Effect of Cu on thermal and magnetocaloric properties of (GdTbHo)CoAl high-entropy metallic glasses

(Gd_1/3Tb_1/3Ho_1/3)₅₅Co_17.5Al_27.5-xCu_x (x = 5, 10, 15, and 20) metallic glasses (MGs) with a high configurational entropy (ΔS_conf) of 1.723–1.754 R were successfully prepared by arc melting technology. Their glass-forming ability (GFA), thermodynamic behavior, and magnetocaloric effect (MCE) upon Cu addition were investigated thoroughly. Upon Cu addition, both T_g and the T_x significantly decrease with increasing 3d electron number due to the weakened f-d hybridization effect. The (Gd_1/3Tb_1/3Ho_1/3)₅₅Co_17.5Al_12.5Cu₁₅ with 15 at. % Cu exhibits a maximum value of GFA criteria, including T_rg, γ, and γ_m, as well as the lowest degree of structural order, indicating its optimal GFA, which is likely associated with the high-entropy effect and suppressed crystallization behavior. Moreover, it is found that the refrigeration capacity (RCP) also reaches a peak value of 624.83 J kg⁻¹ with a relatively larger peak magnetic entropy change ($|\Delta S_M^{pk}|$) of 8.75 J kg⁻¹ K⁻¹ at the composition with 15 at. % Cu, accompanied by an abnormally high Curie temperature (T_C) of 59 K, which is attributed to the high experimental μ_eff of (Gd_1/3Tb_1/3Ho_1/3)₅₅Co_17.5Al_12.5Cu₁₅ as a result of the intensified magnetic interaction between rare-earth (RE) elements for alloying Cu with small size. Our work indicates that Cu is an effective element for manipulating the thermal and magnetic properties of magnetocaloric materials by influencing their microstructure, orbital hybridization effects, and magnetic exchange interactions.