Correlation between magnetic and transport properties of phase-separated La0.5Ca0.5MnO3

The effect of low-magnetic fields on the magnetic and electrical transport properties of polycrystalline samples of the phase-separated compound La_0.5Ca_0.5MnO₃ is studied. The results are interpreted in the framework of the field-induced ferromagnetic fraction enlargement mechanism. A fraction expansion coefficient _f, which relates the ferromagnetic fraction f with the applied field H, was obtained. A phenomenological model to understand the enlargement mechanism is worked out.     

Charge ordering characteristics in Y0.5Ca0.5MnO3 manganite

The charge ordering characteristics in Y_0.5Ca_0.5MnO₃ manganite, prepared by sol–gel process, have been investigated experimentally. It is found that the superlattice diffractions appear in the electron diffraction patterns recorded at low temperatures, while only basic Brag diffraction spots can be observed when temperature is higher than 300 K. This provides direct evidence for the existence of charge ordering in Y_0.5Ca_0.5MnO₃. The magnetization and specific heat measurements indicate the charge ordering temperature of Y_0.5Ca_0.5MnO₃ is 290 K, around which both the magnetization and specific heat reveal anomalous behaviors. We also observed that the MnO bond length changed remarkably and the effective number of carriers reduced prominently with decreasing temperature around charge ordering temperature through transform infrared spectra measurements.     

Anomalous aging and strain induced time dependent phenomena in ultra-thin La0.65Ca0.35MnO3 films

We have shown that ultra-thin La_0.65Ca_0.35MnO₃ films exhibit strong metastable behavior. The resistance can vary with time significantly, suggesting that a state of dynamic phase separation exists whereby one phase grows at the expense of another. Physical properties associated with the metastable behavior have been investigated on the films grown on different substrates. We have found that ultra-thin films age much faster than the thicker counterparts and more interestingly the metastability in the resistance of these films enhanced when aged.     

Influence of Gd-doping in La0.7Ca0.3MnO3 on its structural and magneto-electrical properties

Bulk samples of gadolinium doped manganites with compositional formula La_0.7−xGd_xCa_0.3MnO₃ (x=0.0 and 0.1) were prepared by conventional solid state reaction method. After characterizing the samples by X-ray diffraction, scanning electron microscope and energy dispersive X-ray spectrometer, a systematic investigation of electrical and magneto-transport properties has been investigated. The replacement of La ion by Gd results in a decrease of the metal-insulator transition temperature T_MI and the magnetoresistance as well as the resistivity are found to increase. The electrical resistivity in the entire temperature range fit well with the phenomenological percolation model, which is based upon an approach that the system consists of the phase separated ferromagnetic metallic and paramagnetic insulating regions.     

Structural and electrical characterisation of La0.5Ca0.5MnO3 thin films grown by pulsed laser deposition

Thin films of perovskite manganite, with nominal composition La_0.5Ca_0.5MnO₃, have been prepared by pulsed laser deposition on (1 0 0) SrTiO₃, (1 0 0) LaAlO₃, (1 0 0) Si and YSZ/CeO₂-buffered (1 0 0) Si substrates. Structural and electrical characterisation was performed on the films. The magneto-transport properties of all the thin films depart from the bulk behaviour. The LCMO film grown on buffered Si shows an insulator–metallic transition around 130–150 K while the one deposited directly on Si displayed a similar behaviour under a melting field of 1 T. However, that transition is absent in the films grown on LAO and STO. We suggest that appropriate stress values induced by the substrate favour the formation of metallic percolative paths.     

Dynamical effects in magnetic and transport properties of phase separated La0.5Ca0.5Mn0.95Fe0.05O3

We have measured transport and magnetic properties of polycrystalline La_0.5Ca_0.5Mn_0.95Fe_0.05O₃, a phase separated manganite with ferromagnetic ground state. Cooling rate dependences and time relaxation were found; the coexistence of ferromagnetic and charge ordered regions determines a dynamics which influences physical properties. We show that a dynamical contribution to the resistivity can account for the observed cooling rate dependence and ageing effects on this phase separated manganite.     

Strain effects in epitaxially grown La0.7Sr0.3MnO3 thin films

We report here on an X-ray absorption study of La_0.7Sr_0.3MnO₃ films epitaxially grown on SrTiO₃ substrate. The local organization around Mn in oriented films with 600 Å in thickness was investigated by polarized Extended X-ray Absorption Fine Structure. The angle between electric field vector and film surface was set equal to 5° and 70° to investigate almost independently the contribution of the manganese neighbors situated in and out of the film plane. The first neighboring shell oxygen is found to be the same in both geometries, but small changes in the next neighboring contribution are observed. These changes are associated with variation in the Mn–Mn bond length. A small in-plane elongation (3%) is observed in the constrained films with respect to the unconstrained case.     

Silver addition in thick films of La2/3Ca1/3MnO3 perovskite

Thick films of La_2/3Ca_1/3MnO₃ with 10 wt% of silver are grown on single crystal SrTiO₃ by using the paint-on method. The electrical properties of the films are quite different compared to the pristine counterpart. The samples with silver addition exhibit a very low resistivity and the insulator to metal transition occurs at higher temperature (270 K) than the undoped samples. The resistance versus temperature dependence in these films is very sharp and the magnetoresistance ratio is restricted to a narrow temperature range, close to the electric transition for both cases. The improved properties of the thick films are associated with the improvement of the connectivity between the grains, although microstructural effects are not ruled out.     

Electrical and magnetic behavior of La0.7Ca0.3MnO3/La0.7Sr0.2Ca0.1MnO3 composites

The electrical transport properties and the magnetoresistance of La_0.7Ca_0.3MnO₃/La_0.7Sr_0.2Ca_0.1MnO₃ composites are investigated as a function of sintering temperature. On the basis of an analysis by X-ray powder diffraction and scanning electron microscopy we suggest that raising the sintering temperature enhanced the interfacial reaction and creates interfacial phases at the boundaries of the La_0.7Ca_0.3MnO₃ and La_0.7Sr_0.2Ca_0.1MnO₃. Results also show that in 3 kOe, and at the Curie temperature, the magnetoresistance value of 14% was observed for the composite sintered at 1300 °C. Based on 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 experimental resistivity—temperature data from 50-300 K and find that the activation barrier decreases as temperature is increased.     

Anelastic relaxation of Pr1/2Ca1/2MnO3 at low and medium frequencies in the 350–500 K range

In previous works (Jornadas SAM 2000; J. Magn. Magn. Mater. 226–230 (2001) 988) the low-frequency anelastic effects of two manganite perovskites of La_2/3Sr_1/3MnO₃ and La_2/3Ca_1/3MnO₃ were studied at 1 Hz and temperatures above those of the metal–insulator transition. An important internal friction peak (P1) appears at 414 and 419 K, respectively, with a modulus variation and no appreciable contribution to electrical resistivity. Another compound Pr_1/2Ca_1/2MnO₃ of the same family has been studied by anelastic mechanical spectroscopy in order to add new data which could elucidate the (P1) peak mechanism. The measurements were made in a sub-resonant forced pendulum (0.1–20 Hz) and in a vibrating reed equipment (kHz range). A stable (P1) peak has been detected at 410 K (1 Hz), with a corresponding modulus step. The appearance of this relaxation peak in the Pr_1/2Ca_1/2MnO₃ compound, and its high activation energy value, similar to the other compounds measured, indicates that the microscopic origin of this relaxation does not correspond to an oxygen jump (characteristic activation energy0.8–0.9 eV).     

Fitting of transport measurements in polycrystalline La2/3Ca1/3MnO3

Using a previous qualitative explanation to describe the transport properties of polycrystalline La_2/3Ca_1/3MnO₃ thick films, we achieved a good fit of the temperature dependence of the resistance R(T). Depending on the sample, we have observed different metal–insulator (MI) transitions while the magnetic behavior is always similar. Small regions of depleted T_c adjacent to the grain boundary could have an important resistance contribution without affecting the magnetic properties in an appreciable manner. In this work, we achieve a quantitative explanation for the different transport behaviors that we have observed experimentally.     

CURRENT-DEPENDENT POSITIVE MAGNETORESISTANCE IN Ga-DOPED La0.5Ca0.5MnO3

The magnetic and transport properties of the Ga-doped charge-ordering state La_0.5Ca_0.5MnO₃ have been studied. A current-dependent large positive magnetoresistance (1080%) at 5 K was observed. These observations are interpreted in terms of the spin-dependent tunnelling process between ferromagnetic clusters embedded in an antiferromagnetic matrix.     

Phonon spectra of La0.5Ca2MnO3

This paper presents a microscopic theory to explain different Raman modes of La0.5Ca0.5MnO3 based on the electronic Hamiltonian of the Kondo lattice model,which adds phonon interaction to the hybridization between the conduction electrons of the system and the l-electrons.The spectral density is calculated by the Green function technique of Zubarev at zero wave vector and in the low temperature limit.It finds that there are three Raman-active modes and the spectral densities of these modes are substantially influenced by model parameters such as the position of l-level(εJT),the effective electron-phonon coupling strength(g) and the hybridization parameter(v).Finally,the intensity changes of those peaks are investigated.     

First order paramagnetic-ferromagnetic phase transition in Tb doped La0.5Ca0.5MnO3 manganite

Effect of A-site disorder in half-doped La_0.5Ca_0.5MnO₃ created by the substitution of Tb at La site is studied through temperature dependent neutron diffraction, resistivity, and magnetization on La_0.375Tb_0.125Ca_0.5MnO₃ (LTC) to identify the evolution of phases as a function of temperature and magnetic field. Contrary to the parent system, the paramagnetic (PM) to ferromagnetic (FM) transition is found to be of first order but becomes observable only in fields above 1 T. Absence of this transition in zero and low fields along with the observation of a very intriguing and irreversible M-H curve at low temperature raises the question about the ground state of the system. It is identified from the specially designed measurement protocol that the ground state of the system becomes FM, contrary to the parent LCMO. However, this remains masked because of the hindrance to the transformation kinetics of the PM-FM first order transition in low fields. Significantly, though the thermal hysteresis of the first-order transition decreases with the measurement field, collapse of such a behavior to a critical point is not observed in LTC, unlike earlier observations in some other manganites.     

相分离La0.33Pr0.34Ca0.33MnO3薄膜体系中的交换偏置效应

在相分离La_0.33Pr_0.34Ca_0.33MnO₃薄膜体系中发现了大的交换偏置效应．在4 K时,交换偏置场的大小达到了约1 kOe．交换偏置效应可能源自薄膜内禀的电子相分离特性或薄膜的表面效应．交换偏置效应表现出强的温度、冷却磁场以及厚度依赖的关系．     

Relevancy of phase separation between electrical and thermal properties in La0.8Sr0.2MnO3 thin films

For the purpose of finding the relevancy of phase separation between electrical and thermal properties, La_0.8Sr_0.2MnO₃ thin films, deposited on quartz glass substrates, were studied for temperature dependent resistance and thermal emittance. Based on the phase separation concept, metal phase volume fraction f was calculated from the temperature dependence of resistance and emittance properties by a phenomenological model and two-energy-level Boltzmann distribution, respectively. The two sets of f coincide with each other very well. The results show f plays an important role in the electrical transportation and thermal properties, and the two properties are essentially correlated by f.     

Electrical transport of (1−x)La0.7Ca0.3MnO3+xAl2O3 composites

We report the resistivity (ρ)-temperature (T) patterns in (1-x)La_0,7Ca_0,3MnO₃+xAl₂O₃ composites (0≤x≤0.05) over a temperature regime of 50-300 K. Al₂O₃ addition has increased the resistivity of these composites. The Curie temperature (T_C) is almost independent on the Al₂O₃ content and is about 250 K for all the samples, while the metal-insulator transition temperature (T_MI) decreases with increasing Al₂O₃ content. Based on the phenomenological equation for conductivity under a percolation approach, which is dependent on the phase segregation of ferromagnetic metallic clusters and paramagnetic insulating regions, we fitted the experimental data (ρ−T) from 50 to 300 K and find that the activation barrier increases as Al₂O₃ content increases.     

相分离Nd0.5Ca0.5MnO3体系的再入型自旋玻璃行为和电荷有序

研究了Nd_0.5Ca_0.5MnO₃体系的结构和输运特性. 结构 分析表明，在300K下，体系表现为O′型正交结构并存在典型的Jahn-Teller畸变.在8 T磁场 下，体系出现顺磁绝缘-铁磁金属的转变，庞磁电阻效应发生. 磁测量发现，样品的奈尔温 度T_N和电荷有序转变温度T_CO分别在150和240K左右，在41K左右出 现典型再入型自旋玻璃行为，同时观察到了负的磁化率异常. 结果表明，Nd关键词： 庞磁电阻 自旋玻璃态 负磁化强度 电荷有序     

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.     

Structure, magnetic and transport properties of La1−xBixMnO3

We have investigated the structural, magnetic and transport properties of La_1−xBi_xMnO₃ samples. As the Bi content increases, a structural transition from rhombohedral to pseudocubic and a magnetic phase transition from ferromagnetic ordering to cluster glass are identified. Metal–insulator (MI) transitions and large magnetoresistance (MR) effects are observed at low Bi doping levels, while insulating behavior of resistivity is found in the whole measured temperature range at high-doping levels. Two distinct ferromagnetic insulating (FI) states are found at low temperatures in this system. One can be suppressed and the other can be enhanced by applying magnetic fields. Possible reasons for the observed structural, magnetic phase transitions and changes of resistivity behavior with Bi doping are discussed.