Acoustomagnetic systems for security elements consist of extremely small plastic boxes around 40mm long, 8–14mm wide, depending upon design, and just 1mm high. The boxes contain two metal strips, a hard magnetic metal strip permanently connected to the plastic box, plus a strip made of amorphous metal, positioned such that it is free to vibrate mechanically (Zechbauer, 1999).

Ferromagnetic metals (nickel, iron etc.) change slightly in length in a magnetic field under the influence of the field strength H . This effect is called magnetostriction and results from a small change in the interatomic distance as a result of magnetisation. In a magnetic alternating field a magnetostrictive metal strip vibrates in the longitudinal direction at the frequency of the field. The amplitude of the vibration is especially high if the frequency of the magnetic alternating field corresponds with that of the (acoustic) resonant frequency of the metal strip. This effect is particularly marked in amorphous materials.

The decisive factor is that the magnetostrictive effect is also reversible. This means that an oscillating magnetostrictive metal strip emits a magnetic alternating field. Acoustomagnetic security systems are designed such that the frequency of the magnetic alternating field generated precisely coincides with the resonant frequencies of the metal strips in the security element. The amorphous metal strip begins to oscillate under the influence of the magnetic field. If the magnetic alternating field is switched off after some time, the excited magnetic strip continues to oscillate for a while, like a tuning fork, and thereby itself generates a magnetic alternating field that can easily be detected by the security system.

The great advantage of this procedure is that the security system is not itself transmitting while the security element is responding and the detection receiver can thus be designed with a corresponding degree of sensitivity.

In their activated state, acoustomagnetic security elements are magnetised, i.e. the abovementioned hard magnetic metal strip has a high remanence field strength and thus forms a permanent magnet. To deactivate the security element the hard magnetic metal strip must be demagnetised. This detunes the resonant frequency of the amorphous metal strip so it can no longer be excited by the operating frequency of the security system. The hard magnetic metal strip can only be demagnetised by a strong magnetic alternating field with a slowly decaying field strength. It is thus absolutely impossible for the security element to be manipulated by permanent magnets brought into the store by customers.

Full-and Half-Duplex Procedure
In contrast to 1-bit transponders, which normally exploit simple physical effects (oscillation stimulation procedures, stimulation of harmonic processes by the nonlinear characteristic of diodes or the nonlinear hysteresis curve of metals), the transponders described in this and subsequent sections use an electronic microchip as the data-carrying device. This has a data storage capacity of between a few bytes and more than 100 kilobytes. To read from or write to the data-carrying device it must be possible to transfer data between the reader and the transponder and then back from the transponder to the reader. This transfer takes place according to one of two main procedures: full-duplex and half-duplex procedures, which are described in this section, and sequential systems, which are described in the following section.

In the half-duplex procedure (HDX) the data transfer from the transponder to the reader alternates with data transfer from the reader to the transponder. At frequencies below 30 MHz this is most often used with the load modulation procedure, either with or without a subcarrier, which involves very simple circuitry. Closely related to this is the modulated reflected cross-section procedure that is familiar from radar technology and is used at frequencies above 100 MHz. Load modulation and modulated reflected cross-section procedures directly influence the magnetic or electromagnetic field generated by the reader and belong therefore among the harmonic procedures.

In the full-duplex procedure (FDX) the data transfer from the transponder to the reader (up-link) takes place at the same time as the data transfer from the reader to the transponder (down-link). This includes procedures in which data is transmitted from the transponder at a fraction of the frequency of the reader, i.e. a subharmonic, or at a completely independent, i.e. an anharmonic, frequency.

However, both procedures have in common the fact that the transfer of energy from the reader to the transponder is continuous, i.e. it is independent of the direction of data flow. In sequential systems (SEQ), on the other hand, the transfer of energy from the transponder to the reader takes place for a limited period of time only (pulse operation → pulsed system). Data transfer from the transponder to the reader occurs in the pauses between the power supply to the transponder.

Unfortunately, the literature relating to RFID has not yet been able to agree a consistent nomenclature for these system variants. Rather, there has been a confusing and inconsistent classification of individual systems into full-and half-duplex procedures. Thus pulsed systems are often termed half-duplex systems – this is correct from the point of view of data transfer – and all unpulsed systems are falsely classified as full-duplex systems. For this reason, in this book pulsed systems – for differentiation from other procedures, and unlike most RFID literature(!) – are termed sequential systems (SEQ).