Soft magnetic microwires

Soft magnetic of a microwiresSoft magnetic of a microwires in a quasi-static mode of magnetic reversal are characterized by the following parameters:

Properties soft magnetic of microwires on high frequencies:

Hk(A/m) fr(MHz) m st m r¢ m r²
45 5 8000 4000 3000
120 12 4000 2000 1200
280 25 2500 1200 400
500 30 1000 500 60

m st - static permeability and m r¢ m r² - permeability measured at fr.

Soft magnetic of a microwires:

Material of a core Reference Number Diameter core (mm) Thickness of glass coating (mm) Properties
m st Hk
(A/m) 		Br/Bs Hc
(A/m)
Co Mn B Si (a1) AT SM 8– a1 8 ± 1 12 ± 1 1000 -2000 300 - 500 0,2 10
AT SM14 – a1 14 ± 1,5 16 ± 1,5 1000 -2000 300 - 500 0,2 10
AT SM 18– a1 18 ± 2,0 21 ± 2,0 1500 -2500 200 - 400 0,2 10
AT SM 24– a1 24 ± 2,5 24 ± 2,5 3000 -5000 100 - 200 0,3 15
AT SM 30– a1 30 ± 3,0 30 ± 3,0 10000 –15000 40 – 60 0,8 15
CoNiFeBSi (b1) AT SM 8– b1 8 ± 1 12 ± 1 1000 -2000 300 - 500 0,2 10
AT SM 12–b1 12 ± 1,5 16 ± 1,5 1500 -2500 200 - 400 0,2 10
AT SM 16–b1 16 ± 2,0 21 ± 2,0 3000 -5000 100 - 200 0,3 15
AT SM 22–b1 22 ± 2,5 24 ± 2,5 10000 -15000 40 - 60 0,8 15
CoNiFe MoBSi (c1) AT SM8 – c1 8 ± 0,5 12 ± 1 1000 -2000 300 - 500 0,2 10
AT SM 10– c1 10 ± 0,7 16 ± 1,5 1000 -2000 300 - 500 0,2 10
AT SM14 – c1 14 ± 1,0 21 ± 2,0 1500 -2500 200 - 400 0,2 10
AT SM18 – c1 18 ± 1,5 24 ± 2,5 3000 -5000 100 - 200 0,3 15
AT SM 20– c1 20 ± 2,0 30 ± 3,0 10000 -15000 40 - 60 0,8 15

Soft magnetic of a microwires with GMI

The GMI effect consists in the strong dependence of the electrical impedance of a ferromagnetic conductor on the axial applied magnetic field (Hz) when an ac electric current of frequency (f) is flowing along the sample. Although the GMI effect has been observed in a wide variety of materials, the nearly-zero magnetostriction amorphous wires exhibit the best conditions for the GMI effect.

Initially, the GMI effect was interpreted in terms of the classical skin effect in a magnetic conductor with scalar 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:

Z = Rdc kr Jo(kr) / 2 J1(kr) (1)

with k = (1+j)/d where Jo and J1 are the Bessel functions, r –wire`s radius and d the penetration depth given by:

d = (p s mf f)-1/2   (2)

where s is the electrical conductivity, f the frequency of the current along the sample, and mf the circular magnetic permeability assumed to be scalar. The dc applied magnetic field introduces changes in the circular permeability, mf. Therefore, the penetration depth also changes through and finally results in a change of Z.

During the last few years the giant magneto-impedance effect, GMI, became a topic of great interest in the field of applied magnetism owing to the large sensitivity of the total impedance with the applied DC field at low magnetic fields and high frequencies. Recently such sensitivity of about 600% relative change of the impedance has been observed in amorphous microwires with vanishing magnetostriction.

The magnetoimpedance ratio, DZ/Z, has been defined as:

DZ/Z = [ Z (H) - Z (Hmax)] / Z (Hmax)

where Hmax is a maximum DC longitudinal magnetic field, of the order of 2400 A/m supplied by a long solenoid. All the measurements were performed at room temperature, with the axis of the microwire perpendicularly aligned to the earth' s field.

Alloy Reference
Number 		Nominal metallic
core diameter
(mm) 		Total microwire
diameter (mm) 		DZ/Z % Properties
Co Mn B Si AT GMI – a1 14 ± 1 18 ± 1 60 - 80 ρ = 1,12  μOhm/cm
AT GMI – a2 18 ± 1,5 22 ± 2 80 - 100
AT GMI – a3 22 ± 2 26 ± 3 100 - 120
AT GMI – a4 30 ± 4 36 ± 3 120 - 140
AT GMI – a5 40 ± 5 40 ± 4 140 - 180
Co Ni Fe B Si AT GMI – b1 10 ± 1 14 ± 1 60 - 80 ρ = 1,05  μOhm/cm
AT GMI – b2 14 ± 1,5 16 ± 2 90 - 120
AT GMI – b3 18 ± 2 22 ± 2 100 - 180
AT GMI – b4 25 ± 3 30 ± 3 150 - 250
AT GMI – b5 30 ± 4 35 ± 4 200 - 250

Microwires with magnetic bistable behavior

The microwires with positive, nearly zero or small negative (l > -1´ 10-6) magnetostriction constant show magnetic bistable behavior. The relationship Br/Bs for this microwires can reaches to 0.9 – 1.0, the switching field can be regulated in large limits by changing the alloy composition and geometrical size of microwire – from 10 to 400 A/m with precision 10–20%. All typical application for such kind materials may be provide using microwire.

The microwire hysteresis loop

The microwire hysteresis loop.

Microwires with Natural Ferromagnetic Resonance

The rectangular hysteric's loop can be obtained for the microwires with metal core in amorphous or micro (nano)-crystalline state if magnetostriction constant has a positive value. The large internal stresses (up to 1 GPa), which induce in microwire metal core during the manufacturing process as the result of difference between the thermal expansion coefficients of glass and metal, lids to increasing of the natural ferromagnetic resonance frequency to 1 – 10 GHz region. Imaginary parts of permeability m² reaches 1000 Gs/Oe. Such characteristics of NFMR were not observed yet on other materials.

Frequency dependence of permeability
Frequency dependence of permeability (real and imaginary parts)
for alloy AT FMR 4

Reference Number Nominal metallic core diameter (mm) Total diameter, D,(mm) Resonance Frequency, GHz Df(GHz) m ² (Gs/Oe)
AT FMR 3 from 6 up to 9 from 14 up to 20 from 2 up to 4,4 1 200 - 260
AT FMR 5 from 5 up to 7 from 12 up to 18 from 4,5 up to 6,5 1.5 450 – 550
AT FMR 7 from 3 up to 5 from 8 up to 14 from 6,8 up to 8,2 1.6 800 - 950
AT FMR 9 from 3 up to 5 from 20 up to 25 from 8,3 up to 9,6 1.8 1000 - 1200

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