Response of the electrical resistivity and magnetoresistance of La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> films to biaxial tensile strains

The structure, electrical resistivity, and magnetoresistance of (50-nm)La_0.67Ca_0.33MnO₃ epitaxial films grown on a [(80 nm)Ba_0.25Sr_0.75TiO₃/La_0.3Sr_0.7Al_0.65Ta_0.35O₃] substrate with a substantial positive lattice misfit have been studied. The tensile biaxial strains are shown to account for the increase in the cell volume and in the relative concentration of Mn⁺³ ions in the manganite films as compared to those for the original material (33%). The peak in the temperature dependence of the resistivity ρ of La_0.67Ca_0.33MnO₃ films was shifted by 30–35 K toward lower temperatures relative to its position in the ρ(T) graph for a manganite film grown on (001)La_0.3Sr_0.7Al_0.65Ta_0.35O₃. For T < 150 K, the temperature dependences of ρ of La_0.67Ca_0.33MnO₃/Ba_0.25Sr_0.75TiO₃/La_0.3Sr_0.7Al_0.65Ta_0.35O₃ films could be well fitted by the relation ρ = ρ₀ + ρ₁T^4.5, where ρ₀ = 0.35 mΩ cm and the coefficient ρ₁ decreases linearly with increasing magnetic field. In the temperature interval 4.2–300 K, the magnetoresistance of manganite films was within the interval 15–95% (μ₀H = 5 T).     

Magnetoresistance of La<Subscript>0.67</Subscript>Sr<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> epitaxial films grown on a substrate with low lattice mismatch

The structure, electrical resistivity, and magnetoresistance of La_0.67Sr_0.33MnO₃ heteroepitaxial films (120-nm thick) practically unstrained by lattice mismatch with the substrate were studied. A strong maximum of negative magnetoresistance of ≈27% (for μ₀H = 4 T) was observed at T ≈360 K. While the magnetoresistance decreased monotonically in magnitude with decreasing temperature, it was still in excess of 2% at 150 K. For T < 250 K, the temperature dependence of the electrical resistivity ρ of La_0.67Sr_0.33MnO₃ films is fitted well by the relation ρ = ρ₀ + ρ ₁(H)T^2.3, where ρ₀ = 1.1×10^?4 Ω cm, ρ₁(H = 0) = 1.8×10^?9 Ω cm/K^2.3, and ρ₁(μ₀H = 4 T)/ρ₁(H = 0) ≈0.96. The temperature dependence of a parameter γ characterizing the extent to which the electrical resistivity of the ferromagnetic phase of La_0.67Sr_0.33MnO₃ films is suppressed by a magnetic field (μ ₀H = 5 T) was determined.     

纳米多晶La0.7Sr0.3MnO3－δ的输运性质和磁电阻效应研究

利用溶胶-凝胶法制备了纳米多晶La_0.7Sr_0.3MnO_3-δ(LSM)块体样品.详细研究了在不同烧结温度下的LSM样品电阻率随测量温度的变化关系和磁电阻效应.随着测量温度从室温降低,电阻率ρ都在250K附近存在最大值,低于该温度后,样品表现为金属导电特性,随后在50K左右存在一极小值.即随着温度从50K左右降低到4.2K,ρ反而逐渐升高,表现为绝缘体性的导电特性.研究表明,在低温下(<50K),ρ随温度降低而升高的现象与隧穿效应的理论模型(lnρ∝T^1/2)符合得很好,表明这种现象是由于传导电子在通过邻近LSM晶粒间表面/界面层时的隧道效应所致.而在50—250K的温度范围内,其电阻率与T²成正比,表现为LSM本征的金属导电特性.因此这种低温下电阻率的极小值现象来源于隧穿效应和LSM晶粒本征的金属导电特性的相互竞争.本文还详细研究了相应的隧道磁电阻效应. 关键词： 0.7Sr_0.3MnO_3-δ')" href="#">多晶La_0.7Sr_0.3MnO_3-δ 隧道效应 隧道磁电阻效应     

Effect of lateral tensile stresses on the low-temperature electrical resistivity and magnetoresistance of La0.67Ca0.33MnO3 nanofilms

The thickness of the buffer layer of strontium titanate introduced between an La_0.67Ca_0.33MnO₃ manganite film and a (001)La_0.29Sr_0.71Al_0.65Ta_0.35O₃ substrate is varied (d ₁ = 7–70 nm) to influence effective misfit m in their lattice parameters. As m increases, electrical resistivity ρ of the film increases sharply and the maximum in the ρ(T) dependence shifts toward low temperatures. At T < 150 K, the temperature dependence of ρ of the manganite film obeys the relationship ρ = ρ₁ + ρ₂ T ^4.5, where parameter ρ₁ is independent of the temperature and magnetic field. Coefficient ρ₂ decreases with increasing magnetic field and increases with the misfit between the lattice parameters of the film and substrate, i.e., when the effective hole concentration in the manganite layer decreases.     

Transport behavior and mechanism of conduction of simultaneously substituted Y and Fe in La0.7Ba0.3MnO3 perovskite

The electrical properties and the mechanism of conduction of the simultaneously substituted La_0.7−xY_xBa_0.3Mn_1−xFe_xO₃ perovskite (0≤x≤0.30) have been studied. The insertion of Y³⁺ and Fe³⁺ ions in the parent compound La_0.7Ba_0.3MnO₃ leads to an increase of the resistivity. The undoped sample (x=0) shows a metallic behavior, which can be fitted by the relation ρ(T)=ρ₀+ρ₂T²+ρ_4.5T^4.5, indicating the importance of electron-magnon scattering effects in this material. All the other samples (x≥0.10) are semiconductors throughout the studied temperature range (80-290 K). Several models have been used to fit their temperature-dependent resistivity: thermal activation, adiabatic nearest-neighbor hopping of small polarons (Holstein theory) and variable range hopping (VRH) models. The fits show that the electronic transport in semiconducting La_0.7−xY_xBa_0.3Mn_1−xFe_xO₃ is well described and dominated by the VRH mechanism, for which the hopping distance (a) grows with increasing Fe³⁺ doping, thus increasing the average hopping energy W.     

Electrical resistivity and magnetoresistance of nonuniformly mechanically strained films (30 nm) of the La0.67Ca0.33MnO3 manganite

The structure, electrical resistivity, and magnetoresistance of predominantly oriented La_0.67Ca_0.33MnO₃(30 nm)/LaAlO₃ films are investigated after partial relaxation of biaxial mechanical stresses. The negative magnetoresistance MR of the films reaches a maximum at T = 235–240 K. The full width at half-maximum of the peak in the curve MR(T) for a film is five to six times greater than that for a manganite layer grown on a substrate with a small lattice mismatch. For T < 150 K, the temperature dependence of the electrical resistivity ρ of the films is fitted well by the relationship ρ = ρ₀ + ρ₁ (H)T ^4.5, where ρ₀ ≡ ρ(T = 4.2 K) and ρ₁(H) is a parameter that is independent of temperature but dependent on the magnetic field H. The parameter ρ₁(H = 0) for the La_0.67Ca_0.33MnO₃(30 nm)/LaAlO₃ films is several times larger than that for thin manganite layers only weakly strained by the substrate. The electrical resistivity ρ₁ decreases almost linear as the quantity μ₀ H increases in the field range 1–5 T.     

Effect of pressure and anionic substitution on the electrical properties of HgTeS crystals

The resistivity ρ and the Hall constant R for the HgTe_1?x S_x (0.04≤x≤0.6) crystals have been investigated in the temperature range 4.2–350 K in the magnetic fields B up to 14 T. The pressure dependences of the resistivity ρ have been measured at the pressures P as high as 1 GPa at temperature T=77–300 K and magnetic field B=0–2 T. It is found that the samples with x≤0.20 exhibit a decreasing dependence ρ(T) typical of zero-gap semiconductors, whereas the samples with x≥0.27 show the dependence ρ(T) characteristic of semimetals. For the semiconducting crystals with x≈0.20 and x≈0.14, the temperature coefficient of ρ(T) changes sign at T=265 and T>300 K, respectively. Under a pressure of ≈1 GPa, the temperature of the sign inversion decreases by ≈30 K. An increase in the magnetic field B and a rise in the temperature T lead to a change in the sign of the Hall constant R for the semiconducting samples, but do not affect the electronic sign of R for the semimetallic samples. The behavior of R and ρ correlates with the thermoemf data obtained at the quasi-hydro-static pressure P up to 3 GPa. It is demonstrated that the substitution of sulfur atoms for tellurium atoms brings about an increase in the concentration of electrons and a decrease in their mobility. The transition to the wide-gap semiconductor phase is observed at P>1–1.5 GPa. The conclusion is drawn that the semimetallic crystals HgTe_1?x S_x with x≥0.27 and HgSe are similar in properties.     

多晶La0.7Sr0.3MnO3的低温输运性质和磁电阻效应

详细研究了由纳米晶粒组成的块体多晶La_0.7Sr_0.3MnO₃(LSM)的电阻率和磁电阻效应,以及它们的温度依赖性.随着温度从室温降低,电阻率(ρ)在250K附近存在一最大值,低于该温度后,样品表现为金属导电特性,随后在50K附近存在一极小值.也就是说在低于50K的温度范围内,随着温度降低ρ反而升高,表现为绝缘体性的导电特性.经研究发现,这种随温度降低ρ反而增加的现象与隧穿效应的理论模型(lnρ∝T^-1/2)符合得很好 关键词： 0.7Sr_0.3MnO₃')" href="#">多晶La_0.7Sr_0.3MnO₃ 隧道效应 隧道磁电阻效应     

Transport properties of CeCu6 single crystals

The electrical resistivity ? and the thermoelectric power of CeCu₆ single crystals are strongly anisotropic. The inverse of the temperature of the Kondo resistivity maximum (T_max) roughly scales the linear temperature coefficient B of ? as well as the residual value (?₀ ÷ B ÷ 1/T_max). Along the [1 0 0] direction ? follows a T² Fermi-liquid law between 30 and 90 mK. The thermoelectric power is positive over the investigated temperature range (1–300 K) and shows two contributions.     

Electroresistive and magnetoresistive properties of Nd0.7Sr0.3MnO3 after quenching under pressure of 9 GPa

Polycrystalline Nd_0.7Sr_0.3MnO₃ was quenched from 1300 K to 300 K and 80 K after it had been subjected to a high quasihydrostatic pressure of 9 GPa. Such high pressure and high temperature treatment (HPT) results in significant changes of the crystallochemical parameters—Mn-O lengths and Mn-O-Mn angles within unchanged lattice symmetry of the Pnma-type. A strong increase of the resistivity and a large decrease of the FM-PI transition temperature were detected for the Nd_0.7Sr_0.3MnO₃ HPT treated samples. The intrinsic characteristic T_MI(T_C) (T_MI is the metal-insulator and T_C is the ferromagnetic-paramagnetic transition temperature) correlates with the change of the Mn-O(1)-Mn angle, which is consistent with the double exchange model of the ferromagnetic metallic state in manganites. Remarkable electroresistive (ER) and magnetoresistive (MR) effects appear after HPT treatment, which were not present in the starting Nd_0.7Sr_0.3MnO₃ sample. The structure sensitive properties such as resistivity, MR and ER effects correlate with the change of the nanograin sizes after HPT treatment. Nonlinear current-voltage characteristics showing a hysteresis appear for HPT treated samples at low temperatures. The transport in granular Nd_0.7Sr_0.3MnO₃ samples is likely defined by spin-dependent scattering of charge carriers inside the ferromagnetic metallic grains with embedded small charged isolating islands and by jumping over charged insulating barriers at the intergrain boundaries, which can be strongly affected by the external electric and magnetic fields.     

Structural, magnetic and magnetotransport behavior of La0.7SrxCa0.3−xMnO3 compounds

A systematic investigation of the structural, magnetic and electrical properties of a series of nanocrystalline La_0.7Sr_xCa_0.3−xMnO₃ materials, prepared by high energy ball milling method and then annealed at 900 °C has been undertaken. The analysis of the XRD data using the Win-metric software shows an increase in the unit cell volume with increasing Sr ion concentration. The La_0.7Sr_xCa_0.3−xMnO₃ compounds undergo a structural orthorhombic-to-monoclinic transition at x=0.15. Electric and magnetic measurements show that both the Curie temperature and the insulator-to-metal transition temperature increase from 259 K and 253 K correspondingly for La_0.7Ca_0.3MnO₃ (x=0) to 353 K and 282 K, respectively, for La_0.7Sr_0.3MnO₃ (x=0.3). It is argued that the larger radius of Sr²⁺ ion than that of Ca²⁺ is the reason to strengthen the double-exchange interaction and to give rise to the observed increase of transition temperatures. Using the phenomenological equation for conductivity under a percolation approach, which depends on the phase segregation of ferromagnetic metallic clusters and paramagnetic insulating regions, we fitted the resistivity versus temperature data measured in the range of 50-320 K and found that the activation barrier decreased with the raising Sr²⁺ ion concentration.     

Synthesis and characterization of layered perovskite oxides La1 + x Sr2 − x MnCrO7 (x = 0.2, 0.5)

Intergrowth perovskite type complex oxides of composition La_1.2Sr_1.8MnCrO₇ and La_1.5Sr_1.5MnCrO₇ have been synthesized by ceramic method. Rietveld profile analysis shows that the phases crystallize with tetragonal unit cell in the space group I4/mmm. Both the phases behave as insulators in the high temperature region and the linearity of log ρ versus T ^?1/4 plot in the temperature range 150–300 K shows that the electronic conduction occurs by a 3D variable range hopping mechanism. The phases show insulator-metal transition at low temperature which could be due to the mixed valence state of Mn³⁺/Mn⁴⁺ by double exchange mechanism. The ferromagnetic interactions observed for the samples arises from double exchange interaction between Mn ³⁺ and Mn⁴⁺ and Cr³⁺ and Mn³⁺ ions.     

Magnetic interaction in Mg, Ti, Nb doped manganites

An effect of Mn substitution with Me=Mg²⁺, Ti⁴⁺, Nb⁵⁺ in manganites has been investigated by preparing La_0.7Sr_0.3(Mn_1-xMe_x)O₃ and La_1-xSr_x(Mn_{1 - x/2}Nb_x/2)O₃ series. It was established that substitution of manganese with magnesium up to x = 0.16 leads to a collapse of a long-range ferromagnetic order whereas La_0.7Sr_0.3(Mn ^{3 +} _0.85Nb ^{5 +} _0.15)O₃ is ferromagnet with T _C = 123 K and exhibits a large magnetoresistance below Curie point despite an absence of four-valent manganese. Hypothetical magnetic phase diagrams are constructed for La_0.7Sr_0.3(Mn_1-xMe_x)O₃ and La_1-xSr_x(Mn_{1 - x/2}Nb_x/2)O₃. Our results show that Mn³⁺-O-Mn³⁺ exchange interaction is ferromagnetic in the orbitally disordered manganites as well as an increase of Mn⁴⁺ content above 50% from a total amount of manganese ions leads to formation of a spin glass state due to a competition between antiferromagnetic Mn⁴⁺-O-Mn⁴⁺ and ferromagnetic Mn³⁺-O-Mn⁴⁺(Mn³⁺) superexchange interactions. Received 24 January 2002 Published online 9 July 2002     

1D-3D crossover in hopping conduction of carbynes

D.C. and A.C. conductivities of carbyne samples were studied over the temperature range 1.8–300 K at frequencies 10 MHz–1 GHz. It was established that a variation in the fraction of sp ² bonds in carbynes induces transition from one-to two-and then to three-dimensional conduction. In the one-dimensional hopping conduction regime, the resistivity of carbynes depends on temperature as ρ=ρ₀exp[(T ₀/T)^1/2], whereas Coulomb correlations are insignificant. The Hunt model of one-dimensional A.C. conductivity is experimentally confirmed.     

Two-dimensional antiferromagnetic correlations in an La<Subscript>1.4</Subscript>Sr<Subscript>1.6</Subscript>(Mn<Subscript>0.9</Subscript>Co<Subscript>0.1</Subscript>)<Subscript>2</Subscript>O<Subscript>7</Subscript> single crystal

The temperature and field dependences of the magnetization, the electrical resistivity, and the magnetostriction of bilayer lanthanum manganite La_1.4Sr_1.6Mn₂O₇ single crystals and cobalt-doped La_1.4Sr_1.6(Mn_0.9Cu_0.1)₂O₇ are measured. The magnetostriction of the cobalt-doped compound increases as compared to the initial La_1.4Sr_1.6Mn₂O₇ compound, and the magnetization and the magnetoresistance of the former compound change substantially. Powder and single-crystal neutron diffraction patterns are used to detect ferromagnetic ordering in La_1.4Sr_1.6(Mn_0.9Co_0.1)₂O₇ at a temperature below T _C ~ 45(2) K, and this ordering coexists with antiferromagnetic correlations, which develop at temperatures below T _C ~ 80(5) K.     

Study of some co-precipitated manganite perovskite samples-doped iron

Polycrystalline La_0.70Sr_0.30Mn_1−yFe_yO₃ (0.05?y?0.07) samples are prepared by the co-precipitation method and have been studied. The substitution of Mn³⁺ by Fe³⁺ reduces the number of available hopping sites for the Mn e_g(↑) electron and suppresses the double exchange (DE), resulting in the reduction of the metal–semiconductor transition temperature (T_P) and the flux density saturation (B_s). Low-temperature resistivity (ρ) data (below T_P) well fit with the relation ρ(T)=ρ₀+ρ₂T², indicating the importance of grain/domain boundary effects and electron–electron scattering processes in the conduction of these materials. On the other hand, at high temperature (T_P<T<θ_D/2) conductivity data satisfy the variable range hopping (VRH) model. For T>θ_D/2 the small polaron hopping model is more appropriate than the VRH model.     

钙钛矿锰氧化物La0.7Sr0.3MxMn1-xO3(M=Cr,Fe)的巨磁电阻效应与磁性

研究了溶胶-凝胶法制备氧化物巨磁电阻材料的工艺,制备了La_0.7Sr_{0.3< /sub>CrxMn1-xO3(x=0,0.10,0.15)和La0.7Sr0.3FexMn1-xO3(x=0.05,0.10,0.16)两 系列的单相钙钛矿锰氧化物多晶样品,并研究了Cr,Fe替代La0.7Sr0.3MnO3中部分Mn后对其结构、磁性和巨磁电阻性质的影响.观察到La0.7 Sr0.3Cr0.15Mn0.85O3和La0. 7Sr0.3Fe0.05Mn0.95O3两个样品的 电阻-温度曲线都出现了双峰.定性讨论了可能产生双峰的机制.随Cr(或Fe)替代量的增加, 材料的居里温度很快下降,铁磁性减弱,导电性降低,巨磁电阻效应增强.但与Fe掺杂相比 ,相同数量的Cr掺杂对材料的影响要小. 关键词： 巨磁电阻效应 溶胶-凝胶工艺 电阻-温度曲线 金属绝缘体转变}     

Influence of Fe doping on electrical properties of LCMO

The polycrystalline samples La_0.67Ca_0.33Mn_(1?x)Fe _x O₃ (x?=?0.00,?0.01,?0.03, and 0.1) have been grown in single phase by solid state route. The analysis of the reaction has been done by thermogravimetry and differential thermal analysis measurements. DC electrical resistivity measurements have been carried out down to 15?K. The samples with x?=?0.00, 0.01, and 0.03 exhibit metal–insulator (MI) transition at temperatures 221.5?K, 217?K, and 215?K respectively, whereas the sample with x?=?0.1 is insulating in nature for entire temperature range. Interestingly, the electric transport properties of these samples are not consistent with their magnetic phase transitions and the samples show MI transition at a temperature, T _MI, which is significantly lower than the paramagnetic to ferromagnetic transition temperature (T _c). The resistivity data below T _MI has been analyzed using the empirical relation ρ?=?ρ₀?+?ρ₁ T ⁿ and the data above this temperature has been analyzed using two existing models, Mott's variable range hopping model and spin polaronic conduction model.     

Structural and magnetic phase diagram and room temperature CMR effect of La1−xAgxMnO3

The study of the structural and magnetic phase diagram of the manganites La_1−xAg_xMnO₃ shows similarity with the La_1−x′Sr_x′MnO₃ series, involving a metallic ferromagnetic domain at relatively high temperature (≈300 K). The Ag-system differs from the Sr-one by a much smaller homogeneity range (x≤1/6) and the absence of charge ordering. But the most important feature of the Ag-manganites deals with the exceptionally high magnetoresistance (−25%) at room temperature under 1.2 T, that appears for the composition x=1/6. The latter is interpreted as the coincidence of the optimal double exchange condition (Mn³⁺:Mn⁴⁺=2) with T_max=300 K (maximum of the ρ(T) curve in zero field).     

Effect of Fe substitution on magnetocaloric effect in La0.7Sr0.3Mn1−xFexO3 (0.05≤x≤0.20)

We have studied the effect of Fe substitution on magnetic and magnetocaloric properties in La_0.7Sr_0.3Mn_1−xFe_xO₃ (x=0.05, 0.07, 0.10, 0.15, and 0.20) over a wide temperature range (T=10-400 K). It is shown that substitution by Fe gradually decreases the ferromagnetic Curie temperature (T_C) and saturation magnetization up to x=0.15 but a dramatic change occurs for x=0.2. The x=0.2 sample can be considered as a phase separated compound in which both short-range ordered ferromagnetic and antiferromagnetic phases coexist. The magnetic entropy change (−ΔS_m) was estimated from isothermal magnetization curves and it decreases with increase of Fe content from 4.4 J kg⁻¹ K⁻¹ at 343 K (x=0.05) to 1.3 J kg⁻¹ K⁻¹ at 105 K (x=0.2), under ΔH=5 T. The La_0.7Sr_0.3Mn_0.93Fe_0.07O₃ sample shows negligible hysteresis loss, operating temperature range over 60 K around room temperature with refrigerant capacity of 225 J kg⁻¹, and magnetic entropy of 4 J kg⁻¹ K⁻¹ which will be an interesting compound for application in room temperature refrigeration.