A ferromagnetic resonator for use in a marker in a magnetomechanical electronic article surveillance system has improved magnetoresonant properties and/or reduced eddy current losses by virtue of being annealed so that the resonator has a fine domain structure with a domain width less than about 40 ÿm, or less than about 1.5 times the thickness of the resonator. This produces in the resonator an induced magnetic easy axis which is substantially perpendicular to the axis along which the resonator is operated magnetically by a magnetic bias element also contained in the marker. The annealing which produces these characteristics can take place in a magnetic field of at least 1000 Oe, oriented at an angle with respect to the plane of the material being annealed so that the magnetic field has a significant component perpendicular to this plane, a component of at least about 20 Oe across the width of the material, and a smallest component along the direction of transport of the material through the annealing oven.