FeCoBSiO2磁性纳米颗粒膜的微波电磁特性

采用交替沉积磁控溅射工艺制备了超薄多层的FeCoBSiO2磁性纳米颗粒膜.利用x射线衍射仪、扫描探针显微镜、透射电子显微镜分析了薄膜的微结构和形貌特征.采用振动样品磁强计、四探针法、微波矢量分析仪及谐振腔法测量薄膜试样的磁电性能和微波复磁导率.重点对SiO2介质相含量、薄膜微结构对电磁性能产生重要影响的机理做了分析和探讨.结果 表明：这类FeCoBSiO2磁性纳米颗粒膜具有良好的软磁性能和高频电磁性能，2GHz时的 磁导率μ′高于70，可以应用于高频微磁器件或微波吸收材料的设计. 关键词： 磁性纳米颗粒膜 高频特性 复磁导率 磁控溅射     

FeCoB-SiO2磁性纳米颗粒膜的微波电磁特性

采用交替沉积磁控溅射工艺制备了超薄多层的FeCoB SiO2 磁性纳米颗粒膜 .利用x射线衍射仪、扫描探针显微镜、透射电子显微镜分析了薄膜的微结构和形貌特征 .采用振动样品磁强计、四探针法、微波矢量分析仪及谐振腔法测量薄膜试样的磁电性能和微波复磁导率 .重点对SiO2 介质相含量、薄膜微结构对电磁性能产生重要影响的机理做了分析和探讨 .结果表明 :这类FeCoB SiO2 磁性纳米颗粒膜具有良好的软磁性能和高频电磁性能 ,2GHz时的磁导率 μ′高于 70 ,可以应用于高频微磁器件或微波吸收材料的设计     

逾渗驱动的高自旋极化氧化物材料磁电阻增强效应——网络效应与调控

锰氧化物具有内禀的多尺度非均匀性,这与同时活跃的多个自由度--自旋、电荷、晶格和轨道--非线性耦合,以及多种相互作用的共存密切相关.更重要的是, 这种极为特殊的物理现象可能是庞磁电阻效应的微观起源--铁磁金属相在磁场作用下的逾渗而驱动的磁电阻效应.另一方面,在某组分导电逾渗阈值附近的磁电阻显著增强效应,是高自旋极化氧化物颗粒体系所具有的普遍现象之一.因此,针对各种高自旋极化氧化物的非均匀和颗粒复合体系,逾渗驱动磁电阻增强效应的研究具有重要的学术意义和应用价值,其中输运网络理论为重要的理论研究.在充分认识电磁输运微观机制的基础之上,通过调控输运网络的结构,探讨逾渗驱动磁电阻增强的必要条件,可以找出实现可控性高且幅值较大的磁电阻的新途径、新方法.本文主要基于电阻网络模型,综述高自旋极化氧化物材料中多相共存体系的磁输运性质研究的主要背景和发展现状,充分结合相关的实验结果,介绍逾渗驱动磁电阻效应增强的物理机制,以及各类电输运网络的构建,并展望未来的发展.     

多层膜磁性微泡的非线性声振动特性

超顺磁性氧化铁纳米粒子与造影剂微泡结合形成磁性微泡,用于产生多模态造影剂,以增强医学超声和磁共振成像.将装载有纳米磁性颗粒的微泡包膜层看作由磁流体膜与磷脂膜组合而成的双层膜结构,同时考虑磁性纳米颗粒体积分数a对膜密度及黏度的影响,从气泡动力学基本理论出发,构建多层膜结构磁性微泡非线性动力学方程.数值分析了驱动声压和频率等声场参数、颗粒体积分数、膜层厚度以及表面张力等膜壳参数对微泡声动力学行为的影响.结果表明,当磁性颗粒体积分数较小且a≤0.1时,磁性微泡声响应特性与普通包膜微泡相似,微泡的声频响应与其初始尺寸和驱动压有关;当驱动声场频率f为磁性微泡共振频率f0的2倍(f=2f0)时,微泡振动失稳临界声压最低;磁性颗粒的存在抑制了泡的膨胀和收缩但抑制效果非常有限;磁性微泡外膜层材料的表面张力参数K及膜层厚度d也会影响微泡的振动,当表面张力参数及膜厚取值分别为0.2—0.4 N/m及50—150 nm时,可观察到气泡存在不稳定振动响应区.     

快速傅立叶变换微磁学方法

提出了一种具有快速傅立叶变换的微磁学(FFTM)方法,并应用于二维纳米磁性系统.计算结果表明,利用该方法得到的结果与采用截短相互作用距离直接求和计算偶极相互作用能的方法得到的结果基本一致,同时其计算精度和计算速度得到很大的改善.结果证明FFTM方法能快速有效地计算包含远距离偶极相互作用的磁性系统的磁特性.     

逾渗驱动的高自旋极化氧化物材料磁电阻增强效应——网络效应与调控

锰氧化物具有内禀的多尺度非均匀性,这与同时活跃的多个自由度——自旋、电荷、晶格和轨道——非线性耦合,以及多种相互作用的共存密切相关。更重要的是,这种极为特殊的物理现象可能是庞磁电阻效应的微观起源——铁磁金属相在磁场作用下的逾渗而驱动的磁电阻效应。另一方面,在某组分导电逾渗阈值附近的磁电阻显著增强效应,是高自旋极化氧化物颗粒体系所具有的普遍现象之一。因此,针对各种高自旋极化氧化物的非均匀和颗粒复合体系,逾渗驱动磁电阻增强效应的研究具有重要的学术意义和应用价值,其中输运网络理论为重要的理论研究。在充分认识电磁输运微观机制的基础之上,通过调控输运网络的结构,探讨逾渗驱动磁电阻增强的必要条件,可以找出实现可控性高且幅值较大的磁电阻的新途径、新方法。本文主要基于电阻网络模型,综述高自旋极化氧化物材料中多相共存体系的磁输运性质研究的主要背景和发展现状,充分结合相关的实验结果,介绍逾渗驱动磁电阻效应增强的物理机制,以及各类电输运网络的构建,并展望未来的发展。     

基于交换耦合模型纳米双相(硬磁/软磁)自旋体系的磁性

采用MonteCarlo方法对由异类自旋组成混合Heisenberg自旋体系进行了数值计算,以模拟和预测基于交换耦合模型纳米双相(硬磁软磁)磁性体系的磁性.区别于以往所报道的那些直接针对硬磁NdFeB与软磁纳米αFe复合磁体的微磁学计算工作,着眼于符合Heisenberg模型的两类完全不同的原子自旋集团(硬磁自旋和软磁自旋)之间的直接交换耦合作用,模拟计算了由这两类自旋组成的复合自旋体系的内禀矫顽力Hc、剩余磁化强度Mr、最大磁能级(M×H)max等宏观磁性参量随软磁自旋集团的尺度和体积百分比、两类自旋集 关键词： Heisenberg模型 MonteCarlo模拟 纳米复合磁体 Nd-Fe-B     

磁性薄膜的晶格结构和磁性

采用Monte-Carlo模拟方法对六边形、正方形和三角形晶格结构磁性薄膜的磁学特性及磁畴结构进行了模拟,结果表明,磁性薄膜的磁性特征及其磁相变温度和薄膜结构密切相关并存在临界膜厚,当薄膜厚度大于临界膜厚时薄膜磁性特征稳定.在低温区,不同结构磁性薄膜的磁滞回线均出现台阶现象,结果同相关实验一致.     

稀土永磁薄膜材料

简要地介绍在纳米复合稀土永磁薄膜材料、各向异性稀土永磁薄膜材料方面的进展.在纳米复合稀土永磁薄膜材料中实现磁性交换耦合和剩磁增强效应,系统地研究了其结构与磁性的关系.制备成功高磁能积的各向异性稀土永磁薄膜材料,比较了Ti 或Mo 缓冲层对Nd-Fe-B薄膜的表面形貌、磁畴结构和磁性能的影响.发现薄膜的表面形貌强烈地依赖于缓冲层的厚度.由于它极大地影响薄膜的成分,溅射速率被证明是控制薄膜的显微结构、表面形貌和磁性能的一个重要因素.在微磁学模型的基础上,通过分析从5到300 K的矫顽力温度依赖关系.研究了各向异性Pr-Fe-B薄膜的矫顽力机制.在晶粒表面,由于磁各向异性的降低和局域退磁场的提高导致的反转畴的形核被确定为控制各向异性Pr-Fe-B薄膜的磁化反转过程的首要机制.     

混合磁性薄膜矫顽力及阶梯效应的微磁学及Monte Carlo研究

采用能量极小原理的微磁学及Monte Carlo方法对铁磁/反铁磁混合磁性薄膜的磁特性进行了模拟计算，研究了基态下系统的磁滞回线、自旋组态及铁磁交换作用常数JAA、单轴各向异性常数K、偶极相互作用常数D和铁磁性原子掺杂量X对矫顽力Hc的影响. 同时还模拟计算了矫顽力Hc的温度特性.模拟结果表明，在混合磁性薄膜中磁滞回线存在明显的阶梯效应，利用简单的Ising模型揭示这种阶梯效应主要起源于包含不同反铁磁原 子的掺杂量的不同尺寸的原子团对外加磁场所产生不同响应；在基态下当0.5≤X≤1.0时矫顽力Hc随K，J 关键词： 蒙特卡罗 微磁学 阶梯效应 混合磁系统 矫顽力     

Dielectric sensing by charging energy modulation in a nano-granular metal

Several sensing concepts using nanostructures prepared by focused-electron-beam-induced deposition have been developed over the last years. Following work on highly miniaturized Hall sensors for magnetic sensing with soft magnetic Co, strain and force sensing based on nano-granular platinum–carbon structures (Pt(C)) was introduced. Very recently, the capability of nano-granular Pt(C) structures to detect the presence of adsorbate water layers by conductance modulations was demonstrated. For magnetic and strain sensing, the underlying physical mechanisms of the sensing effect have been analyzed in detail and are now quite well understood. This is not the case for the adsorbate layer-induced conductance modulation effect. Here, we provide a theoretical framework that allows for a semi-quantitative understanding of the observed water-sensing effect. We show how the near-interface renormalization of the Coulomb charging energy in the nano-granular metal caused by the dielectric screening of the polarizable adsorbate layer leads to a conductance modulation. The model can account for the conductance modulation observed in the water adsorbate experiments and can also be applied to understand similar effects caused by near-interface dielectric anomalies of ferroelectric thin films on top of nano-granular Pt(C). Our findings provide a pathway toward optimized nano-granular layer structures suitable for a wide range of dielectric or local potential sensing applications.     

退火对Co35(SiO2)65颗粒膜巨磁电阻效应的影响

采用多靶离子束溅射镀膜机制备了一系列不同退火温度的Co₃₅(SiO₂)₆₅（体积百分数）颗粒膜样品,发现样品的巨磁电阻效应随着退火温度T_a的升高而单调下降.应用磁力显微镜对样品的磁结构进行了观测,发现随着退火温度的升高,近邻的Co颗粒的磁矩倾向于平行排列,形成磁畴结构,从而导致Co₃₅(SiO₂)₆₅颗粒膜巨磁电阻效应单调下降. 关键词：     

Evolution of a Magnetoresonance Line Near to a Limit of Percolation in Dilute Magnetics

In paper the results of numerical modeling of a magnetic resonance in dilute magnetics near to a threshold of a percolation are discussed. The classical equation of motion of magnetic moments is used in view of an exchange interaction such as RKKI and imitation of spin-phonon interaction by Monte-Carlo method. It is shown, that cluster structure of a magnetic and threshold of percolation are determined by critical distance, on which there is a change of a sign of an exchange interaction. In an examination of percolation phase transition the jump change of breadth of a line of a magnetic resonance is set, that can form the basis for experimental definition of a threshold percolation and parameters of an exchange interaction by methods of a radiospectroscopy.     

Dipolar coupling effect in magnetic bilayer system

Equilibrium micromagnetic structures in a bilayer system composed of two thin cobalt films separated by a non magnetic spacer are systematically analysed. These 2D magnetization distributions are obtained by numerical computations according to different set of magnetic and geometric parameters. The coupling effect due to the dipolar long range interaction (or stray-field effect) between the two Co layers is studied through the evolution of the magnetic pattern in the stack with or without an applied field and compared to a continuous film of same thickness. Special attention is paid to the hysteresis process in a bilayer. Even though the general aspect of the magnetization distribution looks like a Landau-Lifshitz structure, the absence of any core in the vortex of the magnetic structure is analysed in relation to a possible disappearence of (topological) hysteresis. Received 12 December 2001     

Structure–conductivity relations of simulated highly porous nanoparticle aggregate films

Electrical conductivity of porous films composed of nanoparticle aggregates is theoretically evaluated with respect to aggregate structure and film packing density. The aggregates are fractals composed of 5–30 primary particles with diameter of 10 nm. The film properties are derived from simulated boxes in the range of 0.5–1 μm. The electrical conductivity across the films of packing densities ranging from 0.01 to 0.15 was studied. All films prepared by an aerosol deposition technique, which uses nanoparticle aggregates, exhibited percolation behavior between planes parallel to the moving direction of the aggregates. They also followed the classical percolation relation for electrical conductivity while the critical percolation packing density depends on the aggregate size and structure used to build the films. Films using larger aggregates as building blocks have higher electrical conductance than smaller aggregates close to the percolation limit. For validation and supplementary information, two independent models are developed: one model follows the percolation theory to get detailed physical insights and another one computes the exact conductivities but at the cost of some details. This analysis gives new insights into the conduction backbone structures of these films with regard to neck contacts within an aggregate and grain boundary contacts between aggregates. The results shown are important for solar application of these films and especially for gas sensors where high sensitivity is often counteracted by low conductivity.     

Peculiarities of Photoprocesses in Poly(9,9-di-<Emphasis Type="Italic">n</Emphasis>-octylfluorenyl-2,7-diyl) Films Doped with Potassium Iodide

The spectral-luminescence properties of poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO) films doped with potassium iodide are investigated. It is found that the addition of potassium iodide at concentrations of 0.1–1% leads to a drop in the degree of ordering of the PFO films and decreases the fluorescence intensity and lifetime of the polymer. The kinetics of photoluminescence of PFO–KI films is studied in nano-, micro-, and millisecond time bands. Analysis of the experimental data on long-term luminescence using the percolation model showed that the addition of potassium iodide to the polymer increases the degree of disordering of the film. Investigations of the effect of an external magnetic field on the PFO luminescence over a wide time range indicate a complex character of the time dependence of the magnetic effect. The changes in both the magnitude and sign of the magnetic effect g(B) are observed over the entire measured time range.     

Monte Carlo simulation of magnetic nanostructured thin films

Using Monte Carlo simulation, we have compared the magnetic properties between nanostructured thin films and two-dimensional crystalline solids. The dependence of nanostructured properties on the interaction between particles that constitute the nanostructured thin films is also studied. The result shows that the parameters in the interaction potential have an important effect on the properties of nanostructured thin films at the transition temperatures.     

Magnetism of Co layers in a Co/Si multilayer film

The magnetic properties of Co/Si multilayer films produced through rf ion sputtering were studied in the temperature range 4.2–300 K. The dependences of the spontaneous magnetization and hysteresis characteristics of the films on the thicknesses of the magnetic layers and nonmagnetic spacers are established. It is shown that these dependences are determined to a large extent by interlayer interfaces, in which the effective magnetic moment of the Co atoms and the exchange interaction decrease and magnetic-anisotropy dispersion arises. A probable cause of the interface formation is interlayer mixing (which is estimated to penetrate to a depth of 15 Å) and the strong effect of Si on the Co electronic structure.     

Photoluminescence properties of SHI induced F2 and F3 color centers in nano-granular LiF thin films

Photoluminescence (PL) properties of swift heavy ions-induced F₂ and F₃⁺ color centers in nano-granular lithium fluoride (LiF) thin film were studied. LiF films were deposited on glass and silica substrates and irradiated with various ion species (Ag, Ni and Au) at different irradiation temperatures. The role of ion species, their fluence and the irradiation temperature on the PL intensity of color centers induced in LiF thin films is discussed.