Speed limit for electrical switching
Iron ore switches ten times faster than graphene
Scientists from the University of Amsterdam and the Foundation for Fundamental Research on Matter FOM have clocked how fast the mineral magnetite can switch electrical states. As part of an international research consortium, the researchers succeeded in measuring that it only takes one picosecond for this oxide to switch from an insulating to an electrically conducting state, which is ten times faster than the best graphene-based devices. Their results could drive innovations in the tiny transistors that control the flow of electricity in computer silicon chips, enabling faster, more powerful computing and communication devices. The researchers' findings are published online today in the leading journal Nature Materials.
Magnetite (Fe3O4) is a mineral, and as ore an important source of iron for steel making. In these experiments an electrically insulating crystal of magnetite was hit by a visible laser pulse, which fragmented the material's electronic structure at an atomic scale, rearranging it to form the islands. The laser blast was followed closely by an ultrabright, ultrashort X-ray pulse from the Linac Coherent Light Source (LCLS) at SLAC National Laboratory. Use of this X-ray laser enabled the researchers to study, for the first time, the timing and details of changes in the sample excited by the initial laser strike.
Dutch angle: the Verwey transition
Magnetite was the very first metal oxide in which a transition from conducting to insulating behaviour was recorded. In 1939, Evert Verwey - who went on to be director of the famous Philips Physics Laboratory - published these results in the journal Nature. Verwey was a graduate in maths & physics from the forerunner of the University of Amsterdam and also a long-serving member of the FOM Board. His discovery that electrons could become frozen in place in an oxide such as magnetite was the first of its kind, and consequently this process, which in Fe3O4 takes place at minus 150 Celsius, is dubbed the Verwey transition.
Freeze and thaw
There has been a decades-long search for the microscopic mechanism behind the Verwey transition. How exactly do the patterns of frozen-in electrons melt, enabling the charges to move and so once again allow the material to display metallic conductivity? In the new experiments, the researchers - among them professor Hermann Dürr, FOM work group leader professor Mark Golden and ex-UvA PhD student Sanne de Jong - used the ultrafast X-ray flashes from LCLS to film the melting of the electronic order in magnetite. Just like the making of an animation film, they recorded many, fast snapshots of the material in the first phases of the thaw of the frozen charges during the Verwey transition.
Ultrafast switching
The insulating state in magnetite is mainly due to groups of three iron sites in the lattice, dubbed trimerons. When the incoming visible light pulse is intense enough, up to one on four of the trimerons is annihilated. This process is very fast - taking at most one quarter of a picosecond. At the locations in which the trimeron order has been destroyed, the charges start to move again. When this leads to a network of conducting regions that contact each other, the material switches from the 'off' to the 'on' conducting state. By using the LCLS as a huge stopwatch, the researchers could time the Verwey transition, clocking the minimum time for switching to a conducting state to be only one picosecond. This is the ultimate speed limit for future oxide electronics.
Dürr and Golden are very enthusiastic about their results. Not only is the long-standing scientific puzzle of the Verwey transition solved, but also it is now proven that oxides of complex transition metals such as iron are able to switch conducting states ten times faster that the best present transistors made of graphene. At present, further research is underway in the scientists' groups to explore alternative oxide materials which can display the same sort of ultrafast switching at room temperature, thus bringing applications in information processing and communication closer.
Reference
Speed limit of the insulator–metal transition in magnetite, S. de Jong et al., Nature Materials, 28 July 2013 (10.1038/NMAT3718)
Contact
Prof. M. S. Golden, Van der Waals-Zeeman Institute, University of Amsterdam, (020) 525 63 63.