Soft magnetic alloy of the iron-nickel type, the chemical composition of which comprises, in % by weight: 34%≦Ni≦40%; 7%≦Cr≦10%; 0.5%≦Co≦3%; 0.1%≦Mn≦1%; O≦0.007%; S≦0.002%; N≦0.004%; with N+S+O≦0.01%; iron and impurities 5 resulting from the production process. Use in motors especially suited for use in horology.