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.     

Low temperature magnetic and magnetostrictive properties in Pr(Fe_(1-x)Co_x)_(1.9) cubic Laves alloys

The structures,spin reorientations,magnetic,and magnetostrictive properties of the polycrystalline Pr(Fe_(1-x)Co_x)_(1.9)(x=0–1.0)cubic laves phase alloys between 5 K and 300 K are investigated.Large low-field magnetostrictions are observed at 5 K in the alloys with x=0.2 and 0.4 due to the low magnetic anisotropies of these two alloys.A large negative magnetostriction of about-1130 ppm is found in PrCo_(1.9) alloy at 5 K.The magnetizations of the alloys with 0≤x≤0.6decrease abnormally at the spin reorientation temperature T_(sr),and an abnormity is detected in the alloy with x=1.0 at its Curie temperature T_c(45 K).The substitution of Fe by Co increases the value of T_(sr) in the alloy with x value increasing from 0.0 to 0.4,and then reduces the value of Tsr with x value further increasing to 0.6.     

Anomalous temperature dependence of magnetization in single-crystal Fe65Ni35 alloy

Magnetization (0–10 Oe) and magnetic relaxation measurements were carried out in the temperature range between 4.2 and 300 K for three picture-frame samples of Fe₆₅Ni₃₅ alloy whose edges were parallel to 100, 110 and 111, respectively. The typical temperature T_g and the magnetic field H_g which correspond to the anomalous temperature in the χ-T curve and inflection field in the σ-H curve, respectively are summarized and H-T_g and H_g-T diagrams are obtained. A strong magnetic relaxation is observed along the H_g-T line. The dependence of H_g on the crystallographic direction and on the temperature are discussed by the thermal activation process of the 180° domain wall which is pinned strongly by the antiferromagnetic clusters below T_g. The anomaly of magnetization of Fe₆₅Ni₃₅ alloy can be interpreted by the macroscopic picture of the coexistence of ferromagnetic and antiferromagnetic-like regions which may be caused by a statistical fluctuation of alloy composition.     

Formation of L10-FeNi hard magnetic material from FeNi-based amorphous alloys

L1₀-FeNi hard magnetic alloy with coercivity reaching 861 Oe was synthesized through annealing Fe₄₂Ni_41.3Si₈B₄P₄Cu_0.7 amorphous alloy, and the L1₀-FeNi formation mechanism has been studied. It is found the L1₀-FeNi in annealed samples at 400 ℃ mainly originated from the residual amorphous phase during the second stage of crystallization which could take place over 60 ℃ lower than the measured onset temperature of the second stage with a 5 ℃/min heating rate. Annealing at 400 ℃ after fully crystallization still caused a slight increase of coercivity, which was probably contributed by the limited transformation from other high temperature crystalline phases towards L1₀ phase, or the removal of B from L1₀ lattice and improvement of the ordering quality of L1₀ phase due to the reduced temperature from 520 ℃ to 400 ℃. The first stage of crystallization has hardly direct contribution to L1₀-FeNi formation. Ab initio simulations show that the addition of Si or Co in L1₀-FeNi has the effect of enhancing the thermal stability of L1₀ phase without seriously deteriorating its magnetic hardness. The non-monotonic feature of direction dependent coercivity in ribbon segments resulted from the combination of domain wall pinning and demagnetization effects. The approaches of synthesizing L1₀-FeNi magnets by adding Si or Co and decreasing the onset crystallization temperature have been discussed in detail.     

Magnetic properties and spin reorientation of iron rare earth based intermetallics

The Curie temperature and saturation magnetization M_s(μ_B/f.u.) of R₂Fe₁₄B have been discussed. The spin reorientations of Nd₂Fe₁₄B compounds have been studied by many authors with various methods, but have not been checked with the neutron diffraction method. We investigated the spin reorientation of Nd₁₅Fe₇₈B₇ by neutron diffraction and obtained θ = 26°34' at 77 K which is in good agreement with other authors' results. The small amount substitution of Si for Fe in Nd₂(Fe_1−xSi_x)₁₄B increases T_c and _cH_c of the compound. These will be able to make an advantage for Nd-Fe -B magnets.     

Nature of the morin transition in al-substituted hematite

The Mössbauer spectra of (Fe_1-cAl_c)2O3 with c=0.048 indicate the presence of two distinct spin phases below 245 K, one of which gradually disappears with increasing temperature. A careful study of the hyperfine parameters as a function of temperature reveals complex spin reorientation phenomena in both phases.     

Magnetostriction and spin reorientation in ferromagnetic Laves phase Pr(Ga_xFe_(1-x))_(1.9) compounds

The magnetostriction, magnetization, and spin reorientation properties in Pr(Ga_xFe_(1-x))_(1.9) alloys have been investigated by high-precision x-ray diffraction(XRD) step scanning, magnetization, and Mo¨ssbauer spectra measurements. Ga substitution reduces the magnetostriction(λ_(||)) with magnetic field H ≥ 8 kOe(1 Oe = 1.33322×10~2 Pa), but it also increases the λ|| value when H ≤ 8 kOe at 5 K. Spin-reorientations(SR) are observed in all the alloys investigated, as determined by the step scanned XRD, Mo¨ssbauer spectra, and the abnormal temperature dependence of magnetization. An increase of the spin reorientation temperature(T_(SR)) due to Ga substitution is found in the phase diagram, which is different from the decrease one in many R(T_x Fe_(1-x))_(1.9)(T = Co, Al, Mn) alloys. The present work provides a method to control the easy magnetization direction(EMD) or T_(SR) for developing an anisotropic compensation system.     

Structure and frustrated magnetism of the two-dimensional triangular lattice antiferromagnet Na2BaNi(PO4)2

A new frustrated triangular lattice antiferromagnet Na₂BaNi(PO₄)₂ was synthesized by high temperature flux method. The two-dimensional triangular lattice is formed by the Ni²⁺ ions with S =1. Its magnetism is highly anisotropic with the Weiss constants θ_CW =-6.615 K (H⊥c) and -43.979 K (H||c). However, no magnetic ordering is present down to 0.3 K, reflecting strong geometric spin frustration. Our heat capacity measurements show substantial residual magnetic entropy existing below 0.3 K at zero field, implying the presence of low energy spin excitations. These results indicate that Na₂BaNi(PO₄)₂ is a potential spin liquid candidate with spin-1.     

YTi(Fe1-xCox)11的磁性和穆斯堡尔谱的研究

YTi(Fe_1-xCo_x)₁₁在012型四方结构,居里温度随Co含量的增加而提高,本文为了揭示YTi(Fe_1-xCo_x)₁₁磁性与微观结构的联系,对这一系列化合物进行穆斯堡尔谱研究,在室温下,测量YTi(Fe_1-xCo_x)₁₁(其中x=0.0,0.2,0.4)的穆斯堡尔谱,最佳拟合结果证实在这种化合物中,Co原子优先占据j和f晶位,在所测样品中,超精细场在x=0.2处出现极值,这与YTi(Fe_1-xCo_x)₁₁饱和磁化强度测量结果相符合。并对YTi(Fe_1-xCo_x)₁₁,Y₂(Fe_1-xCo_x)₁₄和YTi(Fe_1-xCo_x)₁₁的穆斯堡尔谱的实验结果进行比较,分析在YTiFe₁₁中与Y₂Fe₁₄B中的3d电子能带结构的差异。 关键词：     

磁感应强度和冷却速率对Tb_(0.27)Dy_(0.73)Fe_(1.95)合金凝固过程中取向行为的影响

实验研究了磁感应强度和冷却速率对Tb_(0.27)Dy_(0.73)Fe_(1.95)合金凝固过程中(Tb,Dy)Fe_2相取向行为及合金磁性能的影响.结果表明,将强磁场作用于Tb_(0.27)Dy_(0.73)Fe_(1.95)合金的凝固过程可以制备出(Tb,Dy)Fe_2相沿111取向的组织,同时显著提高了合金的磁致伸缩性能;通过提高磁感应强度可以在更快的冷却速率下得到111取向的组织;在4-10 T范围内,随着冷却速率的增加,(Tb,Dy)Fe_2相沿111取向所需的磁感应强度增加,而发生(110)取向的磁感应强度减小.随着冷却速率的增加,合金的饱和磁化强度增加,而强磁场的施加对合金饱和磁化强度的变化没有明显影响.(Tb,Dy)Fe_2相的取向行为受*Tb,Dy)Fe_3相取向行为的影响,且由磁晶各向异性能与磁场作用时间共同控制.     

The anomalous behaviour of Gd atoms (Fe1−xMnx)2 compounds

The educed Gd atoms in the X-structure (Th₆Mn₂₃-type) of Gd(Fe_1−xMn_x)₂ were magnetically investigated by comparing with Gd₆(Fe_1−yMn_y)₂₃ whose structure is Th₆Mn₂₃-type. The magnetic properties of Gd(Fe_1−xMn_x)₂ (0.4≤x≤0.7) were observed to be quite similar to those of Gd₆(Fe_1−yMn_y)₂₃ (0.4≤y≤0.7).     

{Cu_3}单分子磁体在热平衡和磁场作用下的三体纠缠

本文研究了Na_9[Cu_3Na_3(H_2O)_9(α-As W_9O_(33))_2]·26H_2O(简记为{Cu_3})单分子磁体在热平衡和外加磁场作用下的三体纠缠性质,利用等效自旋模型和实验拟合参数,数值计算了{Cu_3}型三角自旋环中三体负性纠缠度(tripartite negativity).分别考虑沿垂直于三角自旋环方向的磁场、平行于三角自旋环方向的磁场,以及倾斜磁场的情形.结果表明,磁场的方向、大小以及温度对系统三体负性纠缠度有着重要影响.文中给出了在不同磁场方向下,临界温度随磁场强度的变化图,由此可以得到三体纠缠存在的参数区域.同时发现在特定的参数区域,该系统存在纠缠恢复现象.因此适当调节温度、磁场强度大小和磁场方向可以有效调控{Cu_3}型三角自旋环中的三体纠缠性质.     

Magnetic entropy change and magnetic properties of LaFe_(11.5)Si_(1.5) after controlling the Curie temperature by partial substitution of Mn and hydrogenation

Magnetic properties and magnetic entropy changes of La(Fe_(1-x)Mn_x)_(11.5)Si_(1.5)H_y compounds are investigated. Their Curie temperatures are adjusted to room temperature by partial Mn substitution for Fe and hydrogen absorption in 1-atm(1 atm = 1.01325×10~5Pa) hydrogen gas. Under a field change from 0 T to 2 T, the maximum magnetic entropy change for La(Fe_(0.99)Mn_(0.01))_(11.5)Si_(1.5)H_(1.61)is-11.5 J/kg. The suitable Curie temperature and large value of ?S_m make it an attractive potential candidate for the room temperature magnetic refrigeration application.     

Shape memory and associated properties in Fe–Mn–Si-based ribbons produced by melt-spinning

Four Fe–Mn–Si alloys, Fe₆₂Mn₃₂Si₆, Fe₆₂Mn₂₀Si₅Cr₈Ni₅, Fe₆₂Mn₁₆Si₅Cr₁₂Ni₅ and Fe₆₅Mn₉Si₇Cr₁₀Ni₉, were obtained by the melt-spinning method. The samples were structurally, magnetic and shape memory effect (SME) investigated, both “as quenched” and thermally treated. The Mn-rich compositions show different phase, magnetic behavior and SME in comparison with Mn-poor compositions. The thermal treatments generate transformation between the two existing majority phases ( and γ), related magnetization and SME behavior. The features are derived from the corroboration of structural, magnetic interaction and magnitude of SME data.     

正交相Fe2(MoO4)3的制备与表征及其负膨胀行为的第一性原理研究

采用水热合成法制备出Fe₂(MoO₄)₃样品, 并用高温X-射线衍射、热重和差示扫描量热同步热分析仪对其进行表征, 发现样品在510 ℃附近发生低温单斜相和高温正交相之间的可逆相变, 且正交相表现出负膨胀特征. 采用第一性原理计算了正交相Fe₂(MoO₄)₃ 的原子、电子结构以及声子谱、声子态密度, 并和可获得的实验结果进行了系统的比较. 结果显示正交相Fe₂(MoO₄)₃中MoO₄四面体较之FeO₆八面体具有更强的刚性. 发现最低频的光学支处具有最负的格林乃森(Grüneisen)系数, MoO₄四面体和FeO₆ 八面体相连的桥氧原子的横向振动、FeO₆八面体柔性扭曲转动以及MoO₄四面体的刚性翻转共同导致了Fe₂(MoO₄)₃负膨胀现象的发生.     

基于分数阶微分流变模型的非晶合金黏弹性行为及流变本构参数研究

近年来, 基于非晶合金名义弹性区的流变力学行为探索其结构及形变机理是非晶合金领域研究的热点之一. 本文根据非晶合金结构不均匀性的特征, 提出能够比拟树状分形网络结构的分数阶微分流变模型研究非晶合金的黏弹性行为. 通过室温纳米压痕实验, 对三种不同泊松比和玻璃化转变温度的非晶合金的黏弹性变形行为进行了研究. 实验结果表明: 在表观弹性区, 非晶合金的变形表现出与加载速率相关的线性黏弹性性质. 根据Riemann-Liouville分数阶微积分定义, 分别由分数阶微分及整数阶Kelvin模型对实验结果进行了分析. 分析结果表明, 相对于整数阶流变模型, 分数阶微分流变模型能更精细地表征材料的黏弹性变形特征; 在流变模型参数中, 黏性系数η_A和分数阶次α反映出材料的流变特性和流动趋势, 流变参数与玻璃转变温度、泊松比之间具有较好的相关性, 上述相关性有助于从微观结构角度理解材料塑性与泊松比的关联.     

Energy Spectra and g Factors of α-A1203:Cr3+ and α-A1203:Mn4+

By diagonalizing the complete d³ energy matrix in a trigonally distorted cubicfield and using the wavefunctions from it, unified calculations of the whole energy spectrum as well as the g factors of the ground state and t₂^{3 2} E excited states for α-A1₂0₃:Cr³⁺ and α-A1₂0₃:Mn⁴⁺ have been carried out respectively. A11 the calculated results are in very good agreement with the experimental data. The comparison between the results of the two crystals has been made, which demonstrates that the covalency of α-A1₂0₃:Mn⁴⁺ is stronger than the one of α-A1₂0₃:Cr³⁺. For the zero-field splittings of the ground state and t₂^{3 2} E , their physical origins are revealed; the comparison and analysis of their values of the two crystals have been made.     

Mg-Sn-Si系合金的热力学基础及合金相演变过程分析

研究了Mg-Sn-Si系合金的热力学基础及合金相的演变过程. 结果表明: 对于Mg-Sn-Si系合金, 合金相的比热容随着温度增加而增加, 在低温下变化迅速, 而在高温下变化平缓, 其热膨胀系数在低温范围内随温度升高呈指数形式增加, 而在高温范围内呈线性增大. 在Mg₂ (Si_x, Sn_1-x)、Mg₂ (Sn_x, Si_1-x)相结构中, Sn(Si)原子的取代位置不固定, 可以是面心, 也可以是顶点. 常规凝固过程中, 由于处于非平衡状态, x的取值范围有所波动, 对于Mg₂ (Si_x, Sn_1-x)和Mg₂ (Sn_x, Si_1-x) 两种结构, x的取值范围在0.25或0.75附近. Mg₂ (Si, Sn)的生成温度较高, 可从液相中直接析出, 也可由Mg₂Si转化而来, 而Mg₂ (Sn, Si)的生成温度较低, 只能从基体中析出, 随着Sn含量的增加, 开始析出Mg₂ (Sn, Si)相的温度升高.     

{Cu3}单分子磁体在磁场中的热纠缠

本文研究单分子磁体Na₉[Cu₃Na₃(H₂O)₉ (α-AsW₉O₃₃)₂]·26H₂O中三角自旋 环在磁场作用下的热纠缠性质, 利用数值计算求出任意两个Cu²⁺离子量子比特之间的配对纠缠度, 分别记为C₁₂, C₂₃和C₁₃. 研究结果表明, 磁场的方向和大小以及温度对配对纠缠度具有重要影响, 而且参数的变化对C₁₂, C₂₃和C₁₃的影响也是各不相同. 给出外加三个不同方向的磁场时, 配对纠缠度C₁₂, C₂₃和C₁₃各自对应的临界温度T_c随磁场强度的变化图, 由此可以得到单分子磁体三角自旋环中存在纠缠态的参数范围. 通过选择适当的磁场方向和大小以及温度等实验参数, 可以有效地调节和提高单分子磁体中的配对纠缠度. 关键词： 配对纠缠 单分子磁体 三角自旋环     

Revealing the A_(1g)-type strain effect on superconductivity and nematicity in FeSe thin flake

The driving mechanism of nematicity and its twist with superconductivity in iron-based superconductors are still under debate.Recently,a dominant B_(1 g)-type strain effect on superconductivity is observed in underdoped iron-pnictides superconductors Ba(Fe_(1-x)Co_x)_2 As_2,suggesting a strong interplay between nematicity and superconductivity.Since the long-range spin order is absent in FeSe superconductor,whether a similar strain effect could be also observed or not is an interesting question.Here,by utilizing a flexible film as substrate,we successfully achieve a wide-range-strain tuning of FeSe thin flake,in which both the tensile and compressive strain could reach up to ～0.7%,and systematically study the strain effect on both superconducting and nematic transition(T_c and T_s) in the FeSe thin flake.Our results reveal a predominant A_(1 g)-type strain effect on T_c.Meanwhile,T_s exhibits a monotonic anti-correlation with T_c and the maximum T_c reaches to 12 K when T_s is strongly suppressed under the maximum compressive strain.Finally,in comparison with the results in the underdoped Ba(Fe_(1-x)Co_x)_2 As_2,the absence of B_(1 g)-type strain effect in FeSe further supports the role of stripe-type spin fluctuations on superconductivity.In addition,our work also supports that the orbital degree of freedom plays a key role to drive the nematic transition in FeSe.