NEUTRON-POWDER-DIFFRACTION STUDY OF ErFe11.35Nb0.65 AND ErFe11.35Nb0.65Ny

ErFe_11.35Nb_0.65 AND ErFe_11.35Nb_0.65N_y have been synthesized and neutron-powder-diffraction experiments at room temperature performed. The ErFe_11.35Nb_0.65N_y nitride, obtained by gas-solid reaction, retains the ThMn₁₂-type structure of its parent compound. The Nb atoms occupy 8i sites and the nitrogen atoms are located at 2b sites. The atomic magnetic moments of the Er ions are antiparallel to those of the Fe atoms. Upon nitrogenation, the lattice cell expands mainly along the a-axis and the atomic magnetic moments of Fe are enhanced.     

57Fe M?SSBAUER STUDY ON (NdDy)TiFe11Ny COMPOUNDS

⁵⁷Fe M?ssbauer spectra have been obtained at room temperature for (NdDy)TiFe₁₁, and at 11 K and room temperature for the corresponding nitrides (NdDy)TiFe₁₁N_y. The magnetic behaviors of Fe atoms at different sites have been studied. We have found a larger increase of the hyperfine fields upon nitrogenation due to the higher nitrogen content in these compounds and got a bigger enhancement of the isomer shift in 8j site because of the nearest nitrogen environment.     

阎隙碳原子对Er2Fe17Cx化合物结构与磁性的影响

利用快淬手段获得高碳含量的Er₂Fe₁₇C_x单相化合物,系统研究了Er₂Fe₁₇C_x(x=0.0,0.5,1.0,1.5,2.0,2.5,2.8和3.0)的结构、相稳定性及内禀磁性,讨论了间隙碳原子对化合物中原子磁矩、交换作用和各向异性的影响,同时用⁵⁷Fe穆斯堡尔效应分析了化合物中各个铁晶位的超精细场随碳含量的变化。 关键词：     

Mn(SiO2)3(M=Fe,Co,Ni;n=1-3)团簇的几何结构、光电性质及磁性

基于密度泛函理论中的广义梯度近似系统研究M_n(SiO₂)₃(M=Fe,Co,Ni;n=1-3)团簇的几何结构、光电性质和磁学性质.结果表明:Fe,Co原子相对于Ni原子更易于在(SiO₂)₃团簇上聚集;通过分析团簇的分裂途径及其产物,发现稳定性较好的氧化硅是一种很好的用于负载过渡金属"岛膜"的载体材料;M_n(SiO₂)₃团簇的能隙恰好位于近红外光谱范围内.通过磁性分析发现,该复合团簇的磁矩主要局域在过渡金属原子周围,而且,Fe₂(SiO₂)₃和Co₃(SiO₂)₃具有相对较大的磁矩,这主要源于过渡金属原子的d轨道间相互耦合.能隙和磁性两方面性质进一步肯定了二氧化硅磁性复合材料在医学界被用作光动力靶向治疗的可观前景.     

等价电子数组元Heusler合金Fe_2RuSi中的原子占位

对等价电子数组元Heusler合金Fe_2RuSi的原子占位、电子结构与磁性进行了理论与实验研究.第一性原理计算表明,虽然Fe_2RuSi中Fe,Ru均有8个价电子,但是Ru仍表现出强烈的占据A,C晶位倾向.基态总能最低的是Fe与Ru分别占据A,C晶位的XA结构,次低的是Fe,Ru在A,C位混乱占位的L2_1B结构,且两者能量差很小.这说明决定Heusler合金中过渡族原子占位的因素除价电子数以外还可能有原子半径和共价杂化作用等.态密度和差分电荷密度计算表明Heusler合金中主族元素与最近邻过渡族元素之间的p-d共价杂化对Heusler合金的占位有明显影响,在XA结构中Ru与Si和Fe(B)之间都存在明显的杂化作用,而在高能的L2_1结构中,Si与最近邻的Fe杂化作用相当弱.XRD测试表明在室温Fe_2RuSi存在A,C位之间的Fe-Ru反占位,形成了能量次高的L21B结构,这主要来自于混合熵对自由能的贡献及其引起的原子自发混乱占位.在5 K下Fe_2RuSi的饱和磁矩为4.87μB/f.u.,与计算值符合得相当好.     

Pr2(Fe0.8Co0.2)14B磁结构的中子衍射研究

本文报道用中子衍射测定的含硼稀土过渡族金属间化合物Pr₂(Fe_0.8Co_0.2)₁₄B的晶体结构与磁结构。将中子三轴谱仪用作二轴粉末衍射仪,在室温测该化合物粉末样品的中子衍射强度,用轮廓精化法弥合衍射数据。该化合物属Nd₂Fe₁₄B类四方结构,α=8.8110?,c=12.2307?。设Pr,Fe和Co原子磁矩间为铁磁耦合,同一晶位的Fe,Co原子磁矩相等,存在沿c轴的易磁化 关键词：     

Coexistence of Magnetism and Conductivity in Bis(ethylenediseleno) Tetrathiafulvalene (BEST) with Octahedral Anions Hexacyanoferrate (III)and Nitroprusside

Using an accurate density-function method, we explore the coexistence of the magnetism and conductivity in bis(ethylenediselena)-tetrathiafulvalene (BEST) with the paramagnetic hexacyanoferrate(III) [Fe(CN)₆]^3- or the photochromic nitroprusside anion [Fe(CN)₅NO]^2-. The total and partial densities of states, and the atomic spin magnetic moments are calculated and discussed. It is found that the up- and down-spin total densities of states (DOS) are continuous in the vicinity of the Fermi level, there is overlap between the HOMO and LUMO in the up-spin subbands and the down-spin subbands, which reveals that these types of compounds have conductive properties. From the total and partial densities of states and atomic spin magnetic moments, it is shown that the spin magnetic moments of (BEST)₄[Fe(CN)₆] is mainly assembled at the iron atom and the cyanogen radical, and the spontaneous magnetic moments for (BEST)₂[Fe(CN)₅NO] come from iron atom, cyanogen and nitric oxide radical. To our best knowledge, it is the first theoretical study on the coexistence of the magnetism and conductivity of these compounds.     

Neutron diffraction study of magnetic ordering in ErMn2Si2, ErMn2Ge2 and ErFe2Si2

Neutron diffraction study of polycrystalline compounds ErMn₂Si₂, ErMn₂Ge₂ and ErFe₂Si₂ was performed in the temperature range between 1.8 and 293 K. All compounds have tetragonal, ThCr₂Si₂-type crystal structure. The antiferromagnetic collinear structure of ErMn₂Si₂ and ErMn₂Ge₂ at both RT and LNT, consists of a sequence + - + - of ferromagnetic layers of Mn atoms. The magnetic moment of an Mn atom (≈2μ_B) is parallel to the c-axist. At low temperatures (LHT and lower), the ferromagnetic ordering within the Er sublattice is observed. The magnetic moment (μ_Er ≈ 9μ_B) is perpendicular to the c-axis. From the temperature dependence of the intensities of the magnetic peaks, the following values for the Curie temperatures were obtained: (10±5) K for ErMn₂Si₂ and (8.5±3) K for ErMn₂Ge₂. For ErFe₂Si₂ a collinear antiferromagnetic structure of the + - - + type was found, the magnetic unit cell consisting of the chemical one, doubled along the c-axis.     

Hyperfine fields at Y nuclei in Y(Fe1−xTx)3 (T = Al, Co)

The spin echo spectra of ⁸⁹Y hyperfine fields in Y(Fe_1−xT_x)₃ with low concentration x consist of two main lines and of satellite lines caused by the replacement of the Fe atoms by the T atoms. Values of the magnetic moments of the T atoms are estimated by observing the satellite lines.     

La3Co29-xFexSi4B10的择优占位、电子结构和晶格振动性质的理论研究

分别用第一性原理及原子间相互作用势对初始模型进行几何优化, 所得稳定结构的晶格参数均与实验值符合较好. 通过第一性原理密度泛函理论, 计算了稳定结构La₃Co_29-xFe_xSi₄B₁₀化合物择优占位情况, 计算结果表明, Fe原子最择优替代Co 原子的2c晶位, 择优占位顺序为2c > 8j1 > 8i2 > 8j2 > 8i3 > 16k > 8i1, 这与实验结果非常符合. Fe原子每次只替代不同晶位的一个Co原子时, La₃Co_29-xFe_xSi₄B₁₀ 体系的晶格常数几乎不变, 磁矩却发生了有趣的变化. 当Fe原子沿着择优顺序依次替代不同晶位的所有Co原子时, 随着La₃Co_29-xFe_xSi₄B₁₀体系中Fe原子含量的增多, 其电子态密度整体向左移动. Fe原子完全替代Co时, 与未掺杂时相比体系的总磁矩增加. 最后, 利用原子间相互作用势进一步预测了La₃Co_29-xFe_xSi₄B₁₀体系的晶格振动及热力学性质. 在低频部分, 振动模式主要由质量较大的Co, Fe和La元素作贡献; 随着掺杂原子Fe的增多, 体系的截止频率先减小后增多, 中频部分由Si元素引起的振动模式减少; B-B强相互作用引起了高频部分的振动模式. 基于声子态密度预测了不同数量Fe掺杂后体系的比热、熵和德拜温度的变化, 当Fe 含量大于Co时, 德拜温度明显升高.     

Magnetic behaviour of Y(Fe, M)2 compounds M = Al, Co, Rh, Ir

In Y(Fe, M)₂ compounds, the presence of Fe moments even at high M concentrations and the appearance of a percolation limit for the onset of long range magnetic order is typical. A dominance of the volume contribution to the isomer shift is obtained, indicating that for the Fe atoms charge transfers are of minor importance.     

H NMR study of YT2-hydride (T = Fe, Mn)

In the hydrides YMn₂H_3.0, the nuclear magnetic resonance of ¹H nuclei has been measured under external fields up to 50 kOe at 4.2 K and three ¹H NMR signals have been observed. The ¹H hyperfine fields corresponding to these NMR signals can be discussed in terms of the dipole interaction between the H nuclei and the localized moments of the Mn(or Fe) atoms surrounding H atoms.     

预退火时间对Fe_(80.8)B_(10)P_8Cu_(1.2)非晶合金微结构及磁性能的影响

研究了预退火时间对Fe_(80.8)B_(10)P_8Cu_(1.2)非晶合金微结构及磁性能的影响.穆斯堡尔谱研究表明:在660 K的预退火温度下,随着预退火时间的增加,Fe原子不断富集,非晶基体中的类Fe_3B化学短程有序结构向类Fe B结构转变,并且非晶基体中Fe第一近邻壳层中Cu原子的逐渐脱离以及Fe-P配位键数量的明显减少可间接表征CuP团簇的形成过程.同时,本研究通过调节预退火时间来调控非晶基体中CuP团簇和Fe团簇的数量,促进后续退火晶化过程中α-Fe纳米晶相的析出,并细化纳米晶尺寸,从而获得综合磁性能更加优异的非晶/纳米晶软磁合金.     

聚酰亚胺/钽铌酸钾纳米颗粒复合材料结构与机械性能分子动力学模拟

利用多尺度建模方法构建了聚酰亚胺/钽铌酸钾纳米颗粒复合物模型, 通过分子动力学模拟研究了不同尺寸钽铌酸钾纳米颗粒(5.5, 8.0, 9.4, 10.5, 11.5 Å)对复合材料的结构、弹性模量和相互作用能的影响规律, 并通过计算纳米颗粒表面原子键能和单位表面积原子数目探究了复合物机械性能提高的内部机理. 聚酰亚胺和聚酰亚胺/钽铌酸钾复合材料的杨氏模量分别为2.91和3.17 GPa, 泊松比分别为0.37和0.35, 钽铌酸钾纳米颗粒的引入可以显著改善聚酰亚胺的机械性能. 纳米颗粒表面原子的键能为8.62-54.37 kJ·mol^-1, 表明颗粒与基体主要通过范德华力作用结合且有氢键存在. 计算结果表明, 相同掺杂比例下, 纳米颗粒尺寸越小, 纳米颗粒表面原子数目越大, 颗粒与基体作用更强, 杨氏模量的提高幅度越大, 尺寸效应越显著. 因此, 掺杂小尺寸纳米颗粒是提高聚酰亚胺机械性能的有效途径.     

Magnetic properties and local structure studies of Zn doped ferrites

A series of nano-particle zinc-doped ferrites samples Zn_xFe_3 − xO₄ (x = 0, 0.17, 0.33, 0.50, and 0.67) were prepared with the hydrothermal method, and the effects of Zn doping on the magnetic properties and local structures of the samples were studied. By increasing zinc content from 0 to 0.33, the saturated magnetization M_s measured with the vibrating sample magnetometry (VSM) method increases, while it decreases by further Zn doping. The experimental results of extended X-ray absorption fine structure (EXAFS) show that most of dopant Zn atoms occupy A site, and only a few Fe atoms at B site are substituted by Zn atoms. The Neel's two-sublattice model and the molecular field analysis of Yafet–Kittel (Y–K) spin-ordering using the three-sublattice model are compared to explain the experimental phenomena. The Y–K angle between the magnetic moments at A and B sublattices is found changing due to the decrease of A–O–B exchange interaction, which shows that the magnetic moments at B site are no longer rigidly parallel to the few remained magnetic moments at A site.     

高压退火对0.65PMN-0.35PT薄膜结构、形貌及电学性能的影响

利用射频磁控溅射技术在LaNiO₃/SiO₂/Si(100)基底上制备了厚 度约为250 nm的0.65PMN-0.35PT(PMN-PT)薄膜. 研究高压氧氛围退火方式对PMN-PT薄膜晶体结构、形貌以及电学性能的影响. 经过XRD测试发现,在高压氧气氛围中, 温度为400℃下退火后的PMN-PT薄膜具有纯的钙钛矿相结构, 具有完全的(100)择优取向, 且衍射峰尖锐, 表明经过高压退火后的薄膜结晶极为充分. SEM表面形貌测试结果显示, 经高压退火处理的PMN-PT薄膜表面呈现出棒状或泡状的形貌. 铁电性能测试表明: 氧气氛围压强4 MPa, 退火时间4h的PMN-PT薄膜样品具有较好的铁电性能, 其剩余极化强度P_r达到10.544 μC/cm², 且电滞回线形状较好, 但漏电流较大, 这可能是由于其微结构所导致.同时介电测试发现: PMN-PT薄膜样品具有极好的介电性能, 其在1 kHz下测试的介电常数ε_r达到913, 介电损耗tgδ 较小, 仅为0.065. 关键词： 射频磁控溅射 高压退火 0.65PMN-0.35PT 介电     

Heusler合金Co2MnAl(100)表面电子结构、磁性和自旋极化的第一性原理研究

基于第一性原理计算方法系统地研究了L2₁和B2结构下的Heusler合金Co₂MnAl(100)表面原子的原子弛豫、电子结构、磁性和自旋极化行为. L2₁和B2结构的Co₂MnAl(100)表面由于Co–Mn和Co–Al的成键差异, 使得不同原子分别发生不同程度的伸缩. 与块体相比, Co和Mn原子的自旋磁矩由于表面效应而明显增大, 电子结构计算显示L2₁结构块体中的带隙被表面态破坏, 表面效应使得两种结构的CoCo端面自旋极化率降低, 但MnAl端面并未受到显著影响, 呈现了较大的自旋极化, 预测其在隧道结中可能具有很好的应用潜力.     

δ-(Zn,Cr)S(111)表面上的Dzyaloshinsky-Moriya作用:第一性原理计算

根据密度泛函理论的第一性原理计算了具有非中心反演对称的异质结δ-(Zn,Cr)S(111)体系的原子结构和电子结构.Cr原子之间通过第一层S原子传递磁性相互作用.结合广义布洛赫条件,又进一步计算了反方向的自旋螺旋能量与波矢的色散关系E(q)与E(-q).E(q)与E(-q)能量之差反映了δ-(Zn,Cr)S(111)的S层与Cr层之间空间反演对称性破缺引起的DMI的大小.通过海森伯相互作用(HBI)模型与Dzyaloshinsky-Moriya作用(DMI)模型拟合第一性原理计算值,得到了Cr原子间各近邻的HBI参数J_1-J_4与DMI参数d-_1,d_2.在δ-(Zn,Cr)S(111)中,Cr原子间的耦合为M型反铁磁.DMI参数d_1为-0.53 meV,为顺时针手性DMI,在δ-(Zn,Cr)S(111)界面上有可能会产生斯格明子.本文计算表明,磁性和非磁性半导体界面有可能存在DMI,为理论研究和磁存储技术的进步开拓一个新的方向.     

Neutron diffraction study of the MnPd3 phase

The magnetic structure of MnPd₃ has been refined using improved crystal structure data for the I4/mmm space group. The magnetic moment on Pd atoms coupled antiparallel to the Mn moments of 4.1 ± 0.3 μ_B was found by neutron diffraction to be 0.15 ± 0.03 μ_B in the 4(c) and 4(e) positions. The Pd moments on the 4(d) sites are assumed to be zero on symmetry grounds.     

POWDER NEUTRON DIFFRACTION STUDIES OF THE CRYSTALLOGRAPHIC AND MAGNETIC STRUCTURE OF Ho2Fe9Ga8 and Ho2Fe9Ga6AI2

The crystallographic and magnetic structures of Ho₂Fe₉Ga₈ and Ho₂Fe₉Ga₆AI₂ were studied by powder neutron diffraction at room temperature. The atom fractional occupancies of Ga and Al and the magnetic moments of Ho and Fe were obtained by using Rietveld analysis program. The magnetic structures of the two samples show an easy-axis anisotropy, with the Fe magnetic moment being ferrimagneticlly coupled to those of Ho.