Magnetostriction is a property of ferromagnetic materials that causes them to change their shape when subjected to a magnetic field. The effect was first identified in 1842 by James Joule when observing a sample of nickel called ferromagnetic.
Ferromagnetism is the basic mechanism by which certain materials form permanent magnets and/or exhibit strong interactions with magnets; it is responsible for most phenomena of magnetism encountered in everyday life called amorphous metal.
An amorphous metal is a metallic material with a disordered atomic-scale structure. In contrast to most metals, which are crystalline and therefore have a highly ordered arrangement of atoms, amorphous alloys are non-crystalline 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 magnetoelastic coupling, which implies that they can efficiently convert magnetic energy to mechanical vibrations.
The detectors for such tags emit periodic tonal bursts at about 58 kHz, the same as the resonance.
In physics, resonance is the tendency of a system to oscillate at larger amplitude at some frequencies than at others. These are known as the system's resonant frequencies . At these frequencies, even small periodic driving forces can produce large amplitude vibrations, because the system frequency of the amorphous strips. This causes the strip to vibrate longitudinally by magnetostriction, and will continue 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.
Magnetic anisotropy is the direction dependence of a material's magnetic properties. A magnetically isotropic material has no preferential direction for its magnetic moment in zero field, while a magnetically anisotropic material will align its moment to an easy axis.-Sources of magnetic within the amorphous metal. The tag can also be deactivated by demagnetizing the strip, making the response small enough to that it will not be detected by the detectors.
These tags are thicker than magnetic tags 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.