Data at the End of the Tunnel

Just like Dr. Manuel Bibes and his colleagues from the French research organisation CNRS south of Paris, Dr. Sergio Valencia and Dr. Florian Kronast from the Helmholtz-Zentrum Berlin (HZB) für Materialien und Energie are classical basic researchers. And yet they show a way towards the development of fast starting computers with low energy consumption with their work on electric control of electron spins. Each electron possesses a so-called spin producing a small magnetic field. “Electric control of the spins could not only revolutionise the main memory in computers but also improve other electronic parts”, explains Sergio Valencia.

In today’s computers, the main memory in principle consists of electric capacitors, that is, many small memories for electric charges. If the power supply is interrupted, the capacitors discharge and the data is gone. At the next start, the computer pulls the information for the operating system and the installed programmes, which are magnetically permanently stored on the hard drive, back into the main memory. Yet depending on device and programmes, this can take up to some minutes. Only after this “boot process” can the computer be used. In addition, the capacitors in the main memory constantly lose their charge, which has to be refreshed several times per second during normal operation. As a result, the main memory uses a relatively large amount of energy.

Developers therefore work diligently on systems to magnetically store data. They use the so-called Tunnel Magneto Resistance. “Two thin magnet layers, for example out of iron, are separated from one another by an even thinner isolator”, explains Valencia. In principle, this intermediary layer prevents electrons flowing from one magnetic layer to the other. However, if the isolator is only one millionth part of a millimetre thick, quantum mechanical effects allow some electrons to “tunnel” through this barrier layer.

Yet electrons carry not only electric charges, which can be stored in conventional capacitors, but feature also a socalled spin producing a tiny magnetic field. And this magnetic field remains even when the power is switched off. When this happens, the spin can have two different directions, called “up” and “down” by physicists. If both of a TMR’s magnetic layers contain mainly spins of the same orientation, the electrons tunnel much more easily than is the case with a magnetic layer, which contains mainly “up” spins, whereas the other contains mainly “down” spins. In principle, with this part one can thus produce a memory, which permanently maintains the binary number “1”, for instance, by way of a tiny section with badly tunnelling electrons and the “0” by good tunnelling. Such a memory based on spins can be rewritten in a manner similarly fast and as often as a conventional electro-capacitor main memory. The read heads of modern hard drives in computers work with such TMR elements, for instance.

Yet in order to write the data, such TMR main memories require relatively strong magnetic fields and therefore also a lot of energy. The CNRS researchers Vincent Garcia and Manuel Bibes therefore created the isolator using a ferro-electric compound called barium titanate. Then, the HZB researchers Sergio Valencia and Florian Kronast examined in detail the chemical composition of the involved magnet layers by using x-ray absorption spectroscopy.

With very little energy, an electric field switches this isolator in such a way, that it carries a slightly positive electric charge on one side and a slightly negative one on the other. If one reverses the electric field, the charge distribution in the barium titanate also reverses. However, each of these two switches influences also the spins in the immediately neighbouring layers and thus obviously also the tunnelling. This organisation has a huge advantage: Data once written there remains even when the power is switched off.

“Following this model, one could, for example, build main memories for computers, which require significantly less energy than current parts, but still store the data permanently and thus boot very quickly when switched on”, hopes Sergio Valencia. With this, the basic researchers from CNRS and HZB opened a door, which could lead towards a fast booting computer that is considerably more environmentally friendly than conventional computers.