release of latent heat and temperature of the sample as evidenced from differential scanning calorimetry and thermal analysis experiments. A large magnetic entropy change of ΔS_m=−7.2 J kg⁻¹ K⁻¹ at T=212.5 K and refrigeration capacity of 228 J kg⁻¹ are found for a field change of ΔH=5 T. It is suggested that destruction of magnetic polarons and growth of ferromagnetic phase accompanied by a lattice volume change with increasing magnetic field is responsible for the large magnetocaloric effect in this compound.     

Detection of structural and magnetic transitions in La0.67Ba0.23Ca0.1MnO3 using the rf resonance technique

We report radio-frequency (rf) electrodynamics in polycrystalline La_0.67Ba_0.23Ca_0.1MnO₃ as a function of temperature and magnetic field using a home-built LC resonant circuit powered by an integrated chip oscillator. The resonance frequency (f_r) of the oscillator and the power (P) absorbed by the sample are measured simultaneously. The paramagnetic to ferromagnetic phase transition in the absence of an external magnetic field is accompanied by a rapid decrease in both P and f_r around the Curie temperature T_C=300 K. However, much below T_C, the f_r shows a step-like anomaly around 165 K (195 K) while cooling (warming), which we attribute to a structural phase transition from high temperature rhombohedral () to low temperature orthorhombic (Imma) phase. The step-like anomaly in f_r versus T disappears in a field of 300 G. Fractional changes as large as 19% in Δf_r/f_r and 10% in ΔP/P are observed under H=1 kG around T_C. Our study suggests that the rf resonance technique is a versatile tool to study the magnetization dynamics as well as to investigate the structural phase transition in manganites.     

Magnetocaloric effect in La0.67Sr0.33MnO3 manganite above room temperature

The La_0.67Sr_0.33MnO₃ composition prepared by sol-gel synthesis was studied by dc magnetization measurements. A large magnetocaloric effect was inferred over a wide range of temperature around the second-order paramagnetic-ferromagnetic transition. The change of magnetic entropy increases monotonically with increasing magnetic field and reaches the value of 5.15 J/kg K at 370 K for Δμ0H=5 T. The corresponding adiabatic temperature change is 3.3 K. The changes in magnetic entropy and the adiabatic temperature are also significant at moderate magnetic fields. The magnetic field induced change of the specific heat varies with temperature and has maximum variation near the paramagnetic-ferromagnetic transition. The obtained results show that La_0.67Sr_0.33MnO₃ could be considered as a potential candidate for magnetic refrigeration applications above room temperature.     

脱氧La0.5Ca0.5MnO3样品的铁磁-反铁磁转变和电阻率变化

研究了氧空位对La_0.5Ca_0.5MnO₃ （LCMO）多晶块材的电输运和磁性质的影响. 随着氧空位的增加, 样品在高温段的电阻率一直增加, 并满足绝热小极化子模型, 而低温段的电阻率先下降后上升, 并出现明显的dR/dT>0的行为, 直至最后变为绝缘的. 氧空位的增加抑止了反铁磁相的出现, 使得脱氧的LCMO样品不发生反铁磁转变, 进一步增加氧空位则会抑制铁磁相. 关键词： 0.5Ca_0.5MnO₃')" href="#">La_0.5Ca_0.5MnO₃ 反铁磁相变 铁磁相变 脱氧     

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.