AM labels (DR labels) 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 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 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 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.

The newer acousto-magnetic system, which has the ability to protect wide exits and allows for high-speed label application, uses a transmitter to create a surveillance area where tags and labels are detected. The transmitter sends a radio frequency signal (of about 58 kHz) in pulses, which energize a tag in the surveillance zone. When the pulse ends, the tag responds, emitting a single frequency signal like a tuning fork. While the transmitter is off between pulses, the tag signal is detected by a receiver. A microcomputer checks the tag signal detected by the receiver to ensure it is at the right frequency, is time-synchronized to the transmitter, at the proper level and at the correct repetition rate. If all these criteria are met, the alarm occurs.

A typical AM tag from Wal-Mart

AM material is highly magnetostrictive, which means that when you put the tag material in a magnetic field, it physically shrinks. The higher the magnetic field strength the smaller the metal becomes. The metal actually shrinks about one-thousandth of an inch over its full 1.50 inch length.

As a result of driving the tag with a magnetic field, the tag is physically getting smaller and larger. So if it is driven at a mechanically resonant frequency, it works like a tuning fork, absorbing energy and beginning to ring.

This tag also requires bias magnet material in addition to active element material. The active material will shrink no matter which direction the magnetic field is placed upon it. If the tag is driven with Frequency, F, it gets smaller as the magnetic field increases and larger as it's driven towards zero. This means that while it is being driven at F, the tag is trying to work at 2F, because at both positive and negative halves of the drive signal, the tag is getting smaller. To get the tag to work at F, a bias field is required. The bias is provided by a semi-hard magnetic element in the label. When magnetized, the bias prevents the active element from ever being in a zero field condition. So for an entire half of the drive signal, the tag shrinks. Then it expands for the other half. This results in an F response.

When you walk through the gate with a tag, the transmitter in the gate energizes the material and causes it to resonate at F. The transmitter then stops. The tag will continue to "ring" at F for a short period of time, and the receiver listens for that frequency. If it hears it, it knows there is a tag and sounds the alarm.

When the AM tag is demagnetized, it is deactivated. When it's magnetized, it is activated. (This is the opposite of how the deactivation of EM tags works.) (Home Depot began using acousto-magnetic tags from Sensormatic. because the tags work well when they're close to metal and the stores use metal shopping carts -- not all systems work well with metal objects.)