Magnetism at the atomic layer limit
We work on magnetic films, and their properties when their thickness is of the order of a few atomic layers. When you go down to such level of detail, you can find new phenomena. Is a single layer ferromagnetic? How does its properties change with thickness? What about a single layer of an antiferromagnetic material? This problems have been the subject of research in the last 50 years. Surprisingly, there is still a lot we can learn with the right combination of techniques.
As an example, and given the recent Nobel prize on physics for the discovery of the giant magnetiresistance that we use in our hard-drives. The original material combination where the effect was discovered was chromium and iron. Surprisingly enough, we still did not know whether a single layer of chromium by itself is magnetic or not. We recently used a series of techniques to find out that even a single layer actually presents antiferromagnetic order.
As another example, consider the easy-axis of magnetization of a ferromagnetic material: the direction along which the magnetization is oriented in a sample in the absence of an applied magnetic field. The magnetization direction of the film can depend on the film thickness. We recently found that atomic films just one atomic layer thick of cobalt on a substrate of ruthenium has the easy-axis of magnetization in the plane of the film. That is actually expected for a thin film. But surprisingly enough, a film -or islands- of two atomic layers changes the magnetization (i.e., has a spin-reorientation transition) to an out-of-plane orientation. 
And films three atomic layers (and thicker) change again their magnetization direction (the figure shows 3 atomic layer islands on top of a two atomic layer film. Of course, the only way to see this kind of behaviur is look close enough. Cobalt on ruthenium, such many other materias, is very difficult to grown in perfect layers, so we can observe this effect. In fact, only by looking in terraces of the substrate several micrometers wide we could find islands perfect enough to do our experiment.
The technique we use to locally image the magnetic domains in a given direction is spin-polarized low energy electron microscopy. This is a low energy electron microscope that employs a beam of electrons that is spin-polarized. Reflection of the beam from a ferromagnetic sample produces some contrast due to exchange scattering between the sample and the beam electrons. By subtracting images with opposite spin polarizations, we can map the magnetic domains of the sample. There are only a few systems in the world. We did this work in collaboration with Andreas Schmid at Berkeley lab.
And schematic overlaid on a topograhic image can be show here. This work was selected as a Physics Review Focus story, you might want to look at it there.
We have also tried to cover the Co films with different materials, such as Cu, Ag, and Au. And we have found that also the growth of a non-magnetic capping layer produces spin-reorientation transitions. As an example, covering the cobalt films with silver we found the following changes in the magnetic easy axis, the arrows indicate the magnetization direction of films of Co with a given coverage of each of the coin-metals. As you can see, we have found quite a few combinations that present out-of-plane magnetization.