Spin-Dependent Tunnelling through Nanoclusters

物理学家用自旋这个词来描述量子粒子的磁性，比如电子。自旋电子学的新领域旨在使用电子自旋来存储和处理信息，就像传统微电子中使用电荷一样。当然，所有的电子都有负电荷，但它们的自旋可以指向磁场方向，也可以指向向上或向下的磁场方向。随机事件可以反转自旋，自旋状态保持的平均时间被称为自旋寿命，通常只有几千分之一纳秒。在这里，我们想要研究只有几千个原子的金属颗粒中的单自旋，以试图了解延长自旋寿命的方法。通常，自旋电子器件包含磁性元素，提供所需的自旋极化电子，因为铁等材料的磁性是由于向上自旋的电子多于向下的电子。一个简单的例子是自旋阀，其中电流必须依次流经两个磁性物体，这可以解释为偏振器-分析器实验。如果两个磁铁中的磁性指向相同的方向，则电流可以相对自由地通过，而如果它们指向相反的方向，则装置的电阻要高得多/这很容易用作磁场传感器。这类设备的基本形式是高密度磁盘驱动器中的读取头，支持依赖高容量但廉价的数据存储的技术，如iPod、Google Gmail或硬盘电视录像机。未来的应用包括存储器、逻辑或量子信息组件。在这个项目中，我们的目标是结合另一项纳米电子学创新--单电子电子学--来研究自旋电子学。在这里，一个微小的导体岛通过隧道结连接到导体导线上。隧道结是一种超窄的绝缘屏障，正常的传导不可能穿过，但电子可以通过量子力学隧道穿过。有可能导致电子一个接一个地在岛上跳上跳下，并在它们经过时进行计数。对于传统的半导体材料来说，这一点现在已经相对成熟。我们想用磁性材料来探索单电子自旋水平上的自旋电子效应。去年，日本东北的一个研究小组在《自然材料》杂志上发表了一篇报告，称在金属触点之间夹着一层包含数百万纳米尺寸金属颗粒的绝缘层。一次偶然的机会，主要的传导路径是通过这些颗粒中的一个，它是由磁性金属钴制成的。这些科学家发现了一些不同寻常的东西：他们的钴颗粒中的自旋寿命比通常的散装钴长一万倍，这在某种程度上与颗粒的微小尺寸有关。不幸的是，由于他们的实验依赖于偶然，日本团队无法控制，甚至无法测量这种颗粒的任何特征/甚至确切的大小尚不清楚。然而，如此巨大的改进的偶然发现表明，受控搜索可能会产生更好的结果。现在，英国至少有两家供应商提供了能够制造如此微小的颗粒并控制其大小的商业工具。我们正在申请资金购买其中一台仪器，并将其安装到我们最先进的层沉积系统中，该系统已经可以准备结构的所有其他部分。这将使我们能够测量自旋寿命，在那里我们可以选择和测量岛屿的大小和材料，使我们能够寻找我们认为可能的极大地延长的自旋寿命。为什么希望有一种能够隔离单自旋并使其长时间处于给定状态的固态技术，原因有很多，也许最令人兴奋的是基于自旋的量子信息技术的前景。

The word spin is used by physicists to describe the magnetic properties of quantum particles, such as electrons. The new field of spintronics aims to use electron spin to store and manipulate information in the same way that charge is used in conventional microelectronics. Of course, all electrons have a negative charge, but their spin can point either along or against a magnetic field direction, called either up or down . Random events can flip the spin, and the average time that a spin state is maintained is called the spin lifetime, usually only a few thousandths of a nanosecond. Here we want to study single spins in metal grains consisting of only a few thousand atoms to attempt to understand the ways in which this spin lifetime can be extended. Usually, spintronic devices contain magnetic elements that provide the spin-polarised electrons that are needed, since materials such as iron owe their magnetic properties to an excess of spin up electrons over down ones. A simple example is a spin-valve where the current has to flow through two magnetic objects in sequence, and can be explained as a polariser-analyser experiment. If the magnetism in the two magnets points in the same direction then the current can pass relatively freely, whilst if they point in opposing directions the device resistance is much higher / this is easy to use as a magnetic field sensor. Such devices are available in a basic form as the read-heads in high density disk drives, enabling technologies that rely on high-capacity but cheap data storage such as iPods, Google Gmail, or hard disk TV recorders. Future applications include memory, logic or quantum information components. In this project we aim to study spintronics combined with another nanoelectronic innovation, that of single electron electronics. Here, a tiny island of conductor is connected to conducting leads through tunnel junctions: ultra-narrow insulating barriers through which normal conduction is impossible, but electrons can quantum mechanically tunnel through. It is possible to cause electrons to hop on and off the island one by one, and count them as they pass. This is now relatively well-established for conventional semiconductor materials. We want to explore this using magnetic materials to search for spintronic effects at the level of a single electron spin. Last year there was a report in Nature Materials by a group from Tohuku in Japan, where an insulating layer containing millions of nanometer-sized metallic grains had been sandwiched between metal contacts. By chance, the main conduction path was through a single one of these grains, which was made from the magnetic metal cobalt. These scientists discovered something remarkable: the spin lifetime in their cobalt grain was ten thousand times longer than in usual, bulk cobalt, which is somehow related to the tiny size of the grain. Unfortunately, since their experiment relies on chance, the Japanese group were unable to control, or even measure, any of the characteristics of this grain /even the exact size is unknown. Nevertheless the chance discovery of such a huge improvement suggests that a controlled search could yield even better results. Commercial tools that can fabricate such tiny grains and control their sizes are now commercially available from at least two suppliers in the UK. We are requesting the funds to purchase one of these instruments, and fit it to our state-of-the-art layer deposition system that can already prepare all the other parts of the structure. This will let us measure the spin-lifetime where we can choose and measure the island size and material, allowing us to look for the enormously extended spin lifetimes that we think could be possible. There are many reasons why it would be desirable to have a solid state technology that can isolate single spins and keep them in given state for long times / perhaps the most exciting is the promise of a quantum information technology based on spin.