Off-diagonal impedance in amorphous wires and its application to linear magnetic sensors

We investigated the magnetic-field behavior of the off-diagonal impedance in Co-based amorphous wires under sinusoidal (50 MHz) and pulsed (5 ns rise time) current excitations. For comparison, we measured the field characteristics of the diagonal impedance as well. In general, when an alternating current is applied to a magnetic wire, the voltage signal is generated not only across the wire but also in a pickup coil wound on it. These voltages are related to the diagonal and off-diagonal impedances, respectively. We demonstrate that these impedances have a different behavior as functions of axial magnetic field: the diagonal impedance is symmetrical, whereas the off-diagonal one is antisymmetrical with a near-linear portion within a certain field interval. For the off-diagonal response, the dc bias current is necessary to eliminate circular domains. In the case of the sinusoidal excitation without a dc bias current, the off-diagonal response is very small and irregular. In contrast, the pulsed excitation, combining both high- and low-frequency harmonics, produces the off-diagonal voltage response without additional biasing. This behavior is ideal for a practical sensor circuit design. We discuss the principles of operation of a linear magnetic sensor based on a complementary metal-oxide-semiconductor transistor circuit.

I. Introduction

Magnetoimpedance (MI) is an expanding area of current research because of its importance for micromagnetic sensor applications (see selected reviews [1]–[6] and references therein). Recently developed MI sensor technology uses CoFeSiB amorphous wires as MI-element incorporated into complementary metal–oxide–semiconductor (CMOS) integrated circuit (IC) multivibrator pulse current circuit. Typical parameters of sensor operation with 1-mm-long MI head are: a field resolution of Oe (0.1 nT) for the full scale of Oe (0.1 mT), a response speed of 1 MHz, and a power consumption of about 10 mW. The sensor sensitivity is at least an order of magnitude higher than that for giant magnetoresistive (GMR) sensors. These advanced characteristics are associated with a large change in the range of 100% in high-frequency impedance of Co-based amorphous wires subjected to a moderate magnetic field (1–5 Oe). Sensor operation needs high sensitivity combined with linearity. On the other hand, the impedance versus field behavior in amorphous wires is essentially nonlinear, especially near zero-field point. Customarily, applying a dc bias field or utilizing an asymmetric MI effect [7]–[10] achieves linearity. On the other hand, off-diagonal impedance may have almost linear region near zero-field point [11], [12] and hence can be used for linear sensing, as demonstrated in our present work.