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 I
_ac
in the wire is changed by an external magnetic field H
_ex
. When the current I
_ac
is passed through the wire, a voltage e
_tot
is induced across the wire ends. The voltage has two components: a voltage e
_R
due to R
_w
I
_ac
(where R
_w
is the electrical resistance of the wire), and a voltage e
_L
equal to dϕ
_θ
/dt (where ϕ
_θ
is the circumferential flux in the wire cross-section). e
_L
is detected by subtracting e
_R
from e
_tot
using a simple resistor circuit. The amplitude |e
_L
| of e
_L
decreases with decreases in L caused by increases in |H
_ex
|. That is, e
_L
is amplitude-modulated by |H
_ex
|. The waveform of |H
_ex
| 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 μm, 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.