A magnetostrictive element for use in a magneto-mechanical marker has a resonant frequency characteristic that is at a minimum at a bias field level corresponding to the operating point of the magnetomechanical marker. The magnetostrictive element has a magnetomechanical coupling factor k in the range 0.28 to 0.4 at the operating point. The magnetostrictive element is formed by applying cross-field annealing to an iron-rich amorphous metal alloy ribbon (45 to 82 percent iron) which includes a total of from 2 to 17 percent of one or more of Mn, Mo, Nb, Cr, Hf, Zr, Ta, V. Cobalt, nickel, boron, silicon and/or carbon may also be included. The metal alloy may include one early transition element selected from the group consisting of Zr, Hf and Ta, and also a second early transition element selected from the group consisting of Mn, Mo, Nb, Cr, and V.