58 kHz AM AKA Acoustic-Magnetic systems
These are similar to magnetic tags in that they are made of two strips: a strip of magnetostrictive, ferromagnetic amorphous metal and a strip of a magnetically semi-hard metallic strip, which is used as a biasing magnet (to increase signal strength) and to allow deactivation. These strips are not bound together but free to oscillate mechanically.
Amorphous metals are used in such systems due to their good magneto elastic coupling, which implies that they can efficiently convert magnetic energy into mechanical vibrations.
The detectors for such tags emit periodic tonal bursts at about 58 kHz, the same as the resonance frequency of the amorphous strips.
This causes the strip to vibrate longitudinally by magnetostriction, and it continues to oscillate after the burst is over. The vibration causes a change in magnetization in the amorphous strip, which induces an AC voltage in the receiver antenna. If this signal meets the required parameters (correct frequency, repetition, etc.), the alarm is activated.
When the semi-hard magnet is magnetized, the tag is activated. The magnetized strip makes the amorphous strip respond much more strongly to the detectors, because the DC magnetic field given off by the strip offsets the magnetic anisotropy within the amorphous metal. The tag can also be deactivated by demagnetizing the strip, making the response small enough so that it will not be detected by the detectors.
AM tags are three dimensional plastic tags, much thicker than electro-magnetic strips and are thus seldom used for books. However, they are relatively inexpensive and have better detection rates (fewer false positives and false negatives) than magnetic tags.