Magnetoresistance of Fe3O4/Au/Fe3O4 and Fe3O4/Au/Fe spin-valve structures

A detailed study of the in-plane magnetotransport properties of spin valves with one and two Fe₃O₄ electrodes is presented. Fe₃O₄/Au/Fe₃O₄ spin valves exhibit a clear anisotropic magnetoresistance in small magnetic fields but no giant magnetoresistance (GMR). The absence of GMR in these structures is due to simultaneous magnetization reversal in the two Fe₃O₄ layers. By contrast, a negative GMR effect is measured on Fe₃O₄/Au/Fe spin valves. The negative GMR is attributed to an electron spin scattering asymmetry at the Fe₃O₄/Au interface or an induced spin scattering asymmetry in the Au interfacial layers.     

双磁性中心内嵌富勒烯Y_2C_2@C_(82)-C_2(1)中的超快自旋动力学行为

自旋翻转和自旋转移是实现基于内嵌富勒体系自旋逻辑功能器件设计的先决条件.本文以双磁性中心内嵌富勒烯Y_2C_2@C8_2-C_2(1)体系为例,采用第一性原理计算方法,结合Λ进程理论模型和自编的遗传算法程序,在该内嵌富勒烯体系中分别实现了亚皮秒时间尺度内的自旋翻转和自旋转移过程.计算结果表明,优化后的内嵌Y_2C_2团簇结构和实验得到的各项数据基本吻合,并且会对外部的C8_2-C_2(1)笼结构产生一定的排斥力,但由于富勒烯笼状结构具有很强的稳定性,所以整个体系仍然保持碳笼结构的完整性.通过对自旋密度分布与激光脉冲作用下自旋期望值演化的具体分析,经由Λ进程的自旋翻转是基于两个Y元素的整体自旋翻转;自旋转移则源自两个磁性中心以及碳笼之间在激光脉冲作用下的自旋密度重新分布.本文结果揭示了Y_2C_2@C8_2-C_2(1)体系中的超快自旋动力学机理,可望为基于实际内嵌富勒烯分子的自旋逻辑功能器件设计提供理论依据.     

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：     

NMR study of local hole distribution, spin fluctuation and superconductivity in Tl2Ba2Ca2Cu3O10

⁶³Cu, ¹⁷O and ²⁰⁵Tl NMR have been performed in the high-T_c superconductor Tl₂Ba₂Ca₂Cu₃O₁₀ whose T_c(max) is 127 K. The hole densities at Cu and oxygen sites in the CuO₂ plane have been extracted from the nuclear quadrupole frequency ν_Q. The striking feature is that the Cu holes are significantly transferred to oxygen site due to strong hybridization between Cu and oxygen. From an analysis of T₁ and T_2G, it has been found that the spectral weight of the spin fluctuation is transferred to higher energy compared to YBa₂Cu₃O₇, while the magnetic correlation length ξ does not differ much. Thus, it is suggested that the higher T_c is due to higher characteristic energy of spin fluctuations, i.e. the superconductivity is spin fluctuation mediated. The superconducting properties are consistently explained by a d-wave superconductivity model with a finite density of states (DOS) at the Fermi level. We show that the disorder of the Ca/TlO layer caused by the partial inter-substitution of Tl and Ca is responsible for the potential scattering to produce such a DOS. It is found that if such a potential scattering were absent, T_c would go up to 132 K which is quite close to the record T_c realized in the Hg based compound.     

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     

Magnetic pair making and breaking effect of Ru in La0.7Sr0.3Mn0.9Ru0.1O3 and La0.5Sr0.5Co0.9Ru0.1O3

The substitutional effect of Ru on the magnetic and transport properties of double exchange ferromagnets, La_0.7Sr_0.3MnO₃ and La_0.5Sr_0.5CoO₃ has been investigated. It is found that substitution of 10% Ru at the Mn site of La_0.7Sr_0.3MnO₃ decreases the Curie temperature by 20 K than that of the parent compound. However, a large decrease in the Curie temperature, ΔT_c80 K and the system undergoes a transition from metallic state to insulating state is observed when 10% Ru is doped in La_0.5Sr_0.5CoO₃. The marginal effect of Ru in the Mn–O–Mn sublattice in comparison to the Co–O–Co sublattice could be due to the magnetic exchange interaction between Mn and Ru by virtue of the fact that Ru exhibits variable valence states, Ru⁺⁴/Ru⁺⁵. The e_g and t_2g parentage of Ru⁺⁵ is similar to Mn⁺⁴ and therefore, Ru⁺⁵ ion appears to participate in the double exchange mediated ferromagnetic (FM) interaction. On the other hand, Ruthenium (IV) ion disrupts an intermediate spin state of cobalt (Co⁺³: t_2g⁵e_g¹), forcing a double exchange FM state to anti-FM state.     

BaCrSi4O10与AgGaSe2:Cr2+吸收光谱的精细结构及自旋单态对零场分裂参量的影响

在强场图像中构造了四角对称环境中Cr²⁺离子包括自旋单态在内的完全能量矩阵, 通过对角化能量矩阵方法, 计算得到了BaCrSi₄O₁₀与AgGaSe₂:Cr²⁺吸收光谱的精细结构及自旋单态对零场分裂参量的影响. 从理论上给出了BaCrSi₄O₁₀与AgGaSe₂:Cr²⁺吸收光谱的精细结构及BaCrSi₄O₁₀的零场分裂参量值. 计算结果显示自旋单态对零场分裂参量D的影响完全可忽略, 但对a和F的影响比较大. 这种影响主要来自自旋-轨道耦合导致的自旋五重态与自旋三重态和自旋单态的相互作用, 而自旋轨道耦合的选择定则显示自旋单态并非直接影响五重态而是通过自旋三重态间接地影响基态的五重态. 因此, 为了得到准确的零场分裂参量值, 所有的自旋态都应该考虑.     

Magnetic structure determination of NdNiC2 and TmNiC2 by neutron diffraction

Isostructural orthorhombic NdNiC₂ and TmNiC₂ reveal collinear antiferromagnetic structures with magnetic propagation vectors [1/21/20] and [0, 0, 1], respectively. In NdNiC₂, ferromagnetic (110) planes are coupled pairwise in opposite spin orientations; in TmNiC₂, adjacent ferromagnetic (001) planes are coupled antiferromagnetically. The magnetic moments are oriented parallel to the a-axis and have values of 2.7μ_B (Nd) and 3.3μ_B (Tm) at 4 K.     

The electronic structure and properties of the C15 compounds CeAl2, LaAl2 and YAl2

Results of self-consistent band calculations are reported for the C15 structured XAl₂ materials (X = Y, La, and Ce) using the local spin density functional formalism for assumed ferromagnetic and antiferromagnetic states as well as the paramagnetic state. The X-atoms are found to be the dominant factor is determining the electronic structure near the Fermi energy and this is enhanced by the presence of f-bands close to (LaAl₂) or at (CeAl₂) the Fermi energy. In paramagnetic CeAl₂, the f-bands are about 1 eV wide and, although principally above the Fermi energy, extend down to accomodate the additional electron compared to LaAl₂. The ferromagnetic state is found not to be stable. By contrast, the antiferromagnetic state is found to be stable with a magnetic moment of 0.88μ_B per Ce atom in very good agreement with the maximum moment, 0.89μ_B found in the neutron measurements of Barbara et al. A significant narrowing of the f-bandwidth is observed in the antiferromagnetic state. The antiferromagnetic spin density ordering appears to be related to nesting features in this underlying Fermi surface in LaAl₂ (i.e., no 4f electron) rather than that of CeAl₂.     

Cl nuclear quadrupole resonance studies of molecular motions of ClO3 and water diffusion in Ca(ClO3)2·2H2O

A ³⁵Cl nuclear quadrupole resonance (³⁵Cl-NQR) investigation of polycrystalline Ca(ClO₃)₂·2H₂O is described. The ³⁵Cl-NQR frequencies (ν_Q) for two resonance lines (ν_Q1 and ν_Q2), the spin lattice relaxation time (T_1Q) for ν_Q2 only and the line width δν_Q2 were measured in the temperature range 292–345 K, except for the frequency measured up to 455 K. The observed decrease in the resonance frequencies with increasing temperature permitted the determination of the frequencies of librations of the ClO₃⁻ ion about two axes perpendicular to the three-fold axis of the ion mainly responsible for this effect. The temperature dependence of the relaxation time T_1Q proved the occurrence of water diffusion and hindered rotation of ClO₃⁻ ions. The activation energies of these two molecular motions were determined, and their effect on the electric field gradient at the site of a chlorine nucleus was discussed. Temperature measurements of the line width δν_Q2 confirmed the conclusions drawn from the analysis of T_1Q(T).     

Diamagnetic impurity effects on the Néel temperature in La2CuO4 and related materials

To investigate why the sensitivity of the Néel temperature T_N of the antiferromagnetic (AF) layered copper perovskites (typically La₂CuO₄) to diamagnetic impurities such as Zn is reportedly much larger than in the AF members of the K₂NiF₄ family, we first treat the effect of a concentration c of impurities on the uncorrelated electronic states in the coherent potential approximation (CPA). Then we consider the Heisenberg hamiltonian as the large correlation limit of the Hubbard hamiltonian for a single band of impurity-modified electronic states. The correlation effects are treated variationally. The model is solved explicity by using a rectangular density of states, and we obtain the c-dependent exchange J, staggered moment S_q, spin wave velocity and transverse susceptibility at zero temperature. We take into consideration several recently proposed formulae for T_N in the clean limit, and include the impurity effects by exploiting the results obtained, in order to test their predictions against the experimental T_N(c) data for La₂Cu_1−cZn_cO₄. Our results suggest that, to explain the difference between the K₂NiF₄ and the La₂ CuO₄ families, one should consider both the sign and the magnitude of the difference I≡ε^B−ε^A between impurity (B) and host (A) ionic potentials. The slowly decreasing trend of T_N(c) in the K₂NiF₄ family is reproduced if I is negative and sizeable, or positive but very small, while the quick decrease typical of the copper perovskites requires a positive and rather large I. For reasonable values of the interaction parameters, among the several models we compare, only the model of Chakravarty, Halperin and Nelson is able to semi-quantitatively reproduce the non-linear behaviour of T_N(c) reported for La₂Cu_1−cZn_cO₄, provided the spin stiffness is assumed to scale with c as appropriate to Fermi liquids.     

BaO-Li2O-B2O3三元系中BaB2O4-Li2B2O4和BaB2O4-Li2O截面相平衡关系的研究

本文用X射线和差热分析方法对BaO-Li₂O-B₂O₃三元系中的两个截面:BaB₂O₄-Li₂B₂O₄和BaB₂O₄-Li₂O作了研究。在BaB₂O₄-Li₂B₂O₄赝二元系中发现了一个新的化合物4BaB₂O₄·Li₂B₂O₄。化合物在930±3℃由包晶反应形成,并与Li₂B₂O₄形成共晶反应。共晶温度为797±3℃,共晶点组分为79mol％Li₂B₂O₄。在BaB₂O₄-Li₂O截面中也存在化合物4BaB₂O₄·Li₂B₂O₄,其包晶反应温度从930±3℃随Li₂O含量增加下降到908±3℃。在组分60mol％Li₂O处形成另一个新的化合物2BaB₂O₄·3Li₂O。该化合物在630±3℃也是由包晶反应形成,并与Li₂O和Li₂CO₃分别形成共晶反应,共晶温度分别为400±3℃和612±3℃。在BaB₂O₄-Li₂B₂O₄和BaB₂O₄-Li₂O体系中都没有观察到固溶体。用计算机程序分别对化合物4BaB₂O₄·Li₂B₂O₄和2BaB₂O₄·3Li₂O的X射线粉末衍射图案进行了指标化,其结果:4BaB₂O₄·Li₂B₂O₄的空间群为Pmma,a=13.033?,b=14.630?,c=4.247?,每个单胞包含两个化合式单位;2BaB₂O₄·3Li₂O的空间群为Pmmm,a=4.814?,b=9.897?,c=11.523?,每个单胞也含有两个化合式单位。 关键词：     

Al0.6Ga0.4N/GaN/Al0.3Ga0.7N/Al0.6Ga0.4N量子阱中的Rashba自旋劈裂

正如人们所知, 可以通过电场或者设计非对称的半导体异质结构来调控体系的结构反演不对称性(SIA)和Rashba自旋劈裂. 本文研究了Al_0.6Ga_0.4N/GaN/Al_0.3Ga_0.7N/Al_0.6Ga_0.4N量子阱中第一子带的Rashba 系数和Rashba自旋劈裂随Al_0.3Ga_0.7N插入层(右阱)的厚度w_s以及外加电场的变化关系, 其中GaN层(左阱)的厚度为40-w_s Å. 发现随着w_s的增加, 第一子带的Rashba系数和Rashba自旋劈裂首先增加, 然后在w_s>20 Å 时它们迅速减小, 但是w_s>30 Å时Rashba自旋劈裂减小得更快, 因为此时k_f也迅速减小. 阱层对Rashba系数的贡献最大, 界面的贡献次之且随w_s变化不是太明显, 垒层的贡献相对比较小. 然后, 我们假w_s=20 Å, 发现外加电场可以很大程度上调制该体系的Rashba系数和Rashba自旋劈裂, 当外加电场的方向同极化电场方向相同(相反)时, 它们随着外加电场的增加而增加(减小). 当外加电场从-1.5×10⁸ V·m^-1到1.5×10⁸ V· m^-1变化时, Rashba系数随着外加电场的改变而近似线性变化, Rashba自旋劈裂先增加得很快, 然后近似线性增加, 最后缓慢增加. 研究结果表明可以通过改变GaN层和Al_0.3Ga_0.7N层的相对厚度以及外加电场来调节Al_0.6Ga_0.4N/GaN/Al_0.3Ga_0.7N/Al_0.6Ga_0.4N量子阱中的Rashba 系数和Rashba自旋劈裂, 这对于设计自旋电子学器件有些启示.     

On the orbital moment of Co in YCo5

The experimental result of the orbital and spin moments for Co in YCo₅ is analyzed by the use of the approximate d bands for YCo₅. The calculation of the anistropy constants K_u1 and K_u2 is worked out. Finally, the contributions to the anistropy constants due to 3d bands of Co_I, those of Co_II, those of Co_I and Co_II, and 3d and 4d bands of Co_I, Co_II and Y are discussed.     

Thermal control magnetic switching dominated by spin reorientation transition in Mn-doped PrFeO3 single crystals

Spin reorientation transition (SRT) has attracted substantial attention due to its important role in the ultrafast control of spins. However, the transition temperature is usually too low for its practical applications. Here, we demonstrate the ability to modulate the SRT temperature in PrFe_1−xMnxO₃ single crystals from 196 K to 317 K across the room temperature by varying the Mn concentration. Interestingly, the Γ₄ to Γ₁ spin reorientation of the Mn-doped PrFeO₃ is distinct from the Γ₄ to Γ₂ spin reorientation transition as in the parent material. Because of the coupling between rare-earth ions and transition-metal ions in determining the SRT temperature, the demonstrated control scheme of spin reorientation transition temperature by Mn-doping is expected to be used in temperature control magnetic switching devices and applicable to many other rare-earth orthoferrites.     

Magnetic properties and critical behavior of quasi-2D layered Cr4Te5 thin film

Quasi-2D layered Cr₄Te₅ thin film has attracted great attention because it possesses the high Curie temperature close to room temperature and relatively large saturation magnetization. However, the magnetic interactions and the nature of magnetic phase transition in the Cr₄Te₅ film have not been explored thoroughly. In this paper, we focused on the critical behavior of its magnetic phase transition through the epitaxial Cr₄Te₅ film fabricated by pulsed laser deposition (PLD). The final critical exponents β = 0.359(2) and γ = 1.54(2) were obtained by linear extrapolation together with Arrott-Noakes equation of state, and their accuracy was confirmed by using the Widom scaling relation and scaling hypothesis. We find that some magnetic disorders exist in the Cr₄Te₅ film system, which is related to Cr₄Te₅ critical behavior why its critical behavior is quite far from any conventional universality class. Furthermore, we also determined that the Cr₄Te₅ film exhibits a quasi-2D long-range magnetic interaction. Finally, the itinerant ferromagnets of Cr₄Te₅ films were confirmed by the Takahashi’s self-consistent renormalization theory of spin fluctuations. Our work provides a new idea for understanding the mechanism of magnetic interactions in similar 2D layered films.     

Growth and magnetism of Ni nanoparticles in Ni/Al2O3/Si or Si3N4 multilayers

We report on the fabrication of Ni/Al₂O₃/Si and textured Ni/Al₂O₃/Si₃N₄ multilayers containing Ni nanoparticles that exhibit significantly improved results. The secondary phases arising from thermal reaction between Ni and Si can be remarkably suppressed with increasing layers of Al₂O₃ and deposition of Ni/Al₂O₃ multilayers on Si₃N₄ substrates. Atomic force microscopy shows the formation of large as well as nanoclusters of Ni when grown on Si, whereas textured Ni nanoparticles are formed on Si₃N₄ substrates. The magnetization measurements on Ni/Al₂O₃/Si containing a single buffer layer of Al₂O₃ shows higher coercivity field with magnetic nanowire-like behavior, whereas with several Al₂O₃ alternate layers almost a superparamagnetic-like behavior is observed. However, significantly improved magnetic hysteresis was observed in textured Ni/Al₂O₃/Si₃N₄ multilayers due to preferred alignment of Ni nanocrystallites.     

Two separate Li ionic motions observed by Li and B NMR in xLi2S + (1 − x)B2S3 glassy fast ionic conductors

Studies of ion dynamics in the highly conductive glassy fast ionic conductor (FIC) xLi₂S + (1 − x)B₂S₃ (x = 0.65 and 0.70) were made with NMR nuclear spin lattice relaxation (NSLR) R₁(ω, T) of both mobile ⁷Li and immobile ¹¹B ions, and ⁷Li NMR line narrowing δν(T). The possible dependence of ion dynamics on the short range order structures (SRO) and the distribution of activation energies (DAE) in this highly conductive FIC was investigated. Two Gaussian DAE were employed to fit ⁷Li NSLR data, where each Gaussian DAE was correlated to a separate ¹¹B NSLR in a BS₃ and in a BS₄ group. The long range diffusion of Li ions among BS₃ groups and a seemingly localized ionic hopping motion around BS₄ group is suggested as a microscopic model for the ion dynamics in thioborate glasses, namely a ‘two channel relaxation’.     

Reversible H2 passivation of Si ≡ Si3 interface defects in (1 1 1)Si/SiO2

K-band electron spin resonance (ESR) at 4.3 K has revealed the dipole-dipole (DD) interaction effects between [1 1 1]P_b centers (^*Si ≡ Si₃ defects with unpaired sp³ hybrid [1 1 1]) at the 2 dimensional (1 1 1)Si/SiO₂ interface. This has been enabled by the perfectly reversible H₂ passivation of P_b, which affects the defect's spin state. Sequential hydrogenation at 253–353°C and degassing treatments in high vacuum at 743–835°C allowed to vary the P_b density in the range 5 × 10¹⁰ < [P_b] (1.14 ± 0.06) × 10¹³ cm^-2. With increasing [P_b] fine structure doublets are clearly resolved. It is found that (1 1 1)Si/SiO₂ interfaces, dry thermally grown at ≈920°C, naturally comprise a ^*Si ≡ Si₃ defect density — passivated or not — of 1.14 × 10¹³ cm^-2.     

X-ray diffraction studies of epitaxy in orthorhombic- DyMnO3/Er1Ba2Cu3O7-δ thin film heterostructures on LaAlO3 (100) substrates

The heteroepitaxy in DyMnO₃/Er₁Ba₂Cu₃O_7-δ bilayer thin films on LaAlO₃ (100) substates was characterized by four-circle X-ray diffractometry. The Er₁Ba₂Cu₃O_7-δ thin films on LaAlO₃ (100) substrates were prepared by molecular-beam deposition (MBD) and post-growth annealing in wet and dry O₂ at 880°C, whereas the DyMnO₃ thin films on the Er₁Ba₂Cu₃O_7-δ/LaAlO₃ (100) heterostructure were deposited by MBD and post-growth annealing in dry O₂ at 750°C. The conventional X-ray diffraction (XRD) patterns as well as pole figures (φ-scans) for specific (hkl) reflections were acquired. The Er₁Ba₂Cu₃O_7-δ thin film in the DyMnO₃/Er₁Ba₂Cu₃O_7-δ/LaAlO₃ (100) heterostructure showed [001] oriented epitaxial growth, as expected. The DyMnO₃ thin film on the Er₁Ba₂Cu₃O_7-δ epilayer in the heterostructure grew with (110) epitaxy in its metastable orthorhombic phase (lattice constants: a_o=5.272 Å, b_o=5.795 Å and c_o=7.38 Å). The heteroepitaxial relationships at the orthorhombic-DyMnO O₃ (110) /Er₁Ba₂Cu₃O_7-δ (001) interface was determined as the following: DyMnO₃ (110) Er₁Ba₂Cu₃O_7-δ (001), DyMnO₃ [1 0] ¶r; Er₁Ba₂Cu₃O_7-δ[100] or Er₁Ba₂Cu₃O_7-δ[010], and DyMnO₃ [001] ¶r; Er₁Ba₂Cu₃O _7-δ[010] or Er₁Ba₂Cu₃O_7-δ [100].