An acoustic-magneto (AM) anti-theft marker is formed with the bias piece made from a soft magnetic material, instead of a “semi-hard” magnetic material that has been used in conventional anti-theft AM security markers. The method of manufacturing such soft magnetic bias pieces includes cold deforming a soft magnetic material with at least an eighty percent reduction rate, while keeping its DC coercivity below 12.5 Oe. The strip or wire of soft magnetic material is then cut to size as required for the bias piece. The anti-theft AM security marker has the soft magnetic bias piece placed inside or outside of the resonating cavity of the housing for the security tag with the resonator pieces inside the resonating cavity with a cover film placed over the housing. The soft magnetic bias piece or pieces effectively operate close to resonator piece or pieces with or without a non-magnetic separating film.