Lattice-spin mechanism in colossal magnetoresistive manganites

We present a single-orbital double-exchange model, coupled with cooperative phonons (the so called breathing modes of the oxygen octahedra in manganites). The model is studied with Monte Carlo simulations. For a finite range of doping and coupling constants, a first-order metal-insulator phase transition is found, which coincides with the paramagnetic-ferromagnetic phase transition. The insulating state is due to the self-trapping of every carrier within an oxygen octahedron distortion.     

Glass transition in the polaron dynamics of colossal magnetoresistive manganites

Neutron scattering measurements on a bilayer manganite near optimal doping show that the short-range polaron correlations are completely dynamic at high T, but then freeze upon cooling to a temperature T(*) approximately equal 310 K. This glass transition suggests that the paramagnetic/insulating state arises from an inherent orbital frustration that inhibits the formation of a long-range orbital- and charge-ordered state. Upon further cooling into the ferromagnetic-metallic state (T(C) = 114 K), where the polarons melt, the diffuse scattering quickly develops into a propagating, transverse optic phonon.     

Electronic confinement and ordering instabilities in colossal magnetoresistive bilayer manganites

We present angle-resolved photoemission studies of (La{1-z}Pr{z}){2-2x}Sr{1+2x}Mn{2}O{7} with x=0.4 and z=0.1, 0.2, and 0.4 along with density functional theory calculations and x-ray scattering data. Our results show that the bilayer splitting in the ferromagnetic metallic phase of these materials is small, if not completely absent. The charge carriers are therefore confined to a single MnO{2} layer, which in turn results in a strongly nested Fermi surface. In addition to this, the spectral function also displays clear signatures of an electronic ordering instability well below the Fermi level. The increase of the corresponding interaction strength with z and its magnitude of ～400 meV make the coupling to a bare phonon highly unlikely. Instead we conclude that fluctuating order, involving electronic and lattice degrees of freedom, causes the observed renormalization of the spectral features.     

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.     

Direct observation of oxygen superstructures in manganites

We report the observation of superstructures associated with the oxygen 2p states in two prototypical manganites using x-ray diffraction at the oxygen K edge. In the stripe order system Bi0.31Ca0.69MnO3, hole-doped O states are orbitally ordered, at the same propagation vector as the Mn orbital ordering, but no oxygen charge stripes are found at this periodicity. In La7/8Sr1/8MnO3, we observe a 2p charge ordering described by alternating hole-poor and hole-rich MnO planes that is consistent with some of the recent predictions.     

Exchange field induced magnetoresistance in colossal magnetoresistance manganites.

The effect of an exchange field on the electrical transport in thin films of metallic ferromagnetic manganites has been investigated. The exchange field was induced both by direct exchange coupling in a ferromagnet/antiferromagnet multilayer and by indirect exchange interaction in a ferromagnet/paramagnet metallic superlattice. The electrical resistance of the metallic manganite layers was found to be determined by the magnitude of the vector sum of the effective exchange field and the external magnetic field.     

Size of the interpolated cation and hole carrier density: two key parameters for the optimisation of colossal magnetoresistive properties of Pr-based manganites

The exploration of the magnetic and transport properties of four series of manganese perovskites, Pr_0.7Ca_0.34−xA_xMnO_3−δ (A=Sr, Ba), Pr_0.7−xLa_xCa_0.3 MnO_3−δ and Pr_0.66Ca_0.34−x Sr_xMnO_3−δ has allowed four phases with colossal magnetoresistive (CMR) properties to be isolated: Pr_0.7Ca_0.25Sr_0.025MnO_3−δ and Pr_0.66Ca_0.26Sr_0.08MnO_3−δ that exhibit a variation of resistance of 2.5. 10⁷% and 10⁹% at μ₀ H=5 T for T=88 K and 50 K respectively, Pr_0.58La_0.12Ca_0.3 MnO_3−δ that exhibits a variation of 6.10⁶% for μ₀ H=5 T at T=80 K and Pr_0.7Ba_0.025Ca_0.275MnO_3−δ for which a resistance variation of 5.10⁹%, at T=50 K, for μ₀ H=5 T is evidenced. for each compound of this series except the barium phase, one observes that the temperature T_max, which corresponds to the resistance maximum on the R(T) curves in zero magnetic field, increases dramatically as the mean size of the interpolated cations increases, and that the CMR effect correlatively decreases dramatically. The comparison of the two series Pr_0.7Ca_0.3−xSr_xMnO_3−δ and Pr_0.66Ca_0.34−xSr_xMnO_3−δ shows also the crucial role of the hole carrier density: for a same mean ionic radius of the interpolated cation T_max is decreased of about 50 K by introducing 0.034 hole per Mn mole.     

Current oscillation and low-field colossal magnetoresistance effect in phase-separated manganites

Current-induced switching from a metallic to an insulating state is observed in phase-separated states of (La(1-y)Pr(y))0.7Ca0.3MnO3 (y=0.7) and Nd(0.5)Ca(0.5)Mn(1-z)Cr(z)O3 (z=0.03) crystals. The application of magnetic fields to this current-induced insulating state causes a pronounced low-field negative magnetoresistance effect [rho(H)/rho(0)=10(-3) at H=1 kOe]. The application of a constant voltage also causes the breakdown of the Ohmic relation above a threshold voltage. At voltages higher than this threshold value, oscillations in currents are observed. This oscillation is well reproduced by a simple model of local switching of a percolative conduction path.     

A new theory of doped manganites exhibiting colossal magnetoresistance

Rare earth manganites doped with alkaline earths, namely Re_1-xA_xMnO₃, exhibit colossal magnetoresistance, metal insulator transitions, competing magnetic, orbital and charge ordering, and many other interesting but poorly understood phenomena. In this article I outline our recent theory based on the idea that in the presence of strong Jahn-Teller, Coulomb and Hund’s couplings present in these materials, the low-energy electronic states dynamically reorganize themselves into two sets: one set (ℓ) which are polaronic, i.e., localized and accompanied by large local lattice distortion, and another (b) which are non-polaronic and band-like. The coexistence of the radically different ℓ andb states, and the sensitive dependence of their relative energies and occupation upon dopingx, temperatureT, magnetic fieldH, etc., underlies the unique effects seen in manganites. I present results from strong correlation calculations using dynamical mean-field theory and simulations on a new 2-fluid model which accord with a variety of observations.     

Size of the interpolated cation and hole carrier density: two key parameters for the optimisation of colossal magnetoresistive properties of Pr-based manganites

The exploration of the magnetic and transport properties of four series of manganese perovskites, Pr_0.7Ca_0.34?xA_xMnO_3?δ (A=Sr, Ba), Pr_0.7?xLa_xCa_0.3 MnO_3?δ and Pr_0.66Ca_0.34?x Sr_xMnO_3?δ has allowed four phases with colossal magnetoresistive (CMR) properties to be isolated: Pr_0.7Ca_0.25Sr_0.025MnO_3?δ and Pr_0.66Ca_0.26Sr_0.08MnO_3?δ that exhibit a variation of resistance of 2.5. 10⁷% and 10⁹% at μ₀ H=5 T for T=88 K and 50 K respectively, Pr_0.58La_0.12Ca_0.3 MnO_3?δ that exhibits a variation of 6.10⁶% for μ₀ H=5 T at T=80 K and Pr_0.7Ba_0.025Ca_0.275MnO_3?δ for which a resistance variation of 5.10⁹%, at T=50 K, for μ₀ H=5 T is evidenced. for each compound of this series except the barium phase, one observes that the temperature T_max, which corresponds to the resistance maximum on the R(T) curves in zero magnetic field, increases dramatically as the mean size of the interpolated cations increases, and that the CMR effect correlatively decreases dramatically. The comparison of the two series Pr_0.7Ca_0.3?xSr_xMnO_3?δ and Pr_0.66Ca_0.34?xSr_xMnO_3?δ shows also the crucial role of the hole carrier density: for a same mean ionic radius of the interpolated cation T_max is decreased of about 50 K by introducing 0.034 hole per Mn mole.     

Theory of insulator metal transition and colossal magnetoresistance in doped manganites

The persistent proximity of insulating and metallic phases, a puzzling characteristic of manganites, is argued to arise from the self-organization of the twofold degenerate e(g) orbitals of Mn into localized Jahn-Teller (JT) polaronic levels and broad band states due to the large electron-JT phonon coupling present in them. We describe a new two band model with strong correlations and a dynamical mean-field theory calculation of equilibrium and transport properties. These explain the insulator metal transition and colossal magnetoresistance quantitatively, as well as other consequences of two state coexistence.     

Direct observation of large electronic domains with memory effect in doped manganites

We use a spatially resolved, direct spectroscopic probe for electronic structure with an additional sensitivity to chemical compositions to investigate high-quality single crystal samples of La(1/4)Pr(3/8)Ca(3/8)MnO3, establishing the formation of distinct insulating domains embedded in the metallic host at low temperatures. These domains are found to be at least an order of magnitude larger in size compared to previous estimates and exhibit memory effects on temperature cycling in the absence of any perceptible chemical inhomogeneity, suggesting long-range strains as the probable origin.     

Direct magneto-optical observation of a structural phase transition in thin films of manganites

The spontaneous formation of twins in La(2/3)Ca(1/3)MnO3 films below T(S) approximately 105 K is observed by a magneto-optical technique. The twins are revealed as stripes along the {110} directions where magnetization tilts out of the film plane due to the stresses in twins. Their appearance is associated with a martensitic phase transition in the film triggered by the cubic-to-tetragonal transition in the SrTiO3 substrate. It is found that magnetization of the films proceeds by inhomogeneous rotation of magnetic moments. This is due to the presence of microscopic structural inhomogeneities. Their dominating role in the low-temperature transport can explain small effects of the transition at T(S) on the resistivity.     

Magnetorefractive effect in manganites with a colossal magnetoresistance in the visible spectral region

The magnetotransmission, magnetoreflection, and magnetoresistance of the La_0.7Ca_0.3MnO₃ and La_0.9Ag_0.1MnO₃ epitaxial films have been investigated. It has been found that the films exhibit a significant magnetorefractive effect in the case of reflection and transmission of light in the fundamental absorption region both in the vicinity of the Curie temperature and at low temperatures. It has been shown that the magnetorefractive effect in the infrared spectral region of the manganites is determined by a high-frequency response to magnetoresistance, whereas the magnetorefractive effect in the visible spectral region of these materials is associated with a change in the electronic structure in response to a magnetic field, which, in turn, leads to a change in the electron density of states, the probability of interband optical transitions, and the shift of light absorption bands. The obtained values of the magnetotransmittance and magnetoreflectance in the visible spectral region are less than those observed in the infrared region of the spectrum, but they are several times greater than the linear magneto-optical effects. As a result, the magnetorefractive effect, which is a nongyrotropic phenomenon, makes it possible to avoid the use of light analyzers and polarizers in optical circuits.     

Interplay of charge, spin, orbital and lattice correlations in colossal magnetoresistance manganites

We derive a realistic microscopic model for doped colossal magnetoresistance manganites, which includes the dynamics of charge, spin, orbital and lattice degrees of freedom on a quantum mechanical level. The model respects the SU(2) spin symmetry and the full multiplet structure of the manganese ions within the cubic lattice. Concentrating on the hole doped domain ( 0≤x≤0.5) we study the influence of the electron-lattice interaction on spin and orbital correlations by means of exact diagonalisation techniques. We find that the lattice can cause a considerable suppression of the coupling between spin and orbital degrees of freedom and show how changes in the magnetic correlations are reflected in dynamic phonon correlations. In addition, our calculation gives detailed insights into orbital correlations and demonstrates the possibility of complex orbital states. Received 4 September 2002 / Received in final form 8 November 2002 Published online 31 December 2002     

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

Monte‐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 Sm_0.55Sr_0.45MnO₃, 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.

     

Magnetic anisotropy in colossal magnetoresistive FeCr2S4 single crystals

The magnetic properties of spinel FeCr₂S₄ single crystals were investigated by ferromagnetic resonance (FMR). The FMR spectrum displays a single absorption line in the whole temperature range measured for both H∥(111) and H⊥(111). With decreasing temperature, the line with H∥(111) shifts to lower fields, while that with H⊥(111) shifts to higher fields. By superposing all the FMR spectra measured in different directions at 110 K, a double-peak is obtained, which clarifies the origin of the FMR double-peak in polycrystalline sample. By taking account of magnetic anisotropy and demagnetizing effect, the orientation dependence of resonance field is well fitted. It is found that the magnetic anisotropy strengthens with decreasing temperature; however, it has no evident influence on transport and colossal magnetoresistance behavior.     

Competing ferromagnetic and charge-ordered states in models for manganites: the origin of the colossal magnetoresistance effect

The one-orbital model for manganites with cooperative phonons and superexchange coupling JAF is investigated via large-scale Monte Carlo simulations. The results for two orbitals are also briefly discussed. Focusing on the electron density n=0.75, a regime of competition between ferromagnetic metallic and charge-ordered (CO) insulating states is identified. In the vicinity of the associated bicritical point, colossal magnetoresistance (CMR) effects are observed. The CMR is associated with the development of short-distance correlations among polarons, above the spin ordering temperatures, resembling the charge arrangement of the low-temperature CO state.