MnO3 Compound Research Articles

A comparative study of magnetic training effect in bulk and nanocrystalline La0.46Ca0.54MnO3 compound

Transport, magneto-transport, and magnetic properties of La0.46Ca0.54MnO3 compounds having average grain size down to ∼15 nm have been studied. A magnetic training effect due to the external magnetic field cycling was distinctly observed in charge ordered antiferromagnetic bulk compound. Our present study indicates that the training effect was markedly modified along with the modification of the charge ordering due to the reduction of the grain size, and eventually both phenomenons disappeared in case of our lowest particle size sample (∼15 nm). Enhanced ferromagnetic correlation with the reduction of particle size plays the key role for the gradual diminishing of the training effect in the region of nanometer length scale.

Synthesis, characterization and electrochemical performance of Al-substituted Li2MnO3

Abstract Li2MnO3 is known to be electrochemically inactive due to Mn in tetravalent oxidation state. Several compositions such as Li2MnO3, Li1.5Al0.17MnO3, Li1.0Al0.33MnO3 and Li0.5Al0.5MnO3 were synthesized by a sol–gel Pechini method. All the samples were characterized with XRD, Raman, XPS, SEM, Tap density and BET analyzer. XRD patterns indicated the presence of monoclinic phase for pristine Li2MnO3 and mixed monoclinic/spinel phases (Li2 − xMn1 − yAlx + yO3 + z) for Al-substituted Li2MnO3 compounds. The Al substitution seems to occur both at Li and Mn sites, which could explain the presence of spinel phase. XPS analysis for Mn 2p orbital reveals a significant decrease in binding energy for Li1.0Al0.33MnO3 and Li0.5Al0.5MnO3 compounds. Cyclic voltammetry, charge/discharge cycles and electrochemical impedance spectroscopy were also performed. A discharge capacity of 24 mAh g−1 for Li2MnO3, 68 mAh g−1 for Li1.5Al0.17MnO3, 58 mAh g−1 for Li1.0Al0.33MnO3 and 74 mAh g−1 for Li0.5Al0.5MnO3 were obtained. Aluminum substitutions increased the formation of spinel phase which is responsible for cycling.

Structural, magnetic and magnetocaloric properties of K-doped Pr0.8Na0.2−xKxMnO3 manganites

In the present study we report the effect of A-site substitution by Potassium on the structural, magnetic and magnetocaloric properties in Pr0.8Na0.2−xKxMnO3 (0 ≤ x ≤ 0.15) samples synthesized using the solid state reaction at high temperatures. The Rietveld refinement of the X-ray powder diffraction shows that all our samples crystallize in the orthorhombic symmetry with Pbnm space group. The surface morphology of these samples was carried out using scanning electron microscopy (SEM). Magnetization measurements versus temperature in a magnetic applied field of 0.05 T indicate that K doping leads to a paramagnetic–ferromagnetic transition with a Curie temperature TC increasing from 117 K for x = 0.05 to 132 K for x = 0.15. The maximum of the magnetic entropy change |ΔSMMax| under a magnetic field of 5 T is found to be 3.20, 3.44, 3.32, and 3.30 Jkg−1 K−1 for x = 0, 0.05, 0.1 and 0.15, respectively. Large relative cooling power (RCP) value of 355.6 Jkg−1 is obtained for Pr0.8Na0.2MnO3 compound, at a field change of 5 T. This relatively large value associated to a Curie temperature of 90 K make the present compound a promising candidate for magnetic refrigeration at low temperatures.

Critical behavior near the ferromagnetic–paramagnetic phase transition in Sm0.55−xPrxSr0.45MnO3 compounds (0.3 ≤ x ≤ 0.4)

We present a detailed study of the static magnetization M (H, T) in the vicinity of the ferromagnetic–paramagnetic (FM–PM) phase–transition temperature (TC) in polycrystalline Sm0.55−xPrxSr0.45MnO3 (0.3 ≤ x ≤ 0.4) manganites. The critical exponents β, γ, and δ are determined by using various techniques such as modified Arrott plot, Kouvel–Fisher method and critical isotherm analysis, and are found to be β = 0.255 ± 0.021 and γ = 0.957 ± 0.014 with TC = 226 K for Sm0.25Pr0.3Sr0.45MnO3 and β = 0.324 ± 0.024 and γ = 1.212 ± 0.027 with TC = 254 K for Sm0.15Pr0.4Sr0.45MnO3. A careful analysis of the magnetic data recorded in Sm0.55−xPrxSr0.45MnO3 demonstrates the existence of a tri-critical point that separates discontinuous first-order magnetic transition (FOMT) from continuous second-order magnetic transition (SOMT).

Investigation on magnetic, electrical and thermoelectric power of Bi-substituted La0.8Ca0.2MnO3 manganites

In the present work, the structural, magnetic, magneto-transport and thermoelectric properties of polycrystalline compounds of La0.8−xBixCa0.2MnO3(0≤x≤0.1) compounds are reported. Structure refinement using Rietveld method confirms that the samples are single phased and crystallize in rhombohedral structure with R-3C space group. Magnetic studies reveal that the pristine sample exhibits TC at 221K and it shift towards lower temperature with Bi doping. Deviation of the temperature dependent of inverse susceptibility curves from the Curie–Weiss law confirms the existence of Griffiths-like phase. All the samples exhibit metal to insulator transition at temperature TMI, which is found to decrease with increase in Bi doping which is consistent with the magnetic studies. Magneto-resistance MR% data shows that its peak value increases with Bi-doping. The analysis of electrical resistivity data in the metallic region (T<TMI) suggests that the electron–electron scattering is predominant in this region whereas the insulating region (T>TMI) is well described using Small polaron hopping (SPH) and Mott's variable range hopping (VRH) models. Analysis of thermoelectric power data in the high temperature region suggests that the SPH mechanism is operative in this regime, while the analysis of low temperature data reveals that the electron–magnon scattering is dominant in the region.

Magnetic polarons in doped La0.7Ca0.3MnO3, La0.7Ba0.3MnO3, and La0.7Sr0.3MnO3 manganites

The characteristics of the paramagnetic state of doped manganites containing 30% Mn4+ ions are studied. It is shown that the ferromagnetic Curie temperature for the materials with chemical composition La0.7A0.3MnO3 (A = Ca and Ba) characterized by semiconductor conductivity at T > T C can be determined from the measured χ–1(T) dependence. In the conducting La0.7Sr0.3MnO3 compound, the nonlinear behavior of χ–1 (T) is interpreted as a result of summation of different contributions related to individual Mn ions, as well as to the paramagnetic and ferromagnetic polarons with enhanced magnetic moments. The order (first or second) of the magnetic transition and the crystal lattice symmetry do not affect the formation of correlated polarons in the paramagnetic range.

Effect of exceeding the concentration limit of solubility of silver in perovskites on the dielectric and electric properties of half doped lanthanum–calcium manganite

Dielectric and electric properties of La0.5Ca0.2Ag0.3MnO3 (LCMO–Ag) manganite have been investigated using admittance spectroscopy technique. AC conductivity analysis shows that the conductivity verifies the Jonscher universal power law. The deduced exponent ‘s’ values prove that hopping model is the dominating mechanism in the material. From dc-electrical resistivity study, conduction process is found to be dominated by thermally activated small polaron hopping (SPH) mechanism. Complex impedance analysis (CIA) indicates the presence of relaxation phenomenon and allows to modelize the sample in terms of an electrical equivalent circuit. Also, impedance study confirms the contribution of grain boundary in the electrical properties. Dielectric studies indicate that the La0.5Ca0.2Ag0.3MnO3 compound has a Debye-like relaxation. The temperature dependence of permittivity is well fitted by the modified Curie–Weiss law. It is found that dielectric permittivity behavior and the estimated relaxation parameter value (γ≈2), support the evidence of the relaxor nature of La0.5Ca0.2Ag0.3MnO3 material. The high dielectric constant and the low loss tangent indicate the material is promising for tunable capacitor applications.

Theoretical investigation of the magnetocaloric effect of La0.7(Ba, sr)0.3MnO3 compound at room temperature with a second-order magnetic phase transition

In this paper, the prediction of magnetic and magnetocaloric properties of La0.7(Sr,Ba)0.3MnO3 (LSBMO) compound have been conducted using a phenomenological model. Indeed, the investigation of magnetic properties has revealed that the sample exhibits a paramagnetic to ferromagnetic phase transition. Interestingly, the Curie temperature (TC) was found to decrease from 322 to 300K with the decrease of magnetic fields. Moreover, the magnetocaloric data displays a large value of the magnetic entropy change, 1.5Jkg−1K−1 under an applied magnetic field change of 20kOe, which increases with the increase of the applied magnetic field, making it a promising compound for ecologically-friendly magnetic refrigeration at room temperature. Finally, the construction of the universal curve of the magnetic entropy change proves that the studied manganite undergoes a second order magnetic phase transition.

Magnetic field induced insulator-metal transition in nanocrystalline Pr0.67Ca0.33MnO3 compounds: Evidence of large temperature co-efficient of resistance

We report the electronic transport, magneto-transport, and magnetic properties of nanocrystalline Pr0.67Ca0.33MnO3 compound. A magnetic field induced insulator—metal transition appears for the external magnetic field higher than 50 kOe. We have obtained large value of the temperature coefficient of resistance (TCR) along with magnetoresistance and field coefficient of resistance (FCR). The value of TCR is 135%/K at 48 K. The calculated magnetoresistance is about −9.8 × 107% for 70 kOe and maximum FCR is about 320%/kOe around 75 K. Due to the application of the external magnetic field, charge ordered state of the compound is destabilized leading to such large values of TCR and FCR. Large values of TCR and FCR along with the large magnetoresistance exhibited by the material is interesting from the application point of view.

Effect of yttrium substitution on the electrical and magnetic properties of La0.7Ba0.3MnO3 compound

In the recent years manganese oxides La0.7A0.3MnO3 (A=Ca, Sr and Ba) received great attention because of their unique electrical and magnetic properties. Colossal changes in the magneto resistance upto the order of 106–108 have been observed in thin films, single crystals and bulk materials. We have studied the effect of Yttrium substitution on the electrical and magnetic properties of La0.7Ba0.3MnO3 compound. Samples of the composition La0.7−xYx Ba0.3MnO3 with x=0.0–0.4 were prepared using solid state reaction method. XRD patterns of these samples show the single phase nature of the samples. Electrical resistance of samples was measured in the presence of zero and various applied magnetic fields. All the samples have shown a metal–insulator transition and Yttrium substitution has resulted in a systematic decrease in this transition temperature TMI. Magneto resistance of samples was also found to increase with Yttrium substitution. Susceptibility measurements on these samples have indicated a ferro-paramagnetic transition. The results are discussed in terms of size effects and changes in Mn3+–O–Mn4+ bond angle.

Critical behavior and spontaneous magnetization estimation in La0.5Sr0.5MnO3 compound

We report the critical behavior in La0.5Sr0.5MnO3 perovskite oxide manganite using methods including modified Arrott plot, Kouvel–Fisher plot, and critical isotherm analysis. By means of the above techniques around the ferromagnetic–paramagnetic (FM–PM) transition, the magnetic data analyzed shows the sample attending the second-order magnetic phase transition with the critical parameters β = 0.553±0.020 with TC=352.02±0.63K (from the spontaneous magnetization MS(T) below TC) and γ =1.169±0.058 with TC=352.49±0.85K (from the inverse initial susceptibility χ0−1(T) above TC). The exponent δ =3,216 independently obtained from the critical magnetization isotherm satisfies the Widom scaling relationδ=1+β/γ. Moreover, the critical exponents are in the theoretically expected values for mean-field interaction model rather than the other universal theory and also follow the single scaling equation of M(H,ε)ε−β=f±(H/εβ+γ). Furthermore, to estimate the spontaneous magnetization of La0.5Sr0.5MnO3 perovskite, we used the magnetic entropy change (−ΔSM), obtained from isothermal magnetization. The results of this approach are compared to results obtained from classical analysis using Arrott curves.

Structural characterization, magnetic, magnetocaloric properties and phenomenological model in manganite La0.75 Sr0.1Ca0.15 MnO3 compound

The La0.75Sr0.1Ca0.15MnO3 sample have been prepared by sol–gel method. The crystallographic study shows that the sample crystallize in the rhombohedral structure with R3¯c space group. The field dependence of magnetocaloric properties of manganite La0.75Sr0.1Ca0.15MnO3 with second order phase transition material is investigated using a phenomenological model. The model parameters were determined from the magnetization data adjustment and were used to give better fits to magnetic transition and to calculate the magnetocaloric thermodynamic quantities. The magnetic entropy change reaches a peak of about 5.8J/kgK and the relative cooling power (RCP) value of 195J/kg at 305K upon 5T applied magnetic field. The critical exponents associated with ferromagnetic transition have been determined from magnetocaloric effect methods.

Comparative study of magnetic ordering in bulk and nanoparticles of Sm0.65Ca0.35MnO3: Magnetization and electron magnetic resonance measurements

To explore the effect of size reduction to nanoscale on the hole doped Sm0.65Ca0.35MnO3 compound, dc magnetic measurements and electron magnetic resonance (EMR) were done on bulk and nanoparticle samples in the temperature range 10 ≤ T ≤ 300 K. Magnetization measurement showed that the bulk sample undergoes a charge ordering transition at 240 K and shows a mixed magnetic phase at low temperature. However, the nanosample underwent a ferromagnetic transition at 75 K, and the charge ordered state was destabilized on size reduction down to nanoscale. The low-temperature ferromagnetic component is found to be enhanced in nanoparticles as compared to their bulk counterpart. Interestingly around room temperature, bulk particles show higher magnetization where as at low temperature nanoparticles show higher magnetization. Ferromagnetism in the bulk is due to super exchange where as ferromagnetism in nanoparticles is due to uncompensated spins of the surface layer. Temperature variation of EMR parameters correlates well with the results of magnetic measurements. The magnetic behaviour of the nanoparticles is understood in terms of the core shell scenario.

Magnetocaloric Properties of La0.85Ag0.15MnO3 and (La0.80Pr0.20)0.85Ag0.15MnO3 Compounds

In this work, the structural and magnetic properties of the polycrystalline La0.85Ag0.15MnO3 (LAM) and (La0.80Pr0.20)0.85Ag0.15MnO3(LPAM) compounds prepared by the sol–gel method were investigated. The structural properties of both polycrystalline compounds were investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), and energy dispersive X-ray spectrum (EDX) methods. The XRD results showed that both samples crystallized in the hexagonal symmetry with $R\overline 3 c$ space group. The SEM images showed that the particles are closely packed with each other, and their size range from 0.5 to 2.0 μm with the average sizes of 1.23 and 1.03 μm, for LAM and LPAM, respectively. From the EDX spectrum, in both compounds, we did not see loss of any integrated elements during the sintering process and no other impurity elements. The magnetic properties were studied by employing temperature M(T) and external magnetic field M(H) dependence of magnetization measurements. Both compounds showed magnetic phase transition from paramagnetic to ferromagnetic phase. With the substitution of Pr for La, the Curie temperature, T C, decreased from 262 to 216 K. At a field change of 5 T, LAM and LPAM showed large magnetic entropy changes of 7.90 and 4.96 J kg−1 K−1, respectively. Furthermore, their relative cooling power (RCP) values were found as 213.32 and 263.02 J kg−1, respectively. These findings make both LAM and LPAM compounds as good candidates for the practical magnetic refrigeration.

Effects of monovalent cation doping on the structure, microstructure, lattice distortion and magnetic behavior of single crystalline NdMnO3 compounds.

Pure and 15 mol% Na, K-doped NdMnO3 compounds with perovskite structures are prepared by sol-gel method. Tiny single crystals are formed after sintering the compounds at 1000 °C. The effect of Na and K doping as well as the effect of sintering temperature on the formation and microstructure of NdMnO3 are studied in detail by the Rietveld refinement technique using X-ray powder diffraction data. Single phase formation and single crystalline growth are also confirmed by high resolution transmission electron microscopy (HRTEM). Bond angles and bond lengths are calculated and shown by 3D diagrams. Monovalent doping induces noticeable changes in the microstructure and yields better structural stability in these compounds. Doping results in the change of Mn-O, Nd-O and Mn-O-Mn bond lengths which in turn reduces the lattice and octahedral distortion in the system along with an increase in the tolerance factor. The magnetic properties of these compounds are also modified as a result of doping. The temperature dependent magnetization results show that the Neel temperature of antiferromagnetic NdMnO3 compound is 67.2 K and the Curie temperatures of ferromagnetic Nd0.85Na0.15MnO3 and Nd0.85K0.15MnO3 compounds are 99.1 K and 98.6 K respectively. Both 15% Na and K doping results in a similar TC in doped NdMnO3 compounds.

Influence of disorder on the appearance of Griffiths phases in La0.8−x□xCa0.2MnO3 (x=0.15 and 0.2) compounds

The La0.8−x□xCa0.2MnO3 (x=0.15 and 0.2) compounds were synthesized by the solid state reaction method. The X-ray diffraction (XRD) and magnetic measurements were used to study their physical and chemical properties. The structural characterization has revealed that these samples are crystallized in the orthorhombic structure with Pnma space group. Then, the magnetic properties of these compounds are discussed in detail, building on the magnetization and the susceptibility. The temperature dependence of magnetic susceptibility at higher temperature confirms the presence of the Griffiths phase above the Curie temperature which proves the existence of ferromagnetic clusters in the paramagnetic domain.

Effect of particle size reduction on the structural, magnetic properties and the spin excitations in ferromagnetic insulator La0.9Sr0.1MnO3 nanoparticles

The La0.9Sr0.1MnO3 (LSMO) compound was prepared by the citrate-gel method and annealed at different temperatures (H6, H8, H10 and H12). X-ray diffraction (XRD), transmission electron microscopy (TEM) and magnetic measurements were used to investigate the particle size effects on the physical properties. All samples were found to be single phase and crystallize in rhombohedral symmetry with R3¯c space group except for the H12 sample, whose refinement has revealed the coexistence of both R3¯c rhombohedral and Pbnm orthorhombic phases. It was also found that the reduction of grain size intensively affects the magnetic properties of these compounds. The variation in the magnetic properties as a function of the particle size may be explained in terms of a core–shell model. The reduction of the Curie temperature with increasing grain size is explained as the strain effect of grain induced by the distortion at grain boundaries and the orthorhombic strain caused by the strong JT coupling. The temperature dependence of magnetization based on a thermal distribution of spin waves and fit results to a power law Tα shows a deviation from Bloch low and α deviates from 1.50 to 2.88 as the particle size decreases from 94.9 to 44.98nm. The experimental magnetization measurements at 5K were interpreted in the framework of a random magnetic anisotropy model. From the analysis of the approach to saturation magnetization some fundamental parameters have been extracted, quantifying the anisotropy forces.

Effect of Crystallite Size on the Thickness of Nonmagnetic Shell and Magnetic Properties of La0.7Ca0.3MnO3

La0.7Ca0.3MnO3 compounds with the crystallite size of D = 45-200 nm were synthesized by utilizing the solid-state reaction and mechanical ball milling methods. The reduction of the crystallite size (D) resulted in the decrease of both TC and magnetization, broadening of the phase transition region, and appearance of Griffiths phase features in the materials. The temperature dependence of saturation magnetization (MS), the variations of the coercivity (HC), and the deviation of magnetization from expected Bloch’s law T3/2 for these samples were also investigated. Based on the core-shell model, the core diameter and the nonmagnetic shell thickness for the samples were determined.

Large magnetocaloric effect in Ln0.5Ca0.5MnO3 (Ln=Gd, DY) compounds: Conseqence of magnetic precursor effect of rare earth ions

Abstract Magnetic, specific heat and magnetocaloric studies have been performed on rare earth calcium manganites; Ln 0.5 Ca 0.5 MnO 3 (Ln=Gd, Dy). The observed isothermal magnetic entropy change is fairly large at low temperature in the manganites family, which is attributed to the magnetic precursor effect of rare-earth ions. For Gd 0.5 Ca 0.5 MnO 3 , the isothermal magnetic entropy change (−Δ S ) at 4 K, obtained for 7 T magnetic field, is as high as 22.8 J/kg K. On the other hand, −Δ S is 8.5 J/kg K for Dy 0.5 Ca 0.5 MnO 3 . The large value of magnetic entropy change at the cryogenic temperature range for these compounds is interesting from application point of view.

Properties of La0.9Sr0.1MnO3 and tourmaline compound catalytic materials for methane combustion

A novel catalytic material La0.9Sr0.1MnO3 and tourmaline compound catalytic material was synthesized in the base of traditional catalytic material La0.9Sr0.1MnO3 which exhibited excellent catalytic activity for methane combustion. Different contents of tourmaline were added to give a series of La0.9Sr0.1MnO3 and tourmaline catalytic material through a sol-gel method. Samples above were characterized and analyzed by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer-Emmett-Teller method (BET), temperature programmed reduction (TPR), catalytic activity test and contact angle test. The as-prepared sample with 2% (m/m) tourmaline showed good homogeneity surface morphology and displayed the optimal catalytic activity. The light-off temperature reduced by 10 °C and the T90 decreased by 15 °C. In addition, the mechanism of the reinforcement of catalytic activity was explored.