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Effect of annealing on magnetic properties and magnetostriction coefficient of Fe–Ni-based amorphous microwires
Journal of Alloys and Compounds 651 (2015) 718e723
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Effect of annealing on magnetic properties and magnetostriction coefficient of FeeNi-based amorphous microwires A. Zhukov a, b, c, *, M. Churyukanova d, S. Kaloshkin d, V. Semenkova d, S. Gudoshnikov d, M. Ipatov a, b, A. Talaat a, b, J.M. Blanco b, V. Zhukova a, b a
Dept. Phys. of Materials, Basque Country University, UPV/EHU, San Sebastian, Spain Dpto. de Física Aplicada, EUPDS, UPV/EHU, 20018 San Sebastian, Spain IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain d National University of Science and Technology «MISIS», Moscow 119049, Russia b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 August 2015 Received in revised form 15 August 2015 Accepted 18 August 2015 Available online 28 August 2015
We studied the correlation between the magnetostriction coefficient, coercivity and Giant magnetoimpedance effect of FeeNi-rich microwires. As-prepared and annealed Fe47.42Ni26.6Si11B12.99C1.99 microwires present rectangular hysteresis loops. Varying the time and the temperature of annealing we observed changes of the coercivity and Giant magnetoimpedance effect (GMI). Increasing the annealing time we observed first some magnetic softening and then magnetic hardening. These changes correlate with the evolution of the magnetostriction coefficient. Decreasing of the magnetostriction coefficient observed after annealing is related to formation of nanocrystalline structure. © 2015 Elsevier B.V. All rights reserved.
Keywords: Magnetic glass-coated microwires Giant magnetoimpedance effect Soft magnetic properties TayloreUlitvosky technique Magnetostriction Heat treatment
1. Introduction Soft-magnetic wires present a number of excellent magnetic properties interesting from the viewpoint of basic physics and suitable for various technological applications. Therefore studies of magnetically soft wires attract attention along many years [1e3]. In the case of conventional crystalline soft magnetic wires these properties are limited by the magneto crystalline anisotropy and defects typical for crystalline materials. Therefore starting from 1980-th growing interest in amorphous soft magnetic wires is related to their liquid-like structure characterized by the absence of long range ordering and of magneto-crystalline anisotropy [1,3]. Among unusual magnetic properties of amorphous wires magnetic bistability and giant magneto-impedance, GMI, effect are the most promising for technical applications [1,3e7]. These properties have been proposed for various magnetic sensor applications [8,9]. For most of modern applications the sensor size is an important
issue. Therefore recently certain tendency in development of thin magnetically soft wires gained growing interest [3,6,7]. One of the main advantages of the so-called TayloreUlitovsky is that it allows preparation of long (up to few km long continuous microwire), homogeneous thin composite wires glass-coated metallic microwires with thinnest (from 0.2 up to 100 mm) diameters [3,6,7,10]. These materials consist of metallic nucleus with diameter from 0.2 up to 70 mm and glass coating sheath with diameter from 2 to 20 mm. Most of published papers on amorphous microwires have been reported for FeeCo based glass-coated microwires with different magnetostriction constants [3,7,8]. The main reason is that in the absence of magneto-crystalline anisotropy a magnetoelastic anisotropy is the main factor determining the magnetic properties of amorphous glass-coated microwires. This magnetoelastic anisotropy, Kme, is determined by internal stresses, and magnetostriction coefficient, which is given by the equation [3,7];
Kme z3=2ls si ; * Corresponding author. Dept. Phys. of Materials, Basque Country University, UPV/EHU, San Sebastian, Spain. E-mail address: [email protected] (A. Zhukov). http://dx.doi.org/10.1016/j.jallcom.2015.08.151 0925-8388/© 2015 Elsevier B.V. All rights reserved.
(1)
where ls is the saturation magnetostriction and si is the internal stress induced by the glass coating layer during the fabrication
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technique of glass-coated microwires. The magnetostriction constant depends mostly on the chemical composition, which is vanishing in amorphous FeeCo based alloys with Co/Fe z 70/5 [11,12]. Therefore changing the chemical composition in the classical (CoxFe1x)75Si15B10 system one can control the magnetostriction constant, ls, that changes with x from 5 x 106 at x ¼ 1, to ls z 35 106 at x z 0.2 [11,12]. On the other hand FeeNi compositions are considered the most promising for crystalline soft magnets allowing obtaining high magnetic permeability. Thus permalloy-type NieFe alloys present excellent soft magnetic properties (after special heat treatment) exhibiting maximum magnetic permeability for a composition range close to 80Ni% (the classical Permalloy) [1]. The other FeeNi potentially interesting magnetic alloy is the Invar containing about 36% Nie 64% Fe which has the lowest thermal expansion coefficient among all metals and alloys in the range from room temperature up to approximately 230 C [13]. In fact all the Fe-rich face-centered cubic FeeNi alloys show Invar anomalies in their measured thermal and magnetic properties that evolve continuously in intensity with varying alloy composition [13e15]. The Invar's behavior is discussed considering either a high-magnetic-moment to low-magnetic-moment transition occurring in the face centered cubic FeeNi series or a highmagnetic-moment frustrated ferromagnetic state in which the FeeFe magnetic exchange bonds have a large magneto-volume effect of the right sign and magnitude to create the observed thermal expansion anomaly [14,15]. The main problems related to crystalline FeNi soft magnetic alloy are the right (very well defined) content of the main chemical components, the influence of the minor elements (the chemical purity) and the non-metallic inclusion, precipitated particles etc (physical purity) and rather restricted conditions the heat treatment. Consequently FeeNi system is quite interesting form the point of view of improvement of magnetic softness of magnetic materials. Previously few attempts to prepare and study magnetic properties of FeeNi-based wires have been reported [16e18]. The problem is that highly Ni-containing amorphous alloys have low saturation magnetization or even no ferromagnetic order at room temperature [18]. The reasons for studies of FeeNi-rich microwires are that in most of the cases the additional annealing for improvement of magnetic softness is not needed and as-prepared amorphous materials usually present high magnetic softness. Moreover substitution of Co by less expensive FeeNi is beneficial from the viewpoint of applications. Recently we reported on successful preparation Fe77.5xNixSi7.5B15 (0 x 38.75) microwires and observation or rectangular hysteresis loops and fast DW propagation for the Fe77.5xNixSi7.5B15 (0 x 38.75). For Fe62Ni15.5Si7.5B15 samples we observed DW velocities up to 2.5 km/s [19]. On the other hand annealing is the effective tool for engineering of magnetic properties of glass-coated microwires. Recently we observed considerable dependence of hysteresis loops character of Co-rich microwires on annealing conditions attributed to the stress relaxation and effect of annealing on the magnetostriction coefficient. Annealed Co-rich microwires with acquired magnetic bistability present unusual behavior, i.e. in the same sample fast domain wall propagation and GMI effect have been observed. Previously it was considered that the magnetic permeability of microwires with rectangular hysteresis loop is rather low and they do not present remarkable GMI effect. Consequently measurements of the magnetostriction coefficient and understanding the mechanisms of the influence of the internal and applied stresses on the magnetostriction coefficient is essentially important for tailoring the magnetic properties of amorphous
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ferromagnetic microwires. The most appropriate method for evaluation of the magnetostriction coefficient of amorphous materials is so-called small angle magnetization rotation (SAMR) method introduced in 1980-th [20,21]. Initially this method has been developed only for amorphous materials where the magnetization rotation presents the determining role. Therefore initially this method has been employed for evaluation of the magnetostriction coefficient of Cobased glass-coated microwires with negative magnetostriction coefficient [22,23]. Afterward we showed that the SAMR method can be extended for the case of microwires with positive magnetostriction coefficient presenting important contribution of domain wall propagation [24,25]. As has been demonstrated experimentally domain structure of wires and microwires with positive magnetostriction coefficient is rather different from wires and microwires with negative magnetostriction coefficient [26]: wires with positive magnetostriction present 180 surface domain structure with magnetization perpendicular to the wire surface, while the surface domain structure of wires with negative magnetostriction coefficient consists of rather big (as-compared with wires with negative magnetostriction coefficient) circular 180 domains. The magnetic permeability corresponding to the magnetization rotation of the outer domain shell of microwires with positive magnetostriction coefficient is rather low. Therefore the accuracy of the SAMR method for the magnetostriction coefficient evaluation employed for the microwires with positive magnetostriction coefficient is lower. Recently novel experimental set-up with improved resolution has been successfully realized for measurements of the magnetostriction coefficient, ls, of any metallic nucleus composition (either with positive magnetostriction coefficient or with negative magnetostriction coefficient) microwires [27,28]. Consequently, the purpose of this paper is to present new experimental results on effect of annealing on magnetic properties and GMI effect of FeeNi based glass-coated microwires and on its correlation with the magnetostriction coefficient. 2. Experimental technique We studied Fe47.42Ni26.6Si11B12.99C1.99 glass-coated microwires (metallic nucleus diameter, d ¼ 29 mm, total diameter, D ¼ 32.2 mm) produced by TayloreUlitovsky technique [3,7,10,11]. The magnetostriction coefficient of studied microwires has been measured using aforementioned SAMR method described elsewhere [27,28]. In this method the sample is saturated by an axial magnetic field, Hz, while applying simultaneously a small ac transverse field, Hy, created by an ac electric current flowing along the sample. The combination of these fields leads to a reversible rotation of the magnetization within a small angle, q, out of the axial direction. The induction voltage, V(2u), due to the magnetization rotation is detected by a pick-up coil wound around the microwire. The magnetostriction constant is determined from the measurement of the dependence on axial magnetic field, Hz, versus on applied stress s at fixed value of induction voltage V(2u). The moMs values of the investigated microwires obtained from room temperature measurements of the magnetization curves at high magnetic field. The AC current value, i~, flowing through the wire is selected to avoid possible Joule heating of the sample: the current amplitude does not exceed 10e30 mA. As has been shown [20e24] under conditions q ¼ const and Ht ¼ const the value of magnetostriction coefficient, ls, is given by
ls ¼ ðm0 Ms =3ÞðdH=dsÞ; where m0 Ms is the saturation magnetization.
(2)
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We measured the magnetostriction coefficient several times and then found the average value. As mentioned above the SAMR method can be extended to microwires either with positive or negative magnetostriction coefficient. The condition for the SAMR method is the existence of the outer shell, where magnetization is perpendicular to the external field. For Co-rich microwires circular orientation of the magnetization is assumed from experimental observation of the magnetic domains structure. For Fe-rich microwires radial magnetization orientation was experimentally observed [26]. Hysteresis loops have been measured either by flux-metric method (low field measurements up to 8 kA/m) described elsewhere [28] or using PPMS device. We represent the normalized magnetization, M/Ms versus magnetic field, H, where M is the magnetic moment at given magnetic field and Ms is the magnetic moment of the sample at the maximum magnetic field amplitude, Ho. We measured magnetic field dependences of impedance, Z, and GMI ratio, DZ/Z using specially designed micro-strip sample holder placed inside a sufficiently long solenoid that creates a homogeneous magnetic field, H. There one wire end was connected to the inner conductor of a coaxial line through a matched microstrip line while the other was connected to the ground plane. This sample holder allows measuring the samples of 6 mm length. This sample length is sufficiently long allowing neglecting the effect of the demagnetizing factor [29]. The magnetoimpedance ratio, DZ/Z, has been defined as:
DZ=Z ¼ ½ZðHÞ ZðHmax Þ$100=ZðHmax Þ;
(3)
An axial DC-field with maximum value, Hmax, up to 20 kA/m was supplied by a magnetization coils. Structure and phase composition have been checked using a BRUKER (D8 Advance) X-ray diffractometer with Cu Ka (l ¼ 1.54 Å) radiation.
Fig. 1. Hysteresis loops (a) and experimentally measured DHz(Ds) dependence (b) measured in as-prepared Fe47.42Ni26.6Si11B12.99C1.99 microwires.
3. Experimental details and results Hysteresis loops of Fe47.42Ni26.6Si11B12.99C1.99 microwires present rectangular character typical for glass-coated microwires with positive magnetostriction constant (see Fig. 1a). Using described above SAMR method we evaluated the saturation magnetostriction values ls from the experimental dependences [DHz/Ds]V(2u)-const using eq. (2). From experimentally observed dependences of change of DHz on applied stress, Ds, for Fe47.42Ni26.6Si11B12.99C1.99 microwire (shown in Fig. 1b) we obtained ls z 24 106. As can be appreciated from Fig. 2, annealing considerably affects the coercivity of studied samples. After annealing we observed considerable changes of magnetic properties. As-prepared and annealed samples present rectangular hysteresis loops, but coercivity, Hc, considerably changes: for fixed annealing temperature, Tann ¼ 410 C, after short-time annealing coercivity decreases. But after further increasing of tann considerable rising of coercivity, Hc, is observed. As discussed elsewhere [29], the magnetoelastic energy, Kme, plays the determining role in the formation of magnetic properties of amorphous microwires. Kme depends on the magnetiostriction coefficient, ls, as well as on the internal stresses, s ¼ sappl þ si, (see eq. (1)). We studied the effect of annealing on structure of studied sample using X-ray diffraction. As can be observed from Fig. 3 after annealing partial crystallization of Fe47.42Ni26.6Si11B12.99C1.99 microwires from amorphous precursor has been observed. Considering the main crystalline peak, by estimating the width
of the peak ε as well as the crystallization angle 2q, then by substituting these parameters on the DeybeeScherrer equation (eq. (4)), one can obtain the value of the grain size (Dg).
Dg ¼ kl=εcos2Q
(4)
where, ε is the half height width of the crystalline peak and 2q is the angular position of the maximum crystalline peak. The diffraction pattern is the superposition of the total peak area consisting of crystalline and amorphous peaks together. Therefore the background that corresponds to the amorphous halo was subtracted. It must be noted that the crystalline structure of the well crystallized samples becomes complex and detailed analysis of the crystallization process require more careful attention. From the very basic analysis of the XRD patterns we can assume that as prepared samples present amorphous structure. After annealing at 435 C for 60 min the sample still presents mixed structure consisting of amorphous phase, bcc (Fe, Ni) grains (Space group Im-3m) with average size about 8 nm and silicides. Further annealing results in increasing of the content or crystalline phases and in further grain size increasing up to 24 nm at Tann ¼ 600 C (Fig. 3). Magnetic softening for short-time annealing can be related to internal stress relaxation. The increasing of Hc for longer annealing at 410 C must be attributed to the crystallization process as observed by XRD in Fig. 3. We measured GMI ratio in as-prepared and annealed Fe47.42Ni26.6Si11B12.99C1.99 microwires (Figs. 4 and 5). As can be
A. Zhukov et al. / Journal of Alloys and Compounds 651 (2015) 718e723
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Fig. 2. Hysteresis loops measured in as-prepared Fe47.42Ni26.6Si11B12.99C1.99 microwires (a) and annealed at 410 C for different time:16 min (b), 128 min (c), 256 min (d) and 480 min (e).
appreciated from Fig. 4 as-prepared microwire presents relatively high GMI ratio with optimum frequency range about 400e700 MHz where maximum GMI ratio, DZ/Zm z 45% is achieved. Usually GMI ratio of Fe-rich amorphous microwires with rectangular hysteresis is below 10%, i.e. maximum GMI ratio, DZ/Zm, of as-prepared (amorphous) Fe70.8Cu1Nb3.1Si14.5B10.6 microwires is about 4% (Fig. 5b). After short-time annealing we observed slight increasing of GMI ratio that must be attributed to magnetic softening of
Fig. 4. DZ/Z(H) dependence of as prepared Fe47.42Ni26.6Si11B12.99C1.99 microwires measured at different frequencies (a) and frequency dependence of maximum GMI ratio DZ/Zm (b) of the same microwire.
Fe47.42Ni26.6Si11B12.99C1.99 microwires annealed for short time. With further increasing of tann a noticeable decreasing of GMI ratio has been observed, although for tann z 256 min (that correlates with decreasing of ls observed in Fig. 6a) DZ/Zm z 22% is observed (Fig. 5a). This relatively high GMI might be attributed to low magnetostriction values obtained after partial crystallization of Fe47.42Ni26.6Si11B12.99C1.99 microwires. In Fig. 6a we present the evolution of the magnetostriction coefficient, ls, after annealing at Tann ¼ 410 C. As can be appreciated from Fig. 6a first we observed some ls increasing (up to 28 106) and then monotonic decreasing of ls with tann. The magnetostriction coefficient, ls, depends on the chemical composition of amorphous metallic alloy as well as on stresses (as we discussed above). On the other hand earlier has been demonstrated that the magnetostriction coefficient is affected by the stresses [30]. Stress dependence of the magnetostriction has been expressed as:
ls;s ¼ ls;0 Bs Fig. 3. XRD microwires.
patterns of
as-prepared
and
annealed
Fe47.42Ni26.6Si11B12.99C1.99
(5)
where ls,s is the magnetostriction coefficient under stress, ls,0 is the zero-stress magnetostriction coefficient and B is a positive
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Fig. 6. Dependences of the magnetostriction coefficient, maximum GMI ratio and coercivity of Fe47.42Ni26.6Si11B12.99C1.99 on annealing time, tann (Tann 410 C).
ls;eff ¼ Fig. 5. DZ/Z(H) dependence of as prepared and annealed at 410 C Fe47.42Ni26.6Si11B12.99C1.99 microwires (a) and Fe70.8Cu1Nb3.1Si14.5B10.6 amorphous microwires (b) measured at 600 MHz.
coefficient of order 1010 MPa and se stresses [30]. Considering two factors affecting the magnetoelastic energy given by eq. (1) we can assume that we must consider two opposite effects of internal stresses. The first contribution is increasing of the total magnetoelastic energy (as usually discussed elsewhere, see Ref. [29]). The second one must be related to the stress dependence of the magnetostriction coefficient described by eq. (1). Therefore under stress relaxation some increasing of the magnetostriction coefficient can be expected. This consideration can explain ls increasing observed after shorttime annealing duration (tann 16 min) Fe47.42Ni26.6Si11B12.99C1.99 microwires. Observed decreasing of the magnetostriction coefficient after annealing of Fe47.42Ni26.6Si11B12.99C1.99 microwire composition that achieves nearly-zero values at certain annealing conditions can provide a new approach in development of novel magnetically soft microwires. After further increasing of tann we observed ls decreasing with increasing of annealing time that correlates with beginning of the crystallization and precipitation of (Fe,Ni) (Figs. 3 and 6). Similarly to nanocrystalline materials, average magnetostriction coefficient can take nearly-zero values owing to the different values and sign of the magnetostriction coefficients of residual amorphous and precipitating crystalline phases, i.e.:
X
Vcri ls;cri þ ð1 Vcr Þls;am
(6)
being, ls,eff the magnetostriction coefficient and Vcri the crystalline volume fraction Vcri and ls,cri are the magnetostriction coefficients and crystalline volume fractions of crystalline phases [31e35]. For comparison we plotted dependences of ls, Hc and maximum GMI ratio, DZ/Zm on tann (Fig. 6b,c). Decreasing of DZ/Zm for longer annealing duration (i.e. at tann ¼ 480 min) can be related to the average grain size growth and magnetic hardening (coercivity growth increasing tann) (see Fig. 6c). Consequently magnetocrystalline anisotropy can play more important role. On the other hand increasing of the coercivity observed simultaneously with nanocrystallization and magnetostriction decreasing seems to indicate some kind of deviation from the random anisotropy model proposed by Herzer for the nanocrystalline Finemet material [32,33]. In according to such model, an enhanced magnetic softness correlates with the grain size and the best magnetic softness achieved under conditions of averaging grain size, typically of 10e15 nm, is much below than the exchange correlation length, being around 35e40 nm. These differences could be ascribed to the strength and complexity of the internal stresses acting on the metallic nucleus due to the glass coating. Therefore further studies of correlation of magnetic properties and miscrostructure of Fe47.42Ni26.6Si11B12.99C1.99 microwire are needed. Another reason to explain the relationship between the grain size and the coercivity obtained for these thermally treated microwires could be assigned to the difference of the content and volume fraction of the precipitating crystalline phase. As recently discussed, formation of silicides can deteriorate magnetic softness of FeeNi based alloys [18]. As mentioned above studied amorphous and partially
A. Zhukov et al. / Journal of Alloys and Compounds 651 (2015) 718e723
crystalline Fe47.42Ni26.6Si11B12.99C1.99 microwires presenting considerable GMI effect and soft magnetic properties have Invarlike composition with about 40%Ni-60%Fe content. After annealing of this microwire we obtained mixed structure consisting of amorphous phase and bcc (Fe, Ni) nanocrystallites (Space group Im-3m). Although amorphous Co-rich microwires usually present higher GMI effect and better soft magnetic properties, substitution of Corich amorphous microwires by FeeNi-rich less expensive microwires can be essentially important from application's point of view. Observed correlations of evolution of the magnetostriction coefficient, magnetic properties and GMI confirm the importance of control of the magnetostriction coefficient in the case of glasscoated microwires. 4. Conclusions We present the results on comparative studies of soft magnetic properties, GMI effect and magnetostriction coefficient of Fe47.42Ni26.6Si11B12.99C1.99 microwires. As-prepared and annealed Fe47.42Ni26.6Si11B12.99C1.99 microwires present rectangular hysteresis loops. We found that the hysteresis loop as well as the magnetostriction coefficient of Fe47.42Ni26.6Si11B12.99C1.99 microwires depend on internal stresses and can be tailored by annealing. Varying the time and the temperature of annealing we observed changes of the hysteresis loops. In as-prepared and annealed for short time Fe47.42Ni26.6Si11B12.99C1.99 microwires we observed considerable GMI effect. We observed correlation of the magnetostriction coefficient, magnetic properties and crystallization process. Decreasing of the magnetostriction coefficient observed after annealing is related to formation of nanocrystalline structure. Acknowledgments This work was supported by Spanish MINECO under MAT201347231-C2-1-P, by the Basque Government under Saiotek 13 PROMAGMI (S-PE13UN014) and DURADMAG (S-PE13UN007) projects and by the Russian Ministry of Education and Science under the MISIS Grant K3-2015-023. Technical and human support provided by SGIker (UPV/EHU, MICINN, GV/EJ, ERDF and ESF) is gratefully acknowledged. References [1] D.C. Jiles, Recent advances and future directions in magnetic materials, Acta Mater. 51 (2003) 5907e5939. [2] K.J. Sixtus, L. Tonks, Propagation of large Barkhausen discontinuities. II, Phys. Rev. 42 (1932) 419e435. [3] M. Vazquez, H. Chiriac, A. Zhukov, L. Panina, T. Uchiyama, On the state-of-theart in magnetic microwires and expected trends for scientific and technological studies, Phys. Status Solidi (a) 208 (2011) 493e501. [4] L.V. Panina, K. Mohri, Magneto-impedance effect in amorphous wires, Appl. Phys. Lett. 65 (1994) 1189e1191. [5] R. Beach, A. Berkowitz, Giant magnetic field dependent impedance of amorphous FeCoSiB wire, Appl. Phys. Lett. 64 (1994) 3652e3654. zquez, G. Basheed, G. Infante, R.P. Del Real, Trapping and injecting single [6] M. Va domainwalls in magnetic wire by local fields, Phys. Rev. Lett. 108 (2012) 037201. [7] P.A. Ekstrom, A. Zhukov, Spatial structure of the head-to-head propagating domain wall in glass-covered FeSiB microwire, J. Phys. D. Appl. Phys. 43 (2010) 205001. [8] Y. Honkura, Development of amorphous wire type MI sensors for automobile use, J. Magn. Magn. Mater. 249 (2002) 375e381. [9] K. Mohri, T. Uchiyama, L.P. Shen, C.M. Cai, L.V. Panina, Amorphous wire and CMOS IC-based sensitive micro-magnetic sensors (MI sensor and SI sensor) for intelligent measurements and controls, J. Magn. Magn. Mater. 249 (2002) 351e356.
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Effect of annealing on magnetic properties and magnetostriction coefficient of FeeNi-based amorphous microwires A. Zhukov a, b, c, *, M. Churyukanova d, S. Kaloshkin d, V. Semenkova d, S. Gudoshnikov d, M. Ipatov a, b, A. Talaat a, b, J.M. Blanco b, V. Zhukova a, b a
Dept. Phys. of Materials, Basque Country University, UPV/EHU, San Sebastian, Spain Dpto. de Física Aplicada, EUPDS, UPV/EHU, 20018 San Sebastian, Spain IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain d National University of Science and Technology «MISIS», Moscow 119049, Russia b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 August 2015 Received in revised form 15 August 2015 Accepted 18 August 2015 Available online 28 August 2015
We studied the correlation between the magnetostriction coefficient, coercivity and Giant magnetoimpedance effect of FeeNi-rich microwires. As-prepared and annealed Fe47.42Ni26.6Si11B12.99C1.99 microwires present rectangular hysteresis loops. Varying the time and the temperature of annealing we observed changes of the coercivity and Giant magnetoimpedance effect (GMI). Increasing the annealing time we observed first some magnetic softening and then magnetic hardening. These changes correlate with the evolution of the magnetostriction coefficient. Decreasing of the magnetostriction coefficient observed after annealing is related to formation of nanocrystalline structure. © 2015 Elsevier B.V. All rights reserved.
Keywords: Magnetic glass-coated microwires Giant magnetoimpedance effect Soft magnetic properties TayloreUlitvosky technique Magnetostriction Heat treatment
1. Introduction Soft-magnetic wires present a number of excellent magnetic properties interesting from the viewpoint of basic physics and suitable for various technological applications. Therefore studies of magnetically soft wires attract attention along many years [1e3]. In the case of conventional crystalline soft magnetic wires these properties are limited by the magneto crystalline anisotropy and defects typical for crystalline materials. Therefore starting from 1980-th growing interest in amorphous soft magnetic wires is related to their liquid-like structure characterized by the absence of long range ordering and of magneto-crystalline anisotropy [1,3]. Among unusual magnetic properties of amorphous wires magnetic bistability and giant magneto-impedance, GMI, effect are the most promising for technical applications [1,3e7]. These properties have been proposed for various magnetic sensor applications [8,9]. For most of modern applications the sensor size is an important
issue. Therefore recently certain tendency in development of thin magnetically soft wires gained growing interest [3,6,7]. One of the main advantages of the so-called TayloreUlitovsky is that it allows preparation of long (up to few km long continuous microwire), homogeneous thin composite wires glass-coated metallic microwires with thinnest (from 0.2 up to 100 mm) diameters [3,6,7,10]. These materials consist of metallic nucleus with diameter from 0.2 up to 70 mm and glass coating sheath with diameter from 2 to 20 mm. Most of published papers on amorphous microwires have been reported for FeeCo based glass-coated microwires with different magnetostriction constants [3,7,8]. The main reason is that in the absence of magneto-crystalline anisotropy a magnetoelastic anisotropy is the main factor determining the magnetic properties of amorphous glass-coated microwires. This magnetoelastic anisotropy, Kme, is determined by internal stresses, and magnetostriction coefficient, which is given by the equation [3,7];
Kme z3=2ls si ; * Corresponding author. Dept. Phys. of Materials, Basque Country University, UPV/EHU, San Sebastian, Spain. E-mail address: [email protected] (A. Zhukov). http://dx.doi.org/10.1016/j.jallcom.2015.08.151 0925-8388/© 2015 Elsevier B.V. All rights reserved.
(1)
where ls is the saturation magnetostriction and si is the internal stress induced by the glass coating layer during the fabrication
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technique of glass-coated microwires. The magnetostriction constant depends mostly on the chemical composition, which is vanishing in amorphous FeeCo based alloys with Co/Fe z 70/5 [11,12]. Therefore changing the chemical composition in the classical (CoxFe1x)75Si15B10 system one can control the magnetostriction constant, ls, that changes with x from 5 x 106 at x ¼ 1, to ls z 35 106 at x z 0.2 [11,12]. On the other hand FeeNi compositions are considered the most promising for crystalline soft magnets allowing obtaining high magnetic permeability. Thus permalloy-type NieFe alloys present excellent soft magnetic properties (after special heat treatment) exhibiting maximum magnetic permeability for a composition range close to 80Ni% (the classical Permalloy) [1]. The other FeeNi potentially interesting magnetic alloy is the Invar containing about 36% Nie 64% Fe which has the lowest thermal expansion coefficient among all metals and alloys in the range from room temperature up to approximately 230 C [13]. In fact all the Fe-rich face-centered cubic FeeNi alloys show Invar anomalies in their measured thermal and magnetic properties that evolve continuously in intensity with varying alloy composition [13e15]. The Invar's behavior is discussed considering either a high-magnetic-moment to low-magnetic-moment transition occurring in the face centered cubic FeeNi series or a highmagnetic-moment frustrated ferromagnetic state in which the FeeFe magnetic exchange bonds have a large magneto-volume effect of the right sign and magnitude to create the observed thermal expansion anomaly [14,15]. The main problems related to crystalline FeNi soft magnetic alloy are the right (very well defined) content of the main chemical components, the influence of the minor elements (the chemical purity) and the non-metallic inclusion, precipitated particles etc (physical purity) and rather restricted conditions the heat treatment. Consequently FeeNi system is quite interesting form the point of view of improvement of magnetic softness of magnetic materials. Previously few attempts to prepare and study magnetic properties of FeeNi-based wires have been reported [16e18]. The problem is that highly Ni-containing amorphous alloys have low saturation magnetization or even no ferromagnetic order at room temperature [18]. The reasons for studies of FeeNi-rich microwires are that in most of the cases the additional annealing for improvement of magnetic softness is not needed and as-prepared amorphous materials usually present high magnetic softness. Moreover substitution of Co by less expensive FeeNi is beneficial from the viewpoint of applications. Recently we reported on successful preparation Fe77.5xNixSi7.5B15 (0 x 38.75) microwires and observation or rectangular hysteresis loops and fast DW propagation for the Fe77.5xNixSi7.5B15 (0 x 38.75). For Fe62Ni15.5Si7.5B15 samples we observed DW velocities up to 2.5 km/s [19]. On the other hand annealing is the effective tool for engineering of magnetic properties of glass-coated microwires. Recently we observed considerable dependence of hysteresis loops character of Co-rich microwires on annealing conditions attributed to the stress relaxation and effect of annealing on the magnetostriction coefficient. Annealed Co-rich microwires with acquired magnetic bistability present unusual behavior, i.e. in the same sample fast domain wall propagation and GMI effect have been observed. Previously it was considered that the magnetic permeability of microwires with rectangular hysteresis loop is rather low and they do not present remarkable GMI effect. Consequently measurements of the magnetostriction coefficient and understanding the mechanisms of the influence of the internal and applied stresses on the magnetostriction coefficient is essentially important for tailoring the magnetic properties of amorphous
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ferromagnetic microwires. The most appropriate method for evaluation of the magnetostriction coefficient of amorphous materials is so-called small angle magnetization rotation (SAMR) method introduced in 1980-th [20,21]. Initially this method has been developed only for amorphous materials where the magnetization rotation presents the determining role. Therefore initially this method has been employed for evaluation of the magnetostriction coefficient of Cobased glass-coated microwires with negative magnetostriction coefficient [22,23]. Afterward we showed that the SAMR method can be extended for the case of microwires with positive magnetostriction coefficient presenting important contribution of domain wall propagation [24,25]. As has been demonstrated experimentally domain structure of wires and microwires with positive magnetostriction coefficient is rather different from wires and microwires with negative magnetostriction coefficient [26]: wires with positive magnetostriction present 180 surface domain structure with magnetization perpendicular to the wire surface, while the surface domain structure of wires with negative magnetostriction coefficient consists of rather big (as-compared with wires with negative magnetostriction coefficient) circular 180 domains. The magnetic permeability corresponding to the magnetization rotation of the outer domain shell of microwires with positive magnetostriction coefficient is rather low. Therefore the accuracy of the SAMR method for the magnetostriction coefficient evaluation employed for the microwires with positive magnetostriction coefficient is lower. Recently novel experimental set-up with improved resolution has been successfully realized for measurements of the magnetostriction coefficient, ls, of any metallic nucleus composition (either with positive magnetostriction coefficient or with negative magnetostriction coefficient) microwires [27,28]. Consequently, the purpose of this paper is to present new experimental results on effect of annealing on magnetic properties and GMI effect of FeeNi based glass-coated microwires and on its correlation with the magnetostriction coefficient. 2. Experimental technique We studied Fe47.42Ni26.6Si11B12.99C1.99 glass-coated microwires (metallic nucleus diameter, d ¼ 29 mm, total diameter, D ¼ 32.2 mm) produced by TayloreUlitovsky technique [3,7,10,11]. The magnetostriction coefficient of studied microwires has been measured using aforementioned SAMR method described elsewhere [27,28]. In this method the sample is saturated by an axial magnetic field, Hz, while applying simultaneously a small ac transverse field, Hy, created by an ac electric current flowing along the sample. The combination of these fields leads to a reversible rotation of the magnetization within a small angle, q, out of the axial direction. The induction voltage, V(2u), due to the magnetization rotation is detected by a pick-up coil wound around the microwire. The magnetostriction constant is determined from the measurement of the dependence on axial magnetic field, Hz, versus on applied stress s at fixed value of induction voltage V(2u). The moMs values of the investigated microwires obtained from room temperature measurements of the magnetization curves at high magnetic field. The AC current value, i~, flowing through the wire is selected to avoid possible Joule heating of the sample: the current amplitude does not exceed 10e30 mA. As has been shown [20e24] under conditions q ¼ const and Ht ¼ const the value of magnetostriction coefficient, ls, is given by
ls ¼ ðm0 Ms =3ÞðdH=dsÞ; where m0 Ms is the saturation magnetization.
(2)
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We measured the magnetostriction coefficient several times and then found the average value. As mentioned above the SAMR method can be extended to microwires either with positive or negative magnetostriction coefficient. The condition for the SAMR method is the existence of the outer shell, where magnetization is perpendicular to the external field. For Co-rich microwires circular orientation of the magnetization is assumed from experimental observation of the magnetic domains structure. For Fe-rich microwires radial magnetization orientation was experimentally observed [26]. Hysteresis loops have been measured either by flux-metric method (low field measurements up to 8 kA/m) described elsewhere [28] or using PPMS device. We represent the normalized magnetization, M/Ms versus magnetic field, H, where M is the magnetic moment at given magnetic field and Ms is the magnetic moment of the sample at the maximum magnetic field amplitude, Ho. We measured magnetic field dependences of impedance, Z, and GMI ratio, DZ/Z using specially designed micro-strip sample holder placed inside a sufficiently long solenoid that creates a homogeneous magnetic field, H. There one wire end was connected to the inner conductor of a coaxial line through a matched microstrip line while the other was connected to the ground plane. This sample holder allows measuring the samples of 6 mm length. This sample length is sufficiently long allowing neglecting the effect of the demagnetizing factor [29]. The magnetoimpedance ratio, DZ/Z, has been defined as:
DZ=Z ¼ ½ZðHÞ ZðHmax Þ$100=ZðHmax Þ;
(3)
An axial DC-field with maximum value, Hmax, up to 20 kA/m was supplied by a magnetization coils. Structure and phase composition have been checked using a BRUKER (D8 Advance) X-ray diffractometer with Cu Ka (l ¼ 1.54 Å) radiation.
Fig. 1. Hysteresis loops (a) and experimentally measured DHz(Ds) dependence (b) measured in as-prepared Fe47.42Ni26.6Si11B12.99C1.99 microwires.
3. Experimental details and results Hysteresis loops of Fe47.42Ni26.6Si11B12.99C1.99 microwires present rectangular character typical for glass-coated microwires with positive magnetostriction constant (see Fig. 1a). Using described above SAMR method we evaluated the saturation magnetostriction values ls from the experimental dependences [DHz/Ds]V(2u)-const using eq. (2). From experimentally observed dependences of change of DHz on applied stress, Ds, for Fe47.42Ni26.6Si11B12.99C1.99 microwire (shown in Fig. 1b) we obtained ls z 24 106. As can be appreciated from Fig. 2, annealing considerably affects the coercivity of studied samples. After annealing we observed considerable changes of magnetic properties. As-prepared and annealed samples present rectangular hysteresis loops, but coercivity, Hc, considerably changes: for fixed annealing temperature, Tann ¼ 410 C, after short-time annealing coercivity decreases. But after further increasing of tann considerable rising of coercivity, Hc, is observed. As discussed elsewhere [29], the magnetoelastic energy, Kme, plays the determining role in the formation of magnetic properties of amorphous microwires. Kme depends on the magnetiostriction coefficient, ls, as well as on the internal stresses, s ¼ sappl þ si, (see eq. (1)). We studied the effect of annealing on structure of studied sample using X-ray diffraction. As can be observed from Fig. 3 after annealing partial crystallization of Fe47.42Ni26.6Si11B12.99C1.99 microwires from amorphous precursor has been observed. Considering the main crystalline peak, by estimating the width
of the peak ε as well as the crystallization angle 2q, then by substituting these parameters on the DeybeeScherrer equation (eq. (4)), one can obtain the value of the grain size (Dg).
Dg ¼ kl=εcos2Q
(4)
where, ε is the half height width of the crystalline peak and 2q is the angular position of the maximum crystalline peak. The diffraction pattern is the superposition of the total peak area consisting of crystalline and amorphous peaks together. Therefore the background that corresponds to the amorphous halo was subtracted. It must be noted that the crystalline structure of the well crystallized samples becomes complex and detailed analysis of the crystallization process require more careful attention. From the very basic analysis of the XRD patterns we can assume that as prepared samples present amorphous structure. After annealing at 435 C for 60 min the sample still presents mixed structure consisting of amorphous phase, bcc (Fe, Ni) grains (Space group Im-3m) with average size about 8 nm and silicides. Further annealing results in increasing of the content or crystalline phases and in further grain size increasing up to 24 nm at Tann ¼ 600 C (Fig. 3). Magnetic softening for short-time annealing can be related to internal stress relaxation. The increasing of Hc for longer annealing at 410 C must be attributed to the crystallization process as observed by XRD in Fig. 3. We measured GMI ratio in as-prepared and annealed Fe47.42Ni26.6Si11B12.99C1.99 microwires (Figs. 4 and 5). As can be
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Fig. 2. Hysteresis loops measured in as-prepared Fe47.42Ni26.6Si11B12.99C1.99 microwires (a) and annealed at 410 C for different time:16 min (b), 128 min (c), 256 min (d) and 480 min (e).
appreciated from Fig. 4 as-prepared microwire presents relatively high GMI ratio with optimum frequency range about 400e700 MHz where maximum GMI ratio, DZ/Zm z 45% is achieved. Usually GMI ratio of Fe-rich amorphous microwires with rectangular hysteresis is below 10%, i.e. maximum GMI ratio, DZ/Zm, of as-prepared (amorphous) Fe70.8Cu1Nb3.1Si14.5B10.6 microwires is about 4% (Fig. 5b). After short-time annealing we observed slight increasing of GMI ratio that must be attributed to magnetic softening of
Fig. 4. DZ/Z(H) dependence of as prepared Fe47.42Ni26.6Si11B12.99C1.99 microwires measured at different frequencies (a) and frequency dependence of maximum GMI ratio DZ/Zm (b) of the same microwire.
Fe47.42Ni26.6Si11B12.99C1.99 microwires annealed for short time. With further increasing of tann a noticeable decreasing of GMI ratio has been observed, although for tann z 256 min (that correlates with decreasing of ls observed in Fig. 6a) DZ/Zm z 22% is observed (Fig. 5a). This relatively high GMI might be attributed to low magnetostriction values obtained after partial crystallization of Fe47.42Ni26.6Si11B12.99C1.99 microwires. In Fig. 6a we present the evolution of the magnetostriction coefficient, ls, after annealing at Tann ¼ 410 C. As can be appreciated from Fig. 6a first we observed some ls increasing (up to 28 106) and then monotonic decreasing of ls with tann. The magnetostriction coefficient, ls, depends on the chemical composition of amorphous metallic alloy as well as on stresses (as we discussed above). On the other hand earlier has been demonstrated that the magnetostriction coefficient is affected by the stresses [30]. Stress dependence of the magnetostriction has been expressed as:
ls;s ¼ ls;0 Bs Fig. 3. XRD microwires.
patterns of
as-prepared
and
annealed
Fe47.42Ni26.6Si11B12.99C1.99
(5)
where ls,s is the magnetostriction coefficient under stress, ls,0 is the zero-stress magnetostriction coefficient and B is a positive
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Fig. 6. Dependences of the magnetostriction coefficient, maximum GMI ratio and coercivity of Fe47.42Ni26.6Si11B12.99C1.99 on annealing time, tann (Tann 410 C).
ls;eff ¼ Fig. 5. DZ/Z(H) dependence of as prepared and annealed at 410 C Fe47.42Ni26.6Si11B12.99C1.99 microwires (a) and Fe70.8Cu1Nb3.1Si14.5B10.6 amorphous microwires (b) measured at 600 MHz.
coefficient of order 1010 MPa and se stresses [30]. Considering two factors affecting the magnetoelastic energy given by eq. (1) we can assume that we must consider two opposite effects of internal stresses. The first contribution is increasing of the total magnetoelastic energy (as usually discussed elsewhere, see Ref. [29]). The second one must be related to the stress dependence of the magnetostriction coefficient described by eq. (1). Therefore under stress relaxation some increasing of the magnetostriction coefficient can be expected. This consideration can explain ls increasing observed after shorttime annealing duration (tann 16 min) Fe47.42Ni26.6Si11B12.99C1.99 microwires. Observed decreasing of the magnetostriction coefficient after annealing of Fe47.42Ni26.6Si11B12.99C1.99 microwire composition that achieves nearly-zero values at certain annealing conditions can provide a new approach in development of novel magnetically soft microwires. After further increasing of tann we observed ls decreasing with increasing of annealing time that correlates with beginning of the crystallization and precipitation of (Fe,Ni) (Figs. 3 and 6). Similarly to nanocrystalline materials, average magnetostriction coefficient can take nearly-zero values owing to the different values and sign of the magnetostriction coefficients of residual amorphous and precipitating crystalline phases, i.e.:
X
Vcri ls;cri þ ð1 Vcr Þls;am
(6)
being, ls,eff the magnetostriction coefficient and Vcri the crystalline volume fraction Vcri and ls,cri are the magnetostriction coefficients and crystalline volume fractions of crystalline phases [31e35]. For comparison we plotted dependences of ls, Hc and maximum GMI ratio, DZ/Zm on tann (Fig. 6b,c). Decreasing of DZ/Zm for longer annealing duration (i.e. at tann ¼ 480 min) can be related to the average grain size growth and magnetic hardening (coercivity growth increasing tann) (see Fig. 6c). Consequently magnetocrystalline anisotropy can play more important role. On the other hand increasing of the coercivity observed simultaneously with nanocrystallization and magnetostriction decreasing seems to indicate some kind of deviation from the random anisotropy model proposed by Herzer for the nanocrystalline Finemet material [32,33]. In according to such model, an enhanced magnetic softness correlates with the grain size and the best magnetic softness achieved under conditions of averaging grain size, typically of 10e15 nm, is much below than the exchange correlation length, being around 35e40 nm. These differences could be ascribed to the strength and complexity of the internal stresses acting on the metallic nucleus due to the glass coating. Therefore further studies of correlation of magnetic properties and miscrostructure of Fe47.42Ni26.6Si11B12.99C1.99 microwire are needed. Another reason to explain the relationship between the grain size and the coercivity obtained for these thermally treated microwires could be assigned to the difference of the content and volume fraction of the precipitating crystalline phase. As recently discussed, formation of silicides can deteriorate magnetic softness of FeeNi based alloys [18]. As mentioned above studied amorphous and partially
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crystalline Fe47.42Ni26.6Si11B12.99C1.99 microwires presenting considerable GMI effect and soft magnetic properties have Invarlike composition with about 40%Ni-60%Fe content. After annealing of this microwire we obtained mixed structure consisting of amorphous phase and bcc (Fe, Ni) nanocrystallites (Space group Im-3m). Although amorphous Co-rich microwires usually present higher GMI effect and better soft magnetic properties, substitution of Corich amorphous microwires by FeeNi-rich less expensive microwires can be essentially important from application's point of view. Observed correlations of evolution of the magnetostriction coefficient, magnetic properties and GMI confirm the importance of control of the magnetostriction coefficient in the case of glasscoated microwires. 4. Conclusions We present the results on comparative studies of soft magnetic properties, GMI effect and magnetostriction coefficient of Fe47.42Ni26.6Si11B12.99C1.99 microwires. As-prepared and annealed Fe47.42Ni26.6Si11B12.99C1.99 microwires present rectangular hysteresis loops. We found that the hysteresis loop as well as the magnetostriction coefficient of Fe47.42Ni26.6Si11B12.99C1.99 microwires depend on internal stresses and can be tailored by annealing. Varying the time and the temperature of annealing we observed changes of the hysteresis loops. In as-prepared and annealed for short time Fe47.42Ni26.6Si11B12.99C1.99 microwires we observed considerable GMI effect. We observed correlation of the magnetostriction coefficient, magnetic properties and crystallization process. Decreasing of the magnetostriction coefficient observed after annealing is related to formation of nanocrystalline structure. Acknowledgments This work was supported by Spanish MINECO under MAT201347231-C2-1-P, by the Basque Government under Saiotek 13 PROMAGMI (S-PE13UN014) and DURADMAG (S-PE13UN007) projects and by the Russian Ministry of Education and Science under the MISIS Grant K3-2015-023. Technical and human support provided by SGIker (UPV/EHU, MICINN, GV/EJ, ERDF and ESF) is gratefully acknowledged. References [1] D.C. Jiles, Recent advances and future directions in magnetic materials, Acta Mater. 51 (2003) 5907e5939. [2] K.J. Sixtus, L. Tonks, Propagation of large Barkhausen discontinuities. II, Phys. Rev. 42 (1932) 419e435. [3] M. Vazquez, H. Chiriac, A. Zhukov, L. Panina, T. Uchiyama, On the state-of-theart in magnetic microwires and expected trends for scientific and technological studies, Phys. Status Solidi (a) 208 (2011) 493e501. [4] L.V. Panina, K. Mohri, Magneto-impedance effect in amorphous wires, Appl. Phys. Lett. 65 (1994) 1189e1191. [5] R. Beach, A. Berkowitz, Giant magnetic field dependent impedance of amorphous FeCoSiB wire, Appl. Phys. Lett. 64 (1994) 3652e3654. zquez, G. Basheed, G. Infante, R.P. Del Real, Trapping and injecting single [6] M. Va domainwalls in magnetic wire by local fields, Phys. Rev. Lett. 108 (2012) 037201. [7] P.A. Ekstrom, A. Zhukov, Spatial structure of the head-to-head propagating domain wall in glass-covered FeSiB microwire, J. Phys. D. Appl. Phys. 43 (2010) 205001. [8] Y. Honkura, Development of amorphous wire type MI sensors for automobile use, J. Magn. Magn. Mater. 249 (2002) 375e381. [9] K. Mohri, T. Uchiyama, L.P. Shen, C.M. Cai, L.V. Panina, Amorphous wire and CMOS IC-based sensitive micro-magnetic sensors (MI sensor and SI sensor) for intelligent measurements and controls, J. Magn. Magn. Mater. 249 (2002) 351e356.
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