Honglei Du

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.

Large E-field tunability of microwave ferromagnetic properties in Fe59.3Co28.0Hf12.7/PZN-PT multiferroic composites

Strong converse magnetoelectric coupling was observed in a multiferroic heterostructure of Fe59.3Co28.0Hf12.7 film on (011) cut lead zinc niobate-lead titanate (PZN-PT) slab, which exhibited a large electric field (E-field) tunability of microwave magnetic properties. With the increase of E-field from 0 to 6 kV/cm on PZN-PT, the ferromagnetic resonance (FMR) field Hr shifts downwards by 430.7 Oe along [011¯] direction and upwards by 492.9 Oe along [100] direction of the PZN-PT. Accordingly, the strong magnetoelectric coupling led to a significantly enhanced self-biased FMR frequency from 4.2 to 7.9 GHz under zero bias magnetic field, and the magnetic damping constant α was decreased from 0.0260 to 0.0185 at the same time. These features demonstrate that this multiferroic laminate is promising in fabrication of E-field tunable microwave components.

Large E-field tunability of magnetic anisotropy and ferromagnetic resonance frequency of co-sputtered Fe50Co50-B film

Fe27.45Co30.19B42.36 (referred to as FeCoB) films with 100 nm in thickness were co-sputtered on (011)-cut lead zinc niobate-lead titanate (PZN-PT) single crystal substrate under RF powers of 80 W for Fe50Co50 target and 120 W for B target, respectively. The anisotropy field HK of the FeCoB/PZN-PT multiferroic composite is increased by more than 10 times, from 56 to 663 Oe under the E-field from 0 to 7 kV/cm due to the strong magnetoelectric coupling, corresponding to a large tunability of HK of 86.7 Oe cm/kV. At the same time, the self-bias ferromagnetic resonance frequency fFMR is dramatically shifted upwards by an electric field from 2.57 to 9.02 GHz with an increment of 6.45 GHz, corresponding to E-field tunablity of fFMR 921.4 MHz.cm/kV. These features demonstrate that FeCoB/PZN-PT multiferroic laminates prepared under an integrated circuits process are promising in fabrication of E-field tunable monolithic microwave integrated circuits (MMIC) devices and their components.

Stress competition and vortex magnetic anisotropy in FeCoAlO high-frequency soft magnetic films with gradient Al-O contents

A vortex magnetic anisotropy (VMA) was formed via the competition of residual stresses between radial and tangential directions in the FeCoAlO soft magnetic films (SMFs), prepared by a composition gradient sputtering (CGS) method. The VMA of the magnetic films gives rise to a rotating excitation direction of the ferromagnetic resonance. As a results, the as-deposited FeCoAlO films exhibit good high-frequency ferromagnetic properties with high permeability about 100, cut-off frequency over 2 GHz, and Qm factor over 50 along its individual excitation direction. These SMFs with the VMA are promising in the integration with the circular spiral inductors due to the geometrical match between the excitation direction of the SMFs and the circular inductor lines.

A Frequency Reconfigurable Microstrip Antenna Based on

A novel frequency reconfigurable microstrip antenna based on (Ba, Sr)TiO3 (BST) substrate is presented. Compared to the complicated structures in traditional reconfigurable antennas, our work is featured by a quite concise design. A theoretical model is proposed to calculate the initial antenna parameters. Both the simulation and the experiment indicate that the coupled aperture structure can efficiently overcome impedance mismatch of BST substrate and obtain a frequency tunability of 10% in Ku band by a DC electric field changing from 0 to 10 V/μm. Besides, similar radiation patterns are obtained in the operating band for both the E plane and H plane. Our study shows that the using of BST material on the frequency reconfigurable antenna is promising, and can be easily extended to reconfigurable antenna arrays.

({\rm Ba}, {\rm Sr}){\rm TiO}_{3}

An orthogonal biaxial magnetic anisotropy configuration was built in the FeCoAlO/PZN-PT multiferroic composite using two perpendicular uniaxial magnetic anisotropies (UMAs). One is stable and induced by composition gradient, and the other is electric field (E-field) dependent resulting from the magnetoelectric coupling between FeCoAlO ferromagnetic film and the PZN-PT ferroelectric substrate. As a result, the easy axis of a FeCoAlO soft magnetic film can be rotated nearly 90° by the E-field due to the competition between the two UMAs. The magnitude and direction of the ferromagnetic resonance frequency can be manipulated by E-field, and very high resonance frequency up to 9 GHz is achieved in this orthogonal biaxial multiferroic composite.

Substrate

A Fe0.7Co0.3-B multilayer was laminated by three Fe0.7Co0.3-B ferromagnetic sublayers prepared by composition gradient sputtering. Three Fe0.7Co0.3-B ferromagnetic sublayers have their individual directions of uniaxial magnetic anisotropy, and the easy magnetic axis of neighboring sublayer successively rotates 60° in the film plane. It is exciting that a quasi magnetic isotropy was achieved in the designed multilayer with a quasi-isotropic hysteresis loop and quasi-isotorpic ferromagnetic resonance around 3.7 GHz. This omnidirectional multilayer is promising for the application in inductors since the 100% hard-axis excitation is achieved for any shaped inductors.

E-Field Tuned Rotation of Magnetic Anisotropy and Enhanced Microwave Performance in FeCoAlO/PZN–PT Multiferroic Composite Prepared by Composition Gradient Sputtering

Co2FeAl Heusler alloy film with 100 nm in thickness was sputtered on (011)-cut lead zinc niobate-lead titanate (PZN-PT) single crystal slabs. It was revealed that this multiferroic laminate shows very large electric field (E-field) tunability of microwave soft magnetic properties. With the increase of electric field from 0 to 8 kV/cm on PZN-PT, the anisotropy field, HK, of the Co2FeAl film along [100] direction of PZN-PT is dramatically enhanced from 65 to 570 Oe due to the strong magnetoelectric (ME) coupling between ferromagnetic Co2FeAl film and ferroelectric substrate. At the same time, the damping constant α of Co2FeAl film dramatically decreases from 0.20 to 0.029. As a result, a significantly shift of self-biased ferromagnetic resonance frequency, fFMR, from 1.86 to 6.68 GHz with increment of 3.6 times was obtained. These features demonstrate that Co2FeAl/PZN-PT multiferroic laminate is promising in fabrication of E-field tunable microwave components.

Quasi magnetic isotropy and microwave performance of FeCoB multilayer laminated by uniaxial anisotropic layers

Patterned Ni80Fe20 permalloy films with strip length of 5 mm, strip distance of 40μm and various ferromagnetic (FM) strip widths from 20 to 40μm were prepared by pulse laser deposition and photolithography for study the microwave performance of permalloy films. Comparing with the continuous magnetically isotropic permalloy film, the anisotropy fields HK increase with the decrease of the FM strips’ width. As a result, the ferromagnetic resonance frequency fr is enhanced from 1.71 GHz of continuous film to 2.1 GHz of patterned film with FM strip width of 20μm. The comparison between fr and HK demonstrates that the enhancement of fr can be attributed to the increase of HK due to the shape anisotropy of permalloy strips.

Electric field tunability of microwave soft magnetic properties of Co2FeAl Heusler alloy film

Dual-mode ferromagnetic resonance is observed in FeCoB/Ru/FeCoB trilayer synthetic antiferromagnets with uniaxial in-plane magnetic anisotropy. The optical mode is present in the (0–108 Oe) magnetic field range, where the top and bottom layer magnetizations are aligned in opposite directions. The strong acoustic mode appears, when the magnetic field exceeds the 300 Oe value, which corresponds to the flop transition in the trilayer. Magnetic field and angular dependences of resonant frequencies are studied for both optical (low-field) and acoustic (high field) modes. The low-field mode is found to be anisotropic but insensitive to the magnetic field value. In contrast, the high field mode is quasi-isotropic, but its resonant frequency is tunable by the value of the magnetic field. The coexistence of two modes of ferromagnetic resonance as well as switching between them with the increase in the magnetic field originates from the difference in the sign of interlayer coupling energy at the parallel and antiparallel configurations of the synthetic antiferromagnet. The dual-mode resonance in the studied trilayer structures provides greater flexibility in the design and functionalization of micro-inductors in monolithic microwave integrated circuits.Dual-mode ferromagnetic resonance is observed in FeCoB/Ru/FeCoB trilayer synthetic antiferromagnets with uniaxial in-plane magnetic anisotropy. The optical mode is present in the (0–108 Oe) magnetic field range, where the top and bottom layer magnetizations are aligned in opposite directions. The strong acoustic mode appears, when the magnetic field exceeds the 300 Oe value, which corresponds to the flop transition in the trilayer. Magnetic field and angular dependences of resonant frequencies are studied for both optical (low-field) and acoustic (high field) modes. The low-field mode is found to be anisotropic but insensitive to the magnetic field value. In contrast, the high field mode is quasi-isotropic, but its resonant frequency is tunable by the value of the magnetic field. The coexistence of two modes of ferromagnetic resonance as well as switching between them with the increase in the magnetic field originates from the difference in the sign of interlayer coupling energy at the parallel and antipa...