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GMI effect in the low magnetostrictive Co70Fe5Si15B10 alloys
ARTICLE IN PRESS
Physica B 384 (2006) 152–154 www.elsevier.com/locate/physb
GMI effect in the low magnetostrictive Co70Fe5Si15B10 alloys L.A.P. Gonc- alvesa,,1, J.M. Soaresa, F.L.A. Machadoa,, W.M. de Azevedob a
Departamento de Fı´sica, Universidade Federal de Pernambuco, 50670-901 Recife-PE, Brazil Departamento de Quı´mica Fundamental, Universidade Federal de Pernambuco, 50670-901 Recife-PE, Brazil
b
Abstract The giant magnetoimpedance (GMI) effect was investigated in pieces of as-quenched ribbons of the high-magnetic permeability amorphous ferromagnetic Co70 Fe5 Si15 B10 alloy. The measurements were made varying the length (l) of the sample, the frequency (f) and amplitude of the AC electrical current ðI AC Þ, and the amplitude of a DC current ðI DC Þ applied simultaneously with the AC current. GMI as high as 107% was observed for l ¼ 30 mm; f ¼ 1 MHz and I AC ¼ 10 mA. For larger values of l the asymmetry in the GMI induced by I DC was found to be small but it is substantially high when l is reduced, yielding 165% for the GMI measured in a sample 30 mm long with I DC ¼ 30 mA. We believe that the magnetostriction plus the demagnetizing field are main reason of these new finds. r 2006 Elsevier B.V. All rights reserved. PACS: 75.47.Np; 75.50.Kj; 75.40.Cx Keywords: GMI; Amorphous; Susceptibility
1. Introduction The giant magnetoimpedance (GMI) effect has attracted considerable scientific and technological interest especially because of its applicability in magnetic sensing and as an additional tool to investigate soft magnetic materials properties [1]. The GMI consists of large variations in the electrical impedance even when small magnetic fields are applied. Its origin is a combination of classical electrodynamics with the dynamics of the p some magnetization ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi processes. The penetration depth d ¼ r=pf mT , where r is the sample electrical resistivity, f is the frequency of the electrical current, and mT is the magnetic permeability (transversal, in the case of ribbons), plays a key role in determining the magnetoimpedance response. The GMI effect is very sensitive to composition [2], sample shape [3], annealing conditions [4] and quenched-in internal stresses [5]. A theoretical model [6] based on the skin depth effect and on the domain wall motion that influenced by both the applied magnetic field and by the field created by the AC Corresponding authors. Tel.: +55 81 21268450; fax: +55 81 32710359.
E-mail addresses: [email protected] (L.A.P. Gonc- alves), fl[email protected] (F.L.A. Machado). 1 Permanent address: CEFET - Ceara´. 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.05.210
current was proposed to explain the GMI. The GMI spectrum is essentially made of a pair of symmetric peaks with a maximum value occurring at magnetic fields of the order of few oersteds. It was also found that a large asymmetry is obtained when a DC electrical current is passed simultaneously with the AC one. This asymmetric GMI makes the phenomenon even more interesting for applications where both the magnitude and direction of the applied fields are desirable. Asymmetric GMI induced by DC electrical currents was observed in wires [7] and ribbons of magnetic amorphous alloys [8,9]. In this work, we present a detailed investigation of the GMI in the magnetostrictive Co70 Fe5 Si15 B10 alloy in a broad range of frequency and amplitude of the AC electrical current, length of the ribbon, and magnitude and direction of a DC electrical current used to induce asymmetry in the GMI. For this sample composition it was found that the GMI and the asymmetry are maximized for a particular length of sample. 2. Samples and experimental techniques Ribbons of the slightly positive magnetostrictive amorphous alloy with composition Co70 Fe5 Si15 B10 were pre-
ARTICLE IN PRESS L.A.P. Gonc- alves et al. / Physica B 384 (2006) 152–154
pared by a melt-spinning technique in an argon atmosphere. Pieces of the as-quenched ribbon 60 mm thick and 1.5 mm wide were cut in samples with length varying in the range 5plp100 mm. The degree of amorphousness was determined by X-ray diffraction using the Cu-Ka radiation and the composition and homogeneity of the ribbons were found using the energy dispersive X-ray analysis (EDS). We found that the samples were indeed homogeneous in composition and presented a high degree of amorphousness. A phase sensitive four-probe technique was used to measure the room temperature magnetoimpedance ZðHÞ. The frequency and amplitude of the AC electrical current, and the magnetic field applied along the length of the sample, were varied in the ranges 0:1pf p3 MHz; 5 pI AC p50 mA; 30pHp30 Oe, respectively. In order to investigate the asymmetric GMI in this alloy, a variable DC electrical current whose magnitude could be increased up to 50 mA was applied simultaneously with the AC current. The low frequency (10 Hz) AC susceptibility wAC was measured as a function of H using a first-order gradiometer coil system.
153
(%), defined as 102 ½ZðHÞ ZðH max Þ=ZðH max Þ, where ZðH max Þ is the magnetoimpedance measured at the maximum value of H, is shown in Fig. 2 for l ¼ 30 mm; I AC ¼ 10 mA and for f ¼ 1; 1:5 and 2.0 MHz. For 1 MHz the maximum GMI is 107%. The data for the GMI
3. Experimental results The room temperature wAC vs. H curves for several values of l are shown in Fig. 1. The data were normalized by the length of the samples to eliminate the filling factor contribution to the magnitude of the signal induced in the pick-up coil. Note that the values of wAC are very large near H ¼ 0 and decrease when the H is increased. For samples that are shorter in length the magnitude are smaller while the peak become broader. The inset in Fig. 1 is a plot of the wAC , normalized to its value at room temperature, vs. T, obtained for the sample with l ¼ 30 mm indicating our sample are ferromagnetic below 586 K. The percent GMI
Fig. 2. GMI spectra measured at different values of f, for l ¼ 30 mm and I AC ¼ 10 mA.
(a)
(b)
Fig. 1. wAC vs. H data for l ¼ 5; 10; 20; 30 and 100 mm for Co70 Fe5 Si15 B10 . The inset show the temperature dependence of wAC for l ¼ 30 mm and f ¼ 10 Hz.
Fig. 3. (a) GMI and (b) asymmetric GMI ratios for I DC ¼ 30 mA, for f ¼ 1:0 MHz and I AC ¼ 10 mA, for several values of l.
ARTICLE IN PRESS 154
L.A.P. Gonc- alves et al. / Physica B 384 (2006) 152–154
for I AC ¼ 10 mA; f ¼ 1:0 MHz varying l is shown in Fig. 3(a). The value of H where the GMI reaches its maximum ðH max Þ diminishes rapidly from 18.8 Oe for l ¼ 5 mm to 1.3 Oe for l ¼ 30 mm and higher values of l. Note that for this sample composition the GMI reaches its maximum value for l ¼ 30 mm. Fig. 3(b) shows the asymmetric GMI curves measured with I AC ¼ 10 mA; I DC ¼ 30 mA and f ¼ 1:0 MHz, for several values of l. The asymmetric GMI was found to increase when l diminishes but it reaches a maximum value (165%) for l ¼ 30mm, as well. For values of l smaller than 30 mm the asymmetry diminishes again.
composition the material has a magnetostriction that is slightly positive (lS ¼ 9:2 107 [10]), favoring the formation of domains that are transversal to the length of the ribbon which, in turn, couples more efficiently with the transversal magnetic field created by the AC electrical current. The samples used in the present work are thicker than the ones used in previous investigations and this may also be important in establishing the magnetic domain configuration. Direct observation of the magnetic domains, using the magnetic optical kerr effect, for instance, is desirable and it is being planned for better clarify the origin of the above reported anomalous behavior.
4. Discussion and conclusions
Acknowledgment
Because of the high magnetic permeability characteristic of these materials, the penetration depth d may become small compared to the thickness of the sample, and the electrical current flows near the surface of the sample. An increase in H diminishes mT due to their magnetic field dependence, driving the AC electrical current towards the bulk of the sample. Susceptibility and magnetization measurements shows that small samples of soft-ferromagnetic materials become magnetically harder since the magnetic domain configurations are strongly influenced by the demagnetizing field, as shown in Fig. 1. There is an increase in the demagnetizing factor (N) along the direction of the length of the sample when l is reduced. An increase in N favors the formation of domains, which reduce the longitudinal magnetic permeability. Consequently, one usually obtains small GMI for shorter samples. However, in as-quenched samples of Co70 Fe5 Si15 B10 we found that both the GMI and the asymmetric GMI become surprising larger when the length of samples is reduced, reaching a maximum value in the sample with l ¼ 30 mm. We are attributing this new find to the fact that for this sample
Work partially supported by FINEP, CNPq and FACEPE. References [1] I. Betancourt, M.L. Sa´nchez, B. Hernando, R. Valenzuela, J. Magn. Magn. Mater. 294 (2005) 159. [2] N.H. Nghi, N.M. Hong, T.Q. Vinh, N.V. Dung, P.M. Hong, Phys. B 327 (2003) 253. [3] J.M. Baradiaran, A. Garcı´ a-Arribas, J.L. Mun˜oz, G.V. Kurlyandskaya, IEEE Trans. Magn. 38 (2002) 3051. [4] K.C. Mendes, F.L.A. Machado, J. Magn. Magn. Mater. 177–181 (1998) 111. [5] C. Go´mez-Polo, K.R. Pirota, M. Knobel, J. Magn. Magn. Mater. 242–245 (2002) 294. [6] F.L.A. Machado, S.M. Rezende, J. Appl. Phys. 79 (1996) 6558. [7] T. Kitoh, K. Mohri, T. Uchiyama, IEEE Trans. Magn. 31 (1995) 3137. [8] F.L.A. Machado, A.R. Rodrigues, A.A. Puc- a, A.E.P. de Arau´jo, Mater. Sci. Fo´rum 302–303 (1999) 202. [9] S.H. Song, K.S. Kim, S.C. Yu, C.G. Kim, M. Va´zquez, J. Magn. Magn. Mater. 215–216 (2000) 532. [10] M. Knobel, R.S. Turtelli, R. Gro¨ssinger, J. Magn. Magn. Mater. 116 (1992) 154.
Physica B 384 (2006) 152–154 www.elsevier.com/locate/physb
GMI effect in the low magnetostrictive Co70Fe5Si15B10 alloys L.A.P. Gonc- alvesa,,1, J.M. Soaresa, F.L.A. Machadoa,, W.M. de Azevedob a
Departamento de Fı´sica, Universidade Federal de Pernambuco, 50670-901 Recife-PE, Brazil Departamento de Quı´mica Fundamental, Universidade Federal de Pernambuco, 50670-901 Recife-PE, Brazil
b
Abstract The giant magnetoimpedance (GMI) effect was investigated in pieces of as-quenched ribbons of the high-magnetic permeability amorphous ferromagnetic Co70 Fe5 Si15 B10 alloy. The measurements were made varying the length (l) of the sample, the frequency (f) and amplitude of the AC electrical current ðI AC Þ, and the amplitude of a DC current ðI DC Þ applied simultaneously with the AC current. GMI as high as 107% was observed for l ¼ 30 mm; f ¼ 1 MHz and I AC ¼ 10 mA. For larger values of l the asymmetry in the GMI induced by I DC was found to be small but it is substantially high when l is reduced, yielding 165% for the GMI measured in a sample 30 mm long with I DC ¼ 30 mA. We believe that the magnetostriction plus the demagnetizing field are main reason of these new finds. r 2006 Elsevier B.V. All rights reserved. PACS: 75.47.Np; 75.50.Kj; 75.40.Cx Keywords: GMI; Amorphous; Susceptibility
1. Introduction The giant magnetoimpedance (GMI) effect has attracted considerable scientific and technological interest especially because of its applicability in magnetic sensing and as an additional tool to investigate soft magnetic materials properties [1]. The GMI consists of large variations in the electrical impedance even when small magnetic fields are applied. Its origin is a combination of classical electrodynamics with the dynamics of the p some magnetization ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi processes. The penetration depth d ¼ r=pf mT , where r is the sample electrical resistivity, f is the frequency of the electrical current, and mT is the magnetic permeability (transversal, in the case of ribbons), plays a key role in determining the magnetoimpedance response. The GMI effect is very sensitive to composition [2], sample shape [3], annealing conditions [4] and quenched-in internal stresses [5]. A theoretical model [6] based on the skin depth effect and on the domain wall motion that influenced by both the applied magnetic field and by the field created by the AC Corresponding authors. Tel.: +55 81 21268450; fax: +55 81 32710359.
E-mail addresses: [email protected] (L.A.P. Gonc- alves), fl[email protected] (F.L.A. Machado). 1 Permanent address: CEFET - Ceara´. 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.05.210
current was proposed to explain the GMI. The GMI spectrum is essentially made of a pair of symmetric peaks with a maximum value occurring at magnetic fields of the order of few oersteds. It was also found that a large asymmetry is obtained when a DC electrical current is passed simultaneously with the AC one. This asymmetric GMI makes the phenomenon even more interesting for applications where both the magnitude and direction of the applied fields are desirable. Asymmetric GMI induced by DC electrical currents was observed in wires [7] and ribbons of magnetic amorphous alloys [8,9]. In this work, we present a detailed investigation of the GMI in the magnetostrictive Co70 Fe5 Si15 B10 alloy in a broad range of frequency and amplitude of the AC electrical current, length of the ribbon, and magnitude and direction of a DC electrical current used to induce asymmetry in the GMI. For this sample composition it was found that the GMI and the asymmetry are maximized for a particular length of sample. 2. Samples and experimental techniques Ribbons of the slightly positive magnetostrictive amorphous alloy with composition Co70 Fe5 Si15 B10 were pre-
ARTICLE IN PRESS L.A.P. Gonc- alves et al. / Physica B 384 (2006) 152–154
pared by a melt-spinning technique in an argon atmosphere. Pieces of the as-quenched ribbon 60 mm thick and 1.5 mm wide were cut in samples with length varying in the range 5plp100 mm. The degree of amorphousness was determined by X-ray diffraction using the Cu-Ka radiation and the composition and homogeneity of the ribbons were found using the energy dispersive X-ray analysis (EDS). We found that the samples were indeed homogeneous in composition and presented a high degree of amorphousness. A phase sensitive four-probe technique was used to measure the room temperature magnetoimpedance ZðHÞ. The frequency and amplitude of the AC electrical current, and the magnetic field applied along the length of the sample, were varied in the ranges 0:1pf p3 MHz; 5 pI AC p50 mA; 30pHp30 Oe, respectively. In order to investigate the asymmetric GMI in this alloy, a variable DC electrical current whose magnitude could be increased up to 50 mA was applied simultaneously with the AC current. The low frequency (10 Hz) AC susceptibility wAC was measured as a function of H using a first-order gradiometer coil system.
153
(%), defined as 102 ½ZðHÞ ZðH max Þ=ZðH max Þ, where ZðH max Þ is the magnetoimpedance measured at the maximum value of H, is shown in Fig. 2 for l ¼ 30 mm; I AC ¼ 10 mA and for f ¼ 1; 1:5 and 2.0 MHz. For 1 MHz the maximum GMI is 107%. The data for the GMI
3. Experimental results The room temperature wAC vs. H curves for several values of l are shown in Fig. 1. The data were normalized by the length of the samples to eliminate the filling factor contribution to the magnitude of the signal induced in the pick-up coil. Note that the values of wAC are very large near H ¼ 0 and decrease when the H is increased. For samples that are shorter in length the magnitude are smaller while the peak become broader. The inset in Fig. 1 is a plot of the wAC , normalized to its value at room temperature, vs. T, obtained for the sample with l ¼ 30 mm indicating our sample are ferromagnetic below 586 K. The percent GMI
Fig. 2. GMI spectra measured at different values of f, for l ¼ 30 mm and I AC ¼ 10 mA.
(a)
(b)
Fig. 1. wAC vs. H data for l ¼ 5; 10; 20; 30 and 100 mm for Co70 Fe5 Si15 B10 . The inset show the temperature dependence of wAC for l ¼ 30 mm and f ¼ 10 Hz.
Fig. 3. (a) GMI and (b) asymmetric GMI ratios for I DC ¼ 30 mA, for f ¼ 1:0 MHz and I AC ¼ 10 mA, for several values of l.
ARTICLE IN PRESS 154
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for I AC ¼ 10 mA; f ¼ 1:0 MHz varying l is shown in Fig. 3(a). The value of H where the GMI reaches its maximum ðH max Þ diminishes rapidly from 18.8 Oe for l ¼ 5 mm to 1.3 Oe for l ¼ 30 mm and higher values of l. Note that for this sample composition the GMI reaches its maximum value for l ¼ 30 mm. Fig. 3(b) shows the asymmetric GMI curves measured with I AC ¼ 10 mA; I DC ¼ 30 mA and f ¼ 1:0 MHz, for several values of l. The asymmetric GMI was found to increase when l diminishes but it reaches a maximum value (165%) for l ¼ 30mm, as well. For values of l smaller than 30 mm the asymmetry diminishes again.
composition the material has a magnetostriction that is slightly positive (lS ¼ 9:2 107 [10]), favoring the formation of domains that are transversal to the length of the ribbon which, in turn, couples more efficiently with the transversal magnetic field created by the AC electrical current. The samples used in the present work are thicker than the ones used in previous investigations and this may also be important in establishing the magnetic domain configuration. Direct observation of the magnetic domains, using the magnetic optical kerr effect, for instance, is desirable and it is being planned for better clarify the origin of the above reported anomalous behavior.
4. Discussion and conclusions
Acknowledgment
Because of the high magnetic permeability characteristic of these materials, the penetration depth d may become small compared to the thickness of the sample, and the electrical current flows near the surface of the sample. An increase in H diminishes mT due to their magnetic field dependence, driving the AC electrical current towards the bulk of the sample. Susceptibility and magnetization measurements shows that small samples of soft-ferromagnetic materials become magnetically harder since the magnetic domain configurations are strongly influenced by the demagnetizing field, as shown in Fig. 1. There is an increase in the demagnetizing factor (N) along the direction of the length of the sample when l is reduced. An increase in N favors the formation of domains, which reduce the longitudinal magnetic permeability. Consequently, one usually obtains small GMI for shorter samples. However, in as-quenched samples of Co70 Fe5 Si15 B10 we found that both the GMI and the asymmetric GMI become surprising larger when the length of samples is reduced, reaching a maximum value in the sample with l ¼ 30 mm. We are attributing this new find to the fact that for this sample
Work partially supported by FINEP, CNPq and FACEPE. References [1] I. Betancourt, M.L. Sa´nchez, B. Hernando, R. Valenzuela, J. Magn. Magn. Mater. 294 (2005) 159. [2] N.H. Nghi, N.M. Hong, T.Q. Vinh, N.V. Dung, P.M. Hong, Phys. B 327 (2003) 253. [3] J.M. Baradiaran, A. Garcı´ a-Arribas, J.L. Mun˜oz, G.V. Kurlyandskaya, IEEE Trans. Magn. 38 (2002) 3051. [4] K.C. Mendes, F.L.A. Machado, J. Magn. Magn. Mater. 177–181 (1998) 111. [5] C. Go´mez-Polo, K.R. Pirota, M. Knobel, J. Magn. Magn. Mater. 242–245 (2002) 294. [6] F.L.A. Machado, S.M. Rezende, J. Appl. Phys. 79 (1996) 6558. [7] T. Kitoh, K. Mohri, T. Uchiyama, IEEE Trans. Magn. 31 (1995) 3137. [8] F.L.A. Machado, A.R. Rodrigues, A.A. Puc- a, A.E.P. de Arau´jo, Mater. Sci. Fo´rum 302–303 (1999) 202. [9] S.H. Song, K.S. Kim, S.C. Yu, C.G. Kim, M. Va´zquez, J. Magn. Magn. Mater. 215–216 (2000) 532. [10] M. Knobel, R.S. Turtelli, R. Gro¨ssinger, J. Magn. Magn. Mater. 116 (1992) 154.