Scientists use scanning tunnelling microscopy to understand how a material's electronic or magnetic properties relate to its structure on the atomic scale. When using this technique, however, they can normally investigate only the uppermost atomic layer of a material. Prof Anika Schlenhoff and postdoctoral researcher Dr Maciej Bazarnik from the Institute of Physics at the University of Münster (Germany) have now succeeded for the first time in using a modified measurement method to image structural and magnetic properties that lie beneath the surface. The team investigated an ultra-thin layer of a magnetic material (iron) beneath a two-dimensional graphene layer.
In conventional scanning tunnelling microscopy, so-called electronic states on the sample surface are used for the measurement signal (the 'tunnel current' that flows between the probe tip and the sample). In the resonant measurement variant used by the team, however, states located in front of the surface were investigated. Seemingly contradictory, but known for some time, these special states can be used to investigate electronic charge transfer at buried interfaces inside the sample. As the researchers have now shown, these special states can be used to detect the local magnetic properties of an iron film covered by graphene. The physical reason for this is that the electronic states located above the surface penetrate beneath the graphene into the sample down to the magnetic iron layer and become magnetic themselves through interaction with the iron.
'This opens up new possibilities for investigation,' Anika Schlenhoff explains. 'We can now use the same scanning tunnelling microscope to investigate the top layer of a layered system and a buried interfacial layer beneath it in terms of their structural, electronic and magnetic properties. Both layers can be analysed with a uniquely high-spatial resolution that extends down to the atomic scale.'
The team also showed that their method can be used to obtain information about the local position of the layers relative to each other. For example, the position of the carbon atoms of the graphene varies locally with respect to the underlying iron atoms due to different stacking sequences. 'The differences in the vertical stacking could not previously be resolved for this material system using conventional scanning tunnelling microscopy,' explains Maciej Bazarnik. As it now turns out, the states near the surface, which are used in resonant scanning tunnelling microscopy, are sensitive to the stacking sequence and thus allow these differences to be visualised.
