La_(0.4)Ca_(0.6)MnO_3中Mn-位Fe和Cr掺杂对磁性质的影响

研究了Fe和Cr掺杂对La0.4Ca0.6Mn O3中电荷有序反铁磁基态的调控作用.磁性质的测量结果表明,两种离子掺杂均能有效抑制原型样品中的长程电荷有序相,但是Fe离子掺杂样品均具有反铁磁的基态,而Cr掺杂样品中则出现了显著的铁磁性.结合电输运测量结果显示,Cr掺杂引起的铁磁态同时具有金属性,表明其中是电子双交换作用占主导.对比两种掺杂离子的电子结构发现,Cr离子空的e g电子轨道促进了电子双交换作用,而Fe掺杂则只是引入了不同的自旋交换作用,导致自旋无序.     

钙钛矿型锰氧化物(La$lt;sub$gt;0.8$lt;/sub$gt;Eu$lt;sub$gt;0.2$lt;/sub$gt;)$lt;sub$gt;4/3$lt;/sub$gt;Sr$lt;sub$gt;5/3$lt;/sub$gt;Mn$lt;sub$gt;2$lt;/sub$gt;O$lt;sub$gt;7$lt;/sub$gt;的磁性和电性研究

采用传统固相反应法制备钙钛矿型锰氧化物 (La_0.8Eu_0.2)_4/3Sr_5/3Mn₂O₇多晶样品, X-射线衍射分析表明, 样品(La_0.8Eu_0.2)_4/3Sr_5/3Mn₂O₇结构呈现良好的单相. 通过磁化强度随温度的变化曲线(M-T)、不同温度下磁化强度随磁场的变化曲线(M-H)和电子自旋共振谱发现: 在300 K以下, 随着温度的降低, 样品先后经历了二维短程铁磁有序转变 (T_C^2D ≈ 282 K)、三维长程铁磁有序转变(T_C^3D ≈ 259 K)、奈尔转变(T_N ≈ 208K)和电荷有序转变(T_CO ≈ 35 K); 样品 (La_0.8Eu_0.2)_4/3Sr_5/3Mn₂O₇在T_N以下, 主要处于反铁磁态; 在T_C^3D达到370 K时, 样品处于铁磁-顺磁共存态, 在370 K以上时样品进入顺磁态. 此外, 分析电阻率随温度的变化曲线(ρ-T)得到: 样品在金属-绝缘转变温度(T_P ≈ 80 K)附近出现最大磁电阻值, 其位置远离T_C^3D, 表现出非本征磁电阻现象, 其磁电阻值约为61%. 在T_CO以下, 电阻率出现明显增长, 这是由于温度下降使原本在高温部分巡游的e_g电子开始自发局域化增强所致. 通过对 (La_0.8Eu_0.2)_4/3Sr_5/3Mn₂O₇的ρ-T 曲线拟合, 发现样品在高温部分的导电方式基本遵循小极化子的导电方式. 关键词： 磁性 电性 金属-绝缘转变温度 电子自旋共振     

Ti掺杂W$lt;sub$gt;18$lt;/sub$gt;O$lt;sub$gt;49$lt;/sub$gt;纳米线的电子结构与NO$lt;sub$gt;2$lt;/sub$gt;敏感性能的第一性原理研究

采用基于密度泛函理论框架下的第一性原理平面波超软赝势方法,通过理论建模,研究了Ti掺杂的非化学计量比W₁₈O₄₉纳米线的几何与能带结构以及电子态密度,并通过进一步计算NO₂/Ti-W₁₈O₄₉纳米线吸附体系的吸附能、电荷差分密度与电荷布居,分析了Ti掺杂W₁₈O₄₉纳米线的气体吸附与敏感性能. 计算发现,Ti掺杂改变了W₁₈O₄₉纳米线的表面电子结构,引入的额外的杂质态密度和费米能级附近能带结构的显著变化,使掺杂纳米线带隙与费米能级位置改变,纳米线导电性能增强. 吸附在W₁₈O₄₉纳米线表面的NO₂作为电子受体从纳米线导带夺取电子,导致纳米线电导降低,产生气体敏感响应. 与纯相W₁₈O₄₉纳米线相比,NO₂/Ti-W₁₈O₄₉纳米线吸附体系内部存在更多的电子转移,从理论上定量地反映了Ti掺杂对改善W₁₈O₄₉纳米线气敏灵敏度的有效性. 对Ti掺杂纳米线不同气体吸附体系电子布居的进一步计算表明,Ti掺杂纳米线对NO₂气体具有良好的灵敏度和选择性. 关键词： 密度泛函计算 Ti掺杂 18O₄₉纳米线')" href="#">W₁₈O₄₉纳米线 气敏     

Tb$lt;sub$gt;0.3$lt;/sub$gt;Dy$lt;sub$gt;0.6$lt;/sub$gt;Pr$lt;sub$gt;0.1$lt;/sub$gt;(Fe$lt;sub$gt;1-$lt;i$gt;x$lt;/i$gt;$lt;/sub$gt;Al$lt;sub$gt;$lt;i$gt;x$lt;/i$gt;$lt;/sub$gt;)$lt;sub$gt;1.95$lt;/sub$gt;合金的磁性、磁致伸缩和穆斯堡尔谱研究

系统研究了室温下Tb_0.3Dy_0.6Pr_0.1（Fe_1-xAl_x）_1.95 （x=0.05,0.1,0.15,0.2,0.25,0.3）合金中元素Al替代Fe对结构、磁性、磁致伸缩性能和自旋重取向的影响.测量结果发现,x<0.2时Tb_0.3Dy_0.6Pr_0.1（Fe_1-xAl_x）_1.95合金基本上是纯的单相,x=0.2时出现其他杂相,杂相随Al替代量的增加不断增多.随Al替代量x的增加,点阵常数a接近于线性增大,Curie温度T_C逐渐下降,而矫顽力H_c急剧下降.振动样品磁强计（VSM）测量发现,磁化强度M随Al替代量x的变化较为复杂.VSM计和磁致伸缩效应测量共同表明,少量Al的替代有利于降低磁晶各向异性,而且随着Al替代量x的增多磁致伸缩系数快速减小,x>0.15时巨磁致伸缩效应消失.穆斯堡尔效应研究发现,随Al含量的增加Tb_0.3Dy_0.6Pr_0.1（Fe_1-xAl_x）_1.95合金中易磁化轴可能在{110}面逐渐偏离了立方晶体的主对称轴,发生自旋重取向,从而引起合金宏观磁性、磁致伸缩性能的变化. 关键词： 磁致伸缩 立方Laves相 自旋重取向 穆斯堡尔谱     

PrNi$lt;sub$gt;2$lt;/sub$gt;的磁和磁热性能研究

在一些磁性材料内, 磁性离子间交换作用和磁性离子的自旋涨落对材料磁性有影响. 本文根据磁比热实验值确定了晶场参数后, 利用包含自旋涨落的交换作用有效场H_m= n₀ (1 + γ T + β e^{ω T})M, 计算了PrNi₂晶体晶场能级的Zeeman劈裂. 在温度为3.8 K ≤T≤ 30 K范围内, 计算了该晶体多晶磁矩随外磁场的变化, 以及外磁场H=5000 Oe时磁化率倒数随温度的变化, 计算结果和实验值符合较好. 当外磁场在0–50000 Oe时, 计算的该晶体的磁熵变与已有文献的理论结果相似. 计算结果说明, 提出的包含自旋涨落的交换作用有效场不仅适合亚铁磁性晶体, 而且也适合顺磁性晶体. 关键词： 2')" href="#">PrNi₂ 磁比热 交换作用有效场 磁矩 磁熵变     

阻挫三角反铁磁AgCrO2螺旋自旋序的第一性原理研究

基于密度泛函理论的广义梯度近似(GGA)和投影缀加波(PAW)方法,分别从共线和非共线磁性结构出发,研究了自旋阻挫三角反铁磁AgCrO₂的基态、磁性以及电子结构,从理论计算的角度给出了基态磁性结构.计算结果表明:AgCrO₂具有120°螺旋自旋序反铁磁基态,其自旋螺旋面平行于(110)面或(11^-0)面;由于Cr离子间的自旋几何阻挫,导致沿晶体的a,b和a+b方向上均形成了螺旋自旋转动角为120°的 关键词： 第一性原理 交换相互作用 阻挫 反铁磁     

笼状Au$lt;sub$gt;20$lt;/sub$gt;内掺$lt;i$gt;M$lt;/i$gt;$lt;sub$gt;13$lt;/sub$gt;($lt;i$gt;M$lt;/i$gt;=Fe，Ti)团簇磁性的密度泛函计算研究

采用密度泛函理论中的广义梯度近似方法,对M₁₃（M=Fe,Ti）以及M₁₃内掺Au₂₀团簇的几何结构和磁性进行了计算研究.结果表明：M₁₃和M₁₃内掺Au₂₀团簇的几何结构在0.006—0.05 nm误差范围内保持着I_h对称性.Fe₁₃团簇最低能态的总磁矩为44 μ_B,内掺到Au₂₀笼中后形成的Fe₁₃内掺Au₂₀团簇的最低能态总磁矩为38 μ_B,且Au原子与内掺Fe₁₃团簇之间存在着弱铁磁相互作用.Ti₁₃团簇在总磁矩为6 μ_B时能量最低,掺入Au₂₀笼后形成的Ti₁₃内掺Au₂₀团簇最低能态总磁矩是4 μ_B,内表面12个Ti原子与表面Au壳之间是弱铁磁相互作用,而与中心Ti原子之间是弱反铁磁相互作用.由于Au₂₀笼状外壳的影响,Fe₁₃内掺Au₂₀和Ti₁₃内掺Au₂₀团簇中Fe₁₃和Ti₁₃的磁矩比无金壳的Fe₁₃和Ti₁₃团簇的磁矩分别减少了6.81 μ_B和2.88 μ_B. 关键词： 几何结构 磁性 密度泛函理论     

强磁场对金属离子掺杂CaTiO$lt;sub$gt;3$lt;/sub$gt;结构和光学性能的影响

研究了800 ℃条件下不同制备磁场强度(最高12 T)对CaTiO₃ 及其浸渍掺杂样品在结构和光学性能方面的影响. 研究表明：样品吸光性能随浸渍掺杂的离子浓度的增大而提升,且发生红移现象;相同掺杂浓度下,磁场下制备样品的吸光性能均较非磁场下制备的样品有所提高,但不同磁场强度下所制备样品的吸光曲线彼此差异不大;此外,磁制备纯CaTiO₃晶体粉末的X-射线衍射曲线峰左移,紫外-可见漫反射光谱吸收截止波长增长,这表明强磁场可使CaTiO₃晶面间距和晶格常数增大、禁带宽度减小. 关键词： 3')" href="#">CaTiO₃ 强磁场 掺杂     

多铁材料BaCoF$lt;sub$gt;4$lt;/sub$gt;电子结构的第一性原理研究

采用基于密度泛函理论的广义梯度近似方法和赝势平面波法,对多铁材料BaCoF₄的铁电反铁磁相和可能的顺电相的电子结构进行了第一性原理研究.研究表明,反铁磁态很可能有利于低温下的铁电稳定性,F的强负电性使得体系内原子间主要是离子键相互作用.Co离子与在bc面上的F（2）,F（3）离子间完全是离子键作用,而与F（4）间有较弱的共价作用,与F（1）间作用介于两者之间.铁电畸变主要来源于Ba离子与F（1）, F（2）, F（3）离子沿着c轴方向的相对位移,F（4）对铁电性的贡献最少.铁电相中F（2）, F（3）离子的能量低于中心对称相,最大位移贡献者F（1）的化学键性由弱共价作用到离子键的变化也是最大的,这均有利于体系的稳定. 关键词： 第一性原理 铁电性 铁电畸变 反铁磁性     

Co掺杂对Mn$lt;sub$gt;3$lt;/sub$gt;Sn$lt;sub$gt;1-$lt;em$gt;x$lt;/em$gt;$lt;/sub$gt;Co$lt;sub$gt;$lt;em$gt;x$lt;/em$gt;$lt;/sub$gt;C$lt;sub$gt;1.1$lt;/sub$gt;化合物的磁性质、熵变以及磁卡效应的影响

利用固态反应法制备了Mn₃Sn_1-xCo_xC_1.1 （x=0.05,0.1,0.2） 系列化合物,研究了Co掺杂对其磁性质、相变、熵变的影响. 随着Co掺杂量的增加,样品的居里温度由283 K先降到212 K （Mn₃Sn_0.9Co_0.1C_1.1） 后又升到332 K （Mn₃Sn_0.2Co_0.8C_1.1）,相变类型由一级相变逐渐转变为二级相变. 增大Co的掺杂量,Mn₃Sn_1-xCo_xC_1.1化合物的熵变峰值逐渐减小,磁熵变温区由9 K展宽到300 K. 当Co掺杂量为0.2时,相对制冷量达到最高,为103 J/kg （磁场强度为1.6 MA/m）. 由于室温附近良好的磁致冷效应,该类材料在磁制冷领域可能具有重要的应用前景. 关键词： 磁性质 相变 磁卡效应 相对制冷量     

Effect of Co substitution on magnetic properties of Sr14Cu24O41

A series samples of Sr₁₄(Cu_1−xCo_x)₂₄O₄₁ (x=0, 0.02, 0.06, 0.14, 0.18) were prepared by standard solid-state reaction. X-ray diffraction measurements show that all the samples are single phase and their lattice parameter hardly changes by Co dopant. Electron diffraction experiments and X-ray photo-emission spectroscopy measurements reveal that Co ions substituted Cu ions in the chain. The measurements of magnetic susceptibility from 10 to 300 K in an applied magnetic field of 1.0 T show that Co dopant induces increase in susceptibility. The spin gaps are observed in all the samples, and decrease with increase in Co doping concentration. Fitting of the date indicates that strong antiferromagnetic interaction is induced and antiferromagnetic dimeried state may be formed due to Co³⁺ ions doping in these compounds.     

Electronic and magnetic structures of ternary iron telluride KFe2Te2

We examine the electronic and magnetic structures of iron telluride KFe₂Te₂ using first-principle calculations. We demonstrate that the ground state of this compound is in bicollinear antiferromagnetic order with Fe local moments (~ 2.6 μ_B) that are ferromagnetically aligned along a diagonal direction and antiferromagnetically aligned along the other diagonal in the Fe-Fe square lattice, similar to the alignment discovered in the parent compound of superconductor α-FeTe. This bicollinear antiferromagnetic order results from the interplay among the nearest, next-nearest, and next-nextnearest neighbor exchange interactions, which are mediated by Te 5p orbitals. This finding may aid our understanding of the interplay between magnetism and superconductivity in the family of iron-based materials.     

Eu_(0.9)M_(0.1)TiO_3(M=Ca,Sr,Ba,La,Ce,Sm)的磁性和磁热效应

EuTi0_3是直接带隙半导体材料,在液氦温度附近呈现反铁磁性,且具有较大的磁熵变,但是当其转变为铁磁性时,可以有效提高低磁场下的磁熵变.本文通过元素替代,研究晶格常数的变化和电子掺杂对磁性和磁热效应的影响.实验采用溶胶凝胶法制备EuTiO_3和Eu_(0.9)M_(0.1)TiO_3 (M=Ca, Sr, Ba, La, Ce, Sm)系列样品.结果表明:大离子半径的碱土金属离子替代提高了铁磁性耦合,有利于提高低磁场下的磁热效应.电子掺杂可以抑制其反铁磁性耦合从而使其表现为铁磁性.当大离子半径的稀土La和Ce离子替代Eu离子时,既增大了晶格常数也实现了电子掺杂,表现出较强的铁磁性.在1 T的磁场变化下,Eu_(0.9)La_(0.1)TiO_3和Eu_(0.9)Ce_(0.1)TiO_3的最大磁熵变分别为10.8和11 J/(kg·K),均大于EuTi0_3的9.8 J/(kg·K);制冷能力分别为39.3和51.8 J/kg,相对于EuTi0_3也有所提高.     

Pb掺杂对Cd_2Ru_2O_7反常金属态的调控

具有烧绿石结构的Cd_2Ru_2O_7在形成长程反铁磁序的同时进入反常的金属态.采用高压高温方法制备了一系列Pb掺杂的Cd_(2-x)Pb_xRu_2O_7(0≤x≤2)多晶样品,并系统研究了其晶体结构和电阻率、磁化率、热电势等物理性质.尽管Pb_2Ru_2O_7是泡利顺磁金属,但少量Pb~(2+)掺杂的样品Cd1.8Pb0.2Ru2O7却呈现出明显的金属-绝缘体转变,与施加静水压和少量Ca~(2+)掺杂的效果类似.通过与类似的烧绿石Ru~(5+)氧化物进行对比,提出Cd_2Ru_2O_7中的Ru~(5+)-4d~3电子态恰好处于巡游到局域过渡的区域,少量Pb~(2+)掺杂造成的晶格无序增强了电子的局域性,使得形成反铁磁序的同时伴随出现了金属-绝缘体转变.这表明具有烧绿石结构的Ru~(5+)氧化物是研究巡游-局域电子转变的理想材料体系.     

An estimation of the magnetic disorder at Fe/Cr interfaces in coupled and non-coupled Fe/Cr multilayers

The antiferromagnetic coupling at the Fe/Cr interfaces, inferred from the orientation of the Cr magnetic moments, is used to estimate the magnetic disorder resulting from the interfacial roughness in Fe/Cr multilayers. A crossover from in-plane to out-of-plane orientation of Cr moments depends on the energy cost in either case: (i) to break the interfacial Fe–Cr antiferromagnetic coupling or (ii) having sites with frustrated Cr–Cr antiferromagnetic coupling in the Cr interlayers. A quantitative model of the magnetic frustration due to interfacial disorder in Fe/Cr multilayer systems is described. The step edge density, or terrace size, required to break the interfacial Fe–Cr coupling and destroy the Fe–Fe interlayer exchange coupling is estimated.     

Observing spin polarization of individual magnetic adatoms

We have used spin-polarized scanning tunneling spectroscopy to observe the spin polarization state of individual Fe and Cr atoms adsorbed onto Co nanoislands. These magnetic adatoms exhibit stationary out-of-plane spin polarization, but have opposite sign of the exchange coupling between electron states of the adatom and the Co island surface state: Fe adatoms exhibit parallel spin polarization to the Co surface state while Cr adatoms exhibit antiparallel spin polarization. First-principles calculations predict ferromagnetic and antiferromagnetic alignment of the spin moment for individual Fe and Cr adatoms on a Co film, respectively, implying negative spin polarization for Fe and Cr adatoms over the energy range of the Co surface state.     

Effect of Co doping on the magnetic properties of La0.85Ag0.15(Mn1−yCoy)O3

The mixed valent manganites (La_0.85Ag_0.15)MnO₃ with perovskite structure has been prepared by doping up to 50% of Co at the Mn site. Paramagnetic (PM) to ferromagnetic (FM) transitions have been observed in all the prepared materials. However, the long-range magnetic ordering observed in (La_0.85Ag_0.15)MnO₃ is systemically reduced to cluster glass-type (short-range) of FM ordering due to the introduction of Co. The FM transition temperature was found to decrease with increase in Co doping up to 20% and for further increase in Co doping, the T_c was found to increase. They are explained on the basis of competition between FM double exchange interactions in Mn–O–Mn and Co–O–Co networks. In addition to PM–FM transition, evidences of FM to antiferromagnetic (AFM), and AFM to reentrant spin-glass transitions have been observed. The shift in spin-glass freezing temperature, T_f has been observed from the frequency variation of ac susceptibility measurements. The observed magnetic transitions are explained on the basis of magnetic interactions in different Mn–O–Mn, Mn–O–Co and Co–O–Co networks and such transitions are also observed from the measurement of third harmonic susceptibility. Metal–insulator transition and colossal magneto-resistivity have been observed up to 10% of Co doping.     

Jahn-Teller transition and electron-phonon interaction in Cr-doped manganites Sr0.9Ce0.1Mn1−yCryO3

Cr-doped manganites Sr_0.9Ce_0.1Mn_1−yCr_yO₃ (y=0, 0.05, and 0.10) have been systematically investigated by X-ray, magnetic, transport, and elastic properties measurements. For parent compound Sr_0.9Ce_0.1MnO₃, it undergoes a metal-insulator (M-I) transition at 318 K, which is suggested to originate from a first-order structural transition accompanied by Jahn-Teller (JT) transition. With increasing Cr doping content, the JT transition temperature decreases. The Cr doping suppresses the antiferromagnetic (AFM) state and makes the system spin-glass (SG) behavior at low temperatures. In the vicinity of JT transition temperatures, the softening of Young's modulus originating from the coupling of the orbital (quadrupolar) moment of the e_g orbital of Mn³⁺ ion to the elastic strain has been observed. The anomalous Young's modulus properties imply the electron-phonon coupling due to the JT effect may play an important role in the system.     

量子顺电EuTiO_3材料基态磁性的第一性原理研究

EuTiO_3是钙钛矿结构的量子顺电体,实验发现其基态具有平面各向异性G类反铁磁结构,本文运用基于密度泛函理论的第一性原理计算研究了EuTiO_3处于量子顺电相和应力作用下处于铁电四方相时可能的自旋取向和自旋交换耦合作用,分析了自旋耦合作用的路径,探讨了应力对磁性交换路径的作用,结果发现:当体系自由时,EuTiO_3具有自旋沿[110]方向平面内单轴各向异性的G类反铁磁结构,该结构下Eu离子4f电子自旋通过处于面心位置的O 2p实现自旋反铁磁性的超交换耦合,而在外加应力诱导的铁电四方结构下,由于自旋交换路径中Eu—O—Eu键角改变,Eu 4f电子自旋实现了[110]方向的铁磁交换耦合。     

First-principles investigations of Cr doping effects on electronic structure and magnetic properties in Sr2FeReO6

The electronic structures and magnetic properties of double perovskites Sr₂Fe_1−xCr_xReO₆ (x=0.0, 0.25, 0.5, 0.75, 1.0) have been studied within the local spin density approximation (LSDA) and LSDA+U schemes. The calculated results reveal that with increasing Cr content the cell volume shrinks 2.61%; the Fe/Cr site magnetic moment decreases while the Re-site moment increases. The total spin magnetic moment linearly decreases with the Cr doping from 3.00μB for x=0.00 down to 1.00μB for x=1.00 per formula unit. The magnetic coupling constants increase with increasing x. The electronic structure calculations indicate that the electronic concentration in the Re spin-down subband slightly increases resulting from the increase of bonding-antibonding interaction between the localised and the delocalised states in spin-down band; the coupling of O-2p_↑ and transition-metal-3d_↑ is substantially enhanced with the Cr doping. We discuss the origin of the anomalously high TC of Cr-doped Sr₂FeReO₆ compounds in terms of band hybridization effects.