Fluctuation population mediated order to disorder stripe phase transition in amorphous Fe-Ge thin film*

Chiral spin-structures like stripes, helices, skyrmions, merons and hopfions has been of great research interest due to their technological implication in magnetic and spintronic devices. Usually, the topological skyrmion phase evolves from a ground state of helical phase in single crystals or stripe phase in thin films. Understanding how the system evolves near phase transition is a key to get insights into the formation of these exotic spin-phases through proper tuning of the external parameters. In this work we study the spin-dynamics near the stripe phase transition of amorphous Fe-Ge thin films using soft x-ray photon correlation spectroscopy (XPCS). Amorphous Fe_xGe_1-x thin films shows an order to disorder stripe phase transition at around 250K and paramagnetic ordering above room temperature for x=0.53. Using the coherence of the x-ray photons we constructed 2D maps of the nanoscale magnetic speckle fluctuations near the phase transition. Such fluctuation maps in Q-space give details about the dynamics of domains of different morphologies present in the real space. We observe that the fluctuation population grows as we increase the temperature, and it peaks near the stripe phase transition before falling off. Interestingly the fluctuation density continues to increase as the paramagnetic transition is approached. This new study provides a more specific way of locating critical phase-transition points through growth of fluctuation population and can be used over a wide variety of systems.