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1.
S. M. Yusuf 《Pramana》2004,63(1):133-141
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 La0.7−xCaxMnO3 (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 La0.67Ca0.33Mn0.9Fe0.1O3 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 (La1−x
Dy
x
)0.7Ca0.3MnO3, 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. 相似文献
2.
We report measurements of non-linear charge transport in epitaxial (La1−x
Pr
x
)0.7Ca0.3MnO3 thin films fabricated on (100) oriented SrTiO3 single crystals by pulsed laser deposition. The end members of this series, namely Pr0.7Ca0.3MnO3 and La0.7Ca0.3MnO3 are canonical charge-ordered (CO) and ferromagnetic manganites, respectively. The onset of the CO state in Pr0.7Ca0.3MnO3 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. 相似文献
3.
Interplay of charge, spin, orbital and lattice correlations in colossal magnetoresistance manganites
A. Weiße H. Fehske 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,30(4):487-494
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 相似文献
4.
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 R0.5Ca0.5MnO3 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. 相似文献
5.
H. Gamari-Seale I.O. Troyanchuk K.L. Stefanopoulos 《Physica B: Condensed Matter》2008,403(17):2924-2929
In this paper we report the study of the perovskites La0.7Ca0.3Mn0.5Co0.5O3 and La0.8Sr0.2Mn0.5Co0.5O3 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 La0.7Ca0.3MnO3 and La0.8Sr0.2MnO3 destroys their long-range ferromagnetism exhibiting a cluster glass ferromagnetic order similar to the one observed in many cobaltites. 相似文献
6.
N. Petit C. Daulan J.-C. Soret A. Maignan F. Gervais 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,14(4):617-625
The temperature dependence of the reflectivity spectra of three manganites ceramics with compositions Pr0.7Ca0.3MnO3, Pr0.7Ca0.25Sr0.05MnO3 and Pr0.7Ca0.1Sr0.2MnO3 has been investigated by infrared reflectivity spectroscopy in the wave number range 0.005-1.1 eV. The compound Pr0.7Ca0.25Sr0.05MnO3 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 相似文献
7.
B. P. Gorshunov E. S. Zhukova E. G. Maksimov A. S. Prokhorov V. I. Torgashev T. Zhang D. Wu M. Dressel 《JETP Letters》2010,91(7):336-340
The nature of the low-energy excitations of polycrystalline and nanostructured La0.25Ca0.75MnO3 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 La0.25Ca0.75MnO3 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. 相似文献
8.
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, Si1-xGex (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 相似文献
9.
A. Maignan Ch. Simon V. Caignaert B. Raveau 《Zeitschrift für Physik B Condensed Matter》1995,99(1):305-310
The exploration of the magnetic and transport properties of four series of manganese perovskites, Pr0.7Ca0.34−xAxMnO3−δ (A=Sr, Ba), Pr0.7−xLaxCa0.3 MnO3−δ and Pr0.66Ca0.34−x
SrxMnO3−δ has allowed four phases with colossal magnetoresistive (CMR) properties to be isolated: Pr0.7Ca0.25Sr0.025MnO3−δ and Pr0.66Ca0.26Sr0.08MnO3−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, Pr0.58La0.12Ca0.3 MnO3−δ that exhibits a variation of 6.106% for μ0 H=5 T at T=80 K and Pr0.7Ba0.025Ca0.275MnO3−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 Tmax, 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 Pr0.7Ca0.3−xSrxMnO3−δ and Pr0.66Ca0.34−xSrxMnO3−δ shows also the crucial role of the hole carrier density: for a same mean ionic radius of the interpolated cation Tmax is decreased of about 50 K by introducing 0.034 hole per Mn mole. 相似文献
10.
Weber F Rosenkranz S Castellan JP Osborn R Zheng H Mitchell JF Chen Y Chi S Lynn JW Reznik D 《Physical review letters》2011,107(20):207202
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. 相似文献
11.
K. Ishii T. Inami K. Kuzushita Y. Murakami S. Ishihara S. Maekawa Y. Moritomo 《Journal of Physics and Chemistry of Solids》2005,66(12):2157-2162
We report a resonant inelastic X-ray scattering (RIXS) study on perovskite manganese oxides La1−xSrxMnO3 (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 La0.6Sr0.4MnO3 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 La0.8Sr0.2MnO3 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 La1−xSrxMnO3 which indicates persistence of short-range orbital correlation. 相似文献
12.
The phenomenological theory of phase transition in Pr0.6 Ca0.4 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 Pr1?xCax MnO3 manganites with 0.3≤x≤0.5 may be in either of the two electronic states, depending on the thermodynamic parameters. 相似文献
13.
R.R. Doshi P.S.R. Krishna D.G. Kuberkar 《Journal of magnetism and magnetic materials》2009,321(19):3285-3289
We report the results of the temperature-dependent neutron diffraction measurements on the nearly half-doped (La0.325Tb0.125)(Ca0.3Sr0.25)MnO3 manganite sample. The simultaneous doping of magnetic Tb3+ and divalent Sr2+ in the La0.7Ca0.3MnO3 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 〈rA〉 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 (TIM) 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 (TC)~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. 相似文献
14.
M. Merz P. Reutler B. Büchner D. Arena J. Dvorak Y. U. Idzerda S. Tokumitsu S. Schuppler 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,51(3):315-319
O1s and Mn2p near-edge X-ray absorption spectroscopy on
La1-xSr1+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 d3z2-r2 to in-plane d3x2-r2/d3y2-r2 orbitals. Furthermore, a
non-vanishing electron occupation of in-plane dx2-y2 and out-of-plane d3z2-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 La1-xSr1+xMnO4 as a charge-transfer insulator. 相似文献
15.
D. S. Rana C. M. Thaker K. R. Mavani J. H. Markna R. N. Parmar N. A. Shah D. G. Kuberkar S. K. Malik 《Hyperfine Interactions》2005,160(1-4):193-197
The effect of simultaneous increase in carrier density and size-disorder on the transport and magnetic properties of La0.7Ca0.3MnO3 has been investigated by studying the (La0.7 − 2x
Eu
x
)(Ca0.3Sr
x
)MnO3 (0.05 ≤ x ≤ 0.2) (LECSMO) compounds. These compounds have been compared with standard La1 − 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. 相似文献
16.
We report here the low-temperature resistivity of the chemical solution deposited La1−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. 相似文献
17.
I. O. Troyanchuk S. V. Trukhanov H. Szymczak J. Przewoznik K. Bärner 《Journal of Experimental and Theoretical Physics》2001,93(1):161-167
The crystal structure parameters and magnetic and electrical properties of La1?x CaxMnO3?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 La0.5Ca0.5MnO2.75 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 La1?x CaxMnO3?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. 相似文献
18.
S. Hébert A. Maignan V. Hardy C. Martin M. Hervieu B. Raveau R. Mahendiran P. Schiffer 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,29(3):419-424
The low temperature magnetic and transport properties of the Pr0.5Ca0.5Mn1-xNixO3 manganites ( 0≤
x
≤0.1) have been investigated. The presence of Ni hinders the charge and orbital ordering observed in Pr0.5Ca0.5MnO3 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 Pr0.5Ca0.5Mn1-xMxO3, 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 相似文献
19.
The surface layer effects on transport in epitaxial La2/3Ca1/3MnO3 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 La2/3Ca1/3MnO3 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. 相似文献
20.
Elena Fertman Anatoly Beznosov Vladimir Desnenko Alexander Feher 《Journal of magnetism and magnetic materials》2009,321(4):316-320
The shifts of the magnetic and charge ordering transition temperatures caused by Nd substitution for Y in Nd2/3Ca1/3MnO3 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 (Nd0.9Y0.1)2/3Ca1/3MnO3 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. 相似文献