Effect of Adding Co on Crystallization Behavior and Magnetoimpedance Effect of Amorphous / Nanocrystalline FeCuNbSiB Alloy Strips

After one of the B atoms in Fe73.5Cu1Nb3Si13.5B9 alloywas replaced by both 0.7 Si and 0.3 Co, Fe73.5Co0.3Cu1Nb3Si14.2B8 alloy ribbons were prepared by single roll fast quenching method. The obtained alloy ribbons were subsequently wound into ring magnetic cores, and then these magnetic cores were annealed at different temperatures in air. The effects of adding Co on the crystallization behavior and soft magnetic properties of the as-quenched alloy ribbons with and without heat treatment were studied. The results show that the amorphous structure of the prepared Fe73.5Co0.3Cu1Nb3Si14.2B8 alloy ribbons is transformed into the coexistence of amorphous and nanocrystalline structures after heat treatment at 550C. Comparing with Fe73.5Cu1Nb3Si13.5B9 alloy ribbons, the first initial crystallization temperature (Tx1) and crystallization peak temperature (Tp1) of Fe73.5Co0.3Cu1Nb3Si14.2B8 alloy ribbons were reduced by 1.6 and 1.7C, respectively, While the second initial crystallization temperature (Tx1) and crystallization peak temperature (Tp2) were increased by 6.5 and 5.7C, respectively, resulting in that the difference between the first and the second initial crystallization temperatures ( Tx) are increased by 8.1C; the initial permeability (μi) and saturation induction density (Bs) of the amorphous/nanocrystalline Fe73.5Co0.3Cu1Nb3Si14.2B8 magnetic cores were reduced by 0.15 H/m and 0.39 T, respectively, while the coercivity (Hc) is increased by 0.34 A/m.


Introduction
Since a fast quenching process was employed to prepare the amorphous alloys by Klement et al. in 1960 [1], it has been attracting more and more attention of researchers because of its unique structure, efficient preparation process, excellent material properties, and wide application prospects [2].The amorphous soft magnetic alloy is mainly composed of Fe, Co, and Ni belonging to ferromagnetic metal elements, as well as Si, B, P, and C belonging to amorphous metal elements.Generally, a small amount of transitional elements or rareearth elements is usually added into the amorphous soft magnetic alloy [3,4], to improve the ability and thermal stability of the amorphous alloy.Yashizawa et al. found that the iron-based alloy with an amorphous and nanocrystalline dual-phase structure can be conveniently obtained by adding a small amount of Cu and M (M=Nb, Ta, Mo and W, etc.) into the amorphous Fe-Si-B alloy family and annealing them at an appropriate temperature [5].
Panina and Mohrifirstly noticed the giant magnetoimpedance (GMI) effect in the amorphous CoFeSiB soft magnetic alloy wire in 1994, where the impedance value changes with the applied magnetic field along the axis of the wire when the amorphous wire is excited by alternating current [6].Afterwards, the GMI were also observed in Febased nanocrystalline ribbons, films, and wires with high permeability [7,8].The various miniature sensors based on GMI have been applied in measurement of weak magnetic fields, detection of azimuth, and magnetic recording technology due to the advantages of this effect such as high sensitivity, small size, and fast response [9].
Both the high temperature properties and high frequency performance of the alloys can be improved by replacing Fe in finemet soft magnetic alloy with Co [10][11][12].In this work, The annealing of the alloy strip at different temperatures is conducted to study the effect of adding Co on the crystallization behavior of the alloy strip.The direct-current (DC) soft magnetic properties of the amorphous/nanocrystalline cores, as well as the magnetic impedance effect of the amorphous/nanocrystalline strips, are also investigated.

Sample Preparation and Test Method
The signal roll quenching is employed to prepare the alloy strip of Alloy-I and Alloy-II (Co) with the width 20 mm and thickness 25 m, respectively.Firstly, the alloy strip is wound into a ring core with an outer diameter of 40 mm and inner diameter of 25 mm by a tape winding machine.Then the end of the strip is welded with amorphous spot welder.Afterwards, the ring core is annealed in a nonmagnetic stainless steel tubular atmosphere resistance furnace in nitrogen atmosphere at the temperature of 350, 400, 450, 500, and 550 ∘ C, respectively, and, at each temperature, the duration is 100 min.Finally, the annealed strip is air cooled.The X-ray diffractometer (XRD) Bruker D-9 is employed to conduct phase analysis.In detail, the copper target   rays with wavelength of 0.154 nm, diffraction angle 2 range of 20∼90 ∘ , and step size of 0.02 ∘ is utilized.The working current and voltage are 40 kV and 40 mA, respectively.The differential thermal analysis is carried on with synchronous thermal analyzer SDT Q600 instrument (TA, America), and the heating rate is 10 ∘ C⋅min −1 .The Argon atmosphere with purity of 99.99% is put to use for protection.Besides, the soft magnetic DC testerMATS-2010SD is employed to test the DC soft magnetic performance.During the process, the amorphous/nanocrystalline cores annealed at 550 ∘ C are loaded into the tray in the first place, and then 10 turns of primary coil and 3-turn secondary coil are wound on the guard plate with enameled wires.The effective magnetic path length of amorphous/nanocrystalline cores is 98.44 mm and the effective cross-sectional area is 127.3 mm 2 .In addition, the impedance meter 4284A is employed to test the impedance of amorphous/nanocrystalline, and the current amplitude and alternating current frequency are set as 10 mA and 0.1∼ 1 MHz, respectively.The magnetic field drawing instrument is employed to generate magnetic field and the intensity is 0∼2800 A/m.Both the DC magnetic field and the current direction are along the length direction of the strip and perpendicular to the geomagnetic field.
As shown in Figure 1, it is the scheme of testing principle of magnetoimpedance effect.The 200-turn copper enameled wire (Φ = 0.21 mm) is layered and evenly wound around the induction coil skeleton (plastic rectangular hollow tube with a section of 7.76 mm in length and 2.15 mm in width and 8.23 mm in length and 3.05 mm in width, respectively).Afterwards, a 2 cm long amorphous/nanocrystalline strip annealed at 550 ∘ C is placed in the middle of the inductor skeleton.Finally, the inductor is placed in the center of the magnetic field, and the axial direction of the inductance coil is set to be parallel to the magnetic field.
The effect of impedance is defined as amplitude of impedance variation like where  0 is impedance of inductance coil without magnetic field and   is impedance of inductance coil after axial magnetic field applied.
In Figure 2, the equivalent circuit model is displayed to reveal the magnetoimpedance effect.The impedance of alloy strip can be equivalent to the series circuit model of resistance  and inductance , and the impedance is where, in (2),  is the frequency for testing and  and  are the resistance and inductance of the coil, respectively.In (3),  and  stand for the round and length (which is equal to the length of amorphous/nanocrystalline strip) of the coil, respectively. and  represent the magnetic permeability of the amorphous/nanocrystalline strip and cross section area of coil, respectively.
From ( 1), (2), and (3), the impedance of amorphous/ nanocrystalline strip can be calculated by where  0 is the permeability of amorphous/nanocrystalline strip without magnetic field, while  ℎ denotes the permeability of amorphous/nanocrystalline strip after axial magnetic field is applied.

Effect of Adding Co on Amorphous and Crystallization
Behavior of Alloy Strip.As can be seen from Figure 3, both quenched alloy strips have a two-stage crystallization process.
In the first stage, the starting crystallization temperatures are 514.4 and 512.8 ∘ C, respectively, while the crystallization peak temperatures are 533.4 and 531.7 ∘ C, respectively.Regarding the second stage, the starting crystallization temperatures are 665.4 and 671.9 ∘ C, respectively, and the crystallization peak temperatures are 684.1 and 689.8 ∘ C, respectively.The differences of the starting crystallization temperature of the two stages are 151 and 159.1 ∘ C, respectively.As shown in Figure 4, the diffraction spectra of two kinds of alloy strip with different quenched states perform typical diffuse peak characteristics of amorphous structure, and there are no sharp diffraction peaks in the crystalline phase.It indicates that both of the alloy strips with different quenched states possess noncrystalline structure.The strength of the diffuse peaks of the two alloy strips increases but the diffuse peak width is gradually narrowed at 2 of=45 ∘ along with the increasing of annealing temperature.When the annealing temperature reaches 500 ∘ C, the sharp diffraction peaks appear at 2 with=44.7 ∘ , 65 ∘ , and 82 ∘ for the both alloy strips.With comparison to the PDF card (35-0519), the crystal surfaces including (110), (200), and (211) belonging to the Fe 3 Si phase with body centered cubic (BCC) structure can be clearly identified.On the other hand, when the annealing temperature comes to 550 ∘ C, the diffraction peak intensity of Alloy-II (Co) alloy strip gets stronger, and the width of the peak is narrower compared to Alloy-I alloy strip.Judged from Scherrer equation, the grain sizes of Alloy-I and Alloy-II (Co) alloy strips can be calculated to be about 11.044 and 12.734 nm at 2 of =44.7 ∘ , respectively.It reveals that both alloy strips can form a composite structure with the coexistence of the amorphous and nanocrystalline phases superior to heat treatment at 550 ∘ C.
From the results in Figures 3 and 4, we find that the first crystallization peak corresponds to the precipitation of soft magnetic solid solution (precipitation of -Fe phase).And the second crystallization peak is the crystallization of the remaining amorphous phase, which is mainly related to the precipitation of Co-B, Fe-B, and Nb-Co compounds [13][14][15].Compared to the Alloy-I alloy strip, the Alloy-II (Co) one possesses lower starting crystallization temperature and crystallization peak temperature, which indicates that the Co addition will lead to worse thermal stability of amorphous alloy [16,17].However, the addition of Co increases the difference between the one-and two-stage crystallization temperature of the alloy strip, which is conducive to the precipitation of a single Fe-Si phase and the precipitation of Fe-B phase which affects the magnetic properties of the alloy.Actually, Co element possesses similar chemical property to Fe, and if Co element is added to the amorphous alloy system, solid solution will be formed.In this process, Co atoms increase the chaos of the alloy melt by occupying the spatial lattice position of the Fe atoms, resulting in formation of amorphous phase.Besides, the addition of Co element can delay the occurrence of crystallization reaction and restrict the precipitation of amorphous phase, which also promotes the formation of amorphous phase.

Effect of Adding Co on DC Soft Magnetic Properties
of Amorphous/Nanocrystalline Crystalline Cores.The effect of adding Co on DC soft magnetic properties of amorphous/nanocrystalline cores is displayed in Table 1, where   and   mark the initial and maximum permeability, respectively;   and   represent the saturation magnetic induction intensity and residual magnetism, respectively.The hysteresis loss and coercive force are denoted by   and   , respectively.Compared with the amorphous/nanocrystalline Alloy-I core, the amorphous/nanocrystalline Alloy-II (Co) magnetic cores possesses lower initial permeability, maximum magnetic permeability, hysteresis loss, saturation magnetic induction, and residual magnetism, but it yields to coercive force.This trend shows that adding Co element reduces the DC soft magnetic properties of amorphous/nanocrystalline cores.The decrease of initial permeability can improve the antisaturation ability and antibiasing ability of amorphous/nanocrystalline magnetic core under DC bias, to some extent.In particular, the stability of the amorphous/nanocrystalline core can be greatly improved in the electromagnetic application environment with DC component.

Effect of Adding Co on Magnetic Impedance of FeCuNbSiB
Amorphous/Nanocrystalline Strip.The magnetoimpedance effect of the amorphous/nanocrystalline Alloy-I strip annealed at 550 ∘ C is shown in Figure 5.The results indicate that the impedance  and amplitude of the impedance û increase as the test frequency  increases when the magnetic field strength  is fixed at constant.On the other side, when the test frequency  remains unchanged, the impedance  will decrease but the amplitude of the impedance û is just the opposite along with the increase of  which is the magnetic field strength.In fact, given a coil with resistance , number of turns , cross section area , and length , the permeability  of amorphous/nanocrystalline strip is constant if the magnetic field intensity  remains unchanged, leading to increase of impedance as the increase of test frequency  from (2) and (3).However, the permeability  of the amorphous/nanocrystalline strip decreases with the increase of magnetic field intensity  when the test frequency  is constant [8], following decrease of the impedance  as the increase of magnetic field strength  according to (2) and (3).
In addition to the above discussion,   will decrease along with the increase of magnetic strength  since  0 is a constant if the test frequency  remains to be unchanged, resulting in the increase of û as the increase of  according to (4), while   is constant if the magnetic strength  stays in the same condition, with the increase of û as increase of  from (4).
In Figure 6, the magnetoimpedance effect of the amorphous/nanocrystalline Alloy-I and Alloy-II (Co) strips annealed at 550 ∘ C is tested and shown, where the test frequency is 1.0MHz.The results show that the impedance variation amplitude û of the amorphous/nanocrystalline Alloy-II (Co) strip increases compared to the Alloy-Ione.

Conclusion
In conclusion, the following two aspects can be attributed in this work: (1).    (2).The initial magnetic permeability, saturation magnetic induction, and the DC soft magnetic properties of the amorphous/nanocrystalline Fe 73.5 Co 0.3 Cu 1 Nb 3 Si 14.2 B 8 magnetic core are all reduced, but it yields to the coercive force compared with the amorphous/nanocrystalline Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 magnetic core.In addition, the impedance amplitude of the amorphous/nanocrystalline

Figure 5 :
Figure 5: The magneto impedance effect of the Alloy-I amorphous/nanocrystalline strips after heat treatment at 550 ∘ C.

Figure 6 :
Figure 6: The magnetoimpedance effect of Alloy-I and Alloy-II(Co) amorphous/nanocrystalline strips after heat treatment at 550 ∘ C.

Table 1 :
The amorphous Fe 73.5 Co 0.3 Cu 1 Nb 3 Si 14.2 B 8 strip is successfully prepared by single roll quenching method by DC soft magnetic properties of the amorphous/nanocrystalline cores heat treatment at 550 ∘ C.