Anti-thief Technology depends on Amorphous Acousto Magnetic Label

When we walk into a supermarket, we can found that each goods has a design against thief, which is the application of the amorphous Acousto magnetic.
It is more common that the Electronic Article Surveilance(EAS) is used in our business system. There are 6 kinds of EAS system in our daily life, Electromagnetic System, Microwave System, Radio Frequency System, Intelligent System and Acousto Magnetic. The Acousto Magnetic is most widely used because its low price and convenience as well as high rate of detection.

I. What is Acousto Magnetic

Acousto Magnetic is depending on the principle that amorphous alloy ribbons can send out strong resonance signal because of its high magnetism (elastic coupling coefficient). In most case, the Acousto Magnetic is composed of detector, decoder, sensormatic etc. The detector send out a 58KHZ impulsion wave in every 20 millisecond, and if the detector tests a resonance signal 5 times after the impulsion wave, and each of these 5 signals has the same capacity and frequency, the alarm bell would ring. And if the signals are not the same in one second, the alarm would not ring. This technology ensures the accuracy of the alarm bell. The decoder reduces the strength of the resonance signal by demagnetizing the bias films in the anti-thief device so the alarm bell will not ring.

The Acousto Magnetic technology has been widely used in the anti-thief devices for more that 20 years. The US4510489 which records US patent said that some amorphous ribbons, such as FeNiMoB, can send out high resonance signals because of its high magnetism and this theory are successfully used in the anti-thief system in commercial use. The amorphous ribbon is made into resonator and the bias films are made by semi-hard magnetic materials (US5729200) or soft magnetic materials (US7626502). There rarely have any researches on acoustic magnetic label.

II. Acousto Magnetic Theory
Magnetostrictive Effect

Under the effect of the external magnetic field, the size of the ferromagnetic substance changes and after the removal of the external magnetic field, it turns into the original length. Because of the effect of the magnetic field, the length of the magnetostrictive material has a liner transformation and will causes displacement; or it repeatedly changes under the effect of the alternating magnetic field so that it will forms vibration or acoustic wave. This kind of material can turn the electromagnetic energy into the mechanical or acoustic energy. On the contrary, the mechanical energy can also be turned into electromagnetic energy. The former are called magnetostrictive effect and the latter are called magnetoelastic effect.

Under a certain condition, the Ferrite magnetic metals have a change on length, and it was caused by the budge between atoms when magnetizing. In an alternating magnetic field, we can see that magnetostrictive metallic stripe vibrating according to the frequency of the alternating magnetic field. If the frequency of the alternating magnetic field and the frequency of the resonance of the metallic stripe are the same, when the amplitude became the highest then the resonance occurs. This is especially apparent to Perm alloy (Fe-Ni).

On the other hand, this magnetostrictive effect is reversible, which is piezomagnetism. Thus, when the frequency of the alternating magnetic field is consistence with the resonance of the metallic stripe, the perm alloy stripe starts to vibrate. When the alternating magnetic field is turned off, the acoustic magnetic label will do the damping viberation for a moment like a tuning fork, and give out resonance signal. As a extension of the space of the alternating magnetic field, it can be tested by the receiver. The magnetostrictive effect can be described by magnetorestrictive coefficient λ, λ= (LH-L0)/L0, L0 is the original length of the substance, LH is the changed length of the substance under the effect of the external magnetic field.

Magnetomechanical Coupling Factor K

When the amorphous stripe is inspired by the alternating magnetic field under the bias magnetic field, the inner of the stripe has a exchange between magnetic energy and the mechanical energy. It is called magnetomechanical coupling, it is evaluated by the factor K according to the following ways. The core component of the acoustic magnetic label is amorphous alloy (Perm alloy).

According to phenomenological theory, the factor k is expressed as: The formula for the resonance frequency Fr and FA for anti - resonance frequency.

Apparently, the label have its resonance point and anti-resonance point, it can spread out large resonance signal under a small inspiring magnetic field. Furthermore, the dropout voltage between these two points is apparent, which means the label has high magnetomechanical coupling factor. Sharp resonance curve indicates that the label has high Q factor and Narrow bandwidth as well as strong selectivity. Thus, if we set appropriate bias magnetic field, and give it good work condition, we can acquire high resonance signal and high frequency stability.   

Tuning-fork Effect
 The acoustic magnetic label is composed by little plastic box, about 40mm in length and 8-14mm in width, and 1mm in depth. In the box, a tuning-fork structure is formed by two kinds of metallic stripe, one hard metallic stripe is fixed on the box and the other is free vibrated amorphous stripe. According to the special material and structure of the label, which has a certain resonance frequency; when the Applied alternating magnetic field frequency and resonance frequency of the label aligned, it will have resonance. Due to magnetostriction effect and Piezomagnetism, when applied alternating magnetic field disappeared, the label will still have a damped Oscillation, forming alternating magnetic energy and mechanical energy conversion mode, Resonance signal attenuation; this is a kind of compound acoustic magnetic signal.
The working frequency of typical acoustic magnetic label is 58 kHz, tuning-fork signal is just like ultrasonic wave, and thus, it has strong anti-interference capability and high penetrating force, which makes the label more brilliant than other labels.
In the process of identification when using tuning-fork effect is actually a process of interaction between electromagnetic energy and mechanical energy conversion. But as the transition of the efficiency of the magnetic devices is very low, it needs strong transmitted power. Typical values such as minimum to activate the magnetic field strength is greater than 16A/m, thus, the antenna detector of the acoustic magnetic label is very large. 

III. Present Situation at Home and Abroad

Resonators and bias films are the key components in the acousto magnetic label. On the one hand, the resonant film is made of Fe-based and Fe-Ni-based amorphous strips. The composition, geometrical size and shape of the resonator, the heat treatment conditions and the number of resonant plates have an important effect on the acousto-magnetic properties of the acousto-magnetic label. On the other hand, the bias film is widely uses semi-hard magnetic material and the performance of the film can influence the frequency of the label and depends whether the acoustic magnetlc label can be tested and decoded. Thus the research of the material of the resonator and the bias film are going on.

Patent WO2007143707 proposes a novel composition of the resonating plate, which is believed to be an amorphous ribbon of FeaNIbMc (where M is one of Co, C, B, P, etc., or a combination of several , A + b + c = 100, a in the range of 40 to 70 wt%, b in 10 to 50 wt%, c in 10 to 50 wt%), annealing in a 300 degree magnetic field for at least one hour can achieve similar results to that used in today's market Sound and magnetic properties, but at the same time greatly reduced the manufacturing cost of the resonant film.

Patent US6057766 discloses an iron-element-based magnetostrictive material suitable for use as a resonant element having a magnetoelastic coupling coefficient of between 0.28 and 0.4, wherein the iron content is between 45 and 82 wt%, while the rest are mainly cobalt and nickel and some glass-forming elements. Patent US 6,181,245 discloses a low coercive bias sheet material that allows an acousto-magnetic tag to be decoded at a greater distance. Patent US 6,689,490 discloses a semi-hard magnetic material suitable for use in a bias sheet in an acousto-magnetic label comprising 8 to 25 wt% nickel, 1.5 to 4.5 wt% aluminum and 0.5 to 3 wt% titanium, with the remainder being iron. This semi-hard magnetic material is different to the typical biased material and has very brilliant magnetic property and high corrosion resistance. Further more, it is very suitable for cold working and tempering when producing.

IV: Related Enterprises
Although resonant and biased films are currently being developed around the world, only the United States prior to the company to produce high-performance sound and magnetic labels in line with market requirements, the preparation technology has been monopolized for many years, especially the use of semi-hard magnetic alloy strip preparation bias sheet production process is complex, the world's qualified suppliers only Germany's vacuum metallurgy company and the United States’ Carpenter special steel company two. At home, only the production of amorphous soft magnetic materials for the bias of the magnetic tag of the company, for semi-hard magnetic bias with the magnetic tag of the preparation process is basically in the blank phase.

V. Market Size and Forecast

As one of the most advanced EAS in the world, acousto magnetic is widely used in different situation. At the same time, European countries are striving to implement the source label scheme, which requires that the manufacturer put the anti-thief label into the products when producing, This behavior will reduce the cost of the production and increase the quantity of the use of the label.

As a result, the use of acousto magnetic label is about 8 billion, the transaction amount of more than 200 million US dollars, and annual demand growth remained at 25%, with a very broad market space.

However, the label preparative technology has been the United States of America's first monopoly of the company for nearly five years, although the domestic company's strong anti-theft label, The share of the market share of less than 2%, at present dominated the country for the kind of sound and magnetic anti-theft tag preparation process to be improved. With the progress of research, things began to enter our lives; amorphous acousto magnetic security systems will bring us more and more reliable security.