Fabrication, structural and magnetic characterization of thin microwires with novel composition Cu70(Co70Fe5Si10B15)30

Journal of Alloys and Compounds 483 (2009) 566–569

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Fabrication, structural and magnetic characterization of thin microwires with novel composition Cu70 (Co70 Fe5 Si10 B15 )30 C. García a , A. Zhukov a,b , J. González a , J.J. del Val a , J.M. Blanco c , M. Knobel d , V. Zhukova a,b,∗ a

Departamento de Física de Materiales, Facultad de Químicas, Universidad del País Vasco, 20018 San Sebastian, Spain TAMAG Ibérica S.L., Parque Tecnológico de Miramón, Paseo Mikeletegi 56, 1a Planta, 20009 San Sebastián, Spain c Departamento de Física Aplicada I, EUPDS, UPV/EHU, Plaza Europa 1, 20018 San Sebastián, Spain d Instituto de Física “Gleb Wataghin”, Universidade Estadual de Campinas, Sao Paulo, Brazil b

a r t i c l e

i n f o

Article history: Received 30 August 2007 Received in revised form 21 July 2008 Accepted 31 July 2008 Available online 19 December 2008 PACS: 75.50 Kj 75.80 +q Keywords: Nanostructures Magnetic measurements X-ray diffraction

a b s t r a c t We report on fabrication, structural and magnetic characterization of Cu70 (Co70 Fe5 Si10 B15 )30 microwires with total diameter of 28.2 ␮m and metallic nucleus diameter of 15.2 ␮m produced by the Taylor-Ulitovski method containing 30% of alloy composition commonly used amorphous soft magnetic material and 70% Cu. The structure consists of crystalline Cu, and mixture of ferromagnetic phases. The as-prepared Cu70 (Co70 Fe5 Si10 B15 )30 microwires present coercivity of about 85 Oe at room temperature. Temperature dependence of coercivity is measured. Significant increase of coercivity till 135 Oe is observed at about 50 K. Magnetization measured under applied magnetic field (FC) and without magnetic field (ZFC) exhibit significant difference attributed to the inhomogeneity of as-prepared samples consisting of crystalline paramagnetic Cu matrix with ferromagnetic entities. MR related with existence of ferromagnetic grains in paramagnetic Cu matrix has been observed. This MR increases with decreasing temperature (up to 1.6% at 5 K). © 2008 Elsevier B.V. All rights reserved.

1. Introduction Studies of thin glass-coated microwires produced by TaylorUlitovski technique gained special attention within last few years due to unusual magnetic properties and low dimensionality of these materials. The Taylor-Ulitovski method allows the fabrication of long glass-coated metallic microwires (∼10 km) with typical radius of the metallic nucleus ranging from 1 and 15 ␮m and the thickness of the insulating glass coating between 1 and 10 ␮m. High enough quenching rate can be achieved while producing such microwires, allowing fabrication of amorphous, nanocrystalline, microcrystalline or granular samples [1]. On the other hand granular alloys formed by immiscible elements (Co, Fe, Ni)–(Cu, Pt, Au, Ag) attracted recently special attention mostly because of the Giant Magneto-resistance (GMR) observed in these alloys [2,3]. These materials consist of ferromagnetic nanoparticles, initially of Fe or Co embedded in a non-magnetic matrix (typically Cu, Ag, Au or Pt). At zero applied field, when magnetic moments of the parti-

∗ Corresponding author at: Departamento de Física de Materiales, Facultad de Químicas, Universidad del País Vasco, 20018 San Sebastian, Spain. Tel.: +34 943 018 611; fax: +34 943 017 130. E-mail address: [email protected] (V. Zhukova). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.07.183

cles are not aligned (e.g., oriented randomly), the resistivity of the material is high, and when an external magnetic field aligns these moments, the resistivity decreases, as in the previously discovered GMR multilayered materials [4]. The field-dependent resistivity in granular materials is related to a spin-dependent scattering of conduction electrons within the magnetic particles as well as the interfaces between magnetic and non-magnetic regions (however, it is assumed that the interfacial effect contributes to a dominant extent to GMR [5–8]). Because of the complex structure of the granular materials, the relationship between microstructure and GMR is still not fully understood. Initially main attention has been paid to studies of magnetically soft microwires with amorphous and/or nanocrystalline structure. Recently few attempts have been made to obtain granular or nanocrystalline microwires with unusual properties [1,9,10]. Thus, semi-hard magnetic properties and GMR effect have been reported in Fe–Ni–Cu and Co–Cu microwires with coercivities of about 700 Oe and GMR up to 18% [9,10]. Another novel composition of granular sample where part of the alloy composition is commonly used on amorphous soft magnetic materials and the other part is based on Cu has been recently reported [11]. Unusual multiphase structure consisting of Cu, small amount of ␣-Fe and amorphous phase has been observed in microwires with CuFeSiBC composition, exhibiting non-regular hysteresis loop. In this work

C. García et al. / Journal of Alloys and Compounds 483 (2009) 566–569

we report structural, magnetic and magneto-transport properties of a new family of glass-coated microwire of nominal composition Cu70 (Co70 Fe5 Si10 B15 )30 , fabricated by the Taylor-Ulitovski technique where initially Co70 Fe5 Si10 B15 alloy was mixture with Cu with the proportions indicated. 2. Experimental details Thin glass-coated microwires with total diameter of 28.2 ␮m and metallic nucleus diameter of 15.2 ␮m of nominal composition Cu70 (Co70 Fe5 Si10 B15 )30 were produced by the Taylor-Ulitovski method containing 30% of alloy composition commonly used amorphous soft magnetic material and 70% Cu [9]. The master alloy of the composition was prepared by arc melting of the pure elements in Ar atmosphere. Subsequently, when the metallic alloy and the Pyrex glass coating was molten, it was drawn and rolled onto a rotating cylinder and quenched to room temperature. The sample obtained was in the form of a tiny metallic wire with the dimensions above mentioned. Structural analysis by X-ray diffraction (XRD) has been carried out in a powder diffractometer from PHILIPS, provided with an automatic divergence slit and a graphite monochromator, with Cu K␣ radiation ( = 0.154 nm) [12]. The measurements were carried out using the step scanning technique between 5◦ and 90◦ (2) in steps of 0.05◦ (2) with accumulation times of 5 s at each point. Magnetic properties (coercive field and magnetization) were deduced from the hysteresis loop of the microwire using a vibrating sample magnetometer (VSM) in the range of temperature 5–300 K. The hysteresis loops M(H) of these samples were measured in the temperature range from 10 to 300 K and maximum field of 90 kOe. Magnetoresistance response (MR) was measured using a physical properties of measurements system (PPMS) device operating from 5 to 300 K. Measurements have been carried out with the current parallel to the applied field and how it was showed this orientation shows negative magnetoresistance. The magnetoresistance response is defined as the change of the electrical resistance R under an applied magnetic field, Happ , as: MR (%) =

(H) − (0) × 100 (0)

(1)

where (H) and (0) is the resistivity at maximum (70 kOe) and 0 kOe applied magnetic field, respectively.

3. Experimental results and discussion Fig. 1 shows the XRD pattern for the as-prepared Cu70 (Co70 Fe5 Si10 B15 )30 microwire. We must point out the existence in the XRD pattern of an amorphous halo with its maximum located at around 21◦ (2), arising from both, the pyrex coating and the non-crystallized part of the metallic core. We can observe the following in Fig. 1:

Fig. 1. XRD pattern for Cu70 (Co70 Fe5 Si10 B15 )30 as-cast glass-coated microwire with total diameter of 28.2 ␮m and metallic nucleus diameter of 15.2 ␮m. The inset shows enlarged the region where the crystalline peaks appear in a logarithmic intensity scale.

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(a) Three peaks with atomic spacing of 0.205, 0.178 and 0.129 nm, respectively corresponding to the (1 1 1), (2 0 0), and (2 2 0) reflections of fcc Cu lattice. The first one is very small, although evident, in our pattern because of texture (owing to the stress during the fabrication) and orientation phenomena linked to the production of the microwire. This fact frequently occurs in this type of materials and is linked to the proper fabrication method [9]. (b) Close to the (1 1 1) and (2 0 0) Cu peaks we can observe a contribution of the (0 0 2) hcp Co phase. Nevertheless, this contribution is lower than the one from Cu phase owing to the lower Co content in the core. The other hcp Co reflections are not detected. Structural information for the microwires is extracted once the crystalline peak of each pattern is identified and the amorphous background is subtracted. The grain size, D, of the formed crystallites is derived from Scherrer’s equation applied to the highest (2 0 0) Cu peak: D=

K ε cos m

(2)

where 2 m is the scattering angle corresponding to maximum of the peak and ε is its FWHM: full width at half maximum. The parameter K is assumed to be around 0.9 (or closer to the unity) in certain cases. Calculations show the existence of crystallites of around 40 nm in grain size. The M(H) curve (hysteresis loop) of the glass coating microwires at room temperature shown in the Fig. 2 resembles superparamagnetic behaviour, previously observed in Cu–Co systems [13,14]. Similar features were observed at different measuring temperatures. From the hysteresis loop at different temperatures, the temperature dependence of the coercivity, Hc , and remanence, Mr , of the glass coating microwire were deduced (Fig. 3). Slight increase of both coercivity and remanence increasing the temperature has been observed below 50 K. The monotonic decrease observed above 100 K on H(T) and Mr (T) generally fit previously predicted and experimentally observed dependences explained assuming coexistence of blocked and unblocked particles [14–16]. In real few factors can be taken into account in order to explain Hc (T) dependence: (1) coexistence of blocked superparamgnetic grains related with complex crystalline structure; (2) thermal fluctuations; (3) internal stress arising from dependence of thermal expansions coefficients of the glass and metal. Thus if temperature dependence of magnetization above 100 K can be explained in terms of the coexistence of

Fig. 2. Hysteresis loop for the Cu70 (Co70 Fe5 Si10 B15 )30 as-cast microwires with total diameter of 28.2 ␮m and metallic nucleus diameter of 15.2 ␮m at room temperature.

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Fig. 3. Coercive field Hc vs. temperature and remanent magnetization Mr vs. temperature for the sample investigated.

both blocked and superparamagnetic particles, the initial increase of the coercive field (from 128 to 135 Oe), could be attributed to the thermal fluctuations, as described in [14]. Increasing the temperature the large particles start to unblock and from T ≈ 50 K Hc decreases [14]. The distribution of particle sizes deduced from Fig. 4 should affect observed decrease of the coercive field. Fig. 4 presents the temperature dependence of magnetization of field cooling (FC) curve (50 Oe applied magnetic field) and zerofield cooling (ZFC) curve. Magnetization measured under 50 and 200 Oe applied magnetic field (FC) field and without magnetic field (ZFC) shows a similar behaviour. The blocking temperature, TB , was found around 25 K. The volume, Vo , of nano-grains can be estimated from well-known relation between TB and Vo [14] TB ≈

Ka Vo 25kB

Fig. 5. Magnetoresistance of the as-cast microwires taken at 300, 100 and 5 K from −7 to 7 T. Magnetoresistance reaches significant values around 1.6% in magnetic field H = 7 T at 5 K.

Internal stresses are mostly determined by the difference in thermal expansion coefficients of the glass and metal. Therefore studies of abovementioned composition with different geometry can be interesting in order to reveal the internal stress contribution in magnetic and magneto-transport properties. Similar studies in Co–Cu microwires performed by us allowed to find that indeed crystalline structure is affected by the glass coating thickness and metallic nucleus diameter [18]. These studies in Cu70 (Co70 Fe5 Si10 B15 )30 microwires are in progress.

(3)

where kB is the Bolzmann constant, Ka is the magnetic anisotropy density. Assuming spherical shape of these nano-grains Eq. (3) gives around 3.6 nm, which correspond with the value obtained by XRD. Fig. 5 shows the magnetoresistance response of the as-cast microwire measured at 5, 100 and 300 K. Relatively small and negative magnetoresistance has been observed at all temperatures. Decreasing the temperature MR increases from 0.05% to 1.6%. In most systems exhibiting GMR the GMR ratio increases decreasing the temperature [17]. Usually this increase is attributed to the superparamagnetic origin of grains responsible for the GMR behaviour as well as related with temperature dependence of resistivity [17].

4. Conclusions We report on fabrication, structure and magnetic characterization of novel Cu70 (Co70 Fe5 Si10 B15 )30 microwires with total diameter of 28.2 ␮m and metallic nucleus diameter of 15.2 ␮m produced by the Taylor-Ulitovski method. The structure consists of crystalline Cu, and mixture of ferromagnetic entities. The asprepared Cu70 (Co70 Fe5 Si10 B15 )30 microwires present coercivity of about 85 Oe. Temperature dependence of coercivity shows the maximum at around 50 K, where Hc ≈ 135 Oe. Considerable difference between ZFC and FC magnetization unusual dependences of Hc and Mr and magnetoresistance (up to 1.6% at 5 K) have been attributed the inhomogeneity of as-prepared samples consisting of crystalline paramagnetic Cu matrix with embedded small grains. References

Fig. 4. Zero-field cooled and field cooled curves for Cu70 (Co70 Fe5 Si10 B15 )30 measured at HDC = 50 Oe up to 300 K.

the

as-prepared

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