Depression of direct exchange couplings in metallic glasses : A comparative study of critical and electronic behavior in Gd₆Co_4.85 intermetallic compound and metallic glass

Gd-based metallic glasses enable a wider exploration for excellent magnetocaloric materials due to no requirement of compositional stoichiometry, but these metallic glasses generally show a lower ordering temperature compared with corresponding intermetallic compounds. In this paper, critical behaviors for magnetic transitions in both Gd₆Co_4.85 intermetallic compound and metallic glass were carefully studied based on isothermal magnetization to reveal the exchange interactions in these two different structural stages, together with experimental electronic density analyses on the compound. Obtained critical exponents close to the theoretical values predicted by the mean-field theory (MFT) in both structures suggest the majority of Gd–Gd long-range exchange interactions, while the Co-Co direct interaction accounts for the deviation of these exponents from the theoretical values in MFT. This is likely a common feature for rare earth – transition metal ferrimagnets. The ordered Co atoms in the intermetallic compound exhibit a high-spin state (~4.6 μ_B), in contrast with the low-spin state of Co (~1.2 μ_B) in the metallic glass. The reduced spin is associated with the liquid-like structure in the metallic glass (e.g. longer Co-Co distances) and responsible for the low ordering temperature. This work also indicates that the substitution of p-block elements like Al prefers the site to diminish Co moments, leading to the significantly reduced ordering temperature, and thus emphasizes the critical role of d-electron elements to maintain enough direct exchange interactions for proper transition temperature in the development of new metallic glasses with giant magnetocaloric effects.