铁磁／有机／绝燃磁多层膜的隧道磁电阻研究

基于Slonczewski的自由电子近似理论,利用转移矩阵的方法计算了铁磁层／有机层／绝缘层／铁磁层磁性多层结构的隧穿磁电阻（tunnehng magnetic resistance,TMR）。保持有机层的厚度以及绝缘层的势垒高度不变,分别计算了在同一个有机层势垒高度且不同的自旋过滤因子β下的TMR随绝缘层厚度的变化;同时,还研究了在有机层和绝缘层的厚度不变,不同的β下,TMR随有机层势垒U的变化。结果表明,选取适当的β和绝缘层厚度能够获得大的TMR值;TMR随有机层势垒U的增加而增大。我们的计算结果对有机自旋注入、输运以及设计新的有机自旋电子器件的研究有一定的指导意义。     

Non-saturating magnetoresistance in heavily disordered semiconductors

The resistance of a homogeneous semiconductor increases quadratically with magnetic field at low fields and, except in very special cases, saturates at fields much larger than the inverse of the carrier mobility, a number typically of the order of 1 T (refs 1, 2). A surprising exception to this behaviour has recently been observed in doped silver chalcogenides, which exhibit an anomalously large, quasi-linear magnetoresistive response that extends down to low fields and survives, even at extreme fields of 55 T and beyond. Here we present a simple model of a macroscopically disordered and strongly inhomogeneous semiconductor that exhibits a similar non-saturating magnetoresistance. In addition to providing a possible explanation for the behaviour of doped silver chalcogenides, our model suggests potential routes for the construction of magnetic field sensors with a large, controllable and linear response.     

Geometrical enhancement of low-field magnetoresistance in silicon

Inhomogeneity-induced magnetoresistance (IMR) reported in some non-magnetic semiconductors, particularly silicon, has generated considerable interest owing to the large magnitude of the effect and its linear field dependence (albeit at high magnetic fields). Various theories implicate spatial variation of the carrier mobility as being responsible for IMR. Here we show that IMR in lightly doped silicon can be significantly enhanced through hole injection, and then tuned by an applied current to arise at low magnetic fields. In our devices, the 'inhomogeneity' is provided by the p-n boundary formed between regions where conduction is dominated by the minority and majority charge carriers (holes and electrons) respectively; application of a magnetic field distorts the current in the boundary region, resulting in large magnetoresistance. Because this is an intrinsically spatial effect, the geometry of the device can be used to enhance IMR further: we designed an IMR device whose room-temperature field sensitivity at low fields was greatly improved, with magnetoresistance reaching 10% at 0.07?T and 100% at 0.2?T, approaching the performance of commercial giant-magnetoresistance devices. The combination of high sensitivity to low magnetic fields and large high-field response should make this device concept attractive to the magnetic-field sensing industry. Moreover, because our device is based on a conventional silicon platform, it should be possible to integrate it with existing silicon devices and so aid the development of silicon-based magnetoelectronics.     

铁磁/绝缘层/有机半导体/铁磁多层膜隧道结的隧穿磁电阻的温度和偏压特性研究

采用Slonczewski的近自由电子模型, 利用转移矩阵的方法, 研究了铁磁/绝缘层/有机半导体/铁磁隧道结的自旋极化载流子隧穿的温度和偏压特性. 计算了T=4 K和T=300 K时, 隧穿磁电阻(Tunneling Magnetic Resistance, TMR)随偏压的变化关系, 同时还研究了零温时在有限偏压下隧穿磁电阻TMR与绝缘层厚度、有机半导体层厚度以及铁磁/有机半导体界面势垒U的变化关系. 我们的计算结果较好地解释了有关的实验 结论.     

石墨微晶粉末系统磁阻研究

用加压的方法制备了石墨微晶粉末样品,利用Quantum Design公司的Squid测量提供的磁场和低温,在温度为1.8 K～300 K和4.5 T的磁场范围内测量了样品的电阻,某些样品的电阻与温度关系符合涨落引起的隧穿导电机理,即满足R∞exp[T1/(T1+T2)],样品磁阻(MR)为正值.其它样品的曲线符合R∞1/Tα这一指数规律,其中α接近于数值1,提出声子激发的束缚态电子隧穿导电机制,其电输运特性取决于势垒有效宽度,含有负磁阻成分.研究表明样品的负磁阻是由于磁场中费米面附近电子能级的分裂,增强了声子激发的束缚态电子隧穿导电效应.     

半导体-金属复合结构中的异常磁电阻效应分析

从电磁方程的解析解出发,分析高对称性vander Pauwdisk装置的异常磁电阻效应,重点研究该体系的几何非均匀性与物理非均匀性对整体磁电阻效应的影响．通过定义三个非均匀参数表征体系的非均匀性,给出该装置中电磁输运性质随非均匀系数的变化．计算结果表明,体系的磁电阻比值在一定的几何非均匀结构下达到最优化,整个装置在磁场下呈现出良好的开关效应．同时,材料在迁移率和电阻率上的非均匀性对异常磁电阻效应产生明显影响．特别地,组分电阻率的差别导致异常磁电阻效应出现符号翻转．     

掺杂磁垒纳米结构中的GMR效应及调控

通过在半导体异质结上的上下表面沉积两条平行的铁磁条带可获得一个巨磁阻器件.为了更好地调控其性能,我们从理论上研究了在该器件中加入δ势后,该器件的透射系数、电导和磁阻比率的变化情况.研究发现,加入δ势后,该器件同样具有明显的巨磁阻效应,且其磁阻比率与加入δ势的权重和位置密切相关.因此,我们可以通过改变加入δ势的权重和位置来调控该巨磁阻器件,设计出磁阻比率可调的磁信息存储器.     

外加磁场中镍线纳米点接触的电镀制备研究

在传统的电镀沉积纳米镍点接触方法的基础上，在电镀时外加一个恒定的磁场，改变了其纳米点结的微观磁结构，改善了样品的成功率和重复性．这种性能的改善被认为是在电镀过程中的纳米镍晶在外磁场下择优取向所致，这种择优取向引起了纳米点结的磁畴结构的变化，从而揭示了纳米点接触中的弹道磁电阻效应．     

Giant synaptosomes

Investigations using synaptosomes, pinched-off nerve ending particles from brain, have greatly improved our knowledge of presynaptic function. However, these structures, like most nerve endings, are too small to be penetrated with microelectrodes. We have treated synaptosomal preparations from rat brain with a neutral protease and obtained fused structures large enough to be recorded from directly with microelectrodes; we report here that these particles (30-250 microM in diameter) contain mitochondria and structures resembling synaptic vesicles, morphological features characteristic of synaptosomes. These 'giant synaptosomes' have resting membrane potentials in the range -45 to -76 mV, are depolarized by increasing concentrations of K+, show responses to a variety of neuroactive substances and exhibit active membrane responses to depolarizing current pulses. These results suggest that this preparation will be of value in further studies of nerve terminal function.     

Spontaneous ferromagnetism and magnetoresistance hysteresis in Ge1-xSnx alloys

Introducing ferromagnetism into non-magnetic systems without the participation of magnetic elements is promising for all-electric spintronic devices[1,2].Many a...     

Magnetic properties and tunneling magnetoresistance in FeCo-SiO2 granular films

Fefv(SiO2)1-fv granular films were fabricated by rf sputtering (fv represents the Fe volume fraction). The mi-crostructure, magnetic properties as well as the tunneling magnetoresistance effect (TMR) were systematically studied. It was found that the maximum TMR ratio is about -3.3% at fv = 0.33. Under the same condition, a series of (Fe100-x Cox)0.33(SiO2)0.67 were prepared. TMR value reaches -4.5% at x = 53, while the microstructure of the film still keeps as that of Fe0.33(SiO2)0.67. The higher TMR ratio is contributed to the elevating of the spin polarization of particles. This is consistent with Inoue's theory.     

颗粒间隧穿体系H-A模型的磁电阻翻转效应

基于HELMAN-ABELES模型,超越传统的一级近似,对磁场相关库仑阻塞下体系的磁输运性质进行了系统的理论分析.我们发现,磁场相关的库仑能隙将引起低温下的正磁电阻发散行为,该行为与自旋极化引起的低温负磁电阻相互竞争,使体系的宏观磁电阻随参数的区域变化呈现出复杂的特征.我们分析了该正磁电阻出现的根本原因,从而指出影响磁电阻的因素除了传统的自旋极化率之外,还有与之效果相反的自旋相关库仑能隙的作用,后者为颗粒体系中的磁电阻效应提供了新的可能来源.     

小世界网络中的相分离和超大磁电阻效应

由于自旋、轨道、电荷和晶格的关联作用,具有超大磁电阻效应的锰氧化物表现出自发的相分离特征和复杂网络的输运特性.小世界网络是介于规则网络与无规网络之间的一种新的网络构形,它能够很好地描述锰氧化物的相分离特征.我们运用蒙特卡罗方法研究了基于小世界网络的一维伊辛自旋链中的磁输运性质,发现小世界关联会导致自发的长程序.随着小世界长程作用的增强,铁磁相的区域扩大,同时体系的磁性相变温度升高.相应的电阻网络模拟显示,在铁磁—顺磁转变温度附近,磁电阻效应有了很大的提高,很好地解释了锰氧化物中的相分离现象与超大磁电阻效应.     

Tunnel magnetoresistance (TMR) in ferromagnetic metalinsulator granular films

We review the recently discovered tunnel-type giant magnetoresistance (GMR) in ferromagnetic metalinsulator granular thin films, which is the magnetoresistance (MR) associated with the spin-dependent tunneling between two ferromagnetic metal particles. The theoretical and experimental results including electrical resistivity, magnetoresistance and their temperature dependence are described. Limitations to the applications of the ferromagnetic metalinsulator granular films are also discussed. Additionally, a brief survey of another two magnetic properties, high-frequency property and giant Hall effect (GHE) associated strongly with the granular structures is also presented.     

Peculiarity of magnetoresistance of discontinuous ferromagnetic thin films

The magnetoresistive properties of discontinuous ferromagnetic Fe and Co thin films deposited by electron-beam sputtering onto glass substrates at room temperature were investigated. Tunnel magnetoresistance (MR) was observed for all of the as-deposited samples. The maximum MR was observed for Fe thin films with an effective thickness of 17 nm. In the case of the Co thin films, the annealing process led to a change of the type of MR to anisotropic at Co film thicknesses (d_Co) of 15 ≤ d_Co ≤ 25 nm and to positive isotropic at thicknesses of d_Co < 15 nm. By contrast, the MR type of Fe thin films did not change.     

磁性颗粒复合介质中磁电阻机制诱导的庞磁电容效应

最近的实验和理论研究表明,磁电阻效应结合Maxwell-Wagner电路模型,磁性颗粒复合介质中也可能产生庞磁电容效应.这种庞磁电容效应与传统的磁电材料中由于直接的磁电耦合机制而产生的磁电容效应不同,它将会具有很高的实际应用价值.从磁性颗粒复合体系微观结构出发,应用Maxwell-Garnett理论(MGT)和磁电阻机制,研究了磁性颗粒复合体系中的庞磁电容效应随外加磁场和组分浓度p的变化关系,很好地解释了实验结果.     

巨磁电阻效应

2007年诺贝尔物理学奖被授予了法国物理学家阿尔贝·费尔(Albert.Fert)和德国物理学家彼得·格伦贝格尔(Peter.Griinberg),以表彰他们发现了巨磁电阻(Giant Magnetoresistance,GMR)效应.这个效应的应用导致了计算机硬盘记录密度的极大提高.此外,GMR效应的发现也是全新的自旋电子学的开端.我们可以期待电子自旋的量子效应带给我们更多的发现和创新技术.     

巨磁电阻效应

重点综述了磁性多层膜、颗粒膜、钙钛矿型氧化物及铁磁薄膜隧道结等几种不同结构类型的巨磁电阻效应的研究现状及其进展情况,并简述了巨磁电阻的物理机制及磁传感器、随机存储器和高密度读出头等几方面的应用,还涉及到了制备这些巨磁电阻材料的常用方法,并列举了10种不同组分的巨磁电阻材料,还说明了特大磁电阻和巨磁电阻的不同。