Enhanced giant magnetoimpedance effect and field sensitivity in Co-coated soft ferromagnetic amorphous ribbons

Abstract

A 50 nm-thick Co film has been grown either on the free surface (surface roughness, $6 nm) or on the wheel-side surface (surface roughness, $147 nm) of Co 84.55 Fe 4.45 Zr 7 B 4 amorphous ribbons. A comparative study of the giant magnetoimpedance (GMI) effect and its field sensitivity (g) in the uncoated and Co-coated ribbons is presented. We show that the presence of the Co coating layer enhances both the GMI ratio and g in the Co-coated ribbons. Larger values for GMI ratio and g are achieved in the sample with Co coated on the free ribbon surface. The enhancement of the GMI effect in the Co-coated ribbons originates mainly from the reduction in stray fields due to surface irregularities and the enhanced magnetic flux paths closure. These findings provide good guidance for tailoring GMI in surface-modified soft ferromagnetic ribbons for use in highly sensitive magnetic sensors. The discovery of the so-called giant magnetoimpedance (GMI) effect in soft ferromagnetic ribbons makes them attractive for magnetic sensor applications. 1,2 GMI is a large change in the ac impedance of a ferromagnetic conductor subject to a dc magnetic field. 1 The impedance (Z) of a ferromagnetic ribbon can be calculated by 3 Z ¼ R dc jka cothðjkaÞ;( 1) where a is half of the thickness of the ribbon, R dc is the electrical resistance for a dc current, j ¼ imaginary unit, andThe impedance is related to the skin effect characterized by the skin depth (d m ), which, in a magnetic medium, is given bywhere q is the electrical resistivity, m T is the transverse magnetic permeability, and f is the frequency of the ac current. The application of a dc magnetic field H dc changes m T , and consequently d m and Z. Since GMI is observed at high frequencies (>1 MHz), the skin effect is significant enough to confine the ac current to a sheath close to the surface of the conductor; GMI is therefore a surface-related magnetic phenomenon. 2,3 As such, the surface roughness of a material is important and can considerably reduce the GMI magnitude if the surface irregularities exceed the skin depth. [4][5][6][7] As an example, reducing the surface irregularities of Co-based amorphous ribbons by chemical polishing was found to greatly enhance the GMI effect. 8 Peksoz et al. recently reported that the coating of the Co-based ribbon surface with CuO or a diamagnetic organic thin film improved the GMI effect. 9,10 While the origin of the enhanced GMI effect in the samples 9,10 is not well understood, these investigations open up new opportunities for improving GMI effect in soft ferromagnetic ribbons. We report here a comparative study of the GMI effect and its field sensitivity (g) in Co 84.55 Fe 4.45 Zr 7 B 4 amorphous ribbons with and without 50 nm thick Co layers deposited on either the free ribbon surface (surface roughness, $6 nm) or on the wheel-side ribbon surface (surface roughness, $147 nm). This composition has a high Curie temperature compared to Fe-based amorphous alloys 11 and is responsive to field annealing. 12 We find a large enhancement of th...

High speed electric motors based on high performance novel soft magnets

“…A new class of Fe-Co-based amorphous and nanocomposite materials, HiTperms, [6][7][8][9][10][11][12] show promise for high frequency applications. Low losses, tunable anisotropies, and robust mechanical properties suggest their use in high efficiency, high speed motors both at room temperature and operating temperatures >300 C. For motors where material strength is critical, Co-rich compositions offer good soft magnetic properties without the brittleness found in Fe-rich alloys.…”

Preparation and magnetic properties of (Co0.6Fe0.3Ni0.1)70−x (B0.811Si0.189)25+x Nb5 bulk glassy alloys

“…Further increasing the frequency, the n decreases smoothly to 4.9 % at f = 10 MHz. Table 2 summarizes the maximum GMI ratio and field sensitivity of the (Co 0.6 Fe 0.3 Ni 0.1 ) 68 (B 0.811 Si 0.189 ) 27 Nb 5 glassy alloy ribbon compared with different alloy systems reported previously [25,[28][29][30][31]. The glassy alloy ribbon exhibits rather high maximum GMI ratio of 138 % which is higher than those of the previously reported alloy systems, together with high field sensitivity of 12.5 %/Oe.…”

Tailoring circular magnetic domain structure and high frequency magneto-impedance of melt-extracted Co69.25Fe4.25Si13B13.5 microwires through Nb doping

“…1 indicate that the surface roughness did not change significantly with Nb substitution. It has been shown previously 26,27 that surface roughness has a tremendous impact on the GMI effect even at frequencies below 10 MHz. The preservation of low surface roughness with doping is important to maintaining a high GMI ratio and large low-field sensitivity of Co-based amorphous microwires.…”