Manganite System Research Articles

Granular and intergranular conduction in La1.32Sr1.68Mn2O7 layered manganite system

We report a comprehensive study of the electrical and magneto-transport properties of La1.32Sr1.68Mn2O7 layered manganite system under different pressures and in the temperature range of 4.2K to 300K. An increase in the resistivity at low temperature has been observed and explained in terms of Coulomb Blockade effect. Magnetoresistance data are explained by assuming that the transport occurs through the mechanism of intergranular as well as granular paths with a pronounced manifestation of Spin Polarization Tunneling phenomenon. By qualitatively comparing the relative fractions of these channels, we explain the resistivity behavior in each temperature range.

Structural and impedance analysis of copper doped LSM cathode for IT-SOFCs

Copper-doped lanthanum strontium manganite (LSM) system, La0.8Sr0.2Mn1−xCuxO3−δ (0⩽x⩽0.3), was synthesized using the EDTA-combined citrate process and their characteristics were investigated for solid oxide fuel cell (SOFC) cathode applications. In all compositions, a single perovskite phase was obtained after calcination at 750°C for 10h. The maximum electrical conductivity at elevated temperatures was obtained at a composition of x=0.2 (190Scm−1 at 750°C), whereas the sample with x=0.3 showed the minimum value (100Scm−1 at 750°C). The increase in electrical conductivity with increasing Cu contents was attributed to a charge compensating transition of the Mn3+–Mn4+ and the reason for the decreased value, at x=0.3, is mainly due to the presence of a second phase. The cathode area specific resistance of the sample with x=0.2 was 4.3Ωcm2 at 750°C, which is lower than that of LSM (x=0). The enhanced performance with the addition of Cu can be explained by the additional Mn4+ promoting the formation of surface oxygen vacancies when Mn4+ is converted to Mn3+ at high temperatures. Copper ion doping at the B-site of LSM affects the formation of oxygen vacancies, which can enhance the oxygen reduction reaction.

Influence of pressure on magnetization and magnetostriction jumps in the manganite (Eu1−xGdx)0.6Sr0.4MnO3(x=0,0.1)

We have demonstrated the influence of external pressure on magnetization jump in Gd substituted Eu0.58Sr0.42MnO3. The collapse of magnetization jump of the Eu based manganite caused by the applied pressure up to 1.2 GPa is explained by a suppression of the metastable blocked state, which is inherent to the phase separated manganite system exhibiting the magnetic avalanche. On the other hand, the magnetization jump of the Gd substituted sample is enhanced under the same pressure. To further examine the nature of the magnetization jump, the influence of pressure on the isothermal magnetostriction of the parent manganite is carried out. These findings indicate the crucial role on the abrupt transition played by the frozen phase separated phase.

Coulomb blockade and magnetoresistance in granular La1.32Sr1.68Mn2O7 under hydrostatic pressure

The transport properties in the La1.32Sr1.68Mn2O7 layered manganite system have been studied in the presence of magnetic field up to 5T. An analysis of the low temperature (T<45K) dependence of the resistivity under hydrostatic pressure up to 25kbars shows the spin-dependent Coulomb Blockade phenomenon. The surface phase and the link condition in grain boundaries are suggested to be responsible for the magnetoresistance data while influencing the charge transfer probability between grains.

ChemInform Abstract: Origin of Hysteresis in La0.67Ca0.33MnO3

AbstractReview: 39 refs.

Scaling of non-Ohmic conduction in strongly correlated systems

A new scaling formalism is used to analyze nonlinear I-V data in the vicinity of metal-insulator transitions (MIT) in five manganite systems. An exponent, called the nonlinearity exponent, and an onset field for nonlinearity, both characteristic of the system under study, are obtained from the analysis. The onset field is found to have an anomalously low value corroborating the theoretically predicted electronically soft phases. The scaling functions above and below the MIT of a polycrystalline sample are found to be the same but with different exponents which are attributed to the distribution of the MIT temperatures. The applicability of the scaling in manganites underlines the universal response of the disordered systems to electric field.

Spin dynamics of (Pr0.5-xCex)Ca0.5MnO3 (x = 0.05, 0.10, and 0.20) system studied by muon spin relaxation

Muon spin relaxation (μSR) measurements were performed on the (Pr0.5-xCex)Ca0.5MnO3 (x = 0.05, 0.10, and 0.20) manganite system to study the influence of Ce substitution on the spin dynamics. A long-range antiferromagnetic state at low Ce substitution levels (x = 0.05) transforms to a spin-glass state by a marginal increase in x to 0.10. The manganite with x = 0.05 shows simple exponential relaxation down to low temperatures, whereas the manganites with x = 0.10 and 0.20 show a coexistence of two distinct relaxation mechanisms below spin glass-like transition temperature (TG). The μSR data for x = 0.10 and 0.20 provide evidence for the existence of a component with a root-exponential relaxation below TG, suggesting a nondiffusive relaxation mechanism similar to that in a magnetic glass. Above TG, the relaxation follows a stretched exponential function with distributed time-scales up to ∼150 K. Although similar types of relaxation dynamics exist in these two manganites (x = 0.10 and 0.20), the temperature dependent behavior slightly differ. In the light of the present results, we construct the phase-diagram of the (Pr0.5-xCex)Ca0.5MnO3 manganite system which encompasses different structural and magnetic correlations evolving as a function of Ce substitution.

Pressure-enhanced ferromagnetism and metallicity in La1.24Sr1.76Mn2O7 bilayered manganite system

We report the low temperature magnetization, electrical resistance, and magnetoresistance of La1.24Sr1.76Mn2O7 bilayer manganite system under hydrostatic pressure. At ambient pressure, the compound shows a sharp ferromagnetic transition (TC) accompanied by a metal–insulator transition (TMI) at 130 K. We observe that the TC and TMI increase with hydrostatic pressure at a rate of dTC/dP = 2.08 K/kbar and dTMI/dP = 2 K/kbar, respectively. Also, we observe an appreciable increase of magnetic moments at low temperatures with increasing pressure. The high temperature regime of temperature dependence of resistivity curves was fitted with the Emin–Holstein’s polaron hopping model and the calculated activation energy values suggest that the applied pressure weakens the formation of Jahn–Teller polarons. The magnetoresistance ratio (MRR) was measured at TC and at 4.2 K upon an external magnetic field of 5 T. The observed MRR at TC is about 210 % and the applied pressure increases the MRR significantly. These results can be interpreted by the pressure-enhanced overlap between the orbitals of Mn–O–Mn, which facilitates the charge transfer and hence enhances the ferromagnetism and metallicity.

First-principles study of phonon properties in magnetic double-layer manganites

We have studied the lattice dynamical properties of the tetragonal double-layer manganite system La${}_{2\ensuremath{-}2x}$Sr${}_{1+2x}$Mn${}_{2}$O${}_{7}$ ($x=0.4$) within the framework of density functional perturbation theory, using a mixed-basis pseudopotential method and the virtual-crystal approximation for modeling the alloy. Performing phonon dispersion calculations for both ferromagnetic (FM) and nonmagnetic (NM) ground states in the plane ${q}_{z}=0$ revealed a strong influence of the magnetic order on high-frequency modes which involve oxygen vibrations with Mn-O bond-stretching modulations. They can be traced back to changes in the dynamical coupling of medium-ranged bonds like Mn-Mn or O-O, while the nearest-neighbor Mn-O bonds remain practically unchanged. Electronically they originate from a replacement of the predominant Mn $d({t}_{2g})$ orbital polarization in the NM phase by a $d({e}_{g})$-type polarization in the FM phase. Good agreement is found between theoretical predictions for phonon frequencies and intensities in the [110] direction and a previous neutron scattering experiment performed in the FM phase. Finally, calculations of the electron-phonon interaction revealed that dips in the dispersion of the high-frequency bond-stretching branches are connected to an enhanced electron-phonon coupling, but on average only a moderate coupling constant of $\ensuremath{\lambda}=0.65$ is obtained.

Improved crystallinity, spatial arrangement and monodispersity of submicron La0.7Ba0.3MnO3 powders: A citrate chelation approach

The perovskite manganite systems have been the materials of tremendous interest due to their strong correlation between structure, transport and magnetism. These materials in their single-crystal form show colossal magneto-resistance (CMR), but the applied fields are very high (∼1–5T). The polycrystalline samples do show high low-field magneto-resistance (LFMR), but good amount of control over particle sizes and grain-boundary distribution is required, which is well known but less realized in practical approaches. In this context, we report on synthesis and manipulation of polycrystalline La0.7Ba0.3MnO3 (LBMO) submicron powders using citric acid chelation. The Citrate-gel route is used to synthesize poly-dispersed LBMO powders which are subjected to citrate chelation for a duration of 0 (LB0) to 4h(LB4) . The samples show improved ordering in X-ray diffraction patterns. Raman spectroscopy scans indicate changed mode signatures due to the probable chelating process, which alters the surface morphology. X-ray photoelectron microscopy shows an evidence of fine citrate layer on the grain boundaries. Low temperature B–H curves exhibit fine hysteresis loops for all samples, while room temperature B–H curves shows paramagnetic response. Scanning electron microscopy images showed the formation of well arranged, connected, mono-dispersed grains of LB4 sample, as against polydispered LB0. The magneto-resistance (at H=100kOe) is seen to enhance for LB4 at its transition temperature (75%, as compared to LB0, where it is 60%), which can be attributed to the well-controlled inter-grain tunneling phenomenon and thin insulating regions in between, created due to citrate chelation, which probably enhances the scattering phenomenon and its susceptibility to applied fields. As citric acid is known to chelate Mn ions, it probably chelates the smaller LB particulate structure and leaves behind citrate-connected submicron grains of LBMO, which are seen to be well engineered.

Influence of Layer-Thickness Ratio on Magnetic Properties in F-La2/3Ca1/3MnO3/AF-La1/3Ca2/3MnO3 Bilayers

Magnetic properties were investigated in ferromagnetic/antiferromagnetic bilayers based on a Ca-doped lanthanum manganite system via hysteretic loops, La1−x Ca x MnO3 grown in situ on (001) oriented SrTiO3 crystalline substrates by using a dc sputtering technique at high-pressure (3.5 mbar) pure oxygen as sputter gas and at a substrate temperature of 850 ∘C. We grew the antiferromagnetic layer La1/3Ca2/3MnO3 (AF-LCMO) with a given thickness, t AF, and the ferromagnetic layer La2/3Ca1/3MnO3 (F-LCMO) with a given thickness, t F. We produced two series of samples: the first one with thickness ratio, t F/t AF, of 1/6 and 6/1, maintaining constant the total thickness of the bilayer to 70 nm, and the second one with t F/t AF of 3/25, 9/25, 15/25, maintaining constant the antiferromagnetic thickness to 25 nm. Magnetization versus applied field loop was strongly dependent on the t F/t AF ratio, which was influenced by both the strains presented in very thin films originated during the growth of the bilayer and the magnetic moment interaction at the ferromagnetic/antiferromagnetic interface due to changes of the electronic structure in this strongly correlated system.

Grain morphology and size disorder effect on the transport and magnetotransport in Sol–Gel grown nanostructured manganites

Nanostructured polycrystalline La0.7Ca0.3MnO3 (LCMO) and Nd0.7Sr0.3MnO3 (NSMO) manganites have been successfully synthesized at various temperatures ranging between 700 and 1100°C using Sol–Gel method. Crystalline nature and morphology of the samples have been studied using XRD (Rietveld analysis) and TEM measurements. SEM studies reveal nanostructured grain morphology of LCMO and NSMO samples. It is observed that, with increase in sintering temperature, crystallinity and grain morphology get improved resulting in better transport. A strong dependence of transport on size variance has been observed in the systems studied. The modified transport and better magnetotransport properties of the manganites have been observed due to the large surface to volume ratio (surface contribution) in the nanostructured manganites. The observation of large low temperature MR in both the manganite systems and the interplay between the intrinsic and extrinsic MR components has been explained in the light of spin polarized tunneling effect, grain morphology, grain boundary nature and size disorder effect.

Scaling description of non-Ohmic transport in manganites

The current–voltage characteristics of several manganite systems like La0.87MnO3–(Mn2O3)0.13, La0.60Y0.07Ca0.33MnO3 and Nd0.55Sm0.3375Sr0.1125MnO3 show non-Ohmic behaviour at different temperatures. The onset voltage V0(T) at which conductance starts deviating from its zero-bias value Σ0(T) scales as V0(T)∼Σ0(T)xT with xT being the onset exponent. A detailed scaling analysis with these manganite systems reveal that the onset exponents xT are different in the paramagnetic insulating and ferromagnetic metallic phases. Besides, the magnitudes of xT are different in different manganite systems. Non-Ohmic conduction and the onset exponent are described with the help of inelastic multi-step tunneling model and scaling concepts.

Korringa-like relaxation in the high-temperature phase ofA-site ordered YBaMn2O6

We report on high-temperature electron spin resonance studies of A-site ordered YBaMn2O6 and disordered Y0.5Ba0.5MnO3. In the disordered sample we find that the linewidth is governed by spin-spin relaxation processes as in many other manganite systems. In contrast we find a Korringa-like spin relaxation with a slope of about 1 Oe/K above the charge-ordering transition and extending up to 930K in the metal ordered YBaMn2O6 samples. A Korringa-law is a clear feature of a truly metallic state, pointing to an unconventional high-temperature phase of these ordered manganites. In agreement with the ESR intensity this suggests that in the metallic high-temperature phase the ESR signal stems from Mn^4+ core spins which relax via the quasi-delocalized e_g electrons.

Synthesis, structural, electrical and magnetic studies of La0.5Ca0.45−xSrxBa0.05MnO3

Half doped mixed valence manganite system La0.5Ca0.45−xSrxBa0.05MnO3 (with x=0.1, 0.2, and 0.3) synthesized through a low temperature nitrate route is systematically investigated in this paper. The electronic transport and magnetic properties are analyzed and compared apart from the study of unit cell structure and composition. The system is found to crystallize only in orthorhombic structure (Pnma) and the electronic phase transitions are observed to be of second order. The charge and orbital ordering have been observed to coexist with ferromagnetism in x=0.1 compound. Application of small polaron and variable range hopping models to resistivity data of the system corresponding to high temperature range shows increasing mobility of eg electrons with x, with the later model describing the electronic transport very closely than the former. The temperature dependent magnetization of the compounds shows monotonic increase of paramagnetic to ferromagnetic transition (TC) with x. Ferromagnetism is exhibited for the complete temperature range down from respective TC in contrast to antiferromagnetism usually exhibited by half-doped compounds in the low temperature range. The plots of magnetization versus magnetic field reveal a transition from soft to hard magnetic character for all the compounds as the temperature is lowered.

Elastic behavior of neodymium based manganites

With a view to investigate the elastic behavior of Nd0.67A0.33MnO3 (where A=Ca, Sr, Ba, Pb) manganite system, the samples were prepared by the sol gel method. After characterizing the samples structurally, a systematic investigation of ultrasonic longitudinal and transverse sound velocities of all the samples was undertaken by pulse transmission technique in the temperature range, 100–300K. It has been found that all the elasticity parameters, including Debye temperature, are found to increase continuously with increasing ionic radii of the dopant ion. All the samples are also found to exhibit anomalies in both the longitudinal and transverse velocities near their ferro to para magnetic transition (TC) temperatures. Apart from this, Nd0.67Ca0.33MnO3 sample is also found to exhibit, a transition at its charge ordering temperature (Tco). An explanation for the observed elastic anomalies based on a mean field theory has been given.

Tuning A-site ionic size in R0.5Ca0.5MnO3 (R = Pr, Nd and Sm): robust modulation in dc and ac transport behavior

Structural investigations on Pr0.5Ca0.5MnO3, Nd0.5Ca0.5MnO3 and Sm0.5Ca0.5MnO3 display a systematic tuning in orthorhombic distortion which appears due to a change in A-site ionic size. Comprehensive investigations on dc conduction using various models indicate strong correlation with structural changes. Careful analysis of the ac conductivity mechanism as a function of frequency and temperature brings out a pronounced effect of structural modifications on the polaron conduction mechanism. Contrary to typical manifestations, dc conductivity and relaxation of charge carriers reveal non-Arrhenius behavior. This work provides a detailed and systematic addition to the phase diagram of electronic transport in the rare-earth manganite system doped with a divalent alkali atom.

The effect of band Jahn–Teller distortion on the magnetoresistivity of manganites: a model study

We present a model study of magnetoresistance through the interplay of magnetisation, structural distortion and external magnetic field for the manganite systems. The manganite system is described by the Hamiltonian which consists of the s–d type double exchange interaction, Heisenberg spin–spin interaction among the core electrons, and the static and dynamic band Jahn–Teller (JT) interaction in the eg band. The relaxation time of the eg electron is found from the imaginary part of the Green’s function using the total Hamiltonian consisting of the interactions due to the electron and phonon. The calculated resistivity exhibits a peak in the pure JT distorted insulating phase separating the low temperature metallic ferromagnetic phase and the high temperature paramagnetic phase. The resistivity is suppressed with the increase of the external magnetic field. The eg electron band splitting and its effect on magnetoresistivity is reported here.

Compressibility of CaMnO3: A study using a large-volume diffraction press

CaMnO3 is a parent compound for various manganite systems exhibiting useful physical properties. Therefore, its structural and elastic properties are of general interest. In this paper, P–V equation of state of stoichiometric CaMnO3 is determined using energy dispersive X-ray diffraction. The measurements were carried out at a synchrotron beamline F2.1 (Hasylab, DESY) with samples compressed in a cubic-anvil diffraction press, MAX80, for pressures ranging up to 4.84 GPa. The experimental bulk modulus of CaMnO3, derived from the variation in the unit-cell volume with pressure by fitting the Birch–Murnaghan equation of state, is 154.4(3.3) GPa. The results are discussed on the basis of experimental and theoretical data for CaMnO3 and related compounds.

Synthesis, oxygen permeation, and electrical properties of (La1−xSrx)(Mn0.85Fe0.05Co0.05Ni0.05)O3+δ -YSZ composite

New dense composites comprised of a lanthanium manganite system, (La1xSrx)(Mn0.85Fe0.05Co0.05Ni0.05)O3+δ (LSMFCN, Sr2+ (x) = 0∼0.4 mole) and yttria-stabilized zirconia (YSZ) were fabricated for use as oxygen separation membranes. The densification behavior of the LSMFCN/YSZ composites were studied by sintering with different ratios of LSMFCN and YSZ at various temperatures in air. A density of about 95% was achieved for a sintering temperature of 1200°C. The electrical conductivity of the LSMFCN0/YSZ composites as a function of reciprocal absolute temperature with various YSZ contents was measured. The activation energies for conduction below and above 500°C were about 89 kJ/mol and 68 kJ/mol, respectively. The oxygen permeation flux increased until the YSZ concentration was 60 wt. %, then decreased at 80 wt. % of YSZ. The oxygen permeation flux of the 0.5 mm composite membrane was 1.06 sccm/cm2 at a 4:6 mixed ratio of LSMFCN0 /YSZ at 1200°C.