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Dynamic magnetization of NiZn ferrite doped FeSiAl thin films fabricated by oblique sputtering
Journal of Magnetism and Magnetic Materials 432 (2017) 373–381
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Dynamic magnetization of NiZn ferrite doped FeSiAl thin films fabricated by oblique sputtering Xiaoxi Zhong a,⇑, Nguyen N. Phuoc b, Wee Tee Soh c, C.K. Ong b,c, Lezhong Li a a
Sichuan Province Key Laboratory of Information Materials and Devices Application, Chengdu University of Information Technology, Chengdu 610225, PR China Temasek Laboratories, National University of Singapore, 5A Engineering Drive 2, Singapore 117411, Singapore c Center for Superconducting and Magnetic Materials, Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore b
a r t i c l e
i n f o
Article history: Received 5 October 2016 Received in revised form 29 December 2016 Accepted 12 February 2017 Available online 14 February 2017 Keywords: FeSiAl thin film Oblique deposition Doping Magnetic anisotropy Thermal stability
a b s t r a c t In this study, we comprehensively investigate the dynamic magnetic properties of FeSiAl-NiZnFeO thin films prepared by the oblique deposition method via a shorted microstrip perturbation technique. For the films with higher oblique angle and NiZn ferrite doping amount, there are two ferromagnetic resonance peaks observed in the permeability spectra, and both of the two peaks originate from FeSiAl. Furthermore, the magnetic anisotropy field HK of the ferromagnetic resonance peak at higher frequency is enhanced with increasing doping amount, which is interpreted in terms of the contribution of reinforced stress-induced anisotropy and shape anisotropy brought about by doping elements and oblique sputtering method. In addition, the thermal stability of the ferromagnetic resonance frequency fFMR of FeSiAl-NiZnFeO films with oblique angles of 35° and 45° with respect to temperature ranging from 300 K to 420 K is deteriorated with increasing ferrite doping amount, which is mainly ascribed to the influence of pair-ordering anisotropy and/or the reduction of the FeSiAl grain size. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Similar to traditional magnetic alloys such as FeCo and NiFe which are widely applicable to electronics information industry, FeSiAl alloy (also known as sendust) exhibit favorable soft magnetic properties including high saturation magnetization and high permeability as well. Besides, it possesses higher resistivity, lower eddy current loss and much lower cost. Therefore, during the past decades, much efforts have been taken to further improve the soft magnetic properties of FeSiAl-based magnetic powder cores [1–6]. With the high-speed development of electronic information technology, the requirements for high frequency miniaturized magnetic devices are increasing. Therefore, investigations on the improvement of high-frequency properties of magnetic thin films have attracted increasing attention from researchers in recent years [7–9]. However, studies focusing on FeSiAl-based thin films are limited, let alone their high-frequency properties [10,11]. One of the most important properties for high-frequency use is ferromagnetic resonance frequency fFMR. According to Kittel’s equation [12] for films with in-plane uniaxial magnetic anisotropy, in order to increase fFMR, one can increase the effective magnetic anisotropy field HK. In our previous study [10], we also named the ⇑ Corresponding author. E-mail address: [email protected] (X. Zhong). http://dx.doi.org/10.1016/j.jmmm.2017.02.021 0304-8853/Ó 2017 Elsevier B.V. All rights reserved.
effective magnetic anisotropy field HK as dynamic magnetic anisotropy to distinguish it from static magnetic anisotropy Hsta K . From the experimental point of view, HK can be deduced from the magnetic permeability spectra and is composed of static magnetic anirot sta sotropy Hsta K and rotatable magnetic anisotropy HK , while HK can be directly deduced from M-H loop. Then, Hrot K can be estimated by subtracting the Hsta K from the HK [10]. From another perspective, anisotropy in magnetic thin films includes magnetocrystalline anisotropy, shape anisotropy, surface anisotropy, pair ordering anisotropy and strain anisotropy, etc. In order to tailor the static and dynamic magnetic properties including magnetic anisotropy of the thin films to meet the specific demands of various applications, great attempts have been made to develop a variety of deposition methods based on magnetron sputtering [10,13–19]. Among them, oblique deposition is considered to be a convenient and useful method [10,17–19]. It is well established in literatures that for magnetic film deposited by oblique deposition, steering [20] and self-shadowing effect [21–23] is the most important mechanisms causing uniaxial magnetic anisotropy of the film. The shadowing effect leads to tilted columnar grains and elongated grains [22,24] which induce magnetocrystalline, shape and strain anisotropies [21,25–26]. In addition, to further reduce the eddy current loss which largely restricts the high-frequency applications of devices based on magnetic thin films, electrical resistivity needs to be increased,
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which can be accomplished by inducing nonmagnetic insulator phases [11,13,27]. However, the saturation magnetization and resonance frequency of the films will also be decreased accordingly. It is well acknowledged that NiZn ferrite with the spinel structure exhibits high resistivity and some good soft magnetic properties. So it is an appropriate doping candidate to decrease the eddy current losses and increase the high frequency properties of magnetic thin films based on alloys [28–30]. Therefore, in this work, NiZn ferrite doped FeSiAl thin films have been prepared using oblique deposition method. The effect of doping amount on the magnetic characteristics of the films together with their thermal stability behavior which is quite important for films’ industrial applications in the temperature range of 300–420 K was investigated. 2. Experiment 100 nm-thick NiZn ferrite doped FeSiAl-based (denoted FeSiAlNiZnFeO in the following) thin films were deposited onto 5 mm 10 mm 0.10 mm Si(100) substrates at room temperature by an oblique deposition method. The FeSiAl target with content of 6 wt% Al and 9 wt% Si and diameter of 75 mm was used during sputtering with varying numbers of 3 mm 3 mm Ni0.5-
Zn0.5Fe2O4 ferrite chips placed on top of it, as illustrated in Fig. 1. The substrates are fixed on various tilted holders and each holder is placed right above the target during sputtering. The side view of each tilted holder is a right triangle with one hypotenuse and two legs – the lengths of which determine the size of oblique angle b. The base pressure was around 7 107 Torr. During deposition, the Argon pressure was kept at 4 103 Torr. The deposition power for the target is 80 W. To assist in inducing magnetic anisotropy, a magnetic field of 200 Oe was provided along the easy axis during deposition. The compositions of the films were measured by an energy dispersive X-ray spectroscope (EDS). The static hysteresis loops were measured using an M-H loop tracer at room temperature. The structure of the films was characterized using an X-ray diffractometer (XRD) with Cu-Ka radiation. The permeability spectra in the temperature range from 300 K up to 420 K was measured by a homemade microstrip fixture using the transmission line perturbation method [31] with an Agilent series N5230A vector network analyzer (VNA). In our setup, an external in-plane magnetic field can be applied using a Helmholtz coil, and the permeability spectra at different magnetic fields can be measured. From the resonance frequency extracted from the permeability spectra at different magnetic fields, the effective magnetic anisotropy and saturation magnetization can be obtained from fitting based on Kittel’s equation. In addition, deeper analysis of the films can be taken as the external in-plane magnetic field can be applied along various angular orientations, which will be discussed later.
3. Results and discussion
Fig. 1. Schematic diagram of the oblique deposition system.
The composition analysis of the FeSiAl-NiZnFeO films carried out by energy dispersive X-ray spectroscope (EDS) reveals that with the number of ferrite chips increase from 0 to 6, the concentrations of Ni and Zn elements are increased while that of Fe is almost unchanged. Specifically, the concentrations of Ni + Zn are about 2.1%, 4.1% and 7.2% when the numbers of ferrite chips are 2, 4 and 6, respectively. For convenience, we use the number of ferrite chips to represent the doping amount. Fig. 2 shows the X-ray diffraction profiles for the FeSiAlNiZnFeO films with oblique angle of 35° and chip number changed from 0 to 6. At the diffraction angle 2h of about 45°, we see only one obvious peak which can be ascribed to the (220) plane of FeSiAl from the JCPDS Card No. 45-1206. In addition, there is no peak corresponding to NiZn ferrite. As the chips number is increased, the position of the peak stemming from FeSiAl (220) shifts to the lower 2h range indicative of an extension of the lattice spacing. Specifically, the lattice spacing is increased from 2.014 Å to 2.021 Å as
Fig. 2. (a) X-ray diffraction patterns for FeSiAl-NiZnFeO thin films grown at b = 35° with various number of doped ferrite chips. (b) Lattice spacing as a function of ferrite chips.
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the chip number is changed from 0 to 6. As both the atomic radius of Ni (1.62 Å) and Zn (1.53 Å) are smaller than that of Fe (1.72 Å) and Al (1.82 Å), the extension of lattice spacing is presumably due to interstitial effect of nickel ions and zinc ions rather than substitution effect. Another remarkable change in the XRD profiles of the FeSiAl-NiZnFeO films with the NiZn ferrite doping amount is the broadening of the FeSiAl (2 2 0) peak when the NiZn ferrite doping amount is increased. This unambiguously indicates that the grain size of FeSiAl is reduced with the increasing of the NiZn ferrite doping. Fig. 3 shows the in-plane M-H loops of FeSiAl-NiZnFeO films with various doping amounts grown at different oblique angles b (16.7–45°) measured at ambient temperature. It can be seen that for all the films, the easy axis curves show a square shape. However, the conditions are different for the hard axis curves. For the films with same doping amount, the loops are more sloped when b is higher, and for the films with same b, the loops are more sloped when doping amount is larger. Such trend is more obvious when b is above 35°. Therefore, one can deduce that the static magnetic anisotropy is increased with b and doping amount. As discussed
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previously [10], the increase of magnetic anisotropy with b is largely due to the self-shadowing effect which leads to shape anisotropy. On the other hand, the stress-induced anisotropy is perceived as the derivation of the magnetic anisotropy [32] since the interstitial effect of Ni and Zn atoms will induce some internal stress upon the films. As shown in Fig. 3-(b), the coercivity HC of FeSiAl-NiZnFeO films is increased with b and doping amount due to the mechanism stated above for the improvement of the static magnetic anisotropy. Shown in Fig. 4 are the real and imaginary permeability spectra of the FeSiAl-NiZnFeO films at ambient temperature. From the imaginary permeability spectra, it can be seen that for films with same ferrite doping amount the resonance frequency is increased as b increases. And for films with b = 35° and b = 45° the resonance frequency is increased as ferrite doping amount increases. In accordance with Kittel’s equation [10], this phenomenon is on account of the increment of magnetic anisotropy with the increasing of b and doping amount. In addition, it can be found that two resonance peaks were appeared for the films with larger doping amount and higher b. It is known that the reason for the appearance of double
Fig. 3. M-H loops for FeSiAl-NiZnFeO films with various doped ferrite concentrations grown at different oblique angles of (a) b = 16.7°, (b) b = 26.6°, (c) b = 35° and (d) b = 45°. (e) Variations of coercivity as a function of doped ferrite concentrations for films with different oblique angles.
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Fig. 4. Permeability spectra measured at room temperature of FeSiAl-NiZnFeO thin films with various doped ferrite concentrations deposited at different oblique angles.
resonance peaks in the films is mainly because of the existence of double magnetic phases with different dynamic behaviors [33]. For a more quantitative analysis of the magnetic properties of the two resonance peaks, additional experimental investigations of the imaginary permeability spectra using a shorted microstrip perturbation technique with a varying external static magnetic field were performed. Based on this method, the properties and the essential mechanisms of the two resonance peaks can be further analyzed. The process and results are as follows. Fig. 5(a)–(c) show the permeability spectra measured at external magnetic field H decreasing from 150 to 0 Oe for film with 6 ferrite chips and b varied from 26.6° to 45°. It can be observed that all of the three samples have two resonance peaks (named as low peak and high peak) whose trends with external magnetic fields
are quite different. Then it can be deduced that both of the saturation magnetization MS and magnetic anisotropy HK are different for the two peaks and the values of MS and HK of low peak are higher and lower than that of high peak respectively based on the Kittel’s equation. In our case, the value of MS of FeSiAl is definitely larger than that of NiZn ferrite, and the doping amount of NiZn ferrite can be considered as small. On account of this, we argue that neither low peak nor high peak belongs to NiZn ferrite, and the existence of double resonance peaks is more likely ascribed to two magnetic phases of FeSiAl with different magnetic anisotropy fields. However, one can see from Fig. 5 that the intensity of the low peak for all the three samples are decreased dramatically with the increasing of H, it is almost impossible to get more information such as the specific values of MS and HK corresponding to the low
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Fig. 5. The imaginary part of the permeability spectra measured at different external magnetic fields for FeSiAl-NiZnFeO films with 6 ferrite chips and (a) b = 26.6°, (b) b = 35° and (c) b = 45°.
peak through fitting based on Kittel’s equation. For the high peak, the situation is quite different. Therefore, in the following part, we shall only focus on the discussion of the high peak hereinafter, and that the values of MS and HK of high peak cannot fully reflect those of the films. To further analyze the high peaks of the FeSiAl-NiZnFeO films, the permeability spectra of the films with H applied along various angular orientations were obtained [31,34]. The angular dependence of the HK can be expressed as the following [30]: rot HK Hsta K cos 2h þ HK
Hsta K
ð1Þ
Hrot K
where and are static and rotatable anisotropy respectively and h is the in-plane angle between the easy axis and H. As discussed earlier, Hsta K can be presented from the difference between the M-H loops along easy and hard axis of the film, but it also can be obtained from the permeability spectra. However, things are different for Hrot K since it cannot be detected by static magnetic measurement. Hrot is usually found in exchange biased systems and K some normal magnetic thin films [35–36]. It gets its name because the direction of it always ‘‘follows” the magnetization. Based on Eq. (1), the Kittel’s equation can be rewritten as following:
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 sta rot sta f FMR c=2p ðHrot K þ HK cos 2h þ HÞðH K H K sin h þ H þ M s Þ ð2Þ If the sample is saturated, the magnetization is not dependent on H, then the dependence between fFMR and h can be obtained rot for specific H and the values of MS, Hsta K and HK can be deduced from fitting curves. To be more specific, for the FeSiAl-NiZnFeO film with specific b and doping amount, the values of fFMR were obtained firstly when specific H were applied along various angular orientations, namely various h. Then the dependence of fFMR as a function of h for this specific H was obtained. Finally, fitting process rot was proceeded based on Eq. (2) and the values of MS, Hsta K and HK for this specific film were obtained accordingly. The process and results obtained based on this method for films with various b and doping amounts are as follows.
Fig. 6 shows the dependence of fFMR as a function of h when H is 150 Oe for the FeSiAl-NiZnFeO films with various b and doping amounts. From the fitting lines presented, the values of MS, Hsta K and Hrot K were obtained, which are summarized in Fig. 7 along with the values of HK. As noticed from Fig. 7-(a), MS is decreased for FeSiAl-NiZnFeO films with different b as the doping amounts increase, due to the increased doping of NiZn ferrite into FeSiAl when b is increased. In particular, the value of MS of NiZn ferrite is much smaller than that of FeSiAl, and increasing the concentration of NiZn ferrite in FeSiAl-NiZnFeO films will dilute the original magnetization of FeSiAl in the films, leading to the decrease of MS. From Fig. 7(b), (c), it can be seen that with doping chips number being increased from 0 to 6, the values of HK for high peaks of FeSiAl-NiZnFeO films are enhanced from 22 Oe to 41 Oe when b is 26.6°, from 30 Oe to 52 Oe when b is 35°, and from 53 Oe to 65 Oe when b is 45°, respectively. In addition, although both of Hsta K and Hrot K are increased with the increasing of ferrite doping amount, rot Hsta K is much larger than HK and accounts for a very high share of HK for all of the cases, which is quite different from the observation of Chai et al. [30]. They found that for the NiZn ferrite doped FeCo thin films, with the increasing of doping amount, Hrot K was dramatically increased, leading to the enhancement of HK and thus the fFMR [30]. Such a phenomenon was explained by the existence of exchange coupling between ferrimagnet and ferromagnet in the films. Such ‘‘exchange coupling” is similar to ferromagnet/antiferromagnet exchange biased system and hard/soft magnet coupling in exchange spring magnets as FeCo was much ‘‘harder” than NiZn ferrite [30]. However, in our case, the fact that the contribution of the rotational anisotropy is very small (Hrot K < 15 Oe) indicates that such an exchange coupling is very weak between FeSiAl and NiZr ferrite. As a consequence, the increasing of HK is largely ascribed rot to the increase of Hsta K rather than HK as presented in Fig. 7. rot Based on the above analysis of Hsta K and HK , we argue that the increment of fFMR with the increasing of doping amount in our case is closely related to doping elements and oblique deposition method itself. Specifically, the interstitial effects of Ni and Zn ions which lead to the extension of lattice spacing induce some stress
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Fig. 6. (a) The dependence of fFMR as a function of h for the FeSiAl-NiZnFeO films with (a) b = 26.6°, (b) b = 35° and (c) b = 45° respectively. The lines are the fitted curves.
rot Fig. 7. (a) Variations of MS of the FeSiAl-NiZnFeO films with different oblique angles as a function of ferrite chips. HK, Hsta K and HK of the FeSiAl-NiZnFeO films with (b) b = 26.6°, (c) b = 35° and (d) b = 45° as a function of ferrite chips.
into the film, which will give rise to the generation of stressinduced magnetic anisotropy. For films with b = 0°, although the stress is increased with the increasing of doping amount, it is almost homogeneous and isotropic, so that the generation of stress-induced anisotropy is limited. With the increasing of b, another factor should be taken into account: self-shadow effect which was applied to explain the increment phenomenon of magnetic anisotropy with b in magnetic films fabricated by oblique
sputtering [10]. In particular, the self-shadow effect leads to the tilt and/or elongation of the grains, which is ascribed to the inducement of shape anisotropy and thus the increment of HK [10]. In our cases, the combined effect of both of the interstitial effect and self-shadow effect give rise to the increasing of HK with increasing doping amount for FeSiAl-NiZnFeO films with higher b. Specifically, on one hand, with the increasing of doping amount, the lattice deformation is more drastic, leading to the larger
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elongation of the grains and thus the increment of shape anisotropy; on the other hand, for films with higher b, the tilt degree and elongation of the grains are relatively larger so that the homogeneity of stress induced by doping elements is broken, leading to the increment of stress-induced anisotropy. Overall, the enhancement of both of shape anisotropy and stress-induced magnetic anisotropy with increasing doping amount of NiZn ferrite when b is relatively large makes the increment of total effective magnetic anisotropy. In addition, there is another factor named as pairordering magnetic anisotropy which is brought from the magnetic field applied during sputtering contributing to the total effective magnetic anisotropy. It is related with the directional magnetic pair ordering of the magnetic atoms where atomic pairs rearrange with bond axes in preferred orientations with respect to the applied field [37–39]. In our case, this pair-ordering magnetic anisotropy arises from the atomic arrangement of Fe-Fe pairs resulting in anisotropic distributions of atomic ordering [37–40]. Theoretically, with the increasing of doping amount of NiZn ferrite, the directional pair ordering of Fe-Fe pairs becomes more difficult to establish. However, one should note that it is precisely because magnetic field is provided during deposition of the films, the influ-
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ence of foreign elements on directional pair ordering of Fe-Fe pairs is depressed. Therefore, the pair-ordering magnetic anisotropy is not supposed to change much with the increase of doping amount and b, but it is of great importance to the thermal stability behavior of the FeSiAl-NiZnFeO films which will be discussed next. Now we focus our discussion on the thermal stability for the ferromagnetic resonance frequency fFMR of the FeSiAl-NiZnFeO films with various doping amounts deposited at high oblique angles. As mentioned in the introduction part, the thermal stability of the magnetic films is quite important for their industrial applications. In Fig. 8, the permeability spectra of FeSiAl-NiZnFeO films doped with 6 ferrite chips deposited at b of 35° and 45° measured at different temperatures in the range of 300–420 K are presented. It can be observed that the fFMR of both of the two resonance peaks are decreased as the temperature increases. We plot the temperature dependences of the fFMR of high peaks for FeSiAl-NiZnFeO films with various doping amounts fabricated at b of 35° and 45° in Fig. 9. It can be observed in Fig. 9 that the thermal stability of fFMR of FeSiAl-NiZnFeO films decrease with the increasing of doping amount when b are 35° and 45°in the temperature range of
Fig. 8. Permeability spectra measured at various temperatures in the range of 300 K to 420 K of FeSiAl-NiZnFeO films with 6 doped ferrite chips when (a) b = 35° and (b) b = 45°.
Fig. 9. Temperature dependence of fFMR of FeSiAl-NiZnFeO thin films with (a) b = 35° and (c) b = 45°. (b) and (d) are the parameters normalized to the value obtained at 300 K.
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300 K to 420 K. In addition, the overall thermal stability of fFMR for films with b = 35° are better than that with b = 45°, which is in accordance with our previous research results [10]. Specifically, for films with b = 35°, the temperature increase causes fFMR to decrease by about 4%, 6%, 7% and 9% when the numbers of doping ferrite chips are 0, 2, 4 and 6, respectively. For films with b = 45°, fFMR decrease by about 5%, 8%, 11% and 14% when the numbers of doping ferrite chips are 0, 2, 4 and 6, respectively. Briefly, the decrease of fFMR with increasing of temperature could be ascribed to the reducing of HK and MS. It is well established that the reason for the decrement of MS with increasing temperature is the exacerbated thermal fluctuation of magnetization. However, the situation for HK is more complicated. As has been noted, the total effective magnetic anisotropy existing in the FeSiAl-NiZnFeO films in our case includes reinforced stress-induced and shape anisotropies brought about by doping elements and oblique deposition method and pair-ordering anisotropy brought from the magnetic field applied during sputtering. Normally, the thermal stability of shape anisotropy is supposed to be good [10], so that the stability as a function of temperature of HK is closely related to the doping elements. It is acknowledged that the atomic and ionic diffusion is intensified with temperature rising, leading to the destruction of some directional pair ordering then the reduction of anisotropic distribution of ordered atoms. Such effect is more significant with the increase in the amount of impurity elements so that the thermal stability of pair ordering anisotropy is decreased with increasing doping amount. In addition, the diffusion of atoms and ions, especially the interstitial ones, still have some influence on the stress. But, as the interstitial ions is very difficult to be increased or decreased under such temperature conditions, the stress-induced anisotropy can be treated as stable with increasing temperature in our case. Therefore, the reduction of thermal stabilization of the total effective magnetic anisotropy with increasing doping amount may possibly be ascribed to the thermal behavior of pair ordering anisotropy as discussed above. Another possibility for the worsening thermal stability with increased doping amount that should not be ruled out is the grain size reduction of FeSiAl when the doping amount of NiZn ferrite is increased. It is well known that when the grain size of ferromagnetic thin films is reduced the thermal stability of the magnetic properties becomes worse [41]. Hence, in this case, it is quite possible that the doping of NiZn ferrite decreases the FeSiAl grain size, thus leading to the worsening of the thermal stability. This argument is consistent with the XRD profiles in Fig. 2 showing a remarkable broadening of the FeSiAl (220) peak when the NiZn ferrite doping amount is increased.
4. Summary and conclusion In summary, the dynamic magnetic properties of FeSiAlNiZnFeO thin films prepared by the oblique deposition method were studied systematically. Our research illustrated that the ferromagnetic resonance frequency fFMR of the films is increased with the increase of oblique angle b from 0° to 45° and NiZn ferrite doping amount from 0% to 7.2% (the concentrations of Ni + Zn) which is ascribed to the increase of magnetic anisotropy field HK. For the films with higher b and doping amount, there are two ferromagnetic resonance peaks observed, named as low peak and high peak shown at lower and higher frequency range respectively, and both of them are shown to originate from FeSiAl. Through further analysis we found that for the films with higher b, the enhancement of HK of the high peak with the increase of doping amount is largely due to the increase of static anisotropy Hsta K rather than rotatable anisotropy Hrot K . The essential mechanism can be explained in terms of the contribution of reinforced stress-induced anisotropy
and shape anisotropy brought from the combination effect of interstitial ions and self-shadow effect. In addition, for FeSiAl-NiZnFeO films with b of 35° and 45°, the thermal stability of the ferromagnetic resonance frequency fFMR is deteriorated with the increase of NiZn ferrite doping amount in the temperature of 300 K to 400 K. This mainly results from the thermal behavior of pair-ordering anisotropy and/or the reduction of the FeSiAl grain sizes. Overall, the dynamic magnetic properties in terms of fFMR and HK of FeSiAlNiZnFeO thin films prepared by the oblique deposition method can be enhanced by increasing the oblique angle and doping amount, but the consequent deterioration of thermal stability of fFMR should be taken into account. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51502025) and the Scientific Research Foundation of CUIT (Grant No. KYTZ201620). CKO acknowledges the fund from AOARD-13-4039. References [1] D. Liu, C. Wu, M. Yan, Investigation on sol–gel Al2O3 and hybrid phosphatealumina insulation coatings for FeSiAl soft magnetic composites, J. Mater. Sci. 50 (2015) 6559–6566. [2] K.J. Sunday, K.A. Darling, F.G. Hanejko, B. Anasori, Y. Liu, M.L. Taheri, Al2O3 ‘‘self-coated” iron powder composites via mechanical milling, J. Alloys Compd. 653 (2015) 61. [3] H. Xu, S. Bie, J. Jiang, W. Yuan, Q. Chen, Y. Xu, Electromagnetic and microwave absorbing properties of the composites containing flaky FeSiAl powders mixed with MnO2 in 1–18 GH, J. Magn. Magn. Mater. 401 (2016) 567–571. [4] X. Fan, J. Wang, Z. Wu, G. Li, Core-shell structured FeSiAl/SiO2 particles and Fe3Si/Al2O3 soft magnetic composite cores with tunable insulating layer thicknesses, Mater. Sci. Eng., B 201 (2015) 79–86. [5] M. Huang, C. Wu, Y. Jiang, M. Yan, Evolution of phosphate coatings during high-temperature annealing and its influence on the Fe and FeSiAl soft magnetic composites, J. Alloys Compd. 644 (2015) 124–130. [6] X. Zhong, Y. Liu, J. Li, Y. Wang, Structure and magnetic properties of FeSiAlbased soft magnetic composite with AlN and Al2O3 insulating layer prepared by selective nitridation and oxidation, J. Magn. Magn. Mater. 324 (2012) 2631– 2636. [7] X.L. Liu, L.S. Wang, Q. Luo, Q.S. Xie, Q.F. Zhang, X. Liu, F.M. Bai, D.L. Peng, Magnetic properties of [FeCoSiN/SiNx]18 multilayer thin films for applications in GHz range, J. Alloys Compd. 680 (2016) 531–537. [8] D. Cao, Z. Zhu, H. Feng, L. Pan, X. Cheng, Z. Wang, J. Wang, Q. Liu, Applied magnetic field angle dependence of the static and dynamic magnetic properties in FeCo films during the deposition, J. Magn. Magn. Mater. 416 (2016) 208–214. [9] L. Zhang, L. Liu, S. Ishio, Investigation of magnetic anisotropy and magnetization process of tetragonal distorted FeCo multilayer films, Mater. Lett. 160 (2015) 238–241. [10] X. Zhong, N.N. Phuoc, G. Chai, Y. Liu, C.K. Ong, Thermal stability and dynamic magnetic properties of FeSiAl films fabricated by oblique deposition, J. Alloys Compd. 610 (2014) 126–131. [11] X. Zhong, N.N. Phuoc, W.T. Soh, C.K. Ong, L. Peng, L. Li, Thermal stability of the dynamic magnetic properties of FeSiAl-Al2O3 and FeSiAl-SiO2 films grown by gradient-composition sputtering technique, J. Electron. Mater. 46 (1) (2017) 208–217. [12] C. Kittel, Interpretation of anomalous larmor frequencies in ferromagnetic resonance experiment, Phys. Rev. 71 (1947) 270. [13] Y.G. Ma, C.K. Ong, Soft magnetic properties and high frequency permeability in [CoAlO/oxide] multilayer films, J. Phys. D Appl. Phys. 40 (2007) 3286–3291. [14] N.N. Phuoc, C.K. Ong, Comparative study of thermal stability of NiFe and NiFeTa thin films grown by co-sputtering technique, J. Electron. Mater. 45 (2016) 4061–4066. [15] A. Akbulut, S. Akbulut, F. Yildiz, Origin of spontaneous exchange bias in co/ NiMn bilayer structure, J. Magn. Magn. Mater. 417 (2016) 230–236. [16] S. Bedanta, T. Eimuller, W. Kleemann, J. Rhensius, F. Stromberg, E. Amaladass, S. Cardoso, P.P. Freitas, Overcoming the dipolar disorder in dense CoFe nanoparticle ensembles: superferromagnetism, Phys. Rev. Lett. 98 (2007) 176601. [17] S. Mallick, S. Mallik, S. Bedanta, Effect of substrate rotation on domain structure and magnetic relaxation in magnetic antidot lattice arrays, J. Appl. Phys. 118 (2015) 083904. [18] N. Chowdhury, S. Bedanta, Controlling the anisotropy and domain structure with oblique deposition and substrate rotation, AIP Adv. 4 (2014) 027104. [19] N. Chowdhury, S. Mallick, S. Mallik, S. Bedanta, Study of magnetization relaxation in Co thin films prepared by substrate rotation, Thin Solid Films 616 (2016) 328.
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Dynamic magnetization of NiZn ferrite doped FeSiAl thin films fabricated by oblique sputtering Xiaoxi Zhong a,⇑, Nguyen N. Phuoc b, Wee Tee Soh c, C.K. Ong b,c, Lezhong Li a a
Sichuan Province Key Laboratory of Information Materials and Devices Application, Chengdu University of Information Technology, Chengdu 610225, PR China Temasek Laboratories, National University of Singapore, 5A Engineering Drive 2, Singapore 117411, Singapore c Center for Superconducting and Magnetic Materials, Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542, Singapore b
a r t i c l e
i n f o
Article history: Received 5 October 2016 Received in revised form 29 December 2016 Accepted 12 February 2017 Available online 14 February 2017 Keywords: FeSiAl thin film Oblique deposition Doping Magnetic anisotropy Thermal stability
a b s t r a c t In this study, we comprehensively investigate the dynamic magnetic properties of FeSiAl-NiZnFeO thin films prepared by the oblique deposition method via a shorted microstrip perturbation technique. For the films with higher oblique angle and NiZn ferrite doping amount, there are two ferromagnetic resonance peaks observed in the permeability spectra, and both of the two peaks originate from FeSiAl. Furthermore, the magnetic anisotropy field HK of the ferromagnetic resonance peak at higher frequency is enhanced with increasing doping amount, which is interpreted in terms of the contribution of reinforced stress-induced anisotropy and shape anisotropy brought about by doping elements and oblique sputtering method. In addition, the thermal stability of the ferromagnetic resonance frequency fFMR of FeSiAl-NiZnFeO films with oblique angles of 35° and 45° with respect to temperature ranging from 300 K to 420 K is deteriorated with increasing ferrite doping amount, which is mainly ascribed to the influence of pair-ordering anisotropy and/or the reduction of the FeSiAl grain size. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Similar to traditional magnetic alloys such as FeCo and NiFe which are widely applicable to electronics information industry, FeSiAl alloy (also known as sendust) exhibit favorable soft magnetic properties including high saturation magnetization and high permeability as well. Besides, it possesses higher resistivity, lower eddy current loss and much lower cost. Therefore, during the past decades, much efforts have been taken to further improve the soft magnetic properties of FeSiAl-based magnetic powder cores [1–6]. With the high-speed development of electronic information technology, the requirements for high frequency miniaturized magnetic devices are increasing. Therefore, investigations on the improvement of high-frequency properties of magnetic thin films have attracted increasing attention from researchers in recent years [7–9]. However, studies focusing on FeSiAl-based thin films are limited, let alone their high-frequency properties [10,11]. One of the most important properties for high-frequency use is ferromagnetic resonance frequency fFMR. According to Kittel’s equation [12] for films with in-plane uniaxial magnetic anisotropy, in order to increase fFMR, one can increase the effective magnetic anisotropy field HK. In our previous study [10], we also named the ⇑ Corresponding author. E-mail address: [email protected] (X. Zhong). http://dx.doi.org/10.1016/j.jmmm.2017.02.021 0304-8853/Ó 2017 Elsevier B.V. All rights reserved.
effective magnetic anisotropy field HK as dynamic magnetic anisotropy to distinguish it from static magnetic anisotropy Hsta K . From the experimental point of view, HK can be deduced from the magnetic permeability spectra and is composed of static magnetic anirot sta sotropy Hsta K and rotatable magnetic anisotropy HK , while HK can be directly deduced from M-H loop. Then, Hrot K can be estimated by subtracting the Hsta K from the HK [10]. From another perspective, anisotropy in magnetic thin films includes magnetocrystalline anisotropy, shape anisotropy, surface anisotropy, pair ordering anisotropy and strain anisotropy, etc. In order to tailor the static and dynamic magnetic properties including magnetic anisotropy of the thin films to meet the specific demands of various applications, great attempts have been made to develop a variety of deposition methods based on magnetron sputtering [10,13–19]. Among them, oblique deposition is considered to be a convenient and useful method [10,17–19]. It is well established in literatures that for magnetic film deposited by oblique deposition, steering [20] and self-shadowing effect [21–23] is the most important mechanisms causing uniaxial magnetic anisotropy of the film. The shadowing effect leads to tilted columnar grains and elongated grains [22,24] which induce magnetocrystalline, shape and strain anisotropies [21,25–26]. In addition, to further reduce the eddy current loss which largely restricts the high-frequency applications of devices based on magnetic thin films, electrical resistivity needs to be increased,
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which can be accomplished by inducing nonmagnetic insulator phases [11,13,27]. However, the saturation magnetization and resonance frequency of the films will also be decreased accordingly. It is well acknowledged that NiZn ferrite with the spinel structure exhibits high resistivity and some good soft magnetic properties. So it is an appropriate doping candidate to decrease the eddy current losses and increase the high frequency properties of magnetic thin films based on alloys [28–30]. Therefore, in this work, NiZn ferrite doped FeSiAl thin films have been prepared using oblique deposition method. The effect of doping amount on the magnetic characteristics of the films together with their thermal stability behavior which is quite important for films’ industrial applications in the temperature range of 300–420 K was investigated. 2. Experiment 100 nm-thick NiZn ferrite doped FeSiAl-based (denoted FeSiAlNiZnFeO in the following) thin films were deposited onto 5 mm 10 mm 0.10 mm Si(100) substrates at room temperature by an oblique deposition method. The FeSiAl target with content of 6 wt% Al and 9 wt% Si and diameter of 75 mm was used during sputtering with varying numbers of 3 mm 3 mm Ni0.5-
Zn0.5Fe2O4 ferrite chips placed on top of it, as illustrated in Fig. 1. The substrates are fixed on various tilted holders and each holder is placed right above the target during sputtering. The side view of each tilted holder is a right triangle with one hypotenuse and two legs – the lengths of which determine the size of oblique angle b. The base pressure was around 7 107 Torr. During deposition, the Argon pressure was kept at 4 103 Torr. The deposition power for the target is 80 W. To assist in inducing magnetic anisotropy, a magnetic field of 200 Oe was provided along the easy axis during deposition. The compositions of the films were measured by an energy dispersive X-ray spectroscope (EDS). The static hysteresis loops were measured using an M-H loop tracer at room temperature. The structure of the films was characterized using an X-ray diffractometer (XRD) with Cu-Ka radiation. The permeability spectra in the temperature range from 300 K up to 420 K was measured by a homemade microstrip fixture using the transmission line perturbation method [31] with an Agilent series N5230A vector network analyzer (VNA). In our setup, an external in-plane magnetic field can be applied using a Helmholtz coil, and the permeability spectra at different magnetic fields can be measured. From the resonance frequency extracted from the permeability spectra at different magnetic fields, the effective magnetic anisotropy and saturation magnetization can be obtained from fitting based on Kittel’s equation. In addition, deeper analysis of the films can be taken as the external in-plane magnetic field can be applied along various angular orientations, which will be discussed later.
3. Results and discussion
Fig. 1. Schematic diagram of the oblique deposition system.
The composition analysis of the FeSiAl-NiZnFeO films carried out by energy dispersive X-ray spectroscope (EDS) reveals that with the number of ferrite chips increase from 0 to 6, the concentrations of Ni and Zn elements are increased while that of Fe is almost unchanged. Specifically, the concentrations of Ni + Zn are about 2.1%, 4.1% and 7.2% when the numbers of ferrite chips are 2, 4 and 6, respectively. For convenience, we use the number of ferrite chips to represent the doping amount. Fig. 2 shows the X-ray diffraction profiles for the FeSiAlNiZnFeO films with oblique angle of 35° and chip number changed from 0 to 6. At the diffraction angle 2h of about 45°, we see only one obvious peak which can be ascribed to the (220) plane of FeSiAl from the JCPDS Card No. 45-1206. In addition, there is no peak corresponding to NiZn ferrite. As the chips number is increased, the position of the peak stemming from FeSiAl (220) shifts to the lower 2h range indicative of an extension of the lattice spacing. Specifically, the lattice spacing is increased from 2.014 Å to 2.021 Å as
Fig. 2. (a) X-ray diffraction patterns for FeSiAl-NiZnFeO thin films grown at b = 35° with various number of doped ferrite chips. (b) Lattice spacing as a function of ferrite chips.
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the chip number is changed from 0 to 6. As both the atomic radius of Ni (1.62 Å) and Zn (1.53 Å) are smaller than that of Fe (1.72 Å) and Al (1.82 Å), the extension of lattice spacing is presumably due to interstitial effect of nickel ions and zinc ions rather than substitution effect. Another remarkable change in the XRD profiles of the FeSiAl-NiZnFeO films with the NiZn ferrite doping amount is the broadening of the FeSiAl (2 2 0) peak when the NiZn ferrite doping amount is increased. This unambiguously indicates that the grain size of FeSiAl is reduced with the increasing of the NiZn ferrite doping. Fig. 3 shows the in-plane M-H loops of FeSiAl-NiZnFeO films with various doping amounts grown at different oblique angles b (16.7–45°) measured at ambient temperature. It can be seen that for all the films, the easy axis curves show a square shape. However, the conditions are different for the hard axis curves. For the films with same doping amount, the loops are more sloped when b is higher, and for the films with same b, the loops are more sloped when doping amount is larger. Such trend is more obvious when b is above 35°. Therefore, one can deduce that the static magnetic anisotropy is increased with b and doping amount. As discussed
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previously [10], the increase of magnetic anisotropy with b is largely due to the self-shadowing effect which leads to shape anisotropy. On the other hand, the stress-induced anisotropy is perceived as the derivation of the magnetic anisotropy [32] since the interstitial effect of Ni and Zn atoms will induce some internal stress upon the films. As shown in Fig. 3-(b), the coercivity HC of FeSiAl-NiZnFeO films is increased with b and doping amount due to the mechanism stated above for the improvement of the static magnetic anisotropy. Shown in Fig. 4 are the real and imaginary permeability spectra of the FeSiAl-NiZnFeO films at ambient temperature. From the imaginary permeability spectra, it can be seen that for films with same ferrite doping amount the resonance frequency is increased as b increases. And for films with b = 35° and b = 45° the resonance frequency is increased as ferrite doping amount increases. In accordance with Kittel’s equation [10], this phenomenon is on account of the increment of magnetic anisotropy with the increasing of b and doping amount. In addition, it can be found that two resonance peaks were appeared for the films with larger doping amount and higher b. It is known that the reason for the appearance of double
Fig. 3. M-H loops for FeSiAl-NiZnFeO films with various doped ferrite concentrations grown at different oblique angles of (a) b = 16.7°, (b) b = 26.6°, (c) b = 35° and (d) b = 45°. (e) Variations of coercivity as a function of doped ferrite concentrations for films with different oblique angles.
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Fig. 4. Permeability spectra measured at room temperature of FeSiAl-NiZnFeO thin films with various doped ferrite concentrations deposited at different oblique angles.
resonance peaks in the films is mainly because of the existence of double magnetic phases with different dynamic behaviors [33]. For a more quantitative analysis of the magnetic properties of the two resonance peaks, additional experimental investigations of the imaginary permeability spectra using a shorted microstrip perturbation technique with a varying external static magnetic field were performed. Based on this method, the properties and the essential mechanisms of the two resonance peaks can be further analyzed. The process and results are as follows. Fig. 5(a)–(c) show the permeability spectra measured at external magnetic field H decreasing from 150 to 0 Oe for film with 6 ferrite chips and b varied from 26.6° to 45°. It can be observed that all of the three samples have two resonance peaks (named as low peak and high peak) whose trends with external magnetic fields
are quite different. Then it can be deduced that both of the saturation magnetization MS and magnetic anisotropy HK are different for the two peaks and the values of MS and HK of low peak are higher and lower than that of high peak respectively based on the Kittel’s equation. In our case, the value of MS of FeSiAl is definitely larger than that of NiZn ferrite, and the doping amount of NiZn ferrite can be considered as small. On account of this, we argue that neither low peak nor high peak belongs to NiZn ferrite, and the existence of double resonance peaks is more likely ascribed to two magnetic phases of FeSiAl with different magnetic anisotropy fields. However, one can see from Fig. 5 that the intensity of the low peak for all the three samples are decreased dramatically with the increasing of H, it is almost impossible to get more information such as the specific values of MS and HK corresponding to the low
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Fig. 5. The imaginary part of the permeability spectra measured at different external magnetic fields for FeSiAl-NiZnFeO films with 6 ferrite chips and (a) b = 26.6°, (b) b = 35° and (c) b = 45°.
peak through fitting based on Kittel’s equation. For the high peak, the situation is quite different. Therefore, in the following part, we shall only focus on the discussion of the high peak hereinafter, and that the values of MS and HK of high peak cannot fully reflect those of the films. To further analyze the high peaks of the FeSiAl-NiZnFeO films, the permeability spectra of the films with H applied along various angular orientations were obtained [31,34]. The angular dependence of the HK can be expressed as the following [30]: rot HK Hsta K cos 2h þ HK
Hsta K
ð1Þ
Hrot K
where and are static and rotatable anisotropy respectively and h is the in-plane angle between the easy axis and H. As discussed earlier, Hsta K can be presented from the difference between the M-H loops along easy and hard axis of the film, but it also can be obtained from the permeability spectra. However, things are different for Hrot K since it cannot be detected by static magnetic measurement. Hrot is usually found in exchange biased systems and K some normal magnetic thin films [35–36]. It gets its name because the direction of it always ‘‘follows” the magnetization. Based on Eq. (1), the Kittel’s equation can be rewritten as following:
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 sta rot sta f FMR c=2p ðHrot K þ HK cos 2h þ HÞðH K H K sin h þ H þ M s Þ ð2Þ If the sample is saturated, the magnetization is not dependent on H, then the dependence between fFMR and h can be obtained rot for specific H and the values of MS, Hsta K and HK can be deduced from fitting curves. To be more specific, for the FeSiAl-NiZnFeO film with specific b and doping amount, the values of fFMR were obtained firstly when specific H were applied along various angular orientations, namely various h. Then the dependence of fFMR as a function of h for this specific H was obtained. Finally, fitting process rot was proceeded based on Eq. (2) and the values of MS, Hsta K and HK for this specific film were obtained accordingly. The process and results obtained based on this method for films with various b and doping amounts are as follows.
Fig. 6 shows the dependence of fFMR as a function of h when H is 150 Oe for the FeSiAl-NiZnFeO films with various b and doping amounts. From the fitting lines presented, the values of MS, Hsta K and Hrot K were obtained, which are summarized in Fig. 7 along with the values of HK. As noticed from Fig. 7-(a), MS is decreased for FeSiAl-NiZnFeO films with different b as the doping amounts increase, due to the increased doping of NiZn ferrite into FeSiAl when b is increased. In particular, the value of MS of NiZn ferrite is much smaller than that of FeSiAl, and increasing the concentration of NiZn ferrite in FeSiAl-NiZnFeO films will dilute the original magnetization of FeSiAl in the films, leading to the decrease of MS. From Fig. 7(b), (c), it can be seen that with doping chips number being increased from 0 to 6, the values of HK for high peaks of FeSiAl-NiZnFeO films are enhanced from 22 Oe to 41 Oe when b is 26.6°, from 30 Oe to 52 Oe when b is 35°, and from 53 Oe to 65 Oe when b is 45°, respectively. In addition, although both of Hsta K and Hrot K are increased with the increasing of ferrite doping amount, rot Hsta K is much larger than HK and accounts for a very high share of HK for all of the cases, which is quite different from the observation of Chai et al. [30]. They found that for the NiZn ferrite doped FeCo thin films, with the increasing of doping amount, Hrot K was dramatically increased, leading to the enhancement of HK and thus the fFMR [30]. Such a phenomenon was explained by the existence of exchange coupling between ferrimagnet and ferromagnet in the films. Such ‘‘exchange coupling” is similar to ferromagnet/antiferromagnet exchange biased system and hard/soft magnet coupling in exchange spring magnets as FeCo was much ‘‘harder” than NiZn ferrite [30]. However, in our case, the fact that the contribution of the rotational anisotropy is very small (Hrot K < 15 Oe) indicates that such an exchange coupling is very weak between FeSiAl and NiZr ferrite. As a consequence, the increasing of HK is largely ascribed rot to the increase of Hsta K rather than HK as presented in Fig. 7. rot Based on the above analysis of Hsta K and HK , we argue that the increment of fFMR with the increasing of doping amount in our case is closely related to doping elements and oblique deposition method itself. Specifically, the interstitial effects of Ni and Zn ions which lead to the extension of lattice spacing induce some stress
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Fig. 6. (a) The dependence of fFMR as a function of h for the FeSiAl-NiZnFeO films with (a) b = 26.6°, (b) b = 35° and (c) b = 45° respectively. The lines are the fitted curves.
rot Fig. 7. (a) Variations of MS of the FeSiAl-NiZnFeO films with different oblique angles as a function of ferrite chips. HK, Hsta K and HK of the FeSiAl-NiZnFeO films with (b) b = 26.6°, (c) b = 35° and (d) b = 45° as a function of ferrite chips.
into the film, which will give rise to the generation of stressinduced magnetic anisotropy. For films with b = 0°, although the stress is increased with the increasing of doping amount, it is almost homogeneous and isotropic, so that the generation of stress-induced anisotropy is limited. With the increasing of b, another factor should be taken into account: self-shadow effect which was applied to explain the increment phenomenon of magnetic anisotropy with b in magnetic films fabricated by oblique
sputtering [10]. In particular, the self-shadow effect leads to the tilt and/or elongation of the grains, which is ascribed to the inducement of shape anisotropy and thus the increment of HK [10]. In our cases, the combined effect of both of the interstitial effect and self-shadow effect give rise to the increasing of HK with increasing doping amount for FeSiAl-NiZnFeO films with higher b. Specifically, on one hand, with the increasing of doping amount, the lattice deformation is more drastic, leading to the larger
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elongation of the grains and thus the increment of shape anisotropy; on the other hand, for films with higher b, the tilt degree and elongation of the grains are relatively larger so that the homogeneity of stress induced by doping elements is broken, leading to the increment of stress-induced anisotropy. Overall, the enhancement of both of shape anisotropy and stress-induced magnetic anisotropy with increasing doping amount of NiZn ferrite when b is relatively large makes the increment of total effective magnetic anisotropy. In addition, there is another factor named as pairordering magnetic anisotropy which is brought from the magnetic field applied during sputtering contributing to the total effective magnetic anisotropy. It is related with the directional magnetic pair ordering of the magnetic atoms where atomic pairs rearrange with bond axes in preferred orientations with respect to the applied field [37–39]. In our case, this pair-ordering magnetic anisotropy arises from the atomic arrangement of Fe-Fe pairs resulting in anisotropic distributions of atomic ordering [37–40]. Theoretically, with the increasing of doping amount of NiZn ferrite, the directional pair ordering of Fe-Fe pairs becomes more difficult to establish. However, one should note that it is precisely because magnetic field is provided during deposition of the films, the influ-
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ence of foreign elements on directional pair ordering of Fe-Fe pairs is depressed. Therefore, the pair-ordering magnetic anisotropy is not supposed to change much with the increase of doping amount and b, but it is of great importance to the thermal stability behavior of the FeSiAl-NiZnFeO films which will be discussed next. Now we focus our discussion on the thermal stability for the ferromagnetic resonance frequency fFMR of the FeSiAl-NiZnFeO films with various doping amounts deposited at high oblique angles. As mentioned in the introduction part, the thermal stability of the magnetic films is quite important for their industrial applications. In Fig. 8, the permeability spectra of FeSiAl-NiZnFeO films doped with 6 ferrite chips deposited at b of 35° and 45° measured at different temperatures in the range of 300–420 K are presented. It can be observed that the fFMR of both of the two resonance peaks are decreased as the temperature increases. We plot the temperature dependences of the fFMR of high peaks for FeSiAl-NiZnFeO films with various doping amounts fabricated at b of 35° and 45° in Fig. 9. It can be observed in Fig. 9 that the thermal stability of fFMR of FeSiAl-NiZnFeO films decrease with the increasing of doping amount when b are 35° and 45°in the temperature range of
Fig. 8. Permeability spectra measured at various temperatures in the range of 300 K to 420 K of FeSiAl-NiZnFeO films with 6 doped ferrite chips when (a) b = 35° and (b) b = 45°.
Fig. 9. Temperature dependence of fFMR of FeSiAl-NiZnFeO thin films with (a) b = 35° and (c) b = 45°. (b) and (d) are the parameters normalized to the value obtained at 300 K.
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300 K to 420 K. In addition, the overall thermal stability of fFMR for films with b = 35° are better than that with b = 45°, which is in accordance with our previous research results [10]. Specifically, for films with b = 35°, the temperature increase causes fFMR to decrease by about 4%, 6%, 7% and 9% when the numbers of doping ferrite chips are 0, 2, 4 and 6, respectively. For films with b = 45°, fFMR decrease by about 5%, 8%, 11% and 14% when the numbers of doping ferrite chips are 0, 2, 4 and 6, respectively. Briefly, the decrease of fFMR with increasing of temperature could be ascribed to the reducing of HK and MS. It is well established that the reason for the decrement of MS with increasing temperature is the exacerbated thermal fluctuation of magnetization. However, the situation for HK is more complicated. As has been noted, the total effective magnetic anisotropy existing in the FeSiAl-NiZnFeO films in our case includes reinforced stress-induced and shape anisotropies brought about by doping elements and oblique deposition method and pair-ordering anisotropy brought from the magnetic field applied during sputtering. Normally, the thermal stability of shape anisotropy is supposed to be good [10], so that the stability as a function of temperature of HK is closely related to the doping elements. It is acknowledged that the atomic and ionic diffusion is intensified with temperature rising, leading to the destruction of some directional pair ordering then the reduction of anisotropic distribution of ordered atoms. Such effect is more significant with the increase in the amount of impurity elements so that the thermal stability of pair ordering anisotropy is decreased with increasing doping amount. In addition, the diffusion of atoms and ions, especially the interstitial ones, still have some influence on the stress. But, as the interstitial ions is very difficult to be increased or decreased under such temperature conditions, the stress-induced anisotropy can be treated as stable with increasing temperature in our case. Therefore, the reduction of thermal stabilization of the total effective magnetic anisotropy with increasing doping amount may possibly be ascribed to the thermal behavior of pair ordering anisotropy as discussed above. Another possibility for the worsening thermal stability with increased doping amount that should not be ruled out is the grain size reduction of FeSiAl when the doping amount of NiZn ferrite is increased. It is well known that when the grain size of ferromagnetic thin films is reduced the thermal stability of the magnetic properties becomes worse [41]. Hence, in this case, it is quite possible that the doping of NiZn ferrite decreases the FeSiAl grain size, thus leading to the worsening of the thermal stability. This argument is consistent with the XRD profiles in Fig. 2 showing a remarkable broadening of the FeSiAl (220) peak when the NiZn ferrite doping amount is increased.
4. Summary and conclusion In summary, the dynamic magnetic properties of FeSiAlNiZnFeO thin films prepared by the oblique deposition method were studied systematically. Our research illustrated that the ferromagnetic resonance frequency fFMR of the films is increased with the increase of oblique angle b from 0° to 45° and NiZn ferrite doping amount from 0% to 7.2% (the concentrations of Ni + Zn) which is ascribed to the increase of magnetic anisotropy field HK. For the films with higher b and doping amount, there are two ferromagnetic resonance peaks observed, named as low peak and high peak shown at lower and higher frequency range respectively, and both of them are shown to originate from FeSiAl. Through further analysis we found that for the films with higher b, the enhancement of HK of the high peak with the increase of doping amount is largely due to the increase of static anisotropy Hsta K rather than rotatable anisotropy Hrot K . The essential mechanism can be explained in terms of the contribution of reinforced stress-induced anisotropy
and shape anisotropy brought from the combination effect of interstitial ions and self-shadow effect. In addition, for FeSiAl-NiZnFeO films with b of 35° and 45°, the thermal stability of the ferromagnetic resonance frequency fFMR is deteriorated with the increase of NiZn ferrite doping amount in the temperature of 300 K to 400 K. This mainly results from the thermal behavior of pair-ordering anisotropy and/or the reduction of the FeSiAl grain sizes. Overall, the dynamic magnetic properties in terms of fFMR and HK of FeSiAlNiZnFeO thin films prepared by the oblique deposition method can be enhanced by increasing the oblique angle and doping amount, but the consequent deterioration of thermal stability of fFMR should be taken into account. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51502025) and the Scientific Research Foundation of CUIT (Grant No. KYTZ201620). CKO acknowledges the fund from AOARD-13-4039. References [1] D. Liu, C. Wu, M. Yan, Investigation on sol–gel Al2O3 and hybrid phosphatealumina insulation coatings for FeSiAl soft magnetic composites, J. Mater. Sci. 50 (2015) 6559–6566. [2] K.J. Sunday, K.A. Darling, F.G. Hanejko, B. Anasori, Y. Liu, M.L. Taheri, Al2O3 ‘‘self-coated” iron powder composites via mechanical milling, J. Alloys Compd. 653 (2015) 61. [3] H. Xu, S. Bie, J. Jiang, W. Yuan, Q. Chen, Y. Xu, Electromagnetic and microwave absorbing properties of the composites containing flaky FeSiAl powders mixed with MnO2 in 1–18 GH, J. Magn. Magn. Mater. 401 (2016) 567–571. [4] X. Fan, J. Wang, Z. Wu, G. Li, Core-shell structured FeSiAl/SiO2 particles and Fe3Si/Al2O3 soft magnetic composite cores with tunable insulating layer thicknesses, Mater. Sci. Eng., B 201 (2015) 79–86. [5] M. Huang, C. Wu, Y. Jiang, M. Yan, Evolution of phosphate coatings during high-temperature annealing and its influence on the Fe and FeSiAl soft magnetic composites, J. Alloys Compd. 644 (2015) 124–130. [6] X. Zhong, Y. Liu, J. Li, Y. Wang, Structure and magnetic properties of FeSiAlbased soft magnetic composite with AlN and Al2O3 insulating layer prepared by selective nitridation and oxidation, J. Magn. Magn. Mater. 324 (2012) 2631– 2636. [7] X.L. Liu, L.S. Wang, Q. Luo, Q.S. Xie, Q.F. Zhang, X. Liu, F.M. Bai, D.L. Peng, Magnetic properties of [FeCoSiN/SiNx]18 multilayer thin films for applications in GHz range, J. Alloys Compd. 680 (2016) 531–537. [8] D. Cao, Z. Zhu, H. Feng, L. Pan, X. Cheng, Z. Wang, J. Wang, Q. Liu, Applied magnetic field angle dependence of the static and dynamic magnetic properties in FeCo films during the deposition, J. Magn. Magn. Mater. 416 (2016) 208–214. [9] L. Zhang, L. Liu, S. Ishio, Investigation of magnetic anisotropy and magnetization process of tetragonal distorted FeCo multilayer films, Mater. Lett. 160 (2015) 238–241. [10] X. Zhong, N.N. Phuoc, G. Chai, Y. Liu, C.K. Ong, Thermal stability and dynamic magnetic properties of FeSiAl films fabricated by oblique deposition, J. Alloys Compd. 610 (2014) 126–131. [11] X. Zhong, N.N. Phuoc, W.T. Soh, C.K. Ong, L. Peng, L. Li, Thermal stability of the dynamic magnetic properties of FeSiAl-Al2O3 and FeSiAl-SiO2 films grown by gradient-composition sputtering technique, J. Electron. Mater. 46 (1) (2017) 208–217. [12] C. Kittel, Interpretation of anomalous larmor frequencies in ferromagnetic resonance experiment, Phys. Rev. 71 (1947) 270. [13] Y.G. Ma, C.K. Ong, Soft magnetic properties and high frequency permeability in [CoAlO/oxide] multilayer films, J. Phys. D Appl. Phys. 40 (2007) 3286–3291. [14] N.N. Phuoc, C.K. Ong, Comparative study of thermal stability of NiFe and NiFeTa thin films grown by co-sputtering technique, J. Electron. Mater. 45 (2016) 4061–4066. [15] A. Akbulut, S. Akbulut, F. Yildiz, Origin of spontaneous exchange bias in co/ NiMn bilayer structure, J. Magn. Magn. Mater. 417 (2016) 230–236. [16] S. Bedanta, T. Eimuller, W. Kleemann, J. Rhensius, F. Stromberg, E. Amaladass, S. Cardoso, P.P. Freitas, Overcoming the dipolar disorder in dense CoFe nanoparticle ensembles: superferromagnetism, Phys. Rev. Lett. 98 (2007) 176601. [17] S. Mallick, S. Mallik, S. Bedanta, Effect of substrate rotation on domain structure and magnetic relaxation in magnetic antidot lattice arrays, J. Appl. Phys. 118 (2015) 083904. [18] N. Chowdhury, S. Bedanta, Controlling the anisotropy and domain structure with oblique deposition and substrate rotation, AIP Adv. 4 (2014) 027104. [19] N. Chowdhury, S. Mallick, S. Mallik, S. Bedanta, Study of magnetization relaxation in Co thin films prepared by substrate rotation, Thin Solid Films 616 (2016) 328.
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