Structural rearrangement at medium-range and its effects on the magnetic properties and crystallization behaviors of a Fe-based amorphous alloy

The structure-property relationships in amorphous alloys are far from being fully understood. Here we show an unusual structural rearrangement occurring over medium-range length scale prior to crystallization for a (Fe_0.5Co_0.5)₇₂Si₄B₂₀Nb₄ amorphous alloy ribbon upon heating by in-situ synchrotron diffraction, whereas the short-range structure shows no anomaly. During the above process, local stress field generates and results in the enlargement of coercivity. This can be further supported by the magnetic force microscopy images showing an enhancement of magnetic phase contrast after annealing the sample at a temperature where the unusual structural change occurs. Moreover, the average grain size derived from the transmission electron microscopy images of the crystallized (Fe_0.5Co_0.5)₇₂Si₄B₂₀Nb₄ ribbon is much smaller than that of (Fe_0.5Co_0.5)₇₆Si₄B₁₆Nb₄, suggesting the slow crystallization kinetics caused by the local stress field. This work highlights the role of medium-range order in determining the properties of amorphous alloys.