As mentioned previously, tailoring the magnetic microwires
under proper conditions shows the significant improvement of the
GMI ratio. Although obtaining the higher ratio is an important
parameter, the corresponding magnetic field sensitivity ðhÞ, which
determines a sensor's qualities, must be scrutinized. The high frequency
GMI response possesses two peaks due to the existence of
the circular magnetic anisotropy in the microwires [30,31]. This
two-peak feature, which noticeably has a maximum at low field
region (on the order of a few Oersted). The field location of the peak
is related to the anisotropy field (Hk). In order to retain both small
Hk and high magnetic permeability of the microwires, the peaklocation
and peak-depth of the GMI-response must be optimized.
Of particular interest is the large depth of the GMI response near
zero field, which has been extensively exploited for making highly
sensitive GMI-based sensors for a weak field detection [32e36].
In this study, we enhanced the low-field GMI response in meltextracted
amorphous Co69.25Fe4.25Si13B13.5 microwires by means of
Joule-heating. The trade-off between dc current amplitudes and
annealing time was systematically optimized in order to refine the
domain microstructures and circular magnetic anisotropy of the
amorphous microwires over a high frequency range
(20 MHze1 GHz) for the development of highly sensitive, high
frequency GMI-based sensors.