Charge order and spin order in Nd_(0.5)Sr_(0.5)Mn_(1-x)(Ga_x, Ti_x)O_3 system

Magnetic properties of Nd_(0.5)Sr_(0.5)Mn_(1-x)(Ga_x, Ti_x)O_3 system (0.04≤x≤0.4) were inves- tigated through magnetization and electron spin resonance (ESR) measurements. It was observed that a small amount of Ti substitution for Mn will destroy the charge-ordering (CO) phase completely and induce the cluster-spin-glass phase in the system, which displays a procedure of collapse of CO and of an enhancement of spin ordering (SO) phase. In contrast, the Ga substitution for Mn induces a melt- ing of CO phase in the system. It was observed that with substitution the CO phase is suppressed gradually and the remanent CO phase is retained all the while, and withal, there is a co-existence of AFM CO phase and FM SO at low temperature. In addition, an abrupt rise of magnetization was observed in M-T curves. We attributed this abnormal phenomenon to a transition from canted AFM SO to FM SO in CO region.     

Charge ordering characteristics in Y0.5Ca0.5MnO3 manganite

The charge ordering characteristics in Y_0.5Ca_0.5MnO₃ manganite, prepared by sol–gel process, have been investigated experimentally. It is found that the superlattice diffractions appear in the electron diffraction patterns recorded at low temperatures, while only basic Brag diffraction spots can be observed when temperature is higher than 300 K. This provides direct evidence for the existence of charge ordering in Y_0.5Ca_0.5MnO₃. The magnetization and specific heat measurements indicate the charge ordering temperature of Y_0.5Ca_0.5MnO₃ is 290 K, around which both the magnetization and specific heat reveal anomalous behaviors. We also observed that the MnO bond length changed remarkably and the effective number of carriers reduced prominently with decreasing temperature around charge ordering temperature through transform infrared spectra measurements.     

Charge order and spin order in Nd0.5Sr0.5Mn1? x (Ga x , Ti x )O3 system

Magnetic properties of Nd0.5Sr0.5Mn1-x(Gax,Tix)O3 system (0.04≤x≤0.4) were investigated through magnetization and electron spin resonance (ESR) measurements. It was observed that a small amount of Ti substitution for Mn will destroy the charge-ordering (CO) phase completely and induce the cluster-spin-glass phase in the system, which displays a procedure of collapse of CO and of an enhancement of spin ordering (SO) phase. In contrast, the Ga substitution for Mn induces a melting of CO phase in the system. It was observed that with substitution the CO phase is suppressed gradually and the remanent CO phase is retained all the while, and withal, there is a co-existence of AFM CO phase and FM SO at low temperature. In addition, an abrupt rise of magnetization was observed in M-Tcurves. We attributed this abnormal phenomenon to a transition from canted AFM SO to FM SO in CO region.     

Bi0.5Ca0.5Mn1-xCoxO3体系中的电荷有序和相分离

利用固相反应法制备了Bi_0.5Ca_0.5Mn_1-xCo_xO₃(0≤x≤0.12)系列多晶样品.研究了Co掺杂对Bi_0.5Ca_0.5MnO₃电荷有序的影响.结果表明,Co掺杂导致电荷有序相逐渐融化、铁磁相互作用的增强;当x≥0.08时,电荷有序转变峰完全消失,但残留的反铁磁电荷有 关键词： 钙钛矿锰氧化物 电荷有序 团簇玻璃 相分离     

(La1-x Yx)0.5Sr0.5MnO3体系的结构和输运性质研究

实验研究了La0.5Sr0.5MnO3体系La位上Y掺杂对电荷有序相的影响。结果表明掺杂量为0．15以下时，样品均呈现金属—绝缘体转变行为；当掺杂量为0．2，0．25时样品中出现了电荷有序相，通过对样品的结构和输运特性的仔细研究，发现有三种因素对其输运特性具有重要影响：带宽即＜rA＞，eg电子的轨道，无序度δ^2，同时，实验结果表明电荷有序相的形成是因为电子之间的强关联作用的结果。     

Nd0.5Ca0.5Mn0.97Co0.03O3体系的超声研究

系统研究了Nd0.5Ca0.5Mn0.97Co0.03O3单相多晶样品在低温下的电磁性质和超声特性.超声测量结果表明,体系在低温下超声声速出现异常,超声声速从室温开始随着温度的降低逐渐减小,并在220K附近声速达到最小,其相对变化(ΔV/V)超过2%.之后,随着温度的进一步降低,声速急剧硬化,其相对变化达到14.5%.分析表明这是由于体系中电-声子相互作用导致的电荷有序态转变的结果,该电声子耦合来源于Mn3 的JahnTeller效应.同时,该体系在100K左右出现了金属绝缘体转变同时伴随着铁磁相变,分析表明在低温下Co离子与Mn离子之间存在着铁磁交换作用,因此微量的Co掺杂会在反铁磁区域内形成部分铁磁小团簇,当铁磁团簇连通后,体系由反铁磁电荷有序态(AFMCO)向铁磁金属态(FMM)转变.研究结果表明了样品在电荷有序相变温度TCO与金属绝缘体转变温度TMI之间(TMI相似文献   

Nd0.5Ca0.5Mn1-xAlxO3 (x=0,0.03)体系中电荷有序态的超声研究

系统研究了Nd0．5Ca0．5Mn1-xAlxO3(x=0,0．03)单相多晶样品在低温下的电磁性质和超声特性．电阻和磁化率测量表明,Nd0．5Ca0.5O3体系在TCO-257 K处发生了电荷有序相变．超声声速从室温开始随着温度的降低逐渐减小,并在TCO附近达到最小,之后,随着温度的进一步降低,声速急刷增加,同时伴随着一个尖锐的超声衰减峰出现.TCO附近的超声异常表明体系中存在着强烈的电-声子相互作用,该电-声子耦合来源于Mn3 的Jahn-Teller效应．在低温下,出现了另一个超声衰减峰,它的出现归结为反铁磁相与顺磁相之间的相分离现象.随着Al在Mn位的掺入,超声声速的最低点和衰减峰向低温移动,表明体系中的电荷有序态和反铁磁相均被部分抑制,     

Magnetocrystalline anisotropy and dynamic spin reorientation of half-doped Nd_(0.5)Pr_(0.5)FeO_3 single crystal

The single crystals of Nd_(0.5)Pr_(0.5)FeO_3 were successfully grown by optical floating zone method.Room temperature x-ray diffraction and Laue photograph declared the homogeneity and high quality of the crystal.The significant magnetic anisotropy and multiple magnetic transitions illustrate the complex magnetic structure.At high temperatures(T 66 K),it shows the typical characteristics of Γ_4(G_x,A_y,F_z) state.With the decrease of the temperature,it undergoes a first-order spin reorientation transition from Γ_4(G_x,A_y,F_z) to Γ_2(F_x,C_y,G_z) state in the temperature window of 45-66 K under an applied magnetic field of 0.01 T.As the temperature drops to ～17 K,a new magnetic interaction mechanism works,which results in a further enhancement of magnetization.The T-H phase diagram of Nd_(0.5)Pr_(0.5)FeO_3 single crystal was finally constructed.     

半掺杂Sm0.5Ca0.5MnO3体系的电荷有序和再入型自旋玻璃行为

研究了半掺杂锰氧化物Sm_0.5Ca_0.5MnO₃体系的结构、输运和磁特性，结果表明，在半掺杂情况下，该体系呈现O′类正交结构，表明体系存在典型的Jahn-Teller效应畸变；输运结果在整个测量温区均呈现半导体导电行为，没有出现金属－绝缘体(M-I)转变和CMR效应；电荷有序转变发生在T=270K左右，反铁磁转变温度出现在200K附近，且表现出典型的再入型自旋玻璃(spin-glass)行为，自旋玻璃转变温度T_SG在4 关键词： 自旋玻璃 电荷有序 负磁化现象 多相竞争     

相分离Nd0.5Ca0.5MnO3体系的再入型自旋玻璃行为和电荷有序

研究了Nd_0.5Ca_0.5MnO₃体系的结构和输运特性. 结构 分析表明，在300K下，体系表现为O′型正交结构并存在典型的Jahn-Teller畸变.在8 T磁场 下，体系出现顺磁绝缘-铁磁金属的转变，庞磁电阻效应发生. 磁测量发现，样品的奈尔温 度T_N和电荷有序转变温度T_CO分别在150和240K左右，在41K左右出 现典型再入型自旋玻璃行为，同时观察到了负的磁化率异常. 结果表明，Nd关键词： 庞磁电阻 自旋玻璃态 负磁化强度 电荷有序     

Structure and magnetization studies of Nd0.5−xPrxSr0.5MnO3 system

The structural and magnetic properties of Nd_0.5−xPr_xSr_0.5MnO₃ (x=0, 0.1, 0.2, 0.3, 0.4 and 0.5) system have been investigated. With the substitution of Pr in Nd_0.5Sr_0.5MnO₃, it shows a gradual structure transformation from the Imma orthorhombic symmetry to the tetragonal I4/mcm phase, and the crystallographic transition remains incomplete, even in Pr_0.5Sr_0.5MnO₃. A large bifurcation between zero-field-cooled (ZFC) and field-cooled (FC) susceptibility has been observed below Curie temperature (T_C), which is characteristic of coexistence of ferromagnetism (FM) and antiferromagnetism (AFM) at low temperature region. The magnetization of Pr_0.5Sr_0.5MnO₃ is larger than that of Nd_0.5Sr_0.5MnO₃, while Nd_0.5Sr_0.5MnO₃ with more CE-type AFM shows larger magnetization than Nd_0.3Pr_0.2Sr_0.5MnO_3, which mixed with CE-type (majority) and A-type (minority) AFM at low temperature, indicating that the magnetization of Nd_0.5−xPr_xSr_0.5MnO₃ system is affected by A-site disorder combined with orbital ordering of A-type AFM and CE-type AFM.     

Asymmetric phase diagram and phase separation in Pr0.5−δCa0.2+δSr0.3MnO3 (−0.04δ0.04)

We present electric and magnetic properties of polycrystalline Pr_0.5−δCa_0.2+δSr_0.3MnO₃, for δ between −0.04 and 0.04, where the hole concentration is n=0.5+δ. In this series, we study the effects of moving n away from 0.5 on both, the phase diagram and phase separated state. We found that the low temperature ferromagnetic fraction X_FM continuously decreases when n increases. As a result, the samples with n<0.5 (large X_FM) exhibit metallic resistivity at low T while for n>0.5 the insulating state predominates. We construct a detailed T–n phase diagram showing the asymmetries around half-doping.     

Charge ordering and structural transitions in Pr0.5Sr0.41Ca0.09MnO3

The structural and magnetic transitions in Pr_0.5Sr_0.41Ca_0.09MnO₃ have been investigated by neutron diffraction and electron microscopy. Two structural transitions, Imma to I4/mcm and I4/mcm to Pmmn, are observed by decreasing the temperature. Two magnetic transitions, from a paramagnetic insulating to a ferromagnetic metallic and from a ferromagnetic metallic to an antiferromagnetic insulating states at T_C=250 K and T_N=180 K, respectively, are also observed. The structures of these three forms have been determined from neutron powder diffraction data. The first important result concerns the low temperature antiferromagnetic CE type and charge ordered structure, which has been refined in the Pmmn space group, without any constraint. This structure is completely long range ordered, with two Mn-sites, Mn³⁺ in tetragonally elongated octahedra, and Mn⁴⁺, off-centered in nearly regular octahedra. The second important point concerns the abrupt character of the structural transition from the I4/mcm to the Pmmn structure, without any appearance of incommensurability. The magnetic and transport properties of this compound are compared with those of Pr_0.5Sr_0.5MnO₃.     

Physical properties of the doped manganites Ln0.5M0.5Mno3 (Ln = Pr,La; M = Sr,Ca, Ba)

We present the structural, magnetic and electrical properties of some manganese oxides such as Ln_0.5M_0.5MnO₃ (Ln = La, Pr and M = Ca, Sr, Ba) in which the average ionic radius <r_A> of the A site cation is varied while keeping the Mn³⁺/Mn⁴⁺ ratio fixed to 1. All the studied samples have a perovskite structure with rhombohedral or orthorhombic distortion. This distortion was related to the A cation size. The lattice volume increases lineraly with ( r _A) in all the range 1.179–1.3264 Å. The magnetic investigation shows that the magnetic transition temperature T _t, and magnetisation saturation M _s have similar behaviour by increasing and decreasing with (r _A). The electrical measurements show conduction by small polarons attributed to a large canting angle 9 in all the range 4–300 K in the two ferromagnetic compounds La_0.5Sr_0.5MnO₃ and La_0.5Sr_0.5MnO₃.     

Magnetotransport properties of epitaxial Pr0.5Ca0.5MnO3 films grown by a solution technique

Epitaxial Pr_0.5Ca_0.5MnO₃ films have been synthesized on (0 0 1) SrTiO₃ substrate using a chemical solution deposition technique and two-step post-annealing process. The zero field resistivity of the films shows semiconducting behavior and a characteristic of charge ordering is observed at 230 K. The resistivity of the 10 nm film did not show any effect with the magnetic field. However, melting of charge ordering was observed for the 120 nm film at an applied magnetic field of 4 T. Large decrease in the resistivity of the 120 nm film (<100 K) resulted in magnetoresistance of nearly −100% at 75 K.     

Characterisation of the reduction properties of La0.5Sr0.5Fe0.5Co0.5O3

We report here on the characterisation by temperature programmed reduction, ⁵⁷Fe Mössbauer spectroscopy and X-ray absorption spectroscopy of the phases resulting from treatment of the perovskite-related material La_0.5Sr_0.5Fe_0.5Co_0.5O₃ in a flowing 90% hydrogen/10% nitrogen atmosphere. The results show that treatment of La_0.5Sr_0.5Fe_0.5Co_0.5O₃ (which contains approximately 50% Fe⁴⁺ and 50% Fe³⁺) in the flowing 90% hydrogen/10% nitrogen atmosphere at 600°C does not result in the reduction of any of the constituent elements of the material and that the perovskite structure is still retained. The Mössbauer spectrum recorded following heating in the gaseous reducing environment at 1,000°C shows the presence of metallic iron, an Fe³⁺-containing phase with parameters compatible with the presence of SrLaFeO₄ which has a K₂NiF₄-type structure, and a paramagnetic Fe³⁺ phase. The X-ray absorption spectroscopy results show the presence of metallic cobalt. The Mössbauer spectrum recorded following heating at 1,200°C continues to show the Fe³⁺-containing components plus a larger contribution from metallic iron. The X-ray absorption spectroscopy results show the presence of metallic cobalt, SrLaFeO₄, La₂O₃ and SrO.     

磁场对La2－xSrxCuO4单晶热导的影响研究

报道了系列欠掺杂La_2-xSr_xCuO₄（x=0063,0070,0 090,0110,0125）单晶的零场和加磁场情况下ab面和c方向的热导率与温度的关系曲线，测量 温度范围从2到45K. 研究发现ab面和c方向的热导率都受到磁场的压制. 而且在磁场 的作用下，热导压制率随温度变化的关系和场致反铁磁有序的增强与温度的依赖关系有高度 相似性. 认为磁场引起的ab面的热导压制主要是电子热导的变化所致，而c方向的 压制则可能主要来源于声子热导的变化，它们均可能与磁场诱导下欠掺杂La_2-x Sr_xCuO₄的某种电荷有序和磁有序的增强密切相关. 关键词： 热导 电荷有序 磁有序     

Crystal structure and the magnetic state of Pr0.5Sr0.5Co0.5Fe0.5O3

The magnetic properties and crystal structure of the Pr_0.5Sr_0.5Co_0.5Fe_0.5O₃ compound are studied by neutron and x-ray diffractions using synchrotron radiation. These measurements show that this compound is a dielectric spin glass with a magnetic moment freezing temperature of about 70 K. As temperature decreases in the range 30–95 K, a structure phase transition of the first order occurs with an increase in the symmetry from orthorhombic (space group Imma) to tetragonal (space group I4/mcm). It is assumed that the transition is caused by a change in the 4f electron configuration of the Pr³⁺ ions.     

Ab initio study of Os0.5W0.5B2, Re0.5W0.5B2, and Os0.5Re0.5B2 with high shear modulus

By using first‐principles calculations, the authors investigate the structural, mechanical, and electronic properties of experimentally synthesized Os_0.5W_0.5B₂. The calculated structural parameters and elastic properties are in good agreement with the experimental results. In addition, two new 5d transition‐metal diborides (Re_0.5W_0.5B₂ and Os_0.5Re_0.5B₂) are predicted to have promising large shear moduli. The latter mainly come from the non‐uniform distribution of valence charge density, which raises the value of the shear moduli. We discuss potentially high hardness in these materials. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

High-temperature impedance spectroscopy of BaFe0.5Nb0.5O3 ceramics doped with Bi0.5Na0.5TiO3

Bi_0.5Na_0.5TiO₃ (BNT)-doped BaFe_0.5Nb_0.5O₃ (BFN) ceramics were synthesized by a two-step solid-state reaction. Temperature dependence of dielectric properties measured at different frequencies was investigated over broad temperature and frequency ranges. Impedance spectroscopy and universal dielectric response were employed to study the relaxation behavior and conductivity mechanism of the ceramics in a frequency range from 40 Hz to 100 MHz and a temperature range from 300 K to 800 K. The complex plane impedance data revealed the bulk and grain boundary contributions toward conductivity processes in the form of semicircular arcs. The high-temperature conductivity of ceramics is attributable to thermally activated second ionized oxygen vacancy.