Giant magnetoresistance in organic spin-valves

A spin valve is a layered structure of magnetic and non-magnetic (spacer) materials whose electrical resistance depends on the spin state of electrons passing through the device and so can be controlled by an external magnetic field. The discoveries of giant magnetoresistance and tunnelling magnetoresistance in metallic spin valves have revolutionized applications such as magnetic recording and memory, and launched the new field of spin electronics--'spintronics'. Intense research efforts are now devoted to extending these spin-dependent effects to semiconductor materials. But while there have been noteworthy advances in spin injection and detection using inorganic semiconductors, spin-valve devices with semiconducting spacers have not yet been demonstrated. pi-conjugated organic semiconductors may offer a promising alternative approach to semiconductor spintronics, by virtue of their relatively strong electron-phonon coupling and large spin coherence. Here we report the injection, transport and detection of spin-polarized carriers using an organic semiconductor as the spacer layer in a spin-valve structure, yielding low-temperature giant magnetoresistance effects as large as 40 per cent.     

Spin-based logic in semiconductors for reconfigurable large-scale circuits

Research in semiconductor spintronics aims to extend the scope of conventional electronics by using the spin degree of freedom of an electron in addition to its charge. Significant scientific advances in this area have been reported, such as the development of diluted ferromagnetic semiconductors, spin injection into semiconductors from ferromagnetic metals and discoveries of new physical phenomena involving electron spin. Yet no viable means of developing spintronics in semiconductors has been presented. Here we report a theoretical design that is a conceptual step forward-spin accumulation is used as the basis of a semiconductor computer circuit. Although the giant magnetoresistance effect in metals has already been commercially exploited, it does not extend to semiconductor/ferromagnet systems, because the effect is too weak for logic operations. We overcome this obstacle by using spin accumulation rather than spin flow. The basic element in our design is a logic gate that consists of a semiconductor structure with multiple magnetic contacts; this serves to perform fast and reprogrammable logic operations in a noisy, room-temperature environment. We then introduce a method to interconnect a large number of these gates to form a 'spin computer'. As the shrinking of conventional complementary metal-oxide-semiconductor (CMOS) transistors reaches its intrinsic limit, greater computational capability will mean an increase in both circuit area and power dissipation. Our spin-based approach may provide wide margins for further scaling and also greater computational capability per gate.     

有机自旋电子学研究进展

本文从铁磁有机器件与非磁有机器件两方面介绍了近年来有机自旋电子学研究进展,理论上探讨了有机功能材料内的自旋极化注入与输运、自旋的微观动力学过程以及调控载流子的自旋取向对器件整体性能的影响。针对最近发现的有机强磁效应,从实验与理论两方面作了系统的阐述。     

贵金属/半导体异质结中的光致自旋电流效应研究

自旋电子材料因能同时对电子的自旋和电荷两个自由度实施操控,在构筑以低功耗、超高速、大容量和超宽带为特征的新一代信息处理技术中展现出巨大的应用潜力.然而,通过掺杂过渡金属元素和稀土离子而形成的传统稀磁半导体和钙钛矿锰氧化物往往因结构缺陷导致的居里温度不高、自旋磁矩和自旋极化率偏低等不足,阻碍了自旋电子材料的商业化应用.近年来,在高纯半导体上沉积贵金属薄膜所形成的贵金属/半导体异质结中,通过使用偏振光激发该类异质结可产生纯自旋电流.这种基于逆自旋霍尔效应(ISHE)、可在室温下运行的、非接触和非破坏型的自旋极化激励方法理论上可获得高于50%自旋极化率,引起了人们的广泛关注.文章主要介绍光致自旋电流形成机制和测试方法,以及入射光圆偏振度、光强、入射角度等参数对光致自旋注入效率的调控机理,介绍杂质介导和声子介导对光致自旋输运的贡献,最后提出增强光致自旋电子极化率的可行方案,可为揭示自旋载流子产生、注入和输运相关的自旋动力学核心科学问题以及研制高性能自旋电子器件提供有益的参考.     

Persistent sourcing of coherent spins for multifunctional semiconductor spintronics.

Recent studies of n-type semiconductors have demonstrated spin-coherent transport over macroscopic distances, with spin-coherence times exceeding 100 ns; such materials are therefore potentially useful building blocks for spin-polarized electronics ('spintronics'). Spin injection into a semiconductor (a necessary step for spin electronics) has proved difficult; the only successful approach involves classical injection of spins from magnetic semiconductors. Other work has shown that optical excitation can provide a short (<500 ps) non-equilibrium burst of coherent spin transfer across a GaAs/ZnSe interface, but less than 10% of the total spin crosses into the ZnSe layer, leaving long-lived spins trapped in the GaAs layer (ref. 9). Here we report a 'persistent' spin-conduction mode in biased semiconductor heterostructures, in which the sourcing of coherent spin transfer lasts at least 1-2 orders of magnitude longer than in unbiased structures. We use time-resolved Kerr spectroscopy to distinguish several parallel channels of interlayer spin-coherent injection. The relative increase in spin-coherent injection is up to 500% in the biased structures, and up to 4,000% when p-n junctions are used to impose a built-in bias. These experiments reveal promising opportunities for multifunctional spin electronic devices (such as spin transistors that combine memory and logic functions), in which the amplitude and phase of the net spin current are controlled by either electrical or magnetic fields.     

Electrical spin injection and accumulation at room temperature in an all-metal mesoscopic spin valve

Finding a means to generate, control and use spin-polarized currents represents an important challenge for spin-based electronics, or 'spintronics'. Spin currents and the associated phenomenon of spin accumulation can be realized by driving a current from a ferromagnetic electrode into a non-magnetic metal or semiconductor. This was first demonstrated over 15 years ago in a spin injection experiment on a single crystal aluminium bar at temperatures below 77 K. Recent experiments have demonstrated successful optical detection of spin injection in semiconductors, using either optical injection by circularly polarized light or electrical injection from a magnetic semiconductor. However, it has not been possible to achieve fully electrical spin injection and detection at room temperature. Here we report room-temperature electrical injection and detection of spin currents and observe spin accumulation in an all-metal lateral mesoscopic spin valve, where ferromagnetic electrodes are used to drive a spin-polarized current into crossed copper strips. We anticipate that larger signals should be obtainable by optimizing the choice of materials and device geometry.     

铁磁/半导体/铁磁隧道结中的隧穿磁电阻

采用相干量子输运理论和传递矩阵方法研究了具有不同自旋指向的极化电子渡越铁磁/半导体/铁磁隧道结的隧穿几率和隧穿磁电阻。研究表明隧穿几率和隧穿磁电阻随半导体长度的改变发生周期性变化、随Rashba自旋轨道耦合强度的改变发生准周期变化,并且在两铁磁电极中磁矩取向平行时,选择适当的半导体的长度和Rashba自旋轨道耦合强度可以得到较大的隧穿磁电阻。     

铁磁/半导体/铁磁异质结构的自旋极化输运

采用相干量子输运理论和传递矩阵方法,研究了具有不同自旋指向的极化电子渡越铁磁/半导体/铁磁异质结构的隧穿几率和自旋极化率.研究表明,隧穿几率和自旋极化率随半导体长度的改变发生周期性变化、随Rashba自旋轨道耦合强度的改变发生准周期变化,并且在2铁磁电极中磁矩取向平行时;选择适当的半导体的长度和Rashba自旋轨道耦合强度可以得到较大的自旋极化率.     

Electrical detection of spin precession in a metallic mesoscopic spin valve

To study and control the behaviour of the spins of electrons that are moving through a metal or semiconductor is an outstanding challenge in the field of 'spintronics', where possibilities for new electronic applications based on the spin degree of freedom are currently being explored. Recently, electrical control of spin coherence and coherent spin precession during transport was studied by optical techniques in semiconductors. Here we report controlled spin precession of electrically injected and detected electrons in a diffusive metallic conductor, using tunnel barriers in combination with metallic ferromagnetic electrodes as spin injector and detector. The output voltage of our device is sensitive to the spin degree of freedom only, and its sign can be switched from positive to negative, depending on the relative magnetization of the ferromagnetic electrodes. We show that the spin direction can be controlled by inducing a coherent spin precession caused by an applied perpendicular magnetic field. By inducing an average precession angle of 180 degrees, we are able to reverse the sign of the output voltage.     

Direct electronic measurement of the spin Hall effect

The generation, manipulation and detection of spin-polarized electrons in nanostructures define the main challenges of spin-based electronics. Among the different approaches for spin generation and manipulation, spin-orbit coupling--which couples the spin of an electron to its momentum--is attracting considerable interest. In a spin-orbit-coupled system, a non-zero spin current is predicted in a direction perpendicular to the applied electric field, giving rise to a spin Hall effect. Consistent with this effect, electrically induced spin polarization was recently detected by optical techniques at the edges of a semiconductor channel and in two-dimensional electron gases in semiconductor heterostructures. Here we report electrical measurements of the spin Hall effect in a diffusive metallic conductor, using a ferromagnetic electrode in combination with a tunnel barrier to inject a spin-polarized current. In our devices, we observe an induced voltage that results exclusively from the conversion of the injected spin current into charge imbalance through the spin Hall effect. Such a voltage is proportional to the component of the injected spins that is perpendicular to the plane defined by the spin current direction and the voltage probes. These experiments reveal opportunities for efficient spin detection without the need for magnetic materials, which could lead to useful spintronics devices that integrate information processing and data storage.     

Transformation of spin information into large electrical signals using carbon nanotubes

Spin electronics (spintronics) exploits the magnetic nature of electrons, and this principle is commercially applied in, for example, the spin valves of disk-drive read heads. There is currently widespread interest in developing new types of spintronic devices based on industrially relevant semiconductors, in which a spin-polarized current flows through a lateral channel between a spin-polarized source and drain. However, the transformation of spin information into large electrical signals is limited by spin relaxation, so that the magnetoresistive signals are below 1% (ref. 2). Here we report large magnetoresistance effects (61% at 5 K), which correspond to large output signals (65 mV), in devices where the non-magnetic channel is a multiwall carbon nanotube that spans a 1.5 microm gap between epitaxial electrodes of the highly spin polarized manganite La(0.7)Sr(0.3)MnO3. This spintronic system combines a number of favourable properties that enable this performance; the long spin lifetime in nanotubes due to the small spin-orbit coupling of carbon; the high Fermi velocity in nanotubes that limits the carrier dwell time; the high spin polarization in the manganite electrodes, which remains high right up to the manganite-nanotube interface; and the resistance of the interfacial barrier for spin injection. We support these conclusions regarding the interface using density functional theory calculations. The success of our experiments with such chemically and geometrically different materials should inspire new avenues in materials selection for future spintronics applications.     

Thermal spin current from a ferromagnet to silicon by Seebeck spin tunnelling

Heat generation by electric current, which is ubiquitous in electronic devices and circuits, raises energy consumption and will become increasingly problematic in future generations of high-density electronics. The control and re-use of heat are therefore important topics for existing and emerging technologies, including spintronics. Recently it was reported that heat flow within a ferromagnet can produce a flow of spin angular momentum-a spin current-and an associated voltage. This spin Seebeck effect has been observed in metallic, insulating and semiconductor ferromagnets with temperature gradients across them. Here we describe and report the demonstration of Seebeck spin tunnelling-a distinctly different thermal spin flow, of purely interfacial nature-generated in a tunnel contact between electrodes of different temperatures when at least one of the electrodes is a ferromagnet. The Seebeck spin current is governed by the energy derivative of the tunnel spin polarization. By exploiting this in ferromagnet-oxide-silicon tunnel junctions, we observe thermal transfer of spins from the ferromagnet to the silicon without a net tunnel charge current. The induced spin accumulation scales linearly with heating power and changes sign when the temperature differential is reversed. This thermal spin current can be used by itself, or in combination with electrical spin injection, to increase device efficiency. The results highlight the engineering of heat transport in spintronic devices and facilitate the functional use of heat.     

Coherent spin manipulation without magnetic fields in strained semiconductors

A consequence of relativity is that in the presence of an electric field, the spin and momentum states of an electron can be coupled; this is known as spin-orbit coupling. Such an interaction opens a pathway to the manipulation of electron spins within non-magnetic semiconductors, in the absence of applied magnetic fields. This interaction has implications for spin-based quantum information processing and spintronics, forming the basis of various device proposals. For example, the concept of spin field-effect transistors is based on spin precession due to the spin-orbit coupling. Most studies, however, focus on non-spin-selective electrical measurements in quantum structures. Here we report the direct measurement of coherent electron spin precession in zero magnetic field as the electrons drift in response to an applied electric field. We use ultrafast optical techniques to spatiotemporally resolve spin dynamics in strained gallium arsenide and indium gallium arsenide epitaxial layers. Unexpectedly, we observe spin splitting in these simple structures arising from strain in the semiconductor films. The observed effect provides a flexible approach for enabling electrical control over electron spins using strain engineering. Moreover, we exploit this strain-induced field to electrically drive spin resonance with Rabi frequencies of up to approximately 30 MHz.     

Atom-by-atom substitution of Mn in GaAs and visualization of their hole-mediated interactions

The discovery of ferromagnetism in Mn-doped GaAs has ignited interest in the development of semiconductor technologies based on electron spin and has led to several proof-of-concept spintronic devices. A major hurdle for realistic applications of Ga(1-x)Mn(x)As, or other dilute magnetic semiconductors, remains that their ferromagnetic transition temperature is below room temperature. Enhancing ferromagnetism in semiconductors requires us to understand the mechanisms for interaction between magnetic dopants, such as Mn, and identify the circumstances in which ferromagnetic interactions are maximized. Here we describe an atom-by-atom substitution technique using a scanning tunnelling microscope (STM) and apply it to perform a controlled study at the atomic scale of the interactions between isolated Mn acceptors, which are mediated by holes in GaAs. High-resolution STM measurements are used to visualize the GaAs electronic states that participate in the Mn-Mn interaction and to quantify the interaction strengths as a function of relative position and orientation. Our experimental findings, which can be explained using tight-binding model calculations, reveal a strong dependence of ferromagnetic interaction on crystallographic orientation. This anisotropic interaction can potentially be exploited by growing oriented Ga(1-x)Mn(x)As structures to enhance the ferromagnetic transition temperature beyond that achieved in randomly doped samples.     

Detecting excitation and magnetization of individual dopants in a semiconductor

An individual magnetic atom doped into a semiconductor is a promising building block for bottom-up spintronic devices and quantum logic gates. Moreover, it provides a perfect model system for the atomic-scale investigation of fundamental effects such as magnetism in dilute magnetic semiconductors. However, dopants in semiconductors so far have not been studied by magnetically sensitive techniques with atomic resolution that correlate the atomic structure with the dopant's magnetism. Here we show electrical excitation and read-out of a spin associated with a single magnetic dopant in a semiconductor host. We use spin-resolved scanning tunnelling spectroscopy to measure the spin excitations and the magnetization curve of individual iron surface-dopants embedded within a two-dimensional electron gas confined to an indium antimonide (110) surface. The dopants act like isolated quantum spins the states of which are governed by a substantial magnetic anisotropy that forces the spin to lie in the surface plane. This result is corroborated by our first principles calculations. The demonstrated methodology opens new routes for the investigation of sample systems that are more widely studied in the field of spintronics-that is, Mn in GaAs (ref. 5), magnetic ions in semiconductor quantum dots, nitrogen-vacancy centres in diamond and phosphorus spins in silicon.     

铁磁/有机系统的自旋扩散漂移性质研究

基于自旋扩散漂移方程和欧姆定律,理论研究了电场对铁磁/有机半导体界面的电流自旋极化性质的影响.考虑到有机半导体内特殊的载流子以及电场对其自旋扩散长度的影响,计算了界面处的电流自旋极化率.结果表明,高电场可以使界面处的电流自旋极化率得到有效提高.同时还进一步研究了电场下有机半导体中极化子比率、自旋相关界面电阻等因素对电流自旋极化的影响.     

Electrical detection of the spin resonance of a single electron in a silicon field-effect transistor

The ability to manipulate and monitor a single-electron spin using electron spin resonance is a long-sought goal. Such control would be invaluable for nanoscopic spin electronics, quantum information processing using individual electron spin qubits and magnetic resonance imaging of single molecules. There have been several examples of magnetic resonance detection of a single-electron spin in solids. Spin resonance of a nitrogen-vacancy defect centre in diamond has been detected optically, and spin precession of a localized electron spin on a surface was detected using scanning tunnelling microscopy. Spins in semiconductors are particularly attractive for study because of their very long decoherence times. Here we demonstrate electrical sensing of the magnetic resonance spin-flips of a single electron paramagnetic spin centre, formed by a defect in the gate oxide of a standard silicon transistor. The spin orientation is converted to electric charge, which we measure as a change in the source/drain channel current. Our set-up may facilitate the direct study of the physics of spin decoherence, and has the practical advantage of being composed of test transistors in a conventional, commercial, silicon integrated circuit. It is well known from the rich literature of magnetic resonance studies that there sometimes exist structural paramagnetic defects near the Si/SiO2 interface. For a small transistor, there might be only one isolated trap state that is within a tunnelling distance of the channel, and that has a charging energy close to the Fermi level.     

半导体中纯自旋流的探测

自旋电子学是利用电子的自旋而非电子的电荷作为信息载体而发展的物理和电子器件研究的分支领域.半导体中自旋流的测量在自旋电子学中起关键作用.本文从自旋流的基本性质出发,简要回顾了目前国际上探测自旋流的实验手段,以及作者最近提出的有关自旋流的光学效应和以此直接测量半导体中纯自旋流的理论.     

铁磁/有机系统的自旋扩散漂移性质研究王

基于自旋扩散漂移方程和欧姆定律, 理论研究了电场对铁磁/有机半导体界面的电流自旋极化性质的影响. 考虑到有机半导体内特殊的载流子以及电场对其自旋扩散长度的影响, 计算了界面处的电流自旋极化率. 结果表明, 高电场可以使界面处的电流自旋极化率得到有效提高. 同时还进一步研究了电场下有机半导体中极化子比率、自旋相关界面电阻等因素对电流自旋极化的影响.     

T型有机器件自旋注入效率的提高

考虑到有机半导体中特殊的载流子电荷自旋关系,建立了一个自旋注入有机半导体的简单的T型器件模型,运用自旋扩散理论计算得出了此模型的电流白旋极化率并与铁磁／有机半导体异质结构的注入效率进行了比较．理论计算发现T型模型中通过调节分支电流的大小会使自旋极化率较铁磁／有机半导体模型有明显的提高,并讨论了极化子比率、外加电场、自旋相关界面电阻以及有机半导体电导率等因素对电流自旋极化性质的影响．