Coherent potential approximation for quaternary alloys: Application to phonon spectra in one dimension

The coherent potential approximation is generalized for application to quaternary alloys of the types A_xC_yB_1?x?yD and A_xB_1?xC_yD_1?y. Formalisms are developed for and application is made to the calculation of the phonon spectra of random, mass-disordered quaternary alloys of both types in one dimension.     

Spin-disorder resistivity in Ni2Mn(Sn1−xInx) heusler alloys

Electrical resistivity measurements have been performed on Ni₂Mn(Sn_1?xIn_x) Heusler alloys, where x = 0, 0.02, 0.05, 0.10 and 0.15, in the temperature range of 4–300 K. The experimental data clearly show the existence of two distinct kinds of resistivity behaviour, both described by an aTⁿ law, for 7 ? T ? 20 K. Between 170 and 300 K the data are well described by an AT + BT² phenomenological fit, and the results obtained are in good agreement with an interpretation based on electron-phonon and spin-disorder scattering.     

Magnetoresistance in ferromagnetic shape memory alloy NiMnFeGa

The magnetoresistance (MR){=[R(H)−R(0)]/R(0)} properties in ferromagnetic shape memory alloy of NiMnFeGa ribbons and single crystals, and NiFeGa ribbons have been investigated. It is found that the NiMnFeGa melt-spun ribbon exhibited GMR effect, arising from the spin-dependent scattering from magnetic inhomogeneities consisting of antiferromagnetically coupled Mn atoms in B2 structure. In the absence of these magnetic inhomogeneities, Heusler alloys seem to show a common linear MR behavior at around 0.8T_C, regardless of sample structures. This may be explained by the s-d model. At low temperatures, conventional AMR behaviors due to the spin-orbital coupling are observed. This is most likely due to the diminished MR from s-d model because of much less spin fluctuation, and is not associated with martensite phase. MR anomaly at intermediate field (ρ_⊥>ρ_||) is also observed in single crystal samples, which may be related to unique features of Heusler alloys.     

有限温度下二维Heisenberg铁磁系统的声子衰减

在二维正方Heisenberg铁磁系统的基础上建立了磁振子-声子相互作用模型. 利用松原格林函数理论研究了系统的声子衰减,计算了布里渊区的主要对称点线上的声子衰减曲线. 发现在第一布里渊区,在Δ线上,横向声频支声子无衰减,在Z线上,纵向声频支声子无衰减;横向声频支声子衰减比纵向声频支声子衰减至少大一个数量级,并讨论了各项参数的变化对横向声频支声子衰减与纵向声频支声子衰减的影响. 根据声子衰减与声子寿命的关系,声子衰减与声子态密度的关系,可以讨论横向声频支声子与纵向声频支声子的寿命与态密度. 关键词：磁振子-声子相互作用横向声频支声子衰减纵向声频支声子衰减声子寿命     

Fe/Si薄膜中相干声学声子的光激发研究

本文通过抽运-探测技术, 利用飞秒激光脉冲激发并探测了Fe/Si薄膜中的高频相干声学声子. 通过经典的阻尼谐振函数, 对声学声子的动力学行为进行了拟合. 实验及拟合结果表明, 该声学声子的共振频率约为0.25 THz, 其退相时间约为12 ps, 且都与激发光的波长和能量密度无关. 声学声子的振幅随着激发光能量密度的增加而线性地增强. 临界参数12_τe-ph/T约为0.6, 表明相干声学声子的驱动力主要来源于电子热应力的贡献. 最后, 结合薄膜的厚度和质量密度, 可以得到室温下垂直于该Fe/Si薄膜表面(out of plane) 的弹性常数C_⊥约为283 GPa.     

Co50Fe25-xMnxSi25系列合金的结构、磁性和半金属性研究

利用实验和能带计算相结合的方法,对介于两种预期的半金属Heusler合金Co₂FeSi和Co₂MnSi间的四元合金Co₅₀Fe_25-xMn_xSi₂₅的晶体结构、磁性、能带结构和半金属性进行了研究.采用考虑库仑相互作用的的广义梯度近似方法计算了系列合金的能带结构,通过与实验结果进行对比,揭示了成分变化过程中合金分子磁矩及原子磁矩的变化规律.研究发现,关键词：磁性半金属Heusler合金     

铁基Heusler合金Fe2Co1-xCrxSi的结构、磁性和输运性质的研究

基于多种实验手段和能带计算的方法, 对四元合金Fe₂Co_1-xCr_xSi的晶体结构、 磁性、输运性质及能带结构进行了研究. 研究发现, 随着Cr的增加, 合金Fe₂Co_1-xCr_xSi保持了高度有序结构, 逐渐从Hg₂CuTi结构的Fe₂CoSi 过渡到L2₁结构的Fe₂CrSi; 由于次晶格网络的破坏, 居里温度逐渐下降; 系列合金的分子磁矩呈现线性下降, 符合半金属特性; 剩余电阻比率与原子占位有序程度密切相关, 呈现两端大、 中间小的特点. 在Cr替代Co的过程中, 材料半金属能隙逐渐打开, 表现半金属特征. 同时费米能级从Fe₂CoSi半金属能隙的价带顶上移至Fe₂CrSi能隙的导带底. 最大的能隙宽度出现在x= 0.75处, 这表明四元合金有可能成为具有更高自旋极化率和更强抗干扰能力的自旋电子学材料.     

Magnetostructural transition in NiCuCoMnGa alloys

The effects of the addition of Co on the martensitic transformation and Curie transition temperatures of polycrystalline Ni46-xCu4CoxMn33.sGa16.5 （x = 0, 1, 3, 5） alloys are investigated. An abrupt decrease in the martensitic transformation temperature and an obvious increase in the Curie transition temperature of austenite （TA） are observed when Co is doped in the NiCuMnGa alloy. As a result, the composition range for obtaining the magnetostructural transition is extended. Furthermore, the effect of a strong magnetic field on the magnetostructural transition is analyzed. This study offers a possible method to extend the composition range for obtaining magnetostructural transition in Heusler alloys.     

Magnetocaloric effect in high Ni content Ni52Mn48−xInx alloys under low field change

Ni-rich Heusler alloys Ni₅₂Mn_48−xIn_x (x=15.5, 16 and 16.5) were prepared by the arc melting method. X-ray diffraction analysis revealed that the martensite has orthorhombic structure (S.G. Pmm2) at room temperature. The only alloy with x=15.5 has structural transmission from martensite to austenite without any magnetic transmission. The temperature dependence and the field dependence of the magnetization measurement indicated that the magnetization increased with the decreasing of the concerntration of Mn. The lesser the Mn atoms located in the In atom sites, the weaker the total AFM interaction in the system. Giant entropy changes ΔS_M(T, H) were found in Ni₅₂Mn_48−xIn_x alloys with the maximum ΔS_M value of 22.3 J kg K for the sample with x=16.5 at 270 K under the magnetic field change of 1.5 T.     

超快光谱技术及其在凝聚态物理研究中的应用

近年来备受关注的超快光谱技术拥有诸多特色,例如极高的时间分辨率,丰富的光与物质的非线性相互作用,可以用光子相干地调控物质的量子态,其衍生和嫁接技术带来许多凝聚态物理实验技术的变革等等.文章介绍了超快光谱技术的一般原理、时间分辨的技术实现和典型构型,并通过具体实例来展示该实验手段在凝聚态物理研究中的应用.     

Impact of iron composition on the calculated electronic and magnetic properties of Fe3−xNixSi

ABSTRACT

The structural, electronic and magnetic properties of Fe_3?xNi_xSi alloys with variable iron composition (0?≤?x?≤?1) have been investigated within by using Projector augmented wave (PAW) method. The exchange–correlation potential was treated with the generalised gradient approximation (GGA) for the calculation of the structural properties and for the computation of the electronic and magnetic properties in order to treat the d states. These alloys crystallize in cubic Heusler structures; The Fe₃Si and Fe₂NiSi have a regular structure DO₃ and L2₁ respectively. To describe the experimental proprieties we use the on-site Coulomb interactions of U_eff(Ni)?=?3.1?eV and U_eff(Fe)?=?3.4?eV. A good agreement between calculated and experimental magnetic moments is found for the cubic Heusler phases without the addition of Hubbard-model. The obtained results of the density of states and the spin-polarized band structure show that the Fe₂NiSi alloy has half-metallic property. Through the obtained values of the total spin magnetic moment, we conclude that in general, the Fe₂NiSi alloy is half-metallic ferromagnet material whereas the Fe₃Si alloy has a metallic nature.     

CoNiZ系列合金的结构和马氏体相变性质

利用X射线衍射研究了CoNiZ(Z＝Si,Sb,Sn,Ga 等)合金在不同热处理条件下的相组成.当Z元素为Sn,Sb时,材料是完全的B2结构;但Z为Si时,材料变成面心立方的γ相.形成B2还是γ相由电子浓度和原子尺寸效应两种因素共同决定.而CoNiGa的研究结果表明,在合金中除了形成B2结构的同时还容易形成γ相,常表现出两相共存的特性.对材料进行不同方式的热处理可以使合金中两相的含量有所消长,γ相含量的多少对CoNiGa合金的马氏体相变有很大的影响.分析指出,两相共存及其所带来的物性变化是CoNiGa铁磁性形状记忆合金非常有利用价值的物理性质.关键词：Heusler合金马氏体相变γ相     

超快光谱技术及其在凝聚态物理研究中的应用

近年来备受关注的超快光谱技术拥有诸多特色,例如极高的时间分辨率,丰富的光与物质的非线性相互作用,可以用光子相干地调控物质的量子态,其衍生和嫁接技术带来许多凝聚态物理实验技术的变革等等.文章介绍了超快光谱技术的一般原理、时间分辨的技术实现和典型构型,并通过具体实例来展示该实验手段在凝聚态物理研究中的应用.