In this paper, Fe24Co11.82Ni47.3Si1.47B15 amorphous ribbons were prepared by melt rapid quenching, and annealed under different conditions to improve their response performance. The influence of annealing conditions on the acoustic and magnetic properties of the ribbons used in the magneto mechanical electronic monitoring system was studied. The results show that the annealing temperature is 320 ℃, which has the maximum resonance amplitude. In addition, the magnetic field annealing perpendicular to the strip surface is conducive to enhancing the acoustic and magnetic properties of the strip.
At present, electronic article surveillance (Système EAS) is widely used in supermarkets and other occasions. The RF system was used earlier, and the marker is a resonant circuit composed of coils and capacitors. At present, there are two widely used systems: harmonic system and magnetic mechanical resonance system. The harmonic type uses the nonlinearity of the magnetization curve of the functional components in the anti-theft tag, but the signal amplitude is small, so it is vulnerable to power frequency interference and false alarm. The magneto mechanical type is a new system developed in recent years. It uses the magnetostrictive effect of amorphous magnetic sensors. It has the advantages of small label size, strong signal, high system operating frequency, and is not easy to be disturbed. Therefore, it has attracted the attention of foreign commercial systems and become the focus of research and development at home and abroad. Germany’s Vacumschmelze GmbH and the United States’ Sensitive Electronics Corporation are leading in this field, And partly realize commercialization. At present, the research and development of this system has begun to attract domestic attention, and preliminary discussion has been carried out [1,3].
Le site anti-theft label is composed of amorphous soft magnetic alloy belt and magnetic biasing bias piece, which is stapled or pasted on the commodity, and constitutes an electronic commodity monitoring system with the detection and alarm devices juxtaposed at both ends of the mall or supermarket outlet. The working principle of this system is to use the transmitting coil in the detection alarm device to generate an alternating magnetic field of a specific frequency, so that the strip in the anti-theft tag generates a mechanical resonance caused by magnetostriction, generate a response signal through magnetomechanical coupling, and generate an alarm through detection and identification of the receiving coil.
There are many factors that affect the resonance frequency and amplitude of the tag, such as the composition of the alloy strip, geometric size, and heat treatment. Heat treatment is an important means to improve its acoustic and magnetic properties.
Fe24Co11.82Ni47.3Si1.47B15 (subscript represents atomic percentage) amorphous ribbons were prepared by melt quenching method, and the width and thickness of the samples were 12.7mm and 27 mm respectively μ m。 Cut the 38.5mm long strip and anneal it in the furnace at different temperatures (260 ℃, 280 ℃, 300 ℃, 320 ℃, 340 ℃) for 10min. Apply a DC magnetic field perpendicular to the long axis of the strip during annealing. The angle between the magnetic field direction and the film surface θ It varies from 0 ° to 90 ° (as shown in Figure 1). 0 ° represents transverse magnetic field annealing and 90 ° represents vertical magnetic field annealing.
Fig. 1 Orientation of magnetic field applied to the strip during annealing
The hysteresis loop of the sample is measured by the vibrating sample magnetometer (VSM) of Lake Shore7400 model. The length of the test sample is 1cm and the width is 5mm. The resonance frequency and resonance amplitude test equipment is shown in Figure 2 (a). A 200 turn excitation coil with a diameter of 34mm is fixed at 35mm from the detection coil. The Helmholtz coil is used to apply a uniform bias magnetic field to the alloy strip. In order to eliminate the influence of the excitation coil signal, a symmetrical coil is used to detect the resonance signal of the sample, which is received and displayed by a digital oscilloscope. From Figure 2 (b), it can be seen that the resonance signal of the strip has a significant attenuation process, Adjust the bias field, observe the change rule of the sample resonance frequency and the resonance amplitude after the excitation signal stops for 1ms .
Fig. 2 Test diagram (a) Instrument block diagram; (b) Test signal
3. Results and discussion
Fig. 3 shows the change curve of resonant frequency and amplitude with applied bias field after annealing in a transverse magnetic field of 320 ℃. It can be seen from the figure that the resonance frequency first decreases and then increases with the increase of bias field, while the resonance amplitude first increases and then decreases. When the frequency of the alternating magnetic field is close to the harmonic frequency of the strip’s magnetomechanical resonance, the vibration intensity of the soft magnetic strip reaches the maximum, and the corresponding bias field is 10.5Oe.
Fig. 3 Variation Curve of Resonance Frequency and Amplitude of Strip Annealed in 320 ℃ Transverse Magnetic Field with Bias Magnetic Field
The variation of frequency with bias field can be well explained by Formula (1).
In the formula: fr is the resonance frequency, λS is the saturated magnetostriction coefficient, Js is the saturated polarization intensity, Es is the saturated Young’s modulus, and HK is the anisotropic field . When the bias field is 0, the resonance frequency of the strip depends on the length, density and Young’s modulus of the sample, , In the formula, l is the sample length, ρ Is the density, and n is the fundamental frequency multiple. Under the action of bias field Hb, the strip length and Young’s modulus will change. It can be seen from the formula that the relationship between fr and Hb/HK also gets the change trend of frequency with bias field Hb, which is consistent with Figure 3.
Fig. 4 shows the change curve of the amplitude of the magnetomechanical resonance of the amorphous ribbon with the annealing temperature. The resonance amplitude first increases and then decreases with the increase of annealing temperature. Too low annealing temperature makes the internal stress release of the sample incomplete. Too high annealing temperature will cause the change of strip anisotropy field, and the signal will weaken. The amplitude is maximum at 320 ℃.
Figure 5 shows the magnitude of the magnetic bias field corresponding to the position of the maximum resonance peak of the sample after annealing at different temperatures. It can be seen that the bias field first increases and then decreases with the increase of annealing temperature. The crystallization temperature of this material is about 400 ℃ , so it is still amorphous after annealing. After heat treatment, the stress inside the material is mainly released, the stress anisotropy can be reduced, and the soft magnetic properties are improved. At the same time, the magnetic field annealing also changes the magnetic moment orientation inside the material.
In order to explain this phenomenon, we measured the hysteresis loop of the amorphous ribbon after annealing at different temperatures (Fig. 6), and the figure shows the curve of the anisotropy field changing with the annealing temperature. The hysteresis loop is linear until the strip reaches magnetization saturation, indicating that the amorphous alloy strip shows uniaxial anisotropy after annealing in magnetic field, and the direction of easy magnetization axis is close to the direction of magnetic field applied by annealing . This is because the external magnetic field will produce induced anisotropy in the strip during annealing, and the magnetic moment of the induced material will tend to arrange in the direction of the magnetic field. Moreover, the ability of the external magnetic field to change the magnetic moment orientation of the material is different at different annealing temperatures. Therefore, the change of strip magnetic properties after magnetic field annealing depends on the combined effect of stress anisotropy and induced anisotropy. In our experiment, it is found that transverse magnetic field annealing optimizes the acoustic and magnetic properties of anti-theft labels. With the increase of temperature, the anisotropic field HK first increases and then decreases, reaching the maximum at 320 ℃. The magnetic bias field corresponding to the maximum resonance peak is associated with the anisotropic field HK of the sample. The peak value generally appears at the position where the magnetic bias field Hb=0.5-0.6 HK . Although limited by the experimental conditions, the sample size for measuring the hysteresis loop is relatively small, and the HK value obtained is larger, but it is consistent with the trend of the magnetic bias field change in Figure 5.
Fig. 4 Resonance amplitude curve of amorphous alloy strip versus annealing temperature
Fig. 5 Bias Magnetic Field Corresponding to the Maximum Resonance Peak of Annealed Samples at Different Temperatures
Figure 7 shows the change trend of the amplitude of the magnetomechanical resonance of the amorphous magnetostrictive strip with the annealing magnetic field angle. It can be seen from the figure that compared with the transverse magnetic field annealing, the resonance amplitude of the strip is greatly improved by using the vertical magnetic field or the magnetic field annealing at a certain angle with the strip. Since the anisotropic field induced by the annealing of the included angle magnetic field is not completely in the strip plane and perpendicular to the long axis of the strip, the anisotropic field must have an out of plane component. In order to reduce the energy of the dissipative field related to the out of plane component of HK, the out of plane HK component produces the refinement of the closed magnetic domain structure. With annealing magnetic field angle θ With the increase of, the width of the magnetic domain decreases gradually. This refined closed magnetic domain structure effectively suppresses the eddy current loss caused by the rotation of the magnetization vector in a single magnetic domain, and accordingly increases the resonance amplitude [8,9].
Fig. 6 Hysteresis Loop of Amorphous Strip Annealed in Transverse Magnetic Field at Different Temperatures
Fig. 7 Variation Curve of Resonance Amplitude of Amorphous Alloy Strip with Angle of Annealing Magnetic Field
Fe24Co11.82Ni47.3Si1.47B15 amorphous alloy strip made by melt rapid quenching was annealed in magnetic field, and the magnetoelastic properties of soft magnetic strip applied to anti-theft labels were studied. The results show that the soft magnetic strip annealed in 320 ℃ magnetic field has the maximum magnetic resonance amplitude, and when annealed in a vertical magnetic field, it has the maximum resonance amplitude. The exploration in this paper has certain reference value for the practical application of amorphous magnetostrictive strip in acoustic magnetic anti-theft tag.
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