Structure and magnetization studies of Nd0.5−xPrxSr0.5MnO3 system

The structural and magnetic properties of Nd_0.5−xPr_xSr_0.5MnO₃ (x=0, 0.1, 0.2, 0.3, 0.4 and 0.5) system have been investigated. With the substitution of Pr in Nd_0.5Sr_0.5MnO₃, it shows a gradual structure transformation from the Imma orthorhombic symmetry to the tetragonal I4/mcm phase, and the crystallographic transition remains incomplete, even in Pr_0.5Sr_0.5MnO₃. A large bifurcation between zero-field-cooled (ZFC) and field-cooled (FC) susceptibility has been observed below Curie temperature (T_C), which is characteristic of coexistence of ferromagnetism (FM) and antiferromagnetism (AFM) at low temperature region. The magnetization of Pr_0.5Sr_0.5MnO₃ is larger than that of Nd_0.5Sr_0.5MnO₃, while Nd_0.5Sr_0.5MnO₃ with more CE-type AFM shows larger magnetization than Nd_0.3Pr_0.2Sr_0.5MnO_3, which mixed with CE-type (majority) and A-type (minority) AFM at low temperature, indicating that the magnetization of Nd_0.5−xPr_xSr_0.5MnO₃ system is affected by A-site disorder combined with orbital ordering of A-type AFM and CE-type AFM.     

Electron magnetic resonance (EMR) in sonochemically prepared doped nano-manganites

X-band electron magnetic resonance method was explored for study of sonochemically prepared nano-powders of La-based doped manganites: La_0.7Sr_0.3MnO₃ and La_0.9Ca_0.1MnO₃ at 115 ⩽ T ⩽ 600 K temperature interval, including main characteristic points of its magnetic transitions. The data obtained were compared with those for crushed bulk single crystals of the same compositions. It is shown that nano-powder of La_0.7Sr_0.3MnO₃ has the same ferromagnetic ordering as its bulk counterpart, being more homogeneous and less anisotropic. While, nano-powder of La_0.9Ca_0.1MnO₃ shows two distinct ferromagnetic phases in a marked contrast to the bulk sample, where complex magnetic structure comprising canted antiferromagnetic matrix and ferromagnetic clusters is observed. To explain the observed phenomena, the crucial role of nano-scale grain size and its interplay with spin and charge degrees of freedom in considered systems are discussed.     

Pseudogap behavior in Pr0.5Sr0.5MnO3: A photoemission study

The valence band electronic structure of Pr_0.5Sr_0.5MnO₃ has been investigated across its paramagnetic metallic (PMM)–ferromagnetic metallic (FMM)–antiferromagnetic insulator (AFMI) transition. Using surface sensitive high resolution photoemission we have conclusively demonstrated the presence of a pseudogap of magnitude 80 meV in the near Fermi level electronic spectrum in the PMM and FMM phases and finite intensity at the Fermi level in the charge ordering (CO)-AFMI phase. The pseudogap behavior is explained in terms of the strong electron–phonon interaction and the formation of Jahn Teller (JT) polarons, indicating the charge localizations. The finite intensity at the Fermi level in the insulating phase showed a lack of charge ordering in the surface of the Pr_0.5Sr_0.5MnO₃ samples.     

Maximum magnetic entropy change modulated toward room temperature in perovskite manganites La0.7−xNdx(Ca,Sr)0.3MnO3

With Nd³⁺ doping and Ca²⁺, Sr²⁺ modulating in the sol–gel technique, a series of polycrystalline perovskite samples La_0.7?xNd_x(Ca,Sr)_0.3MnO₃ (x = 0, 0.05, 0.1, 0.15, 0.20, 0.25) was prepared, their maximum magnetic entropy changes were tuned to room temperature (ΔS_H = ?1.47 J/kg K at 298 k for La_0.45Nd_0.25(Ca,Sr)_0.3MnO₃), an enhancement of the maximum magnetic entropy change (ΔS_H = ?1.89 J/kg K at 315 k) and its refrigerant capacity (about 45.3 J/kg) had also been obtained under 9 kOe magnetic field variation for La_0.55Nd_0.15(Ca,Sr)_0.3MnO₃ contrast to La_0.7(Ca,Sr)_0.3MnO₃.     

Effect of Ca doping on the magnetic properties of Nd0.5Sr0.5MnO3 studied by electron spin resonance

The magnetic properties of Ca-doped Nd_0.5Sr_0.5MnO₃ have been studied by electron spin resonance (ESR) and dc magnetization measurements. The antiferromagnetic order and charge order are found to occur separately at T_N=200 K and T_co=150 K, respectively. Compared to the undoped Nd_0.5Sr_0.5MnO₃, the ferromagnetic correlations are suppressed by doping of the small Ca²⁺ ion. In addition, the antiferromagnetic transition temperature is enhanced to 200 K, which can be explained by an increase of superexchange interaction between Mn³⁺ and Mn⁴⁺ ions as their distance decreases.     

Size-induced ferromagnetism and metallicity in Nd0.5Sr0.5MnO3 nanoparticles: A neutron diffraction study

The phenomenon of destabilization of antiferromagnetic insulating state into a ferromagnetic metallic one in Nd_0.5Sr_0.5MnO₃ with the variation of particle/grain size is critically investigated. Based on our neutron diffraction study, magnetic and transport experiments, we observe ferromagnetism and metallic behavior in Nd_0.5Sr_0.5MnO₃ (∼40 nm grain size) as against A-type antiferromagnetic order in the sample with the largest grain size (∼800 nm). The latter shows a systematic change in the lattice parameters with temperature, and an antiferromagnetic ground state similar to that of a bulk system. Interestingly, the sample with the smallest grain sizes exhibits insignificant structural changes (compared to the largest grain size sample) but a complete change in the magnetic state (ferromagnetic behavior) as revealed from the neutron diffraction study. Magnetic measurements also confirm a ferromagnetic state in the small-grained sample. Electronic transport measurements exhibit a metal-insulator transition in this sample. The effects are primarily attributed to enhanced surface disorder.     

Electronic and vibrational Raman spectroscopy of Nd0.5Sr0.5MnO3 through the phase transitions

Raman scattering experiments have been carried out on single crystals of Nd_0.5Sr_0.5MnO₃ as a function of temperature in the range of 320–50 K, covering the paramagnetic insulator-ferromagnetic metal transition at 250 K and the charge-ordering antiferromagnetic transition at 150 K. The diffusive electronic Raman scattering response is seen in the paramagnetic phase which continue to exist even in the ferromagnetic phase, eventually disappearing below 150 K. We understand the existence of diffusive response in the ferromagnetic phase to the coexistence of the different electronic phases. The frequency and linewidth of the phonons across the transitions show significant changes, which cannot be accounted for only by anharmonic interactions.     

Exchange interaction between ferromagnetic and antiferromagnetic phases in La1/3Nd1/3Ca1/3MnO3

Magnetization measurements of a La_1/3Nd_1/3Ca_1/3MnO₃ perovskite at magnetic field up to 6 T have revealed an anomalous behaviour – above 130 K the material exhibits a loop displacement about a field of 2 T. We assume that this is the result of an exchange interaction between ferromagnetic and antiferromagnetic phases in a magnetically inhomogeneous compound. At about 115 K a transition from a semiconducting to a metallic-like state has been observed.     

Electrical conductivity and thermal expansion of La0.8Sr0.2(Mn,Fe,Co)O3-δ perovskites

La_1−xSr_xMeO₃ (Me = Mn, Co, Fe) perovskites are used as cathodes and are also attractive materials for application as the contact layer between cathode and interconnect in solid oxide fuel cells. In this contribution, three perovskite series, La_0.8Sr_0.2Mn_1−xCo_xO_3-δ (series 1), La_0.8Sr_0.2Fe_1−xCo_xO_3-δ (series 2) and La_0.8Sr_0.2Mn_1−x/2Fe_(1−x)/2Co_xO_3-δ (series 3) with x = 0, 0.25, 0.5, 0.75 and 1 were re-investigated under identical synthesis and measurement conditions with the aim of obtaining a full overview of the quasi-ternary system La_0.8Sr_0.2MnO_3-δ–La_0.8Sr_0.2FeO_3-δ–La_0.8Sr_0.2CoO_3-δ. The distribution of the different crystallographic phases in the selected series, the DC electrical conductivity and the thermal expansion coefficients are presented.     

Influence of the granule size on the magnetocaloric properties of manganite La0.5Ca0.5MnO3

The influence of the granule size on the magnetic and magnetocaloric properties of ceramic sample of manganite La_0.5Ca_0.5MnO₃ has been investigated. Anomalies indicating the coexistence of the ferromagnetic metallic and antiferromagnetic charge-ordered phases below T _C have been found in the temperature dependence of the magnetocaloric effect. It has been shown that a decrease in the granule size to 90 nm leads to the complete suppression of the antiferromagnetic charge-ordered phase.     

Effects of A-site disorder on magnetic,electrical and thermal properties of La0.5−xLnxCa0.5−ySryMnO3manganites

The magnetic, electrical and thermal properties in the La_0.5?xLn_xCa_0.5?ySr_yMnO₃ (Ln=Pr, Nd, Sm) bulk system were investigated. Detailed dc magnetization and linear ac susceptibility measurements reveal that the samples first undergo phase transition from paramagnetic to ferromagnetic phase and then to an antiferromagnetic phase upon further cooling. It is found that both the Curie and Neel temperatures decrease systematically with increasing A-site disorder in these manganites. The electrical resistivity exhibits semiconducting behavior throughout the temperature range investigated and the electronic conduction mechanism can be conveniently described within the framework of the variable range hopping model above T=150 K. The Seebeck coefficient (S) in the magnetically ordered regime infers that the complicated temperature dependence of S is an indication of electron–magnon scattering. Specific heat measurements depict a broadened hump in the vicinity of T_C, indicating the existence of magnetic ordering and magnetic inhomogeneity in the samples. The temperature dependence of thermal conductivity, κ(T), reveals a positive dκ/dT in the paramagnetic region, which may be related to the local anharmonic lattice distortions associated with small polarons.     

Giant strain associated with microstructure control by magnetic field in Fe-31.2Pd, CoO and Nd0.5Sr0.5MnO3

We have made in situ optical microscope observation for the microstructure control driven by magnetic field in Fe-31.2Pd (at%), CoO and Nd_0.5Sr_0.5MnO₃. These materials exhibit structural transitions, and their low-temperature phases are composed of several crystallographic domains (variants), which are separated by twinning planes. In the case of ferromagnetic Fe-31.2Pd and antiferromagnetic CoO, the magnetic field promotes the twinning plane movement. This movement gives a large strain of several percent and is essentially explained by the fact that the magnetic shear stress, which corresponds to the magnetic anisotropy energy divided by the twinning shear, is larger than the twinning stress. In the case of Nd_0.5Sr_0.5MnO₃, the twinned microstructure of the charge-ordered phase disappears under a magnetic field in association with the melting of the charge-ordered phase.     

Electron spin resonance analysis of magnetic structures in La2/3Ca1/3MnO3

Measurements of electron spin resonance (ESR) of La_2/3Ca_1/3MnO₃ (LCMO) in the ferromagnetic and paramagnetic phases were carried out. Phase transition and temperature dependence of the peak-to-peak ESR linewidth were determined. The transition temperature between ferromagnetic and paramagnetic phases was observed at 265 K. A prominent increase of the peak-to-peak linewidth with decreasing temperature below T_c was observed. Using the dynamic scale theory and block spin transformation in critical phenomenon, the quantitative calculation of peak-to-peak linewidth at near T_c was made, which was in good agreement with the experimental data. It was believed that the long interactions between the ferromagnetic microregions for LCMO played a key role in determining the ESR linewidth.     

Transport and magnetic properties of La1.48Nd0.4Sr0.12CuO4/La0.67Sr0.33MnO3 bilayer

We have grown La_1.48Nd_0.4Sr_0.12CuO₄/La_0.67Sr_0.33MnO₃ (LNSCO/LSMO) bilayer structure on SrTiO₃ (0 0 1) substrate. Both temperature dependences of resistivity and magnetization curves show anomalies between 60 < T < 80 K, where a low-temperature orthorhombic (LTO) to low-temperature tetragonal (LTT) structural transition is observed in LNSCO bulk crystal. It is suggested that the formation of domains in LSMO layer can relax the strains caused by the LTO–LTT transition in LNSCO layer.     

Microwave properties of Nd0.5Sr0.5MnO3: a key role of the (x2−y2)-orbital effects

Transmittance of the colossal magnetoresistive compound Nd_0.5Sr_0.5MnO₃ showing metal–insulator phase transition has been studied by means of the submm- and mm-wavelength band spectroscopy. An unusually high transparency of the material provided direct evidence for the significant suppression of the coherent Drude-weight in the ferromagnetic metallic state. Melting of the A-type antiferromagnetic states has been found to be responsible for a considerable increase in the microwave transmission, which was observed at the transition from the insulating to the metallic phase induced by magnetic field or temperature. This investigation confirmed a dominant role of the (x²−y²)-orbital degree of freedom in the low-energy optical properties of Nd_0.5Sr_0.5MnO₃ and other doped manganites with planar (x²−y²)-orbital order, as predicted theoretically. The results are discussed in terms of the orbital-liquid concept.     

Transport and magnetic properties of bulk polycrystalline (YBa2Cu3O7)1−x(Nd0.7Sr0.3MnO3)x nanocomposites

We have prepared a series of bulk polycrystalline samples with the nominal compositions (YBa₂Cu₃O₇)_1?x(Nd_0.7Sr_0.3MnO₃)_x (x = 0–1) by a conventional solid-state reaction method using the larger difference in sintering temperature of the two constituent oxides and a well conceived sintering sequence. XRD patterns show that the samples are composites consisting of YBa₂Cu₃O₇ and Nd_0.7Sr_0.3MnO₃ particles with average grain size of ～65 nm. For x ? 0.55, with increasing x, the zero-resistance superconducting transition temperature, T_C0, measured at zero magnetic field decreases and the normal state resistivity increases rapidly. The T_C0 for the sample with x ～ 0.48 is estimated to be 0 K. The M–H hysteresis loops indicate the coexistence of ferromagnetism and superconductivity in the samples. The depression of T_C0 can be attributed to the proximity effect between ferromagnetism and superconductivity.     

The role of orbital ordering in the formation of electron structure in undoped LaMnO3 manganites in the regime of strong electron correlations

The electron structure of undoped LaMnO₃ and slightly doped La_1?x Sr_xMnO₃ manganites has been calculated within the framework of a generalized tight binding method with explicit allowance for strong intraatomic electron correlations. According to the results of these calculations, the ground state in orbitally disordered undoped LaMnO₃ ferromagnets would be metallic despite the Mott-Hubbard correlation gap in the spectrum of quasiparticles. Owing to the orbital ordering, the insulating state is stabilized in both antiferromagnetic and paramagnetic phases. In-gap states of a polaron nature with a spectral weight proportional to the dopant concentration have been found near the top of the valence band in La_1?x Sr_xMnO₃. As the doping level increases, a metal state appears in the ferromagnetic phase, which has a metallic character for one spin subband and an insulating character for the other subband (representing the so-called half-metallic state).     

Phase separation of the spin system in the La<Subscript>0.93</Subscript>Sr<Subscript>0.07</Subscript>MnO<Subscript>3</Subscript> crystal

The magnetic state of the manganite La_0.93Sr_0.07MnO₃ in the range 4.2–290 K was studied using elastic neutron scattering. The magnetic state of this compound was found to occupy a particular place in the La_1?xSr_xMnO₃ solid-solution system, in which the antiferromagnetic type of order (LaMnO₃, T_N=139.5 K) switches to ferromagnetic ordering (La_0.9Sr_0.1MnO₃, T_C=152 K) with increasing x. In the transition state, this compound contains large-scale spin configurations of two types. A fractional crystal volume of about 10% is occupied by regions of the ferromagnetic phase with an average linear size of 200 Å, while the remainder of the crystal is a phase with a nonuniform canted magnetic structure. Arguments are presented for the phase separation of the La_0.93Sr_0.07MnO₃ spin system being accounted for by Mn⁴⁺ ion ordering.     

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.     

Electronic transport in the novel SOFC anode material La1−xSrxCr0.5Mn0.5O3±δ

Electronic conductivity in the potential SOFC anode material La_1−xSr_xCr_0.5Mn_0.5O_3±δ has been investigated in the range 0.2 < x < 0.3. log(σT) vs. 1/T plots indicate conduction via thermally activated polaron hopping. At 900 °C, conductivity in air increases with Sr²⁺ via an increase in [B_B] holes (B—transition metal). X-ray absorption spectroscopy (XAS) studies indicate that compensation for A-site Sr substitution and oxygen vacancy formation is via the Mn cation only; Cr maintains a 3+ oxidation state and 6-fold oxygen coordination. Electronic transport occurs by percolation between Mn cations in a disordered B-site sub-lattice. Conductivity decreases with p(O₂), which is indicative of p-type conduction behaviour, but the relationship cannot be explained by a simple redox equilibrium involving Mn³⁺, Mn⁴⁺ and oxygen, possibly due to co-existence of Mn²⁺, Mn³⁺ and Mn⁴⁺ via disproportionation as with La_1−xSr_xMnO_3±δ.