Sr2Fe1－xCoxMoO6体系中Co的元素替代效应

在成功制备具有双钙钛矿结构Sr₂Fe_1-xCo_xMoO₆系列样品的基础上，对其结构、输运性质和磁性质进行了系统研究.结果发现，随着Co替代浓度x值的增加，样品的电阻率-温度关系由半金属行为转变为半导体行为，其室温电阻率从3.9×10^-5Ω·cm增大到6.0×10^-1Ω·cm；样品由亚铁磁体转变成反铁磁体，其磁相变温度T_N值也随之下降; Co对Fe的部分替代使其磁电阻效应受到抑制.基于对其电子结构的分析，其磁电阻效应的起源以及Co的元素替代效应也在文中进行了讨论. 关键词： 双钙钛矿结构 2FeMoO₆')" href="#">Sr₂FeMoO₆ 磁电阻     

Jahn-Teller distortion and magnetoresistance in electron doped Sr1-xCexMnO3 (x = 0.1, 0.2, 0.3 and 0.4)

Neutron and electron diffraction, electrical transport and magnetic measurements have been carried out on a newly synthesized electron doped Sr_1-xCe _x MnO₃ (x = 0.1, 0.2, 0.3 and 0.4) system. For x=0.1, while cooling, it undergoes a first-order metal-insulator transition at 315 K which is associated with a structural transition from cubic (Pm3m) to tetragonal (I4/mcm) due to Jahn-Teller ordering () which stabilizes a chain like (C-type) antiferromagnetic ground state with . The antiferromagnetic insulator state is insensitive to an applied magnetic field of 7 T. With increase of x, while the nuclear structure at room temperature for x=0.2 and 0.3 remains tetragonal, for x=0.4 it becomes orthorhombic (Imma) where the doping electrons seem to occupy mainly the d _x2-y2 symmetry. Further, the JT distortion and the antiferromagnetic interactions decrease with doping and a small negative magnetoresistance appears for . Magnetic measurements show that the dilution of antiferromagnetic interaction results into a spin glass like behaviour at low temperature for the samples with x=0.3 and 0.4. This behaviour is in contrast with the CMR properties of calcium based electron doped systems and hole doped manganites. The stability of C-type antiferromagnetic ordering in the electron doped system with large A-site cationic size may be responsible for the absence of double exchange ferromagnetism and CMR effect. Received 10 September 1999     

Study of the magnetization and transport properties of the La(2+x)/3Sr(1−x)/3Mn1−xCrxO3 (0≤x≤0.2) system

The samples with the Mn³⁺/Mn⁴⁺ ratio fixed at 2:1 La_(2+x)/3Sr_(1−x)/3Mn_1−xCr_xO₃ (0≤x≤0.20) have been prepared. The magnetic, electrical transport, and magnetoresistance properties have been investigated. Remarkable transport and colossal magnetoresistance (CMR) effect, as well as cluster glass (CG) behaviors have been clearly observed in the samples studied. It was found that the Curie temperature T_c and insulator−metal transition temperature T_p1 are strongly affected by Cr substitution. The experiment observations are discussed by taking into account the variety of tolerance factors t; the effects of A-site radius 〈r_A〉 and the A-site mismatch effect (σ²).     

A theoretical investigation of the special properties of SrFe1−xCoxO3

Using density-functional calculations, we investigate the special properties of SrFe_1−xCo_xO₃. The results show that the ground states have A-type antiferromagnetic order for x=0.1 and ferromagnetic order for x≥0.2 with Co ions distributed averagely. SrFe_1−xCo_xO₃ exhibits half-metallic nature for 0.2≤x≤0.7 and full-metallic nature for other values of x, and the half-metallic gap decreases with increasing x. The tunneling between the half-metallic ferromagnetic phases drives the large magnetoresistance. In addition, the Co cations are in the intermediate-spin state, while the Fe cations are in the intermediate-spin state for x≤0.5 and the high-spin state for x≥0.6.     

Magnetic/structural phase diagram and enhanced giant magnetoresistance in Zn-doped antiperovskite compound Ga1−xZnxCMn3

The structural, magnetic and electrical transport properties of Zn-doped antiperovskite compounds Ga_1−xZn_xCMn₃ (0≤x≤0.30) have been investigated. After partial substitution of Zn for Ga, the Curie temperature increases monotonously and the ground antiferromagnetic (AFM)-ferromagnetic intermediate (FI) phase transition is gradually suppressed. With increasing the doping level x, the saturated magnetizations decreases gradually firstly for x≤0.20, then increases with increasing x. The electrical transport properties of Ga_1−xZn_xCMn₃ are studied at different magnetic fields. Enhanced giant magnetoresistance (GMR) was observed around the AFM-FI transition. With increasing x, the maximal values and peak widths of GMR increase. Particularly, for x=0.20, GMR reaches a maximum value of 75%, spanning a temperature range of 80 K at 50 kOe and displays the behavior of strongly depending on the magnetization history. The possible origins are discussed.     

La1－xPrxMnO3(x=0.1，0.2)薄膜庞磁电阻性质的研究

一种新的庞磁电阻氧化物薄膜La_1-xPr_xMnO₃(x=0.1，0 .2)薄膜用脉冲激光沉积(PLD)方法生长在(100)SrTiO₃单晶基底上.XRD结果显示 薄膜具有很好的外延单晶取向.电输运和磁性质的研究表明薄膜具有显著的庞磁电阻效应(CM R)效应，其中磁电阻比率达95％(在5T的磁场下).X射线光电子能谱(XPS)的结果表明薄膜体 系中Pr离子的价态为+4价，因此该薄膜很可能是电子掺杂的庞磁电阻体系. 关键词： 脉冲激光沉积 1-xPr_xMnO₃')" href="#">La_1-xPr_xMnO₃ 电子 掺杂 庞磁电阻     

Hall effect in the Fe<Subscript>x</Subscript>Mn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>S magnetic semiconductors

The electrical properties and the Hall effect in the Fe_xMn_1?xS magnetic semiconductors (0<x≤0.5) have been experimentally studied in the range 77–300 K in magnetic fields of up to 15 kOe. The cation-substituted sulfides with 0.25≤x≤0.3 possessing colossal magnetoresistance (CMR) were established to be narrow-gap semiconductors with carrier concentrations n ～ 10¹¹–10¹⁵ cm^?3 and high carrier mobilities μ ～ 10²–10⁴ cm² V^?1 s^?1. It is believed that the CMR effect in these sulfides can be explained in terms of the model of magnetic and electron phase separation, which is analogous to the percolation theory in the case of heavily doped semiconductors.     

Transport properties in La<Subscript>0.7−<Emphasis Type="Italic">x</Emphasis></Subscript>Gd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> system

The influence of Gd doping at La-site on the electrical transport properties and the colossal magnetoresistance of La_0.7−x Gd _x Sr_0.3MnO₃ (x=0.00, 0.10, 0.15, 0.20, 0.30, 0.40, 0.50, 0.60, and 0.70) is studied. The experimental results indicate that the transport properties exhibit abnormal behavior under high doping condition. Forx=0.50, we find that a transition from metal to insulator occurs after the occurrence of insulator-metal transition nearT _c, which seldom occurs in ABO₃ structure. For samplesx=0.60 and 0.70, it exhibits insulator behavior far aboveT _c. These abnormal behaviors are attributed to different magnetic background, i.e. the system undergoes a transition from long range ferromagnetic order to the cluster-spin glass state and further to antiferromagnetic order.     

Effect of the oxygen excess on the properties of weakly doped La<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ca<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3</Subscript> lanthanum manganites

The effect of an oxygen excess δ on the magnetic and electrical properties of La_1−x Ca _x MnO_3+δ (x=0.10–0.15) has been studied over wide ranges of temperatures and magnetic fields. As δ increases, the magnetic ordering temperature T_cdecreases 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 μ_eff 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.     

Synthesis and thermoelectric properties of Mn-doped AgSbTe2 compounds

Polycrystalline p-type Ag 0.9 Sb 1.1 x Mn x Te 2.05（x = 0.05,0.10,and 0.20） compounds have been prepared by a combined process of melt-quenching and spark plasma sintering.The sample composition of Ag 0.9 Sb 1.1 x Mn x Te 2.05 has been specially designed in order to achieve the doping effect by replacing part of Sb with Mn and to present the uniformly dispersed Ag 2 Te phase in the matrix by adding insufficient Te,which is beneficial for optimizing the electrical transport properties and enhancing the phonon scattering effect.All the samples have the NaCl-type structure according to our X-ray powder diffraction analysis.After the treatment of spark plasma sintering,only the sample with x = 0.20 has a small amount of MnTe 2 impurities.The thermal analysis indicates that a tiny amount of Ag 2 Te phase exists in all these samples.The presence of the MnTe 2 impurity with high resistance and high thermal conductivity leads to the deteriorative thermoelectric performance of the sample with x = 0.20 due to the decreased electrical transport properties and the increased thermal conductivity.In contrast,the sample with x = 0.10 exhibits enhanced thermoeletric properties due to the Mn-doping effect.A dimensionless thermoelectric figure of merit of 1.2 is attained for the sample with x = 0.10 at 573 K,showing promising thermoelectric properties in the medium temperature range.     

Electronic transport in LCMO/BTO composites

A series of (1 ? x)La_0.67Ca_0.33MnO₃/(x)BaTiO₃ composites has been synthesized by the solid state route. Microstructural and transport studies on these samples show a complete immiscibility between metallic ferromagnet, La_0.67Ca_0.33MnO₃ (LCMO) and insulating ferroelectric, BaTiO₃ (BTO). Temperature dependent electrical transport studies show evidence of both intrinsic and extrinsic colossal magnetoresistance (CMR) effects. Inclusion of BTO in LCMO phase results in high resistive samples with a metal-insulator transition at temperature T_{p 1} originating from intrinsic effect and another dominant metal-insulator transition at temperature T_{p 2} caused because of extrinsic effect. T_{p 2} decreases up to a certain percentage of BTO content above which this trend of variation is reversed.     

Coercive field enhancement in microstructured (La<Subscript>0.4</Subscript>Pr<Subscript>0.6</Subscript>)<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> thin films

The perovskite material (La_0.4Pr_0.6)_0.67Ca_0.33MnO₃ (LPCMO) has complex electronic and magnetic behavior based on phase competition between ferromagnetic metallic (FMM) and insulating phases with similar free energies. Experimental evidence has indicated that in-plane stress anisotropy influences these phases and can affect electronic and magnetic properties. Here we investigate the roles that both stress and shape anisotropies may play in controlling the coercive field of the material. LPCMO thin films of various thicknesses (20, 25, and 30 nm) were deposited on (110) NdGaO₃ (NGO) substrates using pulsed laser deposition and the coercive fields were measured. Photolithography was then used to fabricate microstructured arrays of LPCMO on the NGO substrates for each of the films. The coercive fields of these arrays of LPCMO were compared to the behavior of the corresponding unpatterned LPCMO thin films across a range of temperatures. Microstructure arrays for the thicker (25 and 30 nm) films showed a substantial increase in the coercive field after forming the arrays, whereas a thinner film (20 nm) showed almost no change in the coercive field. Stress anisotropy continues to play a dominant role in the behavior of LPCMO thin films and dimensionality of the magnetic domains also influences the results. The films show 2D behavior when film thickness approaches the size of the critical radius for single-to-multidomain transitions. Making thicker films allows for 3D behavior and a role for shape anisotropy to influence the coercive fields.     

Kinetic properties of Ni<Subscript>3</Subscript>Al<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> alloys in the range of transition from band to spin-localized ferromagnetism

The kinetic properties of ferromagnetic Ni₃Al_{1 − x} Mn _x alloys with _x ≤ 0.6 are studied at T ≤ 800 K and H ≤ 7 MA/m. The behavior of the electrical resistivity, the thermopower, the magnetoresistance, the Hall effect, and the spontaneous Hall effect are analyzed in the range of transition from band (Ni₃Al) to spinlocalized (Ni₃Mn) ferromagnetism at x ∼ 0.15.     

Effect of isovalent doping of manganite (La1?xPrx)0.7Ca0.3MnO3 films (0≤x≤1) on their optical, magnetooptical, and transport properties near the metal-insulator transition

The optical, magnetooptical, and electric properties of epitaxial (La_1−x Pr_x)_0.7Ca_0.3MnO₃ films (0≤x≤1) grown by MOCVD on LaAlO₃ and SrTiO₃ substrates were studied. It is shown that the decrease in the average cation radius resulting from isovalent substitution of the Pr for La ions brings about a lowering of the Curie temperature, the metal-insulator transition temperature, and the temperatures of the maxima in magnetotransmission (MT) and magnetoresistance (MR). These temperatures depend only weakly on the substrate type. Substitution of La by Pr does not change the shape of the spectral response of the transverse Kerr effect. For concentrations x≤0.50, the maximum values of the Kerr effect and of the MT vary insignificantly, which should be assigned to the existence of a singly connected ferromagnetic metallic region at low temperatures. In films with x=0.75, the presence of ferromagnetic metallic drops in an antiferromagnetic insulating matrix was revealed. The totality of the experimental data obtained suggest that nanoscopic magnetic and electronic nonuniformities exist both in films with a singly connected metallic region and in an x=1 film, which is an antiferromagnetic insulator. __________ Translated from Fizika Tverdogo Tela, Vol. 46, No. 7, 2004, pp. 1203–1213. Original Russian Text Copyright ? 2004 by Sukhorukov, Loshkareva, Gan’shina, Kaul’, Gorbenko, Mostovshchikova, Telegin, Vinogradov, Rodin.     

Influence of the size of the interpolated cation upon the CMR properties of the perovskites Nd0.7Ca0.3−xSrxMnO3−δ

The systematic study of the manganese perovskite Nd_0.7Ca_0.3-xSr_xMnO_3-' has allowed colossal magnetoresistance effects (CMR) with resistance ratios (RR), R_B=0/R_B=5T, up to 10⁴ at 60rK in a field of 5 tesla to be evidenced. The influence of the mean size of the interpolated cation (Nd, Ca, Sr), on the magnitude of the RR ratios, has been demonstrated for the first time: RR goes through a maximum for a particular value of the mean radius of the interpolated cation. This effect seems to be correlated with the evolution of the cell volume that exhibits a singular point around x=0.085. A second important feature deals with the irreversibility of the CMR effect at low temperature that decreases as T increases and disappears at T_max. It is also remarkable that for A(T) curves characterized by a maximum, T_max not only increases as the size of the interpolated cation increases, but also as the magnitude of the applied magnetic field increases.     

Structural and magnetic characterization of Bi1–xLaxFeO3 and BiFe1–yMnyO3 nanoparticles synthesized via a sol–gel method

Here, La-doped Bi_1–xLa_xFeO₃ and Mn-doped BiFe_1–yMn_yO₃ (x, y?=?0.1, 0.2, 0.3) nanoparticles were synthesized by a sol–gel process. For Bi_1–xLa_xFeO₃, structural analysis suggested that its structure changed from rhombohedral to tetragonal phase without secondary phase when x increased from 0 to 0.3. In addition, with the increasing doping concentration of La, transmission electron microscopy exhibited the doped nanoparticle size reduced, while the magnetic properties were correspondingly enhanced. However, for synthesis of BiFe_1–yMn_yO₃, phase analysis showed that there easily existed secondary phase for nanoparticles with 10% and 30% Mn-doped, while 20% Mn-doped nanoparticles indicated high crystallinity without any impure phases. Similarly, we observed the enhanced magnetic properties with the increase of Mn concentration.     

纳米结构CoxFe3-xO4多孔微球的磁性及交换偏置效应研究

用乙二醇为溶剂,用三氯化铁、二氯化钴和醋酸铵为起始反应试剂,通过溶剂热反应首次合成了纳米结构Co_xFe_3-xO₄多孔微球.用X射线衍射仪(XRD)、透射电子显微镜(TEM)和扫描电子显微镜(SEM)表征样品的结构和形貌,结果表明,所制备的单分散Co_xFe_3-xO₄多孔微球为立方多晶结构,其直径约300 nm,是由约30 关键词： xFe_3-xO₄')" href="#">Co_xFe_3-xO₄ 多孔微球 磁性 交换偏置效应     

Spin-glass, antiferromagnetism and kondo behavior in Ce2Au1−x Co x Si3 alloys

Recently, the solid solution Ce₂Au_{1− x Co} xSi₃ has been shown to exhibit many magnetic anomalies associated with the competition between magnetic ordering and the Kondo effect. Here we report high pressure electrical resistivity of Ce₂AuSi₃, ac susceptibility (X) and magnetoresistance of various alloys of this solid solution in order to gain better knowledge of the magnetism of these alloys. High pressure resistivity behavior is consistent with the proposal that Ce₂AuSi₃ lies at the left-hand side of the maximum in Doniach’s magnetic phase diagram. The ac X data reveal that there are in fact two magnetic transitions, one at 2 K and the other at 3 K for this compound, both of which are spin-glass-like. However, as the Co concentration is increased, antiferromagnetism is stabilized for intermediate compositions before attaining non-magnetism for the Co end member.     

锰掺杂对Ba(Zr,Ti)O3陶瓷压电与介电性能的影响

采用氧化物固相反应法制备了锰掺杂改性的Ba(Zr_0.06Ti_0.94)O₃陶瓷.研究了锰的掺杂量对Ba(Zr_0.06Ti_0.94)Mn_xO₃ (BZTM)陶瓷的结构、介电和压电性能的影响.实验发现,当锰含量x<0.5 mol%时进入晶格,使材料压电性能提高,损耗减小,表现出受主掺杂的特性;当锰含量x>0.5 mo 关键词： Ba(Zr 3 陶瓷')" href="#">Ti)O₃ 陶瓷 锰掺杂 介电性能 压电性能     

Effect of heavy cation doping on thermal properties of LaMnO3

Effect of heavy cation doping (Ca²⁺ at the A-site) on the thermal properties of perovskite LaMnO₃ has been investigated using the Rigid Ion Model (RIM)). As strong electron-phonon interactions are present in these compounds, the lattice part of the specific heat deserves proper attention. The specific heat of magnetoresistance compound La_0.25Ca_0.75MnO₃ as a function of temperature (10 K ≤ T ≤ 300 K) is reported. Our results on specific heat are in good agreement with the measured values of specific heat at lower temperatures. In addition, the results on the cohesive energy (ϕ), molecular force constant (f), Restrahalen frequency (ν ₀), Debye temperature (Θ _D) and Gruneisen parameter (γ) are also discussed.