缺陷铁纳米环体系的磁特性研究

采用Monte Carlo方法与快速傅里叶变换微磁学方法相结合的方式,模拟含不同缺陷的铁纳米环的磁滞回线、组态、剩磁等磁特性.研究发现:缺陷的大小与位置明显影响系统的磁化过程.当缺陷较小时,系统存在双稳态特征,此性质与无缺陷系统类似;当缺陷增大时,系统过渡状态增加,双稳态特征不再明显.进一步的研究发现,缺陷系统的剩磁随缺陷半径D的增大而增大.上述结果与非对称纳米环系统的磁特性类似,并可以通过零场状态下的系统自旋组态的变化加以解释.当系统圆心与缺陷中心的间距Y增加时,剩磁与Y的关系是非线性的:剩磁先随Y的增大而增大,后随Y的增大而减小.模拟结果可用零场状态下不同Y值的组态变化进行详细解释.上述研究结果表明,缺陷可以明显影响铁纳米环的磁特性.     

强电场作用下反铁电锆钛酸铅的介电行为研究

采用电滞回线方法和偏置直流电场中叠加小交变电场方法研究了锆钛酸铅反铁电陶瓷材料在强电场作用下的介电行为.测量结果显示，锆钛酸铅反铁电材料的介电常数随外加电场强度呈非线性变化，在反铁电 铁电转变的电场区间形成介电峰.表征极化强度随电场强度变化率的微分介电常数εd峰值出现在反铁电 铁电转换电场强度处，最高达到41000.随着偏置电场增加反铁电向铁电体转变过程中，小信号介电常数εc减小；在电场降低铁电回复成反铁电过程中，小信号介电常数εc增大，小信号介电常数εc峰先于微分介电常数εd峰出现.根据电场作用下反铁电 关键词： 锆钛酸铅反铁电陶瓷 介电行为 强电场条件     

大块永磁铁低温剩磁测量技术研究

通过对霍尔探头低温标定系统改进, 建立了大块永磁铁低温剩磁测量系统(CRMS). 以尺寸为40 mm×40 mm×10 mm的矩形NdFeB永磁铁(N50M)为例, 对低温剩磁测量方法进行了研究, 结果表明, 影响低温剩磁测量可靠性主要因素有: 霍尔探头低温标定, 霍尔探头位置, 温度漂移与材料低温热膨胀等. 如果测量方法一样, 永磁铁低温剩磁测量重复性好于0.1%. 实验为低温波荡器等高精度永磁装置大块永磁铁低温剩磁测量与研究创造了条件. 关键词： 大块永磁铁 低温剩磁 测量误差 低温波荡器     

晶粒易轴取向度对纳米晶永磁Pr2Fe14B磁性的影响

构造了立方和不规则形状晶粒的各向异性纳米晶单相Pr₂Fe₁₄B磁体 .利用微磁学的有限元法，模拟计算了样品的磁滞回线.计算结果表明，随着磁体晶粒易轴取向度的变差， 磁体的剩磁、矫顽力均随之下降.不同晶粒尺寸的纳米晶单相Pr₂Fe_{14B磁体，其磁 性能随取向度的变化快慢不同，原因在于磁体中的晶间交换作用 (IGEC) 的强弱不同.随着 晶粒取向度的提高，纳米晶单相磁体的矫顽力逐渐增加，这完全不同于烧结磁体. 关键词： 纳米晶磁体 矫顽力 剩磁}     

稀有气体原子注入缺陷性纳米碳管的分子动力学模拟

采用TLHT势和经典分子动力学方法研究了稀有气体原子(He，Ne，Ar，Kr，Xe)进入带缺陷的单壁纳米碳管(SWCNT)的动力学过程，计算出了稀有气体原子分别从管壁和管口入射时，它们能封装在SWCNT中的能量阈值E_k0，并与理想结构情形做了比较.结果表明：随着管壁缺陷半径r的增加，E_k0减小；当r<4.5 ?时，给定合适的初始动能，稀有气体原子能封装在纳米碳管中；而r=4.5 ?时，稀有气体原子不能封装在碳管中，且此时缺陷对Ar，Kr和Xe的输运特性有很大影响. 关键词： 纳米碳管 缺陷 稀有气体原子 分子动力学模拟     

纳米晶永磁Pr2Fe14B微磁学有限元法的模拟计算研究

根据实验数据，构造了接近实际纳米晶永磁Pr2Fe14B的样品，用微磁学有限元法进行了模拟计算.计算结果表明，晶界处各向异性的下降会导致矫顽力减小、剩磁值增大，而晶界处交换作用常数的减小则会使剩磁值减小、矫顽力增大.通过对实验样品的模拟研究发现，晶界处各向异性和交换作用常数的共同减小能够同时拟合出真实的矫顽力和剩磁值.模拟计算与实验在退磁曲线形状上的差距则说明模拟还存在不足. 关键词： 纳米晶永磁 磁滞回线 矫顽力 剩磁     

层间作用对HgS/CdS/HgS柱状纳米系统电子能量的影响

确立了柱状纳米系统电子势能随距离的变化关系，以HgS/CdS/HgS柱状纳米系统为例，讨论了层间作用对电子能谱的影响.结果表明：电子能量随势垒宽度和波矢的增大而增大，随势阱宽度的增大而减小；层间互作用会增大电子能量，但不会改变电子能量随势阱和势垒以及波矢的变化趋势. 关键词： 层间作用势 HgS/CdS/HgS柱状纳米系统 电子能量     

纳米铜团簇在常温和升温过程中能量特征的分子动力学研究

采用分子动力学方法和F-S多体势函数，模拟研究纳米铜团簇常温下能量特征及其在升温直到熔化过程中的变化，确定了常温下纳米铜团簇的表面原子厚度和表面能，给出在不同温度下纳米铜团簇能量大小分布比例和能量的概率密度，细致描述了团簇升温过程团簇内部原子和表面原子之间不同的变化特征. 关键词： 铜团簇 分子动力学 能量特征 温度     

面心立方铁纳米粒子的相变与并合行为的分子动力学研究

本文采用分子动力学模拟结合Finnis-Sinclair多体势研究了面心立方铁纳米粒子在加温过程中的相变与并合行为. 模拟结果表明: 纳米粒子在熔化之前均发生了由面心立方至体心立方的马氏体相变; 大小相等的两纳米粒子在并合之前发生了相对转动; 而大小不等的两纳米粒子在并合过程中并未出现转动, 小纳米粒子倾向于吸附在大纳米粒子上, 并随着温度的升高而熔化, 最终形成更大的纳米粒子. 关键词： 纳米粒子 相变 并合 分子动力学     

Ti纳米粒子熔化与凝结的原子尺度模拟

本文采用基于嵌入原子势的分子动力学方法模拟研究了不同尺寸Ti纳米粒子在熔化与凝结过程中的原子堆积结构变化.温度变化过程中对Ti纳米粒子中原子平均能量、对分布函数、键对和比热容的计算结果表明,粒子尺寸和温度变化方式对粒子的结构转变具有重要影响.小尺寸Ti粒子更易于形成二十面体构型.随着Ti纳米粒子粒径的增大,室温下粒子趋于保持初始密排六方的堆积结构.升温过程中,大粒径的纳米粒子内出现HCP向BCC的部分结构转变,导致HCP和BCC结构共存现象.大粒径粒子的熔化与体相材料相似,具有一个熔化温度.熔融粒子降温时,纳米粒子内部原子发生熔融态→BCC→HCP堆积结构的转变,且凝结温度较熔化温度滞后.该原子尺度的模拟提供了可用经典理论估算Ti粒子熔化所需能量的临界尺寸.     

Fe基体中包含Cu团簇的Fe-Cu二元体系在升温过程中结构变化的原子尺度计算

采用基于嵌入原子方法的分子动力学方法模拟了具有体心立方晶格结构的Fe基体中包含小尺寸Cu纳米粒子的Fe-Cu二元体系在升温过程中的原子堆积结构变化.进行了Cu原子均方位移、Cu原子对分布函数和原子的径向密度分布函数的计算,并对纯Cu原子区、Fe-Cu界面区和纯Fe基体区的分区域原子堆积结构进行了分析.结果表明,Fe基体内Cu团簇的尺寸及其在Fe基体内所能占据区域的大小,对不同温度下的Cu团簇内原子堆积结构及Fe基体的原子堆积结构具有影响.升温过程中不同尺寸受基体约束Cu团簇对Fe基体结构改变的影响表现出很大差异.对于Fe_(bulk)-Cu_(135)体系,基体的应变临近Fe-Cu界面区,同时在团簇中间的基体区域出现大量空位缺陷和应变集中区;对于Fe_(bulk)-Cu_(141)体系,随温度升高,基体中出现的应变区域表现为小尺寸、数量多向大尺寸、小数量的变化.     

Defect structure of hydrogen-reduced BaTiO3 with additions of Fe,Co and Ni

A discussion is presented of the possibilities of trapping an electron set free by hydrogen reduction in BaTiO₃ crystals without and with additions of Fe, Co and Ni. It is proved with the aid of the absorption spectra of reduced BaTiO₃ measured in the visible region, and of a theoretical calculation of the energy gain accompanying electron trapping that the preferred reduction is that of Fe, Co and Ni ions to divalent ones. The minimum hydrogen reduction temperature at which a band at 2·5 to 4 is initiated, belonging — as our results indicate — to Ti³⁺ in a certain configuration of the defect oxygen octahedron, was calculated from chemical analysis data of BaTiO₃ monocrystals and also verified experimentally.     

Synchronization defect lines in complex-oscillatory target waves

It is well known that one of key features of spiral waves in complex-oscillatory media is the appearance of synchronization defect lines, across which the phase of the oscillation changes by multiplies of 2π. In this Letter, we report the appearance of synchronization defect lines in target waves in complex-oscillatory media by studying a model of two-dimensional Rössler reaction-diffusion system subject to an appropriate periodic force in a small region of the center of domain. The geometric structure and stability of the defect lines are studied.     

Difference approach to the analysis of resistivity recovery data for irradiated short-range ordered alloys

Electrical resistivity recovery (RR) data for irradiated concentrated alloys typically consist of two inseparable parts, one resulting from defect annihilation and the other from short-range order (SRO) effects. These parts exhibit different behaviour and often follow opposite trends. Therefore, in this case, analysis of RR data within the conventional method is too complicated. A new approach to data analysis of such a two-component RR is proposed. The approach involves a new quantity, the difference RR (DRR), which is composed of RR dependences of two similar samples irradiated to different defect concentrations. It is shown that the SRO formation proper and the stages corresponding to the onset of long-range migration of Frenkel pair defects, formed in each part of RR, can be clearly related to certain features of the DRR plots. This interrelationship allows detecting and identifying these stages in each part of RR separately. The validity of the approach is illustrated by analysis of the available pairwise RR data for Fe–16Cr–20Ni and Fe–4Cr alloys. It makes it possible to detect the small contribution from the SRO formation to RR in Fe–4Cr, which we failed to observe previously. It is shown that stage III of Fe–4Cr, which has a negligible contribution to the part of RR induced by defect annihilation, is clearly observed in the part induced by SRO formation.     

Iron-induced magnetic,transport and magnetoresistance behavior in Nd<Subscript>0.67</Subscript>Sr<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> epitaxial films

The effects of Fe doping on Mn site in the colossal magnetoresistive film, Nd_0.67Sr_0.33MnO₃ have been studied by preparing the series Nd_0.67Sr_0.33Mn_1-xFe_xO₃ (x=0,0.05 and 0.1). Upon doping, no structural changes have been found. However, the Curie temperature, the associated metal-to-insulator transition temperature and the magnetization decrease drastically with Fe doping. The resistivity in the paramagnetic regime for all the samples follows Emin–Holsteins theory of small polaron. The polaron activation energy, W_p and resistivity coefficient, A increase with Fe doping. This effect may be ascribed to the fact that upon Fe doping, the long-range ferromagnetic order is destroyed and, therefore, W_p is enhanced in the system. As compared to the La-based system, Fe doping has a stronger tendency to destabilize the long-range ferromagnetic order in the Nd-based system. Large MR (as high as 90%) observed in the epitaxial NSMFO film may be attributed to the good lattice-matching between the grown film and substrate. PACS 75.47.Gk; 75.47.Lx; 75.70.-i     

Exchange-spring magnets based on L10-FePt ordered phase

Thin bi-layers constituted by a hard L1₀-FePt layer and a soft Fe layer were obtained using respectively an rf sputtering device and an UHV e-beam evaporation technique. Magneto-Optical Kerr Effect magnetometry, Atomic/Magnetic Force Microscopy and Conversion Electron Mössbauer Spectroscopy were used in order to correlate the magnetic properties of the bi-layers with the effects of the interdiffusion at the interfaces. It has been found that the evaporated Fe can easily diffuse into the hard film, giving raise to the formation of a region containing small particles of both Fe and Fe-rich FePt which show a superparamagnetic behaviour. The ferromagnetic Fe film can grow only on this region. The system shows (i) a preferred orientation of the easy magnetization axis along the direction normal to the film plane, and (ii) a single-phase magnetic behaviour due to the strong exchange coupling which established between the constituent phases.     

纳米Fe3 O4 颗粒的正电子湮没谱学研究

测量了磁性纳米Fe₃O₄颗粒的X射线衍射谱(XRD)、正电子湮没寿命谱(PALS)和符合多普勒展宽谱(CDBS),研究了不同压力和退火温度对磁性纳米Fe₃O₄颗粒物相、电子结构、缺陷及电子动量分布等的影响. XRD,PALS,CDBS测量结果表明:纳米Fe₃O₄颗粒的缺陷浓度随压力的增加而增大,但物相和缺陷类型并未发生变化;磁性纳米Fe₃O_{4< 关键词： 正电子 3}O₄')" href="#">Fe₃O₄ 寿命谱 多普勒展宽谱     

56,54Fe(n, α)53,51Cr, 56,54Fe(n,p)56,54Mn反应截面计算及普适性模型参数优化研究

基于中子与Fe发生反应产生“氢泡”,及“氦泡”对新型核能利用系统壁材料的影响,开展了中子诱发56,54Fe(n, α)53,51Cr, 56,54Fe(n, p)56,54Mn反应截面计算研究工作。本工作根据现有的56,54Fe(n, α)53,51Cr, 56,54Fe(n, p)56,54Mn反应实验数据、评价数据,对TALYS程序调用的物理模型(包括能级密度、对修正、核温度、光学势参数等)进行参数调校,得到了一组普适性强的模型参数。基于调校的参数,本工作采用核反应程序TALYS理论计算56,54Fe(n, α)53,51Cr, 56,54Fe(n, p)56,54Mn反应的截面、能量微分截面以及双微分截面,全部数据都能与实验数据、评价数据符合较好,且适用于较宽的中子能量区间0~175 MeV。本工作提出了56,54Fe(n, α)53,51Cr, 56,54Fe(n, p)56,54Mn反应的普适性模型参数,促进了核反应理论的发展,为核数据的评价奠定了基础。     

Phase diagram for the <Emphasis Type="Italic">O</Emphasis>(<Emphasis Type="Italic">n</Emphasis>) model with defects of “random local field” type and verity of the Imry–Ma theorem

It is shown that the Imry–Ma theorem stating that in space dimensions d < 4 the introduction of an arbitrarily small concentration of defects of the “random local field” type in a system with continuous symmetry of the n-component vector order parameter (O(n) model) leads to long-range order collapse and to the occurrence of a disordered state is not true if the anisotropic distribution of the defect-induced random local field directions in the space of the order parameter gives rise to the effective anisotropy of the “easy axis” type. In the case of a weakly anisotropic field distribution, in space dimensions 2 ≤ d < 4 there exists some critical defect concentration, above which the inhomogeneous Imry–Ma state can exist as an equilibrium one. At a lower defect concentration, long-range order takes place in the system. In the case of a strongly anisotropic field distribution, the Imry–Ma state is suppressed completely and long-range order state takes place at any defect concentration.