Effect of DC bias current on sensitivity of thin-film magnetoimpedance element

Magnetoimpedance (MI) sensor is commercially applied for compasses in mobile phones [1], and its sensitivity has been enhanced to be applicable to biomedical research field and nondestructive testing [2]. Amorphous wires and ribbons are mostly used in current developed sensor. On the other hand, miniaturization of electronic devices and measurement with higher spatial resolution progress: this increases a strong demand for a miniaturization of MI elements. Because an MI element composed of thin-film is compatible with integrated electronic devices and it can be fabricated by photolithography techniques, thin-film MI element is a solution to such demands. However, a thin-film MI element has generally less sensitivity compared with wires and ribbons, therefore the sensitivity of thin-film elements should be enhanced more. The sensitivity of thin-film MI element strongly depends on the relation between the easy axis direction, the ac small field excitation direction, and the detection field direction. The typical preferable case is that the ac small field is excited parallel to the easy axis direction and the detection field is perpendicular to the easy axis: the easy axis is aligned to the width direction of the element. When the easy axis is inclined to the width direction and the inclined angle is large, MI elements show quite different profiles with a typical sensor, while its profile is similar to typical one when the inclined angle is small. Additionally, a dc bias current applied to the element directly is an effective way to control a MI profile. However, the effects of dc bias current on MI profiles on thin-film elements have less investigated, though the effects on wires and ribbons have some reports [3, 4]. Therefore, we investigated the effects of dc bias current on the MI profile of thin-film elements with inclined easy axis with small angle, and also discussed a potential for enhancement of sensitivity for a field detection with controlling easy axis direction and dc bias current intensity.