Driving ferromagnetic resonance frequency of FeCoB/PZN-PT multiferroic heterostructures to Ku-band via two-step climbing: composition gradient sputtering and magnetoelectric coupling

RF/microwave soft magnetic films (SMFs) are key materials for miniaturization and multifunctionalization of monolithic microwave integrated circuits (MMICs) and their components, which demand that the SMFs should have higher self-bias ferromagnetic resonance frequency fFMR, and can be fabricated in an IC compatible process. However, self-biased metallic SMFs working at X-band or higher frequency were rarely reported, even though there are urgent demands. In this paper, we report an IC compatible process with two-step superposition to prepare SMFs, where the FeCoB SMFs were deposited on (011) lead zinc niobate–lead titanate substrates using a composition gradient sputtering method. As a result, a giant magnetic anisotropy field of 1498 Oe, 1–2 orders of magnitude larger than that by conventional magnetic annealing method, and an ultrahigh fFMR of up to 12.96 GHz reaching Ku-band, were obtained at zero magnetic bias field in the as-deposited films. These ultrahigh microwave performances can be attributed to the superposition of two effects: uniaxial stress induced by composition gradient and magnetoelectric coupling. This two-step superposition method paves a way for SMFs to surpass X-band by two-step or multi-step, where a variety of magnetic anisotropy field enhancing methods can be cumulated together to get higher ferromagnetic resonance frequency.

In this paper, we choose Fe 70 Co 30 -B alloys with high 4pM S and permeability, and demonstrate a novel method to prepare microwave SMFs at room temperature (an IC compatible process). The novel preparation method combined two magnetic anisotropy fields H K together (i.e. stress induced H K by composition gradient and electric field induced tunable H K via magnetoelectric coupling), realizing a two-step climbing of ferromagnetic resonance frequency. As a result, a giant H K of 1498 Oe, which is 1-2 orders of magnitude larger than that by conventional magnetic annealing method, and a record high f FMR of up to 12.96 GHz, reaching Ku-band, were obtained at zero magnetic bias field and bias electric field of 8 kVcm 21 in the asdeposited Fe 70 Co 30 -B/lead zinc niobate-lead titanate (PZN-PT) (hereinafter referred to as FeCoB/PZN-PT) multiferroic heterostructure films. The ferromagnetic resonance frequency of the FeCoB/ PZN-PT multiferroic heterostructures can be manipulated by electric field, instead of large and energy-consuming electromagnets, from 6.30 GHz to 12.96 GHz with the electric field from 0 to 8 kVcm 21 . The net frequency shift Df FMR is as high as 6.66 GHz, and the frequency tunability Df FMR /f FMR is about 106%, equivalent to 832.5 MHz cm kV 21 . This electric field manipulation of f FMR shift has low energy consumption and lightweight, especially suitable for manufacturing tuneable MMIC devices.
The ferromagnetic resonance frequency f FMR of SMFs can be expressed by the Kittle equation as follows, where c is the gyromagnetic ratio, 4pM S is the saturation magnetization. Clearly, high 4pM S and H K are needed to achieve a high f FMR in SMFs. Previous research on achieving high f FMR SMFs has been mostly focused on enhancing uniaxial magnetic anisotropy field H K since it is relatively easier to be enhanced by 1-2 orders of magnitude, compared to saturation magnetization that is capped at 24.5 kGs at room temperature 14,15 . Magnetron sputtering of SMFs in in-situ magnetic fields and/or subsequent magnetic annealing after deposition have been widely employed for inducing a uniaxial magnetic anisotropy 16,17 . However, the induced uniaxial magnetic anisotropy fields are usually in the range of ,50 Oe, which leads to limited ferromagnetic resonance frequency of ,3 GHz for most of the metallic magnetic films 18 . Several different approaches have been investigated for achieving high uniaxial magnetic anisotropy in SMFs, such as oblique sputtering 19,20 , facing-target sputtering 21 , exchange coupling [22][23][24][25] , and magnetoelectric coupling 26,27 , etc. Oblique sputtering and facing-target sputtering can generate magnetic films with H K of 20-300 Oe 19,20 . Exchange coupling, such as antiferromagnetic/ferromagnetic exchange coupling [22][23][24] and exchange coupling between magnetically soft and hard layers 25 , provides high H K around 100-750 Oe. In our previous work, a novel composition gradient sputtering (CGS) method was applied to achieve a high uniaxial magnetic anisotropy in SMFs [28][29][30] , which dramatically increased the in-plane uniaxial magnetic anisotropy field to up to 547 Oe due to the uniaxial stress distribution induced by composition gradient. As a result, good microwave ferromagnetic properties with ferromagnetic resonance frequency over 7 GHz were obtained in composition gradient deposited magnetic films.
Multiferroic composite materials have drawn an increased amount of attention recently due to the strong magnetoelectric coupling demonstrated in multiferroic composites, which allows for elec-tric field manipulation of magnetic properties (converse magnetoelectric effect) or magnetic field control of electric polarization (direct magnetoelectric effect) [31][32][33][34] . The magnetoelectric coupling in magnetic/ferroelectric multiferroic heterostructures can lead to dramatically enhanced electric-field tunable magnetic anisotropy fields approaching 750-880 Oe [35][36][37] . Based on the discussion above, it is difficult to obtain SMFs with H K over 1000 Oe or f FMR over 10 GHz at zero biased magnetic field using a single method. It is necessary to explore novel methods to enhance H K , and therefore to push the ferromagnetic resonance to X-or Ku-band.

Results
The design for enhancing the ferromagnetic resonance frequency via CGS and magnetoelecric coupling. The FeCoB/PZN-PT multiferroic heterostructures were prepared by a composition gradient sputtering method. The detailed experimental procedures are described as follows: firstly, a (100) single crystal Si substrate with dimension of 75 mm 3 5 mm 3 0.5 mm was pasted on the turntable with the length direction along the radial (R) direction for optimizing fabrication condition of the CGS SMFs. The sample prepared by CGS method was named as S CGS . The S CGS was cut into 15 segments along the length direction with equal size of 5 mm 3 5 mm for microstructure and magnetic properties measurement, and the segments were successively numbered as n 5 1 to 15 from inner to outer (see the top of Figure 1a Figure 1a inset ). The CGS FeCoB film was also deposited on PZN-PT substrate under the same optimum sputtering conditions for studying the electric field manipulation of ferromagnetic resonance via magnetoelectric coupling. So it was named as S ME . Therefore, the effects from composition gradient and magnetoelectric coupling on the microwave soft magnetic properties can be verified by the samples S CGS and S ME , respectively. Figure 1a shows the schematic drawing of the composition gradient sputtering method. The main target of Fe 70 Co 30 directly faces the centre of the substrate which sits on a rotating turntable, while the doping target of B is offset radially from the sample centre. The doping gun (B target) is tilted at a certain angle towards the sample turntable. This geometrical structure ensures that the materials from the Fe 70 Co 30 main target are distributed homogeneously across the sample, while those from B doping target will have a composition gradient distribution, i.e. B concentration increases gradually from inner to outer positions along the R orientation.
The composition distribution and soft magnetic properties of CGS FeCoB/Si films. The composition distribution along R direction was detected by a field emission electron probe microanalyzer (FE-EPMA). As illustrated in Figure 1b, the atomic ratio between Fe and Co remains at 2.12, indicating a homogeneous composition comparable to the Fe 70 Co 30 target composition; while the B composition y B increases linearly from 18.07 at.% to 42.12 at.% for the test positions from n 5 1 to 15, undergoing a linear relation of y B 5 18.07 1 1.603*n (see Figure 1b). This composition distribution verified the design idea of using composition gradient sputtering to create a composition gradient film with linear doping. The linear doping of B element gives rise to an almost linear increase of H K and a nonlinear decrease of saturation magnetization 4pM S . As illustrated in Figure 2, the H K of CGS FeCoB films increases almost linearly from 90 to 436 Oe, while 4pM S rapidly reduces from 14.24 kG (n 5 1) to 13.21 kG (n 5 5) at first, then decrease linearly towards 12.72 kG (n 5 15). The H K increases nearly 5 times, while 4pM S reduces only about 10%, so the ferromagnetic resonance www.nature.com/scientificreports SCIENTIFIC REPORTS | 4 : 7393 | DOI: 10.1038/srep07393 frequency is dominated by H K . As expected, the sample position n dependence of f FMR , shown in Figure 3, demonstrates almost the same trend in f FMR with that in H K (shown in Figure 2). With the increase of n, f FMR increases from 3.18 to 6.73 GHz with increment of 3.55 GHz, equivalent to an increase ratio of 212%. The damping constant a of the CGS FeCoB/Si samples, shown in Figure 3, decreases with the increase of n from 1 to 7, then stays on a low platform with value of 0.011 till n 5 13, after that a goes up, implying an increase of magnetic loss. Considering the comprehensive magnetic properties including 4pM S , H K , f FMR , and a for the segments at different n, we chose n 5 13 as an optimum position to deposit magnetoelectric coupling sample on PZN-PT substrate since sample S CGS @n 5 13 shows a relatively large f FMR of 6.45 GHz, large H K of 402 Oe and low a of 0.0112.
Composition gradient induced uniaxial magnetic anisotropy in CGS films. Some typical hysteresis loops of S CGS are summarized in Figure 4a and 4b. As illustrated in Figure 4a, there is an obvious uniaxial magnetic anisotropy with magnetically easy axis (EA) along tangential direction and magnetically hard axis (HA) along R direction for S CGS @n 5 13. The n-dependent hysteresis loops along HA are shown in Figure 4b. It can be seen that the H K increases with the increase of n (or B doping). The detailed H K variation is shown in Figure 2. The uniaxial magnetic anisotropy in CGS films can be explained as stress gradient induced uniaxial magnetic anisotropy. The intrinsic stress in general is randomly dispersed in the magnetic films which gives rise to a high damping constant and decreasing resonant frequency. However, as reported in our previous work [28][29][30] , if a uniaxial stress replaces the randomly dispersed intrinsic stress, a stress-induced uniaxial magnetic anisotropy will be obtained. This uniaxial magnetic anisotropy leads to enhanced ferromagnetic resonance and improved microwave magnetic properties. The composition gradient sputtering is an effective way to induce a uniaxial stress, a high uniaxial magnetic anisotropy and an enhanced ferromagnetic resonance frequency.
Combination of stress-mediated CGS and magnetoelectric coupling, and enhancement of microwave soft magnetic properties. As described above, the composition gradient sputtering method gives rise to a compressive stress, leading to a magnetically hard axis along the radial direction. On the other hand, for a (011)-cut PZN-PT single crystal substrate, when the electric field is applied along [011] direction, compressive and tensile stresses will be generated along [100] and [01-1] directions, respectively. According to magnetoelastic energy equation E K~{ 3 2 l S s cos 2 h, for a positive l S (as the case in this study for the FeCoB films), a compressive or tensile stress s will lead to a magnetic anisotropy that forces the magnetic moments to align perpendicular or parallel to the stresses direction, respectively. In other words, the magnetoelectriccoupling-induced magnetic hard axis direction is along [100] direction. So if the [100] direction is parallel to the R direction, the electric-field-tunable effective uniaxial magnetic anisotropy field (H K ) ME will be parallel to the composition-gradient-induced uniaxial magnetic anisotropy field (H K ) CGS . Therefore they will be combined together leading to an ultrahigh and tunable uniaxial magnetic anisotropy field H K (see the bottom of Figure 1a inset).
In this study, sample position n 5 13 was chosen to paste the PZN-PT substrate on, with substrate [100]//R to realize the superposition    of two magnetic anisotropies. As expected, the hysteresis loops of CGS FeCoB/PZN-PT multiferroic heterostructures show a welldefined uniaxial magnetic anisotropy with a H K of 384 Oe at E 5 0 kVcm 21 , slightly smaller than that of 402 Oe on Si substrate (see Figure 4a and 4c). It is exciting that the H K of S ME dramatically increases with the increase of electric field, and a record high H K of 1498 Oe was obtained at E 5 8 kVcm 21 , implying that an ultrahigh ferromagnetic resonance frequency will be achieved thanks to the combining of composition gradient sputtering and magnetoelectric coupling effect. Figure 5 shows the frequency dependence of complex permeability for S CGS at various n and for S ME at various electric fields. From Figure 5a, it can be seen that the CGS pushed the f FMR from 3.18 to 6.45 GHz for n from 1 to 13. When the Si substrate was replaced by a PZN-PT substrate, it was found that although the f FMR of 6.30 GHz for S ME @E 5 0 kVcm 21 is slightly smaller than that of S CGS @n 5 13 due to the difference between the Si and PZN-PT substrates, the magnetoelectric coupling effect dramatically drives the f FMR of FeCoB/PZN-PT multiferroic heterostructures towards 12.92 GHz, directly reaching Ku-band from C-band across X-band. To the best of our knowledge, it is the first report that the ferromagnetic resonance frequency of as-deposited metallic magnetic films can reach Ku-band at zero-bias magnetic field. The magnetoelectric coupling effect in S ME not only generates a 6.66 GHz shift of f FMR under an electric field of 8 kVcm 21 , but also provides an electric field tunable ferromagnetic resonance frequency shift over a very broad frequency span, realizing electric field controlled frequency tuning. This is of great significance because it provides the possibility to fabricate electric field tunable microwave devices with large tunability, low energy consumption and light-weight.
The electric field and composition gradient-induced ferromagnetic resonance frequency shift can be explained by the strain/stressmediated in-plane magnetic anisotropy field. The in-plane ferromagnetic resonance frequency [Equation (1)] can be rewritten as: where (H K ) CGS is the CGS-induced uniaxial magnetic anisotropic field,   (H K ) ME is the electric-field-induced effective magnetic field which could be positive or negative, and in this study it can be express as [35][36][37] , where Y is the Young's Modulus, n is Poisson's ratio, l is the magnetostriction constant of FeCoB film, d 31 38 . From equation (3), it can be concluded that the (H K ) ME is proportional to the applied electric field due to the magnetoelectric coupling effect, resulting in an electric field tunable f FMR . Similarly, the increase of (H K ) CGS due to the composition gradient will give rise to an upward shift of f FMR . The sample position n and electric field dependence of magnetic anisotropy field H K and ferromagnetic resonance frequency f FMR are summarized in Figure 6. As illustrated, the two-step enhancement of H K and f FMR is clearly observed. Figure 6 is separated into left and right sections by a red dashed line. The left and right sections represent the contributions from composition gradient sputtering and magnetoelectric coupling effect, respectively. In the left section, with the increase of sample position n, the B concentration increases, and the H K increases linearly from 90 to 402 Oe, leading to an increase of f FMR from 3.18 to 6.45 GHz. In the right section, the CGS FeCoB film was deposited on PZN-PT substrate, and the enhancement effect of electric field on H K and f FMR begins to function in addition to the CGS effect. So the H K and f FMR are further pushed up by electric field starting from the corresponding values of S CGS . The H K and f FMR increase from 384 to 1498 Oe and from 6.30 to 12.96 GHz, respectively, with electric field from 0 to 8 kVcm 21 . This two-step superposition method provides a combined effective uniaxial magnetic anisotropy field of up to 1498 Oe at 8 kVcm 21 , which is 1-2 orders of magnitude higher than that by the conventional magnetic annealing method. At the same time, the f FMR also directly reaches Ku-band from C-band across X-band.
It is worth mentioning that the strain occurs in the magnetic films due to the composition gradient and the magnetoelectric coupling effect, so a perpendicular anisotropy may arise 39 . For verifying this issue, the FMR measurement for S ME sample was carried out at various electric fields. In the case of an in-plane applied magnetic field and measuring FMR along the easy axis, the in-plane resonance frequency is well described by Kittle equation, as reported in Ref. 39 Comparing with the out-of-plane saturation magnetic field of more than 15 kOe, such a small perpendicular field is not enough to drive the magnetic moments to the normal direction of the film. Therefore, the magnetic moments are mainly lying in the plane.
In conclusion, a record high ferromagnetic resonance frequency of 12.92 GHz, which directly reaches Ku-band from C-band across Xband, was obtained in as-deposited CGS FeCoB/PZN-PT multiferroic heterostructures at zero bias magnetic fields due to combining the composition gradient sputtering and magnetoelectric coupling effect together. This two-step superposition method can effectively add two kinds of uniaxial magnetic anisotropy fields together, obtaining ultrahigh H K that cannot be reached with any single method. This method paves the way to get higher f FMR by two-step or multi-step method. The CGS FeCoB/PZN-PT multiferroic  heterostructures exhibit ultrahigh f FMR with very broad electric field tunable frequencies, which provides great opportunities for selfbiased voltage tuning microwave multiferroic components working at X-band or higher frequencies without energy consuming electromagnets. All the fabrication processes of the CGS FeCoB/PZN-PT multiferroic heterostructures are carried out at room temperature (i.e. at IC compatible process), which is very beneficial to the integration of these soft magnetic films into monolithic microwave integrated circuits.

Methods
Preparation of the FeCoB/PZN-PT multiferroic hyterestructures. The FeCoB films with average thickness of 100-nm were deposited on (100) single crystal Si substrates with dimension of 75 mm 3 5 mm 3 0.5 mm by composition gradient sputtering method at room temperature under 2.8 mTorr Ar atmosphere with a flow rate of 20 sccm, along with a RF power of 80 W for Fe 70 Co 30 target and various powers from 60 to 180 W for B target. The FeCoB films deposited on Si substrate were used to measure magnetic properties, to observe microstructure, and to explore optimum deposition condition. It is found that the sputtering powers of 80 W for Fe 70 Co 30  Measurement of composition, magnetic, and microwave properties. The composition of films was determined by a FE-EPMA. The magnetic properties were measured by a vibrating sample magnetometer (VSM). The ferromagnetic resonance characteristics of the multiferroic heterostructures were analyzed by a broadband ferromagnetic resonance spectroscopy with the transmission line along the easy axis. The microwave performances were evaluated a vector network analyzer (VNA) with co-planar waveguide. The vector network analyzer acts as a transmitter and a receiver of microwave. The film sample was put on a specially designed co-plane waveguide transmission line fixture. When the microwave passes through the transmission line covered with the soft magnetic film, it will be absorbed by the magnetic film. As a result, the scattering parameter S 21 will show an absorption peak around the ferromagnetic resonance frequency. The vector network analyzer records the scattering parameters, and simulates the measured curves with LLG (Landau-Liftshitz-Gilbert) equation. Thus, useful parameters such as permeability, ferromagnetic resonance frequency, damping constant, etc. can be obtained. prepared the samples. J.X., Y.W. and Q.L. measured the microwave and magnetic properties. Y.L. analyzed the data. All authors discussed and commented on the manuscript.

Additional information
Competing financial interests: The authors declare no competing financial interests.