Effect of Ga doping on magneto-transport properties in colossal magnetoresistive La0.7Ca0.3Mn1−xGaxO3 (0<x<0.1)

Samples of La_0.7Ca_0.3Mn_1−xGa_xO₃ with x=0, 0.025, 0.05 and 0.10 were prepared by standard solid-state reaction. They were first characterized chemically, including the microstructure. The magnetic properties and various transport properties, i.e. the electrical resistivity, magnetoresistivity (for a field below 8 T), thermoelectric power and thermal conductivity measured each time on the same sample, are reported. The markedly different behaviour of the x=0.1 sample from those with a smaller Ga content, is discussed. The dilution of the Mn³⁺/Mn⁴⁺ interactions with Ga doping considerably reduces the ferromagnetic double exchange interaction within the manganese lattice leading to a decrease of the Curie temperature. The polaron binding energy varies from 224 to 243 meV with increased Ga doping.     

Influence of Tb substitution for La on the structure,magnetic and magneto-transport properties of La1.2Sr1.8Mn2O7

The physical properties of Tb³⁺ ions substitution at A-site are investigated in the layered manganite La_1.2Sr_1.8Mn₂O₇. A series of La_1.2−xTb_xSr_1.8Mn₂O₇ (x=0, 0.05, 0.15, and 0.20) shows that doping with a Tb ion of smaller radius in La_1.2Sr_1.8Mn₂O₇ caused diffraction peaks to shift to high angle. Some samples have an impure diffraction at about 30°, but all samples form single-phase. Samples can be well indexed on a Sr₃Ti₂O7-type tetragonal structure with the space group I4/mmm. According to the M-T curves, when x≤0.05, the series of samples shows ferromagnetism at low temperatures. With increasing temperature, they have two magnetic transitions at different temperatures. When x≥0.15, the magnetizations dramatically decrease. The ρ–T curves of samples show the metal–insulator transition for x=0, 0.05, and the maximum MR values in magnetic field 5 T are 74% at about 73 K and 94% at about 86 K. When x≥0.15, the samples remain in the insulator state in the whole observed temperature range, and the maximum MR values of 86% and 69% appeared at 74 K and 42 K.     

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.     

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.     

Magnetocaloric properties of (La0.67−xGdx)Sr0.33MnO3 polycrystalline nanoparticles

In this paper, magnetic property and magnetocaloric effect (MCE) in nanoparticles perovskite manganites of the type (La_0.67−xGd_x)Sr_0.33MnO₃ (x=0.10, 0.15, 0.20) synthesized by using an amorphous molecular alloy as precursor have been reported. From the magnetic measurements as function of temperature and magnetic applied field, we have discovered that the Curie temperature (T_C) of the prepared samples is found to be strongly dependent on Gd content. The Curie temperature of samples is 358.4, 343.2, and 285.9 K for x=0.1, 0.15, and 0.2, respectively. A large magnetocaloric effect close to T_C has been observed with a maximum of magnetoentropy change in all the samples, ∣ΔS_M∣_max of 1.96 and 4.90 J/kg K at 2 and 5 T, respectively, for a substitution rate of 0.15. In addition, the maximum magnetic entropy change observed for samples with different concentration of Gd, exhibits a linear dependence with the applied high magnetic field. These results suggest that (La_0.67−x Gd_x)Sr_0.33MnO₃ (x=0.10, 0.15, 0.20) compounds could be a suitable candidate as working substance in magnetic refrigeration near room temperature.     

Magnetocaloric properties in the Cr-doped La0.7Sr0.3MnO3 manganites

Single-phase polycrystalline samples of La_0.7Sr_0.3Mn_1-xCr_xO₃ with nominal composition of x=0.00, 0.20, 0.40 and 0.50 were prepared by a conventional solid-state reaction method in air. Investigations of magnetization were carried out in the temperature range 5-400 K and magnetic field range 0-8 T. It was found that the Curie temperature T_C decreases with increasing x and the maximum magnetic entropy change (−ΔS_M) for x=0.20 is ∼1.203 and ∼2.653 J/kg K, respectively for 2 and 6 T magnetic field near the temperature of 280 K.     

Structural and magnetic properties in the self-doped perovskite manganites with nominal composition La0.7Sr0.3−xMnO3−δ

Perovskite manganites with nominal composition La_0.7Sr_0.3−xMnO_3−δ (0.00≤x≤0.20) have been prepared by the sol-gel method with the highest heat treatment temperature being 1073 K. The XRD patterns indicate that when the doping level is x≤0.10 the samples have only a single phase, with the R3?c perovskite structure, while for x>0.10, the samples have two phases with the R3?c perovskite being the dominant phase and Mn₃O₄ being the second phase. A quantitative analysis and Rietveld fitting of the X-ray powder diffraction data indicate that on the basis of the thermal equilibrium theory of crystal defects there are Mn²⁺ ions at the A sites and Mn³⁺ plus Mn⁴⁺ ions at the B sites in the ABO₃ perovskite phase. The curves of magnetization versus applied magnetic field at 10 K showed that the magnetic moments of the Mn²⁺ ions at the A sites are antiparallel to those of the Mn³⁺ and Mn⁴⁺ ions at the B sites.     

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.     

A位的Gd掺杂对La0.7Sr0.3MnO3体系磁电性质的影响

通过实验研究了La_0.7-xGd_xSr_0.3MnO₃(x= 0.00，0.10，0.15，0.20，0.30 ，0.40，0.50，0.60，0.70)体系的M-T曲线、M-H曲线、红外光谱、拉曼光谱、ρ-T曲线和M R-T曲线.实验结果表明：随着Gd掺杂的增加，体系从长程铁磁有序向自旋团簇玻璃态和反铁 磁状态转变，高掺杂时的输运性质在其磁背景下发生异常.Gd掺杂引起的磁结构变化和额外 的磁性耦合将导致庞磁电阻效应. 关键词： 磁结构 输运行为 庞磁电阻效应     

Transport behavior and mechanism of conduction of simultaneously substituted Y and Fe in La0.7Ba0.3MnO3 perovskite

The electrical properties and the mechanism of conduction of the simultaneously substituted La_0.7−xY_xBa_0.3Mn_1−xFe_xO₃ perovskite (0≤x≤0.30) have been studied. The insertion of Y³⁺ and Fe³⁺ ions in the parent compound La_0.7Ba_0.3MnO₃ leads to an increase of the resistivity. The undoped sample (x=0) shows a metallic behavior, which can be fitted by the relation ρ(T)=ρ₀+ρ₂T²+ρ_4.5T^4.5, indicating the importance of electron-magnon scattering effects in this material. All the other samples (x≥0.10) are semiconductors throughout the studied temperature range (80-290 K). Several models have been used to fit their temperature-dependent resistivity: thermal activation, adiabatic nearest-neighbor hopping of small polarons (Holstein theory) and variable range hopping (VRH) models. The fits show that the electronic transport in semiconducting La_0.7−xY_xBa_0.3Mn_1−xFe_xO₃ is well described and dominated by the VRH mechanism, for which the hopping distance (a) grows with increasing Fe³⁺ doping, thus increasing the average hopping energy W.     

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.     

Inelastic magnetic X-ray scattering from highly correlated electron systems: La1.2Sr1.8Mn2O7, La0.7Sr0.3MnO3 and Fe3O4

Magnetic Compton profiles have been measured for the colossal magnetoresistance manganites La_1.2Sr_1.8Mn₂O₇ and La_0.7Sr_0.3MnO₃, and for magnetite Fe₃O₄, along various crystallographic directions, over a wide range of temperatures and magnetic fields. The experimental results are interpreted via first-principles computations for the double layer manganite, La_1.2Sr_1.8Mn₂O₇, and by using a simple model involving atomic d-orbitals and free electrons for the other two compounds. For all three materials a preference for the occupation of e_g orbitals is found, particularly, for orbitals of dx²−y² symmetry. An itinerant electron contribution is adduced at all temperatures in magnetite; such a contribution also appears in La_1.2Sr_1.8Mn₂O₇, but it is present only at low temperatures in La_0.7Sr_0.3MnO₃.     

Magnetic properties and Griffiths singularity in La0.45Sr0.55 Mn1−xCoxO3

The magnetic properties and the Griffiths singularity were investigated in Mn-site doped manganites of La_0.45Sr_0.55Mn_1−xCo_xO₃ (x=0, 0.05, 0.10 and 0.15) in this work. The parent sample La_0.45Sr_0.55MnO₃ undergoes a paramagnetic-ferromagnetic transition at T_C=290 K and a ferromagnetic-antiferromagnetic transition at T_N=191 K. The doping of Co ions enhances the ferromagnetism and suppresses the antiferromagnetism. The enhanced ferromagnetism results from the fact that the Co doping enhances the Mn³⁺-Mn⁴⁺ double-exchange interaction and induces the Co²⁺-Mn⁴⁺ ferromagnetic superexchange interaction. Detailed investigation on the magnetic behavior above T_C exhibits that the Griffiths singularity takes place in this series of Mn-site doped compounds. The correlated disorder induced by the Co ionic doping, together with the phase competition from the ferromagnetic and the antiferromagnetic interactions among Mn ions, is responsible for the Griffiths singularity.     

The structural and multiferroic properties of (Bi1−xLax)(Fe0.95Co0.05)O3 ceramics

La and Co co-doped BiFeO₃ ((Bi_1−xLa_x)(Fe_0.95Co_0.05)O₃ (x=0, 0.10, 0.20, 0.30)) ceramics were prepared by tartaric acid modified sol–gel method. The X-ray diffraction patterns indicate a transition from rhombohedral structure to tetragonal structure at x=0.20, which has been confirmed by the Raman measurements. The band gap increases with increasing x to 0.20, and then decreases with further increasing x to 0.30. The structural transition has significant effects on the multiferroic properties. The remnant magnetization and saturate ferromagnetic magnetization decrease abruptly with increasing x to 0.10, and then gradually increase with further increasing x up to 0.30. The coercivity is significantly reduced with increasing La doping concentration. The ferroelectricity has been improved by La doping, and the polarization increases with increasing x to 0.10, then decreases with further increasing x up to 0.30. The simultaneous coexistence of soft ferromagnetism and ferroelectricity at room temperature in tetragonal Bi_0.70La_0.30Fe_0.95Co_0.05O₃ indicates the potential multiferroic applications.     

钙钛矿锰氧化物La0.7Sr0.3MxMn1-xO3(M=Cr,Fe)的巨磁电阻效应与磁性

研究了溶胶-凝胶法制备氧化物巨磁电阻材料的工艺,制备了La_0.7Sr_{0.3< /sub>CrxMn1-xO3(x=0,0.10,0.15)和La0.7Sr0.3FexMn1-xO3(x=0.05,0.10,0.16)两 系列的单相钙钛矿锰氧化物多晶样品,并研究了Cr,Fe替代La0.7Sr0.3MnO3中部分Mn后对其结构、磁性和巨磁电阻性质的影响.观察到La0.7 Sr0.3Cr0.15Mn0.85O3和La0. 7Sr0.3Fe0.05Mn0.95O3两个样品的 电阻-温度曲线都出现了双峰.定性讨论了可能产生双峰的机制.随Cr(或Fe)替代量的增加, 材料的居里温度很快下降,铁磁性减弱,导电性降低,巨磁电阻效应增强.但与Fe掺杂相比 ,相同数量的Cr掺杂对材料的影响要小. 关键词： 巨磁电阻效应 溶胶-凝胶工艺 电阻-温度曲线 金属绝缘体转变}     

Cr-doping effect on the structural, magnetic, transport properties and Raman spectroscopy of La(2+x)/3Sr(1−x)/3Mn1−xCrxO3 perovskites

A series of the double-doping samples La_(2+x)/3Sr_(1−4x)/3Mn_1−xCr_xO₃ (0?x?0.25) with the Mn³⁺/Mn⁴⁺ ratio fixed at 2:1 have been fabricated. The structural, magnetic, transport properties and Raman spectroscopy have been investigated, and no apparent crystal structure change is introduced by Cr doping up to x=0.25. But the Curie temperature T_C and metal-insulator transition temperature T_MI are strongly affected by Cr substitution. The room temperature Raman spectra start exhibiting some new features following the increasing concentration of Cr substitutions. Moreover, it is worth noting that the frequency of the A_1g phonon mode can also be well correlated with the A-site mismatch effect (σ²), which is influenced mainly by the variety of the Sr content.     

Room temperature magnetic entropy change and magnetoresistance in La0.70(Ca0.30−xSrx)MnO3:Ag 10% (x=0.0−0.10)

The magnetic and magnetocaloric properties of polycrystalline La_0.70(Ca_0.30−xSr_x)MnO₃:Ag 10% manganite have been investigated. All compositions are crystallized in single phase orthorhombic Pbnm space group. Both, the insulator–metal transition temperature (T^IM) and Curie temperature (T_c) are observed at 298 K for x=0.10 composition. Though both T^IM and T_c are nearly unchanged with Ag addition, the MR is increased. The MR at 300 K is found to be as large as 31% with magnetic field change of 1 T, whereas it reaches up to 49% at magnetic field of 3 T for the La_0.70Ca_0.20Sr_0.10MnO₃:Ag_0.10 sample. The maximum entropy change (ΔS_Mmax) at near its T_c (300.5 K) is 7.6 J kg⁻¹ K⁻¹ upon the magnetic field change of 5 T. The La_0.70Ca_0.20Sr_0.10MnO₃:Ag_0.10 sample having good MR (31%^1 T, 49%^3 T) and reasonable change in magnetic entropy (7.6 J kg⁻¹.K⁻¹, 5 T) at 300 K can be a potential magnetic refrigerant material at ambient temperatures.     

Dielectric properties of lanthanum-doped CaCu3Ti4O12 synthesized by semi-wet route

Effect of La³⁺ doping at Ca²⁺ site in CaCu₃Ti₄O₁₂ has been examined. Compositions with x=0.10, 0.20 and 0.30 were synthesized in the system Ca_(1−3x/2)La_xCu₃Ti₄O₁₂ by semi-wet method. Powder X-ray diffraction confirmed the formation of monophasic compounds. The structure remains cubic similar to CaCu₃Ti₄O₁₂. Lattice parameter increases slightly with increasing La³⁺ concentration. Microstructure has been studied using scanning electron microscopy (SEM). Average grain size is in the range 2-4 μm for various compositions. Energy-dispersive spectrometer (EDS) studies confirm the stoichiometry of the synthesized materials. Dielectric constant, dielectric loss and conductivity of the samples decrease with increasing lanthanum concentrations.     

钙钛矿型稀土锰氧化物La2/3Ca1/3MnO3中Mn位的Ti替代效应

研究了钙钛矿型锰氧化物La_2/3Ca_1/3Mn_1-xTi_xO₃(0≤x≤0.3)的结构、磁性和输运性质.发现Ti替代Mn强烈地抑制了La_2/3Ca_1/3MnO₃的铁磁性和金属电导,并很大地提高了磁电阻值.在低掺杂情况下(x≤0.04),1%的Mn被Ti替代,居里温度T_C和金属-绝缘体转变温度T_p分别平均下降了31和26.5K.当x=0.06时,铁磁态过渡为团簇玻璃态,并在x=0.20时完全变为自旋玻璃态.指出由于Ti的掺入而引起的磁稀释作用以及局域晶格畸变是产生上述结果的主要原因. 关键词：     

Phase boundary between uniaxial and planar ferromagnets in layered perovskite manganite La2−2xSr1+2xMn2O7 (0.313⩽x⩽0.350)

We have examined magnetizations as a function of temperature and magnetic field in layered perovskite manganites La_2−2xSr_1+2xMn₂O₇ single crystals (x=0.313, 0.315, 0.318, 0.320 and 0.350) in order to determine the phase boundary between two ferromagnets (one is an uniaxial ferromagnet whose easy axis is parallel to the c-axis and the other is a planar ferromagnet whose easy axis is within the ab-plane) and following results are obtained: (i) all the present manganites exhibit magnetic transitions from a ferromagnet to a paramagnet at 76, 107, 116, 120 and 125 K for x=0.313, 0.315, 0.318, 0.320 and 0.350, respectively; (ii) for x=0.318, 0.320 and 0.350, the magnetic structure is a planar ferromagnet below Curie temperature; (iii) for x=0.313 and 0.315, the magnetic structure changes from an uniaxial to a planar ferromagnet at 66 and 85 K, respectively. From the results described above we have constructed the magnetic phase diagram of layered perovskite manganite La_2−2xSr_1+2xMn₂O₇ (0.313?x?0.350).