Narrow Results

We use the double exchange (DE) model via degenerate orbitals and tight-binding approximation to study the magnetoconductivity of a canted A-phase of pseudo-cubic manganites. It is argued that the model is applicable in a broad concentration range for manganites A_{1 - x}B_xMnO₃ with the tolerance factor t close to one. As for the substitutional disorder, scattering on random Jahn–Teller distortions of MnO₆ octahedra is chosen. We emphasize an intimate correlation between the carrier concentration and resistivity value of metallic manganites. Magnetoresistance as a function of magnetization is calculated for a canted A-phase for both in-plane and out-of-plane current directions. A contact between two manganite phases is considered and structure of the transition region near the contact is discussed. Numerical calculations show charge re-distribution near the contact and a large screening length of the order of five inter-atomic distances. We employed our results to interpret data obtained in recent experiments on La_0.4Sr_0.6MnO₃/La_0.55Sr_0.45MnO₃ superlattices. We also briefly discuss the relative importance of the cooperative Jahn–Teller distortions, double exchange mechanism and super-exchange interactions for the formation of the A-phase at increasing Sr concentrations x > 0.45 in La_{1 - x}Sr_xMnO₃ which suggest that the Jahn–Teller contraction of octahedra, c/a < 1, plays a prevailing role.

Valence state of the doped Ru at the Mn sites in Pr_0.5Sr_0.5MnO₃ has been studied using Ru K-edge X-ray absorption near-edge structure (XANES) spectroscopy. In comparison with XANES of reference ruthenates, it is found that the Mn-site doped Ru is dominantly in tetravalent state. This result suggests that the change of charge carrier density by Ru doping is not sufficient to understand drastic enhancement of ferromagnetism observed in the transport properties of Ru-doped Pr_0.5Sr_0.5MnO₃. The prime role of Ru in Mn³⁺–O–Mn⁴⁺ networks is discussed in terms of the valency effect and the magnetic interactions between doped Ru and Mn ions.

We extend our previous EXAFS investigations on the relationship between magnetization and local distortions to the bilayer materials La_2-2xSr_1+2xMn₂O₇ and to higher magnetic fields, for quasicubic La_0.7Ca_0.3MnO₃. For each, there is a significant change in the broadening parameter σ of the Mn-O pair distribution function (PDF) associated with polaron formation as T is increased through T_c. Applying a magnetic field reduces the local distortion of the Ca sample for temperatures near T_c. For samples with sharp transitions, there is still a significant change of σ² with T below T_c when the sample is fully magnetized, i.e. distortions are still present and continuing to be removed as T is lowered well below T_c. A simple model is presented for the magnetization process.

Magnetic properties of Bi³⁺-doped La_0.67-xBi_xCa_0.33MnO₃(x = 0, 0.05, 0.1, 0.2) are investigated. Using the mean-field and ferromagnetic spin-wave theory at low temperatures, total exchange integrals and Curie temperatures are calculated in the range 0≤x≤0.2. Calculated values of T_c are in good agreement with those obtained by magnetic measurements. In addition, the total exchange integral decrease with Bi³⁺ shows the existence of canted ferromagnetic order in the samples, as observed in experimental results.

La_0.67Ca_0.33MnO₃ samples prepared by polymeric precursor route method are investigated using the thermogravimetric, differential thermal analysis, X-ray powder diffraction, scanning electron microscope, infrared spectroscopy and magnetic measurements. The unit cell parameters and grain size for all the samples are refined using X-ray diffraction and scanning electron microscope. It is observed that the IR bands at about 400 cm^-1 is are most likely due to Mn–O–Mn bending mode and the band at 600 cm^-1 is the result of the in plane Mn–O stretching mode. Magnetization measurements indicated that the Curie temperature increases from 241.3 K for the sample sintered at 600°C to 268.86 K for the sample sintered at 1000°C as a function of sintering temperature. The magnetic entropy change reaches the maximum value of about 4.8J/kg·K for the sample sintered at 1150°C at an applied field of 1 T.

We report here the electrical resistivity, ac susceptibility, low field magneto-resistance (LFMR) and structural parameters of the Pr_1-xBa_xMnO₃(x= 0.33–0.80) system. Samples with 33% and 40% Ba-content exhibit two clear transitions in their resistivity-temperature behavior while signatures of higher temperature transition are also seen in the 50% Ba sample. The higher temperature transition (insulator-metal, T_p1), like in other CMR materials, results due to competition between the double exchange and super exchange mechanisms. The lower temperature broad hump (T_p2) in resistivity reflects the strain effect at the grain boundaries due to the ionic radii mismatch between Pr⁺³ and Ba⁺². The low field magneto-resistance (at 0.15T) also corroborates the above results as MR peaks near T_p1 and increases further at lower temperatures, reflecting the contribution from the grain boundaries. Susceptibility measurements show the Curie transition (T_c) to be near T_p1 of the ρ-T data. Results are found to be consistent with the (NdBa) MnO₃ and (LaBa) MnO₃ systems. Resistivity data on the Pr_0.67Sr_0.33MnO₃ system, where the lower temperature transition at T_p2 is not seen due to the smaller ionic size difference of ( Pr⁺³–Sr⁺²) compared to (Pr⁺³–Ba⁺²), also substantiate these results. The observed resistivity upturn at low temperatures is considered in terms of the ensuing localization of carriers due to the varying degree of ionic size mismatch in these systems.

Temperature dependence of phonons spectra and allied properties of rhombohedral La_0.7Sr_0.3MnO₃ are investigated by using the lattice dynamical method. A tendency of phonon mode to instability causing JT lattice distortion is reflected in a softening of the stretching mode in the phonon dispersion curve of La_0.7Sr_0.3MnO₃ at both 1.6 and 300 K. While the A_1g mode softens because of gradual decrease in MnO₆ rotations, the stretching mode hardens upon reduction in temperature. The distinct features of phonon modes at different temperatures are also reflected in the calculated phonon density of states. Other thermal properties such as specific heat, the Debye temperature, and Grüneisen parameter are also presented. The decrease in the Debye temperature at 300 K indicates the effect of temperature in lattice softening. Anomalously high value of the Grüneisen parameter points out the presence of anharmonic lattice modes.

We present a detailed magnetic study of the polycrystalline transition metal oxides Ln_0.7A_0.3TMO₃, Ln = La, Pr; A = Ca, Sr and TM = Mn, Co at low temperatures including electrical resistance, magnetization and ac susceptibility measurements. The effect of the A-site and B-site substitutions on the properties of these compounds is discussed. The results of this analysis indicate that the electrical and magnetic behavior of these compounds have some reminiscences of the manganites having similar formula, but the spin disorder and the grain boundary effects control the properties of these materials.

The effects of sintering temperature on the electrical and magneto-transport properties of La_0.8Sr_0.2 MnO₃ compounds prepared by employing a chemical co-precipitation route sintered at 1120°C, 1220°C and 1320°C are reported. The X-ray powder diffraction patterns for all samples can be indexed to the rhombohedra structure with R3C space group. The insulator–metal transition temperature remained nearly constant (~291 K) for all samples. The magneto resistance (MR) values were found to decrease noticeably as the sintering temperature was increased from 1120°C to 1320°C. Above 0.3 T, the magneto resistance was influenced by T_o values, which indicates the bending of Mn–O–Mn bond. The resistivity data were fitted with several equations in the metallic (ferromagnetic) region. In the insulating (paramagnetic) region, the variable range hopping and small polaron hopping models were used to compute the density of states at the Fermi level, N(E_F), and activation energy (E_a) of the electrons. The resistivities can be fitted well with ρ–T² and ρ–T^2.5 curves. The variable range hopping model and small polaron hopping model fitted very well at high temperature regions, indicating that small polaron hopping might be responsible for the conduction. La_0.8 Sr_0.2MnO₃ sintered at 1220°C demonstrated the highest MR of 6.5% in 1 T magnetic field.

We report the modifications in the transport and magnetotransport in La_0.7Ca_0.3Mn_1-xAl_xO₃ (LCMAO); x = 0.0, 0.02, 0.04, 0.06, 0.08 and 0.1 polycrystalline manganites synthesized by conventional ceramic method. XRD studies reveal the single phasic orthorhombic structure for all the Al-doped LCMO samples without any detectable impurities and structural phase transition. SEM micrographs show island-like grain growth in all the samples studied. Transport measurements indicate an increase in resistivity and decrease in metal-insulator (M-I) transition temperature (T_P) with the increase in Al-concentration. Field and temperature dependent variation in MR for all the Al (x) doped LCMO samples has been discussed in detail in the light of substitution induced structural disorder.

The local atomic and electronic structure of La_0.5Sr_0.5MnO₃ was investigated at different annealing temperatures (T_A) by X-ray absorption spectroscopy (XAS) and photoemission spectroscopy (XPS). The extended X-ray absorption fine structure indicates that the MnO₆ octahedral distortion is reduced by increasing T_A. The chemical shift for the sample with T_A = 1350°C measured by XPS of Mn 2p core level demonstrates the increasing of Mn³⁺ ions content. From the deconvolution of valence band photoemission spectra, the number of e_g electron is also proved to increase with increasing T_A. It is also demonstrated that there is a strongest hybridization between O 2p and surrounding atomic orbital states in sample with T_A = 1350°C, which is consistent with valence band photoemission.

The mixed-valence perovskite manganites attracted much attention because of their interesting electro-magnetic properties, and strain modulation on magnetic properties of perovskite manganites is worth exploring. In this paper, [La${}_{0.8}$Ca${}_{0.2}$MnO₃/La${}_{0.5}$Ca${}_{0.5}$MnO₃]${}_{20}$ superlattice films with different La${}_{0.8}$Ca${}_{0.2}$MnO₃ layer thicknesses are prepared by pulsed laser deposition (PLD). The crystal structures (lattice constants) of samples are measured by X-ray diffraction (XRD). The strain of the films is determined according to their crystal structures. The magnetic hysteresis loops (M-H loops) and magnetization versus temperature (M-T) curves of these superlattice films are measured by the physical property measurement system (PPMS). From the M-H loops, the coercive field (H${}_C)$ of the samples can be measured. From the M-T curves, the Curie temperature (T${}_C)$ of the samples can be obtained. All samples show a ferromagnetic to paramagnetic transition at T_C, and the [La${}_{0.8}$Ca${}_{0.2}$MnO₃/La${}_{0.5}$Ca${}_{0.5}$MnO₃]${}_{20}$ sample with the La${}_{0.8}$Ca${}_{0.2}$MnO₃ layer thicknesses of 72Å has an antiferromagnetic Néel transition at T_N. According to the strain state and magnetic phases, the magnetic properties are comprehensively analyzed. It is found that with the increase of ferromagnetic La${}_{0.8}$Ca${}_{0.2}$MnO₃ layer thickness, the ferromagnetic phase is increased, and the double exchange effect can also be enhanced, resulting in the increase of T_C. When the thickness of La${}_{0.8}$Ca${}_{0.2}$MnO₃ reaches 96Å, the uneven strain distribution in the superlattice can induce the reduction of ferromagnetic phase compared with antiferromagnetic phase (even antiferromagnetic phase starts to appear), and the double exchange effect can be weakened, and finally leading to the decrease of T_C. In addition, with increasing the La${}_{0.8}$Ca${}_{0.2}$MnO₃ layer thickness, H_C gradually decreases. The change of H_C is related to strain states and magnetic phase interactions in the samples. The analysis of the strain and magnetism can contribute to the understanding of perovskite manganite superlattice films.

X-ray diffraction, electrical resistivity and ac susceptibility measurements have been carried out on La_1-xAg_xMnO₃ compounds for x=0.05 to 0.30. These samples are found to be in single phase form with space group and with typical lattice parameters a=b=5.524Å and c=13.349Å for x=0.05 sample. The Mn–O–Mn bond angles and variance, σ² are found to increase with doping. Metal-insulator transitions in the temperature range 254 to 259 K have been observed. These materials exhibit paramagnetic to ferromagnetic transitions in the vicinity of metal-insulator transition temperatures. The paramagnetic susceptibility could be analyzed using Curie–Weiss law. All the above samples exhibit colossal magneto-resistivity and its maximum value is found to be 73% for x=0.15 sample at 50 kOe magnetic field.

We revise some recent results on the nature of the magnetic phase transition of ferromagnetic manganese perovskites with colossal magnetoresistance. It is found that they exhibit first-order magnetic phase transitions mainly due to strong lattice effects. The presence of such first-order transition is strongly linked to the existence of a temperature region above the Curie Temperature with a phase-separated regime of coexisting metallic and insulating clusters. This situation is compared with that of MnAs, paradigm of system with first-order magnetic phase transition, and we observe a parallel phenomenology of these apparently different systems. This serves as a clue for the finding of a phase-separated regime also in MnAs and, moreover, for the finding of a colossal magnetoresistive effect similar to that of manganese perovskites.

LaMn_1-xCu_xO₃ compounds have been prepared in single phase form for x = 0 to 0.30. X-ray diffraction patterns recorded at room temperature could be mostly refined using Pbnm space group. Paramagnetic-to-ferromagnetic transitions have been observed up to x = 0.30, from ac susceptibility measurements. Metal–insulator transition in the vicinity of ferromagnetic transition temperature has been observed for x = 0.05 and the resistivity data in the metallic region could be explained in terms of electron–electron and electron–magnon scattering mechanisms. Further increase in Cu-doping beyond x = 0.05 leads to systematic decrease in ferromagnetic transition temperature and ultimately ferromagnetism is destroyed for x = 0.40. The resistivity data of all samples except x = 0.05 exhibit semiconducting behaviors and they could be mostly explained using the adiabatic small polaron hopping model.

We investigate theoretically manganese oxides where the colossal magnetoresistance (CMR) is observed. Recent studies show that, if there are both an electron and a hole in the e_g-band of Mn, the Jahn–Teller distortions split the band into a conduction band and a localized band . We find a Kondo lattice model with hole conduction in manganese oxides when the electron in the localized e_g-band plays the role of the Kondo impurity. The Curie temperature (T_c) and the resistivity are calculated. We also calculate the antiferromagnetic temperature by a spin-Peierls transition.

We have investigated a different manganese oxide material in which the colossal magnetoresistance (CMR) is observed. As many recent evidences suggest that a superexchange interaction is dominated in the pyrochlore Tl₂Mn₂O₇, it is found that the superexchange interaction is a chief mechanism driving the ferromagnetism. We specially formulate the superexchange interaction to calculate the resistivity in analogy with the t-scattering matrix of the Kondo formalism. We compare our calculation for resistivity with results of experiments and obtain the Curie temperature, T_c.

In the resistivity–temperature curves of La_0.7Ca_0.3MnO₃ and La_0.75Ca_0.25MnO₃, five distinct regions have been identified. For example, in La_0.7Ca_0.3MnO₃, the regions are:

(1) from the lowest temperature of measurement to 200 K where resistivity is directly proportional to temperature with a small slope,

(2) the reciprocal of resistivity follows Bloch's T^3/2 law,

(3) resistivity is directly proportional to temperature with a large slope,

(4) the cusp region, where there are FM clusters of reduced dimensions and varying degrees of correlated polarons or bipolarons and small polarons,

(5) the semiconducting region having small polarons.

The regions 1, 2 and 3 together appear to replicate the spontaneous magnetization curves of the materials. The effect of magnetic field on the resistivity–temperature curves are as follows:

(a) resistivity is reduced. In particular, the resistivity peak with magnetic field occurs at a temperature μH/K downward from the T_c of the curve without magnetic field;

(b) resistivity peak is displaced by μ · μH/K towards the higher temperature side compared to the resistivity peak without magnetic field;

(c) cusp region is broadened;

(d) magnetoresistance is appreciable only within 10% of T_p on both sides.

All these observations suggest that temperature profile of resistivity of manganites is controlled by magnetic effects.

We report on our recent electron-tunneling studies of bulk manganite samples that provide important information about the structure of the near-surface layers of the material and the nature of the charge transport across them. It is shown that the even part of the differential conductance of contacts formed by a metallic injector with the surface of a manganite is a power function of the voltage bias. High voltages applied to the sample are found to locally modify the conductance of the degraded native surface layer. Experiments aimed to monitor the force applied to a metal tip pressed into the surface of a manganite prove the presence of sub-surface layers with properties significantly different from those near the surface. Experimental data are analyzed and interpreted within the Glazman–Matveev theory taking into account inelastic tunneling through two metallic "drops" inside the insulating barrier.

So far tens of multiferroic materials, with various chemical compositions and crystal structures, have been discovered in the past years. Among these multiferroics, some perovskite manganites with ferroelectricity driven by magnetic orders are of particular interest. In these multiferroic perovskite manganites, the multiferroic phenomena are not only quite prominent, but the involved physical mechanisms are also very plenty and representative. In this brief review, we will introduce some recent theoretical and experimental progress on multiferroic manganites, including the fascinating microscopic physics and very recently addressed experimental findings with attractive multiferroicity.