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Thin-film ferromagnetic materials still give unexpected results. Consider the easy axis of magnetization, a property crucial for technological applications. The easy axis can be in the film's plane, out-of-plane, or have more complex orientations. For example, spin-polarized low-energy electron microscopy [1] revealed that ultrathin cobalt islands and films on ruthenium present consecutive spin-reorientation transitions: monatomic films are magnetized in-plane, bilayer islands or films are magnetized out-of-plane, while thicker films are magnetized in-plane again. Such rich behavior results from a balance of very small contributions that often nearly cancel each other. To understand, and even predict the magnetic behavior, separating those different contributions becomes essential. To do this, magnetic measurements from uniform film regions with precisely known properties (such as structure and thickness) are needed. Such measurements are challenging in systems such as cobalt on ruthenium, where the film wants to form three-dimensional islands rather than remain flat. Here we use the imaging capability of photoemission microscopy and low-energy electron microscopy to measure the magnetic properties of uniform cobalt regions a few layers thick on ruthenium. By using circularly polarized X-rays, the L-level dichroism generated in regions as small as a single Co island is detected, providing a direct image of the local structure's magnetization. We have also measured the dichroic difference between the L3 and L2 Co levels. We will relate our local, element-specific magnetic measurements to cobalt's atomic structure [2], both with and without normally non-magnetic capping layers.
[1] Farid El Gabaly et al., Phys. Rev. Lett. 96 147202 (2006).
[2] Farid el Gabaly el al., New J. of Physics (in press). |