装置极化效率对极化中子成像质量的影响及修正分析

极化中子照相技术通过分析极化中子束的自旋相移对样品磁场进行成像, 自旋极化/分析装置是照相系统的主要组成部分. 引入中子自旋极化/分析装置的极化效率参数, 从中子极化矢量与磁场相互作用机理出发, 重新推导探测中子强度与磁场分布的定量关系, 利用谱仪模拟软件VITESS, 选取bender型超镜极化器和 ³He 自旋过滤器作为极化/分析装置, 对量化修正式进行验证, 并综合装置极化效率、单色器能量分辨精度和bender型极化器的几何结构等参数, 初步分析极化中子照相技术的磁场定量检测能力, 相关结果可为极化中子照相的实验数据处理技术研究及装置设计提供参考. 关键词： 极化效率 中子照相 磁场成像     

极化中子照相磁场量化技术方案比较与分析

极化中子照相技术通过分析极化中子束的自旋相移对样品磁场进行成像,目前已发展出多种成像技术方案,其中能量选择法和自旋回波法极化中子成像技术从不同的原理出发,解决了极化中子照相中磁场量化的周期解问题,同时避免装置极化效率等参数的影响,可以实现较高的量化精度.本文对两种极化中子照相技术方案进行研究,通过对单色器能量分辨率和装置极化效率等关键参数的分析和模拟,确定在研究堆上开展相关实验的可行性,并初步明确其量化能力和适用范围.相关结果可为极化中子照相的实验数据处理技术研究及装置设计提供参考.     

自旋极化中子物质的状态方程

在Brueckner–Hartree–Fock理论框架内,研究了自旋极化的中子物质的状态方程及其自旋依赖性,计算了自旋非对称能及相关的物理量如磁化率和朗道参数G₀,并着重讨论了三体核力的影响,结果表明:在整个自旋极化度范围内,中子物质的每核子能量随自旋极化度的变化都满足二次方规律,自旋对称能随密度单调增加,这意味着中子物质中不会发生由自旋非极化态向自旋极化态的自发相变,三体核力的主要效应是使中子物质磁化率随密度减小的速度加快,从而使中子物质的磁化相变更加困难.     

非弹性中子散射谱仪

非弹性中子散射是利用中子与材料晶格或自旋散射发生能量和动量交换来测量材料动力学性质的实验技术.非弹性中子散射谱仪可以直接测量声子谱、自旋波、晶体场等性质,且能量和动量分辨率高,在物理、材料、化学、生物等领域有广泛应用,是前沿科学研究中重要的实验仪器.本文概述了非弹性中子散射的发展过程、基本理论、散射谱仪及应用范例.     

垂直自由层倾斜极化层自旋阀结构中的磁矩翻转和进动

在理论上研究了垂直自由层和倾斜极化层自旋阀结构中自旋转移矩驱动的磁矩翻转和进动.通过线性展开包括自旋转移矩项的Landau-Lifshitz-Gilbert方程并使用稳定性分析方法,得到了包括准平行稳定态、准反平行稳定态、伸出膜面进动态以及双稳态的磁性状态相图.发现通过调节电流密度和外磁场的大小可以实现磁矩从稳定态到进动态之间的转化以及在两个稳定态之间的翻转.翻转电流随外磁场的增加而增加,并且受自旋极化方向的影响.当自旋极化方向和自由层易磁化轴方向平行时,翻转电流最小;当自旋极化方向和自由层易磁化轴方向垂直时,翻转电流最大.通过数值求解微分方程,给出了不同磁性状态磁矩随时间的演化轨迹并验证了相图的正确性.     

用外磁场控制电流感应磁化翻转效应中的临界电流方法

应用基于磁动力学方程的宏观唯象模型,研究了弱外磁场下纳米尺度赝自旋阀结构的电流感应磁化翻转效应.在统一考虑铁磁/非磁界面的自旋相关散射以及铁磁层中的自旋积累和弛豫过程后,给出了赝自旋阀结构在弱外磁场下的磁化翻转条件和临界电流.对该效应的数值计算解释了弱外磁场下赝自旋阀结构的电阻-电流回线的偏移,并给出了用外磁场控制电流感应磁化翻转效应中的临界电流方法. 关键词： 电流感应磁化翻转 外磁场 临界电流 赝自旋阀     

一种研究自旋翻转散射效应的新方法

金属中自旋翻转散射长度远长于电子平均自由程，近来关于自旋翻转散射效应的研究主要集中于扩散区域．文章作者提出了一种使用双势垒磁性隧道结来研究纳米尺度结构中弹道区域的自旋翻转散射效应的新方法．这种方法可以从磁电输运性质的测量，得出中间隔离层中的自旋翻转散射效应的温度和偏压关系，进一步可以得出诸如电子平均自由程和自旋翻转散射长度等自旋散射信息，以及中间层的态密度和量子阱信息．     

自旋极化的对称核物质和中子物质的性质

利用Skyrme有效相互作用对自旋极化的同位旋对称核物质和中子物质的特性进行了研究.用4种核子-核子相互作用参数SⅢ,SKM,SLy230a和SLy230b,分别描绘了核物质状态方程曲线.可以发现不论使用哪一种参数,在自旋极化的同位旋对称核物质和中子物质中都存在着磁化相变转换点.另外还对磁化系数进行了计算,给出了磁化系数比率随密度的变化关系,由于无限不连续点的存在,进一步肯定了在Skyrme-Hartree-Fock理论框架内两种物质会出现磁化相变转换点.     

中子引起单粒子翻转过程的Monte Carlo计算模拟

从中子与硅原子相互作用的物理机理出发,利用Monte Carlo方法编制了中子引起单粒子翻转的计算模拟程序,并对14 MeV中子环境下的16K位静态存储器硅片翻转过程中的物理量进行了计算,同时可为中子引起的单粒子翻转的研究提供截面和描述内部物理过程的参考数据。     

自旋轨道矩协助自旋转移矩驱动磁化强度翻转

以磁隧道结/重金属层组成的三端口磁隧道结为理论模型,通过对包含自旋转移矩和自旋轨道矩的Landau-Lifshitz-Gilbert (LLG)方程做线性化稳定性分析,研究了自旋轨道矩协助自旋转移矩驱动的磁化强度翻转.发现在自旋轨道矩协助下,磁矩的翻转时间极大减小,翻转时间随自旋轨道矩电流密度的增大而减小,且自旋转移矩和自旋轨道矩的结合可实现零磁场的磁化翻转.另外,相比自旋轨道矩的类阻尼项,类场项在磁化强度的翻转中起着主导作用,且自旋轨道矩类场项的出现也可以减小磁化强度的翻转时间,磁化强度翻转时间随自旋轨道矩类场项强度的增大而减小.     

Design of a pulsed spatial neutron magnetic spin resonator

It is well known that upon passage through a spatially alternating transverse magnetic field, produced by a meander-shaped conducting foil, in its rest frame each neutron creates its individual frequency which depends on its velocity and the period of the meander. A resonant spin flip process takes place, if this frequency equals the Larmor frequency determined by a homogeneous vertical field. Clearly, this effect can be used to monochromatise a polarised neutron beam. Here we propose a novel design of such a magnetic resonator consisting of a sequence of separate compact modules, which provide high homogeneity of the transversal field oscillations and allow rapid beam chopping since they meet the specifications of fast electronic switching. The wavelength resolution of this resonator device can be changed in an instant and likewise an arbitrary amplitude modulation of the transversal field can be established, which is required for an efficient suppression of subsidiary maxima of the wavelength-dependent spin flip probability.     

VITESS polarized neutron suite: allows for the simulation of performance of any existing polarized neutron scattering instrument

As neutron simulations packages are used for analysis of the expected performance for practically all newly built neutron instruments, possibilities for simulations with polarized neutrons have been relatively underdeveloped.During the last years we developed a new approach for the representation of time-dependent magnetic fields (both in magnitude and direction) for the VITESS simulation package. This allowed us to simulate the neutron spin dynamics in practically all polarized neutron devices (RF neutron flipper, adiabatic gradient RF flipper, the Drabkin resonator, etc.). In this article the above-mentioned VITESS instrument components (modules) will be presented and the simulated performance of a number of polarized neutron scattering instruments (NRSE, MIEZE, SESANS, etc.) will be demonstrated.Thus, we practically complete the polarized neutron suite of the VITESS, which seems sufficient for the simulation of performance of any existing polarized neutron scattering instrument. Future work will be concentrated on developments of dedicated sample modules (kernels) to allow for virtual experiments with VITESS.     

Neutron spin echo: A new concept in polarized thermal neutron techniques

A simple method to change and keep track of neutron beam polarization non-parallel to the magnetic field is described. It makes possible the establishment of a new focusing effect we call neutron spin echo. The technique developed and tested experimentally can be applied in several novel ways, e. g. for neutron spin flipper of superior characteristics, for a very high resolution spectrometer for direct determination of the Fourier transform of the scattering function, for generalised polarization analysis and for the measurement of neutron particle properties with significantly improved precision.     

Development of neutron resonance spin flipper for high resolution spin echo spectrometer

A new type of neutron resonance spin flipper (RSF) with high frequency oscillating magnetic field has been developed for Modulated IntEnsity by Zero Effort (MIEZE) spectrometer at cold neutron beam line MINE1 at JRR-3M reactor in JAEA. Dipole magnets enable us to provide the strong static fields for the RSFs. MIEZE signals have been demonstrated with the effective frequency of 600 kHz by using the new RSFs. The contrast of the signals was 0.58. The MIEZE spectrometer is under final process to practical use. The spectrometers can also be applied to the pulsed neutrons like J-PARC.     

Inelastic action of a gradient radio-frequency neutron spin flipper

The energy shift due to the inelastic interaction of the neutron with the time dependent magnetic fields of a gradient radio-frequency spin flipper was studied. A nondispersive perfect crystal spectrometer was used to measure extremly small energy changes of E0.2 eV. The gradient of the magnetic field causes a broadband action of the spin flipper which facilitates its use for new active neutron optical components.     

A wide angle neutron spin filter system using polarized He

Polarized ³He neutron spin filters can operate over a wide neutron energy range and provide a large angular acceptance. A compact ³He neutron spin filter system has been developed for the Multi-Axis Crystal Spectrometer at the National Institute of Standards and Technology (NIST) Center for Neutron Research. Sealed ³He cells, polarized by spin-exchange optical pumping, are used as polarizer and analyzer. The polarization of the neutrons incident on the sample is inverted by flipping the polarization of the ³He gas in the polarizer, with only a small effect on the analyzer cells. The cell fabrication process, ³He spin flipper, and the holding magnetic field are discussed and we present the results of a first on-linetest.     

Cross section of inelastic polarized neutron scattering by superconducting rings

A general expression for the cross section of inelastic magnetic scattering of cold polarized neutrons by superconducting rings has been derived. In this scattering process, the metastable superconducting current changes via quantum jumps corresponding to a decrease in the number of fluxoids in the superconducting ring by one or several units and the change in the energy of the ring is transferred to the kinetic energy of the scattered neutron. For rings from type-II superconductors with a thickness smaller than the field penetration depth but larger than the electron mean free path, the cross sections of inelastic scattering with neutron spin flip have been obtained for the first time. The possibility of increasing the cross section of neutron scattering by a system of rings has been discussed.     

Optimised adiabatic fast passage spin flipping for He neutron spin filters

We describe here a method of performing adiabatic fast passage (AFP) spin flipping of polarized ³He used as a neutron spin filter (NSF) to polarize neutron beams. By reversing the spin states of the ³He nuclei the polarization of a neutron beam can be efficiently reversed allowing for the transmission of a neutron beam polarized in either spin state. Using an amplitude modulated frequency sweep lasting 500 ms we can spin flip a polarized ³He neutron spin filter with only 1.8×10⁻⁵ loss in ³He polarization. The small magnetic fields (10-15 G) used to house neutron spin filters mean the ³He resonant frequencies are low enough to be generated using a computer with a digital I/O card. The versatility of this systems allows AFP to be performed on any beamline or in any laboratory using ³He neutron spin filters and polarization losses can be minimised by adjusting sweep parameters.     

串型耦合双量子点处于自旋阻塞区时磁输运性质的研究

使用双杂质安德森模型的哈密顿量,从理论上研究了串型耦合双量子点系统处于自旋阻塞区时的磁输运性质,并用主方程近似方法求解了哈密顿量.结果表明,自旋轨道耦合作用导致的双量子点间的自旋反转隧穿能够解除系统的自旋阻塞.同时也研究了超精细相互作用导致的在量子点内自旋反转和双量子点之间的自旋关联对系统的磁输运性质的影响,取得了一些有价值的结果,并对相关的物理问题进行了讨论.     

Prospects of developing polarized neutron optics on the basis of new method for improving polarizing coatings

A method for improving polarizing neutron coatings by means of antibarrier interlayers has been suggested in PNPI, and the flip ratio for the reflected beam has been increased by an order as compared to prior coatings. In this paper the tasks to be solved for developing polarized neutron optics are formulated, and the factors that can restrict the efficiency of the polarizing devices on the basis of the improved coatings are discussed. The formulas for the polarizing efficiency which take account of the probabilities of both non-spin-flip (NSF) and spin-flip (SF) reflections are obtained, including the case of multiple reflections; the effect of scattering at magnetic inhomogeneities and that of non-collinearity of the magnetization to the magnetizing field are separated. In the case of multiple reflections the SF probability in the last reflection is shown to play a significant or even a primary role in determining the polarizing efficiency. New neutron instrumentation can be designed with improved polarizing devices, and the possibility to efficiently polarize and analyze very cold or even ultracold neutrons may appear. Thence arises the importance of studying oxidation kinetics and stability of Ti and Co nanolayers.