Electro-Magnetic systems – EM systems [Wikipedia]
EM tags are made of a strip of amorphous metal (metglas) which has a very low magnetic saturation value. Except for permanent tags, this strip is also lined with a strip of ferromagnetic material with a moderate coercive field (magnetic “hardness”). Detection is achieved by sensing harmonics and sum or difference signals generated by the non-linear magnetic response of the material under a mixture of low-frequency (in the 10 Hz to 1000 Hz range) magnetic fields.
When the ferromagnetic material is magnetized, it biases the amorphous metal strip into saturation, where it no longer produces harmonics. Deactivation of these tags is therefore done with magnetization. Activation requires demagnetization.
EM system is suitable for items in libraries since the tags can be deactivated when items are borrowed and re-activated upon return. It is also suitable for merchandise in retail stores, due to the small size and very low cost of the tags (Tattle-Tape).
Electro-Magnetic systems are no longer common in Food or Apparel retail environments, most having been replaced by Radio Frequency, Acousto-Magnetic or even RFID platforms. The technology is still popular in Library environments where the need to deactivate and reactivate tags is a necessity, however here too RFID platforms have been gaining ground in the last 10 years due to their greater range of use.