1.  Field of the Invention
This invention relates to article surveillance systems and markers for use therein.  More particularly, the invention provides a coded ferromagnetic metal marker that enhances the sensitivity and reliabilty of the article surveillance system. 
2.  Description of the Prior Art
The problem of protection of articles of merchandise and the like, against theft from retail stores has been the subject of numerous technical solutions.  Among these, a tag or marker is secured to an article to be protected.  The marker responds to an interrogation signal from transmitting apparatus situated either at the exit door of the premises to be protected, or at the aisleway adjacent to the cashier or check out station.  A receiving coil on the opposite side of the exit or aisleway from the transmitting apparatus, receives a signal produced by the marker in response to the interrogation signal.  The presence of the response signal indicates that the marker has not been removed or deactivated by the cashier, and that the article bearing it may not have been paid for or properly checked out. 
Several different types of markers have been disclosed in the literature, and are in use.  In one type, the functional portion of the marker consists of either an antenna and diode or an antenna and capacitors forming a resonant circuit.  When placed in an electromagnetic field transmitted by the interrogation apparatus, the antenna-diode marker generates harmonics of the interrogation frequency in the receiving antenna; the resonant circuit marker causes an increase in absorption of the transmitted signal so as to reduce the signal in the receiving coil.  The detection of the harmonic or signal level change indicates the presence of the marker.  With this type of system, the marker must be removed from the merchandise by the cashier.  Failure to do so indicates that the merchandise has not been properly accounted for by the cashier. 
A second type of marker consists of a first elongated element of high magnetic permeability ferromagnetic material disposed adjacent to at least a second element of ferromagnetic material having higher coercivity than the first element.  When subjected to an interrogation frequency of electromagnetic radiation, the marker causes harmonics of the interrogation frequency to be developed in the receiving coil.  The detection of such harmonics indicates the presence of the marker.  Deactivation of the marker is accomplished by changing the state of magnetization of the second element.  Thus, when the marker is exposed to a dc magnetic field, the state of magnetization in the second element changes and, depending upon the design of the marker being used, either the amplitude of the harmonics chosen for detection is significantly reduced, or the amplitude of the even numbered harmonics is significantly changed.  Either of these changes can be readily detected in the receiving coil. 
Ferromagnetic harmonic generating markers are smaller, contain fewer components and materials, and are easier to fabricate than resonant-circuit or antenna-diode markers.  As a consequence, the ferromagnetic marker can be treated as a disposable item affixed to the article to be protected and disposed of by the customer.  Such markers may be readily deactivated by the application of a magnetic field pulse triggered by the cashier.  Hence, handling costs associated with the physical removal requirements of resonant-circuit and antenna-diode markers are avoided. 
One of the problems with harmonic generating, ferromagnetic markers is the difficulty of detecting the marker signal at remote distances.  The amplitude of the harmonics developed in the receiving antenna is much smaller than the amplitude of the interrogation signal, with the result that the range of detection of such markers has heretofore been limited to aisle widths less than about three feet.  Another problem with harmonic generating, ferromagnetic markers is the difficulty of distinguishing the marker signal from pseudo signals generated by belt buckles, pens, hair clips and other ferromagnetic objects carried by shoppers.  The merchant's fear of embarrassment and adverse legal consequences associated with false alarms triggered by such pseudo signals will be readily appreciated.  Yet another problem with such ferromagnetic markers is their tendency to be deactivated or reactivated by conditions other than those imposed by components of the system.  Thus, ferromagnetic markers can be deactivated purposely upon juxtaposition of a permanent magnet or reactivated inadvertently by magnetization loss in the second ferromagnetic element thereof.  Still another problem with harmonic generating ferromagnetic markers is the difficulty of distinguishing one marker from another.  For these reasons, article surveillance systems have resulted in higher operating costs and lower detection sensitivity and operating reliability than are considered to be desirable.