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Impact of the transverse magnetocrystalline anisotropy of a Co coating layer on the magnetoimpedance response of FeNi-rich nanocrystalline ribbon
Journal of Alloys and Compounds 741 (2018) 1105e1111
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Impact of the transverse magnetocrystalline anisotropy of a Co coating layer on the magnetoimpedance response of FeNi-rich nanocrystalline ribbon d T. Eggers a, **, D.S. Lam a, b, O. Thiabgoh a, J. Marcin c, P. Svec , N.T. Huong b, c , *** a ,* nek , M.H. Phan I. Skorva a
Department of Physics, University of South Florida, Tampa, FL 33620, USA Department of Physics, Hanoi National University, 334 Nguyen Trai, Hanoi, Viet Nam Institute of Experimental Physics, Slovak Academy of Sciences, Watsonova 47, 040 01, Kosice, Slovakia d Institute of Physics, Slovak Academy of Sciences, Dubravska cesta 9, 845 11, Bratislava, Slovakia b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 June 2017 Received in revised form 11 January 2018 Accepted 14 January 2018 Available online 31 January 2018
There is a pressing need for improving the high-frequency magneto-impedance (MI) response of costeffective soft magnetic materials for use in high-performance sensing devices. We report here a comparative study of uncoated (Fe50Ni50)81Nb7B12 nanocrystalline ribbons and ribbons coated with 120 nm of Co on both surfaces. The impact of the Co coating on the high frequency impedance of the (Fe50Ni50)81Nb7B12 ribbon was studied with techniques sensitive to surface magnetism, namely magneto-optical Kerr effect (MOKE), MOKE microscopy, and high frequency impedance as a function of magnetic field. MOKE microscopy on the coated ribbons showed a superposition of high anisotropy domains from the crystalline Co and wide magnetic domains from (Fe50Ni50)81Nb7B12. In full agreement with the MOKE results, MI measurements indicated an increase in anisotropy field in the excitation frequency range 100e1000 MHz in the Co-coated ribbons. Noticeably, the Co-coating improved the MI ratio by 15% at fields near and below the anisotropy field. The improvement of the low-field MI response by the coating encourages the use of magnetic coatings to improve MI-based sensors in ribbon geometry for technological applications such as biomagnetic sensing. This study also yields insights into the effects of negative magnetostrictive coatings on the MI behavior in soft magnetic ribbons. © 2018 Elsevier B.V. All rights reserved.
Keywords: Fe-rich amorphous ribbons Surface anisotropy Magneto-impedance Magnetic sensors
1. Introduction Understanding the relationship between the surface magnetic properties, microstructure and giant magneto-impedance (GMI) effect in soft ferromagnetic ribbons is essential for the development of sensitive magnetic field sensors for a variety of applications [1]. The first groups to consider GMI for technological application were chiefly concerned with sensitive changes in electrical impedance at excitation frequencies less than 10 MHz [2,3]. They described the impedance change as the classical consequence of the skin effect, where the distribution of an ac current of frequency, f, in a soft
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (I. Skorv anek), [email protected] (M.H. Phan).
(T.
https://doi.org/10.1016/j.jallcom.2018.01.206 0925-8388/© 2018 Elsevier B.V. All rights reserved.
Eggers),
[email protected]
magnetic conductor of conductivity, s, flows near the surface at a depth d ¼ ðpsf mT Þ1=2 . The large and sensitive transverse effective permeability, mT(HDC), in the skin depth region is required for unlocking giant changes in impedance. At excitation frequencies greater than 10 MHz, where many of today's current integrated circuit technology operates, the GMI effect is known to suffer due to reduction in the effective permeability by relaxation of domain wall movement in ribbons. While a large GMI effect, on the order of hundreds of percent, has been demonstrated in optimized Fe-rich [4] and Co-rich [5,6] microwires at high frequency (a current review on the subject is given in Ref. [7]), the GMI effect is generally low in the ribbon geometry. Since the platform shape of a ribbon or film is highly desirable to the application of GMI to biomagnetic [8] or flexible stress [9] sensors, there is still effort put toward research into the effects of post-processing techniques to increase the GMI effect in ribbons by way of annealing [10] or coating [11e16]. Some works have reported a significant enhancement of the
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GMI effect upon coating ribbons with other magnetic layers. For example, an increase in the GMI ratio at 2.6 MHz was observed when a nanocrystalline Fe73.5Cu1Nb3Si13.5B9 (FINEMET) ribbon was coated with (130e500 nm) Ni80Fe20 film [12]. The magnetic exchange interaction and low stress between layers due to a lattice match between the Ni80Fe20 film and the Fe73.5Cu1Nb3Si13.5B9 ribbon was suggested to promote the GMI enhancement. In another study conducted by Laurita et al. [17], the authors found that sputter deposition of a 50 nm Co film enhanced both the GMI ratio and sensitivity of a HITPERM ribbon at frequencies of 10 MHz and below. The enhancement was attributed to the reduction of stray fields and an improved magnetic flux path closure at the ribbon's surface. At excitation frequencies below 10 MHz, the effective permeability change with magnetic field is governed by magnetization reorientation of the film/ribbon bulk, while at higher frequencies, the skin depth approaches its minimum value and more of the surface and interface of the film/ribbon system is sampled. A clear understanding of how a magnetic coating affects the effective permeability and the GMI response at high frequency (f > 100 MHz) of the ribbon is not well understood. Since amorphous ribbons with a domain structure or anisotropy transverse to the current direction are excellent candidates for GMI based sensors due to high GMI ratios [18,19], it would follow that the magnetic coating should be capable of supporting a transverse anisotropy [16,17,20e23]. From a structure-magnetic property perspective, Co is chosen as the coating layer since it has a negative magnetostriction. It has been shown that isothermally annealed FeNi-based ribbons contract along their length [24], therefore could be a source of a stress-induced transverse anisotropy in the Co coating. Annealing these ribbons at 500 C achieves uniform, nanosized crystalline grains and thus soft magnetic behavior beneficial to the GMI effect. In a nanocrystallized system, the local magnetocrystalline anisotropy of the nanograins embedded in the amorphous matrix is randomly averaged out by their exchange interaction resulting in a small net anisotropy [25]. In this work, we aim to investigate the impact of a 120 nm polycrystalline Co coating on two sides of an amorphous (Fe50Ni50)81Nb7B12 ribbon, which was subsequently annealed along with a control ribbon to achieve a nanocrystalline morphology. Using magneto-optical Kerr effect to measure and image the low field dc surface magnetization, the positive and negative effects of the Co coating layer with a transverse anisotropy on the magnetoimpedance of the ribbon are assessed. 2. Materials and methods Amorphous ribbons of composition (Fe50Ni50)81Nb7B12 were prepared by planar flow casting with master alloys prepared from elements with purity better than 99.95% [26]. Both sides of a 4 cm section of as-quenched ribbon were coated with 120 nm of Co by RF magnetron sputter deposition. The coated ribbon, along with an uncoated control ribbon, was annealed for 1 h at 500 C under high vacuum in order to achieve a low coercive field and soft magnetic behavior [24]. X-ray diffraction (XRD) of the coated and uncoated ribbons was conducted using Cu Ka radiation. Longitudinal magneto-optical Kerr effect (L-MOKE) hysteresis loops were taken using a homemade setup with a 5 mW (635 nm) laser source. The applied magnetic field was oriented in two directions, along the length of the ribbon and perpendicular to the length of the ribbon. MOKE microscopy was conducted by Magneto-optical Kerr Microscope (EVICO Magnetics), also in the longitudinal geometry, to image the domain structure on the surface of the coated and uncoated ribbons with the magnetic field applied along the ribbon axis. Magneto-impedance was measured as a function of applied field
(Hmax ¼ 110 Oe) along the length of the ribbon at high frequency (100e1000 MHz) using an HP 4191A impedance analyzer. The HP 4191A determines the complex reflection coefficient, G, of a high frequency signal sent down a terminated transmission line. An airline [27] made of a 2 cm section of ribbon suspended over a Cu ground plane served as the transmission line, which was terminated with a 50 U standard that matches the input impedance of the analyzer. The complex reflection coefficient at the beginning of the ribbon airline was measured and converted to complex impedance (Z) by
Z ¼ 50
1G ¼ R þ jX; 1þG
(1)
where R is the resistance, X is the reactance, and j is the imaginary unit. In this work, the magnetoimpedance ratio is defined as
DZ Z
ð%Þ ¼
ZðHÞ ZðHmaxÞ 100; ZðHmaxÞ
(2)
where Z(H) is the impedance (or resistance, reactance) at field H. The impedance ratio is normalized by Z(H¼Hmax). 3. Results and discussion Fig. 1 (a),(b) shows XRD patterns of the control (Fe50Ni50)81Nb7B12 ribbon and Co-coated ribbon after annealing at 500 C for 1 h, respectively. The nanocrystallized control ribbon shows a hump near 2q ¼ 43.9 , which corresponds to the fcc(111) reflection of bulk Fe0.5Ni0.5 and an additional peak at 2q ¼ 50.79 identified as (200) reflection of the fcc phase. This agrees with prior research on these FeNiNbB ribbons [24]. The XRD pattern of the annealed Co-coated ribbon shows the development of a shoulder near 44.5 , which is likely due to the presence of Co hcp(002) and/ or Co fcc(111), which have very close lattice spacings [28,29]. Fig. 1 (c) shows the nanocrystalline surface microstructure of the control FeNi ribbon after the 1 h, 500 C annealing treatment. Fig. 1 (d) is a cross sectional SEM image of the clean interface between the 120 nm layer of Co on the FeNi ribbon. This image shows the Cocoating was continuous and mechanically solid interface was formed between the Co layer and the ribbon. Kerr microscopy was used to image the low-field domain structure of the ribbons after the 1 h anneal at 500 C. Fig. 2(aeb) shows the domain patterns on the air side of the uncoated FeNibased ribbon with increasing magnetic field (0
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Fig. 1. XRD patterns of (a) the nanocrystalline control ribbon annealed at 500 C and (b) the ribbon with 120 nm Co. (c) TEM of nanocrystalline uncoated ribbon and (d) SEM of coated ribbon cross section.
Fig. 2. Domain structure visualized by Magneto-optical Kerr effect microscopy on (a,b) uncoated ribbon and (c,d) Co-coated ribbon with increasing HDC along ribbon length from left to right.
Fig. 3e). Fig. 3 (a) shows L-MOKE on the air side of the uncoated ribbon in the two measuring directions. A super soft easy axis with a coercive field (HC) of about 0.2 Oe. A hard axis hysteresis loop is found when the air side of the ribbon was rotated 900 (perpendicular to the casting direction/ribbon length) and Kerr effect measured. The anisotropy field can be roughly estimated from the
saturation field of the hard axis if it is assumed the anisotropy is uniaxial. The anisotropy field estimated from MOKE of the uncoated ribbon is around 3 Oe. Meanwhile, the Co-coated ribbon has a square hysteresis loop when measured in the perpendicular field orientation, while a hard axis is evident in the parallel orientation (Fig. 3 (c)). This is a clear indication of a transverse (easy axis
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Fig. 3. L-MOKE on the nanocrystalline control ribbon (a) and 120 nm Co coated ribbon (b) in two measuring directions. L-MOKE on the air and wheel side of the control ribbon (c) and 120 nm Co coated ribbon (d). (e) shows the two measuring directions, HDC parallel and perpendicular to ribbon length.
transverse to the ribbon length) magnetocrystalline anisotropy attributed to the polycrystalline Co-coating. The coercive field of about 50 Oe is typical for a 120 nm Co film annealed at 500 C [29,30]. The hard axis MOKE curve of Fig. 3 (c) is atypical, but the humped features around zero field are an artifact of the Kerr effect measurement geometry. When measuring MOKE in the longitudinal geometry, there are two possibilities for the magnetization direction in the sample that would be in the plane of incidence of the experimental setup. Longitudinal magnetization (in plane of the sample) and polar magnetization (out-of-plane of the sample) can both contribute to the polarization rotation or ellipticity change in the plane of incidence. Due to the rough surface of the Co coating, evidenced by microscopy, it is likely that some polar contribution to the magnetization is being measured when the magnetization vector happens to lie in the plane of incidence. The polar signal is not seen in Fig. 3(a) because the light intensity change due to the polarization rotation along the easy axis overwhelms the small polar signal. In Fig. 3 (b), L-MOKE hysteresis loops from the air and wheel sides of the uncoated ribbon are compared. The external field was applied perpendicular to the ribbon axis. On the wheel side, which is typically the rougher side, a split hysteresis loop is seen indicating a hard axis. It is worth mentioning that the rougher wheel side of a rapidly quenched ribbon could have multiple local anisotropies at the surface. In Fig. 3 (d), the Co-coated ribbon shows an increased coercivity and wider switching distribution on the wheel side as compared to the air side. This also points toward increased roughness on the wheel side of the ribbon. The field dependence of the DR/R, DX/X, and DZ/Z ratios are presented in Fig. 4(aec) for selected frequencies of 100, 400, and 900 MHz. At frequencies less than 400 MHz, the DR/R of the coated ribbon shows a beneficial increase while at higher frequencies the coating had a negative effect. The increase in DR/R is seen over a large field range. Conversely, the DX/X of the coated ribbon shows a reduction between jHDCj< ~50 Oe. Since the resistance contributes mostly to the impedance at this frequency range, the DZ/Z of the
coated ribbon ultimately has a beneficial increase up until 400 MHz. The frequency dependence of the maximum change in DR/R, DX/X, and DZ/Z is presented in Fig. 4(def), respectively. The field value of the maximum change in the impedance is given in the following Fig. 5(aec). At excitation frequencies less than 400 MHz, the benefit of the Co coating is seen in the DR/R and DZ/Z. Examination of the real and imaginary components of the impedance can provide insight into the effect of the Co-coating on the ac magnetic behavior of the ribbons. Fig. 4 (a) shows that the Co-coating had a beneficial effect on the DR/R over a large dc field range at frequencies near f ¼ 400 MHz. The change in reactance decreased from ~-50 Oe < HDC < ~50 Oe over all frequencies. This behavior can be understood through inductance formalism [31], which allows one to connect the impedance to a magnetic quantity. Inductance formalism relates the real (imaginary) part of the impedance to the imaginary (real) part of the permeability via a geometrical constant. In this formalism, the real part of the impedance (imaginary part of the permeability) is related to dissipative magnetization processes that contribute to the permeability, such as domain wall movement, irreversible rotation, etc., and the imaginary component of the impedance (real part of permeability) is related to conservative magnetic process such as reversible spin rotation [32]. Therefore, the increase in DR/R due to the coating could indicate an increase in lossy ac magnetization processes contributing to the impedance. However, since the increase in DR/R is over the entire field range, it is more likely the beneficial increase in DR/R is due to the lower resistivity of the Co. However, the DX/X was reduced at fields between ± 50 Oe. This could be evidence of some form of magnetic interaction between the Co layer and FeNi ribbon. The high degree of anisotropy evidenced in the Co layer could be strong enough to restrict the magnetization process of FeNi ribbon near the Co/FeNi interface, which would suppress reversible magnetization processes until the Co anisotropy field is reached, leading to a reduction in DX/X between ± 50 Oe. The symmetric peaks in amplitude and field position as seen in Fig. 4(aec) does not indicate a presence of magnetostatic coupling, as observed in thin film layered systems
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Fig. 4. DR/R (a), DX/X (b), and DZ/Z (c) at selected excitation frequencies as a function of HDC. Open symbols denote control ribbon and closed symbols denote 120 nm Co coated ribbon. (d) Maximum change in resistance, (e) maximum change in reactance, and (f) maximum change in impedance of the control and coated ribbon.
Fig. 5. Anisotropy field, HK, extracted from the peaks in field dependent resistance (a) reactance (b) and impedance (c) of the nanocrystalline control ribbon (open symbols) and Co coated ribbon (closed symbols).
[33,34]. Typically, magnetic film(t > 100 nm)/ribbon structures do not exhibit any features of magnetostatic or exchange coupling in their GMI curves at f < 20 MHz [11,12,16,17]. On the other hand, asymmetric peaks and hysteresis of the GMI curve is observed in
the Co(15 nm)/Co-rich ribbon system of Ref. [14] at f ¼ 10 MHz. The GMI peak position is the field value that makes the effective permeability a maximum and therefore the dc field that makes the Z a maximum is called the effective anisotropy field, HK. In Fig. 5, HK is extracted from the peaks in R(H), X(H), and Z(H). The HK from R (Fig. 5 (a)) is at most increased by 4 Oe due to the Co coating in the frequency range 100e1000 MHz. The HK from X (Fig. 5 (b)) also increased due to the Co coating. Overall, the anisotropy field was only modestly increased by the addition of the Co coating as seen in HK from Z (Fig. 5 (c)). This is expected due to the fact that most of the current is still flowing through the ribbon. It is of interest to those in the field to correlate the HK field from GMI to critical field values of the dc magnetization process such as coercive field and anisotropy field. Depending on the excitation frequency, the HK field from GMI may be related to the HK estimated from dc hysteresis loops. In general, at frequencies in the tens of MHz and below, it has been shown that the HK fields from GMI are close to the bulk anisotropy field of the sample [35,36]. As the exciting frequency reaches into the hundreds of MHz and GHz, however, the HK field is pushed to higher values where some or all of the Co/ribbon structure is uniformly magnetized. Therefore, it is not straightforward to correlate critical fields from the dc magnetization process to HK from GMI at high frequencies. At a frequency of 100 MHz, HK from Z (Fig. 5 (c)) of the uncoated and coated ribbon are both 6 Oe, indicating that at f ¼ 100 MHz the ribbon contributes mainly to the GMI response. Comparing these values to HK extracted from MOKE of the uncoated ribbon, the HK from GMI at f ¼ 100 MHz is slightly larger; H100MHz ¼ 6 Oe as K compared to HMOKE ¼ 3 Oe. It is important to note that MOKE is a K localized measurement; the spot size used in this research is roughly 1 mm in diameter, while GMI is an average over the length of the ribbon. This could be a reason for the small difference in HK value. There is no attempt to correlate the HK estimated from the MOKE of the Co-coated ribbon (HMOKE ~ 100 Oe) since the surface K effect of MOKE is sampling, at most, the top 20 nm of the 120 nmthick Co coating, while the penetration depth of the current is
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estimated to be on the order of microns and flowing mostly within the ribbon. 4. Conclusions In conclusion, a comparative study of the surface magnetization and magnetoimpedance effect in an uncoated and 120 nm Cocoated (Fe0.5Ni0.5)81Nb7B12 nanocrystalline ribbon was conducted. The impact of the transverse anisotropy of the Co coating, evident from MOKE, was to increase the DZ/Z by a majority contribution from the DR/R. However, the high anisotropy of the Co layer reduced DX/X and increased the dc field where DX/X is a maximum. This could point to a coupling or interaction of the hard Co layer and the soft FeNi ribbon, which acts to reduce the reversible rotational permeability. The HK fields of the coated ribbon increased by only a few Oe at frequencies less than 600 MHz, where there is an increase in DZ/Z, implying that the skin depth in this region still contains the majority of the FeNi ribbon bulk. Novelty statement In this work we have demonstrated a new and effective approach to improving the high frequency MI response of a Febased nanocrystalline ribbon by coating it with a magnetic thin film with transverse anisotropy. The impact of the Co coating on the high frequency impedance of the (Fe50Ni50)81Nb7B12 ribbon was studied with techniques sensitive to surface magnetism, namely magneto-optical Kerr effect (MOKE), MOKE microscopy, and high frequency impedance as a function of magnetic field. Our study also provides important insights into the effects of magnetostatic interaction between layers on the MI behavior in soft magnetic layer structures. Acknowledgements Research at USF was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-FG02-07ER46438 (MOKE and magnetoimpedance studies). The Slovak group acknowledges support of the projects APVV-0492-1, VEGA 2/0173/16 and ITMS 26220220037. We thank Prof. Hari Srikanth for useful discussions. References [1] M.H. Phan, H.X. Peng, Giant magnetoimpedance materials: fundamentals and applications, Prog. Mater. Sci. 53 (2008) 323e420, https://doi.org/10.1016/ j.pmatsci.2007.05.003. [2] R.S. Beach, a. E. Berkowitz, Giant magnetic field dependent impedance of amorphous FeCoSiB wire, Appl. Phys. Lett. 64 (1994) 3652e3654, https:// doi.org/10.1063/1.111170. [3] L.V. Panina, K. Mohri, T. Uchiyama, M. Noda, K. Bushida, Giant magnetoimpedance in Co-rich amorphous wires and films, IEEE Trans. Magn. 31 (1995) 1249e1260, https://doi.org/10.1109/20.364815. [4] A. Talaat, V. Zhukova, M. Ipatov, J.M. Blanco, L. Gonzalez-Legarreta, B. Hernando, J.J. Del Val, J. Gonzalez, A. Zhukov, Optimization of the giant magnetoimpedance effect of Finemet-type microwires through the nanocrystallization, J. Appl. Phys. 115 (2014) 6e9, https://doi.org/10.1063/ 1.4863484. [5] A. Zhukov, A.B. Chizhik, M. Ipatov, A. Talaat, J.M. Blanco, A. Stupakiewicz, V. Zhukova, Giant magnetoimpedance effect and domain wall dynamics in Corich amorphous microwires Giant magnetoimpedance effect and domain wall dynamics in Co-rich amorphous microwires, J. Appl. Phys. 117 (2015) 43904, https://doi.org/10.1063/1.4906503. [6] A. Zhukov, A. Talaat, M. Churyukanova, S. Kaloshkin, V. Semenkova, M. Ipatov, J.M. Blanco, V. Zhukova, Engineering of magnetic properties and GMI effect in Co-rich amorphous microwires, J. Alloy. Comp. 664 (2016) 235e241, https:// doi.org/10.1016/j.jallcom.2015.12.224. [7] A. Zhukov, M. Ipatov, M. Churyukanova, A. Talaat, J.M. Blanco, V. Zhukova, Trends in optimization of giant magnetoimpedance effect in amorphous and nanocrystalline materials, J. Alloy. Comp. 727 (2017) 887e901, https://doi.org/ 10.1016/j.jallcom.2017.08.119.
[8] J. Devkota, J. Wingo, T.T.T. Mai, X.P. Nguyen, N.T. Huong, P. Mukherjee, H. Srikanth, M.H. Phan, J. Devkota, J. Wingo, T.T.T. Mai, X.P. Nguyen, N.T. Huong, P. Mukherjee, A highly sensitive magnetic biosensor for detection and quantification of anticancer drugs tagged to superparamagnetic nanoparticles, J. Appl. Phys. 115 (2014), https://doi.org/10.1063/1.4862395, 17B603. [9] B. Li, M.N. Kavaldzhiev, J. Kosel, Flexible magnetoimpedance sensor, J. Magn. Magn. Mater. 378 (2015) 499e505, https://doi.org/10.1016/ j.jmmm.2014.11.067. [10] T. Eggers, A. Leary, M. McHenry, J. Marcin, I. Skorvanek, H. Srikanth, M.H. Phan, Correlation between domain structure, surface anisotropy and high frequency magneto-impedance in Joule annealed CoFe-based melt-spun ribbons, J. Alloy. Comp. 682 (2016) 799e804, https://doi.org/10.1016/j.jallcom.2016.04.312. [11] X. Sun, J. Du, Z. Zhu, J. Wang, Q. Liu, Enhanced GMI effect in NiZn-ferritemodified Fe-based amorphous ribbons, Appl. Phys. A Mater. Sci. Process 119 (2015) 1277e1281, https://doi.org/10.1007/s00339-015-9092-5. [12] Y. Han, X. Li, W.X. Lv, W.H. Xie, Q. Zhao, Z.J. Zhao, Magnetoimpedance effect of FINEMET ribbons coated with Fe20Ni80 permalloy film, J. Alloy. Comp. 678 (2016) 494e498, https://doi.org/10.1016/j.jallcom.2016.04.021. , a V. Svalov, [13] G.V. Kurlyandskaya, L. Elbaile, F. Alves, B. Ahamada, R. Barrue V.O. Vas kovskiy, Domain structure and magnetization process of a giant magnetoimpedance geometry FeNi/Cu/FeNi(Cu)FeNi/Cu/FeNi sensitive element, J. Phys. Condens. Matter 16 (2004) 6561e6568, https://doi.org/ 10.1088/0953-8984/16/36/021. [14] L. Jamilpanah, M.R. Hajiali, S.M. Mohseni, S. Erfanifam, S.M. Mohseni, M. Houshiar, S.E. Roozmeh, Magnetoimpedance exchange coupling in different magnetic strength thin layers electrodeposited on Co-based magnetic ribbons, J. Phys. D Appl. Phys. 50 (2017), 155001, https://doi.org/ 10.1088/1361-6463/aa6098. [15] Y. Zhang, T. Gan, T. Wang, F. Wang, W. Shi, Giant magnetoimpedance effect in Fe75.5Cu1Nb3Si13.5B7 ribbon/FeGa film composite, J. Magn. Magn. Mater. 417 (2016) 37e41, https://doi.org/10.1016/j.jmmm.2016.05.062. [16] D. Mukherjee, J. Devkota, A. Ruiz, M. Hordagoda, R. Hyde, S. Witanachchi, P. Mukherjee, H. Srikanth, M.H. Phan, Impacts of amorphous and crystalline cobalt ferrite layers on the giant magneto-impedance response of a soft ferromagnetic amorphous ribbon, J. Appl. Phys. 116 (2014), https://doi.org/ 10.1063/1.4896583. [17] N. Laurita, A. Chaturvedi, C. Bauer, P. Jayathilaka, A. Leary, C. Miller, M.H. Phan, M.E. McHenry, H. Srikanth, Enhanced giant magnetoimpedance effect and field sensitivity in Co-coated soft ferromagnetic amorphous ribbons, J. Appl. Phys. 109 (2011) 3e5, https://doi.org/10.1063/1.3548857. nchez, B. Hernando, [18] A. Chizhik, V. Vega, A.E.-M.A. Mohamed, V.M. Prida, T. Sa M. Ipatov, V. Zhukova, A.P. Zhukov, A. Stupakiewicz, L. Domínguez, J. Gonz alez, Surface magnetic properties and giant magnetoimpedance effect in Co-based amorphous ribbons, Intermetallics 86 (2017) 15e19, https:// doi.org/10.1016/j.intermet.2017.03.010. [19] H.Q. Guo, H. Kronmüller, T. Dragon, Z.H. Cheng, B.G. Shen, Influence of nanocrystallization on the evolution of domain patterns and the magnetoimpedance effect in amorphous Fe73.5Cu1Nb3Si13.5B9 ribbons, J. Appl. Phys. 89 (2001) 514e520, https://doi.org/10.1063/1.1331649. [20] A.A. Taysioglu, A. Peksoz, Y. Kaya, N. Derebasi, G. Irez, G. Kaynak, GMI effect in CuO coated Co-based amorphous ribbons, J. Alloy. Comp. 487 (2009) 38e41, https://doi.org/10.1016/j.jallcom.2009.08.041. [21] A. Peksoz, Y. Kaya, A.A. Taysioglu, N. Derebasi, G. Kaynak, Giant magnetoimpedance effect in diamagnetic organic thin film coated amorphous ribbons, Sens. Actuators, A Phys. 159 (2010) 69e72, https://doi.org/10.1016/ j.sna.2010.03.002. [22] F.L.A. Machado, B.L. Da Silva, S.M. Rezende, C.S. Martins, Giant ac magnetoresistance in the soft ferromagnet Co70.4Fe 4.6Si15B10, J. Appl. Phys. 75 (1994) 6563e6565, https://doi.org/10.1063/1.356919. [23] F.L.A. Machado, S.M. Rezende, A theoretical model for the giant magnetoimpedance in ribbons of amorphous soft-ferromagnetic alloys, J. Appl. Phys. 78 (1996) 6558, https://doi.org/10.1063/1.361945. , J. Marcin, J. Kov nek, Magnetic [24] J. Tur canova a c, D. Jani ckovi c, P. Svec, I. Skorv a and mechanical properties of nanocrystalline Fe-Ni-Nb-B Alloys, J. Phys. Conf. 144 (2009) 12065, https://doi.org/10.1088/1742-6596/144/1/012065. [25] G. Herzer, Soft magnetic nanocrystalline materials, Scripta Metall. Mater. 33 (1995) 1741e1756, https://doi.org/10.1016/0956-716X(95)00397-E. [26] P. Svec, M. Miglierini, J. Dekan, J. Tur, G. Vlas ak, I. Skorv anek, Influence of structure evolution on magnetic properties of Fe e Ni e Nb e B system 46 (2010) 412e415. [27] L. Brunetti, P. Tiberto, F. Vinai, A new measurement method for magnetoimpedance in soft amorphous ferromagnets at microwave frequency, Sens. Actuators A Phys. 67 (1998) 84e88. [28] A. Sharma, R. Brajpuriya, S. Tripathi, D. Jain, R. Dubey, T. Shripathi, S.M. Chaudhari, A comparative study of Co thin film deposited on GaAs (100) and glass substrates, Mater. Sci. Eng. B 130 (2006) 120e125, https://doi.org/ 10.1016/j.mseb.2006.02.065. [29] A. Sharma, P. Bhatt, R. Brajpuriya, S. Tripathi, S.M. Chaudhari, Correlation study between structural, magnetic and transport properties of annealed Co thin films, Vacuum 78 (2005) 47e51, https://doi.org/10.1016/ j.vacuum.2004.12.013. [30] C.G. Kim, Y.W. Rheem, C.O. Kim, E.E. Shalyguina, E.A. Ganshina, Magnetostatic properties of heterogeneous Co-based amorphous/crystalline phases, J. Magn. Magn. Mater. 262 (2003) 412e419, https://doi.org/10.1016/S0304-8853(03)
T. Eggers et al. / Journal of Alloys and Compounds 741 (2018) 1105e1111 00072-6. [31] R. Valenzuela, Giant magnetoimpedance and inductance spectroscopy, J. Alloy. Comp. 369 (2004) 40e42, https://doi.org/10.1016/ j.jallcom.2003.09.057. [32] J.B. Goodenough, Summary of losses in magnetic materials, IEEE Trans. Magn. 38 (2002) 3398e3408, https://doi.org/10.1109/TMAG.2002.802741. [33] N.A. Buznikov, A.S. Antonov, Asymmetric magnetoimpedance in ferromagnetic biphase films: effects of magnetostatic coupling and number of layers, J. Supercond. Nov. Magn. (2017) 1e6, https://doi.org/10.1007/s10948-0174069-6.
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zquez, [34] E.F. Silva, R.B. Da Silva, M. Gamino, A.M.H. De Andrade, M. Va ^a, F. Bohn, Asymmetric magnetoimpedance effect in ferromagnetic M.A. Corre multilayered biphase films, J. Magn. Magn. Mater. 393 (2015) 260e264, https://doi.org/10.1016/j.jmmm.2015.05.079. [35] L. Kraus, Theory of giant magneto-impedance in the planar conductor with uniaxial magnetic anisotropy, J. Magn. Magn. Mater. 195 (1999) 764e778, https://doi.org/10.1016/S0304-8853(99)00286-3. [36] D. Menard, M. Britel, P. Ciureanu, A. Yelon, Giant magnetoimpedance in a cylindrical magnetic conductor, J. Appl. Phys. 84 (1998) 2805, https://doi.org/ 10.1063/1.368421.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Impact of the transverse magnetocrystalline anisotropy of a Co coating layer on the magnetoimpedance response of FeNi-rich nanocrystalline ribbon d T. Eggers a, **, D.S. Lam a, b, O. Thiabgoh a, J. Marcin c, P. Svec , N.T. Huong b, c , *** a ,* nek , M.H. Phan I. Skorva a
Department of Physics, University of South Florida, Tampa, FL 33620, USA Department of Physics, Hanoi National University, 334 Nguyen Trai, Hanoi, Viet Nam Institute of Experimental Physics, Slovak Academy of Sciences, Watsonova 47, 040 01, Kosice, Slovakia d Institute of Physics, Slovak Academy of Sciences, Dubravska cesta 9, 845 11, Bratislava, Slovakia b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 June 2017 Received in revised form 11 January 2018 Accepted 14 January 2018 Available online 31 January 2018
There is a pressing need for improving the high-frequency magneto-impedance (MI) response of costeffective soft magnetic materials for use in high-performance sensing devices. We report here a comparative study of uncoated (Fe50Ni50)81Nb7B12 nanocrystalline ribbons and ribbons coated with 120 nm of Co on both surfaces. The impact of the Co coating on the high frequency impedance of the (Fe50Ni50)81Nb7B12 ribbon was studied with techniques sensitive to surface magnetism, namely magneto-optical Kerr effect (MOKE), MOKE microscopy, and high frequency impedance as a function of magnetic field. MOKE microscopy on the coated ribbons showed a superposition of high anisotropy domains from the crystalline Co and wide magnetic domains from (Fe50Ni50)81Nb7B12. In full agreement with the MOKE results, MI measurements indicated an increase in anisotropy field in the excitation frequency range 100e1000 MHz in the Co-coated ribbons. Noticeably, the Co-coating improved the MI ratio by 15% at fields near and below the anisotropy field. The improvement of the low-field MI response by the coating encourages the use of magnetic coatings to improve MI-based sensors in ribbon geometry for technological applications such as biomagnetic sensing. This study also yields insights into the effects of negative magnetostrictive coatings on the MI behavior in soft magnetic ribbons. © 2018 Elsevier B.V. All rights reserved.
Keywords: Fe-rich amorphous ribbons Surface anisotropy Magneto-impedance Magnetic sensors
1. Introduction Understanding the relationship between the surface magnetic properties, microstructure and giant magneto-impedance (GMI) effect in soft ferromagnetic ribbons is essential for the development of sensitive magnetic field sensors for a variety of applications [1]. The first groups to consider GMI for technological application were chiefly concerned with sensitive changes in electrical impedance at excitation frequencies less than 10 MHz [2,3]. They described the impedance change as the classical consequence of the skin effect, where the distribution of an ac current of frequency, f, in a soft
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (I. Skorv anek), [email protected] (M.H. Phan).
(T.
https://doi.org/10.1016/j.jallcom.2018.01.206 0925-8388/© 2018 Elsevier B.V. All rights reserved.
Eggers),
[email protected]
magnetic conductor of conductivity, s, flows near the surface at a depth d ¼ ðpsf mT Þ1=2 . The large and sensitive transverse effective permeability, mT(HDC), in the skin depth region is required for unlocking giant changes in impedance. At excitation frequencies greater than 10 MHz, where many of today's current integrated circuit technology operates, the GMI effect is known to suffer due to reduction in the effective permeability by relaxation of domain wall movement in ribbons. While a large GMI effect, on the order of hundreds of percent, has been demonstrated in optimized Fe-rich [4] and Co-rich [5,6] microwires at high frequency (a current review on the subject is given in Ref. [7]), the GMI effect is generally low in the ribbon geometry. Since the platform shape of a ribbon or film is highly desirable to the application of GMI to biomagnetic [8] or flexible stress [9] sensors, there is still effort put toward research into the effects of post-processing techniques to increase the GMI effect in ribbons by way of annealing [10] or coating [11e16]. Some works have reported a significant enhancement of the
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GMI effect upon coating ribbons with other magnetic layers. For example, an increase in the GMI ratio at 2.6 MHz was observed when a nanocrystalline Fe73.5Cu1Nb3Si13.5B9 (FINEMET) ribbon was coated with (130e500 nm) Ni80Fe20 film [12]. The magnetic exchange interaction and low stress between layers due to a lattice match between the Ni80Fe20 film and the Fe73.5Cu1Nb3Si13.5B9 ribbon was suggested to promote the GMI enhancement. In another study conducted by Laurita et al. [17], the authors found that sputter deposition of a 50 nm Co film enhanced both the GMI ratio and sensitivity of a HITPERM ribbon at frequencies of 10 MHz and below. The enhancement was attributed to the reduction of stray fields and an improved magnetic flux path closure at the ribbon's surface. At excitation frequencies below 10 MHz, the effective permeability change with magnetic field is governed by magnetization reorientation of the film/ribbon bulk, while at higher frequencies, the skin depth approaches its minimum value and more of the surface and interface of the film/ribbon system is sampled. A clear understanding of how a magnetic coating affects the effective permeability and the GMI response at high frequency (f > 100 MHz) of the ribbon is not well understood. Since amorphous ribbons with a domain structure or anisotropy transverse to the current direction are excellent candidates for GMI based sensors due to high GMI ratios [18,19], it would follow that the magnetic coating should be capable of supporting a transverse anisotropy [16,17,20e23]. From a structure-magnetic property perspective, Co is chosen as the coating layer since it has a negative magnetostriction. It has been shown that isothermally annealed FeNi-based ribbons contract along their length [24], therefore could be a source of a stress-induced transverse anisotropy in the Co coating. Annealing these ribbons at 500 C achieves uniform, nanosized crystalline grains and thus soft magnetic behavior beneficial to the GMI effect. In a nanocrystallized system, the local magnetocrystalline anisotropy of the nanograins embedded in the amorphous matrix is randomly averaged out by their exchange interaction resulting in a small net anisotropy [25]. In this work, we aim to investigate the impact of a 120 nm polycrystalline Co coating on two sides of an amorphous (Fe50Ni50)81Nb7B12 ribbon, which was subsequently annealed along with a control ribbon to achieve a nanocrystalline morphology. Using magneto-optical Kerr effect to measure and image the low field dc surface magnetization, the positive and negative effects of the Co coating layer with a transverse anisotropy on the magnetoimpedance of the ribbon are assessed. 2. Materials and methods Amorphous ribbons of composition (Fe50Ni50)81Nb7B12 were prepared by planar flow casting with master alloys prepared from elements with purity better than 99.95% [26]. Both sides of a 4 cm section of as-quenched ribbon were coated with 120 nm of Co by RF magnetron sputter deposition. The coated ribbon, along with an uncoated control ribbon, was annealed for 1 h at 500 C under high vacuum in order to achieve a low coercive field and soft magnetic behavior [24]. X-ray diffraction (XRD) of the coated and uncoated ribbons was conducted using Cu Ka radiation. Longitudinal magneto-optical Kerr effect (L-MOKE) hysteresis loops were taken using a homemade setup with a 5 mW (635 nm) laser source. The applied magnetic field was oriented in two directions, along the length of the ribbon and perpendicular to the length of the ribbon. MOKE microscopy was conducted by Magneto-optical Kerr Microscope (EVICO Magnetics), also in the longitudinal geometry, to image the domain structure on the surface of the coated and uncoated ribbons with the magnetic field applied along the ribbon axis. Magneto-impedance was measured as a function of applied field
(Hmax ¼ 110 Oe) along the length of the ribbon at high frequency (100e1000 MHz) using an HP 4191A impedance analyzer. The HP 4191A determines the complex reflection coefficient, G, of a high frequency signal sent down a terminated transmission line. An airline [27] made of a 2 cm section of ribbon suspended over a Cu ground plane served as the transmission line, which was terminated with a 50 U standard that matches the input impedance of the analyzer. The complex reflection coefficient at the beginning of the ribbon airline was measured and converted to complex impedance (Z) by
Z ¼ 50
1G ¼ R þ jX; 1þG
(1)
where R is the resistance, X is the reactance, and j is the imaginary unit. In this work, the magnetoimpedance ratio is defined as
DZ Z
ð%Þ ¼
ZðHÞ ZðHmaxÞ 100; ZðHmaxÞ
(2)
where Z(H) is the impedance (or resistance, reactance) at field H. The impedance ratio is normalized by Z(H¼Hmax). 3. Results and discussion Fig. 1 (a),(b) shows XRD patterns of the control (Fe50Ni50)81Nb7B12 ribbon and Co-coated ribbon after annealing at 500 C for 1 h, respectively. The nanocrystallized control ribbon shows a hump near 2q ¼ 43.9 , which corresponds to the fcc(111) reflection of bulk Fe0.5Ni0.5 and an additional peak at 2q ¼ 50.79 identified as (200) reflection of the fcc phase. This agrees with prior research on these FeNiNbB ribbons [24]. The XRD pattern of the annealed Co-coated ribbon shows the development of a shoulder near 44.5 , which is likely due to the presence of Co hcp(002) and/ or Co fcc(111), which have very close lattice spacings [28,29]. Fig. 1 (c) shows the nanocrystalline surface microstructure of the control FeNi ribbon after the 1 h, 500 C annealing treatment. Fig. 1 (d) is a cross sectional SEM image of the clean interface between the 120 nm layer of Co on the FeNi ribbon. This image shows the Cocoating was continuous and mechanically solid interface was formed between the Co layer and the ribbon. Kerr microscopy was used to image the low-field domain structure of the ribbons after the 1 h anneal at 500 C. Fig. 2(aeb) shows the domain patterns on the air side of the uncoated FeNibased ribbon with increasing magnetic field (0
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Fig. 1. XRD patterns of (a) the nanocrystalline control ribbon annealed at 500 C and (b) the ribbon with 120 nm Co. (c) TEM of nanocrystalline uncoated ribbon and (d) SEM of coated ribbon cross section.
Fig. 2. Domain structure visualized by Magneto-optical Kerr effect microscopy on (a,b) uncoated ribbon and (c,d) Co-coated ribbon with increasing HDC along ribbon length from left to right.
Fig. 3e). Fig. 3 (a) shows L-MOKE on the air side of the uncoated ribbon in the two measuring directions. A super soft easy axis with a coercive field (HC) of about 0.2 Oe. A hard axis hysteresis loop is found when the air side of the ribbon was rotated 900 (perpendicular to the casting direction/ribbon length) and Kerr effect measured. The anisotropy field can be roughly estimated from the
saturation field of the hard axis if it is assumed the anisotropy is uniaxial. The anisotropy field estimated from MOKE of the uncoated ribbon is around 3 Oe. Meanwhile, the Co-coated ribbon has a square hysteresis loop when measured in the perpendicular field orientation, while a hard axis is evident in the parallel orientation (Fig. 3 (c)). This is a clear indication of a transverse (easy axis
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Fig. 3. L-MOKE on the nanocrystalline control ribbon (a) and 120 nm Co coated ribbon (b) in two measuring directions. L-MOKE on the air and wheel side of the control ribbon (c) and 120 nm Co coated ribbon (d). (e) shows the two measuring directions, HDC parallel and perpendicular to ribbon length.
transverse to the ribbon length) magnetocrystalline anisotropy attributed to the polycrystalline Co-coating. The coercive field of about 50 Oe is typical for a 120 nm Co film annealed at 500 C [29,30]. The hard axis MOKE curve of Fig. 3 (c) is atypical, but the humped features around zero field are an artifact of the Kerr effect measurement geometry. When measuring MOKE in the longitudinal geometry, there are two possibilities for the magnetization direction in the sample that would be in the plane of incidence of the experimental setup. Longitudinal magnetization (in plane of the sample) and polar magnetization (out-of-plane of the sample) can both contribute to the polarization rotation or ellipticity change in the plane of incidence. Due to the rough surface of the Co coating, evidenced by microscopy, it is likely that some polar contribution to the magnetization is being measured when the magnetization vector happens to lie in the plane of incidence. The polar signal is not seen in Fig. 3(a) because the light intensity change due to the polarization rotation along the easy axis overwhelms the small polar signal. In Fig. 3 (b), L-MOKE hysteresis loops from the air and wheel sides of the uncoated ribbon are compared. The external field was applied perpendicular to the ribbon axis. On the wheel side, which is typically the rougher side, a split hysteresis loop is seen indicating a hard axis. It is worth mentioning that the rougher wheel side of a rapidly quenched ribbon could have multiple local anisotropies at the surface. In Fig. 3 (d), the Co-coated ribbon shows an increased coercivity and wider switching distribution on the wheel side as compared to the air side. This also points toward increased roughness on the wheel side of the ribbon. The field dependence of the DR/R, DX/X, and DZ/Z ratios are presented in Fig. 4(aec) for selected frequencies of 100, 400, and 900 MHz. At frequencies less than 400 MHz, the DR/R of the coated ribbon shows a beneficial increase while at higher frequencies the coating had a negative effect. The increase in DR/R is seen over a large field range. Conversely, the DX/X of the coated ribbon shows a reduction between jHDCj< ~50 Oe. Since the resistance contributes mostly to the impedance at this frequency range, the DZ/Z of the
coated ribbon ultimately has a beneficial increase up until 400 MHz. The frequency dependence of the maximum change in DR/R, DX/X, and DZ/Z is presented in Fig. 4(def), respectively. The field value of the maximum change in the impedance is given in the following Fig. 5(aec). At excitation frequencies less than 400 MHz, the benefit of the Co coating is seen in the DR/R and DZ/Z. Examination of the real and imaginary components of the impedance can provide insight into the effect of the Co-coating on the ac magnetic behavior of the ribbons. Fig. 4 (a) shows that the Co-coating had a beneficial effect on the DR/R over a large dc field range at frequencies near f ¼ 400 MHz. The change in reactance decreased from ~-50 Oe < HDC < ~50 Oe over all frequencies. This behavior can be understood through inductance formalism [31], which allows one to connect the impedance to a magnetic quantity. Inductance formalism relates the real (imaginary) part of the impedance to the imaginary (real) part of the permeability via a geometrical constant. In this formalism, the real part of the impedance (imaginary part of the permeability) is related to dissipative magnetization processes that contribute to the permeability, such as domain wall movement, irreversible rotation, etc., and the imaginary component of the impedance (real part of permeability) is related to conservative magnetic process such as reversible spin rotation [32]. Therefore, the increase in DR/R due to the coating could indicate an increase in lossy ac magnetization processes contributing to the impedance. However, since the increase in DR/R is over the entire field range, it is more likely the beneficial increase in DR/R is due to the lower resistivity of the Co. However, the DX/X was reduced at fields between ± 50 Oe. This could be evidence of some form of magnetic interaction between the Co layer and FeNi ribbon. The high degree of anisotropy evidenced in the Co layer could be strong enough to restrict the magnetization process of FeNi ribbon near the Co/FeNi interface, which would suppress reversible magnetization processes until the Co anisotropy field is reached, leading to a reduction in DX/X between ± 50 Oe. The symmetric peaks in amplitude and field position as seen in Fig. 4(aec) does not indicate a presence of magnetostatic coupling, as observed in thin film layered systems
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Fig. 4. DR/R (a), DX/X (b), and DZ/Z (c) at selected excitation frequencies as a function of HDC. Open symbols denote control ribbon and closed symbols denote 120 nm Co coated ribbon. (d) Maximum change in resistance, (e) maximum change in reactance, and (f) maximum change in impedance of the control and coated ribbon.
Fig. 5. Anisotropy field, HK, extracted from the peaks in field dependent resistance (a) reactance (b) and impedance (c) of the nanocrystalline control ribbon (open symbols) and Co coated ribbon (closed symbols).
[33,34]. Typically, magnetic film(t > 100 nm)/ribbon structures do not exhibit any features of magnetostatic or exchange coupling in their GMI curves at f < 20 MHz [11,12,16,17]. On the other hand, asymmetric peaks and hysteresis of the GMI curve is observed in
the Co(15 nm)/Co-rich ribbon system of Ref. [14] at f ¼ 10 MHz. The GMI peak position is the field value that makes the effective permeability a maximum and therefore the dc field that makes the Z a maximum is called the effective anisotropy field, HK. In Fig. 5, HK is extracted from the peaks in R(H), X(H), and Z(H). The HK from R (Fig. 5 (a)) is at most increased by 4 Oe due to the Co coating in the frequency range 100e1000 MHz. The HK from X (Fig. 5 (b)) also increased due to the Co coating. Overall, the anisotropy field was only modestly increased by the addition of the Co coating as seen in HK from Z (Fig. 5 (c)). This is expected due to the fact that most of the current is still flowing through the ribbon. It is of interest to those in the field to correlate the HK field from GMI to critical field values of the dc magnetization process such as coercive field and anisotropy field. Depending on the excitation frequency, the HK field from GMI may be related to the HK estimated from dc hysteresis loops. In general, at frequencies in the tens of MHz and below, it has been shown that the HK fields from GMI are close to the bulk anisotropy field of the sample [35,36]. As the exciting frequency reaches into the hundreds of MHz and GHz, however, the HK field is pushed to higher values where some or all of the Co/ribbon structure is uniformly magnetized. Therefore, it is not straightforward to correlate critical fields from the dc magnetization process to HK from GMI at high frequencies. At a frequency of 100 MHz, HK from Z (Fig. 5 (c)) of the uncoated and coated ribbon are both 6 Oe, indicating that at f ¼ 100 MHz the ribbon contributes mainly to the GMI response. Comparing these values to HK extracted from MOKE of the uncoated ribbon, the HK from GMI at f ¼ 100 MHz is slightly larger; H100MHz ¼ 6 Oe as K compared to HMOKE ¼ 3 Oe. It is important to note that MOKE is a K localized measurement; the spot size used in this research is roughly 1 mm in diameter, while GMI is an average over the length of the ribbon. This could be a reason for the small difference in HK value. There is no attempt to correlate the HK estimated from the MOKE of the Co-coated ribbon (HMOKE ~ 100 Oe) since the surface K effect of MOKE is sampling, at most, the top 20 nm of the 120 nmthick Co coating, while the penetration depth of the current is
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estimated to be on the order of microns and flowing mostly within the ribbon. 4. Conclusions In conclusion, a comparative study of the surface magnetization and magnetoimpedance effect in an uncoated and 120 nm Cocoated (Fe0.5Ni0.5)81Nb7B12 nanocrystalline ribbon was conducted. The impact of the transverse anisotropy of the Co coating, evident from MOKE, was to increase the DZ/Z by a majority contribution from the DR/R. However, the high anisotropy of the Co layer reduced DX/X and increased the dc field where DX/X is a maximum. This could point to a coupling or interaction of the hard Co layer and the soft FeNi ribbon, which acts to reduce the reversible rotational permeability. The HK fields of the coated ribbon increased by only a few Oe at frequencies less than 600 MHz, where there is an increase in DZ/Z, implying that the skin depth in this region still contains the majority of the FeNi ribbon bulk. Novelty statement In this work we have demonstrated a new and effective approach to improving the high frequency MI response of a Febased nanocrystalline ribbon by coating it with a magnetic thin film with transverse anisotropy. The impact of the Co coating on the high frequency impedance of the (Fe50Ni50)81Nb7B12 ribbon was studied with techniques sensitive to surface magnetism, namely magneto-optical Kerr effect (MOKE), MOKE microscopy, and high frequency impedance as a function of magnetic field. Our study also provides important insights into the effects of magnetostatic interaction between layers on the MI behavior in soft magnetic layer structures. Acknowledgements Research at USF was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-FG02-07ER46438 (MOKE and magnetoimpedance studies). The Slovak group acknowledges support of the projects APVV-0492-1, VEGA 2/0173/16 and ITMS 26220220037. We thank Prof. Hari Srikanth for useful discussions. References [1] M.H. Phan, H.X. Peng, Giant magnetoimpedance materials: fundamentals and applications, Prog. Mater. Sci. 53 (2008) 323e420, https://doi.org/10.1016/ j.pmatsci.2007.05.003. [2] R.S. Beach, a. E. Berkowitz, Giant magnetic field dependent impedance of amorphous FeCoSiB wire, Appl. Phys. Lett. 64 (1994) 3652e3654, https:// doi.org/10.1063/1.111170. [3] L.V. Panina, K. Mohri, T. Uchiyama, M. Noda, K. Bushida, Giant magnetoimpedance in Co-rich amorphous wires and films, IEEE Trans. Magn. 31 (1995) 1249e1260, https://doi.org/10.1109/20.364815. [4] A. Talaat, V. Zhukova, M. Ipatov, J.M. Blanco, L. Gonzalez-Legarreta, B. Hernando, J.J. Del Val, J. Gonzalez, A. Zhukov, Optimization of the giant magnetoimpedance effect of Finemet-type microwires through the nanocrystallization, J. Appl. Phys. 115 (2014) 6e9, https://doi.org/10.1063/ 1.4863484. [5] A. Zhukov, A.B. Chizhik, M. Ipatov, A. Talaat, J.M. Blanco, A. Stupakiewicz, V. Zhukova, Giant magnetoimpedance effect and domain wall dynamics in Corich amorphous microwires Giant magnetoimpedance effect and domain wall dynamics in Co-rich amorphous microwires, J. Appl. Phys. 117 (2015) 43904, https://doi.org/10.1063/1.4906503. [6] A. Zhukov, A. Talaat, M. Churyukanova, S. Kaloshkin, V. Semenkova, M. Ipatov, J.M. Blanco, V. Zhukova, Engineering of magnetic properties and GMI effect in Co-rich amorphous microwires, J. Alloy. Comp. 664 (2016) 235e241, https:// doi.org/10.1016/j.jallcom.2015.12.224. [7] A. Zhukov, M. Ipatov, M. Churyukanova, A. Talaat, J.M. Blanco, V. Zhukova, Trends in optimization of giant magnetoimpedance effect in amorphous and nanocrystalline materials, J. Alloy. Comp. 727 (2017) 887e901, https://doi.org/ 10.1016/j.jallcom.2017.08.119.
[8] J. Devkota, J. Wingo, T.T.T. Mai, X.P. Nguyen, N.T. Huong, P. Mukherjee, H. Srikanth, M.H. Phan, J. Devkota, J. Wingo, T.T.T. Mai, X.P. Nguyen, N.T. Huong, P. Mukherjee, A highly sensitive magnetic biosensor for detection and quantification of anticancer drugs tagged to superparamagnetic nanoparticles, J. Appl. Phys. 115 (2014), https://doi.org/10.1063/1.4862395, 17B603. [9] B. Li, M.N. Kavaldzhiev, J. Kosel, Flexible magnetoimpedance sensor, J. Magn. Magn. Mater. 378 (2015) 499e505, https://doi.org/10.1016/ j.jmmm.2014.11.067. [10] T. Eggers, A. Leary, M. McHenry, J. Marcin, I. Skorvanek, H. Srikanth, M.H. Phan, Correlation between domain structure, surface anisotropy and high frequency magneto-impedance in Joule annealed CoFe-based melt-spun ribbons, J. Alloy. Comp. 682 (2016) 799e804, https://doi.org/10.1016/j.jallcom.2016.04.312. [11] X. Sun, J. Du, Z. Zhu, J. Wang, Q. Liu, Enhanced GMI effect in NiZn-ferritemodified Fe-based amorphous ribbons, Appl. Phys. A Mater. Sci. Process 119 (2015) 1277e1281, https://doi.org/10.1007/s00339-015-9092-5. [12] Y. Han, X. Li, W.X. Lv, W.H. Xie, Q. Zhao, Z.J. Zhao, Magnetoimpedance effect of FINEMET ribbons coated with Fe20Ni80 permalloy film, J. Alloy. Comp. 678 (2016) 494e498, https://doi.org/10.1016/j.jallcom.2016.04.021. , a V. Svalov, [13] G.V. Kurlyandskaya, L. Elbaile, F. Alves, B. Ahamada, R. Barrue V.O. Vas kovskiy, Domain structure and magnetization process of a giant magnetoimpedance geometry FeNi/Cu/FeNi(Cu)FeNi/Cu/FeNi sensitive element, J. Phys. Condens. Matter 16 (2004) 6561e6568, https://doi.org/ 10.1088/0953-8984/16/36/021. [14] L. Jamilpanah, M.R. Hajiali, S.M. Mohseni, S. Erfanifam, S.M. Mohseni, M. Houshiar, S.E. Roozmeh, Magnetoimpedance exchange coupling in different magnetic strength thin layers electrodeposited on Co-based magnetic ribbons, J. Phys. D Appl. Phys. 50 (2017), 155001, https://doi.org/ 10.1088/1361-6463/aa6098. [15] Y. Zhang, T. Gan, T. Wang, F. Wang, W. Shi, Giant magnetoimpedance effect in Fe75.5Cu1Nb3Si13.5B7 ribbon/FeGa film composite, J. Magn. Magn. Mater. 417 (2016) 37e41, https://doi.org/10.1016/j.jmmm.2016.05.062. [16] D. Mukherjee, J. Devkota, A. Ruiz, M. Hordagoda, R. Hyde, S. Witanachchi, P. Mukherjee, H. Srikanth, M.H. Phan, Impacts of amorphous and crystalline cobalt ferrite layers on the giant magneto-impedance response of a soft ferromagnetic amorphous ribbon, J. Appl. Phys. 116 (2014), https://doi.org/ 10.1063/1.4896583. [17] N. Laurita, A. Chaturvedi, C. Bauer, P. Jayathilaka, A. Leary, C. Miller, M.H. Phan, M.E. McHenry, H. Srikanth, Enhanced giant magnetoimpedance effect and field sensitivity in Co-coated soft ferromagnetic amorphous ribbons, J. Appl. Phys. 109 (2011) 3e5, https://doi.org/10.1063/1.3548857. nchez, B. Hernando, [18] A. Chizhik, V. Vega, A.E.-M.A. Mohamed, V.M. Prida, T. Sa M. Ipatov, V. Zhukova, A.P. Zhukov, A. Stupakiewicz, L. Domínguez, J. Gonz alez, Surface magnetic properties and giant magnetoimpedance effect in Co-based amorphous ribbons, Intermetallics 86 (2017) 15e19, https:// doi.org/10.1016/j.intermet.2017.03.010. [19] H.Q. Guo, H. Kronmüller, T. Dragon, Z.H. Cheng, B.G. Shen, Influence of nanocrystallization on the evolution of domain patterns and the magnetoimpedance effect in amorphous Fe73.5Cu1Nb3Si13.5B9 ribbons, J. Appl. Phys. 89 (2001) 514e520, https://doi.org/10.1063/1.1331649. [20] A.A. Taysioglu, A. Peksoz, Y. Kaya, N. Derebasi, G. Irez, G. Kaynak, GMI effect in CuO coated Co-based amorphous ribbons, J. Alloy. Comp. 487 (2009) 38e41, https://doi.org/10.1016/j.jallcom.2009.08.041. [21] A. Peksoz, Y. Kaya, A.A. Taysioglu, N. Derebasi, G. Kaynak, Giant magnetoimpedance effect in diamagnetic organic thin film coated amorphous ribbons, Sens. Actuators, A Phys. 159 (2010) 69e72, https://doi.org/10.1016/ j.sna.2010.03.002. [22] F.L.A. Machado, B.L. Da Silva, S.M. Rezende, C.S. Martins, Giant ac magnetoresistance in the soft ferromagnet Co70.4Fe 4.6Si15B10, J. Appl. Phys. 75 (1994) 6563e6565, https://doi.org/10.1063/1.356919. [23] F.L.A. Machado, S.M. Rezende, A theoretical model for the giant magnetoimpedance in ribbons of amorphous soft-ferromagnetic alloys, J. Appl. Phys. 78 (1996) 6558, https://doi.org/10.1063/1.361945. , J. Marcin, J. Kov nek, Magnetic [24] J. Tur canova a c, D. Jani ckovi c, P. Svec, I. Skorv a and mechanical properties of nanocrystalline Fe-Ni-Nb-B Alloys, J. Phys. Conf. 144 (2009) 12065, https://doi.org/10.1088/1742-6596/144/1/012065. [25] G. Herzer, Soft magnetic nanocrystalline materials, Scripta Metall. Mater. 33 (1995) 1741e1756, https://doi.org/10.1016/0956-716X(95)00397-E. [26] P. Svec, M. Miglierini, J. Dekan, J. Tur, G. Vlas ak, I. Skorv anek, Influence of structure evolution on magnetic properties of Fe e Ni e Nb e B system 46 (2010) 412e415. [27] L. Brunetti, P. Tiberto, F. Vinai, A new measurement method for magnetoimpedance in soft amorphous ferromagnets at microwave frequency, Sens. Actuators A Phys. 67 (1998) 84e88. [28] A. Sharma, R. Brajpuriya, S. Tripathi, D. Jain, R. Dubey, T. Shripathi, S.M. Chaudhari, A comparative study of Co thin film deposited on GaAs (100) and glass substrates, Mater. Sci. Eng. B 130 (2006) 120e125, https://doi.org/ 10.1016/j.mseb.2006.02.065. [29] A. Sharma, P. Bhatt, R. Brajpuriya, S. Tripathi, S.M. Chaudhari, Correlation study between structural, magnetic and transport properties of annealed Co thin films, Vacuum 78 (2005) 47e51, https://doi.org/10.1016/ j.vacuum.2004.12.013. [30] C.G. Kim, Y.W. Rheem, C.O. Kim, E.E. Shalyguina, E.A. Ganshina, Magnetostatic properties of heterogeneous Co-based amorphous/crystalline phases, J. Magn. Magn. Mater. 262 (2003) 412e419, https://doi.org/10.1016/S0304-8853(03)
T. Eggers et al. / Journal of Alloys and Compounds 741 (2018) 1105e1111 00072-6. [31] R. Valenzuela, Giant magnetoimpedance and inductance spectroscopy, J. Alloy. Comp. 369 (2004) 40e42, https://doi.org/10.1016/ j.jallcom.2003.09.057. [32] J.B. Goodenough, Summary of losses in magnetic materials, IEEE Trans. Magn. 38 (2002) 3398e3408, https://doi.org/10.1109/TMAG.2002.802741. [33] N.A. Buznikov, A.S. Antonov, Asymmetric magnetoimpedance in ferromagnetic biphase films: effects of magnetostatic coupling and number of layers, J. Supercond. Nov. Magn. (2017) 1e6, https://doi.org/10.1007/s10948-0174069-6.
1111
zquez, [34] E.F. Silva, R.B. Da Silva, M. Gamino, A.M.H. De Andrade, M. Va ^a, F. Bohn, Asymmetric magnetoimpedance effect in ferromagnetic M.A. Corre multilayered biphase films, J. Magn. Magn. Mater. 393 (2015) 260e264, https://doi.org/10.1016/j.jmmm.2015.05.079. [35] L. Kraus, Theory of giant magneto-impedance in the planar conductor with uniaxial magnetic anisotropy, J. Magn. Magn. Mater. 195 (1999) 764e778, https://doi.org/10.1016/S0304-8853(99)00286-3. [36] D. Menard, M. Britel, P. Ciureanu, A. Yelon, Giant magnetoimpedance in a cylindrical magnetic conductor, J. Appl. Phys. 84 (1998) 2805, https://doi.org/ 10.1063/1.368421.
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