IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 6, JUNE 2014
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Effect of Nanocrystallization on Magnetic Properties and GMI Effect of Microwires Arcady P. Zhukov1,2, Ahmed Talaat1, Mihail Ipatov1 , Juan M. Blanco3, Lorena Gonzalez-Legarreta4, Blanca Hernando4, and Valentina Zhukova1 1 Departamento
Fisica de Materiales, UPV/EHU, San Sebastian 20009, Spain Basque Foundation for Science, Bilbao 48011, Spain 3 Departamento de Física Aplicada, EUPDS Basque Country University UPV/EHU, San Sebastian 20009, Spain 4 Departamento de Física, Universidad de Oviedo, Oviedo 33007, Spain 2 IKERBASQUE,
We studied giant magnetoimpedance (GMI) effect and magnetic properties of FINEMET-type FeCuNbSiB microwires. We observed that the GMI effect and magnetic softness of glass-coated microwires produced by the Taylor–Ulitovski technique can be tailored either controlling magnetoelastic anisotropy of as-prepared FeCuNbSiB microwires or controlling their structure by heat treatment or changing the fabrication conditions. We observed considerable magnetic softening of studied microwires after annealing. This magnetic softening correlates with the devitrification of amorphous samples. Amorphous microwires exhibited low GMI effect (GMI ratio below 5%). Considerable enhancement of the GMI effect (GMI ratio up to 100%) has been observed in heat treated microwires with nanocrystalline structure. Some of as-prepared Fe-rich exhibited nanocrystalline structure and the GMI ratio up to 45%. Index Terms— Giant magnetoimpedance (GMI) effect, magnetic anisotropy, thin microwires.
I. I NTRODUCTION
S
OFT MAGNETIC materials exhibiting giant magnetoimpedance (GMI) effect are quite attractive for various applications [1]–[3]. It is found that amorphous Co-rich materials with low magnetostriction constant after adequate processing can exhibit outstanding soft magnetic properties and high GMI effect. Excellent soft magnetic properties of the amorphous materials are attributed to the lack of the defects typical for crystalline materials (grain boundaries, dislocations and so on). In the absence of magnetocrystalline anisotropy one of the main factors affecting magnetic softness of the amorphous materials is the magnetoelastic anisotropy. Co-rich amorphous materials present vanishing magnetostriction constant and consequently high soft magnetic properties and the GMI effect [1]–[5]. It is worth mentioning that the magnetostriction constant is determined by the chemical composition of amorphous alloys, achieving nearly zero values in amorphous Fe–Co based alloys with Co/Fe ≈ 70/5 [5]–[7]. The GMI effect, reported for soft ferromagnetic materials consists of large changes in the impedance Z under application of magnetic field [1], [8]. The GMI effect is satisfactory interpreted in terms of classical electrodynamics as a consequence of abrupt change of skin depth of magnetically soft conductor under application of the magnetic field [1]–[3], [8]. The intensive research on the GMI effect is related to its technological importance for the magnetic sensors as well as tuneable metamaterials applications [9]–[13]. From the point of view of applications cheaper Fe-rich materials are preferable. In addition the Fe-rich soft magnetic materials exhibit higher saturation magnetization. However, Manuscript received November 21, 2013; accepted January 21, 2014. Date of current version June 6, 2014. Corresponding author: A. P. Zhukov (e-mail:
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2014.2303396
the amorphous Fe-rich materials as a rule present rather high magnetostriction constant and therefore low GMI effect [14]. One of the ways to optimize the soft magnetic properties of the Fe-rich amorphous materials and to minimize the magnetostriction constant is preparation of two-phase nanocrystalline alloys produced by controlled crystallization of amorphous [15]–[17]. It is found that in the case of Fe-rich FINEMET alloys the size of the crystallites does not exceed 20 nm. This alloy is based on a typical Fe–Si–B metallic glass, with little amount of copper enhancing crystals nucleation and niobium retarding the crystallites growth. As the result, the coercive field of FINEMET is of the order of 4 A/m, which is comparable with the best amorphous Co-rich magnetically soft amorphous alloys. Vanishing effective magnetocrystalline anisotropy is explained due to the size of crystals below the exchange length which is ∼35 nm for pure iron [15], [16]. In such a case, the magnetocrystalline anisotropy of single grains is averaged out and the material behaves like a soft magnet. In addition, the average magnetostriction constant takes nearly zero values [7], because of the control of a crystalline volume fraction λs,eff = Vcr λs,cr + (1 − Vcr )λs,am
(1)
where λs,eff is the saturation magnetostriction coefficient and Vcr is the crystalline volume fraction. One of the latest tendencies in development of industrial applications is the miniaturization of the magnetic sensors. Consequently families of the soft magnetic wires with reduced dimensionality and outstanding magnetic characteristics, such as melt extracted wires (typically with diameters of 40–50 μm) [3], [18] and in particular, glass-coated microwires with even thinner diameters (between 1 and 40 μm) [4], [19], recently attracted growing attention. In the case of Co-rich amorphous microwires excellent soft magnetic properties and high GMI (GMI ratio, Z /Z , up to 600%) have been reported
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IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 6, JUNE 2014
Fig. 1. Hysteresis loops of as-prepared and annealed at 673 K Fe73.8 Cu1 Nb3.1 Si13 B9.1 microwires with d = 20.3, D = 28.6, and ρ = 0.71.
[4], [19], [20]. Reported excellent features of the amorphous Co-rich microwires give rise to the development of industrial applications for lowmagnetic field detection in various industrial sectors [9], [10]. On the other hand, although soft magnetic behavior of nanocrystalline Fe-rich microwires has been reported [21]–[23], only a few publications deal with studies of the GMI effect in FINEMET-type microwires [24]. In addition the reported GMI ratio values are much lower than reported for amorphous microwires or FINEMET-type ribbons [20], [25]–[27]. Consequently we performed studies of annealing conditions on magnetic properties and the GMI effect of FINEMET-type microwires. In this paper we report our last results on the optimization of the GMI effect in Fe-rich FINEMET glass-coated microwires. The superior magnetic properties of these glasscoated microwires make them ideal for high-performance sensor application. II. E XPERIMENTAL D ETAILS We prepared Fe70.8 Cu1 Nb3.1 Si14.5 B10.6 , Fe71.8 Cu1 Nb3.1 Si15 B9.1 , Fe73.8 Cu1 Nb3.1 Si13 B9.1 , and Fe70.8 Cu1 Nb3.1 Si16 B9.1 glass-coated microwires with different metallic nucleus diameter, d, and total microwire diameter, D, using modified Taylor–Ulitovsky method described elsewhere [4]–[6]. We measured the hysteresis loops using flux-metric method and the magnetic field dependences of impedance, Z , and the GMI ratio, Z /Z , for as-prepared samples and after heat treatments previously described by us [5], [19]. For the GMI measurements, we used specially designed micro-strip sample holder placed inside a sufficiently long solenoid that creates a homogeneous magnetic field, H . The sample impedance Z was measured using vector network analyzer from reflection coefficient S11 . The frequency range for the GMI measurements is from 10 MHz to 4 GHz. More details on experimental technique can be found in [19]. The magnetoimpedance ratio, Z /Z , has been defined as Z /Z = [Z (H ) − Z (Hmax )] · 100/Z (Hmax).
(2)
An axial dc field with maximum value, Hmax, up to 8 kA/m was supplied by a magnetization coil. Structure and phase composition have been checked using a Bruker (D8 Advance) X-ray diffractometer with Cu Kα (λ = 1.54 Å) radiation.
Fig. 2. Dependences of coercivity on ρ-ratio measured in as-prepared Fe70.8 Cu1 Nb3.1 Si14.5 B10.6 and Fe73.8 Cu1 Nb3.1 Si13 B9.1 microwires.
Heat treatments have been performed in conventional furnace as described elsewhere [21]. III. R ESULTS AND D ISCUSSION As a rule, hysteresis loops of all as-prepared and even annealed at temperatures below the beginning of the first crystallization process studied microwires present rectangular shapes (Fig. 1). In Fig. 1 we can appreciate that after annealing prior the first crystallization only slight changes of hysteresis loops can be observed; for Fe73.8 Cu1 Nb3.1 Si13 B9.1 microwire with d = 20.3, D = 28.6, and ρ = d/D = 0.71, we observed only some decreasing of the remanent magnetization. Similar character of hysteresis loops is observed for all studied samples. The difference between the microwires of the same composition with different geometric features is the value of coercivity field: decreasing the ρ-ratio values (i.e., increasing the strength of internal stresses) the coercivity values, Hc , increase. This dependence can be well appreciated in Fig. 2, where Hc (ρ) for Fe73.8 Cu1 Nb3.1 Si13 B9.1 and Fe70.8 Cu1 Nb3.1 Si14.5 B10.6 microwires is presented. The main peculiarity of the described above Taylor– Ulitovsky technique for glass-coated microwires fabrication is the composite character of microwires and considerable internal stresses induced by fast solidification of the composite microwire [28]–[30]. The strength of these internal stresses can be manipulated by modifying the aforementioned ρ-ratio. Experimentally and theoretically found that the internal stresses are of the order of 100–1000 MPa, and the strength of these stresses rises with the decreasing ρ-ratio [28]–[30]. The aforementioned influence of internal stresses on magnetic properties must be responsible for considerable increasing of the coercivity observed with decreasing of the ρ-ratio as shown in Fig. 2. Heat treatment is the conventional way to release the internal stresses. In addition, in FINEMET alloys, the annealing inducing the nanocrystallization usually allows achieving of better magnetic softness. Consequently, we studied the effect of annealing on coercivity and the GMI effect of microwires. Coercivity dependence on annealing temperature for Fe70.8 Cu1 Nb3.1 Si14.5 B10.6 microwire is shown in Fig. 3. We observed considerable magnetic softening at annealing temperatures, Tann , between 750 and 850 K followed by abrupt magnetic hardening at Tann > 873 K. This magnetic
ZHUKOV et al.: EFFECT OF NANOCRYSTALLIZATION ON MAGNETIC PROPERTIES
Fig. 3. Annealing temperature dependence of coercivity of Fe70.8 Cu1 Nb3.1 Si14.5 B10.6 FINEMET microwire with different ρ-ratio.
Fig. 4. XRD patterns of Fe70.8 Cu1 Nb3.1 Si14.5 B10.6 microwires with ρ = 0.72 (d = 15.6, D = 21.6 μm) after annealing at different temperatures.
hardening is usually associated with the second crystallization process. Observed dependences are similar to previously observed in other FINEMET-type materials [26], [27]. In order to correlate the observed dependences with the structural changes, we performed the X-ray Diffraction (XRD), studies. The XRD spectra for as-prepared and annealed Fe70.8 Cu1 Nb3.1 Si14.5 B10.6 microwires with ρ = 0.72 are shown in Fig. 4. As-prepared microwires, Fe70.8 Cu1 Nb3.1 Si14.5 B10.6 microwires present amorphous structure. The first crystallization process, we observed at T ≥ 823 K (Fig. 4). The first crystallization process corresponds to the precipitation of α-Fe (Si) bcc crystal structure, similarly to the nanocrystallization of conventional FeCuNbSiB materials [22]–[27]. Using the Debye-Sherrer formula, we estimated the grain size, Dg. As shown in Fig. 5, the average crystallites size for Tann = 823 K is 12 nm increasing up to 27 nm at T = 923 K. Similar dependences of the average crystallites size on annealing temperature have been observed for the other studied Fe70.8 Cu1 Nb3.1 Si14.5 B10.6 microwires with different ρ-ratios. The GMI effect in as-prepared Fe-rich microwires is rather small [Fig. 6(a)–(c)] being ∼1% at measured frequency ∼100 MHz. After annealing, we observed a considerable increasing of the GMI ratio, Z /Z [Fig. 6(a)–(c)].
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Fig. 5. Dependence of average grain size on annealing temperature determined for Fe70.8 Cu1 Nb3.1 Si14.5 B10.6 microwires with ρ = 0.72.
We selected annealing temperatures below the first crystallization process (723 K) and right after the beginning of the nanocrystallization. As can be appreciated from Fig. 6 after annealing at temperatures when we achieved the nanocrystalline structure of the samples, we observed drastic enhancement of the Z /Z effect. This considerable growth of the GMI ratio must be related with magnetic softening of studied microwires after annealing related with the internal stress relaxation and, in particular, with the nanocrystallization. Another factor that affects the GMI ratio is the frequency at which the GMI ratio is measured. It is already pointed out that selecting appropriate frequency range allows optimization of the GMI ratio for a given sample [19]. Consequently, we presented Z /Z (H ) measurements at different frequencies (Fig. 7). As can appreciated in as-prepared samples, we did not observed any appreciable frequency dependence and the GMI effect is low [below 3%, see Fig. 7(a)]. However, in annealed sample at 823 K [Fig. 7(b)], the GMI ratio achieves values ∼90% at 500 MHz for the same Fe70.8 Cu1 Nb3.1 Si14.5 B10.6 microwire. It is well known that magnetic anisotropy considerably affects the Z /Z (H ) dependences [19], [31], [32]. Observed double peak Z /Z (H ) in nanocrystalline microwire is typical for the samples with low-negative magnetostriction constant. Therefore, we can assume that annealing results in formation of double-phase structure with vanishing magnetostriction constant. As observed in Fig. 6, the GMI ratio increases even after annealing at temperatures below the first crystallization temperatures. The GMI effect is affected by the internal stresses distribution [33]. Consequently, this GMI ration increasing must be attributed to the stress relaxation. Analyzing Z /Z (H ) dependences of as-prepared, we found that some of the as-prepared sample present anomalously high GMI effect (Fig. 8). This is the case of the Fe73.8 Cu1 Nb3.1 Si13 B9.1 microwire, which exhibits maximum GMI ratio ∼45%. The XRD studies show that as-prepared Fe73.8 Cu1 Nb3.1 Si13 B9.1 microwire exhibiting a high GMI ratio in fact has nanocrystalline structure, although as-prepared Fe70.8 Cu1 Nb3.1 Si14.5 B10.6 microwires with low GMI effect are amorphous. Usually, applications of the nanocrystalline materials are restricted by poorer mechanical properties. However, one of the advantages of glass-coated microwires is that the glass coating of micrometric thickness is rather flexible. There-
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IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 6, JUNE 2014
Fig. 7. Z /Z (H ) dependences of Fe70.8 Cu1 Nb3.1 Si14.5 B10.6 microwires with (a) ρ = 0.59 in as-prepared, (b) annealed at 823 K measured at different frequencies.
Fig. 6. Z /Z (H ) dependences of Fe70.8 Cu1 Nb3.1 Si14.5 B10.6 microwires with geometric ratio (a) ρ = 0.59, (b) ρ = 0.81, and (c) ρ = 0.62 measured at 100 MHz measured in as-prepared and annealed at different temperatures samples.
fore, the effect of the nanocrystallization on the mechanical properties of these microwires is generally smaller than the one produced by annealing on conventional amorphous alloys (ribbons and conventional wires) due to the existence of the glass coating [5], [34], [35]. In addition, as-prepared Fe73.8 Cu1 Nb3.1 Si13 B9.1 nanocrystalline microwires are less brittle. This is one more interesting feature for the potential technological applications. Consequently, we can assume that the nanocrystalline structure of the FeCuNbSiB microwires is essential for optimization of the GMI effect.
Fig. 8. Z /Z (H ) dependences measured in as-prepared Fe70.8 Cu1 Nb3.1 Si14.5 B10.6 (ρ ≈ 0.81) and Fe73.8 Cu1 Nb3.1 Si13 B9.1 (ρ ≈ 0.6) microwires, measured at 300 MHz.
IV. C ONCLUSION Studies of the magnetic properties of FINEMET-type FeCuNbSiB microwires reveals that annealing considerably affects the hysteresis loop and the GMI effect of this family of microwires. In as-prepared microwires the reduction of the ρ-ratio results in the rise of coercivity. Magnetoelastic anisotropy affects soft magnetic properties of as-prepared
ZHUKOV et al.: EFFECT OF NANOCRYSTALLIZATION ON MAGNETIC PROPERTIES
FeCuNbSiB microwires. We observed magnetic softening and considerable GMI effect in FINEMET-type FeCuNbSiB with nanocrystalline structure even in as-prepared microwires. After adequate annealing of FINEMET-type microwires we observed the GMI ratio ∼100%. The nanocrystallization of the FeCuNbSiB microwires is a key for optimization of the GMI effect. ACKNOWLEDGMENT This work was supported in part by the EU ERANET Program SoMaMicSens under Project MANUNET2010-Basque-3; in part by EU under FP7 “EM-safety” Project; in part by the Spanish MICINN under Project MAT2010-18914; in part by the Basque Government under Projects SAIOTEK-2012 MEMFOMAG (S-PE12UN139), PROMAGMI (S-PE13UN014) and DURADMAG (S-PE13UN007); and in part by Oviedo University through the Principado de Asturias under Grant SV-PA-13-ECOEMP-47. The work of A. Zhukov, and V. Zhukova was supported by the Basque Government through the Program of Mobility of the Investigating Personnel of the Department of Education, Universities and Investigation under Grant MV-2013-2-22 and Grant MV-2013-2-23. R EFERENCES [1] L. V. Panina, K. Mohri, K. Bushida, and M. Noda, “Giant magnetoimpedance and magneto-inductive effects in amorphous alloys (invited),” J. Appl. Phys., vol. 76, no. 10, pp. 6198–6203, 1994. [2] M. Vázquez, J. M. García-Beneytez, J. M. García, J. P. Sinnecker, and A. Zhukov, “Giant magneto-impedance in heterogeneous microwires,” J. Appl. Phys., vol. 88, no. 11, pp. 6501–6505, 2000. [3] P. Ciureanu, G. Rudkowska, L. Clime, A. Sklyuyev, and A. Yelon, “Anisotropy optimization of giant magnetoimpedance sensors,” J. Optoelectron. Adv. Mater., vol. 6, no. 3, pp. 905–910, 2004. [4] M. Vazquez, H. Chiriac, A. Zhukov, L. Panina, and T. Uchiyama, “On the state-of-the-art in magnetic microwires and expected trends for scientific and technological studies,” Phys. Status Solidi A, vol. 208, no. 3, pp. 493–501, 2011. [5] A. F. Cobeño, A. Zhukov, A. R. de Arellano-Lopez, F. Elías, J. M. Blanco, V. Larin, et al., “Physical properties of nearly zero magnetostriction Co-rich glass-coated amorphous microwires,” J. Mater. Res., vol. 14, no. 9, pp. 3775–3783, 1999. [6] A. Zhukov, V. Zhukova, J. M. Blanco, A. F. Cobeño, M. Vazquez, and J. Gonzalez, “Magnetostriction in glass-coated magnetic microwires,” J. Magn. Magn. Mater., vols. 258–259, pp. 151–157, Mar. 2003. [7] Y. Konno and K. Mohri, “Magnetostriction measurements for amorphous wires,” IEEE Trans Magn., vol. 25, no. 5, pp. 3623–3625, Sep. 1989. [8] L. V. Panina, K. Mohri, T. Uchyama, and M. Noda, “Giant magnetoimpedance in co-rich amorphous wires and films,” IEEE Trans. Magn., vol. 31, no. 2, pp. 1249–1260, Mar. 1995. [9] T. Uchiyama, K. Mohri, and S. Nakayama, “Measurement of spontaneous oscillatory magnetic field of guinea-pig smooth muscle preparation using pico-tesla resolution amorphous wire magneto-impedance sensor,” IEEE Trans. Magn., vol. 47, no. 10, pp. 3070–3073, Oct. 2011. [10] L. Ding, S. Saez, C. Dolabdjian, L. G. C. Melo, A. Yelon, and D. Ménard, “Equivalent magnetic noise limit of low-cost GMI magnetometer,” IEEE J. Sensors, vol. 9, no. 2, pp. 159–168, Feb. 2009. [11] Y. Honkura, “Development of amorphous wire type MI sensors for automobile use,” J. Magn. Magn. Mater., vol. 249, nos. 1–2, pp. 375–381, 2002. [12] L. V. Panina, M. Ipatov, V. Zhukova, A. Zhukov, and J. Gonzalez, “Microwave metamaterials with ferromagnetic microwires,” Appl. Phys. A, Mater. Sci. Process. vol. 103, no. 3, pp. 653–657, 2011. [13] F. Qin and H.-X. Peng, “Ferromagnetic microwires enabled multifunctional composite materials,” Progr. Mater. Sci., vol. 58, no. 2, pp. 183–259, 2013. [14] A. Zhukov, M. Ipatov, A. Talaat, M. Churyukanova, S. Kaloshkin, and V. Zhukova, “Giant magnetoimpedance in thin amorphous and nanocrystalline microwires,” Appl. Phys. A, Mater. Sci. Process., Oct. 2013, doi: 10.1007/s00339-013-8028-1.
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