Mixed-valence Manganites Research Articles

The Mn2+/Mn3+ state of La0.7Ce0.3MnO3 by oxygen reduction and photodoping

Films of cerium-doped LaMnO3, which has been intensively discussed as an electron-doped counterpart to hole-doped mixed-valence lanthanum manganites during the past decade, were analyzed by x-ray photoemission spectroscopy with respect to their manganese valence under photoexcitation. The comparative analysis of the Mn 3s exchange splitting of La0.7Ce0.3MnO3 (LCeMO) films in the dark and under illumination clearly shows that both oxygen reduction and illumination are able to decrease the Mn valence towards a mixed 2+/3+ state, independently of the film thickness and the degree of CeO2 segregation. Charge-injection from the photoconductive SrTiO3 substrate into the Mn eg band with carrier lifetimes in the range of tens of seconds and intrinsic generation of electron–hole pairs within the films are discussed as two possible sources of the Mn valence shift and the subsequent electron doping.

Pulsed laser deposition of La1−xSrxMnO3: thin-film properties and spintronic applications

Materials engineering on the nanoscale by precise control of growth parameters can lead to many unusual and fascinating physical properties. The development of pulsed laser deposition (PLD) 25 years ago has enabled atomistic control of thin films and interfaces and as such it has contributed significantly to advances in fundamental material science. One application area is the research field of spintronics, which requires optimized nanomaterials for the generation and transport of spin-polarized carriers. The mixed-valence manganite La1−xSrxMnO3 (LSMO) is an interesting material for spintronics due to its intrinsic magnetoresistance properties, electric-field tunable metal–insulator transitions, and half-metallic band structure. Studies on LSMO thin-film growth by PLD show that the deposition temperature, oxygen pressure, laser fluence, strain due to substrate–film lattice mismatch and post-deposition annealing conditions significantly influence the magnetic and electrical transport properties of LSMO. For spintronic structures, robust ferromagnetic exchange interactions and metallic conductivity are desirable properties. In this paper, we review the physics of LSMO thin films and the important role that PLD played in advancing the field of LSMO-based spintronics. Some specific application areas including magnetic tunnel junctions, multiferroic tunnel junctions and organic spintronic devices are highlighted, and the advantages, drawbacks and opportunities of PLD-grown LSMO for next-generation spintronic devices are discussed.

Substitutional effects on structural and magnetotransport properties of La0.85−xSmxK0.15MnO3 (x = 0.05, 0.1 and 0.15)

A series of polycrystalline La0.85−xSmxK0.15MnO3 (x=0.05, 0.1 and 0.15) have been synthesized by standard solid state reaction method to study effects of substituting La3+ by Sm3+ ion on the structural, electrical and magnetic properties. The Rietveld refinement of X-ray diffraction data revealed that mixed valence manganites crystallize in rhombohedral structure (space group R3¯C, no. 167). Raman spectra at room temperature reveal gradual change in phonon modes on increased doping. The temperature dependent electrical resistivity is measured by a standard four-probe method between 5 and 300K. The temperature dependent electrical resistivity exhibits metal–insulator transition at temperature TMI and metallic state (T<TMI) signifies importance of electron–electron, electron–phonon and electron–spin fluctuation scattering process. The peak resistivity ρm increases rapidly with Sm and TMI shifts to a lower temperature. The high temperature (T>TMI) conductivity data signifies polaronic nature. Application of magnetic field suppresses resistivity in entire temperature range. Low temperatures resistivity infers a field-dependent minimum and is due to electron–electron interaction and Kondo effects of mixed valence manganites. The ferro to paramagnetic transition temperature (TC) decreases with decreasing ionic radius as well as one-electron bandwidth of the A-site cation.

Effect of Ca and Ba Doping on the Magnetotransport Properties of NdMnO&lt;sub&gt;3&lt;/sub&gt;

Mixed valence manganites of the type Nd0.7-xRxMnO3 where, R = divalent cation (Ca and Ba) have been synthesized using citrate pyrolysis process. It is interesting to see that the Ca and Ba doped NdMnO3 compounds have metal - insulating behavior but different TMI values at different magnetic field. Although the Ca and Ba ions have almost same ionic radii but different density. For Ca-doped NdMnO3 compound, a magnetoresistance as high as 79 % was observed at the insulator transition temperature Tp =102 K, while in Ba-doped NdMnO3 compound, lower magnetoresistance ~ 67% was observed at the metal insulator transition temperature Tp = 90 K. For both Ca &amp; Ba doped NdMnO3 structural characterization shows same orthorhombic phase without impurities in the sample sintered at 900 °C, with the crystalline sizes in the nanometer scale. Grain size and morphology results show that the compounds have clear grain morphology and also nanoscale grain sizes.

Synthesis, magnetic and electric transport properties of mixed-valence manganites La0.5+xSr1.5−xMn0.5Cr0.5O4 (x=0.1, 0.2 and 0.3)

Abstract A series of mixed-valence manganites La0.5+xSr1.5−xMn0.5Cr0.5O4 were synthesized by sol–gel method. Rietveld profile analysis shows that the phases crystallize with tetragonal unit cell in the space group I4/mmm. The lattice parameter c increases with increase in x while a parameter almost remains constant. The Weiss constant (θ) is positive for all the phases and found to increase with increase in x. The ferromagnetic interactions becomes more dominant with increase in x due to increase in Mn3+–O–Mn4+ and Mn3+–O–Cr3+ double exchange interactions. Resistivity can be described by the polaron hopping and the variable range hopping model. It was found that the transport mechanism is dominated by Mott's variable range hopping (VRH) model with a decrease of Mott localization energy with increase in x, which explains the decrease of resistivity.

Simulation of Magnetocaloric Effect in La0.7Ca0.3MnO3 Ceramics Fabricated by Fast Sintering Process

The enhanced low-field magnetocaloric effect (MCE) is simulated for La0.7Ca0.3MnO3 (LCMO) ceramics that were fabricated by fast sintering process with different Al2O3 contents. It is shown that LCMO exhibits magnetic entropy change (ΔSM) much more uniform than that of gadolinium. The results show that the peak in the MCE at the ferromagnetic to paramagnetic phase transition is improved as the sintering temperature decreases. Furthermore, the samples open up a new way in which to tune the intrinsic properties of mixed-valence manganites. Through these results, LCMO has some potential applications for magnetic refrigerants in an extended high-temperature range. It is suggested that the fast sintering process with different Al2O3 contents for LCMO is an efficient way to obtain a working material of an apparatus based on the active magnetic regenerator cycle that cools hydrogen gas.

Identification of coherent lattice modulations coupled to charge and orbital order in a manganite

We apply grazing-incidence femtosecond x-ray diffraction to investigate the details of the atomic motion connected with a displacively excited coherent optical phonon. We concentrate on the low frequency phonon associated with the charge and orbital order in the mixed valence manganite La${}_{0.25}$Pr${}_{0.375}$Ca${}_{0.375}$MnO${}_{3}$ for $T\ensuremath{\lesssim}210$ K. We measure the response of three superlattice reflections that feature different sensitivities to the motion of the unit cell constituents. The results support the assignment to a translational mode of the Mn${}^{4+}$ atoms together with the oxygen atoms connecting adjacent Mn${}^{4+}$ sites.

Magnetic properties of mixed valence La2/3Sr1/3Mn1 − x T x O3 (T = Fe,Cr) manganites obtained by Pechini method

Polycrystalline manganites of composition La2/3Sr1/3Mn1 − x T x O3 (T = Cr3 + or Fe3 + , x = 0.0–0.10) were obtained by the Pechini method. Their magnetic properties exhibited a marked dependence on Fe/Cr content, with significant reduction of the magnetic moment per formula unit and their Curie temperatures. In addition, the magnetocaloric effect, determined by isothermal magnetization measurements, displayed a decreasing tendency with increasing Fe/Cr concentration. Results were interpreted on the basis of a deleterious effect on the double exchange interaction provoked by the presence of Fe3 + and Cr3 + ions within the crystal structure. Polycrystalline mixed valence manganites of composition La2/3Sr1/3Mn1 − x T x O3 (T= Cr3 + or Fe3 + , x = 0.0–0.10) were obtained by the amorphous citrate process known as Pechini method. Their magnetic properties (Curie temperature and spontaneous magnetization) exhibited a marked dependence with Fe or Cr content, together with magnetic entropy variations of up to 2.0 J/kgK.

Optical and x-ray time resolved study of the structural transition in mixed valence manganites

Time resolved optical reflectivity and x-ray diffraction techniques are employed to study the laser-induced structural response in two charge and orbitally ordered manganites. Optical data indicate a non-thermal nature of the laser-triggered phase transition via the disappearance of an optical phonon related to the charge and orbitally ordered phase. The x-ray diffraction measurements on superlattice reflections confirm the non-thermal time scale of the initial step of this phase transition but also show that the complete change of structural symmetry is not instantaneous.

Study of doping effect, phase separation and heterojunction in CMR manganites

Mixed-valence manganites have attracted considerable research focus in recent years not only because of the potential application of colossal magnetoresistance (CMR) in magnetic devices, but also because of many intriguing physical properties observed in these materials. Doping elements at A-site can alter the filling of 3d Mn band and the tolerance factor. Therefore the hole- and electron-doped CMR manganites exhibit a rich phase diagram. In addition, more theoretical and experimental results suggest that phase separation is a critical factor for the understanding of CMR phenomena. Recently, there is an increasing interest in the fabrication and investigation on manganite-based heterojunction, which demonstrated excellent rectifying property, large MR, and photovoltaic effect.

Structural and Electrical Properties of Monovalent Doped Manganites Pr0.6Sr0.4−x K x MnO3 (x=0, 0.05 and 0.1)

Structural and electrical properties of the mixed-valence monovalent doped manganites Pr0.6Sr0.4−x K x MnO3 (x=0, 0.05 and 0.1), prepared using the conventional solid-state synthesis method, have been investigated. Rietveld refinement of the X-ray diffraction patterns at room temperature shows a slight increase in unit cell volume with K content and confirms that all powder samples are single phase and crystallize in the orthorhombic structure with Pnma space group. Electrical resistivity measurements under and without magnetic field show a transition from the metallic to insulating behavior. The electrical conductivity is improved with increasing K content. The resistivity data in metallic region were fitted according to the electron–electron scattering process while in insulating region they were fitted using the small polaron hopping SPH model and Mott’s variable range hopping VRH model.

Emergence of Multicolor Photoluminescence in La0.67Sr0.33MnO3 Nanoparticles

Herein, we report the emergence of multicolor photoluminescence in a mixed-valence manganite nanoparticle La0.67Sr0.33MnO3 (LSMO NP) achieved through electronic structural modification of the nanoparticles upon functionalization with a biocompatible organic ligand, sodium tartrate. From UV–vis absorption, X-ray photoelectron spectroscopy (XPS), time-resolved photoluminescence study, and Raman spectroscopic measurements, it is revealed that ligand-to-metal charge transfer transitions from highest occupied molecular orbital (HOMO, centered in tartrate ligand) to lowest unoccupied molecular orbital (LUMO, centered in Mn3+/4+ of the NPs), and d–d transitions involving Jahn–Teller sensitive Mn3+ ions in the NPs plays the central role behind the origin of multiple photoluminescence from the ligand functionalized LSMO NPs.

Femtosecond dynamics of the structural transition in mixed valence manganites

We investigate the structural response of charge and orbitally ordered (CO/OO) manganites to ultrafast optical excitation using optical reflectivity and x-ray diffraction as a probe. We study a La0.42Ca0.58MnO3 (LCMO) thin film and a La0.25Pr0.375Ca0.375MnO3 (LPCMO) single crystal. For both materials we observe oscillations in the optical responses that are assigned to a coherent optical phonon generated via displacive excitation. The coherent phonon disappears either when increasing the temperature above T-CO/OO or when raising the excitation fluence above a certain threshold. At low excitation fluences the amplitude and lifetime of this phonon behave similarly to the order parameter of the structural phase transition.

Competing soft dielectric phases and detailed balance in thin film manganites

Using frequency dependent complex capacitance measurements on thin films of the mixed-valence manganite (La${}_{1\ensuremath{-}y}$Pr${}_{y}$)${}_{1\ensuremath{-}x}$Ca${}_{x}$MnO${}_{3}$, we identify and resolve the individual dielectric responses of two competing dielectric phases. We characterize their competition over a large temperature range, revealing they are in dynamic competition both spatially and temporally. The phase competition is shown to be governed by the thermodynamic constraints imposed by detailed balance. The consequences of the detailed balance model strongly support the notion of an ``electronically soft'' material in which continuous conversions between dielectric phases with comparable free energies occur on time scales that are long compared with electron-phonon scattering times.

Nonlinearity exponent of ac conductivity in disordered systems

We measured the real part of ac conductance Σ(x,f) or Σ(T,f) of iron-doped mixed-valent polycrystalline manganite oxides LaMn1−xFexO3 as a function of frequency f by varying initial conductance Σ0 by quenched disorder x at a fixed temperature T (room) and by temperature T at a fixed quenched disorder x. At a fixed temperature T, Σ(x,f) of a sample with fixed x remains almost constant at its zero-frequency dc value Σ0 at lower frequency. With increase in f, Σ(x,f) increases slowly from Σ0 and finally increases rapidly following a power law with an exponent s at high frequency. Scaled appropriately, the data for Σ(T,f) and Σ(x,f) fall on the same universal curve, indicating the existence of a general scaling formalism for the ac conductivity in disordered systems. The characteristic frequency fc at which Σ(x,f) or Σ(T,f) increases for the first time from Σ0 scales with initial conductance Σ0 as , where xf is the onset exponent. The value of xf is nearly equal to one and is found to be independent of x and T. Further, an inverse relationship between xf and s provides a self-consistency check of the systematic description of Σ(x,f) or Σ(T,f). This apparent universal value of xf is discussed within the framework of existing theoretical models and scaling theories. The relevance to other similar disordered systems is also highlighted.

Impact of Jahn-Teller active Mn3+on strain effects and phase transitions in Sr0.65Pr0.35MnO3

The mixed-valence manganite Sr${}_{0.65}$Pr${}_{0.35}$MnO${}_{3}$ has been prepared and its crystal and magnetic structure investigated between 7 and 1200 K using high-resolution powder neutron diffraction. The structural and lattice parameter data have been used to determine the octahedral tilting and spontaneous strains associated with the structural, electronic, and magnetic phase transitions. At room temperature, the structure is tetragonal and is characterized by cooperative out-of-phase tilts of the MnO${}_{6}$ octahedra about the $c$ axis and a large Jahn--Teller-type distortion due to the presence of Mn${}^{3+}$. The sample exhibits a reversible phase transition from the cubic $Pm\overline{3}m$ perovskite to a tetragonal $I$4/mcm structure at 750 K. The $Pm\overline{3}m$ \ensuremath{\leftrightarrow} I4/mcm phase transition is continuous, and the tetragonal strain, which is dominated by the Jahn--Teller-type distortion of the MnO${}_{6}$ octahedra, exhibits an unusual ${e}_{\mathrm{t}}$${}^{0.5}$ \ensuremath{\propto} (${T}_{c}$ $\ensuremath{-}$ $T$) temperature dependence. At low temperatures, a C-type antiferromagnetic structure develops with a Neel temperature ${T}_{N}$ of 250 K. The Mn magnetic moment at 7 K is 2.99(2) ${\ensuremath{\mu}}_{B}$/Mn. The magnetic ordering introduces additional tetragonal strain, and this strain shows the expected quadratic dependence on the magnetic moment at low temperatures. An increase in the octahedral tilt angle at ${T}_{N}$ demonstrates an effective coupling between the magnetic ordering process and octahedral tilting.

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.

Orbital order in layered manganites probed with 57Fe Mössbauer spectroscopy

In mixed-valence manganites, the doped iron species play the roles of simultaneous probes and ruiners for the orbital order occurring in the Mn-O electronic subsystem. We investigated the Mn3 + -based undoped system RBaMn2O5.5 and the half-doped systems RBaMn2O6, and RBaMn2O5 substituting a tiny portion of Mn3 + sites with Fe3 + . Single-site Mossbauer spectra were observed at room temperature in the charge-orbitally ordered state, i.e., below TCOO, for R=Y, Sm, Gd. Size of the rare earth ion strongly influences the quadrupole splitting in RBaMn2O6, but not in RBaMn2O5.

The effect of strontium deficiency in Pr 0.6Sr 0.4MnO 3 perovskite manganites

The structural, magnetic and magnetocaloric properties of the mixed valent manganites Pr 0.6Sr 0.4− x □ xMnO 3 (0 ≤ x ≤ 0.2) with perovskite structure have been investigated. X-ray diffraction (XRD) analysis using Rietveld refinement show that all our samples are single phase and crystallize in the orthorhombic structure with Pbnm space group. Strontium vacancy leads to a gradual increase of the unit cell volume. Paramagnetic (PM) to ferromagnetic (FM) transitions have been observed in all our synthesized samples. With increasing Sr-deficient amount, the Curie temperature T C decreases from 310 K for x = 0 to 170 K for x = 0.2. From the measured magnetization data of Pr 0.6Sr 0.4− x □ x MnO 3 (0 ≤ x ≤ 0.2) samples as a function of magnetic applied field, the associated magnetic entropy change Δ S M close to the Curie temperature T C has been determined. The Pr 0.6Sr 0.35□ 0.05MnO 3 compound ( x = 0.05) exhibits a maximum entropy change Δ S M Max of 4.81 J/(kg K) at 260 K in a magnetic applied field of 5 T. Δ S M Max decreases to 4.17 J kg/K at 175 K for x = 0.2 in the same field magnitude. Large relative cooling power (RCP) value of 260.7 J/kg is obtained for x = 0.05 sample at a field change of 5 T.

Near-room-temperature control of magnetization in field effect devices based on La0.67Sr0.33MnO3 thin films

The control of the magnetization in ferromagnetic layers via electric fields is a hot topic in view of applications to the next generation of spintronic devices, where writing the magnetic information through current lines could be replaced by electric writing. Mixed valence manganites are good candidates for such a purpose because they present an intriguing coupling between ferromagnetism and charge ordering/doping which can be tuned by the application of an electric field. Here we present results on the near-room temperature control of the magnetization of optimally doped La0.67Sr0.33MnO3 ultrathin films in vertical field effect devices, where they act as top or bottom electrodes. In the latter case a slight decrease in the Curie temperature (∼5 K) is observed after application of 5×107 V/m, i.e., the maximum field preventing electric breakdown, compatible with the induced variation in the charge density and mixed valence within the Thomas Fermi screening length. These results indicate that electric fields achievable in vertical field effect devices, of the same entity of interfacial fields originating from differences in the work function in heterostructures, have only minor influence on the magnetic properties of optimally doped ultrathin La0.67Sr0.33MnO3 films.