New 'Seebeck spin effect' utilises heat in silicon
Researchers from the Foundation for Fundamental Research on Matter (FOM) and the National Institute of Advanced Industrial Science and Technology (AIST) in Tsukuba (Japan) publish an article about the Seebeck spin tunnel effect in the 7 July issue of Nature. In this new phenomenon heat is converted into a 'spin voltage'. By creating a difference in temperature the researchers transferred the magnetic information (spin) from a magnetic material to silicon, without the need for electrical current. Using the new concept it will be possible to develop energy-efficient information technology by functionally using and re-using heat. The article has been available as an advance online publication since 29 June.
Magnetic information
Electric switches and memories make use of charge and its transport. This process costs a relatively large amount of energy and produces heat. This has already led to problems in computer chips. Spintronics, however, investigates a different property of the electron, its spin. The spin is the basis of magnetism. Just like the north and south pole of a magnet the spin points in a certain direction. This can be used to store digital information, for example, by representing a '1' as the spin that points up and a '0' as a spin that points down. Processing magnetic information costs less energy than that needed to transport charge. The ultimate aim is to implement this principle in silicon, the building block of nearly all electronic components.
Thermal spin current
The researchers made a pure spin current in a contact between silicon and a ferromagnet. They only used a temperature difference to achieve this and not an electrical voltage or current. As a result of the temperature difference, electrons with the one spin orientation of the ferromagnet move towards the silicon, while exactly the same number of electrons with the other spin orientation move in the opposite direction. The net balance of charge therefore remains the same, in other words there is a net transfer of spin but no electrical current. The orientation of the spins depends on the ferromagnetic material used and reverses when the warm and cold side of the contact are switched. The effect can therefore be controlled.
Seebeck spin tunnelling
The Seebeck spin tunnel effect is completely determined by the boundary between the ferromagnet and the silicon. In this study, the contact consisted of an ultrathin – less than 1 nanometre thick − oxide layer between the ferromagnet and the semiconductor. This layer works as an electrical barrier that electrons can only move through by means of tunnelling, a quantum mechanical process that is strongly dependent on the spin and thermal energy of the electrons. This means that spin current can be generated by a temperature difference: increasing the thermal energy (temperature difference) increases the spin current. That is the key to further optimising the process.
Reference
The article 'Thermal spin current from a ferromagnet to silicon by Seebeck spin tunnelling' by Jean-Christophe Le Breton, Sandeep Sharma, Hidekazu Saito, Shinji Yuasa and Ron Jansen will be published on 7 July 2011 in Nature and has been available as an advance online publication since 29 June via DOI: 10.1038/nature10224.
Further information
The research was carried out by FOM researchers Jean-Christophe Le Breton and Sandeep Sharma together with their AIST fellow researchers Hidekazu Saito, Shinji Yuasa and former FOM workgroup leader Ron Jansen from Japan. The research received funding from FOM and is part of the FOM programme 'Controlling spin dynamics in magnetic nanostructures'.
Contact: Dr Ron Jansen, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan.