Stoichiometric variations of fluctuation heterogeneity of stripe domains in amorphous Fe-Ge thin films

Magnetic thin-films provide a unique system to study the evolution of exotic spin-textures which are fundamentally intriguing and potentially promising for spintronics application. A key question to address is how symmetric and anti-symmetric exchange coupling between atomic spins modulate the nature of fluctuations as the system approaches phase transition. In amorphous Fe-Ge thin films by controlling the Fe concentration one can tune the magnitude of the symmetric Heisenberg exchange and randomness of anti-symmetric Dzyaloshinskii-Moriya interaction (DMI) respectively. In this work we performed soft x-ray photon correlation spectroscopy (XPCS) measurements to probe the nanoscale magnetic fluctuations of stripe domains in amorphous Fe_xGe_1−x thin films for x=0.51, 0.52 and 0.53. The fluctuations were found to be length-scale specific heterogeneous; starting over a small fraction of the domains well below T_C and the fluctuation amplitude gradually grows into a collective fluctuation encompassing the entire film as T_C was approached. We also observed that stoichiometry has a profound effect on the spectrum of thermal fluctuations which forms the basis of phase transition pathway. Specifically, for the same reduced temperature, we observed enhanced thermally activated fluctuations with decreasing Fe concentration.