Tunable Linear Dependence of Giant Magnetoimpedance Response of Microwires Annealed under Fluid Oil for Sensor Applications

Abstract

Generally, CoFe-based amorphous magnetic materials exhibit excellent soft magnetic properties such as the giant magnetoimpedance (GMI), effect which is promising for high-sensitivity sensor applications. [1][2][3][4][5][6][7] The GMI material for magnetic sensors demands high impedance ratio, high magnetic field sensitivity, and a large magnetic field response range to meet the detection requirement under different magnetic fields. However, it is a challenge to develop such a GMI material in the sensor field. Amorphous ribbons have been used in medical applications as biosensors under a magnetic field of 15 Oe. [8] Nevertheless, the ribbons have low magnetic field sensitivity and cannot serve when driving frequency is lower than 20 MHz. In addition, the large anisotropy field also limits the further promotion of amorphous ribbons as sensor material. [9] Compared with the ribbons, the meltextracted microwires have higher solidification or cooling rate and quasi-1D heat transfer direction, which are more suitable for electronic package and sensor applications. [10] The CoFebased amorphous microwires have better soft magnetic properties and higher sensitivity than other composition microwires [11] due to their typical "core-shell" magnetic structure. [12,13] The unique magnetic structure will lead to a large GMI effect, which is desirable for miniature magnetic sensors. [14] The CoFe-based amorphous alloy has a nearly zero magnetostriction constant, the magnitude is usually between 10 À8 and 10 À7 , which is also appropriate for sensor application. [15][16][17][18] The GMI value is defined as the impedance ratio, ΔZ/Z 0 (%), the peak position of which is regarded as the critical transition field (H k ) of circumferential anisotropy. [2] For as-cast amorphous microwires, the internal stress generated during preparation can induce large magnetic anisotropy, which, together with the heterogeneous structure and the rough surface of the material, will deteriorate GMI performance and magnetic field sensitivity. Thus, the prepared amorphous microwires need heat treatment to release the internal stress and homogenize the structure. [19] The heat treatment methods of microwires mainly include conventional annealing, Joule annealing (JA), magnetic field annealing, and stress annealing. [20][21][22] It was reported that cryogenic JA