Trace Cu doping on ultralow magnetic susceptibility of AuPt alloys: A first-principles KKR-CPA study

Test masses for gravitational wave detectors require ultralow magnetic susceptibility (χ<10-6 cm3/mol). Binary AuPt alloys are promising, but trace Cu is inevitably introduced during fabrication. Using fully relativistic first-principles within the Korringa-Kohn-Rostoker coherent-potential approximation (KKR-CPA) calculations combined with virtual crystal approximation, we quantitatively link composition, electronic structure, and magnetic response of AuPtCu ternary alloys. Our results show that for Au content between 78-81% and Cu up to 5%, the alloy retains χ<10-6 cm3/mol, meeting the ultralow magnetic susceptibility requirement on test masses. The calculated three-dimensional magnetic phase diagram reveals strongly nonlinear susceptibility variation with composition, deviating up to 40% from linear mixing. For the first time, we identify a zero-susceptibility contour for Au concentrations from 70% to 81%, offering a wide and tunable window for experimental synthesis. This work not only clarifies the microscopic origin of ultralow susceptibility in ternary alloys, through reduced Pt d-band density of states and weakened Stoner enhancement, but also provides a quantitative map for designing ultralow-susceptibility alloys for gravitational wave detectors and MRI-compatible devices.