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Electric field tunability of microwave soft magnetic properties of Co2FeAl Heusler alloy film Shandong Li, Jie Xu, Qian Xue, Honglei Du, Qiang Li, Caiyun Chen, Ru Yang, Shiming Xie, Ming Liu, Tianxiang Nan, Nian X. Sun, and Weiquan Shao Citation: Journal of Applied Physics 117, 17B722 (2015); doi: 10.1063/1.4916112 View online: http://dx.doi.org/10.1063/1.4916112 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/117/17?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Large E-field tunability of magnetic anisotropy and ferromagnetic resonance frequency of co-sputtered Fe50Co50-B film J. Appl. Phys. 117, 17D702 (2015); 10.1063/1.4906752 Quasi magnetic isotropy and microwave performance of FeCoB multilayer laminated by uniaxial anisotropic layers J. Appl. Phys. 115, 17A310 (2014); 10.1063/1.4863257 Large E-field tunability of microwave ferromagnetic properties in Fe50Co50-Hf/lead zinc niobate–lead titanate multiferroic laminates J. Appl. Phys. 113, 17C727 (2013); 10.1063/1.4799486 E-field tuning microwave frequency performance of Co2FeSi/lead zinc niobate–lead titanate magnetoelectric coupling composites J. Appl. Phys. 111, 07C705 (2012); 10.1063/1.3670979 Static and dynamic magnetic properties of epitaxial Co2FeAl Heusler alloy thin films J. Appl. Phys. 109, 07D324 (2011); 10.1063/1.3549581 [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: 129.10.132.16 On: Mon, 06 Apr 2015 17:39:10 JOURNAL OF APPLIED PHYSICS 117, 17B722 (2015) Electric field tunability of microwave soft magnetic properties of Co2FeAl Heusler alloy film Shandong Li,1,a) Jie Xu,1 Qian Xue,1 Honglei Du,1 Qiang Li,1 Caiyun Chen,1 Ru Yang,1 Shiming Xie,1 Ming Liu,2 Tianxiang Nan,3 Nian X. Sun,3 and Weiquan Shao1 1 College of Physics Science, Key Laboratory of Photonics Materials and Technology in Universities of Shandong, and Laboratory of Fiber Materials and Modern Textile, The Growing Base for State Key Laboratory, Qingdao University, Qingdao 266071, China 2 Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an 710049, China 3 Electrical and Computer Engineering Department, Northeastern University, Boston, Massachusetts 02115, USA (Presented 4 November 2014; received 6 September 2014; accepted 15 November 2014; published online 24 March 2015) Co2FeAl Heusler alloy film with 100 nm in thickness was sputtered on (011)-cut lead zinc niobate-lead titanate (PZN-PT) single crystal slabs. It was revealed that this multiferroic laminate shows very large electric field (E-field) tunability of microwave soft magnetic properties. With the increase of electric field from 0 to 8 kV/cm on PZN-PT, the anisotropy field, HK, of the Co2FeAl film along [100] direction of PZN-PT is dramatically enhanced from 65 to 570 Oe due to the strong magnetoelectric (ME) coupling between ferromagnetic Co2FeAl film and ferroelectric substrate. At the same time, the damping constant a of Co2FeAl film dramatically decreases from 0.20 to 0.029. As a result, a significantly shift of self-biased ferromagnetic resonance frequency, fFMR, from 1.86 to 6.68 GHz with increment of 3.6 times was obtained. These features demonstrate that Co2FeAl/PZN-PT multiferroic laminate is promising in fabrication of E-field tunable microwave C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4916112] components. V I. INTRODUCTION Recently, multiferroic composite materials have drawn increasing attention due to that the magnetoelectric (ME) coupling structures offer an E-field manipulation of magnetic properties (converse ME effect) or vice versa (direct ME effect).1–4 The ME coupling gives rise to many novel multiferroic materials and devices. Of them, one important branch is electrostatically tunable ferromagnetic/ferroelectric composites, which led to many tunable devices such as a picotesla sensitivity magnetometer filters,5 resonators, phase shifters, etc.6–9 Comparing with the conventional magnetic field tuned microwave magnetic devices, the electrostatically tunable microwave multiferroic devices exhibit some advantages, such as more energy efficient, compact, lightweight, and less noisy, etc.10 The ME coupling strength in multiferroic composites is determined by many factors, such as piezoelectric/magnetoelastic parameters of the ferroic/magnetic phases, the interface interactions, the ME coupling mode, and the orientation of the magnetic and electric fields. The strain/stress-mediated multiferroic composites have effective energy transfer between electric and magnetic fields; however, at microwave frequencies, the strong ME coupling is difficult to be achieved due to the large loss tangents of ferroic/magnetic phases, leading to a very limited tunability in electrostatically tunable microwave multiferroic devices. The typical a) Author to whom correspondence should be addressed. Electronic addresses: [email protected] and [email protected]. 0021-8979/2015/117(17)/17B722/4/$30.00 tunable frequency range is less than 150 MHz, and the tunable magnetic field less than 50 Oe.8,11,12 Layered multiferroic heterostructures with magnetic thin films give rise to great opportunities for obtaining strong ME coupling at microwave frequencies due to improved interfaces, minimized charge leakage of ferroelectric materials, and low loss tangents of magnetic thin films.10,13 Heusler alloys, such as NiMnSb, Co2FeAl, Co2MnSi, Co2FeSi, exhibit low coercivity HC, low damping constant a, and high anisotropy field, HK, due to their itinerant-electron characteristics, showing a potential high-frequency ferromagnetic properties.14–16 (011)-cut PZN-PT single-crystal slabs with 6% lead titanate have high anisotropic piezoelectric coefficients.17 Therefore, in this study, Co2FeAl Heusler alloy films as ferromagnetic films were deposited on (011)-cut single-crystal PZN-PT substrates to explore the ME effect in the layered ferromagnetic/ferroelectric multiferroic heterostructure. II. EXPERIMENTAL PROCEDURE The 100-nm Co2FeAl Heusler alloy films were deposited on PZN-PT substrates at room temperature under 2.8 mTorr Ar atmosphere with a floating rate of 20 sccm, along with a RF power of 80 W for Co2FeAl target. The (011)-cut single crystal PZN-PT substrates with a dimension of 5 mm[100] 5 mm[01-1] 0.5 mm[011] have been pasted on the sample turntable with their [100] direction along the radial (R) direction. The magnetic properties were measured by a vibrating sample magnetometer (VSM). The microwave frequency performances of the multiferroic 117, 17B722-1 C 2015 AIP Publishing LLC V [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: 129.10.132.16 On: Mon, 06 Apr 2015 17:39:10 17B722-2 Li et al. J. Appl. Phys. 117, 17B722 (2015) FIG. 1. The E-field dependence of hysteresis loops for the Co2FeAl/PZN-PT multiferroic laminates along [100] and [01-1] directions, respectively. (a) The as-deposited Co2FeAl film at 0 kV/cm, showing isotropic loops along [100] and [01-1] directions of PZN-PT substrate; (b) at 8 kV/cm, showing a welldefined magnetic anisotropy induced by E-field; and (c) and (d) the E-field dependent loops along [01-1] and [100] directions, respectively. composites were evaluated by use of a vector network analyzer with co-planar waveguide fixture. III. RESULTS AND DISCUSSION Figure 1 shows the E-field dependence of hysteresis loops for the Co2FeAl/PZN-PT multiferroic laminates along [100] and [01-1] directions, respectively. As illustrated in Fig. 1(a), the as-deposited Co2FeAl film does not exhibit detectable magnetic anisotropy; however, when an external E-field is applied on the [011] direction of PZN-PT substrate, a well-fined uniaxial magnetic anisotropy was formed due to the ME coupling [see Fig. 1(b)]. The E-field not only enhances the squareness ratio of film along the easy axis direction of [01-1] [see Fig. 1(c)] but also dramatically increases the magnetic anisotropy field, HK, along hard axis of [100]. As illustrated in Fig. 1(d), the E-field dependent magnetic anisotropy field, HK, dramatically increases from 65 Oe at 0 kV/cm to 570 Oe at 8 kV/cm along the HA [100] direction. A large ME coupling coefficient of 63.1 Oe cm/kV is obtained in Co2FeAl/PZN-PT multiferroic laminates. The ferromagnetic resonance frequency, fFMR, of ferromagnetic films can be expressed by Kittle equation, fFMR ¼ c pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi HK ðHK þ 4pMS Þ; 2p Figure 2 shows the E-field dependence of permeability for the Co2FeAl/PZN-PT multiferroic laminates. As expected, the ferromagnetic resonance frequency was dramatically shifted from 1.86 to 6.68 GHz, when the E-field increased from 0 to 8 kV/cm. The fFMR increases by 3.6 times due to the ME coupling via the interface between Co2FeAl film PZN-PT substrate. A large fFMR enhancement ratio of 602.5 MHz cm/kV was achieved. This E-field-induced fFMR shift can be explained by the strain/stress-mediated electrostatic-field-induced in-plane magnetic anisotropy field Heff. For the Co2FeAl/PZN-PT multiferroic laminates, the Kittel equation [Eq. (1)] can be rewritten as fFMR ¼ c pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðHK þ Heff Þ ðHK þ Heff þ 4pMS Þ; 2p (2) (1) c 2.8 MHz/Oe), HK is where c is the gyromagnetic ratio (2p the anisotropic field in plane, and 4pMS is the saturation magnetization of ferromagnetic films. From Eq. (1), it can be concluded that large enhancement of HK manipulated by E-field will give rise to a large upward shift of fFMR. In other words, the ferromagnetic resonance of Co2FeAl films will be driven to a high frequency by E-field. FIG. 2. The E-field dependence of permeability for the Co2FeAl/PZN-PT multiferroic laminates. [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: 129.10.132.16 On: Mon, 06 Apr 2015 17:39:10 17B722-3 Li et al. J. Appl. Phys. 117, 17B722 (2015) where Heff is the orthogonal in-plane compressive and tensile stress corporately induced internal effective magnetic field, which could be positive or negative, and in our case it can be expressed as Heff ¼ 3kY ðd31 d32 ÞE; M S ð1 þ Þ (3) where Y is the Young’s Modulus, is the Poisson’s ratio, k is the magnetostriction constant, d31 ¼ 3000 pC/N along [100] and d32 ¼ 1100 pC/N along [01-1] are linear anisotropic piezoelectric coefficients of PZN-PT, and E is the applied external E-field strength. Heff is the effective magnetic anisotropy field under various E-fields.18 From Eqs. (3) and (2), it can be concluded that the E-field will give rise to an increase of Heff via ME coupling, and, therefore, drive the ferromagnetic resonance upwards shifting to a high frequency. The free-energy density, Etotal, in the studied system includes the Zeeman energy, Ezeeman , the demagnetization energy; Eshape , the stress energy, Estress , and the uniaxial anisotropy energy, Euni . It can be written as Etotal ¼ Ezeeman þ Eshape þ Estress þ Euni : (4) For the studied system, the main interaction between the magnetic film and PZN-PT substrate occurs at the interface (i.e., in-plane), so the contribution of demagnetization energy can be ignored. In the case of a certain magnetic field, the variation of total free-energy density is dominated by the competition and transformation between Estress and Euni. The uniaxial magnetic anisotropy of magnetic film is induced by E-field tunable stress, and the uniaxial magnetic energy can be manipulated by E-field via the magnetoelectric coupling. Therefore, the anisotropy field exhibits an E-field tunability, e.g., HK increases with the increase of E-field. Figure 3 shows the relationship between magnetic anisotropy field, HK, and ferromagnetic resonance frequency, fFMR, as well as the damping constant a, at various E-fields. The Gilbert damping constant, a, represents magnetic loss of the Co2FeAl magnetic film, which is deduced by fitting the permeability spectra, as shown in Fig. 2, using LandauLifshitz-Gilbert equation. As illustrated in Fig. 3(a), the fFMR and HK increase almost linearly with the E-field, and the Efield dependence of HK and fFMR exhibits a same trend at various E-fields. These facts demonstrated that the E-field induced HK dominates the ferromagnetic resonance frequency, as described in Eqs. (1)–(3). On the other hand, the damping constant, a, dramatically reduces from 0.20 to ca. 0.029 with the increase of E-field from 0 to 8 kV/cm, indicating that the magnetic loss of the multiferroic composites effectively decreases by the E-field. It is excited to note that the ME coupling in the studied multiferroic laminates not only enhances the ferromagnetic resonance frequency but also reduces the magnetic loss at microwave frequencies. This merit of the multiferroic composites gives great opportunity in practical application. The intrinsic stress, in general, is randomly distributed in the magnetic films, which give rise to a high damping FIG. 3. The E-field dependence of (a) magnetic anisotropy field, HK, and ferromagnetic resonance frequency, fFMR, and (b) Gilbert damping constant a. constant and decreasing resonant frequency. The investigated Co2FeAl film at zero E-field exhibits bad ferromagnetic resonance characteristics due to the magnetic isotropy of as-deposited Co2FeAl film. As illustrated in Figs. 1, 2 and 3(b), the Co2FeAl film shows a higher coercivity of 65 Oe, lower fFMR of 1.86 GHz with broad resonance peak, and very large damping constant, a, of 0.20. The large and random distributed intrinsic stress is responsible for the bad microwave performance of the Co2FeAl film at E ¼ 0 kV/cm. However, if the E-field was applied on the PZN-PT substrate, a biaxial anisotropy stress, compressive stress along [100] direction and tensile one along [01-1] direction of the PZN-PT substrate will be induced by E-field.19 According to magnetoelastic energy equation, EK ¼ 32 kS r cos 2 h, for a positive kS (as the case in this study for the Co2FeAl films), a compressive stress will drive the magnetic moments to be aligned parallel to [01-1] direction of PZN-PT substrate. Similarly, the tensile stress also leads the magnetic moments along [01-1] direction. As a result, a uniaxial magnetic anisotropy with the magnetically easy axis along [01-1] direction is formed due to the ME coupling effect. As reported in our previous work,20,21 if the biaxial stress induced uniaxial magnetic anisotropy is formed in the ferromagnetic films, the enhanced ferromagnetic resonance and improved microwave magnetic properties will be obtained due to the alignment of magnetic moments. As illustrated in Figs. 1–3, with the increase of E-field, the coercivity and damping constant reduce, and the HK and fFMR dramatically increase. IV. CONCLUSIONS Strong magnetoelectric coupling effect, corresponding to a large magnetoelectric coefficient of 63.1 Oe cm/kV, was observed in Heusler alloy Co2FeAl/PZN-PT multiferroic heterostructure, leading to continuously E-field tunable microwave frequency characteristics with fFMR upwards [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: 129.10.132.16 On: Mon, 06 Apr 2015 17:39:10 17B722-4 Li et al. shifting from 1.86 to 6.68 GHz, i.e., an increment of DfFMR ¼ 4.82 GHz or increment ratio of DfFMR/fFMR ¼ 256% in E-field range of 0–8 kV/cm. The magnetoelectric coupling between Co2FeAl film and PZN-PT substrate not only enhances the ferromagnetic resonance frequency but also reduces the magnetic loss at microwave frequencies, which gives great opportunity in practical application of the multiferroic composites. ACKNOWLEDGMENTS This work was financially supported by National Nature Science Foundation of China with Grant No. 11074040 and Key Project of Nature Science Foundation of Shandong Province with Grant No. ZR2012FZ006. 1 N. Spaldin and M. Fiebig, Science 309, 391 (2005). W. Eerenstein, N. D. Mathur, and J. F. Scott, Nature 442, 759 (2006). 3 J. F. Scott, Nat. Mater. 6, 256 (2007). 4 C. A. F. Vaz, J. Hoffman, C. H. Anh, and R. Ramesh, Adv. Mater. 22, 2900 (2010). 2 J. Appl. 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