[en] Magnetic domain structure of FeCoBSi antidot array thin films of varying thickness were characterized using surface magneto-optic Kerr effect. Vibrating sample magnetometry and microstrip transmission line measurements helped to associate the microwave magnetic analysis of the antidot arrays with hysteresis studies. The domain structure evolution from quasi-continuous domains to strip domains induced by the competing exchange and dipolar interaction resulted in the change of ferromagnetic resonance from multi-band to single-band. Hence, the mechanisms of multi-resonance are proposed to be related to domain wall motion, natural resonance and spin wave modes. This phenomenon can be used to control the magnetization dynamics in spin wave devices. - Highlights: • A multiresonance mechanism for ferromagnetic antidot array is proposed. • The mechanism relies on the domain structure evolution of antidote arrays. • Domain structure of antidot arrays changes from quasi-continuous patterns to strip domains. • Resonance of domain wall motion is discriminated from the natural resonance and spin wave modes

[en] Flake-shaped particles were fabricated from crushing ingots with ball milling method. Scanning electron microscope (SEM), X-ray diffraction (XRD), vibrating samples magnetometer (VSM) and Mössbauer spectroscopy measurements were carried out to characterize the milled powders. The XRD results show that the DO3 ordered phase coexists with α-Fe(Si,Cr) matrix phase. Generally, grain size decreases with milling time, as internal strain increases. The fitting result of the Mössbauer spectra of the as-prepared powders reveals that the DO3 phase always exists, but the percentage of DO3 phase drops from 52% in the starting powder to 25% after 80 h milling. A planar anisotropy of the flake-shaped powders is also demonstrated by the Mössbauer spectra, and found to increase with milling time. The saturation magnetization M_s has a maximum value of 109 emu/g after milling of 8 h, and then remains at a steady value around 104 emu/g. The maximum complex permeability is obtained by 32 h milling

[en] In this study, microwave magnetic and absorption properties were investigated for BaCo_xTi_xFe_12−2xO₁₉ (x=0.3, 0.4 and 0.5). M-type barium ferrite can be used as high frequency absorber by Co–Ti substitution because of the strong uniaxial anisotropy and high saturated magnetization. The X-ray diffraction and magnetic hysteresis loops of the samples show that the c-axis anisotropy of BaCo_xTi_xFe_12−2xO₁₉ decreases with the substitution, resulting in the decrease of coercivity. The frequency dependence of complex permeability, permittivity and microwave absorption of BaCo_xTi_xFe_12−2xO₁₉ has been studied in the Ka band (26.5–40 GHz), respectively. The results show that the natural resonance frequency decreases with the substitution. The complex permeability increases as the sintering temperature increases in the range from 1280 °C to 1310 °C with 10 °C as a step. The imaginary part of permeability value obtained here is the highest value among the reported ferrite millimeter wave absorbers in the Ka band. The microwave absorption bandwidth at −10 dB of BaCo_xTi_xFe_12−2xO₁₉ is effectively broadened compared with other ferrites. Therefore, BaCo_xTi_xFe_12−2xO₁₉ may be a good candidate for electromagnetic materials with low reflectivity in the Ka band. - Highlights: • The frequency dependence of complex permeability, permittivity and microwave absorption of BaCo_xTi_xFe_12−2xO₁₉ have been studied in the Ka band. • The c-axis anisotropy of BaCo_xTi_xFe_12−2xO₁₉ decreases with Co–Ti substitution, resulting in the decrease of coercivity. • The natural resonance frequency decreases with Co–Ti substitution in the Ka band. • The microwave absorption bandwidth at −10 dB of BaCo_xTi_xFe_12−2xO₁₉ is effectively broadened compared with other ferrites

[en] The spin orientation dependence of magnetic hysteresis and microwave ferromagnetic resonance data are investigated in FeCoBSi amorphous thin films. Demagnetization effect allows the weak interface-rooted out-of-plane anisotropy to build up local spin orientation domains under the dominant in-plane anisotropy. As a result, two phase magnetization reversal and double-peak ferromagnetic resonance traces with varying damping behavior are observed. Due to the distribution of in-plane and out-of-plane spin orientations, the ferromagnetic resonance bandwidth has been extensively expanded with the full width at half maximum increased from 1.2 GHz to 3.5 GHz

[en] A dual-band and high-efficiency reflective linear polarization converter composed of a layer of slotted metal wires has been proposed. Both the simulated and experimental results indicate that the structure can convert a linearly polarized wave to its cross-polarized state for two distinct frequency bands under normal incidence: 9.8–15.1 and 19.2–25.7 GHz. This phenomenon is attributed to a resonance that corresponds to the “trapped mode” at 15.8 GHz. This mode is stable with structural parameters and incident angle at a relatively wide range, and thus becomes promising for dual-band (also multiband) devices design. By surface current distribution and electric field analysis, the operation mechanism has been illuminated, especially for the “trapped mode”, identified by the equally but also oppositely directed currents in each unit cell.

[en] Infrared resonator metamaterials, exhibiting spectral selective absorption of light, have recently been of great interest for passive radiative cooling. In this approach, coolers radiate power passively, with emissivity equals to its absorptivity. However, high-efficient energy dissipation requires radiation windows of broad bandwidth, i.e. 8–14 μm, that challenges the narrow-band feature of resonators. Here, we numerically realize the broadband absorption (above 80% in 8–11 μm) for dielectric resonators over a wide incident angle range (0°–60°), predicting 12 °C cooling below the ambient temperature at nighttime. The electromagnetically resonating eigenmodes of the dielectric resonators are perturbated by losses to reshape the dispersion relation in radiation windows, which are implemented by two methods: inserting metal components and coating lossy dielectrics. Retrieval constitutive parameters, as well as S parameters, map the reshaping process for broadband considerations. Additionally, ideal transparency (nearly 100%) above the wavelength of 0.5 mm is achieved. These crucial features offer an effective solution to the microwave signals shielding problem generally encountered in previous radiative coolers, regarding applied scenes such as buildings and cars. (paper)

[en] We present the design, analysis, and measurement of a broadband reflective converter using a cut-wire (CW) metasurface. Based on the characteristics of LC resonances, the proposed reflective converter can rotate a linearly polarized (LP) wave into its cross-polarized wave at three resonance frequencies, or convert the LP wave to a circularly polarized (CP) wave at two other resonance frequencies. Furthermore, the broad-band properties of the polarization conversion can be sustained when the incident wave is a CP wave. The polarization states can be adjusted easily by changing the length and width of the CW. The measured results show that a polarization conversion ratio (PCR) over 85% can be achieved from 6.16 GHz to 16.56 GHz for both LP and CP incident waves. The origin of the polarization conversion is interpreted by the theory of microwave antennas, with equivalent impedance and electromagnetic (EM) field distributions. With its simple geometry and multiple broad frequency bands, the proposed converter has potential applications in the area of selective polarization control. (paper)

[en] A scattered radar signal by an electrically-large open-ended cavity involves information of the target. Therefore, controlling the Radar cross section (RCS) of the cavity is significant for target stealth. Coding metasurfaces have promised great possibilities for full control of the electromagnetic waves, especially for reducing RCS by random coding metasurfaces. Here, we report an approach to realize broadband RCS reduction of electrically-large open-ended cavities through loading random coding metasurfaces on the inner side walls. The RCS reduction is caused by suppressing the strong backscattering of corner reflectors inside the cavities, which is further attributed to the diffuse reflection performance of the random coding metasurfaces. Both simulation and measurement show a decrease of cavities RCS, which is a broadband (X-band), polarization-independent, high performance (10 dB) RCS reduction. The proposed method may create new opportunities to improve microwave stealth features of cavities working in high temperatures, which could provide crucial benefits in many practical uses, such as engine nozzles. (paper)