We studied GMI effect and magnetic properties of Finemet-type FeCuNbSiB microwires. We observed that GMI magnetic field and frequency dependences and magnetic softness of composite 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. GMI effect has been observed in as-prepared Fe-rich microwires with nanocrystalline structure.This work was supported by EU ERA-NET programme under project "SoMaMicSens" (MANUN ET-2010-Basque-3), by EU under FP7 "EM-safety" project, by Spanish Ministry of Science and Innovation, MICINN under Project MAT2010-18914, by the Basque Government under Saiotek 11 MICMAGN project (S-PE11UN087) and by federal target program "Scientific and scientific-pedagogical personnel of innovative Russia", state contract No. 14.18.21.0783

Blanco Aranguren, Juan María

Churyukanova, Margarita

González Legarreta, Lorena

Hernando Grande, Blanca

Ipatov, Mihail

Kaloshkin, Sergey Dmitrievich

Kostitcyna, Elena

Shuvaeva, Evgenia A.

Sudarchikova, V.

Talaat, Ahmed

Zhukov Egorova, Arkady Pavlovich

Zhukova, Valentina

English

UCrea

1246 PIERS Proceedings, Stockholm, Sweden, Aug. 12–15, 2013Giant Magnetoimpedance Effect in Nanocrystalline MicrowiresA. Talaat1, V. Zhukova1, M. Ipatov1, J. M. Blanco2, M. Churyukanova3,S. Kaloshkin3, E. Kostitcyna3, E. Shuvaeva3, V. Sudarchikova3,L. Gonzalez-Legarreta4, B. Hernando4, and A. Zhukov1, 51Dpto. Fisica de Materiales, UPV/EHU, San Sebastian 20018, Spain2Dpto. de F́ısica Aplicada, EUPDS, UPV/EHU, San Sebastian 20018, Spain3National University of Science and Technology “MISIS”, Moscow 119049, Russia4Dpto. de F́ısica, Universidad de Oviedo, Calvo Sotelo s/n, Oviedo 33007, Spain5IKERBASQUE, Basque Foundation for Science, Bilbao 48011, SpainAbstract— We studied GMI effect and magnetic properties of Finemet-type FeCuNbSiB mi-crowires. We observed that GMI magnetic field and frequency dependences and magnetic soft-ness of composite microwires produced by the Taylor-Ulitovski technique can be tailored eithercontrolling magnetoelastic anisotropy of as-prepared FeCuNbSiB microwires or controlling theirstructure by heat treatment or changing the fabrication conditions. GMI effect has been observedin as-prepared Fe-rich microwires with nanocrystalline structure.1. INTRODUCTIONStudies of glass coated ferromagnetic microwires (typically of 5–30µm in diameter) have attractedgrowing attention in the last few years owing to their outstanding soft magnetic properties (magneticbistability, enhanced magnetic softness, GMI effect, fast domain wall propagation) and possibility toobtain glass-coated microwires with different structure (amorphous, nanocrystalline, granular) [1, 2].Particularly, recent studies have demonstrated that optimization of soft magnetic properties andGMI effect of amorphous glass coated microwires is possible choosing the appropriate chemicalcomposition of metallic nucleus and adequate annealing conditions [1].In some cases, nanocrystallization allows achieving good magnetic softness and enhanced GMIeffect in ferromagnetic microwires. Such soft magnetic character is usually attributed to vanish-ing magnetocrystalline anisotropy and the very small magnetostriction value when the grain sizeapproaches 10 nm [3, 4]. Like for conventional nanocrystalline materials, average anisotropy forrandomly oriented α-Fe(Si) grains is negligibly small when the average grain is about 10–20 nm.Consequently, low values of coercivity in the nanocrystalline microwires were ascribed to smalleffective magnetic anisotropy. In addition to the suppressed magnetocrystalline anisotropy, lowmagnetostriction values provide the basis for the superior soft magnetic properties observed in par-ticular compositions [3, 4]. Low values of the magnetostriction are essential to avoid magnetoelasticanisotropies arising from internal or external mechanical stresses.Generally magnetic properties and overall shape of hysteresis loops of amorphous ferromagneticmicrowires depend on the composition of the metallic nucleus as well as on the composition andthickness of the glass coating. As discovered before, shape of hysteresis loops changes from rectan-gular, typical for amorphous Fe-rich compositions, to inclined, typical for Co-rich compositions [5].Amorphous microwires with vanishing magnetostriction exhibit quite soft magnetic properties.Such strong dependence of the hysteresis loops on these parameters should be attributed to themagnetoelastic energy given by:Kme ≈ 3/2λsσi, (1)where λs is the saturation magnetostriction and σi is the internal stress. The magnetostrictionconstant depends mostly on the chemical composition and is vanishing in amorphous Fe-Co basedalloys with Co/Fe ≈ 70/5 [5–7]. One of the peculiarities of the fabrication technique of glass-coatedmicrowires is that it involves the simultaneous solidification of composite microwire consisting offerromagnetic nucleus surrounded by glass coating. Quite different thermal expansion coefficientsof the glass and the metallic alloys introduce considerable internal stresses inside the ferromagneticnucleus during simultaneous fast solidification of the composite microwire [1, 6, 8, 9]. The estimatedvalues of the internal stresses in these glass coated microwires arising from the difference in the ther-mal expansion coefficients of metallic nucleus and glass coating are of the order of 100–1000 MPa,depending strongly on the ratio between the glass coating thickness and metallic core diameter [8–11], increasing with the glass coating thickness. Such large internal stresses give rise to a drasticReproduced courtesy of The Electromagnetics AcademyProgress In Electromagnetics Research Symposium Proceedings, Stockholm, Sweden, Aug. 12-15, 2013 1247change of the magnetoelastic energy, Kme, even for small changes of the glass-coating thickness atfixed metallic core diameter. Additionally, such a change of the ρ-ratio should be related to thechange of the magnetostriction constant with applied stress [6, 12]:λs = (µoMs/3)(dHk/dσ), (2)where µoMs is the saturation magnetization.It is worth mentioning, that residual stresses of glass-coated microwires arising during simul-taneous solidification of metallic nucleus and glass coating, mostly have been estimated from thesimulations of the process of simultaneous solidification of metallic nucleus inside the glass tube [6–8]and experimental determination of such residual stresses is rather complex. One of the experimen-tal evidence of existence of such stresses is the dependence of hysteresis loops and particularlymagnetic properties (coercivity, remanent magnetization) on ρ-ratio [6, 8].Consequently, tailoring of the magnetoelastic energy, Kme, is essentially important for optimiza-tion of magnetic properties of glass-coated microwires [1, 6, 8].Accordingly, any method allowing estimation of internal stresses in glass-coated microwires isquite suitable for soft magnetic properties optimization.In the case of glass-coated microwires, existence of the outer glass-coating with physical prop-erties completely different from metallic nucleus alloys (different thermal conductivity, thermalexpansion coefficients . . . ) in some cases drastically affects structure and magnetic properties ofglass-coated microwires [1]. Thus, crystalline structure, crystallization temperature and magneticproperties of nanocrystalline microwires are rather different from nanocrystalline ribbons of thesame composition [1–3].In this paper, we studied the effect of the nanocrystallization on magnetic properties, structureand GMI of Finemet-type amorphous and nanocrystalline microwires.2. EXPERIMENTALWe studied Finemet-type Fe70.8Cu1Nb3.1Si14.5B10.6, Fe71.8Cu1Nb3.1Si15B9.1, Fe73.8Cu1Nb3.1Si13B9.1and Fe70.8Cu1Nb3.1Si16B9.1 glass-coated microwires with different metallic nucleus diameter, d, andtotal microwire diameter, D, were produced by modified Taylor-Ulitovsky method [1–3]. It isworth mentioning, that the strength of internal stresses is determined by ratio ρ [6, 9]. Therefore,controllable change of the ρ-ratio allowed us to control the residual stresses.Hysteresis loops have been determined by flux-metric method, as described elsewhere [1, 2]. Wemeasured magnetic field dependences of impedance, Z, and GMI ratio, ∆Z/Z, for as-preparedsamples and after heat treatments. We used specially designed micro-strip sample holder. Thesample holder was placed inside a sufficiently long solenoid that creates a homogeneous magneticfield, H. The sample impedance Z was measured using vector network analyzer from reflectioncoefficient S11. The DC bias current IB was applied to the sample through a bias-tee element.All experimental graphs show both ascending and descending branches of the field dependenciesof the real part of impedance Z so that the magnetic hysteresis can be evaluated. More details onexperimental technique can be found in Ref. [1].Structure and phase composition have been checked using a BRUKER (D8 Advance) X-raydiffractometer with Cu Kα (λ = 1.54 Å) radiation.3. RESULTS AND DISCUSSIONAll as-prepared and even annealed Finemet-type microwires at annealing temperature, Tann, be-low first crystallization process, present squared hysteresis loops similar to Fe-rich amorphous mi-crowires (Fig. 1).As expected from previous studies of Fe-rich amorphous microwires [1], coercivity, HC , of as-prepared Finemet-type microwires depends on ratio ρ = d/D (Fig. 2). Annealing temperaturedependence of coercivity of Fe71.8Cu1Nb3.1Si15B9.1 microwires shown in Fig. 3 present considerablemagnetic softening at annealing temperatures, Tann, between 800 and 900 K as previously observedin other Finemet-type materials and Finemet-type microwires [1, 13]. Consequently, although GMIeffect in as-prepared Fe-rich microwires is rather small, after annealing we observed increasing ofthe GMI effect (Figs. 4(a) and 4(b)). Enhancement of the ∆Z/Z ratio is related with magneticsoftening of studied microwires after annealing and internal stress relaxation. Indeed applied andinternal stresses considerably affect GMI effect [12]. Generally, measurements of the GMI effectin nanocrystalline microwires involve preparation of the electrical connections of rather brittlenanocrystalline samples.Reproduced courtesy of The Electromagnetics Academy1248 PIERS Proceedings, Stockholm, Sweden, Aug. 12–15, 2013-100 0 100-1.0-0.50. 00. 51. 0Reduced magnetizationMagnetic field (A/m)1- as-cast2- Tann=4000Cρ=0.79(a)12-200 -100 0 100 200-1.0-0.50.00.51.0Reduced magnetization1 - as-cast2 - Tann=4000CMagnetic field (A/m)ρ=0.38(b)12Figure 1: Hysteresis loops of as-prepared and after the heat treatment Fe70.8Cu1Nb3.1Si14.5B10.6 microwireswith different ρ = d/D ratios: (a) ρ = 0.79, (b) ρ = 0.38.0.4 0.5 0.6 0.7 0.80306090120150Coercivity (A/m)ρ−ratio as-cast Tann= 4000CFigure 2: Coercivity dependence on ρ-ratio for as-prepared Fe70.8Cu1Nb3.1Si14.5B10.6 microwires.200 300 400 500 600 700 800 900 1000 11005001000150020002500ρ=0.359ρ=0.871Coercivity ( A/m)Temperature ann (K)Figure 3: Annealing temperature dependence of co-ercivity of Fe71.8Cu1Nb3.1Si15B9.1 microwires withdifferent ρ-ratios.-16000 -8000 0 8000 160000481216GMI ratio (%)Magnetic field (A/m)f30MHzf100MHzf200MHzf600MHzTann=400ºC, 1h(a)-16000 -8000 0 8000 160000481216Magnetic field (A/m)1  as-cast2 annealed  400oC 1hf=600MHz(b)12GMI ratio (%)Figure 4: (a) ∆Z/Z(H) dependences of Fe73.8Cu1Nb3.1Si13B9.1 amorphous microwires annealed at 400◦Cmeasured at different frequencies and (b) ∆Z/Z(H) dependences of Fe73.8Cu1Nb3.1Si13B9.1 amorphous mi-crowires measured in as-prepared and annealed at 400C samples at 600 MHz.X-ray studies show, that as-prepared Fe73.8Cu1Nb3.1Si13B9.1 microwire exhibit nanocrystallinestructure. The structure consists of of α-Fe nanocrystallites with average grain size about 12 nm andamorphous matrix (Fig. 5(a)). The grain size has been estimated the average grain size can be esti-mated from the width of the crystalline peak using the Debye-Scherrer equation. For comparison X-ray diffraction pattern of completely amorphous as-prepared Fe70.8Cu1Nb3.1Si14.5B10.6 microwire isshown in Fgi. 5(b). In as-prepared Fe73.8Cu1Nb3.1Si13B9.1 microwires we observed considerable GMIeffect (up to ∆Z/Z ≈ 30% at f = 100MHz, see Fig. 5(c)). Observed considerable GMI effect in as-prepared Fe73.8Cu1Nb3.1Si13B9.1 microwires is much higher that of Fe70.8Cu1Nb3.1Si14.5B10.6. Thisdifference must be attributed to amorphous structure of Fe70.8Cu1Nb3.1Si14.5B10.6 and nanocrys-talline structure of Fe73.8Cu1Nb3.1Si13B9.1 microwires.Reproduced courtesy of The Electromagnetics AcademyProgress In Electromagnetics Research Symposium Proceedings, Stockholm, Sweden, Aug. 12-15, 2013 124940 60 8001020304050 2θ (deg)I (arb. units)(a)40 60 8001020304050 2θ (deg)I (arb. units)(b)-10000- 5000 0 5000 100000102030GMI ratio (%)Magnetic field (A/m)  100 MHz (c)-18000 -9000 0 9000 18000024GMI ratio (%)Magnetic field (A/m)  100 MHz (d)Figure 5: (a) X-ray diffraction patterns of as-prepared Fe73.8Cu1Nb3.1Si13B9.1 (ρ = 0.6) and(b) Fe70.8Cu1Nb3.1Si14.5B10.6 (ρ = 0, 87) microwires and (c), (d) GMI effect in the same samples.4. CONCLUSIONSThe study of magnetic and thermal properties of nano-scaled Finemet-type FeCuNbSiB and Co-Cuglass-coated microwires reveals that increase of stresses in the amorphous metallic nucleus consid-erably affect magnetic properties of these materials. In Finemet-type FeCuNbSiB the reductionof the ρ-ratio, results in the rise of coercivity. We observed magnetic softening and considerableGMI effect in Finemet-type FeCuNbSiB with nanocrystalline structure. Magnetoelastic anisotropyaffects soft magnetic properties and GMI effect of FeCuNbSiB micrfowires.ACKNOWLEDGMENTThis work was supported by EU ERA-NET programme under project “SoMaMicSens” (MANUNET-2010-Basque-3), by EU under FP7 “EM-safety” project, by Spanish Ministry of Science andInnovation, MICINN under Project MAT2010-18914, by the Basque Government under Saiotek11 MICMAGN project (S-PE11UN087) and by federal target program “Scientific and scientific-pedagogical personnel of innovative Russia”, state contract No. 14.18.21.0783.REFERENCES1. Zhukova, V., M. Ipatov, and A. Zhukov, Sensors, Vol. 9, 9216–9240, 2009.2. Jiles, D. C., Acta Materialia, Vol. 51, 5907–5939, 2003.3. Arcas, J., C. Gómez-Polo, A. Zhukov, M. Vázquez, V. Larin, and A. Hernando, NanostructuredMaterials, Vol. 7, No. 8, 823–834, 1996.4. Dudek, C., A. L. Adenot-Engelvin, F. Bertin, and O. Acher, J. Non-Cryst. Solids, Vol. 353,925–927, 2007.5. Zhukov, A. and V. Zhukova, “Magnetic properties and applications of ferromagnetic microwireswith amorphous and nanocrystalline structure,” 162, Nova Science Publishers, Inc. 400 OserAvenue, Suite 1600 Hauppauge, NY 11788, 2009.6. Zhukov, A., V. Zhukova, J. M. Blanco, A. F. Cobeño, M. Vazquez, and J. Gonzalez, J. Magn.Magn. Mater., Vols. 258–259, 151–157, 2003.7. Konno, Y. and K. Mohri, IEEE Trans. Magn., Vol. 25, No. 5, 3623–3625, 1989.8. Chiriac, H., T.-A. Ovari, and A. Zhukov, J. Magn. Magn. Mater., Vols. 254–255, 469–471,2003.9. Velázquez, J., M. Vazquez, and A. Zhukov, J. Mater. Res., Vol. 11, 2499–2505, 1996.10. Chiriac, H., T.-A. Ovari, and G. Pop, Phys. Rev. B, Vol. 52, 10104–10113, 1995.11. Antonov, A. S., V. T. Borisov, O. V. Borisov, A. F. Prokoshin, and N. A. Usov, J Phys D:Appl. Phys., Vol. 33, 1161–1168, 2000.12. Blanco, J. M., A. Zhukov, and J. Gonzalez, J. Appl. Phys., Vol. 87, No. 9, 4813–4815, 2000.13. Herzer, G., J. Magn. Magn. Mat., Vol. 294, 99–106, 2005.Reproduced courtesy of The Electromagnetics Academy

Giant magnetoimpedance effect in nanocrystalline microwires

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Giant magnetoimpedance effect in nanocrystalline microwires

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