Magnetism created out of non-magnetic materials

Ever since the ancient Greeks, or even before them the Chinese, discovered the magnetic force in certain stones researchers have been occupying themselves with magnetism. It appears that materials can be magnetic themselves or that they just can be made magnetic by using another magnet. Not until the twentieth century it has become clear what causes this, up to the level of elementary particles and their interactions out of which matter has been built. Thus, it appears that electrons moving around a nucleus, may have a magnetic moment analogous to the phenomenon that a rotating electric charge may generate a magnetic field (the electromagnet).The interaction between magnetic moments may cause a collective alignment (all moments elaborate into the same direction), which leads to measurable magnetism. In the laboratory researchers nowadays are able to make thin layers of all sorts of material in the structures wanted (something that, for example, semiconductor industry benefit from) and these structures also reveal surprises.

Magnetism at interfaces
Researchers at the MESA+ Institute for Nanotechnology at the University of Twente have been investigating into the material range of perovskites for a long time. Perovskites are oxide materials that have unexpected qualities, such as high-temperature superconductivity and ferroelectricity. Although the qualities of the material have not been all explicated yet, the investigation has been extended to combinations of materials. When two different materials are being piled on one another, it appears that the interface is able to behave quite differently from the bulk of the material. Researchers at the University of Twente have shown earlier that the interface becomes conductive and now it appears that the interface between de non-magnetic insulators SrTiO₃ and LaAlO₃  in fact becomes magnetic.

In order to examine a similar interface, checking on the growth of the materials is needed at an atomic scale. Researchers are able to apply a material on a carrying crystal unit cell per unit cell with the aid of laser pulses. A unit cell is the smallest basic structure a crystal consists of. The researchers are able to monitor the growth of each unit cell minutely during the process. It appears that layers with different charges are found in a unit cell at a level of separate atoms. When alternately layers with a real positive and negative charge are being piled up, there are configurations possible that have a built-in extra positive layer. These layers appear to provide electrons that, in their turn, are able to provide for conductivity and magnetism.

Insight into magnetism at interfaces
Measurements at the High Field Magnet Laboratory at Radboud University Nijmegen - that has one of the largest magnets in the world - have gained more insight into magnetism at the interface between SrTiO₃ and LaAlO₃ . Thus, the electric resistance appears to be a strong function of the applied magnet field. At a large field of 30 tesla the resistance is 30 percent lower than without a field. This means that the interface contains local magnetic moments, of which the alignment influences the resistance. Besides, the connection between resistance and temperature is logarithmic, a phenomenon that also suggests an interaction of the Kondo effect. This quantum mechanical effect describes the screening of localized magnetic moments through free electrons. Hysteresis is found in the resistance measured at a very low temperature (300 millikelvin), which is a strong indication for the creation of magnetic structure over long distances.

New model system
Magnetism in thin layers, and specifically, in semiconducor structures, is already a ‘hot’ item in physics. These new findings in magnetism created at an interface, give rise to a new model system for fundamental research on magnetic interactions in the materials, but in a broad sense, also on phenomena that might not occur in the bulk of the materials, but that now can be artificially aroused by an interface.

For more information, please contact Dr. Alexander Brinkman, University of Twente, phone: (053) 489 31 22.

The article is entitled ‘Magnetic effects at the interface between non-magnetic oxides’, the authors are Alexander Brinkman¹, Mark Huijben¹*, Maarten van Zalk¹, Jeroen Huijben¹, Uli Zeitler², Jan Kees Maan², Wilfred van der Wiel¹, Guus Rijnders¹, Dave Blank¹ and Hans Hilgenkamp¹. The article will be published in Nature Materials on-line, 3 June, and in print in the July-issue of Nature Materials. In Physics Today of June 2007 a dissertation will be published, entitled ‘Evidence suggests that a ferromagnetic metal may lie at the interface between non-magnetic insulators’ by B.G. Levi.

¹  University of Twente/MESA+ Institute
²  Radboud University Nijmegen/HFML/IMM
* currently University of California, Berkely

Note
A large number of the authors come in under the Innovational Research Incentives for talented researchers at NWO (VENI,VIDI, VICI). Brinkman is a VENI-laureate, Van der Wiel, Rijnders and Hilgenkamp are VIDI-laureates and Blank is a VICI-laureate.

A digital version of the  illustration can be obtained via Annemarie Zegers, phone: (030) 600 12 18.