A balance-spring intended to equip a balance of an horological movement, comprising a core made of Nb—Ti made from an alloy consisting of: niobium: balance to 100% by weight, titanium: between 5 and 95% by weight, traces of elements chosen from the group consisting of O, H, C, Fe, Ta, N, Ni, Si, Cu, Al, each of said elements being present in a quantity between 0 and 1600 ppm by weight, the total quantity formed by all of said elements being between 0% and 0.3% by weight, wherein the core made of Nb—Ti is coated with a layer of niobium, said layer of niobium having a thickness between 20 nm and 10 μm.