Quantification of thermal fluctuations in stripe domain patterns

In ultrathin ferromagnetic films with perpendicular anisotropy a spin-reorientation transition from out-of-plane to in-plane orientation of the magnetization vector may occur. The competition of exchange and dipole interaction leads to the formation of stripe domain patterns in the vicinity of the spin reorientation transition. Here we investigate fluctuations of domain patterns in ultrathin epitaxial Ni/Fe films grown on Cu(001) using the technique of threshold photoemission magnetic circular dichroism in combination with photoemission electron microscopy allowing real-time observation of the domain pattern dynamics. The key finding of our experiments is that fluctuations can easily be quantified by calculating thermodynamic susceptibilities from a series of time resolved images. We analyze the strength of fluctuations with respect to temperature and externally applied out-of-plane magnetic fields.