Tailoring the magnetic properties and thermal stability of FeSiAl-Al2O3 thin films fabricated by hybrid oblique gradient-composition sputtering

Journal of Magnetism and Magnetic Materials 429 (2017) 52–59

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Tailoring the magnetic properties and thermal stability of FeSiAl-Al2O3 thin films fabricated by hybrid oblique gradient-composition sputtering

MARK

⁎

Xiaoxi Zhonga, , Nguyen N. Phuocb, Wee Tee Sohc, C.K. Ongb,c, Long Penga, Lezhong Lia a Sichuan Province Key Laboratory of Information Materials and Devices Application, Chengdu University of Information Technology, Chengdu 610225, PR China b Temasek Laboratories, National University of Singapore, 5A Engineering Drive 2, 117411 Singapore c Center for Superconducting and Magnetic Materials, Department of Physics, National University of Singapore, 2 Science Drive3, 117542 Singapore

A R T I C L E I N F O

A BS T RAC T

Keywords: Thermal stability Gradient-composition deposition Oblique deposition Sendust alloy Magnetic properties

In this study, we systematically investigate the dynamic magnetic properties of FeSiAl-Al2O3 thin films fabricated by hybrid oblique gradient-composition sputtering technique with respect to temperature ranging from 300 K to 420 K. The magnetic anisotropy field HK and ferromagnetic resonance frequency fFMR can be tuned from 14.06 to 110.18 Oe and 1.05–3.05 GHz respectively, by changing the oblique angle, which can be interpreted in terms of the contribution of stress-induced anisotropy and shape anisotropy. In addition, the thermal stability of FeSiAl-Al2O3 films in terms of magnetic anisotropy HK and ferromagnetic resonance frequency fFMR are enhanced with the increase of oblique angle up to 35° while the thermal stability of effective Gilbert damping factor αeff and the maximum imaginary permeability μ’’max are improved with the increase of oblique angle up to 45°.

1. Introduction As a traditional magnetic material for various applications, sendust (FeSiAl) alloy possesses a range of favorable properties such as high permeability, high resistivity, low coercivity, low loss and low cost. Therefore, it is widely used in the electronics information industry and related fields in the form of magnetic powder cores[1–7]. However, with the rapid development of electronic information technology, is it difficult for FeSiAl magnetic powder cores to keep up and meet the demand for excellent high-frequency performance of magnetic components. On the other hand, FeSiAl based thin film devices could be a better substitute as it is well-known that thin films could have much higher ferromagnetic resonance frequencies than their bulk counterparts [8]. However, there are only few published studies that have paid close attention to FeSiAl-based magnetic thin films, especially their dynamic magnetic properties [9]. Recently, in order to meet the specific requirements of various applications, attempts have been made by researchers to tailor the microwave characteristics of the films by a variety of techniques including composition gradient deposition[10], oblique sputtering [11,12], multilayered film technique [13] and exchange-biased systems [14,15]. Based on these methods for magnetic thin films, high ferromagnetic resonance frequencies above a few GHz can be achieved.

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In addition, it is also important for such films to have excellent thermal stability so as to meet the stringent demands in industrial applications [16,17]. As a useful and convenient way to tailor the magnetic anisotropy field of thin films, oblique sputtering technique was applied by our group to prepare pure FeSiAl films with ferromagnetic resonance frequency fFMR up to 2.5 GHz [18]. However, the thermal stability of their dynamic magnetic properties are not good enough. In the previous studies, we have proved that FeCo-based [19,20] films with excellent thermal stability in terms of magnetic anisotropy field and ferromagnetic resonance frequency could be obtained by employing composition gradient deposition, which has been rarely observed in magnetic thin films fabricated by other methods. Moreover, being a good insulator, Al2O3 is an appropriate doping candidate to further enhance the electrical resistivity and decrease eddy current losses of FeSiAl thin films. Therefore, in the present study, we investigate the thermal stability in the temperature ranging from 300 K to 420 K of the magnetic characteristics of FeSiAl-Al2O3 thin films fabricated using a hybrid deposition method combining oblique and gradient-composition sputtering. 2. Experiment

Corresponding author. E-mail address: [email protected] (X. Zhong).

http://dx.doi.org/10.1016/j.jmmm.2017.01.002 Received 13 June 2016; Received in revised form 21 December 2016; Accepted 2 January 2017 Available online 05 January 2017 0304-8853/ © 2017 Elsevier B.V. All rights reserved.

100 nm-thick FeSiAl-Al2O3 were deposited onto 5 mm×10 mm

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different magnetic fields by Lorentz fitting, the dynamic magnetic anisotropy and saturation magnetization can be obtained from fitting based on Kittel's equation. The effective Gilbert damping factor can also be calculated based on LLG (Landau-Lifshitz-Gilbert) equation.

3. Results and discussion The composition analysis of the central point of the FeSiAl-Al2O3 films carried out by energy dispersive x-ray spectroscope (EDS) reveals that the concentration of Fe element is decreased while that of Al is increased with oblique angle β increased from 0° to 45°. In addition, the measurement taken at various points along the hard axis indicates that for the films with higher β, the change in concentration of Al from end to end is greater than that for the films with lower β. This observation is similar to our previous study [19]. However, it should be noted that it is very difficult to obtain a completely accurate composition measurement because of the strong contribution of Si substrate to the EDS signal. Therefore, the specific composition of each element of FeSiAl-Al2O3 films is not given here. In Fig. 2 the X-ray diffraction profiles for the FeSiAl-Al2O3 samples with oblique angle β ranging from 0° to 45° were presented. We only see one prominent peak at the diffraction angle 2θ of about 45°. From the JCPDS Card (No. 45-1206), one may ascribe this peak to the (220) plane of FeSiAl. As β is increased, the FeSiAl (220) peak position shifts to lower diffraction angle which is indicative of an extension of the lattice spacing. To be specific, the lattice spacing is found to be increased from 2.006 to 2.018 angstrom as β being changed from 0° to 45°. We argue that the increased lattice spacing in our case can be ascribed to three main contributions. The first one is the so-called selfshadow effect brought from the oblique deposition method and leads to the growth of tilted columnar grains and/or elongate grains and thus the distortion of the lattice and increase of lattice spacing [18]; the second one is the possible substitution of Al atoms with larger atomic radius for Fe and Si atoms in the lattice, which may result in the increasing of lattice spacing; and the last one is the interstitial effect that Al and O atoms may be positioned on the interstitial sites in the lattice, which would also leads to the extension of lattice spacing. Fig. 3 shows the in-plane M-H loops of FeSiAl-Al2O3 films for various oblique angles β (0–45°) 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 since the loops are more sloped for the films with higher β than that for the ones grown at lower β. Therefore, one can deduce that the static magnetic anisotropy is increased with β. As discussed previously [18], the increasing of magnetic anisotropy with oblique angle is due to the self-shadowing effect which leads to shape anisotropy. On the other hand, for the magnetic films produced by composition-graded sputter-

Fig. 1. Schematic diagram of the hybrid deposition system.

×0.10 mm Si(100) substrates at room temperature by a hybrid deposition method combining oblique and gradient-composition sputtering using FeSiAl and Al2O3 targets, as illustrated in Fig. 1. It should be noted that our selection of the thickness of 100 nm comes from the fact that if the thickness is higher than 100 nm we may observe some stripe domains in the films causing multiple peaks of the permeability spectra [12]. Yet, if the film is too thin, the permeability spectra may become too weak to detect. Hence, the selected thickness of 100 nm is favorable for the aim of our study. The diameter of FeSiAl target with content of 6 wt% Al and 9 wt% Si and Al2O3 target are 75 mm and 50 mm, respectively. The substrates are tilted and placed right above the FeSiAl target. The base pressure was 5×10−7 Torr. During deposition, the Argon pressure was kept at 4×10−3 Torr. The deposition power for both of the two targets 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 energy dispersive x-ray spectroscope (EDS). The structure of the films was characterized using an X-ray diffractometer (XRD) with Cu-Kα radiation. The static hysteresis loops were measured using M-H loop tracer at room temperature. 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 [21] with an Agilent series N5230A vector network analyzer (VNA). In this measurement, a small radio-frequency (RF) excitation field generated from the VNA parallel to the hard axis makes the magnetization process around the easy axis so as the permeability spectra can be measured. Moreover, in our setup, an external in-plane magnetic field can be applied using a Helmholtz coil, and we can thus measure the permeability spectra at different magnetic fields. From the resonance frequency, extracted from the permeability spectra at

Fig. 2. (a) XRD patterns for FeSiAl-Al2O3 thin films grown at different oblique angles. (b) Lattice spacing as a function of oblique angle.

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the FeSiAl-Al2O3 films measured at ambient temperature. To make comparison, the data of the FeSiAl film named as “without” deposited with only FeSiAl target is also presented in the figure. It can be seen from the imaginary permeability spectra that the resonance frequency is increased as β increases. In accordance with Kittel's equation[18], this phenomenon is due to the increment of dynamic magnetic anisotropy with the increase of β as mentioned above. Here it should be noted that the static magnetic anisotropy is only part of dynamic magnetic anisotropy [18]. Therefore, the dynamic one is larger than the static one, and they normally have the same variation tendency [18]. For a more quantitative analysis of the dynamic magnetization, additional experimental investigations of the imaginary permeability spectra using a shorted microstrip perturbation technique [21] with a varying external magnetic field along the easy axis were performed. Here we take the film deposited at β of 35° as example. Based on the experimentally obtained permeability spectra measured at room temperature under external magnetic field H in the range of 70–140 Oe as shown in Fig. 5-(a), we can determine the resonance peak fFMR as function of H which can be expressed according to Kittel's equation as follows:

fFMR ≈

γ (HKdyn + H )(HKdyn + H + MS ) 2π

(1)

Here, MS is the magnetization of the films along the applied field H, HKdyn is the dynamic magnetic anisotropy and γ the electron gyromagnetic ratio with value of 1.759×1011 rad/s. T [22]. If the sample is saturated and the magnetization is not dependent on H, then the values of MS and HKdyn can be deduced from fitting curves as shown in Fig. 5(b). It is noticed that even though the Kittel's equation is used for the case of homogeneous materials in uniform mode, we argue that it is still valid for our case with inhomogeneous materials. First, we take a very small area of our sample where the variation of the composition is negligible, then such a small area can be considered homogeneous and the Kittel's equation can be applied. In other words, we can have all the parameters of fFMR, HK and MS of that small area strictly related to one another via the Kittel's formula. That means each small area of our sample can have its own fFMR, HK and MS dependent to each other according to Kittel's equation. Then, considering the whole sample as a combination of many small areas with each fFMR, HK and MS, the values of fFMR, HK and MS of the whole sample will be the average values of many small areas. Therefore, one can deduce that the average values of fFMR, HK and MS should also follow the Kittel's equation. In our measurement, both the static and dynamic magnetic properties are characterized for the entire sample with all the parameters related to the Kittel formula, namely fFMR, HK and MS, being averaged throughout the whole sample. Hence, it can be argued that our experimentally measured values of fFMR, HK and MS being averaged throughout the

Fig. 3. (a) M-H loops measured along easy axis and hard axis at room temperature for FeSiAl-Al2O3 films grown at different oblique angles. (b) Variations of coercivity as a function of oblique angle.

ing, the stress-induced anisotropy is perceived as the derivation of the magnetic anisotropy [20] since the substitution effect and interstitial effect of Al and O atoms will induce some internal stress upon the films. As shown in Fig. 3-(b), the coercivity HC of FeSiAl-Al2O3 films is increased from 4.6 to 6.9 Oe with β increasing from 0° to 45° due to the mechanism stated above for the improvement of magnetic anisotropy. Shown in Fig. 4 are the real and imaginary permeability spectra of

Fig. 4. Permeability spectra measured at room temperature of FeSiAl-Al2O3 thin films deposited at various oblique angles. (“Without” means “pure” FeSiAl thin film deposited with only FeSiAl target at oblique angle β of 0°.).

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Fig. 5. (a) The imaginary part of the permeability spectra measured at room temperature with various external magnetic fields H (70–140 Oe) for the FeSiAl-Al2O3 thin film with β=35°. (b) The dependence of resonance frequency fFMR as a function of H for the FeSiAl-Al2O3 thin film with β=35°. The line is fitting curve and the data of dynamic magnetic anisotropy field HKdyn and saturation magnetization MS in the inset table are obtained during the fitting procedure. The inset figure includes the imaginary part of the permeability spectra for the film with β=35° and the corresponding LLG fitting curve.

Fig. 6. Variations of (a) saturation magnetization (MS), (b) resonance frequency (fFMR), (c) dynamic magnetic anisotropy fields(HKdyn) and (d)effective Gilbert damping coefficient (αeff) as a function of oblique angle.

As noticed from Fig. 6-(a), MS is decreased from 1.214 to 1.118 T for FeSiAl-Al2O3 films with the increasing of β, due to the increased doping of Al2O3 into FeSiAl when β is increased. In particular, increased concentration of Al2O3 in FeSiAl-Al2O3 films will dilute the original magnetization of FeSiAl in the films, thus decreasing the MS. From Fig. 6(b) and (c), it can be seen that the fFMR is enhanced from 1.05 to 3.05 GHz while the HKdyn is evidently increased from 14.06 to 110.18 Oe with β being increased from 0° to 45°for FeSiAl-Al2O3 films. According to Kittel's equation, it can be easily deduced that the reason for the increment of fFMR is exclusively due to the enhancement of HKdyn since the MS is decreasing with increasing β in this case.

sample are still related to one another via the Kittel's equation. The results are presented in Fig. 6-(a) and Fig. 6-(c). Using the values of MS and HKdyn derived from last step along with the values of fFMR, the effective Gilbert damping factor αeff can be calculated based on the expressions of the imaginary parts of permeability spectra obtained by solving the LLG equation as follows [21]:

μ″ = 4πMs ωαeff

2 γ 2 (4πMs + HKdyn ) 2(1 + αeff ) + ω2 2 [ωR2 (1 + αeff ) − ω 2] + [αeff ωγ (4πMs + 2HKdyn )]2

(2)

Here, ωR (ωR =2πfFMR) is the ferromagnetic resonance frequency. The results are summarized in Fig. 6-(d). 55

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Fig. 7. Permeability spectra measured at various temperatures ranged from 300 K to 420 K of FeSiAl-Al2O3 film with (a) β=16.7° and (b) β=42°.

Here, λ is the magnetostriction coefficient, σ being the stress and MS the saturation magnetization of the film. For a tensile stress, the sign of σ is positive and for a compressive stress, it is negative. In our case, the stress is of compressive type along the gradient composition axis; therefore, the stress-induced anisotropy will have an easy axis perpendicular to gradient composition direction. In short, from our above argument, it can be deduced that increased doping of Al and O atoms with increasing of β will induce more compressive stress along the hard axis, leading to a higher stress-induced magnetic anisotropy. Overall, the enhancement of both of shape anisotropy and stressinduced magnetic anisotropy with β makes the increment of total dynamic magnetic anisotropy with β. In addition, it should be noted that the total dynamic magnetic anisotropy includes pair-ordering contribution which is not supposed to change much with the increase of β but will play a vital role in the thermal stability behavior which will be discussed in the following part. As shown in Fig. 6-(d), the effective Gilbert coefficient αeff is increased from 0.0111 to 0.0135 for FeSiAl-Al2O3 films as β increases from 0° to 42°. In our case, the effective Gilbert coefficient αeff includes both intrinsic and extrinsic contributions. The intrinsic contribution is related to the material properties such as the composition of the films while the extrinsic contribution is comprised of two-magnon contribution due to degenerate spin waves induced by defects within the film, the inhomogeneity of the film and the mosaicity contribution due to variation in saturation magnetization and/or angular variation in the anisotropy axis [23] In our previous work, we found that for FeSiAl films prepared by oblique deposition technique, the intrinsic damping constant does not vary significantly for various oblique angles. Specifically, we found that the intrinsic and mosaicity effects dominate for low oblique angles, whereas two-magnon scattering which stems from the defects induced from oblique sputtering due to the shadowing effect becomes dominant for higher oblique angles [24] In this work, however, our composition-graded FeSiAl-Al2O3 films are not uniform

As the hybrid sputtering method used to prepare FeSiAl-Al2O3 films combines both the oblique and gradient-composition deposition techniques, the enhancement of HKdyn with increasing of β can be attributed to two main factors. Firstly, the increasing trend of HKdyn with increasing of β was previously found in other kinds of magnetic films fabricated by oblique sputtering, and the so-called self-shadow effect was applied to explain such a phenomenon [18]. Specifically, the self-shadow effect leads to the tilt and/or elongation of the grains. As a result, the shape anisotropy is induced and increased with β, which contributes to the increment of total dynamic magnetic anisotropy. Secondly, according to our previous study, magnetic anisotropy present in the films grown by the same technique mainly arises from the stressinduced origin [19,20]. In particular, the stress-induced magnetic anisotropy in the FeSiAl-Al2O3 films is proportional to the stress arising from the composition gradient distribution of Al and O along the hard axis brought from the substitution and interstitial effects. Under normal conditions, the stress brought from the substitution and interstitial effect is homogeneous and isotropic, so that no stressinduced anisotropy is induced in the film [19,20]. However, for the films with gradient composition, the composition along the hard axis is differential, causing inhomogeneity and anisotropy of stress which leads to a stress-induced magnetic anisotropy [19,20]. This effect is socalled the inverse magnetostriction effect (which is also known as Villari effect). Specifically, when there is a stress applied along a predefined axis of a ferromagnetic material, there will be a magnetic anisotropy induced along such a pre-defined axis or perpendicular to it depending on the type of the stress (tensile stress or compressive stress). Quantitatively, in our case, the stress-induced magnetic anisotropy field HKstress can be presented via the following formula [19,20].

HKstress = −

3λσ MS

(3)

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Fig. 8. Temperature dependences of (a) saturation magnetization (MS), (c) magnetic anisotropy field (HK), and (e) ferromagnetic resonance frequency (fFMR) of FeSiAl-Al2O3 thin films grown at various oblique deposition angles. (b), (d) and (f) are the parameters normalized to the value obtained at 300 K.

at different temperatures in the range of 300 K to 420 K are presented. It can be observed that both the ferromagnetic resonance frequency fFMR and the maximum value of the imaginary permeability are decreased as the temperature increases. Briefly, the decrease of fFMR with increasing of temperature could be ascribed to the reducing of HKdyn and MS, and the decrease of μ’’max can be attributed to the reduction of static permeability μS and the increase of αeff. We plot the temperature dependences of the MS, HKdyn and fFMR for FeSiAl-Al2O3 films fabricated with different oblique angle in Fig. 8 and those of αeff and μ’’max in Fig. 9. As shown in Fig. 8(a)-(b), MS is decreased with increasing temperature for all the films, which is mainly because of the thermal perturbation of magnetization. It can be observed in Fig. 8-(c) that the thermal stability of HKdyn of FeSiAl-Al2O3 films increases with the increasing of β when β is below 35° since HKdyn increase about 2% and 6% for the films with β=26.6° and 35° respectively while it decrease

in composition. Therefore, as for the extrinsic contribution of the Gilbert damping, we have to take into account the inhomogeneity factor besides the two-magnon and the mosaicity contributions. This makes the situation even more complicated and hence it is very difficult to quantify the contribution of each factor given that our effective Gilbert coefficient is derived from the permeability measurement for the whole sample and the obtained parameters are the average values for the entire sample. Furthermore, the composition of the FeSiAlAl2O3 films is changed with oblique angle β, thus the intrinsic contribution is changed as well. Therefore, the increasing behavior of αeff of the FeSiAl-Al2O3 films with oblique angle β is related to all the factors mentioned above even though it is very difficult to quantify the contribution of each factor. Now we focus our discussion on the thermal stability for the magnetic properties of the FeSiAl-Al2O3 films. In Fig. 7, the permeability spectra of FeSiAl-Al2O3 films with β=16.7° and β=42° measured 57

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Fig. 9. Temperature dependences of (a) effective Gilbert damping coefficient (αeff), (b) static permeability (μs) and (c) maximum imaginary permeability (μ’’max).

On one hand, as discussed previously, for gradient-composition sputtered films, when the temperature is increased, the stress of the films may increase accordingly, which leads to the increment of stressinduced anisotropy with the rising of temperature [19,20]. On the other hand, the stability as a function of temperature of shape anisotropy is supposed to be good [18]. Therefore, the thermal stabilization of the total dynamic magnetic anisotropy is good, as shown. However, when β is larger than 35°, the decrement of pair-ordering magnetic anisotropy with increasing temperature may overcome the increment of stressinduced one. As a result, the thermal stability of HKdyn deteriorates when β is 42° and 45°. Although the HKdyn increases with temperature for some cases, the fFMR decreases with temperature in the range of 300 K to 420 K for all the FeSiAl-Al2O3 films, which is ascribed to the decreasing of MS. Nevertheless, the thermal stability of fFMR for the film with β=35° is still the best. Particularly, fFMR are decreased about 17%, 6% and 13%, respectively for the film with β=0°, 35° and 45° respectively as temperature rising from 300 K to 420 K. The temperature dependences of the effective Gilbert damping factor αeff deduced from the LLG fitting and the values of the static permeability μS and maximum imaginary permeability μ’’max derived from the permeability spectra for films fabricated at various β are summarized in Fig. 9. It is shown that αeff is increased with increasing temperature for each deposition angle, but the increasing rate is smaller for the films with larger β. As discussed above, in our case, αeff includes intrinsic contribution which arises from gradient-composition of the films and dominates for low oblique angles and extrinsic contribution comprised of two-magnon factor which plays vital role for high oblique angles and mosaicity factor [24], but it is found that the separation of each factor is extremely difficult. Therefore, combined with our previous work, we can only deduce that the thermal stability of two-magnon contribution which increases with β is better than that of intrinsic contribution, leading to the improvement of the thermal

Table 1 Summary of the properties of some FeSiAl-based magnetic thin films [29]. Materials

Deposition method

HK (Oe)

fFMR (GHz)

αeff

FeSiAl

Oblique sputtering (β = 45°) Gradient-composition sputtering (β = 15°) Gradient-composition sputtering (β = 15°)

50.7(−6%)

2.1(−12%)

51.4(−6%)

2.0(−6%)

48.4(+11%)

2.0(−14.8%)

0.0148 (+20%) 0.0182 (+44%) 0.0144 (+185%)

Hybrid oblique gradient-composition sputtering (β = 45°)

110.2(−9%)

3.1(−13%)

FeSiAlSiO2[29] FeSiAlAl2O3[29] FeSiAlAl2O3

0.0133 (+21%)

about 15% and 1% for the films with β=0° and 16.7°. When β is beyond 35°, the thermal stability of HKdyn decreases with increasing β. As aforementioned, the total dynamic magnetic anisotropy existing in the FeSiAl-Al2O3 films in our case includes the following three contributions: stress-induced factor brought from gradient-composition deposition, the shape anisotropy brought from oblique deposition and the pair-ordering factor brought from the applied magnetic field in the process of sputtering. For the films with small β, the stress-induced and shape anisotropies are small as the influence of oblique sputtering is insignificant and the change of Al compositional grade is small. Therefore, the pair-ordering factor which leads to the anisotropic distribution of ordered atoms dominates in this case [25]. As the pair-ordering magnetic anisotropy is well-known to reduce substantially with temperature [26], the total dynamic magnetic anisotropy follows the same behavior when β is small. For the films with larger β, stress-induced and the shape anisotropies prevail over the pairordering factor. Therefore, the variation trend of the dynamic magnetic anisotropy with temperature follows the same trends as both of them.

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stability of αeff with the increased oblique angle β when temperature is rising from 300 to 420 K. The static permeability μS is dependent on the saturation magnetization MS and static magnetic anisotropy field HKsta as follows [27]:

μS = 1 +

Ms HKsta

temperature to some extent, leading to the enhancement of thermal stability for the total magnetic anisotropy. Moreover, the effective Gilbert damping factor αeff is increased and the values of static permeability μS and maximum imaginary permeability μ’’max are decreased with temperature for each oblique angle, while their thermal stability is improved with the increase of oblique angle. Overall, the thermal stability of FeSiAl-Al2O3 films prepared by our hybrid sputtering technique is good, but it is important to find an appropriate oblique angle carefully since the thermal stability of HKdyn and fFMR start to worsen if the oblique angle β reaches 45°.

(4)

It can be seen in Fig. 9-(b) that μS is dramatically decreased with oblique angle β as a result of the decreasing of MS and increasing of HKsta. Moreover, it is slightly decreased with the increasing of temperature possibly owing to the fact that the reduction rate of MS with temperature is larger than that of HKsta to some extent. In addition, the thermal stability of HKsta is improved with the increase of β. In Fig. 9-(c), μ’’max is observed to be decreased in the same temperature range. The mechanism of this phenomenon can be interpreted by the following equation [28].

″ = μmax

1 1 (μS − 1) 1 + 2 2 αeff

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.

(5)

References

As the values of μS and αeff are decreased and increased with temperature respectively, the thermal stability of μ’’max can be explained accordingly. Finally, in order to make a comparison, the following table (see Table 1) was provided, in which some of the magnetic properties of FeSiAl-based thin films prepared by various deposition methods we have investigated are presented [29]. All of those films were deposited using the same FeSiAl target. And the value of each kind of magnetic property for all the films was obtained using the same investigation method respectively. Note that the data in the brackets represent the thermal stability of the corresponding properties in the temperature of 300–420 K, and the symbol “-” means decrease while “+” means increase.

[1] H. Xu, S. Bie, J. Jiang, et al., J. Magn. Magn. Mater. 401 (2016) 567. [2] V. Cardoso Schwindt, M. Sandoval, J.S. Ardenghi, et al., J. Magn. Magn. Mater. 389 (2015) 73. [3] Z.H. Yang, Z.W. Li, Y.H. Yang, et al., J. Magn. Magn. Mater. 331 (2013) 232. [4] X. Fan, J. Wang, Z. Wu, et al., Mater. Sci. Eng. B 201 (2015) 79. [5] Y. Qing, J. Su, Q. Wen, et al., J. Alloy. Compd. 651 (2015) 259. [6] M. Huang, C. Wu, Y. Jiang, et al., J. Alloy. Compd. 644 (2015) 124. [7] D. Wei, X. Wang, Y. Nie, et al., J. Appl. Phys. 117 (2015) 17A921. [8] O. Acher, S. Dubourg, Phys. Rev. B 77 (2008) 10440. [9] S.J. Lee, J.E. Snyder, C.C.H. Lo, K.M. Campo, J.W. Andere, D.C. Jiles, J. Appl. Phys. 94 (2003) 4. [10] N.N. Phuoc, L.T. Hung, C.K. Ong, J. Alloy. Compd. 509 (2011) 4010. [11] X. Zhong, N.N. Phuoc, Y. Liu, et al., J. Magn. Magn. Mater. 365 (2014) 8. [12] G. Chai, N.N. Phuoc, C.K. Ong, Appl. Phys. Express 7 (2014) 063001. [13] H. Geng, J.Q. Wei, S.J. Nie, et al., Mater. Lett. 92 (2013) 346. [14] R. Dutra, D.E. Gonzalez-Chavez, T.L. Marcondes, et al., J. Magn. Magn. Mater. 346 (2013) 1. [15] B. Peng, N.N. Phuoc, C.K. Ong, J. Alloy. Compd. 602 (2014) 87. [16] M. Ledieu, F. Schoenstein, J.-H.L. Gallou, O. Valls, S. Queste, F. Duverger, O. Acher, J. Appl. Phys. 93 (2003) 7202. [17] K. O’Grady, L.E. Fernandez-Outon, G. Vallejo-Fernandez, J. Magn. Magn. Mater. 322 (2010) 883. [18] X. Zhong, N.N. Phuoc, G. Chai, et al., J. Alloy. Compd. 610 (2014) 126. [19] N.N. Phuoc, C.K. Ong, Adv. Mater. 25 (2013) 980. [20] N.N. Phuoc, C.K. Ong, Appl. Phys. Lett. 102 (2013) 212406. [21] L.T. Hung, N.N. Phuoc, et al., Rev. Sci. Instrum. 82 (2011) 084701. [22] M.Van Valkenburg, W. Middleton, Reference Data for Engineers: Radio, Electronics, Computer, and Communications, 9th ed. (Butterworth-Heinemann, Boston: Newnes, 2001. [23] R.L. Rodríguez-Suárez, L.H. Vilela-Leão, T. Bueno, et al., Phys. Rev. B 83 (2011) 224418. [24] W.T. Soh, X. Zhong, C.K. Ong, Appl. Phys. Lett. 105 (2014) 112401. [25] E.B. Park, Y. Kim, S. Jang, et al., Met. Mater. Int. 19 (2013) 129. [26] S.J. Kernion, P.R. Ohodnicki Jr, J. Grossmann, et al., Appl. Phys. Lett. 101 (2012) 102408. [27] D. Spenato, S.P. Pogssian, D.T. Dekadjevi, J. Ben Youssef, J. Phys. D: Appl. Phys. 40 (2007) 3306. [28] N.N. Phuoc, G. Chai, C.K. Ong, J. Appl. Phys. 112 (2012) 113908. [29] X. Zhong, N.N. Phuoc, W.T. Soh, C.K. Ong, L. Peng, L. Li, J. Electron. Mater. (2016). http://dx.doi.org/10.1007/s11664-016-4882-x.

4. Summary and conclusion To conclude, the thermal stability of the magnetic properties for temperatures ranging from 300 to 420 K of FeSiAl-Al2O3 thin films prepared by hybrid deposition method combining oblique and gradient-composition sputtering were systematically studied. Our research illustrated that the films produced by this hybrid technique have adjustable ferromagnetic resonance frequency fFMR from 1.05 to 3.05 GHz and effective Gilbert damping coefficient αeff from 0.0111 to 0.0135 as the oblique angle β changes from 0° to 45°. In addition,, the thermal stability of the dynamic magnetic anisotropy HKdyn is better for films with higher β as HKdyn increases by about 6% when β is 35° while it decreases by about 15% when β is 0°. As the dynamic magnetic anisotropy in our case includes stress-induced, shape and pair-ordering anisotropies, this phenomenon can be explained by qualitatively separating each contribution. Specifically, for the films with relatively higher β, the increase of stress-induced anisotropy can compensate for the decrement of pair-ordering one with increasing

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