phys stat sol (a) 201, No 7, 1558 – 1562 (2004) / DOI 10.1002/pssa.200306791 Enhanced GMI effect in a Co70Fe5Si15B10 ribbon due to Cu and Nb substitution for B M H Phan*, 1, H X Peng1, M R Wisnom1, S C Yu2, and N Chau3 Department of Aerospace Engineering, Bristol University, Queen's Building, University Walk, Bristol, BS8 1TR, United Kingdom Department of Physics, Chungbuk National University, Cheongju, 361-763, South Korea Center for Materials Science, National University of Hanoi, 334 Nguyen Trai, Hanoi, Vietnam Received 24 November 2003, revised 28 January 2004, accepted February 2004 Published online 25 March 2004 PACS 75.50.Kj, 75.60.Ch, 75.75.+a We present here, the results of an investigation on giant magnetoimpedance (GMI) effect in both annealed and as-quenched Co70Fe5Si15B10 and Co70Fe5Si15Nb2.2Cu0.8B7 ribbons Substitution of Cu and Nb for B in an initial Co70Fe5Si15B10 composition forming the Co70Fe5Si15Nb2.2Cu0.8B7 composition improves both GMI effect and its field sensitivity The GMI effect was more pronounced in the annealed samples The field sensitivity of both the longitudinal permeability ratio and the magnetoimpedance ratio for the annealed Co70Fe5Si15Nb2.2Cu0.8B7 ribbon increase exponentially as the testing temperature is increased, indicating that the magnetic permeability is very sensitive to the temperature The results obtained are of significant importance in developing quick-response magnetic sensors © 2003 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim Introduction Recently, a giant magnetoimpedance (GMI) effect, which was discovered in soft magnetic amorphous alloys, has generated growing interest owing to its high potential for magnetic sensors application [1, 2] The GMI phenomenon can be understood as the conjunction of a skin effect and a strong field dependence of the transverse magnetic permeability associated with transverse domain wall motions [1] At low frequencies, the GMI effect is demonstrated to originate from the contribution of the induced magnetoinductive voltage to magnetoimpedance (MI) As frequency increases, in the high-frequency regime, the GMI effect can be interpreted in terms of the applied dc magnetic field dependence of impedance as a result of the transverse magnetization with respect to the ac current direction flowing through the sample and the skin effect due to this ac current In such a magnetic material, the transverse permeability µ affects the magnetic penetration depth (δm) through δm = (ρ/πfµ)1/2, where f is the frequency and ρ is the electrical resistivity It is worth noting that as the skin effect becomes dominant (a/δm ӷ 1, a is the thickness of the ribbon), the impedance Z is proportional to (fµ)1/2 [1] Hence, µ decreases rapidly upon the dc applied magnetic field, causing a significant change in MI, i.e the GMI effect In general, Co- and Fe-based amorphous alloys with nearly zero magnetostriction exhibit very large change of magnetoimpedance (MI) caused by the application of an external magnetic field [2–4] Additionally, Knobel et al [5] suggested that the time relaxation of impedance plays an important role in identifying what soft magnetic amorphous material is suitable for a quick-response magnetic sensor application This observed relaxation of the impedance is related to the transverse magnetic permeability, also known as magnetic permeability aftereffect (MAE) or simply disaccomodation [6] There the maxi* Corresponding author: e-mail: M.H.Phan@bristol.ac.uk, Phone: +44 (0)117 928 7697, Fax: +44 (0)117 927 2771 © 2004 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim phys stat sol (a) 201, No (2004) / www.pss-a.com 1559 mum GMI was found corresponding to a maximum transverse permeability It is therefore desirable to seek more suitable GMI candidate material for novel magnetic sensors In this paper, we report the results of an investigation on the GMI effect in annealed and as-quenched Co70Fe5Si15B10 and Co70Fe5Si15Nb2.2Cu0.8B7 amorphous alloys Interestingly, substitution of Cu and Nb for B in an initial Co70Fe5Si15B10 composition forming the Co70Fe5Si15Nb2.2Cu0.8B7 composition favors both GMI and its field sensitivity The GMI effect in both the amorphous samples can be significantly improved by appropriate heat treatment These results are very beneficial for magnetic sensor application Experimental The Co70Fe5Si15B10 and Co70Fe5Si15Nb2.2Cu0.8B7 ribbons with a width of mm and a thickness of 20 µm were prepared by the rapid quenching method The as-quenched ribbons were annealed in vacuum at 550 K for hour X-ray diffraction analysis confirmed the quality of the samples The hysteresis loops, M-H, were measured using a vibrating sample magnetometer The resistivity measurements for the two kinds of samples were carried out using the four-probe method The obtained resistivity values for the Co70Fe5Si15B10 and the Co70Fe5Si15Nb2.2Cu0.8B7 compositions are ρ = 1.1 × 10–5 Ωm and 0.75 × 10–5 Ωm, respectively Magnetoimpedance measurements were carried out along the ribbon axis with the longitudinal applied magnetic field The samples with a length of about 15 mm were used for all MI measurements A schematic diagram of the magnetic impedance measurement system is depicted in Fig A computer-controlled RF signal generator with its power amplifier was connected to the sample with a series of resistors for monitoring the driving ac current We measured the ac current and voltage across the sample using a digital multimeters (DMM) with RF/V probes to compute the impedance The external dc field, applied by a solenoid, can be swept through the entire cycle equally divided by 800 intervals from –150 to 150 Oe The frequency of the ac current was varied from to 10 MHz, while its magnitude was kept constant at 10 mA Results and discussion The magnetoimpedance ratio ∆Z/Z can be defined as ∆Z/Z(%) = Z(H)/Z(Hmax) - 1, where Hmax is the external magnetic field sufficient to saturate the impedance and equals to 150 Oe in the present work Similarly, the longitudinal permeability ratio ∆µ/µ can also be defined as ∆µ/µ(%) = µ(H)/µ(Hmax) – In several works [2–4], it has been shown that the change of MI is closely related to that of the longitudinal Solenoid Cryogenic system RF power Amp R’ R Fig (online colour at: www.interscience.wiley.com) Schematic diagram of the magnetoimpedance measurement system © 2004 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim 1560 M H Phan et al.: Enhanced GMI effect in a Co70Fe5Si15B10 ribbon due to Cu and Nb substitution for B 60 (a) 50 1.1 MHz 2.1 MHz 3.1 MHz 4.1 MHz Fig ∆Z/Z vs the external magnetic field at various frequencies for the as-quenched amorphous samples: (a) Co70Fe5Si15B10 composition and (b) Co70Fe5Si15Nb2.2Cu0.8B7 composition 40 30 permeability Hence, the evaluation for the GMI effect in such a soft magnetic amorphous alloy can be realized either by ∆Z/Z measurements or 10 by ∆µ/µ measurements As shown in Fig 2, the ∆Z/Z curves for both the as-quenched samples, measured at frequen90 1.1 MHz cies up to f = 4.1 MHz, have a single peak at (b) 2.1 MHz zero field At the frequency of 3.1 MHz, it 75 3.1 MHz is noted that the maximum ∆Z/Z is ~55% 4.1 MHz for Co70Fe5Si15B10 while it is ~89% for 60 Co70Fe5Si15Nb2.2Cu0.8B7 The higher ∆Z/Z value 45 for the latter compound at f = 3.1 MHz is likely due to the presence of its special domain struc30 ture as transverse domains formed by a magnetomechanical coupling between internal stress 15 and magnetostriction [1–3] Substitution of Cu and Nb for B in the initial Co70Fe5Si15B10 com0 position seems to have promoted the formation of a transverse domains structure, because of -150 -100 -50 50 100 150 the presence of Cu and Nb allowing the formaH (Oe) tion of well-differentiated microstructures [7] In other words, a higher transverse permeability of the latter compound could result in the larger ∆Z/Z Additionally, the maximum ∆Z/Z for Co70Fe5Si15Nb2.2Cu0.8B7 composition is larger than that for Co70Fe5Si15B10 composition at all the measured frequencies, indicating a favorably formed transverse domain structure in Co70Fe5Si15Nb2.2Cu0.8B7 Another reason causing the difference in ∆Z/Z for the two samples to be noted is the difference in their electrical resistivities As reported earlier in Ref [8], the higher the electrical resistivity of amorphous alloy, the lower the obtained ∆Z/Z is In the present case, the resistivity is ρ = 1.1 × 10–5 Ωm for Co70Fe5Si15B10 composition, and this is higher than ρ = 0.75 × 10-5 Ωm for Co70Fe5Si15Nb2.2Cu0.8B7 composition Hence, the higher ∆Z/Z for Co70Fe5Si15Nb2.2Cu0.8B7 composition was obtained Furthermore, one can see from Fig 2(b) that there is a smaller full width at haft maximum (FWHM) in ∆Z/Z curves for Co70Fe5Si15Nb2.2Cu0.8B7 composition This indicates a high field sensitivity of ∆Z/Z (or the so-called magnetic response), which is ~17–18%/Oe at a current driving frequency of f = 3.1 MHz for Co70Fe5Si15Nb2.2Cu0.8B7 composition More interestingly, the high magnetic response of this sample remains unchanged at high frequencies, which is ideal for magnetic sensors application in the highfrequency regime As compared to the Co70Fe5Si15Nb2.2Cu0.8B7 composition, Co70Fe5Si15B10 shows the much lower magnetic response (~1%/Oe) at the same frequency of 3.1 MHz This difference can be understood in terms of the local magnetic anisotropy in Co70Fe5Si15B10 which was much larger than that in Co70Fe5Si15Nb2.2Cu0.8B7 (as estimated by the hysteresis loops) It is the local anisotropy that considerably reduces the transverse magnetization associated with the transverse permeability, thereby leading to the broadening in ∆Z/Z curves and the smaller ∆Z/Z for Co70Fe5Si15B10 composition [9] It is also interesting to note that, due to the internal stress relief [7–11], the GMI effect was significantly improved in both annealed samples relative to their as-quenched counterparts (see Fig 3) ∆Z/Z (%) 20 © 2004 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim phys stat sol (a) 201, No (2004) / www.pss-a.com 140 Fig [∆Z/Z]max vs frequency for the annealed and as-quenched Co70Fe5Si15B10 (No 1) and Co70Fe5Si15Nb2.2Cu0.8B7 (No 2) samples No (as-quenched) No (annealed) No (as-quenched) No (annealed) 120 1561 [∆Z/Z]max (%) 100 80 60 40 20 Frequency (MHz) As shown in Fig 3, the maximum ∆Z/Z is plotted against frequency for all annealed and as-quenched samples At the measured frequencies, ∆Z/Z is larger for the annealed samples than that for the asquenched samples ∆Z/Z first increases with increasing frequency up to f = 3.1 MHz and then decreases at higher frequencies This feature can be interpreted as follows: at frequencies below 1.1 MHz (a < δm), the maximum ∆Z/Z was not very large because of the contribution of the induced magneto-inductive voltage to magnetoimpedance When 1.1 MHz ≤ f ≤ 3.1 MHz (a ≈ δm), where the skin effect is dominant, the higher ∆Z/Z is reported Beyond f = 3.1 MHz, the maximum ∆Z/Z decreases with increasing frequency The reason here is that, in this frequency region (f ≥ 3.1 MHz), domain wall displacements were strongly damped owing to eddy currents thus contributing less to the transverse permeability, i.e., a small ∆Z/Z Since the temperature dependence of GMI profile plays an important role in identifying what amorphous soft magnetic alloy is useful for a quick-response magnetic sensor In the present study, we investigated the influences of the temperature on the longitudinal permeability and GMI profile in the annealed Co70Fe5Si15Nb2.2Cu0.8B7 alloy The results show that the magnitude of ∆µ/µ decreases with 600 1/2 400 300 200 90 LPRmax(T) = LPRmax(0)exp(cT ) with c = const f = MHz 30 80 25 70 20 60 15 50 100 1/2 MIRmax(T) = MIRmax(0)exp(cT ) with c = const f = 1MHz 40 -30 -20 -10 10 20 30 H (Oe) Fig Permeability change ∆µ/µ as a function of magnetic field at f = 0.1 MHz and at T = 10, 300 K 35 [∆Ζ/Ζ ]max/∆H (%/Oe) [∆µ/µ ]max/∆H (%/Oe) 500 ∆µ/µ (%) 40 100 T = 10 K T = 300 K 10 50 100 150 200 250 300 T (K) Fig (online colour at: www.interscience.wiley.com) Temperature dependence of [∆µ/µ]max/∆H and [∆Z/Z]max/∆H at a frequency of MHz for the annealed Co70Fe5Si15Nb2.2Cu0.8B7 amorphous alloy The solid line indicates the fit © 2004 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim 1562 M H Phan et al.: Enhanced GMI effect in a Co70Fe5Si15B10 ribbon due to Cu and Nb substitution for B increasing frequency from 100 kHz to MHz, but it increases with the testing temperature between 10 and 300 K (for example, see in Fig 4) Note that there was a remarkable change in the shape of ∆µ/µ curves with increase of the measured frequency and temperature It was found that the ∆µ/µ curves became narrower with further increase of the frequency and temperature In contrast, the ∆Z/Z curves became broader with further increases in the measured frequency and temperature This difference is caused by an increase of impedance and a reduction in the longitudinal permeability with increasing frequency [3] Also, the increase in ∆Z/Z and ∆µ/µ with further increase in the temperature results mainly from enhanced effective magnetic permeability of the sample [12, 13], because, at low temperatures (~10 K), the exchange energy between magnetic moments is larger than that at high temperatures (~300 K) Thereby, the circular motion of magnetic moments at low temperature might be frozen thus leading to the lower permeability and the smaller ∆Z/Z As shown in Fig 5, for the temperature dependence of ∆µ/µ and ∆Z/Z at f = MHz, the field sensitivity of ∆µ/µ and ∆Z/Z, defined as [∆µ/µ]max/∆H and [∆Z/Z]max/∆H respectively, increase exponentially as the temperature is increased This reflects the fact that the magnetic permeability is very sensitive to the temperature, which is very useful in developing so-called quick-response magnetic sensors [12–15] The decrease of anisotropy field and the increase of GMI profile with the testing temperature in the present study are believed to be attributed to the increased magnetic softness [16] Conclusions We investigated the GMI effect in annealed and as-quenched Co-based amorphous ribbons Substitution of Cu and Nb for B in the initial Co70Fe5Si15B10 composition forming the Co70Fe5Si15Nb2.2Cu0.8B7 composition favors both GMI and its field sensitivity The GMI effect was significantly improved in the samples annealed at 550 K The results obtained are beneficial for quick-response magnetic sensor application Besides, the field sensitivity of the longitudinal permeability ratio and the magnetoimpedance ratio for the annealed Co70Fe5Si15Nb2.2Cu0.8B7 ribbon increases exponentially as the temperature is increased, reflecting that the magnetic permeability is very sensitive to the temperature The decrease of anisotropy field and the increase of GMI profile with the testing temperature could be attributed to the increased magnetic softness Acknowledgements The authors are grateful to the support from the Korean Science and Engineering Foundation through the Research 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GMI effect in a Co70Fe5Si1 5B1 0 ribbon due to Cu and Nb substitution for B increasing frequency from 100 kHz to MHz, but it increases with the testing temperature between 10 and 300 K (for example,... longitudinal permeability ratio and the magnetoimpedance ratio for the annealed Co70Fe5Si1 5Nb2 . 2Cu0 . 8B7 ribbon increases exponentially as the temperature is increased, reflecting that the magnetic... struc30 ture as transverse domains formed by a magnetomechanical coupling between internal stress 15 and magnetostriction [1–3] Substitution of Cu and Nb for B in the initial Co70Fe5Si1 5B1 0 com0