Colossal Magnetoresistance Manganites Research Articles

Fingerprint of dynamical spin-charge correlations in the tunneling spectra of colossal magnetoresistive manganites

We present temperature-dependent scanning tunneling spectroscopy measurements on $La_{1-x}Ca_{x}MO_{3}$ ($x\sim0.33$) films with different degrees of biaxial strain. A depletion in normalized conductance around the Fermi level is observed both above and below the insulator-to-metal transition temperature $T_{MI}$, for weakly as well as highly-strained films. This pseudogap-like depletion globally narrows on cooling. The zero-bias conductance decreases on cooling in the insulating phase, reaches a minimum close to $T_{MI}$ and increases on cooling in the metallic phase, following the trend of macroscopic conductivity. These results support a recently proposed scenario in which dynamical short-range antiferromagnetic/charge order correlations play a preeminent role in the transport properties of colossal magnetoresistive manganites [R. Yu \textit{et al}., Phys. Rev. B \textbf{77}, 214434 (2008)].

Strong magnetorefractive effect in epitaxial La 2/3Ca 1/3MnO 3 thin films

We report here on the magneto-optical characterization of epitaxial La 2/3Ca 1/3MnO 3 thin films. We observe that the magnetic field dependence of the magneto-optical signal measured in transverse Kerr geometry can be decomposed into even and odd contributions which evolve differently with the temperature. We demonstrate that whereas the odd component is proportional to the magnetization, the even contribution is related to the magnetorefractive effect, which is caused by the changes of the refractive index and optical conductivity with the magnetic field. This phenomenon, previously reported only at infrared wavelengths in some spin valves and granular systems, is shown here to be very relevant at visible frequencies for the colossal magnetoresistance manganites, thus allowing simultaneous optical characterization of the magnetic and magnetotransport properties. We argue that these characteristics result from inherent transport properties of these strongly correlated ferromagnetic oxides.

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.

Elastic behaviour of strontium-doped lanthanum calcium manganites in the vicinity of TC

Colossal magnetoresistance (CMR) manganites with compositional formula, La 0.67Ca 0.33− x Sr x MnO 3 (where x=0, 0.09, 0.11, 0.13, 0.33) were prepared by citrate gel route mainly to understand the elastic behaviour in the vicinity of their magnetic transition temperature T C. The structural characterizations of all the materials clearly indicate that samples upto x=0.13 doping, are having orthorhombic structure with Pbnm space group and sample with x=0.33 is having rhombohedral structure with R 3 ¯ c space group. The magnetic transition temperatures ( T C) are determined by AC susceptibility measurements and are found to increase continuously with increasing strontium concentration. Finally, a systematic investigation of the ultrasonic longitudinal velocities of all the samples was carried out. Further, all the samples are found to exhibit anomalous behaviour in the vicinity of their magnetic transition temperatures and the observed behaviour is explained using Landau's theory.

Effects of correlated disorder on the magneto‐transport in colossal magnetoresistance manganites

AbstractMonte‐Carlo simulations predict that a local correlated disorder is responsible for many of the novel transport and magnetic properties of colossal magnetoresistance (CMR) materials such as manganites. One important prediction of these models is that the resistivity at the metal–insulator transition (MIT) in manganites depends strongly on the correlated quenched disorder. However, experimental confirmation has been challenging since it is difficult to control the amount of disorder in these compounds. We carried out experiments on Sm0.55Sr0.45MnO3, a prototypical CMR manganite with a sharp MIT, whereby the oxygen‐related disorder is systematically enhanced by low temperature thermal activation. We observe dramatic changes in the temperature dependence of resistivity at the MIT as the amount of quenched disorder is increased, occurring in a manner that is in agreement with theoretical predictions.magnified imageTemperature dependence of resistivity of Sm0.55Sr0.45MnO3 for different annealing times at 350 °C in vacuum.(© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Formation and temperature evolution of correlated polarons in colossal magnetoresistivemanganites

We investigate the temperature dependence of charge ordered clusters in manganites. Interms of an approximate treatment of the half-filled spinless Holstein model incorporatingthe double exchange effect and nearest-neighbor Coulomb interactions, we find that chargeordering and polaronic states appear simultaneously which naturally defines a correlatedpolaron state. Without the double exchange, the temperature evolution of the charge(polaron) ordering parameter exhibits a broad peak structure only in the stronglycorrelated quasiadiabatic regime. Including the double exchange, the peak becomessharper and is observed both in the quasiadiabatic and antiadiabatic regimes.The peak structure in the quasiadiabatic regime provides an explanation for thetemperature dependence of the polaron correlation in nanoscale charge ordered polaronclusters observed in the manganites with intermediate to narrow bandwidths.

Green-Function-Based Monte Carlo Method for Classical Fields Coupled to Fermions

Microscopic models of classical degrees of freedom coupled to noninteracting fermions occur in many different contexts. Prominent examples from solid state physics are descriptions of colossal magnetoresistance manganites and diluted magnetic semiconductors, or auxiliary field methods for correlated electron systems. Monte Carlo simulations are vital for an understanding of such systems, but notorious for requiring the solution of the fermion problem with each change in the classical field configuration. We present an efficient, truncation-free O(N) method on the basis of Chebyshev expanded local Green functions, which allows us to simulate systems of unprecedented size N.

Structure and charge ordering behavior of the colossal magnetoresistive manganite Nd0.5Sr0.5MnO3

The structure and charge ordering (CO) behavior of the three-dimensional colossal magnetoresistive manganite Nd0.5Sr0.5MnO3 have been studied by transmission electron microscopy. The electron diffraction analysis suggested its room temperature structure as orthorhombic, with a Pnma space group. By controlling the experimental temperature setting either above or below the CO transition temperature, it was observed that the CO grew through the propagation of the CO front. The twin boundaries were the barriers of the CO front movement. In heavily ion irradiated samples, it was observed that the CO front was pinned by the intracrystalline defects, forming a zigzag shape interface. Moreover, the electron beam was observed to influence the CO.

Giant superconductivity-induced modulation of the ferromagnetic magnetization in a cuprate–manganite superlattice

Artificial multilayers offer unique opportunities for combining materials with antagonistic orders such as superconductivity and ferromagnetism and thus to realize novel quantum states. In particular, oxide multilayers enable the utilization of the high superconducting transition temperature of the cuprates and the versatile magnetic properties of the colossal-magnetoresistance manganites. However, apart from exploratory work, the in-depth investigation of their unusual properties has only just begun. Here we present neutron reflectometry measurements of a [Y(0.6)Pr(0.4)Ba(2)Cu(3)O(7) (10 nm)/La(2/3)Ca(1/3)MnO(3) (10 nm)](10) superlattice, which reveal a surprisingly large superconductivity-induced modulation of the vertical ferromagnetic magnetization profile. Most surprisingly, this modulation seems to involve the density rather than the orientation of the magnetization and is highly susceptible to the strain, which is transmitted from the SrTiO(3) substrate. We outline a possible explanation of this unusual superconductivity-induced phenomenon in terms of a phase separation between ferromagnetic and non-ferromagnetic nanodomains in the La(2/3)Ca(1/3)MnO(3) layers.

X-ray photoemission study in Re0.7Ca0.3MnO3 epitaxial films

We have performed the x-ray photoemission experiments in the colossal magnetoresistive manganites Re0.7Ca0.3MnO3 with Re=La, Pr, and Nd in order to study the Re dependence of electronic structures in this system. The samples are epitaxial films grown on LaAlO3 substrates with pulsed laser deposition system. It is found that the binding energies of Mn 2p3/2 are 641.7 and 643.1 eV for La0.7Ca0.3MnO3 and Nd0.7Ca0.3MnO3, respectively. The shift of this energy level is related to the change in bond angle and/or bond length between Mn and oxygen. Moreover, two lines of O 1s core level are assigned as Mn–O (530 eV) and Re–O (533 eV). The intensity ratio of these two lines in Nd0.7Ca0.3MnO3 is different from those in La0.7Ca0.3MnO3 and Pr0.7Ca0.3MnO3, suggesting that the ion redistribution between Re–O and Mn–O sites occurs with the Re substitution in these films.

Time-resolved studies of phase transition dynamics in strongly correlated manganites

Ultrafast light pulses can be used to control electronic, magnetic and structural phases of complex solids. Here, we investigate the dynamics of insulator-metal phase transitions in colossal magnetoresistive (CMR) manganites by a combination of femtosecond visible-to-midinfrared pump-probe techniques and transport measurements. We show that an insulator-metal transition can be stimulated in CMR manganites by both above bandgap excitation and selective excitation of individual vibrational degrees of freedom. These two approaches rely on the ultrafast manipulation of parameters controlling the electronic filling and the electronic bandwidth respectively, extending the concepts of filling and bandwidth control to the ultrafast timescale. The ultrafast vibrational control of correlated-electron phases may provide new insights into the role played by lattice vibrations in determining the electronic properties of complex solids.

Probing strongly correlated electron dynamics on extreme timescales

We use sub-10 fs laser pulses in the visible and near IR region of the spectrum in order to probe the dynamics of photo-induced phase transitions on their characteristic timescales. We present results from two different cases: the 1D organic Mott insulator ET-F2TCNQ and the more complex, colossal magneto-resistive manganite, Pr0.7Ca0.3MnO3. For the former, we find that the melting of the Mott state takes 20 fs, a timescale which is dictated by the electron tunnelling time, h/t. In comparison, the dynamics of the manganite show coupling to a 14 THz structural mode and a high frequency 30 THz mode, which we assign as an orbiton, a collective electronic excitation.

A-site ordering in colossal magnetoresistance manganite La[sub 1−x]Sr[sub x]MnO[sub 3]? Molecular dynamics simulations and quantum mechanics calculations

Recent experiments have called into question the assumption of a random A-site distribution in mixed-valence colossal magnetoresistance (CMR) manganites. We explored the possibility of an A-site (La(3+)/Sr(2+)) ordering in a CMR manganite La(3/4)Sr(1/4)MnO(3) using molecular dynamics (MD) simulations with a newly developed force field (FF) and quantum mechanics (QM) (density functional theory with the generalized gradient approximation) calculations of the relative stability of structures obtained from MD. Both methods suggest that the degree of stabilization (enthalpy gain) of A-site ordering is not sufficient to overcome the accompanying entropy loss, supporting the assumption of a random A-site distribution in La(3/4)Sr(1/4)MnO(3). This approach combining MD and QM as well as the versatile FF developed in this study should be useful for investigating the structure and functionality of magnetic tunnel junction devices involving composite materials of mixed-valence manganites.

Phase Separation of Electrons Strongly Coupled with Phonons in Cuprates and Manganites

Recent advanced Monte Carlo simulations have not found superconductivity and phase separation in the Hubbard model with on-site repulsive electron–electron correlations. We argue that microscopic phase separations in cuprate superconductors and colossal magnetoresistance (CMR) manganites originate from a strong electron–phonon interaction (EPI) combined with unavoidable disorder. Attractive electron correlations, caused by an almost unretarded EPI, are sufficient to overcome the direct inter-site Coulomb repulsion in these charge-transfer Mott–Hubbard insulators, so that low energy physics is that of small polarons and small bipolarons (real-space electron (hole) pairs dressed by phonons). They form clusters localized by disorder below the mobility edge, but propagate as the Bloch states above the mobility edge. I identify the Frohlich finite-range EPI with optical phonons as the most essential for pairing and phase separation in superconducting layered cuprates. The pairing of oxygen holes into heavy bipolarons in the paramagnetic phase (current-carrier density collapse (CCDC)) explains also CMR of doped manganites due to magnetic break-up of bipolarons in the ferromagnetic phase. Here I briefly present an explanation of high- and low-resistance phase coexistence near the ferromagnetic transition as a mixture of polaronic ferromagnetic and bipolaronic paramagnetic domains due to unavoidable disorder in doped manganites.

Colossal Magnetoresistive Manganite Based Fast Bolometric X-ray Sensors for Total Energy Measurements of Free Electron Lasers

Bolometric detectors based on epitaxial thin films of rare earth perovskite manganites have been proposed as total energy monitors for X-ray pulses at the Linac Coherent Light Source free electron laser. We demonstrate such a detector scheme based on epitaxial thin films of the perovskite manganese oxide material Nd{sub 0.67}Sr{sub x0.33}MnO{sub 3}, grown by pulsed laser deposition on buffered silicon substrates. The substrate and sensor materials are chosen to meet the conflicting requirements of radiation hardness, sensitivity, speed and linearity over a dynamic range of three orders of magnitude. The key challenge in the material development is the integration of the sensor material with Si. Si is required to withstand the free electron laser pulse impact and to achieve a readout speed three orders of magnitude faster than conventional cryoradiometers for compatibility with the Linac Coherent Light Source pulse rate. We discuss sensor material development and the photoresponse of prototype devices. This Linac Coherent Light Source total energy monitor represents the first practical application of manganite materials as bolometric sensors.

Pump-and-probe study inLaMnO3thin films

Subpicosecond time-resolved anisotropic transmission spectroscopy has been applied to one of the parent compounds of the colossal magnetoresistance manganites, orbital-ordered ${\text{LaMnO}}_{3}$, with and without the $A$-type antiferromagnetic order. ${\text{LaMnO}}_{3}$ is one of the simplest Mott insulators which facilitates unambiguous interpretation of the data. Two characteristic decay time constants are found; the fast one $({\ensuremath{\tau}}_{1})$ around 0.5 ps and the slow one $({\ensuremath{\tau}}_{2})$ around 10 ps. ${\ensuremath{\tau}}_{1}$ is the same above and below ${T}_{N}$ and is interpreted as the electron-phonon equilibration time. ${\ensuremath{\tau}}_{2}$ shows very slight increase below ${T}_{N}$. We interpret ${\ensuremath{\tau}}_{2}$ as the time constant for the orbital order to reach thermal equilibrium. The lack of a significant effect due to the magnetic order is explained using a recent band-structure calculation.

Multicritical End Point of the First-Order Ferromagnetic Transition in Colossal Magnetoresistive Manganites

We have studied the bandwidth-temperature-magnetic-field phase diagram of RE0.55Sr0.45MnO3 colossal magnetoresistance manganites with ferromagnetic metal (FM) ground state. The bandwidth was controlled both via chemical substitution and hydrostatic pressure with a focus on the vicinity of the critical pressure p;{*} where the character of the zero-field FM transition changes from first to second order. Below p;{*} the first-order FM transition extends up to a critical magnetic field. It approaches zero on the larger bandwidth side where the surface of the first-order FM phase boundary is terminated by a multicritical end point. The change in the character of the transition and the decrease of the colossal magnetoresistance effect is attributed to the reduced charge-order and orbital-order fluctuations.

Spin-charge coupling in a band ferromagnet: Magnon-energy reduction, anomalous softening, and damping

The effects of correlation-induced coupling between spin and charge fluctuations on spin-wave excitations in a band ferromagnet are investigated by including self-energy and vertex corrections within a systematic inverse-degeneracy expansion scheme which explicitly preserves the Goldstone mode. Arising from the scattering of a magnon into intermediate spin-excitation states (including both magnon and Stoner excitations) accompanied with charge fluctuations in the majority-spin band, this spin-charge coupling results not only in a substantial reduction of magnon energies but also in anomalous softening and significant magnon damping for zone-boundary modes lying within the Stoner gap. Our results are in good qualitative agreement with recent spin-wave excitation measurements in colossal magnetoresistive manganites and ferromagnetic ultrathin films of transition metals.

Electrical transport properties of Nd0.67−xEuxSr0.33MnO3 (0 ≤ x ≤ 0.67) manganites

A systematic investigation of electrical transport properties viz., electrical conductivity and thermopower of Eu-doped Neodymium-based colossal magnetoresistive manganites with compositional formula, Nd0.67−xEuxSr0.33MnO3 (x = 0–0.67) has been undertaken. These materials were prepared by citrate gel route and characterized by X-ray diffraction, scanning electron microscopy, AC susceptibility, and electrical resistivity measurements. With a view to understand the complex conduction mechanism of these materials, electrical resistivity and thermoelectric power (TEP) data have been analyzed using various theoretical models. It has been concluded that the ferromagnetic metallic part of the conduction mechanism is explained by grain/domain boundary, electron–electron, and magnon scattering mechanisms, while that of high temperature paramagnetic insulating region might be due to small polaron hopping mechanism. The sign change of charge carriers observed in TEP measurements is attributed to the oxygen deficiency of the samples.

Polaron Activation Energy as Evidenced by EMR in Colossal Magnetoresistive Nanowires

In the present work an X-band electron magnetic resonance (EMR) investigation in the paramagnetic regime of colossal magnetoresistive manganite La2/3Ca1/3 MnO3 (LCMO) nanowires was carried out. The temperature dependence of the EMR line width has been analyzed in terms of the small polaron hopping scenario. From this analysis, the polaron activation energy Ea in LCMO nanowires was evaluated. A discussion is given concerning the factors which could explain the smaller value of Ea as compared with the corresponding ones for LCMO nanoparticles.