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1.
In this paper, the microscopic theory of the relative change in velocity of sound with temperature of La0.5Ca0.5MnO3 is reported. The phonon Green function is calculated using the Green function technique of Zubarev in the limit of zero wave vector and low temperature. The lattice model electronic Hamiltonian in the presence of the phonon interaction with hybridization between the conduction electrons and the l-electrons is used. The relative change in velocity of sound at various temperatures is studied for different model parameters, namely the position of the l-level, the effective phonon coupling strength and hybridization strength. The phonon anomalies observed experimentally at different temperatures are explained theoretically. An abrupt change in velocity at Neel temperature (TN) is observed clearly. It is observed that different parameters influence the velocity of sound.  相似文献   

2.
The LaxCa1−xMnO3+δ compositions close to charge ordering (x∼0.5) show a gradual relaxation from a metallic/ferromagnetic state to an insulating/antiferromagnetic state with thermal cycling. Here, we report on the magnetic relaxation in the metastable state and also the revival of the metastable state (in a relaxed sample) due to high temperature thermal treatment. We also show the changes in the magnetization and the thermoelectric power as the revived metastable state is cycled. We find that the changes in the thermoelectric power extend well into the region above the charge ordering temperatures. This suggests that the micro-structural changes accompanying the thermal cycling leave their imprint in the paramagnetic insulating state as well.  相似文献   

3.
The influence of dc current on the resistivity ρ and the Young's modulus E of La0.5Ca0.5MnO3 compound has been investigated by means of an in situ measuring method. At low temperatures, both the resistivity ρ and the relative modulus ΔE increase with the current. A relaxation behavior of ρ to the higher resistive state is observed at a fixed temperature and a constant current. After storing the sample for a few days, ρ decreases with the current, accompanying a slight drop of ΔE at low temperatures. Current-induced effects on ρ and ΔE are interpreted according to the current-induced interwinning of Mn3+O6 octahedral distortion modes between Q2- and Q3-types, which is suggested to contribute to the variation of the resistivity.  相似文献   

4.
Strain in the La0.67Ca0.33MnO3 films has been tuned by varying substrate and film thickness, and its effects on magnetic anisotropy are studied based on the measurements of isothermal magnetization. Measuring the strain in the films by the out-of-plane lattice parameter (c), we found a strong dependence of the magnetic anisotropy constant (Ku) on strain. Ku decreases linearly from ∼−1.1×106 erg/cm3 for c=0.763 nm to 1.2×106 erg/cm3 for c=0.776 nm, corresponding to a change from tensile strain to compressive strain. Positive Ku signifies a uniaxial anisotropy with the easy axis perpendicular to the film plane, while negative Ku demonstrates an anisotropy of the easy plane character. Smaller or larger c leads a decrease or increase in Ku, which indicates the presence of other effects in addition to those associated with strain. Three distinctive processes for the magnetization are observed along the hard magnetic axis of the films on (001)SrTiO3, suggesting a possibility of strain relaxation even in ultra-thin films.  相似文献   

5.
Polycrystalline samples of La0.67Ca0.33MnO3 were prepared by solid-state reactions by varying the pelletization force and the sintering temperature. Lowering the sintering temperature gave rise to smaller grains and increased the overall resistivity of the ceramic. Partial melting was observed in the ceramics sintered at higher temperatures (1400-1500 °C). Additionally, these ceramics showed two distinct resistivity peaks. The resistivity peak near the magnetic transition (TC) was sharp, whereas the second peak was a broad one observed below TC.  相似文献   

6.
An enhanced magnetoresistance and a two-fold effect result from impurity dopant were observed in composites of La0.67Ca0.33MnO3/YSZ and La0.67Ca0.33MnO3/Fe3O4. Where YSZ represents yttria-stabilized zirconia and the doping level of both YSZ and Fe3O4 is 1 mol%. Different electrical and magnetic transport properties, in particular a lower field magnetization behavior, were observed between pure La0.67Ca0.33MnO3 and the impurity doped La0.67Ca0.33MnO3 composites. Compared with pure La0.67Ca0.33MnO3, a possible interpretation is presented by considering the influences of YSZ and Fe3O4 on the structure of grain boundaries and/or surfaces of La0.67Ca0.33MnO3grains.  相似文献   

7.
The role of vibrational anisotropy of Mn3+O6 octahedron in the phase separation behavior of La0.67−yPryCa0.33MnO3 (x=0, 0.15, 0.25 and 0.30) has been investigated by means of magnetization M, internal friction Q−1, Young's modulus E along with the X-ray powder diffraction measurements. For the samples with y=0 and 0.15, the Q−1 exhibits three peaks in the ferromagnetic region, which are attributed to the intrinsic inhomogeneity of ferromagnetic phase, i.e. the electronic phase separation with the coexistence of insulating and conducting phases. However, both the samples with y=0.25 and 0.30 undergo a magnetic phase separation with the coexistence of the antiferromagnetic and ferromagnetic phases, and the Q−1 peaks related to the electronic phase separation have not been observed. In addition, the Q−1 exhibits a peak in the paramagnetic region for all samples, which may result from the formation of magnetic clusters. We observed that the evolution from electronic to magnetic phase separation is close related to the rapid increase in the ratio of two kinds of Jahn-Teller distortion modes Q3 and Q2, i.e. Q3/Q2. A schematic phase diagram is given in the text, and it is suggested that the enhancement of vibrational anisotropy of Mn3+O6 octahedron plays a key role in the evolution from electronic to magnetic phase separation.  相似文献   

8.
Effects of doping Na on the structure, electrical and magnetic properties of La2/3Ca1/3MnO3 are investigated. A structural phase transition from orthorhombic to rhombohedral structure takes place at y=0.375. All samples show metal-insulator (M-I) transition at the transition temperature and undergo the transition from paramagnetism to ferromagnetism at the Curie temperature TC. and TC increase monotonically with increasing Na content. However the Na-doped samples have a shoulder in their electrical transport curves found below and shows a widened magnetic transition process. On the other hand, intrinsic colossal magnetoresistance (CMR) peaks are observed in all the samples, but samples with y around 0.25 show two MR peaks which can be attributed to magnetic inhomogeneity induced by the doped Na+ ions. Here we propose a method to broaden the CMR peak of perovskite manganite, which is beneficial for practical applications.  相似文献   

9.
The influence of SiO2 on the electrical transport properties of LCMO/SiO2 composites with different SiO2 contents x is investigated, where LCMO represents La2/3Ca1/3MnO3. Results show that the SiO2 phase not only shifts the metal–insulator transition temperature (Tp) to a high temperature range, but also has an effect on the magnetoresistance (MR) of the composites. The temperature dependence of resistivity indicates that the Tp of the composites is obviously higher than that of pure LCMO, and that the peak resistivity ρmax of the composites is lower than that of pure LCMO. In the SiO2 content x∼0.02, the TP is the highest and ρmax becomes the lowest. The experimental observation is discussed on the basis of the analysis of scanning electron microscopy (SEM) images and X-ray diffraction (XRD) patterns. Compared with pure LCMO, a possible interpretation is presented by considering the influence of SiO2 on the coupling between ferromagnetic (FM) domains of LCMO.  相似文献   

10.
Rare-earth-based manganites ABO3 may present interesting properties when the lanthanide (A-site) and/or the manganese (B-site) are partially substituted by divalent elements. Heavy lanthanides are particularly appealing because of the expected interplay between the intrinsic magnetic properties of the rare-earth element (Ln) and those of the ferromagnetic manganese sublattice. As such, a spin reorientation has been observed during magnetization-versus-temperature cycles due to a negative exchange interaction between Mn and Ln. We present herein high-quality epitaxial thin films (∼200 nm thick) of Gd0.67Ca0.33MnO3 deposited onto (1 0 0) SrTiO3 substrates by pulsed-laser deposition. Enhanced properties were observed in comparison with bulk samples. The magnetic transition temperature Tc of the as-grown films is much higher than the corresponding bulk values. Most interesting, magnetization measurements performed under small applied fields, exhibit magnetization reversals below Tc, no matter whether the film is field-cooled (FC) or zero-field-cooled (ZFC). The reversal mechanism is discussed in terms of a negative exchange f-d interaction and magnetic anisotropy, this latter enhanced by strain effects induced by the lattice mismatch between the film and the substrate.  相似文献   

11.
X-band electron magnetic resonance (EMR) measurements were done at 115?T?600 K on bulk and nanometer size-grain powder single-crystalline samples of La0.9Ca0.1MnO3, in order to study an impact of structural inhomogeneity on magnetic ordering. For the nano-crystal sample, two superimposed EMR lines are observed below 240 K, while for bulk-crystal one, a second line emerges in narrow temperature interval below 130 K. Temperature dependences of resonance field and line width of the main and the secondary line are drastically different. EMR data and complementary magnetic measurements of bulk-crystal sample reveal mixed-magnetic phase, which agrees with the published phase diagram of bulk La1−xCaxMnO3. In a marked contrast, the same analysis for nano-crystal sample shows two phases one of which is definitely ferromagnetic (FM) and other is likely such, or super paramagnetic. The data obtained are interpreted in terms of very different magnetic ground states in the two samples, that is attributed to different randomness of the indirect FM exchange interactions mediated by bound holes.  相似文献   

12.
The highly metastable region across the metal-insulator transition in Nd0.25La0.25Ca0.5MnO3 is probed by transport and magnetic measurements. The metal-insulator transition observed around 130 K is associated with large thermal hysteresis between temperatures 50-150 K. Observation of minor hysteresis loops across the region indicates coexistence of both metallic and insulating phases. The field-cooled heating and the zero-field-cooled heating susceptibilities deviate from each other from 205 K, signifying thermo-magnetic irreversibility arising from magnetic pinning and/or glassy magnetic behaviour. The magnetoresistance measured on the heating and cooling loops are found to be different in magnitude and nature. The cooling-cycle magnetoresistance was found to be highly irreversible with an open-ended hysteresis loop. We also observe change in the nature of the magnetic relaxation data on the heating and cooling protocols. This indicate strong thermal history dependence of magneto-transport behaviour in the present compound.  相似文献   

13.
The temperature dependence of the resistivity for composite samples of (1−x)La0.67Ba0.33MnO3+xYSZ(LBMO/YSZ) with different YSZ doping level of x has been investigated in a magnetic field range of 0-7000 Oe, where the YSZ represents yttria-stabilized zirconia (8 mol% Y2O3+92 mol% ZrO2). With increasing YSZ doping level, the range of 0-10%, the metal-insulator transition temperature (TP) decreases. However, the resistivity, specially the low temperature resistivity, increases. Results also show that the YSZ doping level has an important effect on a low field magnetoresistance (LFMR). In the magnetic field of 7000 Oe, a room temperature magnetoresistance value of 20% was observed for the composite with a YSZ doping level of 2%, which is encouraging for potential application of CMR materials at room temperature and low field.  相似文献   

14.
The influence of Fe3O4 contents on the electrical transport properties (resistivity and ac susceptibility) of a series of composite samples of La0.67Ca0.33MnO3/Fe3O4 is studied. Results show that the Fe3O4 phase not only shifts the intrinsic insulator-metal (I-M) transition temperature TP1 to a lower temperature, but also causes a new I-M transition at a lower temperature TP2 (TP2<TP1). On the basis of an analysis by scanning electron microscopy and X-ray diffraction, we suggest that the decrease of the I-M transition temperature and the formation of the new I-M transition are caused by the segregation of a new phases related to the Fe3O4 at grain boundaries or surfaces of the La0.67Ca0.33MnO3 grains.  相似文献   

15.
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   

16.
Visible photoluminescence and its temperature dependence of La2/3Ca1/3MnO3 in the temperature range 138-293 K were measured. It was observed that the main broad band centered at ∼1.77 eV with the shoulders at ∼1.57 and ∼1.90 eV existed in the entire temperature range. It can be well fitted by three Gaussian curves B1, B2 and B3 centered at ∼1.52, ∼1.75 and ∼1.92 eV, respectively. The intensities of the peak B1 and B2 vary as temperature increases. In the entire temperature range, the intensity of B1 increases with increasing temperature, whereas that of B2 decreases. The photoluminescence mechanisms for La2/3Ca1/3MnO3 are presented based on the electronic structures formed by the interactions among spin, charge and lattice, in which B1 was identified with the charge transfer excitation of an electron from the lower Jahn-Teller split eg level of a Mn3+ ion to the eg level of an adjacent Mn4+ ion, B2 is assigned to the transition between the spin up and spin down eg bands separated by Hund's coupling energy EJ and B3 is attributed to the transition, determined by the crystal field energy EC, between a t2g core electron of Mn3+ to the spin up eg bands of Mn4+ by a dipole allowed charge transfer process.  相似文献   

17.
The evolution of magnetic and electrical phases in La0.8−δCa0.2MnO3 was investigated in terms of La deficiency. We found that the increase of the La deficiency tends to raise the Curie temperature (TC) in La0.8−δCa0.2MnO3. The FM clusters formed in compounds with large La deficiency provide percolation paths above TC. With increasing the La defect, the transport property changes from insulating to metallic state, which is in association with the crossover from a second order to a first order magnetic phase transition in the vicinity of TC.  相似文献   

18.
Upon annealing polycrystalline La0.67Ba0.33MnO3 bulk samples in flowing 95%Ar:5%H2 mixed gas at 700 °C for different time, the insulator–metal transition temperature, TPTP and the amplitude of AC magnetic susceptibility were decreased first, then increased, finally decreased again. While the resistivity was increased monotonically. This anomalous behavior was explained by the combinational effects of oxygen loss and Ba ion vacancies caused by the segregation of Ba ion related impurity phase.  相似文献   

19.
Ferromagnetic La0.7Sr0.3MnO3 (LSMO) and antiferromagnetic La0.33Ca0.67MnO3 (LCMO) layers were grown on SrTiO3 (STO) substrates by the pulsed laser deposition technique. LSMO films had rougher surfaces and larger grain sizes than LCMO films. Fully strained bilayers, in which each layer was as thin as 10 nm, were prepared by changing their stacking sequences, i.e. LSMO/LCMO/STO and LCMO/LSMO/STO. The former had higher TC (350 K) than the latter (300 K), and exchange bias effects were only observed in the former bilayers. This revealed that microstructures could play an important role in the transport and magnetic properties of manganese oxide thin films.  相似文献   

20.
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