- The invention consists of the fabrication of thin (metallic nucleus diameter below 10 µm) microwires with determined chemical composition containing Co, Fe, Si, B, C with additions of Ni, Mo, Cr, Zr, Hf with determined relation between metallic nucleus diameter and glass coating thickness.

- The present patent, as expressed in its title of the present description, consists of a method of fabrication of thin (metallic nucleus diameter below 20 µm) microwires with determined chemical composition containing Co, Fe, Si, B, with additions of Ni, Mo, Cr, Zr, Hf, C with determined relation between metallic nucleus diameter and glass coating thickness.

The scientific aspects forming a base of the invention can be found in the following scientific issues:

1. [1] L.V. Panina and K. Mohri, Appl. Phys. Lett. 65 (1994) 1189.
2. [2] V. Zhukova, A. Chizhik, A. Zhukov, A. Torcunov, V. Larin and J. Gonzalez, IEEE Trans. Magn. 38, 5, partI, (2002) 3090.
3. [3] Spanish patent "Amorphous glass-coated misrowires as an element of magnetic sensors based on the magnetic bistability, magneto-impedance and as material for the protection from the radiation." (Ref. P200202248)
4. [4] V. S. Larin, A. V. Torcunov, A. Zhukov, J. González, M. Vazquez, L. Panina "Preparation and properties of glass-coated microwires" J. Magn. Magn. Mater. 249/1-2 (2002) 39-45

The present patent is closely related with previous Spanish patent "Amorphous glass-coated misrowires as an element of magnetic sensors based on the magnetic bistability, magneto-impedance and as material for the protection from the radiation." (Ref. P200202248) [3], but paying special attention to the GMI effect (absolute value and tensor components) in thin wires (with diameter of metallic nucleus below 20 µm).

Microwires are manufactured by means of a modified Taylor-Ulitovsky process [3,4] based on direct casting from the melt, as schematically depicted in Fig. 1. In the laboratory process, a few grams of the master alloy with the desired composition is put into a Pyrex-like glass tube and placed within a high frequency inductor heater. The alloy is heated up to its melting point, forming a droplet. While the metal melts, the portion of the glass tube adjacent to the melting metal softens, enveloping the metal droplet. A glass capillary is then drawn from the softened glass portion and wound on a rotating coil. At suitable drawing conditions, the molten metal fills the glass capillary and a microwire is thus formed where the metal core is completely coated by a glass shell. The amount of glass used in the process is balanced by the continous feeding of the glass tube through the inductor zone, whereas the formation of the metallic core is restricted by the initial quantity of the master alloy droplet. The microstructure of a microwire (and hence, its properties) depends mainly on the cooling rate, which can be controlled by a cooling mechanism when the metal-filled capillary enters into a stream of cooling liquid (water or oil) on its way to the receiving coil.

The main advantages of this method of microwire fabrication are:

1. i. repeatability of microwire properties at mass-production;
2. ii. wide range of variation in parameters (geometrical and physical);
3. iii. fabrication of continuous long pieces of microwire up to 10 000 m;
4. iv. control and adjustment of geometrical parameters (inner core diameter and glass thickness) during the fabrication process

The giant magneto-impedance, GMI, attracted great attention for the sensor applications owing to the large sensitivity (up to 600%) of the electrical impedance to the DC magnetic field of soft magnetic conductor [1]. The recent tendency in miniaturization of the magnetic sensors requires the development of extremely thin composite wires produced by the Taylor-Ulitovsky method (1 ÷ 30 µm in diameter) consisting of metallic nucleus coated by glass. Recent significant progress in tailoring of magnetically soft Co-rich glass coated microwires with metallic nucleus diameter of about 20 µm fabricated by this method enabled to enhance significantly the GMI ratio (up to about 600%) [2].
The AC current frequency should be high enough (usually above 100 kHz) in order to observe significant change of the electrical impedance. On the other hand special care is needed in order to study GMI effect at frequencies above 10 MHz. Particularly, the sample holder should have special design and the electrical cables should be as shorter as possible and should possess special HF specifications.
As has been mentioned, initially the GMI effect was interpreted in terms of the classical skin effect in a magnetic conductor assuming scalar character for the magnetic permeability, as a consequence of the change in the penetration depth of the ac current caused by the dc applied magnetic field. The electrical impedance, Z, of a magnetic conductor in this case is given by [1,5,6]: $$\mathrm{Z}={\mathrm{R}}_{\mathrm{dc}}{\mathrm{kr\; J}}_{\mathrm{o}}\left(\mathrm{kr}\right)/\mathrm{2}{\mathrm{J}}_{\mathrm{1}}\left(\mathrm{kr}\right)$$ with k = (1+j)/δ, where Jo and J1 are the Bessel functions, r -wire's radius and δ the penetration depth given by: $$\mathrm{\delta }={\left({\mathrm{\pi \; \sigma \; \mu }}_{\mathrm{\phi }}\mathrm{f}\right)}^{-\mathrm{1}/\mathrm{2}}$$ where σ is the electrical conductivity, f the frequency of the current along the sample, and µφ the circular magnetic permeability assumed to be scalar. The dc applied magnetic field introduces significant changes in the circular permeability, µφ. Therefore, the penetration depth also changes through and finally results in a change of Z [1, 5,6].

From aforementioned on the GMI effect for optimization of the GMI effect the materials must present high magnetic permeability, µ..

In the absence of the magnetocrystalline anisotropy (that is the case of amorphous materials) the magnetoelastic anisotropy plays the most important role and must be minimized.
The magnetoelastic anisotropy is essentially determined by the magnetostriction coefficient, λ_s and internal stresses, σ_i: $${\mathit{K}}_{\mathit{me}}\approx \mathit{3}/\mathit{2}{\mathit{\lambda }}_{\mathit{s}}{\mathit{\sigma }}_{\mathit{i}},$$ The magnetostriction constant depends mostly on the chemical composition and is vanishing in amorphous Fe-Co based alloys with the content of Co/Fe ≈70/5 [2].
Recently this "scalar" model was significantly modified taking into account the tensor origin of the magnetic permeability and magneto impedance [7,8]. For instance, it was theoretically shown in [7], that the axial dependence of the GMI spectra is mainly determined by the type of magnetic anisotropy. It was shown particularly, that the circumferential anisotropy leads to the observation of the maximum of the real component of wire impedance (and consequently of the GMI ratio) as a function of the external magnetic field. In contrast, in the case of axial magnetic anisotropy the maximum value of the GMI ratio corresponds at zero magnetic field [8], i.e. results in a monotonic decay of the GMI ratio with the axial magnetic field. Consequently, non-diagonal components of the magnetic permeability tensor and impedance tensor were introduced in [6,7] in order to describe such circumferential anisotropy. In order to achieve highest GMI effect the magnetic anisotropy should be as smaller as possible.
The aim of this patent is to introduce a novel family of thin wires exhibiting high GMI effect including non-diagonal components at high enough frequencies (above 10 MHz).

1. [1] L.V. Panina and K. Mohri, Appl. Phys. Lett. 65 (1994) 1189.
2. [2] V. Zhukova, A. Chizhik, A. Zhukov, A. Torcunov, V. Larin and J. Gonzalez, IEEE Trans. Magn. 38, 5, partI, (2002) 3090.
3. [3] Spanish patent "Amorphous glass-coated microwires as an element of magnetic sensors based on the magnetic bistability, magneto-impedance and as material for the protection from the radiation." (Ref. P200202248)
4. [4] V. S. Larin, A. V. Torcunov, A. Zhukov, J. González, M. Vazquez, L. Panina "Preparation and properties of glass-coated microwires" J. Magn. Magn. Mater. 249/1-2 (2002) 39-45
5. [5] R. S. Beach, A. E. Bertowitz, Appl. Phys. Lett. 64, 3652 (1994).
6. [6] R.L. Sommer and C.L. Chien, J. Appl. Phys. 79, 5139 (1996).
7. [7] N.A. Usov, A.S. Antonov and A.N. Lagar'kov, J. Magn. Magn. Mat. 185, 259 (1998).
8. [8] D.P. Makhnovskiy, L.V. Panina and D.J. Mapps, Phys.Rev.B, 63, 1444241 (2001).

In this patent deals with the method of fabrication of thin glass-coated microwires with elevated GMI effect at frequencies between 10 MHz and 500 MHz.

At 10 MHz the GMI of thin microwires is smaller than in thicker conventional wires, but increasing the frequency GMI effect significantly increases, exhibiting much higher GMI effect at high frequencies and the shape of the Z(H) dependence at least for Co-rich materials is typical for the materials with circular magnetic anisotropy, i.e. with the a maximum at certain dc axial magnetic field (Fig. 2,3).
Besides, the glass-coating technology gives rise to the internal stresses due to the difference in the thermal expansion coefficients of the glass coating and metallic nucleus. This difference in the fabrication technique results in the different magnetic anisotropy in the surface and in different frequency dependence of the GMI effect.

The present patent deals with a method of a method of fabrication of thin (metallic nucleus diameter below 20 µm) microwires with determined chemical composition containing Co, Fe, Si, B, C with additions of Ni, Mo, Cr, Zr, Hf with determined relation between metallic nucleus diameter and glass coating thickness.

The GMI effect is related intrinsically with the hysteresis loops of the samples. In fact the hysteresis loops depend on many factors, such as the metallic nucleus composition which is closely related with the magnetostriction constant, the geometrical parameters (diameter of the metallic nucleus, total diameter of microwires). Effect of the metallic nucleus composition on hysteresis loops of microwires is reflected in Figs.4. On the other hand Fig. 5 shows the effect of the metallic nucleus diameter on hysteresis loop of the microwire with the same composition. As can be observed, the magnetic softness deteriorates when the metallic nucleus diameter becomes thinner. On the other hand best magnet softness (combination of high magnetic permeability with low coercivity) is observed in Co-rich composition with nearly-zero magnetostriction constant (Fig.4). GMI effect is also higher in Co-Fe-rich samples with nearly-zero magnetostriction constant (Fig.3). Therefore further we concentrate on thin microwires with nearly-zero magnetostrictive compositions.

Axial hysteresis loops of studied Co_67.05Fe_3,84Ni_1,44Si_14,47B_11,51Mo_1,69 microwire with metallic diameter of 8.5 µm is presented in Fig.6. As can be noted from Fig.6, excellent magnetic softness with coercivity of the order of 4 A/m has been achieved in this microwire in spite of its reduced diameter (only 8.5 µm).

The GMI effect of as-prepared Co_67.05Fe_3,84Ni_1,44Si_14,47B_11,51Mo_1,69 measured at frequencies, f, up to 500 MHz is shown in Fig.7. As can be observed, considerable GMI ratio is achieved at higher frequencies, although at conventional frequency f = 10 MHz, the GMI ratio is about 60%. The shape of the DC magnetic field dependence of the GMI ratio is typical for the samples with small and negative magnetostriction constant with circular magnetic anisotropy, i.e. with a maximum at certain DC axial magnetic field, H_m. A maximum value of the GMI ratio, ΔZ/Z_max, achieves about 180% at about 200 MHz. Frequency dependence of the ΔZ/Z_max is shown in Fig.8.

As it was shown, imaginary part of the impedance is quite useful for the use in magnetic sensors. Fig.9 shows the imaginary part the impedance, X, measured at different frequencies.

It is worth underlining high enough GMI effect in such thin microwires.

Fig.10 shows, that heat treatment conditions (annealing temperature in this concrete sample) play an important role for tailoring of the GMI effect in glass coated wires. In this particular case stress annealing for 40 min at different temperatures (as indicated in Fig.12: 1- 265°C; 2- 275°C and 3- 400°C) strongly affects the GMI ratio of Fe₇₄B₁₃Si₁₁C₂ microwire, measured at the same conditions (11 MHz).

Finally, even for the same composition, the GMI ratio depends on the samples geometry, as shown in Fig.11 for Co_67.05Fe_3,84Ni_1,44Si_14,47B_11,51Mo_1,69. Samples with metallic nucleus diameter from 8.96 till 9.89 µm and total diameter from 10.29 till 11.94 µm are studied. Samples geometry affects not only GMI ratio but also imaginary part the impedance, X, measured at the same frequency (300 MHz) through the effect of the internal stresses on the magnetic anisotropy. Such dependences for the same samples as in Fig.11 are shown in Fig.12.

The steps of the realisation of the invention are:

• Selection of the sample alloy composition for obtaining better GMI properties
• Selection of the sample geometry for concrete application
• Fabrication of amorphous microwires with thin diameter of the metallic nucleus of determined composition.
• Tailoring of the magnetic properties b y means of heat treatment (without magnetic field, without stress or under magnetic field and/or applied stress)

The advantages of the present invention are:

• high corrosion resistance owing to the insulation coating;
• small size owing to the smaller microwire diameter and therefore smaller demagnetizing factor of short enough samples;
• flexibility of the samples which permits its use in the flexible objects.

To complete this description and in order to help the understanding of the invention the present description includes figures:

• Figure. 1.
  The schematic picture of the glass-coating microwires fabrication process, where 1- glass tube, 2-molten alloy, 3- glass cone, 4- HF inductor, 5- water jet, 6- solidified glass-coated microwire.
• Figure 2.
  DC magnetic field dependence of the impedance of Fe_75.5.B₁₃Si₁₁Mo_0.5 (a), Fe_3.7Co_{6 9.8}Ni₁Si₁₁B₁₃Mo_1.5 (b) and Co_77.5Si_7.5B₁₅ (c) glass-coated amorphous microwires with metallic nucleus diameter of about 15-20 µm
  It represents the DC magnetic field dependence of the absolute value of the electrical impedance of glass -coated microwires of 3 different compositions as Fe_75.5.B₁₃Si₁₁Mo_0.5 (a), Fe_3.7Co_{6 9.8}Ni₁Si₁₁B₁₃Mo_1.5 (b) and C_077.5Si_7.5B₁₅ (c) measured at 3 different frequencies of AC driving current: 10, 100 and 500 MHz.
• Figure 3.
  It represents the frequency dependence of the absolute value of the GMI ratio, ΔZ/Z_max for different glass-coated thin amorphous wires:(a)- Fe₇₆ Si₁₁B₁₃; (b)- C_077.5Si_7.5B₁₅ and (c)- Co_68.5Fe_3.5Cr₃Si₁₁B₁₄ compositions.
• Figure 4.
  It represents the axial hysteresis loops of Fe_75.5.B₁₃Si₁₁Mo_0.5, Co_77.5Si_7.5B₁₅ and Fe_3.7Co_69.8Ni₁₁Si₁₁B₁₃Mo_1.5 glass-coated microwires respectively with different magnetostriction constant.
• Figure 5.
  It represents the hysteresis loops of FeBSiC microwires with metallic nucleus Diameter of 15 µm (a), 13.5 µm (b); 11 µm (c); 6 µm (d) and 4 µm (e).
• Figure 6.
  It represents the axial hysteresis loop of Co_67.05Fe_3.84Ni_1.44Si_14.47B_11.51Mo_1.69 microwire with diameter of 8.5 µm.
• Figure 7.
  It represents the GMI ratio of as-prepared Co_67.05Fe_3.84Ni_1.44Si_14.47B_11.51Mo_1.69 with diameter of 8.5 µm measured at frequencies up to 500 MHz.
• Figure 8.
  It represents the frequency dependence of the ΔZ/Z_max of as-prepared Co_67.05Fe_3.84Ni_1.44Si_14.47B_11.51M_1.69 with diameter of 8.5 µm measured at frequencies up to 500 MHz.
• Figure 9.
  It represents the imaginary part the impedance, X, of as-prepared Co_67.05Fe_3.84Ni_1.44Si_14.47B_11.51Mo_1.69 with diameter of 8.5 µm measured at frequencies up to 500 MHz measured at different frequencies.
• Figure 10.
  Represents GMI effect of stress annealed of Fe₇₄B₁₃Si₁₁C₂ glass-coated microwire subjected to stress annealing for 40 min at different temperatures: 1- 265°C, 2- 275°C and 3- 400°C
• Figure 11
  It represents the GMI ratio of Co_67.05Fe_3.84Ni_1.44Si_14.47B_11.51Mo_1.69 microwires with metallic nucleus, d, and total diameter, D as 1- d=8,96 µm, D =10.29 µm; 2- d=9,17 µm, D =11.94 µm; 3- d=9,64 µm, D =10.66 µm; 4- d=9,78µm, D=11,74µm
• Figure 12.
  It represents the imaginary part the impedance, X, of as-prepared Co_67.05Fe_3.84Ni_1.44Si_14.47B_11.51Mo_1.69 with metallic nucleus, d, and total diameter, D as: 1- d=8,96 µm, D =10.29 µm; 2- d=9,17 µm, D =11.94 µm; 3- d=9,64 µm, D =10.66 µm; 4- d=9,78µm, D=11,74µm

The values of the GMI ratio, ΔZ/Z_max and the hysteresis loop depend on the microwires dimensions: metallic nucleus diameters and thickness of glass coating, as presented in Fig.5 and Fig.11.

The values of the GMI ratio, ΔZ/Z_max and the hysteresis loop depend on the microwires composition, such as presented in Figs.2-4

The values of the GMI tensor components and its imaginary part depend on the microwires composition, such as presented in Figs.2, 7

The values of the GMI tensor components and its imaginary part depend on the microwires geometry, such as presented in Fig.11, 12

The absolute value of the GMI ratio of stress annealed of Fe₇₄B₁₃Si₁₁C₂ glass-coated microwire subjected to stress annealing for 40 min depends on annealing temperature (between 265°C and 400°C), such as presented in Fig.10.