Diameter Dependence of Magnetic Properties of Co-based Metal Fibers¹

Abstract

Naked Co_68.25Fe_4.5Si_12.25B₁₅ metal fibers with diameter from 25 µm to 100 µm are produced by melt extraction method. Significant diameter dependence of magnetic properties is studied. Their microstructure and magnetic properties of hysteresis loops and giant magneto impedance (GMI) effect at frequencies from 0.1MHz to13MHz are investigated. The results show that the coercivity increases with the diameter of fibers and the GMI effect is best in fiber with a diameter of 35µm. It is found that the cooling rate of solidification decreases with the increase of fiber diameter. And fibers are amorphous with the diameter of 50µm and there are nanocrystallines in fibers with a diameter of 85µm. The grain boundary blocks the magnetization process which makes lower circular permeability, larger coercivity and lower field sensitivity of GMI effect. The original microstructure of Co-based fibers decides their magnetic properties. Therefore, material design is important to improve the GMI effect in magnetic field sensor making.

Keywords:
Metal fiber; GMI effect; hysteresis loops

1. Introduction

Giant magneto-impedance (GMI) effect is a large and sensitive change in electrical impedance with the external dc magnetic field applied in a magnetic conductor¹. It has attracted much attention from research community because of its potential applications in magnetic sensors and recording devices². The GMI effect has found in amorphous ribbons, wires and films^3,4. It is found that the Co-rich metallic fibers have good soft magnetic properties and excellent GMI effect⁵. The strong GMI effect of a piece of soft magnetic wire at a low dc magnetic field originates from the dependence of the circular magnetic permeability on the dc magnetic field and skin effect¹. H. Chirac et al studied the impedance response of Co_68.18Fe_4.32Si_12.5B₁₅ wires prepared by rotating water rapid quenching and found thinner wires had better GMI effect at low external field⁶. They also found the reduction of diameter by cold drawing produced a significant increase in GMI response⁶. H. Chirac et al also found the magnitude of the GMI effect was strongly dependent on the diameter of the metallic core and on the glass cover thickness through study on the GMI effect of glass-covered Co_68.18Fe_4.32Si_12.5B₁₅ wires⁷. Naked wires prepared by melt extraction method are different from the hard drawing or glass-covered wires because there is no tensile stress induced by hard drawing process or the covered glass must be taken into account. The aim of our work is to study the magnetic properties of melt extraction Co_68.25Fe_4.5Si_12.25B₁₅ amorphous wires with different diameters.

2. Experimental

Co_68.25Fe_4.5Si_12.25B₁₅ amorphous wires were extracted directly from the melt alloy by a sharp rapidly rotating red copper wheel in an argon atmosphere⁸. The hysteresis loops of fiber with diameter of 45 µm, 55 µm, 60 µm and 100 µm were testes with induction method. The microstructure of fibers with 50 µm and 80 µm were analyzed by high resolution transmission electron microscopy, HRTEM. As-cast fibers of 25 µm,35 µm,44 µm,60 µm in diameter and 20mm in length were selected for impedance analysis with a distance between two voltages contacts of 15mm. Impedance Z was measured using Agient 4294A impedance analyzer at room temperature of about 25℃. External dc axial magnetic field H_ex was supplied by a pair of Helmholtz coils and the maximum field H_max was 4Oe. The ac current amplitude was kept at 10mA and the current frequency ranged from 0.1MHz to 15MHz. The Helmholtz coils were kept perpendicular to the earth’s magnetic field to avoid earth effect.

The impedance ratio can be expressed as:

$∆\mathit{Z}/\mathit{Z}\left(\%\right)=\left[\mathit{Z}\left({\mathit{H}}_{\mathit{ex}}\right)-\mathit{Z}\left({\mathit{H}}_0\right)\right]/\mathit{Z}\left({\mathit{H}}_0\right)\times 100\%$

Where Z(H_ex) and Z(H₀) are impedance values at external magnetic field of H_ex and zero.

3. Results and Discussion

The GMI amplitude as a function of the external magnetic field Hex is shown in Fig 1. The GMI response has different features with different diameters. GMI curves are all two-peak ones under different frequencies, fibers with diameter of 35µm has the strongest GMI effect, shown in Fig 2. The external field at peak impedance is named effective anisotropy field H_m⁹. The result shows that the filed H_m increases gradually with the increase of diameter as shown in Fig 3. At 2MHz, with the diameter increasing from 25 µm to 60µm, the corresponding H_m increases from 14.4A/m to 27.2A/m, and the H_m increases from 27.2A/m to 46.4A/m at 10MHz.

Figure 1
GMI profiles of Co_68.25Fe_4.5Si_12.25B₁₅ metallic fibers with different diameters at different frequencies: (a) 2MHz (b) 5MHz (c) 10 MHz (d) 15 MHz.

Figure 2
Diameter dependency of GMI effect of metallic fibers.

Figure 3
Frequency dependency on the effective anisotropy filed Hm of fibers.

Fig. 4 presents the hysteresis loops of Co-based metal fibers with diameter of 45 µm, 55µm, 60 µm and 100 m, respectively. With the increase of fiber diameter, the coercive field increases linearly. And the magnetization increases gradually which is mainly due to the increase of the quality and the greater the magnetic moment intensity.

Figure 4
Hysteresis loops of Co_68.25Fe_4.5Si_12.25B₁₅ metallic wires with different diameters.

Then the microstructure of fibers with diameter of 50 µm and 85 µm is investigated and shown in Fig 5. There are short-range orders with a few of atoms and the microstructure is long range disorder, as shown in Fig 5(a). The short-range orders are formed by a central circular atomic structure or several close atoms packed into strips. The strip structure of single row atoms is irregular shape. After the fast Fourier transform, there is a diffraction ring or called a diffuse halo, no obvious crystal spots or nano ring appeared, which proves that the fiber is amorphous completely. That means during rapid solidification process, liquid atoms remain chaotic state and the fiber with a diameter of 50µm is amorphous one. Fig 5 (b) shows the microstructure of metal fibers of 85µm in diameter. After the fast Fourier transform, the diffraction ring is composed of several concentric circles, indicating the presence of nanophase in the amorphous matrix. There are long-range ordered regions, which are mainly circular and strip structures based on multi-layer long-range orders of atoms, as microcrystalline or nanocrystalline. The results show that the strip structure or circular structure is of 2nm x 4nm in range with ruled atomic arrangement. When the size of nanocrystalline is not large enough to block the magnetization process, the presence of small size nanocrystals is beneficial to the magnetization of fibers and to the sensitive GMI effect^10,11.

Figure 5
HRTEM maps of Microstructure and Fourier transform maps of metallic fibers in diameter: (a) 50 μm (b) 85μm.

It is proved that domains consist of an axial inner core domains and circular outer shell domains of this kind of Co-based fibers^12,13. What’s more, the thermal induced quenched-in stress in amorphous fibers is correlated with diameter^7,14,15. The resulting domain configuration consists of a core axially magnetized and an outer shell which occupies an increase volume as diameter increases. Due to the increase of circular domains in thicker wires, the circular anisotropy field and permeability µ_Ф increases and determines strong GMI effect. Two-peak GMI response appearance is correlated with the skin effect and magnetization process^16,17. The magnetization driven by the ac current self-generated circular field decides circular permeability µ_Ф. In Co-based amorphous fibers the magnetization in the outer shell and the inner core contribute to the circular permeability¹⁸. As frequency increases, the skin effect gets strong and the domain wall displacement in the outer shell is damped by the eddy current. When the external field compensates for the anisotropy field, the circular permeability gets the largest values and the MI peak appears. It is investigated that the coercivity increases as diameter increases¹⁹ and the effective anisotropy field H_m also increase as frequency²⁰. For this reason, GMI effect will decreases when the diameter is over than 35 µm. Obviously, during rapid solidification process, a small number of atoms in the fibers of 85µm in diameter remain chaotic and disordered state as liquid atoms. Most atoms adjust their morphology to form regular long-range clusters and a certain amount of atoms even grow to be nanocrystals.

The difference in microstructure of the fibers with different diameters is related to the cooling rate of the fibers preparation. Under the same preparation conditions, the cooling rate of the alloy with the same component is determined only by the diameter of the fiber^4,8,21. There is a reciprocal relationship between cooling rate and diameter. The increase of radius induces the decrease of cooling rate rapidly.

4. Conclusions

Through analysis the magnetic properties of naked Co_68.25Fe_4.5Si_12.25B₁₅ melt extraction fibers of different diameters; it is found that their magnetic properties and GMI effect are significantly dependent on the diameter which decides the solidification cooling rate. And there are nanocrystals in the microstructures of fibers with a diameter of 85 µm which induces larger coercivity. And the fibers of 50µm in diameter are completely amorphous with little coercivity. The increase of circular domains in thicker wires induces higher permeability µ_Ф and better GMI effect. However, larger coercivity in thicker fibers also decreases the GMI effect. Consequently, GMI effect is best in fibers with a diameter of 35µm.

• 1
  Key Scientific Research of North Nationalities University (Grant No. 2015KJ15) and National Natural Science Foundation of China (Grant No.51861031).

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Publication Dates

• Publication in this collection
  09 Sept 2019
• Date of issue
  2019

History

• Received
  18 Jan 2019
• Reviewed
  04 May 2019
• Accepted
  19 May 2019