The origin of magneto-structural coupling in Fe-based bulk metallic glasses

All ferromagnetic Fe-based bulk metallic glasses show the Invar effect. It is a magnetic effect that reduces the coefficient of thermal expansion and comes in two forms, a step-type and a peak-type effect. Here, we study the atomic arrangement of an (Fe 71.2 B 24 Y 4.8 ) 96 Nb 4 bulk metallic glass as a function of temperature and across more than six orders of magnitude in length scale. Combining various synchrotron-based X-ray scattering techniques we show that the Invar effect originates at the atomic scale within the Fe–Fe network. We find no signs of increased spatial correlations due to the ferromagnetic interactions. This shows that no structural rearrangement occurs at the Curie temperature and that the Invar effect is purely of energetic nature. We conclude that the Invar effect has a fundamental base that results from the magnetic interactions of Fe–Fe bonds. Based on this, we provide a model for the magnetic interactions that create the Invar effect in amorphous materials. • The Invar effect in metallic glasses does not cause (partial) ordering of atomic positions. • The anomalous thermal expansion is fully controlled by the Fe–Fe network. • The Invar effect can be fluctuation-modifying or bond-modifying. • The transition between them is continuous. • The Invar effect in metallic glasses can be explained by the Heisenberg model.