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     

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.     

(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。     

Nd0.5Ca0.5Mn1-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,     

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     

R-site cation randomness effect in the A-site ordered Y0.5La0.5BaMn2O6 compound

The R/Ba-ordered and R-site mixed compound Y0.5La0.5BaMn2O6 is synthesized, in which (Y, La) and Ba are regularly arranged, while Y and La randomly occupy the R-site. Y0.5La0.5BaMn2O6 has a tetragonal unit cell with a space group of P4/mmm. A structural transition between tetragonal and orthorhombic is observed at about 325 K by X-ray powder diffraction (XRD). Thermal magnetic measurement shows the occurrence of an antiferromagnetic transition at the temperature TN～190 K. Anomalies in magnetization, resistivity and lattice parameters observed around 340 K indicate a charge/orbital order transition accompanying the structural phase transition. The R-site randomness effect is discussed to interpret the different properties of Y0.5La0.5BaMn2O6 between NdBaMn2O6 and SmBaMn2O6.     

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 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.     

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   

Electrical and structural properties of (Pb0.5Fe0.5)Sr2(RE0.5Ca0.5)Cu2Oz (RE: rare-earth element)

(Pb_0.5Fe_0.5)Sr₂(Y_0.5Ca_0.5)Cu₂O_z is known to have so-called “1-2-1-2” crystallographic structure which is one of typical structures of high-temperature superconducting cuprates (HTSC’s). While it possesses structural characteristics of HTSC’s, superconductivity has not been reported so far. In this study, phase formation, electrical properties and structural characteristics of this compound are investigated. It is shown that this phase can be synthesized using several kinds of rare-earth element and that post-annealing in O₂ is effective for decreasing electrical resistivity. The latter suggests that this “1-2-1-2” can be made superconducting by appropriate carrier-doping into the [Cu–O₂] plane. Preliminary Rietveld structural refinement using powder X-ray diffraction data show that the initial structural model based on the nominal (Pb,Fe)Sr₂(Y,Ca)Cu₂O_z is basically correct except for true site-distribution of Fe and Cu.     

三元合金Ga_(0.52)In_(0.48)P的喇曼散射谱

采用室温下微区Raman散射方法 ,观测到了GaInP2 的LO双模行为和禁戒的TO模 ,由于晶格有序导致晶体对称性从Td 降低为C3v,从而使禁戒的TO模变为Raman活性。在所有的样品中都观测到了由晶格无序激活的DALA模。在有序样品中 ,除观测到了二级Raman散射峰LO1+LO2 以外 ,还观测到了由超晶格效应所导致的FLA折叠模和LO模的分裂。对LO模峰谷比b/a的分析表明 ,随着晶格有序度的增加 ,b/a值减小。这是因为 :一方面主要是由于禁戒的TO模变为Raman活性所引起的 ,另一方面 ,还可能有LO1模和LO2 模分裂的贡献。在实验上 ,可以用b/a值或FLA的强度来表示样品的有序度。     

Dielectric loss of half-doped manganite La_(0.5)Ca_(0.5)MnO_3

The dielectric loss tanδ of half-doped manganite La 0.5 Ca 0.5 MnO 3 is investigated using Green's function technique. The La 0.5 Ca 0.5 MnO 3 is described by the Kondo-lattice model in the double exchange limit, taking into account the Jahn-Teller distortion and the super-exchange interaction between the localized electrons. It is found that the intensity of tanδ decreases with increasing | ε JT |, V, and U. It is also observed that the transition temperature T P rises as | ε JT | and U increase. It is worth noting that T P remains unchanged and the strength of tanδ increases with increasing g. The calculated dielectric loss results are explained theoretically, and these behaviors are in qualitative agreement with the experimental results.     

自发有序Ga_（0．5）In_（0．5）P合金的拉曼光谱研究

自发有序Ｇａ＿（０．５）Ｉｎ＿（０．５）Ｐ合金的拉曼光谱研究李国华，刘振先，韩和相，汪兆平，董建荣（中国科学院半导体研究所，半导体扭晶格国家重点实验室北京１０００８３）（中国科学院半导体研究所，半导体材料科学实验室北京１０００８３））ＲａｍａｎＳｃａ...     

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.     

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.     

Magnetic order in hybrid frustrated magnets Gd(2-x)Tb(x)Ti2O7 (x = 0.2 and 0.5)

We report on the specific heat, magnetization and ac susceptibility measurements of single crystals of hybrid frustrated magnets Gd(1.8)Tb(0.2)Ti(2)O(7) and Gd(1.5)Tb(0.5)Ti(2)O(7). The analysis of experimental data revealed that, although partial replacing of the Gd(3+) ions by the Tb(3+) ions in the Gd(2)Ti(2)O(7) host lattice slightly enhances antiferromagnetic coupling, as inferred from the evolution of the paramagnetic Curie-Weiss temperature, the ordering temperature gradually decreases. Paramagnetic correlations introduced by the Tb(3+) ions cause this perturbation, altering the effective further neighbor interactions and destabilizing the ground state in Gd(2)Ti(2)O(7). In addition, the low-energy states of Gd(2-x)Tb(x)Ti(2)O(7) are suggested to possess a nature different from those in parent members Tb(2)Ti(2)O(7) and Gd(2)Ti(2)O(7). Finally, the frequency-dependent magnetic susceptibility behavior in Gd(1.5)Tb(0.5)Ti(2)O(7) is consistent with the formation of a spin-glass-like state indicating a pronounced slowing down of the dynamical response of the studied hybrid magnets.