Effect of electron doping on the magnetic correlations in the bilayered brownmillerite compound Ca(2.5-x)La(x)Sr(0.5)GaMn2O8: a neutron diffraction study

The effect of electron doping on the magnetic properties of the brownmillerite type bilayered compounds has been investigated by neutron powder diffraction in La substituted Ca(2.5-x)La(x)Sr(0.5)GaMn(2)O(8) compounds (x = 0.05 and 0.1), in comparison with the undoped compound (x = 0). In all compounds, a long-range three-dimensional collinear antiferromagnetic (AFM) structure is found below the Néel temperature T(N) of the respective compound, whereas, well above T(N), three-dimensional short-range magnetic ordering is observed. In the intermediate temperature range just above T(N), a strong effect of electron doping (La substitution) on the magnetic correlations has been observed. Here, a short-range AFM correlation with a possible dimensionality of three has been found for substituted compounds (x = 0.05 and 0.1) as compared to the reported two-dimensional long-range AFM ordering in the parent compound. With increasing electron doping, a decrease in T(N) is also observed. The short-range magnetic correlations set in over a large temperature range above T(N). A magnetic phase diagram in the x-T plane is proposed from these results.     

Doping dependent evolution of magnetism and superconductivity in Eu(1-x)K(x)Fe2As2 (x = 0-1) and temperature dependence of the lower critical field H(c1)

We have synthesized polycrystalline samples of Eu(1-x)K(x)Fe2As2 (x = 0-1) and carried out systematic characterization using x-ray diffraction, ac and dc magnetic susceptibility, and electrical resistivity measurements. A clear signature of the coexistence of a superconducting transition (T(c) = 5.5 K) with spin density wave (SDW) ordering is observed in our underdoped sample with x = 0.15. The SDW transition disappears completely for the x = 0.3 sample and superconductivity arises below 20 K. The superconducting transition temperature Tc increases with increase in the K content and a maximum Tc = 33 K is reached for x = 0.5, beyond which it decreases again. The doping dependent Tx phase diagram is extracted from the magnetic and electrical transport data. It is found that magnetic ordering of Eu moments coexists with the superconductivity up to x = 0.6. The isothermal magnetization data taken at 2 K for the doped samples suggest the 2+ valence state of the Eu ions. We also present the temperature dependence of the lower critical field H(c1) of the superconducting polycrystalline samples. The values of H(c1)(0) obtained for x = 0.3, 0.5, and 0.7 after taking the demagnetization factor into account are 202, 330, and 212 Oe, respectively. The London penetration depth λ(T) calculated from the lower critical field does not show exponential dependence at low temperature, as would be expected for a fully gapped clean s-wave superconductor. In contrast, it shows a T2 power law feature up to T = 0.3Tc, as observed in Ba(1-x)K(x)Fe2As2 and BaFe(2-x)Co(x)As2.     

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,     

Charge ordering fluctuation and optical pseudogap in La(1-x)Ca(x)MnO(3)

Optical spectroscopy was used to investigate the optical gap ( 2 Delta) due to charge ordering (CO) and related pseudogap developments with x and temperature ( T) in La(1-x)Ca(x)MnO(3) ( 0.48< or =x< or =0.67). Surprisingly, we found 2 Delta/k(B)T(CO) is as large as 30 for x approximately 0.5, and decreases rapidly with increasing x. Simultaneously, the optical pseudogap, possibly starting from T* far above T(CO) becomes drastically enhanced near x = 0.5, producing non-BCS T-dependence of 2 Delta with the large magnitude far above T(CO), and systematic increase of T* for x approximately 0.5. These results unequivocally indicate systematically enhanced CO correlation when x approaches 0.5 even though T(CO) decreases.     

La0.3Ca0.7Mn1-xWxO3体系电荷有序的建立与融化

用标准的固相反应法制备了La0.3Ca0.7Mn1-xWxO3(0.00≤X≤0.15)多晶样品.通过对样品磁化强度-温度(M-T)曲线、磁化强度-磁场强度(MH)曲线及ESR谱的测量,研究了Mn位W掺杂对La0.3Ca0.7Mn1-xWxO3体系的磁相变的影响.结果表明,随着W掺杂量的增加,体系磁相变发生了复杂的变化过程.当掺杂量为0.00≤X≤0.08时,体系存在电荷有序(CO)相,反铁磁(AFM)/C0态共存于相变温度以下,电荷有序温度(Tco)随着W掺杂量的增加而增加.x≥0.12时,体系电荷有序态逐渐减弱并融化,在极低温度下存在顺磁-铁磁(PM-FM)相变.     

Neutron powder diffraction investigation on the crystal and magnetic structure of (Ho(0.50+x)Ca(0.50-x))(Mn(1-x)Cr(x))O3

The crystal and magnetic structure of (Ho(0.50+x)Ca(0.50-x))(Mn(1-x)Cr(x))O(3) (x = 0.00, 0.01, 0.02, 0.03) has been investigated between 5 and 300 K by means of neutron powder diffraction followed by Rietveld refinement and dc magnetic measurements. During cooling an orthorhombic to monoclinic phase transition occurs on account of the charge and orbital ordering taking place in the Mn sub-lattice; at low temperature phase separation takes place and the main monoclinic phase coexists with a secondary orthorhombic phase, whose amount slightly increases with the increase of Cr content. Cr(3+) is not involved in orbital ordering or superexchange interactions. The charge and magnetic ordering are decoupled: the Mn moments order according to a CE-type structure in all samples.     

Ba2+掺杂钙钛矿Bi0.5Ca0.5MnO3多晶材料的结构、磁性和输运性质研究

本文系统地研究了具有电荷有序(CO)和C型反铁磁(AFM)磁结构的钙钛矿锰氧化物Bi0.5Ca0.5-xBaxMnO3(x=0,0.03,0.05,0.07 and 0.10)的结构、磁性和电输运性质.系统晶格对称性保持Pnma空间群正交结构不变.随着Ba掺杂浓度的提高,电荷有序转变温度和反铁磁转变温度明显降低,表明掺...     

Experimental evidence of coupling interaction between charge ordering and spin ordering

In order to experimentally probe into the complicated interaction between charge ordering and spin ordering in manganites,two sets of samples Nd0.5Sr0.5Mn1-xGaxO3(NSMGO) and Nd 0.5Sr0.5Mn1-y CryO3(NSMCO)(0.0 x,y 0.075),have been studied by means of electrical transport,magnetization,electron spin resonance and transmission electron microscopy analysis.By comparing the influence of Cr-doping and Ga-doping in the Nd0.5Sr0.5MnO3(NSMO) system,large difference between the evolution of charge ordering temperature T co in the Cr-doping and the Ga-doping cases is found.In the NSMCO system,the CE-type antiferromagnetic(AFM)/charge ordering(CO) phase disappears with only 2.5 percent Cr doping;but in the NSMGO system,the CE-type AFM/CO phase always exists.This phenomenon indicates that the charge ordering formation is dominated by the spin ordering.As a result,it is experimentally proved that there is strong coupling interaction between charge ordering and spin ordering in NSMO system.     

Temperature-dependent structures of proton-conducting Ba(Zr(0.8-x)Ce(x)Y(0.2))O(2.9) ceramics by Raman scattering and x-ray diffraction

In situ temperature-dependent micro-Raman scattering and x-ray diffraction have been performed to study atomic vibration, lattice parameter and structural transition of proton-conducting Ba(Zr(0.8-x)Ce(x)Y(0.2))O(2.9) (BZCY) ceramics (x = 0.0-0.8) synthesized by the glycine-nitrate combustion process. The Raman vibrations have been identified and their frequencies increase with decreasing x as the heavier Ce(4+) ions are replaced by Zr(4+) ions. The main Raman vibrations of Ba(Ce(0.8)Y (0.2))O(2.9) appear near 305, 332, 352, 440 and 635 cm(-1). The X-O ( X=Ce, Zr, Y) stretching modes are sensitive to the variation of Ce/Zr ratio. A rhombohedral-cubic structural transition was observed for x = 0.5-0.8, in which the transition shifts toward higher temperature as cerium increases, except for Ba(Ce(0.8)Y(0.2))O(2.9). A minor monoclinic phase possibly coexists in the rhombohedral matrix for x = 0.5-0.8. The lower-cerium BZCYs (x = 0.0-0.4) ceramics do not exhibit any transition in the region of 20-900?°C, indicating a cubic phase at and above room temperature.     

Possible quantum critical point in La(2/3)Ca(1/3)Mn(1-x)Ga x O3

We study the magnetic ground state in La(2/3)Ca(1/3)Mn(1-x)Ga x O3 manganites, where a quantum critical point (QCP) has been theoretically predicted. The metallic ferromagnetic ground state for low Ga doping breaks down for x > or = 0.11, an insulating state being established at low temperatures. Long-range ferromagnetism coexists with short-range magnetic correlations in the concentration range 0.11 < or = x < or = 0.145 while only the short-range correlations survive for x > or = 0.16. We discuss the implications of such a QCP to the physics of manganites and compare to other QCP systems.     

Evolution of structural and magnetic properties with varying oxygen content in low-bandwidth manganite Pr0.9Ca0.1MnO3 thin films

The effects of ex situ vacuum and oxygen annealing treatments on thin films of the low-bandwidth compound Pr(1-x)Ca(x)MnO(3) (PCMO) are investigated. Structural and magnetic measurements reveal that increased ferromagnetism can be achieved by oxygen annealing treatment, which is linked to the increased Mn(4+) ion content, as observed from x-ray photoelectron spectroscopy (XPS) measurements, as well as relaxation of the substrate-induced tensile strain of the PCMO unit cell. The increased number of Mn(4+) ions and partial release of strain lead to stronger double-exchange interaction in the system. Vacuum annealing increases the ferromagnetic (FM) interaction as well; however, the increased FM ordering is not directly related to the improved double-exchange interaction, as XPS measurement reveals an indication of a slight increase in Mn(3+) ions in this case. Trapping of carriers in the oxygen vacancies and formation of magnetic polarons have been suggested as the causes of the increase in ferromagnetic ordering, and this is also supported by the large coercivity and longer spin memory in the vacuum annealed PCMO.     

A Crystallographic and Mössbauer Spectroscopic Study of BaCo0.5x Zn0.5x Ti x Fe12−2x O19 (M-Type Hexagonal Ferrite)

Hyperfine Interactions - A series of pure polycrystalline M-type hexagonal ferrite with the formula BaCo0.5x Zn0.5x Ti x Fe12?2x O19 (x=0.0, 0.4, 0.8, 1.2, 1.6, 2.0) has been synthesized and...     

Nanomagnetic droplets and implications to orbital ordering in LA(1-x)Sr(x)CoO3

Inelastic cold-neutron scattering on LaCoO3 provided evidence for a distinct low energy excitation at 0.6 meV coincident with the thermally induced magnetic transition. Coexisting strong ferromagnetic (FM) and weaker antiferromagnetic correlations that are dynamic follow the activation to the excited state, identified as the intermediate S = 1 spin triplet. This is indicative of dynamical orbital ordering favoring the observed magnetic interactions. With hole doping as in La(1-x)Sr(x)CoO3 , the FM correlations between Co spins become static and isotropically distributed due to the formation of FM droplets. The correlation length and condensation temperature of these droplets increase rapidly with metallicity due to the double exchange mechanism.     

La_(0.7-x)Gd_xSr_(0.3)MnO_3(x=0.20,0.30,0.40,0.50)的TCR研究(英文)

研究了La位Gd掺杂对La0.7-xGdxSr0.3MnO3(x=0.20,0.30,0.40,0.50)体系的电阻温度系数(TCR)的影响.实验结果表明:Gd掺杂将引起电阻率曲线的急剧变化,导致出现大的TCR;而且随Gd掺杂的增加,TCR在x=0.30出现峰值,然后随掺杂量增加逐步降低.体系出现大的TCR,来源于Gd掺杂引起的额外磁性耦合.     

Charge/orbital ordering structure of Pr(1-x)Ca(x)MnO(3)(x = 3/8) examined by low-temperature transmission electron microscopy

The structural phase transition of Pr(1-x)Ca(x)MnO(3)(x = 3/8) was investigated by means of low-temperature transmission electron microscopy. Superlattice reflection spots with a modulation wave vector q(1) = (0,1/2,0) appeared below 230 K, indicating formation of the d(3x(2-r(2))/d(3y(2)-r(2)) type of charge/orbital ordering. Below 150 K, a new series of superlattice reflection spots with a modulation wave vector q(2) = (1/4,1/4,1/2) appeared, suggesting an additional ordering of excess 1/8 Mn(3+), necessary due to the deviation of x from 1/2, with the occupation of the d(3z(2-r(2)) type of e(g) orbital.     

Critical behavior of metal-insulator transition in La1-(x)Sr(x)VO3

Critical behavior of the metal-insulator transition (MIT) coupled with spin/orbital correlations has been investigated for single crystals of La1-(x)Sr(x)VO3. In the paramagnetic (PM) metal phase (x > 0.260), the precursor to the MIT manifests itself as an enhancement of carrier effective mass. In the antiferromagnetic (AF) metal phase (0.178 < or = x < or = 0.260), the carrier density decreases and the correlation of the orbital seems to evolve towards the MIT (x = 0.178). In the AF insulating phase (x < 0.178), the distinct first-order structural phase transition occurs with the decrease of temperature, perhaps concomitantly with the orbital ordering.     

双掺杂铁基1111型超导体Nd1-xBaxFeAsO1-2xF2x的磁传输性能研究

采用两步固相反应法,在母相FeAs=1111结构的NdFeAsO中掺入BaF2,实现电子与空穴的双掺杂．该体系超导转变温度（Tc）在33-50K范围,取决于名义BaF2掺杂量x．在实验范围内,掺杂量越大,转变温度越高,当x=0．2时,Tc达到50K．磁电阻测量表明：高掺杂量样品（例如x=0．2）具有较高的上临界场（Hc...     

Multiple photonic responses in films of organic-based magnetic semiconductor V(TCNE)x, x approximately 2

Concomitant photoinduced magnetic and electrical phenomena are reported for the organic-based magnetic semiconductor V(TCNE)x (x approximately 2; TCNE=tetracyanoethylene; magnetic ordering temperature Tc approximately 400 K). Upon optical excitation (457.9 nm), the system can be trapped in a thermally reversible photoexcited state, which exhibits reduced magnetic susceptibility and increased conductivity with a simultaneous change in IR absorption spectrum. The multiple photonic effects in V(TCNE)x are proposed to originate from structural changes induced by internal excitation in (TCNE)- anions, which lead to relaxation to a long-lived metastable state.     

Y替代La2/3Ca1/3MnO3体系的结构与输运行为

系统研究了(La1-xYx)2/3Ca1/3MnO3(0.0≤x≤0.3)体系的结构和输运行为.结果表明，实验样品具有很好的单相结构，随Y掺杂浓度的增加，金属—绝缘体（M—I）转变温度T-MI向低温区移动，对应的峰值电阻率ρp升高，对x=0.3样品，较未替代样品（x=0.0）增幅达8个数量级.在外加磁场下，材料表现出很强的磁电阻效应.同时，从实验结果出发，直接给出了输运特性与晶体结构之间的关联，并从双交换模型和可变程跃迁理论出发，对实验结果进行了初步讨论. 关键词： La2/3Ca1/3MnO3锰氧化物 Y替代 晶体结构 输运行为     

Defect-induced magnetic structure in (Ga(1)-(x)Mn(x))As

We show that magnetic structures involving partial disorder of local magnetic moments on the Mn atoms in (Ga(1)-(x)Mn(x))As lower the total energy, compared to the case of perfect ferromagnetic ordering, when As defects on the Ga sublattice are present. Such magnetic structures are found to be stable for a range of concentrations of As antisites, and this result accounts for the observed magnetic moments and critical temperatures in (Ga(1)-(x)Mn(x))As. We propose an explanation for the stabilization of the partially disordered magnetic structures and conclude that the magnetization and critical temperatures should increase substantially by reducing the number of As antisite defects.