A marker for employment in a magnetic anti-theft security system is composed of an oblong alarm strip of an amorphous ferromagnetic alloy and at least one activation strip applied onto the alarm strip and composed of a semi-hard magnetic alloy. One or more spatially separated regions that are composed of a non-magnetic phase are introduced into the activation strip with a thermal treatment. The semi-hard magnetic alloy has a coercive force H.sub.c of 15 through 100 A/cm and a remanence B.sub.r of at least 0.8 T.