Nowadays, the integration circuit (IC) technology is developing from system-in-a-package toward system-on-a-chip, and the integration of passive components, such as inductors and capacitors, on onechip is critical for miniaturizing the electromagnetic products.^[1] Microwave soft magnetic films (SMFs) are key materials for reducing the footprint of magnetic components on monolithic microwave integrated circuit (MMIC) boards.^[2,3] The functioning frequency, performances, and size of the magnetic microwave components in MMICs, such as magnetic inductors, phase shifters, circulators, isolators, and filters, primarily dominate ferromagnetic resonance (FMR) frequency f_r and the permeability μ of the SMFs.^[4–9] The developing trend of MMICs toward high integration, high frequency, light weight, low energy consumption, etc., requires that SMFs should exhibit higher f_r, larger μ, and good compatibility with IC fabrication processes. On the other hand, in order to reduce the weight and energy consumption of MMICs, researchers are trying to create magnetic devices that can be controlled with no applied magnetic field,^[10] that is, the magnetic properties are manipulated with nonmagnetic field methods, such as stress, magnetoelectric coupling, and magnetoelastic coupling, instead of bulky and power-consuming electromagnets.

Metallic soft magnetic films exhibit special advantages in MMICs, because they exhibit higher saturation magnetization 4πM_s and permeability μ, and good compatibility with the IC fabrication processes. Therefore, the self-biased metallic SMFs prepared at IC compatible processes are the recent focus in this area.^[11–14] Enhancing uniaxial magnetic anisotropy field H_K of SMFs is the most viable route, as it can be increased by 1–2 orders of magnitude, compared to increasing 4πM_S that is capped at 24.5 kG.^[11] Various approaches were developed for achieving high H_K in SMFs, such as composition gradient sputtering,^[12,13,15] oblique sputtering (OS),^[16–19] facing target sputtering,^[20] exchange coupling,^[21–23] and magnetoelectric (ME) coupling.^[24–27]

Oblique sputtering is a widely used method for preparing SMFs, because it is easy to tailor the magnetic anisotropy.^[16–19] Another effective method to enhance H_K is the magnetoelectric coupling in multiferroic composites, which permits electric field manipulation of magnetic properties (inverse ME effect) or magnetic field control of electric polarization (direct ME effect).^[24–27] The ME coupling in magnetic/ferroelectric heterostructures can lead to large and E-field tunable H_K approaching 750–1100 Oe.^[13,28–30] In this study, an SMF with a nominal composition of Fe_52.5Co_22.5B_25.0 was deposited on (011)-cut Lead Zinc Niobate–Lead Titanate (PZN-PT) substrate using the OS method at room temperature (an IC compatible process), as shown in Fig. 1. Large and tunable H_K up to 600 Oe and f_r reaching 8.73 GHz were achieved by E-field manipulation in the Fe_52.5Co_22.5B_25.0/PZN-PT multiferroic heterostructure owing to the contribution of both OS and ME coupling.

Fig. 1.

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	Fig. 1. (color online) Profiles of oblique sputtering device (a) and FeCoB/PZN-PT multiferroic heterostructure (b).

As illustrated in Fig. 1, a (011)-cut single crystal PZN-PT substrate with the dimension of was pasted onto the sputter chamber’s turntable with the [100] and ] directions of PZN-PT along the radial (R) and tangential (T) directions of the turntable, respectively. A Fe_52.5Co_22.5B_25.0 film with an average thickness of 100 nm was deposited on the PZN-PT substrate using the OS method at room temperature under 2.8 mTorr Ar atmosphere at a flow rate of 20 sccm, along with an RF power of 80 W and an incidence angle θ=38°. The magnetic properties were measured using a vibrating sample magnetometer (VSM, PPMS Ever cool II). The microwave performances of the Fe_52.5Co_22.5B_25.0/PZN-PT multiferroic heterostructure were evaluated using an FMR spectrometer and a vector network analyzer (Agilent N5224A).

As illustrated in Fig. 2(a), a well-defined uniaxial magnetic anisotropy with a anisotropy field H_K of 169 Oe was observed in the oblique-deposited Fe_52.5Co_22.5B_25.0 film with the easy axis (EA) along the tangent direction ( of the PZN-PT substrate) and hard axis (HA) along the radial direction ([100] of the PZN-PT substrate), respectively. A compressive stress and a tensile stress are generated along the [100] and directions of the PZN-PT substrate via the ME coupling effect, respectively.^[28,29,30] For an SMF with a positive magnetostriction coefficient such as λ_S in the studied case, the HA and EA appear along [100] and ] directions, respectively. Thus, the EA directions from the OS and ME coupling are collinear, and the effective anisotropy field H_eff is enhanced simultaneously by the OS and ME coupling. Figure 2(b) shows the E-field dependence of the HA hysteresis loops. With the increase of the E-field, the hysteresis loops along the HA‖[100] direction of PZN-PT become more difficult to saturate, thereby demonstrating a dramatic enhancement of H_eff in Fe_52.5Co_22.5B_25.0 induced by the E-field. The E-field-dependent H_eff linearly increases from 169 to 600 Oe with the increase of the E-field from 0 to 8 kV/cm, which is equivalent to a large ME tunability of 53.9 Oe.cm/kV.

Fig. 2.

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	Fig. 2. (color online) Hysteresis loops along EA and HA directions for oblique-deposited Fe52.5Co22.5B25.0/PZN-PT multiferroic heterostructure (a), and its E-field dependence of HA hysteresis loops (b).

The exact shift of H_eff with the E-field can be established using the FMR spectra. As illustrated in Fig. 3, with the increase of E-field from 0 to 8 kV/cm, the FMR fields H_γ at 11 GHz along the HA∥[100] direction are moved upwards from 1026.6 to 1310.4 Oe with an increment of 283.8 Oe, while those along the EA∥] direction are shifted downwards from 887.5 to 601.9 Oe with a decrement of 285.6 Oe. The E-field-induced FMR peak shift can be explained by the strain/stress-mediated in-plane magnetic anisotropy field. The in-plane FMR frequency can be expressed by the Kittel equation

Fig. 3.

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	Fig. 3. (color online) E-field dependence of FMR curves at 11 GHz along HA (a) and EA (b) directions, and FMR field Hγ analysis (c).

In Eqs. (7) and (8), let . Therefore, we can obtain

The large shifts of ΔH_γ under E-fields imply that a large, E-field-controllable f_r is present in the Fe_52.5Co_22.5B_25.0/PZN-PT multiferroic heterostructure. Figure 4(a) shows the E-field dependence of the film’s permeability. With the increase of the E-field from 0 to 8 kV/cm, the FMR frequencies are shifted from 4.57 to 8.73 GHz with an increment of 4.16 GHz, which is equivalent to an E-field tunability of 520 MHz⋅cm/kV. The dependence of H_eff, f_r, and α versus E-field are summarized in Fig. 4(b). As discussed earlier, the upward shift of H_eff and f_r can be attributed to the ME coupling between the Fe_52.5Co_22.5B_25.0 film and the PZN-PT substrate. It is interesting to note that the damping constant α decreases from 0.021 to 0.0186 with the increase of E-field, which is considerably favorable for applying Fe_52.5Co_22.5B_25.0 films at higher microwave frequencies.

Fig. 4.

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	Fig. 4. (color online) Electric field dependence of permeability (a) and fr, Heff, and α (b).

The ME coupling effect in the Fe_52.5Co_22.5B_25.0/PZN-PT multiferroic heterostructure not only generates an FMR shift of 4.16 GHz under an E-field of 8 kV/cm, but also provides an E-field-tunable FMR shift over a considerably wide frequency range, thereby effectively achieving the E-field manipulation on microwave frequencies. It provides a possible route to fabricate E-field tunable microwave devices with large E-field tunability, low energy consumption, and light weight.

In this study, we obtained the E-field tuning on FMR frequency in as-deposited Fe_52.5Co_22.5B_25.0/PZN-PT multiferroic heterostructure owing to the magnetoelectric coupling effect. The heterostructure exhibits large self-biased f_r with a considerably wide tunable frequency range from 4.57 to 8.73 GHz, which provides great opportunities for self-biased, voltage-tuned microwave components without high energy-consumption electromagnets. All the fabrication processes are conducted at room temperature (i.e., IC compatible processes), which is considerably beneficial to the integration of soft magnetic films using monolithic microwave integrated circuits.