The preferred direction of magnetization in a Fe/Co/Ni trilayer, grown epitaxially on Cu(001), changes twice as a function of the Fe overlayer thickness: Without Fe, the magnetic easy axis of 2.4-3.6 ML Co/15 ML Ni/Cu(001) is in the film plane (bottom part of image). Above 0.6 ML Fe it becomes out-of-plane, and stays out-of-plane until the Fe thickness reaches ≈1.8 ML Fe (middle part of image). Above 1.8 ML Fe thickness it is again in-plane (top).

   The image shows magnetic domains at the Co-L₃ edge of an Fe/Co crossed double wedge on 15 ML Ni/Cu(001). Fe and Co thickness gradients are indicated by arrows. Within the imaged area the Fe thickness varies from 0 to 2.5 ML, the Co thickness from 2.4 to 3.6 ML.

   The dashed lines indicate the so-called spin-reorientation transitions between these regions. Domain branching into smaller and smaller stripe domains is observed below the upper line.

   This is attributed to the competition between the magnetostatic energy and the energy cost for creating domain walls. Closely spaced up and down magnetized out-of-plane domains have a lower magnetostatic energy, but a higher domain wall energy. Close to the spin-reorientation transition the domain wall energy is low, so that more domains can be formed to gain magnetostatic energy.

   It is not clear why such a domain branching does not occur at the lower spin reorientation transition; kinetic barriers for the formation of the domains could be responsible for that.

   In ultrathin Ni films, grown epitaxially on Cu(001), the preferred magnetization direction changes twice as a function of Ni thickness. The images show the magnetic domains in the region of these spin reorientation transitions. The sample, a Cu/Ni bilayer in which Ni was deposited as a wedge, is schematically shown at the bottom.

   The two images have been acquired for near-opposite azimuthal direction of incident x-rays. This helps to distinguish regions with perpendicular (out-of-plane) magnetization and regions with in-plane magnetization: Whereas the contrast of the former does not change upon a change of the incidence angle, the latter show a contrast reversal. This is due to the changes in the relative orientation of light incidence and magnetization direction.

   Spin-reorientation transitions are recognized at 10 and ≈19 ML Ni thickness (left axis). Domain branching into smaller and smaller stripe domains is observed between 14 and 18 ML Ni thickness. As in the example above, this results from the competition between the magnetostatic energy and the energy cost for creating domain walls. Closely spaced up and down magnetized out-of-plane domains have a lower magnetostatic energy, but a higher domain wall energy. Close to the spin-reorientation transition the domain wall energy is low, so that more domains can be formed to gain magnetostatic energy.

   See a movie of zooming in into stripe domains at a spin reorientation transition (1.4 MB). The movie starts with a field of view of 100 μm and ends at 10 μm field of view (movie by Daniel Kroneberg).
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