MAGNETO-INDUCTIVE ELEMENT

Magneto-Inductive effect (MI-effect, for short) means a phenomenon of change of an inductance L for an external magnetic field Hex in a ferromagnetic wire element magnetized with a wire ac current I(ac). When a I(ac) is applied through a ferromagnetic wire, a voltage e(tot) is induced between both ends of the wire which has two voltage components: a voltage e(R) of R(w)I(ac) (R(w): electric resistance of the wire) and a voltage e(L) of dphi(c)/dt (phi(c): a circumferential flux at the cross section of the wire). e(L) is detected by subtraction e(R) from e(tot) using a simple resistor circuit. The amplitude \e(L)\ of e(L) decreases with increasing \H(ex)\ due to the decrease of L. That is, \e(L)\ is an amplitude-modulated wave with \H(ex)\. The waveform of \H(ex)\ is detected using a simple demodulator with a diode and a capacitor. A sensitive MI element is constituted using a slightly negative magnetostrictive amorphous wire (a-wire, for short). The rate of change of \e(L)\ at a folded 5-mm long a-wire element was about 50% for \H(ex)\ of 10 Oe (800 A/m). The a-wire MI element is expected as new sensitive magnetic heads (MI heads) for accurate rotary encoders, hard disks, and various magnetic cards.