Magneto-Inductive Effect in Amorphous Wires and MI Elements

In the magneto-inductive effect (MI effect), the inductance L of a ferromagnetic wire element magnetized by an ac current Iac in the wire is changed by an external magnetic field Hex. When the current Iac is passed through the wire, a voltage etot is induced across the wire ends. The voltage has two components: a voltage eR due to RwIac (where Rw is the electrical resistance of the wire), and a voltage eL equal to d¿¿/dt (where ¿¿ is the circumferential flux in the wire cross-section). eL is detected by subtracting eR from etot using a simple resistor circuit. The amplitude |eL| of eL decreases with decreases in L caused by increases in |Hex|. That is, eL is amplitude-modulated by |Hex|. The waveform of |Hex| is detected using a simple demodulator based on a diode and capacitor. A sensitive MI element is created by using a slightly negative magnetostrictive amorphous wire of length 2 mm and diameter 50 ¿m, without a coil. This MI element is expected to find applications in sensitive magnetic heads for accurate rotary encoders, in rigid disk drives, and in various magnetic cards.