Influence of annealing atmosphere on the properties of La0.5Sr0.5MnO3

The structure, magnetic and electrical transport properties of La_0.5Sr_0.5MnO₃ annealed in different atmosphere have been investigated. No evident change of structural symmetry and the Curie temperature is observed for the samples. The resistivity at zero magnetic field of the samples annealed in air and nitrogen exhibits a metal–insulator transition, while no metal–insulator transition is observed for the sample annealed in oxygen, and for which the resistivity decreases monotonously with increasing temperature. Surprisingly, when an external magnetic field is applied, a metal–insulator transition appears for the sample annealed in oxygen. It is suggested that the annealing atmosphere affects the competition between FM and AFM phases due to the change of Mn⁴⁺/Mn³⁺ ratio and the oxygen/cation vacancies, and has a great influence on the electrical transport properties of La_0.5Sr_0.5MnO₃.     

Research on charge-ordering state in La0.5Sr0.5MnO2.88 and La0.5Sr0.5Mn0.5Ti0.5O3 systems

The structural, magnetic and transport properties of La_0.5Sr_0.5MnO_2.88 and La_0.5Sr_0.5Mn_0.5Ti_0.5O₃ samples have been investigated systematically. Indeed, this series has been considered to understand the influence of physical parameters such as oxygen deficiency and titanium doping effect in undoped La_0.5Sr_0.5MnO₃ sample. Ceramic material based on La_0.5Sr_0.5MnO_2.88 exhibits interesting behaviours of charge-ordering (CO), ferromagnetic (FM) states and a good conductivity down to the lowest temperatures. The substitution of Ti for Mn destroyed drastically the CO, damaged the motion of itinerant e_g electrons and changed the local parameters of perovskite cell. A change of the structure from tetragonal to rhombohedral symmetry is observed causing a weakening of double-exchange interaction. The experiment results show that the suppression of the CO is sensitive to the variety of Mn³⁺/Mn⁴⁺ ratio. In a field of 8 T at 10 K, FM and CO phase can be evaluated to be ∼20:80 according to the μ_exp/μ_cal ratio for La_0.5Sr_0.5MnO_2.88, whereas the CO state is suppressed for La_0.5Sr_0.5Mn_0.5Ti_0.5O₃ sample, FM and anti-ferromagnetic (AFM) phase are coexisted and evaluated to be ∼54:46, respectively.     

Zero-field cooled exchange bias and magnetization reversal in La1.5Sr0.5Co0.4Fe0.6MnO6

The polycrystalline sample La_1.5Sr_0.5Co_0.4Fe_0.6MnO₆ (LSCFMO) was prepared by sol-gel method and its magnetic properties were studied. The interesting magnetization reversal phenomenon and the zero-field cooled exchange bias (ZEB) effect were simultaneously observed in LSCFMO. ZEB effect can exist in a wider temperature range (0–200 K) compared with La_1.5Sr_0.5CoMnO₆ (0–10 K), which is very important in the potential applications. A schematic diagram based on the coupling between the Fe³⁺ spins, Mn³⁺ spins and Co²⁺ or Co³⁺ spins is used to understand the ZEB and the reversal behaviors. Due to the doping of 60% Fe ion, the magnetic microstructure of LSCMFO is more complex than that of LSCMO, resulting in the meta-stable spin structure and more interesting magnetic phenomenon.     

Magnetic and transport properties of Dy substituted layered perovskite La1.3Sr1.7Mn2O7

The effect of Dy substitution for La site in layered manganese oxides La1.3-x Dyx Sr1.7 Mn2O7 on the magnetic and electrical properties has been investigated.With the La 3+ substituting by Dy3+,the long range three-dimensional ferromagnetism transition and the insulator-metal transition disappear.These effects are attributed to the lattice distortion due to the substitution of the smaller Dy3+.Addtionally,the small Dy3+ is inclined to occupy the R site which is in the rock-salt layer,then the distribution of La,Sr,Dy ions in Dy-doped sample should be more orderly than that in La1.3 Sr1.7 Mn2O7,so there is only one insulator-metal transition in the ρ-T curve of the sample with x = 0.05 and x = 0.1.     

Substantial Ca doping site effect on structure of La0.5Ca0.5MnO3 at low temperature

Six possible structures of La_0.5Ca_0.5MnO₃ corresponding to different Ca doping sites are studied at low temperature using atomistic simulation techniques. It is found that Ca doping site has substantial effect on structure of La_0.5Ca_0.5MnO₃ and this effect may explain the existence of four similar phases with slightly different lattice parameters in La_0.5Ca_0.5MnO₃ at low temperature. Structures I and II are found to be favorable La_0.5Ca_0.5MnO₃ structures. They have an ordering arrangement of La³⁺ and Ca²⁺ ions and different arrangements of Mn³⁺ and Mn⁴⁺ ions, which may affect the charge, orbital and magnetic ordering in La_0.5Ca_0.5MnO₃ at low temperature.     

Colossal magnetoresistance in layered manganite Nd2−2x Sr1+2x Mn2O7 (0 ≤ x ≤ 0.5)

The layered manganite Nd_2−2x Sr_1+2x Mn₂O₇, with x varying between 0 and 0.5, has been synthesized using solid-state reaction method. We have found that Nd_2−2x Sr_1+2x Mn₂O₇ do not form the single-phase layered compound with A₃B₂O₇ structure. Instead, mixtures of various phases, such as, orthorhombic perovskite, i.e., Nd_1−z Sr _z MnO₃, layered manganite and unreacted starting compounds, have been obtained. Except for x=0.4, which is found to be an antiferromagnetic insulator, all other x values yielded metal-insulator transition and ferromagnetic ordering.     

Synthesis and characterization of K2NiF4-type phases Ln0.5Sr1.5Mn0.5Fe0.5O4 (Ln = La, Nd, Gd, and Dy)

A systematic investigation of layered perovskite oxides with general formula Ln_0.5Sr_1.5Mn_0.5Fe_0.5O₄ (Ln?=?La, Nd, Gd, and Dy) has been undertaken mainly to understand their structural, magnetic, as well as electrical behavior. The materials were prepared by the ceramic method. X-ray data have been analyzed by using program Checkcell and the variations of various parameters are explained. It has been concluded that not only A-site cation radius, <r _A>, but also the size variance factor (σ ²) influence electrical and magnetic properties. A systematic study of electrical resistivity of all the four materials was undertaken as a function of temperature to understand the conduction mechanism. On analyzing the electrical resistivity data, it has been concluded that variable range hopping model is found to fit well. The magnetic studies suggest that the phases are antiferromagnetic and this behavior could arise from Mn⁴⁺–O–Mn⁴⁺, and Fe³⁺–O–Fe³⁺ superexchange interaction.     

Synthesis,X-ray,magnetic and electrical studies of substituted (Pr,Sr)MnO3 perovskites

A systematic study of the structural, magnetic and electrical properties of the manganites Pr_1-x Sr _x MnO₃(0≤ x ≤ 0.5) has been carried out. X-ray diffraction investigation shows a structural change with composition, from orthorhombic (0 ≤ x ≤ 0.2) to rhombohedral (0.25 ≤ x ≤ 0.5). The magnetic properties of Pr_1-x Sr _x MnO₃ samples could be explained on the basis of a double exchange mechanism between pairs of Mn³⁺ and Mn⁴⁺ ions. These properties are strongly dependent on the ratio of Mn³⁺/Mn⁴⁺. The maximum of the ferromagnetic transition temperature T_c is reached at x ≈ 0.35 corresponding to a value 1.85 of this ratio. The investigation of the electrical properties shows a semiconductor to metal transition as a function of temperature (0.25≤x≤0.4) with a metallic-like behaviour above a critical temperature T_p . A semiconducting-like one is observed for all the range of temperature (50–300 K) for (0 ≤ x ≤ 0.2 and x = 0.5). The evolution of activated energies with the carrier concentration has been investigated.     

Origin of the electron spin resonance signal in the manganites: from polarons to phase separation

In the manganites L_1?xM_xMnO₃ (L = La, Nd, Pr, …; M = Sr, Ba, Ca, …), the doping concentration introduces a mixed valence (Mn³⁺, Mn⁴⁺) which governs the magnetic and electric properties of the compound. Mn³⁺ (S = 2) is scarcely observed in electron spin resonance (ESR). In contrast, Mn⁴⁺ (S = 3/2), is a good ESR probe. However, X-band measurements show an enhanced Mn⁴⁺ susceptibility, which is the signature of some kind of coupling of the Mn⁴⁺ ions with the Mn³⁺ ions, but its exact nature is still controversial. We present multifrequency ESR experiments (9–385 GHz) obtained on different systems (La_1?δMnO₃, La_1?xMnO₃, La_1?xCa_xMnO₃, and Nd_1?xCa_xMnO₃) in the low-concentration range (0 <x< 0.33). In the paramagnetic regime, the Mn³⁺ spectrum cannot be observed because of fast relaxation. The signal arises from polarons, whose size, temperature and magnetic field dependences vary with M andx. The single line observed in the metallic compound evolves towards a double-peak structure visible at high frequency in La_0.97MnO₃. Its evolution with temperature below the magnetic transition reveals the presence of manganese ions in a different magnetic environment, i.e., phase separation. The magnetic order of the separated phase is not ferromagnetic. It is a more complex order, which depends substantially on the nature of the cation M.     

Effects of A-site non-stoichiometry on the structural and electrical properties of 0.96K0.5Na0.5NbO3- 0.04LiSbO3 lead-free piezoelectric ceramics

Effects of A-site non-stoichiometry on the structural and electrical properties of 0.96K0.5+xNa0.5+xNbO3- 0.04LiSbO3 lead-free piezoelectric ceramics were examined for 0 ≤ x ≤ 0.02. The piezoelectric coefficients exhibited a maximum, d33 = 187 pC/N at x = 0.0075, coinciding with the maximum of the grain size and the apparent density at x = 0.0075. The apparent density and the piezoelectric coefficients decreased with increasing x at higher x which was likely due to the crystal geometrical distortion of 0.96K0.5+xNa0.5+xNbO3-0.04LiSbO3. In addition, super-large grains were found and this may be due to liquid phase sintering. Excess (K++Na+) attracted a sum of space charges to keep the charge neutral, resulting in charge leakage during the course of ceramic polarization, influencing the piezoelectric and ferroelectric properties. These findings are of importance for guiding the design of K0.5Na0.5NbO3-based lead-free ceramics with enhanced electrical properties.     

CRYSTAL STRUCTURE AND MAGNETIC PROPERTIES OF La1.2Sr1.8-xCaxMn2O7

The effect of divalent cation substitution on the structure and magnetic properties of La_1.2Sr_1.8-xCa_xMn₂O₇ (x = 0－0.900) is investigated in this paper. Partly replacing divalent cation Sr²⁺ by Ca²⁺ ions results in the weakening and then disappearance of long-range ferromagnetic ordering, and the formation of spin canting and low-temperature spin-glass. Based on structural analysis by Rietveld profile fitting, we suggest that this variation of magnetic property be related to a Jahn－Teller-type attice distortion of MnO₆ octahedra due to the introduction of the smaller sized Ca²⁺ ions.     

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     

Glassy-ferromagnetic behavior in Eu0.5Sr0.5CoO3

Polycrystalline perovskite cobalt oxide Eu_0.5Sr_0.5CoO₃ was prepared by the conventional solid-state reaction method. X-ray powder patterns indicated the prepared samples are pure, cubic perovskite structure (Pm3?m), and with no evidence of any secondary phases. The dc magnetization and ac susceptibility measurements were carried out to investigate the magnetic properties of the sample, and which indicated that cluster-glasses properties are suppressed with the increasing of the coercive field. We denied the possibility of spin-glasses and the existence of the Hopkinson effect in Eu_0.5Sr_0.5CoO₃ through the temperature-dependent ac susceptibility measurements, and explained the magnetic behavior of Eu_0.5Sr_0.5CoO₃ with the competition between magnetic anisotropy and the external magnetic field.     

THE STRUCTURE AND MAGNETIC PROPERTIES OF La2-2xSr1Ca2xMn2O7 (x=0.25－1.00)

In this paper, the effect of divalent cation substitution on the structure and magnetic properties in La_2-2xSr₁Ca_2xMn₂O₇ have been investigated systematically using bulk samples with a wide doping concentration range 0.25≤x≤1.00. Replacing trivalent La ions by divalent Ca ions results in the weakening and then disappearance of the long-range ferromagnetic (FM) ordering, the formation of spin canting, antiferromagnetic (AFM) ordering and low-temperature spin-glass. These results show that increasing the hole-doping concentration significantly suppresses the FM state. We suggest that this variation of magnetic properties is related to the competition of the FM and AFM interactions resulting from the change of Mn³⁺/Mn⁴⁺ ratio and Jahn-Teller-type lattice distortion of MnO₆ octahedra due to the introduction of Ca²⁺ ions.     

Tunable exchange bias in La1.5Sr0.5CoMnO6 double perovskite doped with nonmagnetic Ga ions

The crystal structure, electronic structure, and magnetic behaviors of nonmagnetic Ga ions doped double perovskite La_1.5Sr_0.5CoMnO₆ single phase crystals have been investigated. Different from the traditional magnetic dilution effect of nonmagnetic doping, Ga doping in La_1.5Sr_0.5CoMnO₆ enhances the ferromagnetic (FM) exchange interaction of Co³⁺-O-Mn³⁺. Moreover, both conventional and spontaneous exchange bias (EB) effects can be tuned by modulating the Ga doping content, which is accompanied by the variation of the Co^3+/4+ and Mn^3+/4+ and the effective magnetic moment. The EB field and magnetization can be improved by nonmagnetic Ga³⁺ doping with content lower than 0.2. The evolution of conventional and spontaneous EB effects in La_1.5Sr_0.5Co_1-xGa_xMnO₆ can be understood in terms of the unidirectional interface anisotropic coupling between FM/anti-FM, and/or FM/spin glass, which is affected by antisite disorder, spin glass, and the uncompensated coupling between Co and Mn.     

Effect of Sr content on structure and electrical properties of La1−x Sr x MnO3 from ITSOFC cathode view point

The samples belonging to La_1−x Sr _x MnO₃ series, prepared by combustion route without using fuel, exhibit crystal structural phase transition with the change in Sr content. Less than 40 mol% Sr is partially substituted in the crystal structure of LaMnO₃. The structural phase transition from rhombohedral to cubic, cubic to tetragonal, and tetragonal to orthorhombic takes place on 40, 60, and 80 mol% addition of Sr. The highest electrical conductivity in La_0.4Sr_0.6MnO₃ is understood to be due to the maximum concentration of polaron. The polarons are formed due to the conversion of Mn³⁺ to Mn⁴⁺ so as to achieve electroneutrality after substitution of Sr²⁺ for La³⁺.     

Synthesis and dielectric characteristics of La0.5Bi0.5MnO3 ceramics

La_0.5Bi_0.5MnO₃ ceramics with a single phase were prepared by a solid-state reaction method, and their dielectric properties were characterized. Two dielectric relaxations with a giant dielectric constant were identified in the temperature range from 125 to 350 K. The electron hopping between Mn³⁺ and Mn⁴⁺ was found to be the origin of the dielectric relaxation at low temperatures (125–200 K) with an activation energy of 0.18 eV. The high temperature (200–350 K) dielectric relaxation can be attributed to the conduction.     

Charge ordering and structural transitions in Pr0.5Sr0.41Ca0.09MnO3

The structural and magnetic transitions in Pr_0.5Sr_0.41Ca_0.09MnO₃ have been investigated by neutron diffraction and electron microscopy. Two structural transitions, Imma to I4/mcm and I4/mcm to Pmmn, are observed by decreasing the temperature. Two magnetic transitions, from a paramagnetic insulating to a ferromagnetic metallic and from a ferromagnetic metallic to an antiferromagnetic insulating states at T_C=250 K and T_N=180 K, respectively, are also observed. The structures of these three forms have been determined from neutron powder diffraction data. The first important result concerns the low temperature antiferromagnetic CE type and charge ordered structure, which has been refined in the Pmmn space group, without any constraint. This structure is completely long range ordered, with two Mn-sites, Mn³⁺ in tetragonally elongated octahedra, and Mn⁴⁺, off-centered in nearly regular octahedra. The second important point concerns the abrupt character of the structural transition from the I4/mcm to the Pmmn structure, without any appearance of incommensurability. The magnetic and transport properties of this compound are compared with those of Pr_0.5Sr_0.5MnO₃.     

Effect of cobalt on the magnetoresistance characteristics of rare-earth doped manganites

The effect of cobalt-doping on the magnetic, transport and magnetoresistance characteristics of La_1-xSr_xMnO₃ was investigated. The results show that the magnetoelectric property of rare-earth doped manganites is greatly affected by substitution of Co for Mn sites. The Curie temperature as well as the magnetic moment decreases with the increase of doping concentration, and the samples exhibit obvious characteristics of the spin glass state. Moreover, the magnetoresistance is evidently modulated by doping concentration, and the relevant temperature dependence is also suppressed. In addition, low-temperature magnetoresistance is significantly promoted as doping concentration increases, which renders a value of approximately 50% in the temperature range of 5--200~K and varies within 12.5%. It can be attributed to the effect of spin scattering, induced by cobalt doping, on the itinerant electrons of Mn ions, thus introducing a spin-disorder region into the ferromagnetic region of double-exchange interaction between neighbouring Mn³⁺ and Mn ³⁺ ions.     

Improving microwave dielectric properties of La2.98/3Sr0.01(Mg0.5Sn0.5)O3 ceramics with CuO additive

The microwave dielectric properties of CuO-doped La_2.98/3Sr_0.01(Mg_0.5Sn_0.5)O₃ ceramics were investigated with a view to their application in microwave devices. CuO-doped La_2.98/3Sr_0.01(Mg_0.5Sn_0.5)O₃ ceramics were prepared by the conventional solid-state method. The X-ray diffraction patterns of CuO-doped La_2.98/3Sr_0.01(Mg_0.5Sn_0.5)O₃ ceramics exhibited no significant variation of phase with sintering temperature. By adding 0.75 wt.% CuO, a dielectric constant of 20.07, a quality factor (Q × f) of 63,000 GHz, and a temperature coefficient of resonant frequency τ_f (−77.0 ppm/°C) were obtained when La_2.98/3Sr_0.01(Mg_0.5Sn_0.5)O₃ ceramics were sintered at 1500 °C for 4 h.