Colossal Magnetoresistive Perovskites Research Articles

Modeling of neutron diffractometry facility of Tehran Research Reactor using Vitess 3.3a and MCNPX codes

Abstract The neutron powder diffractometer (NPD) is used to study a variety of technologically important and scientifically driven materials such as superconductors, multiferroics, catalysts, alloys, ceramics, cements, colossal magnetoresistance perovskites, magnets, thermoelectrics, zeolites, pharmaceuticals, etc. Monte Carlo–based codes are powerful tools to evaluate the neutronic behavior of the NPD. In the present study, MCNPX 2.6.0 and Vitess 3.3a codes were applied to simulate NPD facilities, which could be equipped with different optic devices such as pyrolytic graphite or neutron chopper. So, the Monte Carlo–based codes were used to simulate the NPD facility of the 5 MW Tehran Research Reactor. The simulation results were compared to the experimental data. The theoretical results showed good conformity to experimental data, which indicates acceptable performance of the Vitess 3.3a code in the neutron optic section of calculations. Another extracted result of this work shows that application of neutron chopper instead of monochromator could be efficient to keep neutron flux intensity higher than 106 n/s/cm2 at sample position.

Fermi Surface of Three-Dimensional La(1-x)Sr(x)MnO3 Explored by Soft-X-Ray ARPES: Rhombohedral Lattice Distortion and its Effect on Magnetoresistance.

Electronic structure of the three-dimensional colossal magnetoresistive perovskite La(1-x)Sr(x)MnO3 has been established using soft-x-ray angle-resolved photoemission spectroscopy with its intrinsically sharp definition of three-dimensional electron momentum. The experimental results show much weaker polaronic coupling compared to the bilayer manganites and are consistent with the theoretical band structure including the empirical Hubbard parameter U. The experimental Fermi surface unveils the canonical topology of alternating three-dimensional electron spheres and hole cubes, with their shadow contours manifesting the rhombohedral lattice distortion. This picture has been confirmed by one-step photoemission calculations including displacement of the apical oxygen atoms. The rhombohedral distortion is neutral to the Jahn-Teller effect and thus polaronic coupling, but affects the double-exchange electron hopping and thus the colossal magnetoresistance effect.

Exchange bias in phase-segregated Nd2/3Ca1/3MnO3 as a function of temperature and cooling magnetic fields

Exchange bias (EB) phenomena have been observed in Nd2/3Ca1/3MnO3 colossal magnetoresistance perovskite below the Curie temperature TC ∼ 70 K and attributed to an antiferromagnetic–ferromagnetic (FM) spontaneous phase segregated state of this compound. Field cooled magnetic hysteresis loops exhibit shifts toward negative direction of the magnetic field axis. The values of exchange field HEB and coercivity HC are found to be strongly dependent of temperature and strength of the cooling magnetic field Hcool. These effects are attributed to evolution of the FM phase content and a size of FM clusters. A contribution to the total magnetization of the system due to the FM phase has been evaluated. The exchange bias effect decreases with increasing temperature up to TC and vanishes above this temperature with disappearance of FM phase. Relaxation of a non-equilibrium magnetic state of the compound manifests itself through a training effect also observed while studying EB in Nd2/3Ca1/3MnO3.

Nanophase Separation and Magnetic Spin Glass in Nd<sub>2/3</sub>Ca<sub>1/3</sub>MnO<sub>3</sub>

A study of the low temperature magnetic state of polycrystalline colossal magnetoresistance perovskite Nd2/3Ca1/3MnO3 has been carried out. The data obtained, such as strongly divergent ZFC and FC static magnetizations and frequency dependent ac susceptibility, are evident of the glassy magnetic state of the system. Well defined maxima Tmax in the in-phase linear ac susceptibility χ curves were observed, indicating a spin-glass transition. Clear frequency dependence of the cusp temperature Tmax was found. The frequency dependence of Tmax was successfully analyzed by the dynamical scaling theory of a three-dimensional spin glass. Slow relaxation process and variety of relaxation times found imply a cluster glass magnetic state of the compound at low temperatures rather than a canonical spin glass state. The cluster glass state, accompanied by the multiple magnetic transitions of Nd2/3Ca1/3MnO3, might exist due to the competing interaction between the FM clusters and the AFM matrix induced by the complex nanophase segregated state of the compound.

Effect of cation substitution on magnetic properties in Mn1−xA’xCr2S4 (A’ = Cd, Zn) thiospinel series

Colossal magnetoresistance (CMR) materials such as REMnO3 perovskites and AB2S4 thiospinels have been well studied in the past. The authors present thiospinel compounds with A = Mn, since they may be viewed as a link to REMnO3 CMR perovskites. Ferrimagnetic MnCr2S4 is known to present a Tc of 65 K and a magnetic transition towards a non-collinear Yafet-Kittel state. The (Mn1−xA’x)Cr2S4 series (A = Cd, Zn) was synthesised at 850°C. This series crystallises in the space group Fd3m with a close to 10 Å. When A’ = Cd (Tc ranging from 68 K, x = 0·2, to 79 K, x = 0·6), the ferromagnetic features are enhanced, indicating that antiferromagnetism between Mn spins is progressively lost. For A’ = Zn, two magnetic transitions take place, one at 15 K, which is reminiscent of the antiferromagnetic state of ZnCr2S4 and another one at Tc = 115 K. Impurities which could be responsible of these two magnetic states are excluded.

Magnetic and magnetotransport properties of the (La 0.67Pb 0.33)(Mn 1− xFe x)O 3 (0 ≤ x ≤ 0.1) compounds

A study of magnetic and electrical transport properties of Fe-doped colossal magnetoresistive manganese perovskites (La 0.67Pb 0.33)(Mn 1− x Fe x )O 3 ( x = 0, 0.01, 0.03, 0.06 and 0.1) is presented. Polycrystalline compounds were prepared by the sol–gel low-temperature method. The dc magnetization measured in the fields 50 Oe and 1 kOe and hysteresis loops up to 89 kOe were measured from 4 K up to 400 K. Four-probe magnetoresistance measurements have been performed in the same temperature range at the applied magnetic fields up to 80 kOe. It was found that the substitution of Fe for Mn results in a reduction of the saturation magnetic moment, an increase of the resistivity, as well as a decrease of the Curie ( T C) and metal–insulator ( T M–I) transition temperatures. The x = 0.1 compound exhibits the largest magnetoresistance effect of about 300% around its T M–I temperature in the applied field of 80 kOe. The resistivity above T C for the compounds follows the thermally activated behaviour with the activation energy of about 0.12 eV. The results confirm that Fe substitutes for Mn 3+, forming Mn 4+/Fe 3+ pairs which do not support the double exchange interaction. This strongly affects the magnetic and magnetotransport properties as compared with those of the Fe undoped compound.

Re-examination of half-metallic ferromagnetism for dopedLaMnO3 in a quasiparticleself-consistent GW method

We apply the quasiparticle self-consistentGW (QSGW) method to a cubic virtual-crystal alloyLa1−xBaxMnO3 as a theoretical representative for colossal magnetoresistive perovskite manganites. TheQSGW predicts it as a fully polarized half-metallic ferromagnet for a wide range ofx and lattice constant. Calculated density of states and dielectric functions are consistentwith experiments. In contrast, the energies of the calculated spin wave are very low incomparison with experiments. This is affected neither by rhombohedral deformation northe intrinsic deficiency in the QSGW method. Thus we end up with a conjecture thatphonons related to the Jahn–Teller distortion should hybridize with spin waves morestrongly than people thought until now.

Influence of Composition and Particle Size on Spin Dynamics and Thermodynamic Properties of Magnetoresistive Perovskites

The thermodynamic behavior and spin dynamics of the colossal magnetoresistive (CMR) perovskites of general formula La(1-x)(A)xMn(1-y)(B)yO3 (where A is an alkaline earth, and B = Al, In) have been studied in order to evidence the effect of composition and the influence of nanocrystallinity on the thermodynamic and magnetic characteristics. By using electron paramagnetic resonance (EPR) spectroscopy, the behavior of the exchange coupling integral (J) between Mn spins and the polaron activation energy (Ea) have been investigated. The thermodynamic properties represented by the relative partial molar free energies, enthalpies and entropies of oxygen dissolution in the perovskite phase, as well as the equilibrium partial pressures of oxygen have been obtained by using solid electrolyte electrochemical cells method. The influence of the oxygen stoichiometry change on the thermodynamic properties was examined using the data obtained by a coulometric titration technique coupled with measurements of the electromotive force (EMF). The results were correlated with the average Mn valence values as determined by redox titration. The properties of the rare-earth manganites are strongly affected by the A- and B-site substitution and by the oxygen nonstoichiometry. New features related to the modifications in properties connected with the nanocrystalline state were evidenced. The correlation existing between the magnetic and thermodynamic characteristics were discussed in relation to significant changes in the overall concentration of defects.

Mössbauer Studies for Exploring CMR and TMR Perovskites

Doped transition metal compounds with complex electronic and magnetic structures show a wide range of new physical phenomena like high-temperature superconductivity, room temperature magnetic semiconductivity, or colossal magnetoresistance (CMR). Following the discovery of the effect of colossal negative magnetoresistance in manganese based perovskites, several different classes of transition metal compounds such as Sr2FeMoO6 double perovskites or La1-xSrxCoO3 perovskites were found to exhibit unusually high magnetoresistance (MR) partly as a peak around their Curie-temperature (CMR effect), partly as an increasing feature with decreasing temperature (tunneling-type magnetoresistance, TMR)(Table1). The most important difference between these two types of MR is their temperature dependence: while CMR effect manifests only around the magnetic temperature as a peak in the MR vs. T plot, TMR increases monotonically with decreasing temperature. Due to the complexity of the underlying physical and chemical processes in these materials, the understanding of their electronic and magnetic structure is one of the most vital topics in condensed matter physics nowadays. The first models aiming to shed light on the CMR effect found in manganite perovskites, and to explain the unusually strong correlation between the magnetic state of the material and its electric transport properties were based on the theories of double exchange model and strong electron-lattice interactions. The former was based on the fact that in La1-xCaxMnO3 perovskites the doping divalent ions (usually Ca) introduce a number of x extra electrons to the system, either oxidizing Mn ions into Mn or creating oxygen vacancies, although for low doping rates the latter effect was found to be negligible. As a Mossbauer Studies for Exploring CMR and TMR Perovskites

Synthesis and properties of nanograined La-Ca-manganite–Ni-ferrite composites

Composites of La 0.67Ca 0.33MnO 3 (LCMO), a colossal magnetoresistance perovskite and NiFe 2O 4 (NF), an insulating magnetic oxide have been prepared in situ using microwave-refluxing technique. Microstructural studies of the composites show that the two phases are uniformly distributed and the grain size of both the phases is identical, in the range 10–40 nm. The NF appears as a separate phase for concentrations x > 0.10 M, where M is the molecular weight of NF in the starting precursor solution. For concentrations <0.1 M, the X-ray diffraction studies indicate the presence of only the LCMO phase. Pure LCMO exhibits an insulator–metal transition T MI at 215 K whereas the magnetic transition T C occurs at 250 K. The transition temperatures T C of this LCMO phase decreases from 250 to 125 K for the addition of 0.15 M NF while T MI decreases from 215 to 90 K for the addition of just 0.02 M NF to the precursor solution. The magnetoresistance (MR) magnitude or increase in electrical conductivity of LCMO in the presence of an external magnetic field decreases progressively with the addition of NF. The MR of the composite with 0.01 M NF increases with decreasing temperature to ∼16% at 85 K in the presence of 8.5 kOe external field. The suppression of electrical and magnetic transitions in LCMO is found to be due to substitution of Mn by either or both Ni and Fe. The increasing MR response of the composite is due to the nanocrystalline grain size of the two phases and also partly due to the substitution phenomenon.

Surface characterization of colossal magnetoresistive manganites La 1− xSr xMnO 3 using photoelectron spectroscopy

We have studied the temperature and time dependence of the surface chemical composition and atomic structure of in situ fractured colossal magnetoresistive perovskites La 1− x Sr x MnO ( x = 0.3, 0.4) using core-level photoelectron spectroscopy and diffraction, simultaneous with observing marked changes in both core and valence electronic structure on going above the Curie temperature [N. Mannella et al., Phys. Rev. Lett. 92 (2004) 166401]. Stoichiometric analyses via core-level intensity ratios show that the near-surface composition is very nearly the same as that of the nominal (bulk) stoichiometry and further show that, during duration of our experiments, the degree of surface stoichiometry alteration or contamination has been minimal. The effects of photoelectron diffraction on such analyses are also explored. We comment on the degree to which near-surface composition or atomic-structure alterations might influence spectroscopic investigations of these manganites, or other strongly correlated materials.

Model for strain-induced metal-insulator phase coexistence in colossal magnetoresistive perovskite manganites (invited)

There is considerable evidence from new generations of high resolution microscopies and scattering techniques for intrinsically multiscale structures and dynamics in complex transition-metal oxides. In particular, the coexistence of submicrometer-size insulating and metallic domains in the same sample of perovskite manganites is believed to be crucial to the understanding of colossal magnetoresistance in these materials, and has been a puzzle to both theorists and experimentalists. In this work, we demonstrate, using an atomic-scale description of lattice distortions and long-range strains, that the presence of multiple local energy minimum states with different distortions provides a natural mechanism for such multiphase coexistence within the same material. The framework provides a basis for engineering nanoscale patterns of metallic and insulating phases and understanding other novel features observed in manganites, such as precursor short-range ordering and quasielastic scattering near the phase-transition temperature, hysteretic and glassy dynamics, metastability, and photoinduced insulator-metal transition.

Role of disorder and competing ferromagnetic and antiferromagnetic interactions in the magnetic, electrical, and dynamic properties ofLa0.7Pb0.3(Mn1−xFex)O3,0⩽x⩽0.2manganites

The effect of Fe doping on the magnetic properties of ${\mathrm{La}}_{0.7}{\mathrm{Pb}}_{0.3}{\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}{\mathrm{O}}_{3}(0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.2)$ colossal magnetoresistance perovskites is studied by muon spin rotation $(\ensuremath{\mu}\mathrm{SR})$ spectroscopy and macroscopic static magnetization measurements. The latter quantities show a cusp at temperatures below that signaling magnetic ordering (Curie temperature ${T}_{C}$) as well as a kink at lower temperatures which are taken as signatures of magnetic irreversibility. Random substitution of Mn by Fe ions gives rise to antiferromagnetic couplings mediated by conventional superexchange interaction. This manifests itself by the decrease of Curie temperatures and a concomitant decrease of the spontaneous magnetization with increased doping. The muon relaxation data under zero and applied longitudinal fields is interpreted in terms of progressive spin freezing concomitant with a reduction of the ferromagnetic component as Fe substitution is increased. The spin dynamics as ${T}_{C}$ is crossed from above shows clear departures from those expected for exchange-coupled Heisenberg or Ising systems but rather it displays features common to magnetically interacting-cluster systems. This effect leads to the appearance of a peak in the muon relaxation rate versus temperature curves that is located at temperatures below that signaling magnetic ordering $({T}_{C})$. Finally, direct evidence of the existence of antiferromagnetic and ferromagnetically short-range-ordered regions within the $x=0.2$ sample is provided by polarized neutron diffraction. The picture that emerges from our study identifies competing positive (double exchange) and negative (superexchange) interactions at random sites as the entities responsible for the low-temperature freezing monitored by using $\ensuremath{\mu}\mathrm{SR}$ spectroscopy.

X-MCD magnetometry of CMR perovskites La0.67−yREyCa0.33MnO3

Abstract Field dependences of the magnetic circular dichroism at the Mn:K edge and RE:L2,L3 edges in colossal magnetoresistive perovskites La0.67−yREyCa0.33MnO3 (y=0, Tb-doped y=0.1, 0.22 and Nd-doped y=0.33) are reported. Ferromagnetically ordered compounds show a similar dependence for the Mn and RE magnetic sublattices, in contrast to the cluster glass (y=0.22) compound which revealed a weaker Mn–RE magnetic coupling. By comparison of X-MCD and VSM magnetometry an estimate of the relative amount of manganese atoms in the metallic ferromagnetic regions is obtained.

Mössbauer effect probe of local Jahn–Teller distortion in Fe-doped colossal magnetoresistive manganites

The local structure of the Fe-doped La1−xCaxMnO3 (x=0.00–1.00) compounds has been investigated by means of Mössbauer spectroscopy. Fe57 Mössbauer spectra provide direct evidence of Jahn–Teller distortion in these manganites. On the basis of the Mössbauer results, the Jahn–Teller coupling was estimated. It is noteworthy that the Ca-concentration dependence of the Jahn–Teller coupling strength is very consistent with the magnetic phase diagram. Our results reveal that Mössbauer spectroscopy cannot only detect the local structural distortion, but also provide a technique to investigate the Jahn–Teller coupling of Fe-doped La1−xCaxMnO3 colossal magnetoresistive perovskites.

The concentration dependence of the Curie temperature of the colossal magnetoresistance perovskite La 1− xCa xMnO 3

The concentration dependence of the Curie temperature of the colossal magnetoresistance perovskite La 1− x Ca x MnO 3 is explained using a phenomenological model for interacting electronic and magnetic-phase transitions.

FERROMAGNETIC RESONANCES IN POLYCRYSTALLINE La0.8Li0.2MnO3

We report on magnetic resonances observed in the doped lanthanum manganite La0.8Li0.2MnO3 (LLMO) below the Curie temperature Tc ~ 235 K. Fields up to 30 T were employed. The data are compared with that from the archetypal colossal magnetoresistance perovskite La0 7Ca0 3MnO3 (LCMO) taken under similar conditions. In contrast to LCMO, LLMO exhibits two resonances, one above and one below the LCMO resonant field. These are attributed to a ferromagnetic interaction and a canted ferromagnetic interaction.

Ultraviolet photoelectron spectroscopy study of colossal magnetoresistive La 0.7− xPr xCa 0.3MnO 3

We have investigated the electronic structure of colossal magnetoresistive perovskite manganites La 0.7− x Pr x Ca 0.3MnO 3 using ultraviolet photoelectron spectroscopy as a function of temperature and Pr concentration x ( x=0.13, 0.3, and 0.4). Unusual temperature-dependent and x-dependent spectral changes are observed in the entire valence band region. Since Pr doping does not introduce carriers into the system but brings about strong lattice distortion effects which exert significant influence on the p–d hybridization, the degree of mixing between Mn 3d and O2p levels is considered to be the major factor responsible for the spectral weight transfers with temperature and Pr doping level. The observed changes are not consistent with the results of local-density-approximation (LDA) band calculation, but can be understood by the density-of-states from the LDA+U calculation or configuration-interaction cluster calculations. This implies that a strong correlation effect between d electrons plays an important role in these manganites. High resolution spectra near the Fermi level shows a clear metallic edge at low temperature for low Pr concentration sample ( x=0.13), but the metallic Fermi edge disappears at higher Pr doping levels ( x=0.3 and 0.4) even at the temperature below T c where the bulk metal–insulator transition occurs. This may be due to the low effective carrier density resulting from the reduced ferromagnetic ordering and the lattice distortion, or the pseudo-gap phenomena and the surface effect.

Core photoemission spectra of oxygen atoms in perovskite manganites La 1−x A x MnO 3 (A=Sr, Pb)

The electronic structure of the colossal magnetoresistance perovskite manganite materials, La 0.65 Pb 0.35 MnO 3 and La 0.7 Sr 0.3 MnO 3, has been studied by X-ray photoemission spectroscopy (XPS). The 1s core level spectra of oxygen (O(1s)) exhibit double-peak structure with asymmetric intensities characteristic for different chemical environments. Ab-initio calculations allow identifying source of the peaks. The higher peak in the lower binding energy is associated with ten of oxygen atoms, and the rest of oxygen atoms (two) give the weaker peak in the higher binding energy part of the XPS spectra. The weaker peak is associated only with these two oxygen atoms which are located in the lanthanum atoms plane without Sr or Pb impurities.

Structure of nanoscale polaron correlations inLa1.2Sr1.8Mn2O7

A system of strongly-interacting electron-lattice polarons can exhibit charge and orbital order at sufficiently high polaron concentrations. In this study, the structure of short-range polaron correlations in the layered colossal magnetoresistive perovskite manganite, La1.2Sr1.8Mn2O7, has been determined by a crystallographic analysis of broad satellite maxima observed in diffuse X-ray and neutron scattering data. The resulting q=(0.3,0,1) modulation is a longitudinal octahedral-stretch mode, consistent with an incommensurate Jahn-Teller-coupled charge-density-wave fluctuations, that implies an unusual orbital-stripe pattern parallel to the <100> directions.