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     

La0.3Ca0.7Mn1－xVxO3体系的有序相和再入型自旋玻璃行为研究

通过对La_0.3Ca_0.7Mn_1－xV_xO₃(x=0.05,0.10,0.134,0.20)体系的M-T曲线、ρ-T曲线、ESR谱的测量,研究了Mn位掺V对La_0.3Ca_0.7MnO₃体系电荷序和自旋序的影响.结果表明,当0.05≤x≤0.134时, 体系存在电荷有序(CO)相,其自旋序随温度降低发生顺磁(PM)-电荷有序(CO)-反铁磁(AFM)变化.当x＝0.20时,CO相逐步融化,在40 K发生自旋玻璃转变,表现出再入型的自旋玻璃行为,低温下的基态存在着多种复杂的磁相互作用之间的竞争机理. 关键词： 电荷有序 自旋序 相分离 再入型自旋玻璃行为     

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Sm0.5Ca0.5MnO3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TSG在41K附近,同时,观察到了存在于该体系中负的磁化异常,表明半掺杂的Sm0.5Ca0.5MnO3基态存在有多种复杂而丰富的磁相互作用之间竞争机制,其研究将为强关联锰氧化物体系物理机理的理解提供丰富的实验资料.     

相分离Nd0.5Ca0.5MnO3体系的再入型自旋玻璃行为和电荷有序

K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,Nd0.5Ca0.5MnO3体系在低温下存在着典型的相分离和多种复杂的磁相互作用竞争机制,这为强关联锰氧化物体系物理机制的理解提供了重要的实验资料.     

A位的Gd掺杂对La0.7Sr0.3MnO3体系磁电性质的影响

通过实验研究了La_0.7-xGd_xSr_0.3MnO₃(x= 0.00，0.10，0.15，0.20，0.30 ，0.40，0.50，0.60，0.70)体系的M-T曲线、M-H曲线、红外光谱、拉曼光谱、ρ-T曲线和M R-T曲线.实验结果表明：随着Gd掺杂的增加，体系从长程铁磁有序向自旋团簇玻璃态和反铁 磁状态转变，高掺杂时的输运性质在其磁背景下发生异常.Gd掺杂引起的磁结构变化和额外 的磁性耦合将导致庞磁电阻效应. 关键词： 磁结构 输运行为 庞磁电阻效应     

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     

SrMn0.5Fe0.5O3的自旋玻璃态和过渡金属离子价态的研究

采用固相反应法制备了SrMn_0.5Fe_0.5O₃陶瓷样品,并对样品的晶体结构,磁性和离子价态进行了系统的表征与分析. X射线衍射谱的Rietveld拟合表明样品属于理想的立方钙钛矿型结构,Mn离子和Fe离子随机占据B位的O八面体中心. X射线光电子能谱表明Mn离子为+3和+4的混合原子价态,Fe离子为+3价. 样品在大于230K的高温区域呈现Curie顺磁特性,在小于230 K的低温区域样品表现出自旋玻璃态行为,这种特性源于Mn离子和Fe离子之间的交换作用及自身价态和分布的不均匀性. 由于Fe³⁺离子占据O八面体的中心,对顺磁区的Mssbauer谱测量表现为四级分裂. 关键词： 晶体结构 价态 顺磁 自旋玻璃态     

晶粒尺寸对Bi0.2Ca0.8MnO3电荷有序的影响

利用溶胶-凝胶法制备了平均晶粒尺寸在38—1500 nm的系列Bi_0.2Ca_0.8MnO₃多晶样品,研究了它们的结构、电输运性质和磁性.研究结果表明,随着晶粒尺寸的减小,电荷有序逐渐消失、最后被完全压制,导致低温下出现自旋玻璃态.利用马氏体相变图像和表面效应解释了相关现象. 关键词： 钙钛矿锰氧化物 电荷有序 马氏体相变 自旋玻璃     

相分离Eu0.5Sr0.5MnO3体系中的场诱导磁相变及输运特性研究

研究了半掺杂相分离锰氧化物Eu_0.5Sr_0.5MnO₃样品的结构和电磁输运特性.在半掺杂情况下,该样品呈O′型正交结构,表明样品存在典型的Jahn-Teller畸变;在75 K附近样品的顺磁/反铁磁背景中开始出现铁磁相,在更低的温度42 K,4000 A/m磁场下M-T的场冷曲线和零场冷曲线出现明显分岔,样品的交流磁化率实部随温度的变化曲线中也在42 K观察到尖峰的出现,表现出团簇玻璃行为.在无外加磁场下该样品在 关键词： 多相竞争 半掺杂 铁磁团簇     

永磁性大块金属玻璃Nd60Al10Fe20Co10低 温磁性的研究

研究了Nd₆₀Al₁₀Fe₂₀Co₁₀ 大块金属玻璃 磁性随温度的变化关系，结果表明Nd₆₀Al₁₀Fe₂₀Co₁₀ 在室温下 表现为永磁性，随着温 度的降低，矫顽力和磁滞回线形状都有很大的变化.交流磁化率在18 K左右出现尖峰而且峰 值温度随频率变化，表明该大块非晶体系中存在自旋玻璃态. 关键词： 大块金属玻璃 低温磁化率 自旋玻璃     

Asymmetrical phase separation in Nd0.25La0.25Ca0.5MnO3

The highly metastable region across the metal-insulator transition in Nd_0.25La_0.25Ca_0.5MnO₃ is probed by transport and magnetic measurements. The metal-insulator transition observed around 130 K is associated with large thermal hysteresis between temperatures 50-150 K. Observation of minor hysteresis loops across the region indicates coexistence of both metallic and insulating phases. The field-cooled heating and the zero-field-cooled heating susceptibilities deviate from each other from 205 K, signifying thermo-magnetic irreversibility arising from magnetic pinning and/or glassy magnetic behaviour. The magnetoresistance measured on the heating and cooling loops are found to be different in magnitude and nature. The cooling-cycle magnetoresistance was found to be highly irreversible with an open-ended hysteresis loop. We also observe change in the nature of the magnetic relaxation data on the heating and cooling protocols. This indicate strong thermal history dependence of magneto-transport behaviour in the present compound.     

Analysis of low temperature specific heat in Nd0.5Sr0.5MnO3 and R0.5Ca0.5MnO3 (R=Nd,Sm, Dy and Ho) compounds

We report on the specific heat C(T) of doped manganites Nd_0.5Sr_0.5MnO₃, Nd_0.5Ca_0.5MnO₃, Sm_0.5Ca_0.5MnO₃, Dy_0.5Ca_0.5MnO₃ and Ho_0.5Ca_0.5MnO₃ in the temperature range 2?T?300 K using modified rigid ion model (MRIM). The present specific heat results are in general satisfactory agreement with experimental data except at very low temperatures (i.e. T?12 K). Also a sharp peak observed in the experimental results for these compounds around 5 K could not be revealed by our computed results as they arise due to Schottky-like anomaly. Besides, we have reported the cohesive and the thermal properties of these compounds. The results obtained by us are discussed in detail.     

Effect of Y-doping on the magnetic and charge orderings in Nd2/3Ca1/3MnO3

The shifts of the magnetic and charge ordering transition temperatures caused by Nd substitution for Y in Nd_2/3Ca_1/3MnO₃ CMR narrow-band perovskite manganite have been studied. At low temperatures, three different long-range magnetic orderings consistent with a phase separation scenario have been observed in the doped compound (Nd_0.9Y_0.1)_2/3Ca_1/3MnO₃ by neutron-diffraction study: the antiferromagnetic orderings of PCE and DE types existing below ∼110 and ∼60 K, respectively, and the ferromagnetic one of B type existing below ∼42. Magnetic phase transformations temperatures as well as those of charge ordering have been found to be structural-dependent. Y-doping leads to the decrease of the anisotropy of the orthorhombic Pnma crystal lattice b/√2c, which causes a decrease of the indirect exchange parameters in the system and thus a decrease in the magnetic transformation temperatures for 20-30 K in the doped compound. Doping leads as well to the higher level of the coherent Jahn-Teller distortions of the MnO₆ octahedra in the 200-300 K temperature region, which results in the increase of the charge ordering temperature for ∼80 K.     

Effect of Ca doping on the magnetic properties of Nd0.5Sr0.5MnO3 studied by electron spin resonance

The magnetic properties of Ca-doped Nd_0.5Sr_0.5MnO₃ have been studied by electron spin resonance (ESR) and dc magnetization measurements. The antiferromagnetic order and charge order are found to occur separately at T_N=200 K and T_co=150 K, respectively. Compared to the undoped Nd_0.5Sr_0.5MnO₃, the ferromagnetic correlations are suppressed by doping of the small Ca²⁺ ion. In addition, the antiferromagnetic transition temperature is enhanced to 200 K, which can be explained by an increase of superexchange interaction between Mn³⁺ and Mn⁴⁺ ions as their distance decreases.     

Heisenberg-like ferromagnetism and percolative conductivity in the half-doped manganite Nd0.5Ca0.25Sr0.25MnO3

The magnetic behavior and electronic transport in the half-doped manganite Nd_0.5Ca_0.25Sr_0.25MnO₃ have been investigated. The critical exponents are studied by using isothermal magnetization methods. The results show that the paramagnetic-ferromagnetic transition is second order and the magnetic interaction is satisfied with the prediction of three-dimensional Heisenberg model. The electronic transport belongs to the percolation mechanism. These findings demonstrate that the critical behavior of the magnetic transition and conductivity for manganites are related to Mn-site ordering degree.     

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.     

Effect of oxygen non-stoichiometry on the antiferromagnet-ferromagnet transition in Nd0.5Ca0.5MnO3−δ(δ≤0.12)

A decrease in the oxygen content in Nd_0.5Ca_0.5MnO_3?δ down to γ≤0.12 is shown to bring about a strong decrease in the magnetic field inducing a transition from the antiferromagnetic charge-ordered to the ferromagnetic charge-disordered state. The ferromagnetic phase in a Nd_0.5Ca_0.5MnO_2.92 sample is stable in the absence of an external magnetic field. A further increase in the content of oxygen vacancies stabilizes the antiferromagnetic charge-disordered state.     

An electron paramagnetic resonance study of phase segregation in Nd0.5Sr0.5MnO3

We present results of an electron paramagnetic resonance (EPR) study of Nd_1−xSr_xMnO₃ with x=0.5 across the paramagnetic to ferromagnetic, insulator to metal transition at 260 K (T_c) and the antiferromagnetic, charge ordering transition (T_N=T_co) at 150 K. The results are compared with those on Nd_0.45Sr_0.55MnO₃ which undergoes a transition to a homogeneous A-type antiferromagnetic phase at T_N=230 K and on La_0.77Ca_0.23MnO₃ which undergoes a transition to coexisting ferromagnetic metallic and ferromagnetic insulating phases. For x=0.5, the EPR signals below T_c consist of two Lorentzian components attributable to the coexistence of two phases. From the analysis of the temperature dependence of the resonant fields and intensities, we conclude that in the mixed phase ferromagnetic and A-type antiferromagnetic (AFM) phases coexist. The x=0.55 compound shows a single Lorentzian throughout the temperature range. The signal persists for a few degrees below T_N. The behaviour of the A-type AFM phase is contrasted with that of the two ferromagnetic phases present in La_0.77Ca_0.23MnO₃. The comparison of behaviour of A-type AFM signal observed in both Nd_0.5Sr_0.5MnO₃ and Nd_0.45Sr_0.55MnO₃ with the two FM phases of La_0.77Ca_0.23MnO₃, vis-à-vis the shift of resonances with respect to the paramagnetic phases and the behaviour of EPR intensity as a function of temperature conclusively prove that the Nd_0.5Sr_0.5MnO₃ undergoes phase separation into A-type AFM and FM phases.