In an alternating magnetic field, you can see the magnetostrictive metal strip vibrating according to the frequency of the alternating magnetic field. If the frequency of the alternating magnetic field is consistent with the resonant frequency of the metal bar, its amplitude is the largest, that is, resonance occurs. This effect is especially obvious for permalloy (or iron-nickel alloy). On the other hand, this magnetostrictive effect is reversible, that is, piezomagnetic effect.
Therefore, when the frequency of the alternating magnetic field coincides with the resonance frequency of the metal strip in the acousto-magnetic tag, the permalloy strip starts to vibrate. When the alternating magnetic field is turned off, the acousto-magnetic tag will maintain a damped vibration for a certain period of time like a tuning fork, and generate a resonance signal as a spatial extension of the alternating magnetic field, which can be detected by the receiver.
The magnetostriction coefficient λ is used to describe the magnetostriction effect, λ=(LH-L0)/L0, L0 is the original length of the material, and LH is the length of the material after the change under the action of an external magnetic field. Because Permalloy has high magnetostriction coefficient, such as: Ni50 Permalloy λ=25×10-6, Ni80 Permalloy λ=(0.1"0.5)×10-6, so the magnetostriction of Permalloy The coefficients are larger, and the resonance signal generated by the tag is also larger.