Efficient spin voltage/current generation in a ferromagnet/semiconductor lateral spin-valve

自旋极化电子电流已被广泛研究，以实现新的自旋电子器件的应用。例如，由自旋极化电子电流引起的自旋传递转矩为铁磁纳米线中的电流感应磁化开关（CIMS）和畴壁运动提供了基本的物理机制。自旋传递转矩由Berger和Slonczewski独立预测，并通过实验证明。通过对巨磁阻（GMR）纳米柱的自旋散射层插入和形状修改，开关的临界电流密度已经降低，以满足磁随机存储器（MRAM）的gb级要求。飞思卡尔（现为EverSpin Technologies）于2006年推出了4 mbit版本的MRAM。由于非挥发性和更好的热稳定性，MRAM有望取代硅基RAM。最近，Fe/MgO/Fe体系中的相干隧穿被预测可以实现超过1000%的隧穿磁阻（TMR），并在外延/高取向Fe(Co)/MgO/Fe（Co）结中进行了实验观察。将这种相干隧穿技术应用于纳米柱中，在室温下实现了TMR比为160%的CIMS。通过将大TMR比与临界电流密度大幅降低至2.5x10^6 A/cm2相结合，满足了超gbit级MRAM应用的要求。因此，在美国和日本，政府倡议已被应用于开发商用gb MRAM。纳米级制造技术的最新发展将使我们能够将垂直的GMR纳米柱扩展到横向配置，由铁磁性纳米线和非磁性纳米线组成，以桥接自旋注入器和检测器，从而实现尺寸的精确控制。在这种横向自旋阀配置中，自旋极化电子可以注入电子电荷电流（局部几何）而不注入电荷电流（非局部几何）。Jedema等人利用非局部几何进行了开创性的工作，成功地展示了铁磁性Ni80Fe20纳米电极的扩散自旋注入，非磁性Cu纳米线中的自旋积累以及另一种NiFe纳米电极的自旋检测。他们通过在FM/非磁铁（NM）界面上插入一个AlOx隧道势垒（绝缘体，I），进一步扩展了他们对弹道自旋注入的研究。因此，非局部自旋阀系统已被广泛应用于通过最小化扩散和弹道接触中的界面散射来实现有效的自旋注入，并且还可以检测自旋霍尔和逆自旋霍尔效应。这清楚地表明了横向装置配置的优点。在这个提议的项目中，我们将采用横向自旋阀结构而不是传统的纳米柱来演示有效地产生自旋电压和电流，这与电子电荷电流无关，从而最大限度地减少焦耳加热。在我们提出的器件中，自旋电流和自旋极化电子-电荷电流将分别通过改变测量几何形状来检测非局部和局部测量几何形状中的自旋电压/电流产生。在非局部几何结构中，自旋电流可以通过隧道势垒有效地注入到非磁体中，并通过第二隧道势垒检测到大的自旋电压。这使得一个大的自旋电流通过金属界面。因此，我们提出的器件将独立作为一个纯自旋电压和自旋电流源，具有高效率。纯自旋电压和电流的评估将揭示自旋电流输运的基本机制（不带电子电荷），这将鼓励进一步的理论研究，以更好地理解自旋电流，也将引领一种新型的器件架构。

A spin-polarised electron current has been widely investigated to realise new spintronic device application. For example, spin-transfer torque induced by a spin-polarised electron current offers a fundamental physical mechanism on current-induced magnetisation switching (CIMS) as well as domain-wall motion in a ferromagnetic (FM) nanowire. The spin-transfer torque was predicted by Berger and Slonczewski independently, and has been experimentally demonstrated. By spin-scattering layer insertion and shape modification for a giant magnetoresistive (GMR) nanopillar, a critical current density for switching has been reduced to satisfy a Gbit-scale requirement for a magnetic random access memory (MRAM), a 4-Mbit version of which has been introduced by Freescale (now EverSpin Technologies) in 2006. MRAM is expected to replace a Si-based RAM due to the non-volatility and the better thermal stability. Recently, coherent tunnelling in an Fe/MgO/Fe system has been predicted to achieve over 1000% tunnelling magnetoresistance (TMR) and experimentally observed in epitaxial/highly-oriented Fe(Co)/MgO/Fe(Co) junctions. Such coherent tunnelling has been implemented into a nano-pillar to demonstrate the CIMS with 160% TMR ratio at room temperature. By combining the large TMR ratio with the substantial decrease in critical current density down to 2.5x10^6 A/cm2, the requirement for beyond the Gbit-scale MRAM application is satisfied. Hence, government-initiatives have been applied to develop a commercial Gbit MRAM both in the USA and Japan.Recent development in nanometre-scale fabrication techniques will enable us to expand a vertical GMR nanopillar into a lateral configuration, consisting of ferromagnetic nanowires and a non-magnetic nanowire to bridge over the spin injector and detector, enabling precise control of dimensions. In such a lateral spin-valve configuration, spin-polarised electrons can be injected with an electron charge current (local geometry) and without a charge current (non-local geometry). Using non-local geometry pioneering work has been performed by Jedema et al., successfully demonstrating diffusive spin injection from a ferromagnetic Ni80Fe20 nano-electrode, spin accumulation in a non-magnetic Cu nano-wire and spin detection by another NiFe nano-electrode. They have further extended their study into ballistic spin injection by inserting an AlOx tunnel barrier (insulator, I) at the FM/non-magnet (NM) interfaces. Consequently non-local spin-valve systems have been extensively employed to achieve efficient spin injection by minimising interfacial scattering in both diffusive and ballistic contacts and also to detect both spin Hall and inverse spin Hall effects. This clearly indicates the advantages of the lateral device configurations.In this proposed project, we will employ a lateral spin-valve structure instead of a conventional nano-pillar to demonstrate efficient generation of a spin voltage and current, which is not associated with an electron-charge current and hence minimises the Joule heating. In our proposed devices, both a spin current and a spin-polarised electron-charge current will be used to detect the spin voltage/current generation in non-local and local measurement geometries, respectively by changing the measurement geometries. In the non-local geometry , a spin current can be injected efficiently into a non-magnet through a tunnel barrier and detected as a large spin voltage through a second tunnel barrier. This gives a large spin current through a metallic interface. Our proposed device will therefore act independently as a pure spin-voltage and spin-current source with high efficiency. The evaluation of the pure spin-voltage and current will reveal the fundamental mechanism of spin-current transport (without an electron charge), which will encourage further theoretical studies for better understanding of the spin current and will also lead a new type of device architecture.