Spin-Dependent Tunnelling through Nanoclusters

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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.