Influence of DC Joule-heating treatment on magnetoimpedance effect in amorphous Co64Fe21B15 alloy

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Journal of Magnetism and Magnetic Materials 271 (2004) 312–317

Influence of DC Joule-heating treatment on magnetoimpedance effect in amorphous Co64Fe21B15 alloy M. Coissona, S.N. Kaneb,1, P. Tibertoa,*, F. Vinaia a

Istituto Elettrotecnico Nazionale Galileo Ferraris and INFM, UdR TO-Poli, Strada delle Cacce 91, Torino I-10135, Italy b SATIE UMR CNRS, Ecole Normale Sup!erieure de Cachan, 61 Avenue de Pr!esident Wilson, 94235 Cachan, France Received 5 June 2003; received in revised form 12 July 2003

Abstract The influence of DC Joule-heating treatment on magnetoimpedance response in Co64Fe21B15 ribbons has been studied. Monitoring of electrical resistance during the thermal treatment, hysteresis loops measurements, X-ray . diffraction (XRD) and Mossbauer spectroscopy were used to study the structural changes induced by the currentannealing technique. The connection between the magnetoimpedance response and microstructure has been studied in detail. Finally, the normalization effect on the maximum DZ=Z (%) ratio achieved due to the maximum magnetic field applied during the experiment has been considered. r 2003 Elsevier B.V. All rights reserved. PACS: 75.47.Np; 75.50.Kj; 64.60.My . Keywords: Magnetoimpedance; Amorphous alloys; Joule heating; XRD; Mossbauer spectroscopy

1. Introduction The discovery of the giant magnetoimpedance effect (GMI) in soft magnetic materials had caused a large increase of interest from the perspective of application as magnetic field sensors [1]. This effect, consisting in the strong dependence of the AC impedance on the DC magnetic field, has an electromagnetic origin and is related to changes in the dynamics of the magnetization processes. Such *Corresponding author. Tel.: +39-011-3919-857; fax: +39011-3919-834. E-mail address: [email protected] (P. Tiberto). 1 On leave from: School of Physics, D.A. University, Khandwa road Campus, Indore 452017, India.

changes are a function of frequency and affect the magnetic permeability and the penetration depth of the AC current through a magnetic conductor [2,3]. As a consequence, GMI effect turns out to be very sensitive to composition, sample shape, annealing conditions and quenched-in internal stresses. The magnetoimpedance response has been extensively studied in melt-spun Co-rich amorphous ribbons and wires [4,5]. The impedance variation can reach high values (200% or more) for very small applied field values (from 103 to few 105 A/m) in Co-based amorphous alloys, wires and ribbons. The magnetic domain structure of low-magnetostrictive amorphous materials can be easily tailored to optimize the GMI response by inducing transverse magnetic anisotropy using the

0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.09.038

ARTICLE IN PRESS M. Coisson et al. / Journal of Magnetism and Magnetic Materials 271 (2004) 312–317

proper annealing condition. Current-annealing techniques turned out to be easy and handy to induce peculiar domain structure in Co-based amorphous ribbons [6]. In the present work the magnetoimpedance response of melt spun Co64Fe21B15 ribbons submitted to Joule-heating treatments has been studied in detail. Magnetic and magnetotransport properties have been correlated with microstructure induced during the thermal treatments. . In particular, X-ray diffraction (XRD) and Mossbauer spectroscopy were used to check structural changes occurring during current annealing. Finally, the influence of the maximum applied field on the magnetoimpedance variation has been put into evidence.

2. Experimental Amorphous Co64Fe21B15 ribbons (having E6 mm width and E19 mm thickness) were obtained by rapid solidification technique on a rotating drum. Selected strips with a length of 10 cm have been submitted to DC Joule-heating in vacuum [7,8] with electrical current values I ranging from 1 to 2 A for t ¼ 1800 s. Generally, low-current densities are exploited to perform annealing at temperatures below the Curie temperature of the studied alloy. As a consequence, the magnetic field generated by the electrical current allows the annealing under an external, self-generated, circular or transversal magnetic field [6]. This rapid annealing technique, exploiting the heat released in the sample by the electrical current flow, allows to get information on the microstructural changes by monitoring the structure-sensitive properties as the electrical resistance during Joule-heating treatment. In order to carefully evaluate the electrical current value, which induces the amorphous-to-crystalline transformation, one sample has been submitted to a current ramp and the electrical resistance was continuously monitored. The electrical resistance evolution as a function of electrical current is shown in Fig. 1. After a slight increase up to an electrical current value of E1.9 A, the decay of electrical resistance

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Fig. 1. Electrical resistance behaviour as a function of the electrical current.

clearly indicates the onset of microstructural changes within the amorphous matrix (i.e. sample crystallization). Quasi-static hysteresis loops were measured using a conventional fluxmetric technique (f ¼ 10 Hz). X-ray diffraction (XRD) measurements were carried out at room temperature using Siemens D-5000 Diffractometer (using Cu-Ka radiation) equipped with a LiF monochromator in the diffracted beam arm. . Transmission Mossbauer spectra of Jouleheated samples were recorded at room temperature in constant acceleration mode using 57Co:Rh . source. Mossbauer spectra consisting of amorphous and crystalline components were fitted with the assumption of a distribution of hyperfine fields using NORMOS program [9]. Low-frequency (500 kHz–50 MHz) impedance measurements as a function of the external longitudinal DC field HMax have been performed on current annealed samples by means of an impedance analyser using a conventional volt-amperometric technique. Two values of maximum magnetic field have been tested (HMax ¼ 103 A/m and 4  103 A/m) in order to check the influence of the magnetic state on the magnetoimpedance response. The magnetoimpedance ratio (GMI) has been defined as DZ=Zð%Þ ¼ ½ZðHÞ  ZðHMax Þ= ZðHMax Þ  100:

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M. Coisson et al. / Journal of Magnetism and Magnetic Materials 271 (2004) 312–317

3. Results and discussion The electrical resistance behaviour as a function of time is reported in Fig. 2 for selected amorphous Co64Fe21B15 strips submitted to dif-

Fig. 2. Electrical resistance variation vs. time for Co64Fe21B15 amorphous strips submitted to different electrical currents for 1800 s.

ferent electrical currents during Joule-heating annealing. The electrical resistance variation is calculated as DR=R0 ð%Þ ¼ ½RðtÞ  R0 =R0  100; R0 being the room temperature resistance. For low values of electrical currents (Io1:65 A), the electrical resistance increases up to a steady-state value, progressively increasing with increasing I without affecting significantly the value of the sample resistance after the thermal treatment. For higher electrical currents (IX1:6 A), a new feature appears related to the process of crystallization [7,8]. A further increase of I causes the resistance to leave the steady state and drops as a consequence of crystallization. Higher current values are characterized by a DR=R drop at shorter times. Fast crystallization of samples leads to a welldefined resistance bump (I ¼ 2 A). The longitudinal hysteresis loops measured on the as-cast and Joule-heated samples are reported in Fig. 3. As shown in figure, when the electrical current value I increases, the hysteresis curve shape drastically changes. A monotonic increase of the coercive field together with a decrease of the

Fig. 3. Hysteresis loops of as-cast and selected Joule-heated Co64Fe21B15 samples.

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Fig. 4. (a) X-ray diffractogram of the as-cast ribbon and (b) representative X-ray diffractograms of selected Joule-heated Co64Fe21B15 ribbons.

. Fig. 5. (a) Representative Mossbauer spectra of as-cast and Joule-heated Co64Fe21B15 ribbons and (b) variation of crystalline fraction as a function of annealing current.

magnetic permeability is clearly visible for I > 1:6 A. This behaviour can be easily related to either the induced anisotropy and/or the onset of structural changes in the amorphous matrix (i.e. crystallization process). X-ray diffraction pattern of the as-cast sample observed in the angular range 30p2yp60 is reported in Fig. 4(a) showing just the broad maximum typical to the amorphous phase. Representative XRD patterns of Joule-heated specimens vs. electrical current I are shown in Fig. 4(b). First faint traces of crystallization are visible for the sample subjected to I ¼ 1:6 A and indicate the onset of crystallization. Well-defined crystallization peaks are visible on the XRD patterns of the samples subjected to higher currents and they are attributed to a BCC Fe–Co phase [10]. . Representative Mossbauer spectra of as-cast and Joule-heated Co64Fe21B15 ribbons are shown

in Fig. 5(a). As observed by XRD data the precipitation of the crystalline component starts for the Joule-heated sample at 1.6 A for t ¼ 1800 s. . The Mossbauer spectrum of this sample was fitted with both amorphous and crystalline phases revealing the presence of 6.8% crystalline phase (BCC Fe–Co) with an average hyperfine field (Bhf ) . of 30.9 T. The Mossbauer spectrum of the Jouleheated sample at I ¼ 2 A shows the presence of 25% crystalline fraction with Bhf ¼ 33:2 T, which is close to a-Fe. The variation of the crystalline fraction as a function of electrical current treatment is reported in Fig. 5(b) indicating an almost linear increase with the increase of the Jouleheating current I: The field dependence of DZ=Z (%) ratio measured at various frequencies for the as-cast and Joule-heated samples with I ¼ 1:6 and 2.0 A is presented in Figs. 6(a–c), respectively. For all

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M. Coisson et al. / Journal of Magnetism and Magnetic Materials 271 (2004) 312–317 Table 1 Hm values for as-cast and selected Joule-heated samples at 10 MHz Hm (A/m)

Sample As-cast Joule-heated, Joule-heated, Joule-heated, Joule-heated,

Fig. 6. Field dependence of DZ=Z % measured at different frequencies: (a) as-cast; (b) Joule-heated sample; I ¼ 1:6 A for t ¼ 1800 s; and (c) Joule-heated sample, I ¼ 2:0 A for t=1800 s.

studied samples an increase of the magnetoimpedance response with the applied field H is observed reaching a maximum value at a certain field Hm for all frequencies. This maximum, generally connected to the anisotropy field value [11], turned out to strongly depend on the sample microstructure as shown in Table 1, where Hm values of selected Joule-heated samples are reported. A dramatic increase of the field at which the

I I I I

¼ 1:0 A, ¼ 1:6 A, ¼ 1:7 A, ¼ 2:0 A,

t ¼ 1800 s t ¼ 1800 s t ¼ 1800 s t ¼ 1800 s

400 750 1500 1000 2000

maximum magnetoimpedance response occurs is observed with increasing the Joule-heating electrical current value, indicating an increase of the transversal magnetic anisotropy [6] induced by the current annealing treatment and related to the progressive precipitation of crystalline phases. The maximum value of the DZ=Z (%) measured for an applied field Hmax ¼ 4  103 A/m is observed at a frequency of B10 MHz for all studied samples. In particular, maximum magnetoimpedance ratio of the as-cast sample turns out to be B30%, while for the Joule-heated samples values as high as 48% for I ¼ 1:6 A and 29% for I ¼ 2:0 A have been observed. It should be noted that the sample treated at I ¼ 1:6 A displays a non-negligible fraction of crystalline phase, but it is characterized by almost the highest measured magnetoimpedance response. On the other hand, the occurrence of a massive crystallization is seen to be detrimental to the DZ=Z (%) ratio. The influence of the magnetic state on the DZ=Z (%) ratio has been ruled out by performing magnetoimpedance measurements with two different values of HMax : The behaviour of the maximum values of the DZ=Z (%) ratio as a function of frequency measured at two HMax for the Jouleheated sample with I ¼ 1:6 A displaying the maximum magnetoimpedance response are reported in Fig. 7. The normalization effect is evidenced by the fact that, for HMax ¼ 103 A/m, the maximum DZ=Z (%) ratio is always lower. The best magnetoimpedance response is obtained when the sample is in a highly saturated state (is submitted to an higher HMax field value). The behaviour of Hm as a function of frequency, measured at HMax ¼ 4  103 A/m is reported in the inset of Fig. 7. As expected [11], the field at

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ization effect on the GMI response. The maximum DZ=Z (%) ratio has been observed in the sample treated with I ¼ 1:6 A for t ¼ 1800 s indicating that the presence of a very low crystalline fraction enhances the magnetoimpedance response. In this case the measurement has been performed with HMax ¼ 4  103 A/m. It should be noted that the measurements performed on the same Jouleheated sample with HMax ¼ 103 A/m turns out to display a maximum DZ=Z (%) value very close to the one observed in the as-cast sample. As a conclusion, the effect of normalization related to the maximum field value HMax exploited to measure the magnetoimpedance properties turns out to be relevant to optimize the DZ=Z (%) ratio and may lead to ambiguous interpretation.

Fig. 7. DZ/Z (%) maximum as a function of frequency for two HMax values; in the inset Hm vs. frequency measured with HMax ¼ 4  103 A/m.

which the maximum magnetoimpedance response is observed is displaced towards higher values with increasing frequency.

4. Conclusions The connection between microstructure and magnetotransport properties in amorphous Co64Fe21B15 ribbons submitted to DC Joule heating has been analysed. The magnetotransport measurements have been performed exploiting two values of maximum field HMax (103 A/m and 4  103 A/m) in order to evaluate any normal-

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