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We report on the Pinning Field Distribution (PFD) in amorphous and annealed Fe-Nb-Cu-Si-B wires. It is demonstrated that PFD reflects the microstructure evolution of the alloy during the devitrification process on annealing. The PFD were obtained from the amplitude dependence of the complex AC susceptibility for each sample. We found a wide PFD in the amorphous state caused by the internal stresses distribution. Annealing at temperatures below the onset of nanocrystallization (400Â°C) allow the PFD gets narrower, consisting only of a sharp maximum. This effect has to be ascribed to structural relaxation, giving rise to a smoothing of the internal stresses. Just at beginning of the nanocrystallization onset the PFD becomes wider and unfolds in many maxima due to the randomly pinning centers of the sample. After sample annealing at 565Â°C, it becomes the PFD narrower again, exhibiting the alloy its softest magnetic behavior.