Structural,electrical, magnetic,elastic and anelastic behaviour of Nd0.6Ca0.4MnO3

With a view to understanding the structural, electrical and magnetic, elastic and anelastic behaviour of charge ordered Nd_0.6Ca_0.4MnO₃, systematic investigations were undertaken. The sample was synthesized by a sol–gel route. The sample was characterized by X-ray diffraction (XRD) using a Rietveld refinement technique and was found to have orthorhombic structure with space group Pnma. A study on the variation of electrical resistivity with temperature has been carried out in the range 100–300 K and it was found to exhibit a field induced transition. The ac susceptibility studies show two transition temperatures, which are attributed to charge ordering and a Néel transition. Internal friction and longitudinal modulus studies were carried out using the composite oscillator technique. An effort has been made to explain the observed anomalous behaviour by a qualitative model.     

La0.4Ca0.6MnO3系统中Mn位Fe和Cr掺杂效应的比较性研究

利用固相反应法制备了Mn位Fe³⁺和Cr³⁺掺杂的系列锰氧化物La_0.4Ca_0.6(Mn_1-x(y)B_x(y))O₃ (B=Fe³⁺ (0≤x≤0.1); Cr³⁺ (0≤y≤0.1)) 多晶样品,研究了掺杂对样品输运性质的影响.实验结果表明两种离子具有截 关键词： 锰氧化物 反铁磁 磁电阻效应     

Nonlinear optical spectroscopy of (La0.6Pr0.4)0.7Ca0.3MnO3 manganite

The electronic structure of doped orthorhombic (La_0.6Pr_0.4)_0.7Ca_0.3MnO₃ manganite is studied using the second optical harmonic generation method. The analysis of the frequency and polarization dependence of the intensity related to the second harmonic demonstrates that the applied magnetic field stimulates a transition of spin-polarized electrons from the d orbitals of Mn ions to the p orbitals of O₂ ligands. The manifestation of such transition in the orthorhombic RMnO₃ manganite with the distorted crystal lattice is attributed to the dipole-forbidden electronic transitions arising in the applied magnetic field. The mechanism of magnetooptical activity induced by the charge transfer between Mn and O ions is discussed.     

Charge ordering modulations in a Bi0.4Ca0.6MnO3 film with a thickness of 110 nm

The low temperature sample stage in a transmission electron microscope is used to investigate the charge ordering behaviours in a Bi0.4Ca0.6MnO3 film with a thickness of 110 nm at 103 K. Six different types of superlattice structures are observed using the selected-area electron diffraction （SAED） technique, while three of them match well with the modulation stripes in high-resolution transmission electron microscopy （HRTEM） images. It is found that the modulation periodicity and direction are completely different in the region close to the Bi0.4Ca0.6MnO3/SrTiO3 interface from those in the region a little further from the Bi0.4Ca0.6MnO3/SrTiO3 interface, and the possible reasons for this are discussed. Based on the experimental results, structural models are proposed for these localized modulated structures.     

Pressure-induced variation of the magnetic structure of the surface of La0.6Sr0.4MnO3 granules

The transport properties of textured films and tunneling junctions of La_0.6Sr_0.4MnO₃, defined by the surface state of the granule, are studied in low magnetic fields (below 100 Oe) and at pressures of up to 10 kbar. Tunneling junctions of two types are investigated, namely, mechanical break junctions and La_0.6Sr_0.4MnO₃-insulator-superconductor junctions. Although only one electrode represents the magnetic material in the latter case, all samples exhibit a low-field magnetoresistive effect. Hydrostatic compression suppresses the magnetoresistive effect to considerably change the transport properties of ceramic and tunnel samples. The reasons for such behavior are discussed in connection with the model of spin-polarized inelastic tunneling of charge carriers through a potential barrier formed both by the intergranular region and by the surface of contacting granules. Reasons are given for the fact that it is most probable that the magnetic state of the barrier and its height vary under the effect of pressure because of the transition of the surface of granules to the metallic state.     

Magnetocaloric effect in Y-doped La0.6Ca0.4MnO3 enhanced by Griffiths phase and re-entrance of first-order phase transition

It has been known that bulk La_0.6Ca_0.4MnO₃ is an intermediate material of the first- and second-order characters with the tricritical-point exponents, and the doping of a metal ion in it usually causes a continuous second-order transition. The present work reports the re-entrance of a discontinuous first-order transition in orthorhombic La_0.6-xY_xCa_0.4MnO₃ (x = 0.03–0.09) compounds. This enhances the magnetocaloric effect. For the field H = 30 kOe, the maximum magnetic-entropy change (|ΔS_max|) and relative cooling power (RCP) have been evaluated being about 5.45–6.3 J/kg·K and 130–185 J/kg, respectively. If combining these compounds as refrigerant blocks in a rotary ring model, a magnetic cooling device can operate at temperatures T = 85–280 K, with |ΔS_max| ≈ 5.5 J/kg⋅K and RCP ≈ 1073 J/kg. Aside from the re-entranced first-order phase transition, the magnetization and structural analyses have proved the enhanced magnetocaloric effect in La_0.6-xY_xCa_0.4MnO₃ related to a Griffiths singularity, and local Jahn-Teller distortions of the perovskite structure (since the Mn³⁺/Mn⁴⁺ ratio and orthorhombic structural phase are unchanged vs. x).     

Dielectric relaxations,magnetodielectric and magnetoelectric interactions in Bi0.6La0.4MnO3 ceramics

Complex permittivity ε*/ε₀ = ε′/ε₀–iε″/ε₀ of the bismuth–lanthanum manganite Bi_0.6La_0.4MnO₃ ceramics has been measured in the temperature range of 10–220 K at frequencies f = 20–10⁶ Hz and magnetic inductions B = 0–0.846 T. At a temperature of 80 K, the spectra ε′/ε₀(t) and ε″/ε₀(t) demonstrate the dielectric relaxation that is a superposition of contributions of several relaxation processes, each of which is dominant in its frequency range: I (f < 10³ Hz, II (10³ < f < 10⁵ Hz), and III (10⁵ < f < 10⁶ Hz). In the range of 10–120 K, anomalous behavior of ε′/ε₀(T) and ε″/ε₀(T) is observed near the temperature of the transition from the paramagnetic to ferromagnetic phase and is due to the Anderson localization of charge carrier on a spin disorder.     

Transport and transient photovoltaic properties of La0.4Ca0.6MnO3/Nb:SrTiO3 heterojunction at high temperature

We have investigated the electronic transport and transient photovoltaic properties on the La_0.4Ca_0.6MnO₃/Nb(0.05 wt%):SrTiO₃ heterojunction at high temperatures up to 723 K. The temperature-dependent reversal of the rectifying polarity and the decrease of the photosensitivity and response time were observed in the junction with increasing temperature from 293 to 723 K. The mechanism of the energy-band structure evolvement with increasing temperature is presented to understand the experimental results.     

Critical behavior and magnetic relaxation dynamics of Nd0.4Sr0.6MnO3 nanoparticles

Detailed DC and AC magnetic properties of chemically synthesized Nd_0.4Sr_0.6MnO₃ with different particle size (down to 27?nm) have been studied in details. We have found ferromagnetic state in the nanoparticles, whereas the bulk Nd_0.4Sr_0.6MnO₃ is known to be an A-type antiferromagnet. A Griffiths-like phase has also been identified in the nanoparticles. Further, critical behaviour of the nanoparticles has been studied around the second-order ferromagnetic-paramagnetic transition region (|(T?T _C)/T _C|???0.04) in terms of modified Arrott plot, Kouvel–Fisher plot and critical isotherm analysis. The estimated critical exponents (β, γ, δ) are quite different from those predicted according to three-dimensional mean-field, Heisenberg and Ising models. This signifies a quite unusual nature of the size-induced ferromagnetic state in Nd_0.4Sr_0.6MnO₃. The nanoparticles are found to be interacting and do not behave like ideal superparamagnet. Interestingly, we find spin glass like slow relaxation of magnetization, aging and memory effect in the nanometric samples. These phenomena have been attributed to very broad distribution of relaxation time as well as to inter-particle interaction. Experimentally, we have found out that the dynamics of the nanoparticle systems can be best described by hierarchical model of spin glasses.     

Spontaneous generation of the electrical voltage in charge-ordered manganite Pr0.6Ca0.4MnO3

Spontaneous generation of the electrical voltage in a Pr_0.6Ca_0.4MnO₃ single crystal has been found. The charge and orbital ordering in the crystal takes place at T _CO = 240 K, while the antiferromagnetic ordering occurs at T _N = 174 K. As the temperature lowers, spontaneous voltage U increases initially slowly (in the temperature range from 300 K to T _CO) and then more rapidly (in the range T _CO-T _N ). Starting from T _N , the voltage U increases exponentially and, at 85 K, reaches 115 mV (in the ab plane) and 6.5 mV (along the c axis). The magnetic field differently affects the voltage U in different temperature ranges: it decreases the value of U in the temperature range of 85–130 K and increases in the range of 130–240 K. It is assumed that the spontaneous voltage is associated with the existence of ferromagnetic and charge orbital ordered clusters of different topologies in the crystal.     

窄带隙Pr0.6Ca0.4MnO3体系的电荷有序和多重磁化转变

研究了窄带隙Pr0.6Ca0.4MnO3体系的电荷有序及低温下的团簇玻璃现象.实验表明,样品具有典型的相分离特征,在高温区于241K左右发生电荷有序(Charge Ordering,CO)转变,在高的外加磁场下破坏了样品的电荷有序和反铁磁,使材料发生绝缘-金属转变.在低温下(T～41 K),形成自旋玻璃态(Spin Glass,SG).发现了存在于低温团簇玻璃相中的多重磁化跳跃现象,利用外场作用下AFM团簇中磁矩的翻转而导致铁磁成分的增加对实验结果给予了初步解释,证明了磁场作用对Pr0.6Ca0.4MnO3的CO与团簇玻璃态冻结之间的直接关联作用.     

窄带隙Pr0．6Ca0．4MnO3体系的电荷有序和多重磁化转变

研究了窄带隙Pr0．6Ca0．4MnO3体系的电荷有序及低温下的团簇玻璃现象．实验表明，样品具有典型的相分离特征，在高温区于241K左右发生电荷有序（Charge Ordering，CO）转变，在高的外加磁场下破坏了样品的电荷有序和反铁磁，使材料发生绝缘—金属转变．在低温下（T～41K），形成自旋玻璃态（Spin Glass，SG）．发现了存在于低温团簇玻璃相中的多重磁化跳跃现象，利用外场作用下AFM团簇中磁矩的翻转而导致铁磁成分的增加对实验结果给予了初步解释，证明了磁场作用对Pr0．6Ca0．4MnO3的CO与团簇玻璃态冻结之间的直接关联作用．     

Charge ordering and polaron formation in the magnetoresistive oxide La0.7Ca0.3MnO3

Neutron scattering has been used to study the nature of the ferromagnetic transition in a single crystal of the perovskite La0. 7Ca0.3MnO3. Diffuse scattering from lattice polarons develops as the Curie temperature is approached from below, along with short range polaron correlations that are consistent with stripe formation. Both the scattering due to the polaron correlations and the anomalous quasielastic component in the magnetic fluctuation spectrum maximize very close to T(C), in a manner remarkably similar to the resistivity, indicating that they have a common origin.     

Structural and magnetic phase transitions in La0.9Sr0.1MnO3

A study is made of phase transitions in doped La_0.9Sr_0.1MnO₃ compounds using combined x-ray, electrical, and magnetic measurements. Structural phase transitions are observed accompanied by a change in the cell volume at temperatures of 100–110 K and 300–340 K. These structural changes are found to be related to different contributions of the rhombic Jahn-Teller Q ₂ mode to the formation of the crystal lattice. The structural transition at 100–110 K is accompanied by distinctive magnetic and electrical properties. The data are analyzed in detail. Fiz. Tverd. Tela (St. Petersburg) 41, 1064–1069 (June 1999)     

ESR study of magnetic structures in La0.7Ca0.3MnO3

Comprehensive measurements of electron spin resonance (ESR) and magnetization of La_0.7Ca_0.3MnO₃ in the ferromagnetic as well as paramagnetic phases were carried out. Quantitative evidences for the inhomogeneous magnetic structure, consisting of ferromagnetic microregions embedded in the antiferromagnetic surrounding near T_c, were found. It is suggested that the microscopic local magnetic structures above and below T_c are qualitatively similar except that the phase below T_c carries long range order between the ferromagnetic microregions whose sizes grow with decreasing temperature.     

Pressure induced magnetic phase separation in La0.75Ca0.25MnO3 manganite

The pressure dependence of the Curie temperature T(C)(P) in La(0.75)Ca(0.25)MnO(3) was determined by neutron diffraction up to 8 GPa, and compared with the metallization temperature T(IM)(P) (Postorino et al 2003 Phys. Rev. Lett. 91 175501). The behavior of the two temperatures appears similar over the whole pressure range, suggesting a key role of magnetic double-exchange also in the pressure regime where the superexchange interaction is dominant. The coexistence of antiferromagnetic and ferromagnetic peaks at high pressure and low temperature indicates a phase separated regime which is well reproduced with a dynamical mean-field calculation for a simplified model. A new P-T phase diagram has been proposed on the basis of the whole set of experimental data.     

Electrical transport and magnetic properties of nanostructured La0.67Ca0.33MnO3

We report fabrication of nanostructured La_0.67Ca_0.33MnO₃ (NS-LCMO) by pulsed-laser deposition on the surface of porous Al₂O₃. The resistance peak temperature (T_p) of the NS-LCMO increases with increasing average thickness of the films, while their Curie temperatures (T_c) remain unchanged. The coercive field of the samples increases with decreasing film thickness and its temperature dependence can be well described by H_c(T)=H_c(0)[1-(T/T_B)^1/2]. A large magnetoresistance and strong memory effect were observed for the NS-LCMO. The results are discussed in terms of the size effect, Coulomb blockade and magnetic tunneling effect. This work also demonstrates a new way to get nanostructured manganites. PACS 75.70.Ak; 75.75.+a; 72.80.Ga