Valve behavior of giant magnetoimpedance in field-annealed Co 70 Fe Si 15 Nb 2.2 Cu 0.8 B amorphous ribbon Manh-Huong Phan, Hua-Xin Peng, Michael R Wisnom, Seong-Cho Yu, and Nguyen Chau Citation: Journal of Applied Physics 97, 10M108 (2005); doi: 10.1063/1.1854891 View online: http://dx.doi.org/10.1063/1.1854891 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/97/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Optimum condition for magnetic properties of two-phase soft magnetic alloys J Appl Phys 110, 043925 (2011); 10.1063/1.3622583 Giant magnetoimpedance effect in ultrasoft FeAlSiBCuNb nanocomposites for sensor applications J Appl Phys 98, 014316 (2005); 10.1063/1.1953864 Effect of stress and/or field annealing on the magnetic behavior of the ( Co 77 Si 13.5 B 9.5 ) 90 Fe Nb amorphous alloy J Appl Phys 97, 034911 (2005); 10.1063/1.1845577 Giant magnetoimpedance of amorphous ribbon/Cu/amorphous ribbon trilayer microstructures J 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The influence of longitudinal field annealing on the giant magnetoimpedance ͑GMI͒ effect in Co70Fe5Si15Nb2.2Cu0.8B7 amorphous ribbons has been investigated It was found that annealing in the open air at magnetic fields smaller than the anisotropy field along the ribbon gave rise to the GMI-valve phenomenon, while annealing at magnetic fields higher than the anisotropy field significantly reduced the GMI effect The GMI-valve behavior corresponding to the highest field sensitivity of GMI ͑125% / Oe͒ was observed at a frequency of 0.1 MHz in the ribbon annealed under an applied field of Oe This is ideal for developing sensitive and quick-response magnetic sensors The GMI-valve behavior observed in the Co-based amorphous ribbon due to field annealing can be explained by considering the complex permeability spectra in relation to the rotational dc magnetization © 2005 American Institute of Physics ͓DOI: 10.1063/1.1854891͔ I INTRODUCTION Giant magnetoimpedance ͑GMI͒ sensors have greatly benefited from the development of amorphous soft magnetic materials It has been shown that amorphous Co-based ribbons, wires, and glass-covered microwires are promising candidates for GMI sensor applications.1–3 The GMI effect occurs at high frequency range and can be explained by the classical electrodynamics.1 The maximum GMI is generally found in an alloy with the lowest value of magnetostriction, which corresponds to a maximum transverse permeability.1,3 Nonetheless, Sommer and Chien2 pointed out that such a high permeability of an amorphous alloy does not necessarily lead to a high GMI effect, and the observed large effect was due to the presence of transverse magnetic anisotropy induced by the application of an external magnetic field during the annealing process They also showed that the longitudinal field annealing reduced the transverse component of the anisotropy and consequently eliminated the GMI effect.2 This is in contrast to the recently observed asymmetric giant magnetoimpedance ͑AGMI͒ phenomenon, i.e., the so-called GMI valve, in a longitudinally weak-field-annealed Co-based amorphous ribbon.4,5 The reason for this discrepancy is that an annealing field of ϳ2 kOe applied along the ribbon2 might be high enough to entirely suppress the domain wall motion in the transverse direction and lead to the observed AGMI effect.4,5 In order to gain some insights into the nature of the GMI valve or the AGMI phenomena, the aim of the present work is to experimentally verify the AGMI due to longitudinal a͒ Author to whom correspondence should be addressed; electronic mail: M.H.Phan@bristol.ac.uk 0021-8979/2005/97͑10͒/10M108/3/$22.50 weak-field annealing by means of magnetoimpedance and complex permeability spectra In a recently published work,6 we found that the substitution of Cu and Nb for B in an initial Co70Fe5Si15B10 composition forming the Co70Fe5Si15Nb2.2Cu0.8B7 composition improved both the GMI effect and the field sensitivity of the amorphous ribbon A pronounced GMI effect is also expected in the annealed samples Therefore, we selected the Co70Fe5Si15Nb2.2Cu0.8B7 amorphous ribbons for the present study II EXPERIMENT The selected Co70Fe5Si15Nb2.2Cu0.8B7 amorphous samples were annealed at a temperature of 350 °C for h in open air in order to develop an oxide coating on the ribbon surface During the annealing process, an annealing field ͑Ha͒ was applied along the ribbon axis using a solenoid and its magnitude was varied from 50 mOe to Oe Magnetic measurements were carried out using a vibrating sample magnetometer ͑VSM͒ Magnetoimpedance and complex permeability spectra were measured using a HP4129A impedance analyzer.7 The GMI ratio was obtained from the following relation: ⌬Z/Z͑%͒ = 100 % ϫ ͓Z͑H͒ − Z͑Hmax͔͒/Z͑Hmax͒, ͑1͒ where Hmax is the external magnetic field sufficient to saturate the impedance and is equal to 35 Oe in the present work III RESULTS AND DISCUSSION GMI profiles were measured as a function of frequency ͑f͒ and the annealing field ͑Ha͒ It was found that, at relatively low frequencies ͑ϳ0.1 MHz͒, the GMI profiles showed a typical two-peak characteristic for Ha ഛ 100 mOe 97, 10M108-1 © 2005 American Institute of Physics [This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions Downloaded to ] IP: 130.179.16.201 On: Tue, 16 Jun 2015 09:20:27 10M108-2 Phan et al J Appl Phys 97, 10M108 ͑2005͒ FIG Variation of static permeability from domain wall motion ͑Ldw͒ and from rotational magnetization ͑Lrot͒ in the longitudinal direction with the annealing field ͑Ha͒ whereas only a single peak appeared under higher annealing fields with the peak in the antiparallel field region disappearing completely The GMI for the field-annealed samples also showed a distinct variation in AGMI features with increasing frequency The peak in the antiparallel field appeared again at frequencies above MHz, and it developed strongly with increasing frequencies Similar behaviors were observed by Kim et al.4 and Jang et al.5 In the present work, we focused our study on the characterization of the GMI valve behavior or the AGMI features Figure shows the GMI profiles measured at a frequency of 0.1 MHz under various annealing fields As one can see clearly from Fig 1, at a frequency of 0.1 MHz, the amplitude of AGMI increases with Ha up to Oe and then decreases under higher annealing fields, while the shape of the AGMI profile remains almost unchanged In addition, the AGMI peak is found to shift slightly toward a higher value of the external magnetic field ͑Hex͒ as the annealing field is increased These can be of significant importance in developing sensitive and quick-response magnetic sensors.3 Figure shows the field sensitivity of GMI as a function of the annealing field at different frequencies The highest field sensitivity of GMI of 125% / Oe was found at Ha = Oe and at f = 0.1 MHz This is obviously much higher than those of giant magnetoresistance ͑GMR͒ materials,8 and therefore is ideal for developing autobiased linear field sensors It is worth noting from Fig that, with increasing frequency, the field sensitivity of GMI decreases considerably and the maximum sensitivity is found at a higher annealing field Furthermore, it has recently been demonstrated by Phan et al.9 that the permeability spectra can be used as a useful measure to assess the AGMI phenomena in such a Co-based amorphous microwire under a dc bias current We estimated the static permeability as a function of the annealing field ͑Ha = 500 mOe− Oe͒ using the experimental procedures similar to that in Ref The results are given in Fig It can be seen that the longitudinal permeability from domain wall motion ͑Ldw͒ decreases with increasing annealing field, while the longitudinal permeability from magnetization rotation ͑Lrot͒ first increases with annealing field up to Ha = Oe and then deceases under higher annealing fields To further scrutinize this feature, we measured the axial hysteresis loops from which both the coercivity ͑HC͒ and the anisotropy field ͑Hk͒ were deduced and plotted as a function of the annealing field in Fig It shows that Hk decreases with FIG Variation of field sensitivity of GMI with the annealing field for various frequencies FIG Variation of the coercivity ͑HC͒ and the anisotropy field ͑Hk͒ with annealing field FIG GMI profiles measured at a frequency of 0.1 MHz for various annealing fields [This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions Downloaded to ] IP: 130.179.16.201 On: Tue, 16 Jun 2015 09:20:27 10M108-3 J Appl Phys 97, 10M108 ͑2005͒ Phan et al increasing Ha, whereas HC is independent of the annealing field Now, let us discuss the realistic influence of the longitudinal field annealing on the GMI effect in the Co-based amorphous ribbon First, it has been reported that the Cobased amorphous ribbons annealed in vacuum did not show AGMI,4 but an asymmetry in GMI was observed in the samples annealed in open air in the present work Accordingly, it is proposed that the AGMI phenomenon or the socalled GMI valve observed in the weak-field-annealed Cobased amorphous ribbon can be attributed to the crystallization of the surface underlayer, which becomes depleted in B and Si due to the surface oxidation.5 Second, it is known that this type of heat treatment produces asymmetric hysteresis loops in amorphous ribbons due to the interaction between the inner amorphous phase and the magnetically harder crystalline phase on the sample outer layer.10 When the crystallization took place under a weak magnetic field, the crystallites were magnetically ordered, resulting in an effectively unidirectional surface anisotropy It is the influence of the unidirectional surface anisotropy on domain wall motion in the transverse direction that in turn causes the AGMI observed in Fig Because the magnitude of the unidirectional surface anisotropy depends on that of the annealing field, the AGMI and its field sensitivity in the longitudinally weak-fieldannealed Co-based amorphous ribbon also depend on the magnitude and direction of the annealing field ͑Ha͒ as well as the measuring field ͑Hex͒ The permeability from domain wall motion in the transverse direction ͑Tdw͒ should increase with the annealing field up to Ha = Oe since Tdw is proportional to the permeability from magnetization rotation in the longitudinal direction Lrot ͑Fig 3͒ That is why the highest field sensitivity of GMI was observed at Ha = Oe ͑Fig 1͒ The reduction of AGMI under higher annealing fields ͑Figs and 2͒ is a direct consequence of the decrease of the longitudinal permeability from the wall motion and magnetization rotation processes ͑Fig 3͒ This is also attributed to the decrease of the magnetic anisotropy in the transverse direction as the annealing field is increased ͑Fig 4͒ These are consistent with what was reported in Ref However, in the work reported in Ref 2, the ribbons were field annealed in an inert atmosphere of Ar with a large magnetic field, Ha = kOe This annealing field induced a considerable unidirectional magnetic anisotropy that entirely suppressed domain wall motion in the transverse direction, and therefore eliminated the GMI effect It should be further noted that, due to the decrease of asymmetry in GMI at high frequencies where the rotational contribution to GMI is dominant, the field sensitivity of GMI decreases as the measuring frequency is increased ͑Fig 2͒ It is pointed out that annealing at small magnetic fields ͑Ha ഛ Hk͒ along the ribbon in open air introduced a peculiar domain structure.4,5 This enhanced the transverse permeability, and a larger asymmetry in GMI was consequently observed Annealing at magnetic fields slightly higher than the anisotropy field ͑Ha Ͼ Hk͒ induces the unidirectional magnetic anisotropy along the ribbon and this anisotropy can be large enough to hinder domain wall motion in the transverse direction, thereby significantly reducing the GMI effect Annealing at much higher magnetic fields ͑Ha ӷ Hk͒ can eliminate the GMI effect as reported in Ref This finding is important in understanding theoretically the exchange-biased asymmetry ͑i.e., the GMI-valve behavior in Co-based amorphous ribbons annealed in a weak field in the open air͒ by means of the quasistatic model, in which the unidirectional exchange anisotropy can be replaced by an effective dc bias field This warrants further study IV CONCLUSIONS The influence of field annealing on the GMI effect in Co70Fe5Si15Nb2.2Cu0.8B7 amorphous ribbons was investigated systematically Annealing at small magnetic fields ͑Ha ഛ Hk͒ along the ribbon in the open air resulted in AGMI phenomena—the so-called GMI valve, while annealing at higher magnetic fields ͑Ha Ͼ Hk͒ significantly reduced the GMI effect The optimum GMI-valve behavior, which corresponds to the highest field sensitivity of GMI of 125% / Oe, was observed at f = 0.1 MHz in the ribbon annealed under a field of Ha = Oe This is ideal for developing sensitive and quick-response magnetic sensors AGMI and its field sensitivity were induced by the field annealing due to the influence of the dc bias field on domain wall motion in the transverse direction The reduction in AGMI under higher annealing fields ͑Ha Ͼ Hk͒ was caused by the decrease of the transverse permeability from domain wall motion and magnetization rotation processes This can be attributed to the decrease of the transverse magnetic anisotropy as the annealing field is increased ACKNOWLEDGMENTS The authors would like to thank Professor Cheol Gi Kim for helpful comments This work was partially supported by the Korea Science and Engineering Foundation through the Research Center for Advanced Magnetic Materials at Chungnam National University L V Panina and K Mohri, Appl Phys Lett 65, 1189 ͑1994͒ R L Sommer and C L Chien, 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Annealing at small magnetic fields ͑Ha ഛ Hk͒ along the ribbon in the open air resulted in. .. field annealing on the giant magnetoimpedance ͑GMI͒ effect in Co70Fe5Si15Nb2.2Cu0.8B7 amorphous ribbons has been investigated It was found that annealing in the open air at magnetic fields smaller... GMI -valve behavior observed in the Co-based amorphous ribbon due to field annealing can be explained by considering the complex permeability spectra in relation to the rotational dc magnetization