The nature of the atomic-scale Invar effect in Fe-based bulk metallic glasses

Fe-based bulk metallic glasses (BMGs) universally show an anomalously low coefficient of thermal expansion below their Curie temperature. This effect is known as the Invar effect, is rarely seen in crystalline materials and also vanishes after the crystallization of BMGs. While it is known that Co and Ni reduce the strength of the Invar effect, the influence of other elements is unknown. Moreover, it is unclear to what extent structural modifications and magnetic interactions of minor alloying elements contribute to the Invar effect. Using synchrotron-based in-situ X-ray diffraction, we show that (Fe71B24RE5)96Nb4 BMGs with rare earth (RE) = Tm, Er, Ho also reveal the Invar effect. This can be seen in the diffraction peaks that shift in accordance with the macroscopically measured thermal expansion. In comparison to a similar FeBYNb BMG, the Invar effect is not influenced by the substitution of Y with RE. This suggests that the minor alloying elements do not contribute to the Invar effect. Neither their influence on the atomic arrangement, the (anti)ferromagnetic interactions of the 4f electrons of the RE elements nor paramagnetic interactions such as those from Y and Nb have an influence on the Invar effect. These elements only serve to increase glass-forming ability and the formation of a disordered Fe network but do not contribute themselves to the Invar effect. Additionally, their (anti-)ferromagnetic interaction with Fe does not influence the magneto-structural correlations of the Fe network. Therefore, the Invar effect in these materials not only originates at the atomic scale but can be attributed solely to the magnetic interaction in the disordered Fe network.