首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 894 毫秒
1.
The temperature dependence of the resistivity and magnetic moment of La0.85Ba0.15MnO3 and La0.85Sr0.15MnO3 manganite single crystals in magnetic fields up to 90 kOe is investigated. Analysis of the experimental results shows that the magnetoresistance of lanthanum manganites far from the Curie temperature T C can be described quantitatively by the s-d model normally used for ferromagnets and taking into account only the exchange interaction between the spins of charge carriers and magnetic moments. These data also show that the features of lanthanum manganites responsible for colossal magnetoresistance (CMR) are manifested in a narrow temperature interval δT ≈ 20 K near T C. Our results suggest a CMR mechanism analogous to the mechanism of giant magnetoresistance (GMR) observed in Fe/Cr-type multilayers with nanometer layer thickness. The nanostratification observed in lanthanum manganites and required for GMR can be described taking into account the spread in T C in the CMR range δT.  相似文献   

2.
Electrical conductivity and magnetoresistance of a series of monovalent (K) doped La1−xKxMnO3 polycrystalline pellets prepared by pyrophoric method have been reported. K doping increases the conductivity as well as the Curie temperature (TC) of the system. Curie temperature increases from 260 to 309 K with increasing K content. Above the metal-insulator transition temperature (T>TMI), the electrical resistivity is dominated by adiabatic polaronic model, while in the ferromagnetic region (50<T<TMI), the resistivity is governed by several electron scattering processes. Based on a scenario that the doped manganites consist of phase separated ferromagnetic metallic and paramagnetic insulating regions, all the features of the temperature variation of the resistivity between ∼50 and 300 K are described very well by a single expression. All the K doped samples clearly display the existence of strongly field dependent resistivity minimum close to ∼30 K. Charge carrier tunneling between antiferromagnetically coupled grains explains fairly well the resistivity minimum in monovalent (K) doped lanthanum manganites. Field dependence of magnetoresistance at various temperatures below TC is accounted fairly well by a phenomenological model based on spin polarized tunneling at the grain boundaries. The contributions from the intrinsic part arising from DE mechanism, as well as, the part originating from intergrannular spin polarized tunneling are also estimated.  相似文献   

3.
We present the results a study of structure by neutron diffraction and data on the magnetic properties (linear and nonlinear (second and third order) susceptibilities) of polycrystalline La0.88MnO2.95. This compound exhibits an insulator-metal (IM) phase transition at T IM ≈ 253 K (above the Curie temperature, T C ≈ 244 K) and reveals colossal magnetoresistance. The crystal structure is found to be rhombohedral, and the space group is R3c. Analysis of magnetic properties shows that at T* ≈ 258 K > T C , isolated paramagnetic clusters occur in the paramagnetic matrix; their concentration increases upon cooling. We observed no noticeable differences between the temperature evolution of the clustered state of this manganite with its insulator-metal transition and in the insulator La0.88MnO2.91. Possible scenarios of the paramagnet-ferromagnet and I-M transitions in a self-organized clustered structure are discussed.  相似文献   

4.
This paper reports on measurements of the acoustic, magnetic, and electrical properties and on an x-ray microprobe analysis of a La0.825Sr0.175MnO3 single-crystal sample. The acoustic studies were made with a pulsed acoustic spectrometer operating on a 770-MHz carrier. The studies revealed anomalies in the damping coefficients and sound velocity near 300, 200 K, and the Curie temperature TC (283 K) where the colossal magnetoresistance occurs. The effect of a magnetic field on the magnetic texture of lanthanum manganites cooled below TC, observed earlier in samples of other composition, is confirmed. In addition, a region was found wherein the magnetic susceptibility of an unclamped sample behaves anomalously. The electrical resistivity was observed to decrease substantially below TC; this effect exhibits a hysteretic pattern in the interval 200–180 K.  相似文献   

5.
The magnetic, electrical, and optical properties of Ca1 − x Ce x MnO3 (x≤0.12) manganite single crystals are investigated with the aim of revealing the specific features of the multiphase electronic and magnetic state as a function of the cerium concentration and the atmosphere used for growing single crystals. It is found that the concentration dependence of the low-temperature magnetization M(x) of the single crystals is shifted toward the high-concentration range as compared to the corresponding dependence of the polycrystals, which is explained by the predominant cation deficiency. The electrical resistivity and the reflection spectra of the single crystals in the infrared spectral range indicate that charge carriers exhibit a band nature at temperatures close to room temperature. The temperature dependence of the electrical resistivity of the single crystal with x = 0.08, which has the maximum magnetization in the studied series of Ca1 − x Ce x MnO3 compounds, unlike polycrystals, exhibits a metallic behavior over the entire temperature range. The G-type antiferromagnetic phase with the Néel and Curie temperatures T N(G) = T C = 100 K is characterized by maxima of the electrical resistivity ρ and the magnetoresistance Δρ/ρ = |(ρ0 − ρ H )/ρ0| = 38% in the magnetic field H = 90 kOe. The magnetoresistance Δρ/ρ of the single crystals at cerium concentrations x = 0.10 and 0.12 with variations in temperature exhibit three specific features: near the temperature of charge ordering T co, near the temperature of the magnetic phase transition to the C-type antiferromagnetic phase T N(C), and near the temperature of the phase transition to the magnetic charge-ordered phase T N(MCO). An anomalous temperature dependence of the magnetization is revealed for a single crystal with x = 0.10 grown in oxygen at a pressure of 5 atm, which is explained by the presence of regions with hole conductivity due to cation deficiency. The inhomogeneous electronic and magnetic state is associated with the interrelation of the charge, orbital, and spin orderings. Original Russian Text ? N.N. Loshkareva, A.V. Korolev, N.I. Solin, E.V. Mostovshchikova, S.V. Naumov, N.V. Kostromitina, A.M. Balbashov, 2009, published in Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2009, Vol. 135, No. 1, pp. 98–107.  相似文献   

6.
The electrical conductivity and magnetoresistance effect of n and p types of CuInSe2 single crystals were studied within the temperature range of 4.2–300 K and within magnetic fields of up to 6 T. It was found that the hopping mechanism of conductivity dominates in the temperature range of 4.2–100 K. A peculiarity of electrical conductivity behavior, accompanied by a change in the magnetoresistance sign in the vicinity of T ≈ 60 K, was revealed.  相似文献   

7.
We report measurements of the specific heatC p(T), electrical resistivity ϱ(T) and magnetic susceptibility ξ(T) of hexagonal CePd2In, at low temperatures. Anomalies inC p(T), χ(T) and ϱ(T) atT=1.23 K, indicate a phase transition, most likely to an antiferromagnetically-ordered phase. The electronic entropy reachesR ln2 per mole Ce at 9.2K, suggesting that the phase transition involves a doublet state. The ordered phase coexists with moderately correlated itinerant electrons.  相似文献   

8.
Results of investigation of resistivity and magnetoresistance of manganites La1 − x K x MnO3 (x = 0.050–0.175) are presented. Behavior of resistivity ρ(T) in the paramagnetic and ferromagnetic phases has been described. To describe ρ(T) near the phase-transition temperature, notions of the percolation theory have been used. Two maxima have been found in the dependence ρ(T); their appearance has been attributed to the ceramic nature of the studied samples. The observed increase in magnetoresistance with a decrease in temperature is caused by intergranular spin-polarized tunneling of charge carriers.  相似文献   

9.
The effect of an oxygen excess δ on the magnetic and electrical properties of La1−x Ca x MnO3+δ (x=0.10–0.15) has been studied over wide ranges of temperatures and magnetic fields. As δ increases, the magnetic ordering temperature Tcdecreases by 70–90 K, the magnetoresistance increases (the electrical resistivity decreases by a factor of up to 104 in a field of 9 T), and the effective moment μeff of the paramagnetic susceptibility substantially exceeds the theoretical value at temperatures two to four times higher than T c and undergoes a jump, just as the activation energy of electrical resistivity, at T∼270 K. These results are attributed to the formation of cation vacancies, the localization of electrons in their vicinity with the subsequent formation of magnetic clusters, tunneling (or hopping) of carriers among them, changes in the sizes of clusters with variations in the temperature and magnetic field strength, the onset of frustrations initiated by the competition among different types of exchange interaction, and the dependence of the cluster parameters on the annealing conditions. Annealing of the oxygen-excess samples at high temperatures in vacuum (above 1100°C) restores the samples to a nearly initial state with the magnetic and magnetotransport properties characteristic of weakly doped manganites, as a result of the removal of cation vacancies.  相似文献   

10.
The temperature dependences of the magnetic susceptibility χ(T) and the electrical resistivity ρ(T) of ceramic samples of La1 − x Ca x MnO3 with x = 0.67 (LCMO) and La1 − x Ca x Mn1 − y Fe y O3 with x = 0.67 and y = 0.05 (LCMFO) are investigated in magnetic fields B = 50–105 G and the temperature range T = 4.2–400 K. Both samples undergo a transition from the paramagnetic state to a state with charge (orbital) ordering (CO) at temperatures T CO ≈ 272 K for LCMO and T CO ≈ 222 K for LCMFO. The behavior of the paramagnetic phase in the temperature range 320–400 K for LCMO and 260–400 K for LCMFO is described by the Curie-Weiss law with effective Bohr magneton numbers p eff = 4.83 μB (LCMO) and 4.77 μB (LCMFO), respectively. The disagreement between the observed positive Weiss temperatures (θ ≈ 175 K (LCMO) and θ ≈ 134 K (LCMFO)) and negative Weiss temperatures required for the antiferromagnetic ground state can be explained by the phase separation and transition to the charge-ordered state. The magnetic irreversibility for T < T CO is accounted for by the existence of a mixture of the ferromagnetic and antiferromagnetic phases, as well as the cluster glass phase. At low temperatures, doping with iron enhances the frustration of the system, which manifests itself in a more regular behavior of the decay rate of the remanent magnetization with time. The temperature dependence of the electrical resistivity in the range of the charge-ordered phase conforms to the variable-range hopping model. The behavior of the electrical resistivity is governed by the complex structure of the density of localized states near the Fermi level, which includes a soft Coulomb gap Δ = 0.464 eV for LCMO and 0.446 eV for LCMFO. It is established that the ratio between the localization radii of charge carriers a for LCMFO and a und for LCMO is a/a und = 0.88. Original Russian Text ? V.S. Zakhvalinskiĭ, R. Laiho, T.S. Orlova, A.V. Khokhulin, 2008, published in Fizika Tverdogo Tela, 2008, Vol. 50, No. 1, pp. 61–68.  相似文献   

11.
12.
In this work, we report the behavior of electrical resistivity of SmB6 at temperatures between 2.2 and 70 K in pulsed magnetic fields up to 54 T. A strong negative magnetoresistance was detected with increasing magnetic field, when lowering the temperature in the range T<30 K. We show that the amplitude of negative magnetoresistance reaches its maximum dR/R~70% at B=54 T, in the vicinity of phase transition occurring in this strongly correlated electron system at TC~5 K. The crossover from negative magnetoresistance to positive magnetoresistance found at intermediate temperatures at T>30 K is discussed within the framework of exciton-polaron model of local charge fluctuations in SmB6 proposed by Kikoin and Mishchenko. It seems that these exciton-polaron in-gap states are influenced both by temperature and magnetic field.  相似文献   

13.
Variations in the thermopower, electrical resistivity, magnetoresistance, thermal expansion coefficients, and their anisotropy with temperature were detected near room temperature in single crystals of weakly doped lanthanum manganites La1?xAxMnO3 (A = Ca, Sr; x = 0.07–0.125) with orthorhombic structure. The results obtained are discussed in terms of a model of phase separation related to polaron anisotropy. Due to a gain in exchange and elastic energies in the lattice, small-radius magnetic polarons can merge to form polarons of a larger size, which would contain now not one but rather a few electrons (equal in number to the polarons in the cluster). As a result, short-range order in a cluster and phase separation set in at a temperature Tps ≈ 250–300 K, which is approximately equal to the Curie temperature TC of conducting manganites with x ≈ 0.2–0.3.  相似文献   

14.
The temperature dependences of the velocities of transverse and longitudinal hypersound in lanthanum strontium manganites of the composition La1−x Sr x MnO3 (x = 0.125, 0.150, 0.175) have been measured at frequencies of 0.5–0.7 GHz. The structural phase transitions have been revealed, and their positions have been confirmed by data on the electrical resistance and magnetic measurements performed using the same samples. The results obtained have been analyzed in the framework of the model of competing Jahn-Teller distortions and magnetic ordering. The anomalies observed in the behavior of the velocities of longitudinal hypersound have been attributed to the local Jahn-Teller distortions, and their suppression due to the magnetic ordering has been considered a possible factor responsible for the colossal magnetoresistance.  相似文献   

15.
A study is reported of the magnetic, electrical, and crystallographic properties of La1−x SrxMnO3 (0.15⩽x⩽0.23) epitaxial films grown on single-crystal substrates of (001)ZrO2(Y2O3) having the fluorite structure and (001)LaAlO3 having the perovskite structure. It was found that films with close compositions for x=0.15 and 0.16, grown on different substrates, have different properties, namely, the film on a fluorite substrate is semiconducting and has a coercive strength 30 times that of the film on a perovskite substrate; the temperature dependence of electrical resistance of the latter film has a maximum around the Curie point T C and follows metallic behavior for T<T C. These differences are explained as due to different structures of the films. The x=0.23 film on the perovskite substrate has been found to exhibit a combination of giant magnetoresistance at room temperature with a resistance of ≈300 Ω which is useful for applications. The maxima in resistance and absolute value of negative magnetoresistance are accounted for by the existence of two-phase magnetic states in these films. Fiz. Tverd. Tela (St. Petersburg) 40, 290–294 (February 1998)  相似文献   

16.
An experimental study is reported on the temperature dependences of the magnetic susceptibility, electrical resistivity, magnetoresistance, thermo-and magnetothermopower, and on the Hall effect of the polycrystalline lanthanum manganites La0.67?x R x Sr0.33MnO3 (x=0 and 0.07, R=Eu, Gd) within the 77-to 430-K temperature range. Replacement of a small amount of lanthanum by europium or gadolinium was found to result in a considerable decrease in the resistivity and in a change in the pattern of its temperature dependence. The temperature dependence of both the ordinary and anomalous Hall coefficients near the Curie temperature is shown to be determined by the change in the number of carriers in delocalized states. A method is proposed for separation of the hole and electronic contributions to thermopower in the ferromagnetic region. The conduction mechanisms are discussed in terms of the concept of mobility edge motion.  相似文献   

17.
A systematic investigation of structural, magnetic and electrical properties of nanocrystalline La0.67Ba0.33MnO3 materials, prepared by citrate gel method has been undertaken. The temperature-dependant low-temperature resistivity in ferromagnetic metallic (∼50 K) phase shows upturn behavior and is suppressed with applied magnetic field. The experimental data (<75 K) can be best fitted in the frame work of Kondo-like spin-dependant scattering, electron-electron and electron-phonon interactions. It has been found that upturn behavior may be attributed to weak spin disorder scattering including both spin polarization and grain boundary tunneling effects, which are the characteristic features of extrinsic magnetoresistance behavior, generally found in nanocrystalline manganites. The variation of electrical resistivity with temperature in the high temperature ferromagnetic metallic part of electrical resistivity (75K<T<TP) has been fitted with grain/domain boundary, electron-electron and magnon scattering mechanisms, while the insulating region (T>TP) of resistivity data has been explained based on adiabatic small polaron hopping mechanism.  相似文献   

18.
The electrical and galvanomagnetic properties of high-porosity biocarbon preforms prepared from white pine wood by pyrolysis at carbonization temperatures T carb = 1000 and 2400°C have been studied. Measurements have been made of the behavior with temperature of the electrical resistivity, as well as of magnetoresistance and the Hall coefficient in the 1.8–300-K temperature interval and magnetic fields of up to 28 kOe. It has been shown that samples of both types (with T carb = 1000 and 2400°C) are characterized by high carrier (hole) concentrations of 6.3 × 1020 and 3.6 × 1020 cm−3, respectively. While these figures approach the metallic concentration, the electrical resistivity of the biocarbon materials studied, unlike that of normal metals, grows with decreasing temperature. Increasing T carb brings about a decrease in electrical resistivity by a factor 1.5–2 within the 1.8–300-K temperature range. The magnetoresistance also follows a qualitatively different pattern at low (1.8–4.2 K) temperatures: it is negative for T carb = 2400°C and positive for T carb = 1000°C. An analysis of experimental data has revealed that the specific features in the conductivity and magnetoresistance of these samples are described by quantum corrections associated inherently with structural characteristics of the biocarbon samples studied, more specifically with the difference between the fractions of the quasi-amorphous and nanocrystalline phases, as well as with the fine structure of the latter phase forming at the two different T carb.  相似文献   

19.
We report a study of the temperature dependence, down to 30 mK, of the magnetoresistance of Cd-Sb alloy in the insulating phase obtained by annealing the quenched metallic superconducting ( T c ≈4.5 K) phase of the alloy. Even though the sample in this state is no longer superconducting, the observed negative magnetoresistance points to single-particle tunneling in the presence of a superconducting gap in the spectrum. At magnetic fields B<T the ratio α(T,B)=R(T,B)/R(T,B=4 T)is found to be maximum at a temperature of about 0.1 K. This behavior indicates a change of the conductivity mechanism from single-particle tunneling to incoherent two-particle tunneling as the temperature decreases. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 10, 713–718 (25 November 1996) Published in English in the original Russian Journal. Edited by Steve Torstveit.  相似文献   

20.
A study has been made of the magnetic and electrical properties of oxygen-enriched La1 − x Ag x MnO3.1 manganites at 77 K < T < 300 K. The samples have been produced by the sol-gel technique. The study has revealed an increase of the Curie temperature and a shift of the maximum in magnetoresistance toward high temperatures.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号