Magnetic studies of colossal magnetoresistance perovskites on macroscopic,mesoscopic and microscopic length scales

We have investigated magnetic correlations in various CMR manganites on macroscopic, mesoscopic and microscopic length scales by carrying out DC magnetization, neutron depolarization, and neutron diffraction measurements. We present here the effect of substituting Mn with Fe and La with Dy in the ferromagnetic La_0.7−xCa_xMnO₃ (x ∼ 0.3–0.33) compounds. Neutron diffraction has been used in order to characterize the long-range magnetic order and its gradual suppression by the substitution. Neutron depolarization study has been carried out in order to bridge the gap in our understanding regarding the nature of magnetic correlation obtained from the macroscopic and microscopic measurements. In particular, our study on La_0.67Ca_0.33Mn_0.9Fe_0.1O₃ has established the fact that a true double exchange mediated spin-glass is insulating. In another study of La-site ionic size effect and its disorder in (La_1−x Dy _x )_0.7Ca_0.3MnO₃, we have investigated the evolution of the length scale of magnetic ordering with a possible microscopic explanation and the results have been compared with that for the light rare earth substituted compounds.     

Percolative transport in the vicinity of charge-order ferromagnetic transition in a hole-doped manganite

We report measurements of non-linear charge transport in epitaxial (La_1−x Pr _x )_0.7Ca_0.3MnO₃ thin films fabricated on (100) oriented SrTiO₃ single crystals by pulsed laser deposition. The end members of this series, namely Pr_0.7Ca_0.3MnO₃ and La_0.7Ca_0.3MnO₃ are canonical charge-ordered (CO) and ferromagnetic manganites, respectively. The onset of the CO state in Pr_0.7Ca_0.3MnO₃ is manifested by a pronounced insulating behavior below ∼ 200 K. The CO state remains stable even when a large (∼ 2×10⁵ V/cm) electric field is applied across the thin film samples. However, on substitution of Pr with La, a crossover from the highly resistive CO state to a state of metallic character is observed at relatively low electric fields. The current-voltage characteristics of the samples at low temperatures show hysteretic and history dependent effects. The electric field driven charge transport in the system is modelled on the basis of an inhomogeneous medium consisting of ferromagnetic metallic clusters dispersed in a CO background.     

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     

Antiferromagnetic-resonance spectrum in charge-ordered R<Subscript>0.5</Subscript>Ca<Subscript>0.5</Subscript>MnO<Subscript>3</Subscript> manganites (R=La,Pr, Tb): The effect of orbital and charge structures

A theoretical study is made into the effect of the crystal, orbital, and charge structures on the magnetic structure and spin-wave spectra and on the antiferromagnetic resonance (AFMR) for R_0.5Ca_0.5MnO₃ crystals of monoclinic structure. The model assumes fixed crystal, charge, and orbital structures and enables one to determine the orbitally dependent exchange interaction and single-ion anisotropy for R = La, Pr, Tb. A 16-sublattice weakly noncollinear magnetic CE-structure without a ferromagnetic component is obtained. The behavior of magnetic structure in an external magnetic field is simulated, and the values of fields of spin-flop-transition for different Rs are obtained. The law of spin-wave dispersion and the field dependence of the antiferromagnetic-resonance spectrum are calculated.     

Structure and magnetic order in La0.7Ca0.3Mn0.5Co0.5O3 and La0.8Sr0.2Mn0.5Co0.5O3 perovskites

In this paper we report the study of the perovskites La_0.7Ca_0.3Mn_0.5Co_0.5O₃ and La_0.8Sr_0.2Mn_0.5Co_0.5O₃ by neutron powder diffraction at various temperatures and magnetization measurements in zero applied field and at low cooling regimes. The replacement of half Mn by Co in La_0.7Ca_0.3MnO₃ and La_0.8Sr_0.2MnO₃ destroys their long-range ferromagnetism exhibiting a cluster glass ferromagnetic order similar to the one observed in many cobaltites.     

Temperature dependence of infrared conductivity of manganites (x = 0, 0.05 and 0.2)

The temperature dependence of the reflectivity spectra of three manganites ceramics with compositions Pr_0.7Ca_0.3MnO₃, Pr_0.7Ca_0.25Sr_0.05MnO₃ and Pr_0.7Ca_0.1Sr_0.2MnO₃ has been investigated by infrared reflectivity spectroscopy in the wave number range 0.005-1.1 eV. The compound Pr_0.7Ca_0.25Sr_0.05MnO₃ which shows the largest conductivity jump at the ferromagnetic-paramagnetic phase transition has been studied in details. The optical conductivity of this compound is deduced from the best fit to reflectivity spectra of a “double-damping Drude” model, itself derived from the factorized form of the dielectric function. Excellent agreement with Kramers-Kronig transformation is reported. The model allows in particular to discriminate the contributions to the optical conductivity of trapped charges (polarons) and mobile charge carriers. Received 20 July 1999 and Received in final form 15 October 1999     

Nature of the low-energy excitations of a charge-ordered phase of La<Subscript>0.25</Subscript>Ca<Subscript>0.75</Subscript>MnO<Subscript>3</Subscript> manganites

The nature of the low-energy excitations of polycrystalline and nanostructured La_0.25Ca_0.75MnO₃ samples has been analyzed in order to investigate the mechanisms of charge ordering in manganites. It has been found that the electrodynamic response spectra of La_0.25Ca_0.75MnO₃ in the energy range of 0.5 to 90 meV and the temperature range of 5 to 300 K have no resonance features that could be attributed to the collective excitations of the charge-ordered phase. It has been shown that the absorption lines observed at frequencies of 20–40 and 80–100 cm^–1 are attributed to usual acoustic phonons becoming optically active owing to the structure phase transition and the appearance of a fourfold superstructure with a quadruple period along the crystallographic a axis. The suppression of the superstructure has been revealed in samples with nanocrystallites (≤40 nm). This suppression indicates a relatively weak coupling of the charge and magnetic order parameters with the phonon subsystem.     

A method for the experimental determination of the net atomic charge via X-ray photoemission spectroscopy

A method for the experimental determination of the net charge distribution over the atoms in a large class of compounds is described. The method is based on X-ray photoemission spectroscopy and requires the determination of two core-level energies per constituting atomic species. Two special cases of large conceptual and practical interest are discussed: the silicon charge in CoSi₂, Si_1-xGe_x (0≤x≤0.3), SiC, Si₃N₄ and SiO₂, and the aluminum charge in aluminum-doped zeolites. Received: 20 March 2000 / Accepted: 28 March 2000 / Published online: 21 March 2001     

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.     

Response of acoustic phonons to charge and orbital order in the 50% doped bilayer manganite LaSr2Mn2O7

We report an inelastic neutron scattering study of acoustic phonons in the charge and orbitally ordered bilayer manganite LaSr(2)Mn(2)O(7). For excitation energies less than 15 meV, we observe an abrupt increase (decrease) of the phonon energies (linewidths) of a transverse acoustic phonon branch at q = (h, h, 0), h ≤ 0.3, upon entering the low temperature charge and orbital ordered state (T(COO) = 225 K). This indicates a reduced electron-phonon coupling due to a decrease of electronic states at the Fermi level leading to a partial removal of the Fermi surface below T(COO) and provides direct experimental evidence for a link between electron-phonon coupling and charge order in manganites.     

Electronic excitations in hole-doped La1−xSrxMnO3 studied by resonant inelastic X-ray scattering

We report a resonant inelastic X-ray scattering (RIXS) study on perovskite manganese oxides La_1−xSr_xMnO₃ (x=0, 0.2, and 0.4) at Mn K-absorption edge. Hole-doping effect on the electronic excitations in the strongly correlated electron systems is elucidated by comparing with undoped LaMnO₃. The scattering spectra of metallic La_0.6Sr_0.4MnO₃ show that a salient peak appears in low energies indicating the persistence of the Mott gap. At the same time, the energy gap is partly filled by doping holes and the spectral weight shifts toward lower energies. Though the peak position of the excitations shows weak dispersion in momentum dependence, RIXS intensity changes as a function of the scattering angle (2θ), which is related to the anisotropy. Furthermore, anisotropic temperature dependence is observed in La_0.8Sr_0.2MnO₃ which shows a metal-insulator transition associated with a ferromagnetic transition. We consider that the anisotropy in the RIXS spectra is possibly attributed to the correlation of the orbital degrees of freedom. The anisotropy is large in LaMnO₃ with long-range orbital order, while it is small but finite in hole-doped La_1−xSr_xMnO₃ which indicates persistence of short-range orbital correlation.     

Orbital phase transition in Pr<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ca<Subscript>x</Subscript>MnO<Subscript>3</Subscript>

The phenomenological theory of phase transition in Pr_0.6 Ca_0.4 MnO₃ manganite is developed. It is shown that this is the orbital phase transition and that the two electronic states of the manganese ion, which are discussed in the literature, result from two different types of condensation of the same orbital order parameter. Thus, the manganese ions in Pr_1?xCa_x MnO₃ manganites with 0.3≤_x≤0.5 may be in either of the two electronic states, depending on the thermodynamic parameters.     

Magnetic phase coexistence in Tb- and Sr-doped La0.7Ca0.3MnO3 manganite:A temperature-dependent neutron diffraction study

We report the results of the temperature-dependent neutron diffraction measurements on the nearly half-doped (La_0.325Tb_0.125)(Ca_0.3Sr_0.25)MnO₃ manganite sample. The simultaneous doping of magnetic Tb³⁺ and divalent Sr²⁺ in the La_0.7Ca_0.3MnO₃ system results into a large A-site size disorder. Rietveld refinement of neutron diffraction data reveal that the single phase sample crystallizes in a distorted orthorhombic structure. Increased 〈r_A〉 value affects the transport behavior that results into an insulating-like behavior of the sample. Under application of 1 T field sample exhibit insulating-like behavior while insulator-metal transition (T_IM) is exhibited under 5 and 8 T fields. Variable range hoping (VRH) mechanism of charge carriers is exhibited in the insulating region. Field cooled and zero field cooled magnetization measurement shows the Curie temperature (T_C)~47 K. The refinement of the ND data collected at various temperatures below 300 K shows that there is no structural phase transition in the compound. Around 100 K, a magnetic peak appears at lower angle that can be ascribed to the presence of the A-type antiferromagnetic (AFM) phase. Two more peaks are observed around 50 K at lower angles that can be fitted in CE-type antiferromagnetic phase. Splitting of the peaks at lower temperatures is the signature of orbital ordering in the presently studied nearly half-doped manganite system. Results of the detailed structural analysis of the temperature-dependent ND measurements on (LaTb)_0.45(CaSr)_0.55MnO₃ sample has been discussed in the light of coexisting A-type and CE-type antiferromagnetic phases present in the sample at low temperature.     

O1s and Mn2p NEXAFS on single-layered La1-xSr1+xMnO4: crystal field effect versus orbital coupling mechanism

O1s and Mn2p near-edge X-ray absorption spectroscopy on La_1-xSr_1+xMnO₄ (0 ≤x ≤0.5) single crystals shows that Sr doping does not only provide holes to the system but also induces a continuous transfer of electrons from out-of-plane d_3z2-r2 to in-plane d_3x2-r2/d_3y2-r2 orbitals. Furthermore, a non-vanishing electron occupation of in-plane d_x2-y2 and out-of-plane d_3z2-r2 orbitals is observed up to relatively high doping contents. These findings demonstrate that the energy difference between all these orbital types has to be very small and manifest that the orbital degree of freedom is determined not just by crystal field effects but also by orbital coupling. Moreover, the doping-dependent transfer of spectral weight observed in the current data identifies La_1-xSr_1+xMnO₄ as a charge-transfer insulator.     

Transport and Magnetic Properties of Eu and Sr Doped Manganite Compound La0.7Ca0.3MnO3

The effect of simultaneous increase in carrier density and size-disorder on the transport and magnetic properties of La_0.7Ca_0.3MnO₃ has been investigated by studying the (La_{0.7 − 2x} Eu _x )(Ca_0.3Sr _x )MnO₃ (0.05 ≤ x ≤ 0.2) (LECSMO) compounds. These compounds have been compared with standard La_{1 − x} Ca _x MnO₃ (0.3 ≤ x ≤ 0.5) (LCMO) in which carrier density alone varies. In LECSMO, the insulator-metal transition temperature (T _p) decreases from 180 K for x = 0.05 to 80 K for x = 0.15 sample vis-à-vis ∼240 K for x = 0.35 to ∼225 K for x = 0.45 in LCMO system. Similarly, the Curie temperature (T _C) in LECSMO, decreases from 205 K for x = 0.05 to 75 K for x = 0.2 sample against 240 K for x = 0.35 to ∼220 K for x = 0.45 in LCMO system. Also, in LCMO the T _C and T _p are coincident whereas, in LECSMO system, with increasing x there is an increasing disparity between the two. At 5 K, in the metallic region, a large MR (>90%) is observed in x = 0.15 sample and is discussed in terms of phase segregation and inter-grain magnetoresistance.     

Electrical transport in La1−x Ca x MnO3 thin films at low temperatures

We report here the low-temperature resistivity of the chemical solution deposited La_1−x Ca _x MnO₃ (x=0.2, 0.3 and 0.33) thin films on LaAlO₃ substrates. The films were post-annealed in atmosphere at 850°C. The low temperature resistivity data has been studied in order to understand the nature of low-temperature conduction processes. The data showed T ² dependence from 60 K to 120 K consistent with the single magnon scattering process. The deviation from this quadratic temperature dependence at low temperatures is attributed to the collapse of the minority spin band. The two-magnon and electron-phonon processes contribute to scattering of carriers in the temperature range above 120 K.     

Phase transitions in La1−x CaxMnO3−x/2 manganites

The crystal structure parameters and magnetic and electrical properties of La_1?x Ca_xMnO_3?x/2 reduced manganites with 0≤x≤0.5 are established. These investigations contribute to the understanding of magnetic interactions in manganites without Mn⁴⁺ ions. It is found that these manganites show a long-range antiferromagnetic order up to x=0.09 and transform into spin glasses at 0.09<x≤0.35. The compositions in the range 0.35<x≤0.5 show a strong increase in the spontaneous magnetization and critical point associated with the appearance of spontaneous magnetization and can therefore be viewed as inhomogenious ferromagnets. The magnetic and crystal structure peculiarities of La_0.5Ca_0.5MnO_2.75 are established by the neutron diffraction method. The strongly reduced samples show a large magnetoresistance below the point where the spontaneous magnetization develops. The magnetic phase diagram of La_1?x Ca_xMnO_3?x/2 is established by magnetization measurements. The magnetic behavior is interpreted assuming that the Mn³⁺-O-Mn³⁺ magnetic interaction is anisotropic (positive-negative) in the orbitally ordered phase and isotropic (positive) in the orbitally disordered phase. Introduction of the oxygen vacancies changes the magnetic interaction sign from positive to negative, thereby leading to a spin glass state in strongly reduced compounds. The results obtained reveal unusual features of strongly reduced manganites such as a large ferromagnetic component, a high magnetic ordering temperature, and a large magnetoresistance despite the absence of Mn³⁺-Mn⁴⁺ pairs. In order to explain these results, the oxygen vacancies are supposed to be ordered.     

Magnetization and resistivity steps in the phase separated PrCaMn NiO manganites

The low temperature magnetic and transport properties of the Pr_0.5Ca_0.5Mn_1-xNi_xO₃ manganites ( 0≤ x ≤0.1) have been investigated. The presence of Ni hinders the charge and orbital ordering observed in Pr_0.5Ca_0.5MnO₃ and favors the creation of ferromagnetic regions, leading to phase separation. The ferromagnetic fractions induced by the Ni substitution have been estimated from magnetization measurements, they are large and reach 40% for 4% of Ni. Steps are observed in the M ( H ) and ρ( H ) curves of all the samples at T < 5 K. They are similar to the steps observed in Pr_0.5Ca_0.5Mn_1-xM_xO₃, where M is a non magnetic cation (Mg²⁺, Ga³⁺,...), and for which the ferromagnetic fractions are very small (less than 2%), however, their appearance is restricted to lower temperatures (T < 5 K) with Ni dopant than with non magnetic cations. This study shows that steps can be observed in a wide range of phase-separated systems, even when the ferromagnetic fraction is very large. Received 5 April 2002 / Received in final form 8 July 2002 Published online 14 October 2002 RID="a" ID="a"e-mail: antoine.maignan@ismra.fr     

Effects of the surface layer of La2/3Ca1/3MnO3 thin films

The surface layer effects on transport in epitaxial La_2/3Ca_1/3MnO₃ thin films are studied. It was found that the two-probe resistance is nonlinear which is enhanced with decreasing temperature. Similar to the resistance of intrinsic La_2/3Ca_1/3MnO₃ thin films reported in the literature, the apparent dynamic contact resistance behaves semiconducting at high temperatures, passes through a peak, and displays a metallic behavior. At lowest temperatures, the curve of the contact resistance versus temperature shows a little upturn. The temperature dependent work function difference between the surface layer and the thin film underneath, together with the tunneling process across either the resulting charge depleted layer or the semiconducting surface layer is used to explain our observations.     

Effect of Y-doping on the magnetic and charge orderings in Nd2/3Ca1/3MnO3

The shifts of the magnetic and charge ordering transition temperatures caused by Nd substitution for Y in Nd_2/3Ca_1/3MnO₃ CMR narrow-band perovskite manganite have been studied. At low temperatures, three different long-range magnetic orderings consistent with a phase separation scenario have been observed in the doped compound (Nd_0.9Y_0.1)_2/3Ca_1/3MnO₃ by neutron-diffraction study: the antiferromagnetic orderings of PCE and DE types existing below ∼110 and ∼60 K, respectively, and the ferromagnetic one of B type existing below ∼42. Magnetic phase transformations temperatures as well as those of charge ordering have been found to be structural-dependent. Y-doping leads to the decrease of the anisotropy of the orthorhombic Pnma crystal lattice b/√2c, which causes a decrease of the indirect exchange parameters in the system and thus a decrease in the magnetic transformation temperatures for 20-30 K in the doped compound. Doping leads as well to the higher level of the coherent Jahn-Teller distortions of the MnO₆ octahedra in the 200-300 K temperature region, which results in the increase of the charge ordering temperature for ∼80 K.