Nonreciprocal transmission spectrum of a ferrite plate-resonant element grating metasandwich with split resonance

It is established that, when microwaves propagate in a structure comprising a ferrite plate and a grating of conducting resonant elements (situated on or close to the plate), the nonreciprocal wave transmission is accompanied by a nonreciprocal splitting of the microwave resonance at the grating frequency and by an expansion of the resonance bandwidth upon application of a constant magnetic field with a magnitude about one order smaller than that of the field exciting the ferromagnetic resonance (FMR). The sign of the nonreciprocity changes when the ferrite is arranged on the opposite side of the grating and/or when the applied constant magnetic field reaches a level at which the difference between the FMR and the grating resonance frequencies changes sign.     

Giant nonreciprocal effect under conditions of mutual influence of ferromagnetic and chiral resonances

A more than hundredfold increase in the nonreciprocity of electromagnetic wave transmission in a waveguide containing a transversely magnetized ferrite plate has been observed upon the introduction of a grating of bianisotropic elements. This effect takes place at a certain magnitude of a constant magnetic field, under the conditions of mutual influence of ferromagnetic and chiral resonances. In an evanescent regime, the non-reciprocity exhibits a severalfold increase. The effect depends on the mode of excitation of the chiral resonance, being observed in the case of excitation by a magnetic component of the microwave field.     

Electrically controlled frequency bands of nonreciprocal passage of microwaves in metastructures

Measurements of the transmission coefficients of linearly polarized waves in a rectangular waveguide along a metastructure formed by a transversely magnetized ferrite plate and double-split rings with varactors indicated the presence—in addition to the ferromagnetic resonance—of a resonance region of nonreciprocal passage, which is controlled, in contrast to the ferromagnetic resonance, by the electric field. This effect manifests itself in magnetic fields substantially lower than the field exciting the ferromagnetic resonance at these frequencies. Electrically controlled nonreciprocal passage of microwaves cannot be achieved by means of known natural materials or traditional ferromagnets.     

Nonreciprocal left-handed transmission characteristics of microstriplines on ferrite substrate

In this paper, a nonreciprocal left-handed transmission line has been investigated. It is composed of a normally magnetised ferrite microstripline periodically loaded with lumped capacitances and inductive stubs. Based on the simple equivalent circuit model for a predominant edge guided mode, the dispersion relation of the ferrite left-handed transmission line has been estimated, providing the backward wave characteristic in the positive and negative permeability frequency regions. With the aid of the finite element method, it is confirmed that specific three-dimensional structures do show left-handed characteristics as well as the nonreciprocity. Nonreciprocal transmission characteristics with the isolation of more than 30 dB are obtained numerically and demonstrated experimentally in the positive and negative permeability frequency regions, for the edge guided modes. It is explained that the nonreciprocity of the ferrite left-handed transmission line is caused by the leaky wave phenomenon from the microstrip to the ferrite substrate     

Magnetic-type resonance in nonmagnetic line wire excited by surface plasmons in microwave range

A magnetic resonance microwave response has been detected and identified in a structure of parallel nonmagnetic wires or a single line wire perpendicular to the electric field of a plane electromagnetic wave in the case where the wires are arranged near an array (grating) of resonant surface-plasmon-generating elements and oriented along the direction of wave propagation. A giant resonance is observed for a definite (resonance) length of the wire(s) in a certain frequency range corresponding to the existence of surface plasmons (below the resonance frequency of the plasmon-generating array). It is suggested that the magnetic response of the wire(s) is due to the excitation of resonance currents by the magnetic field of surface plasmons. Using the observed phenomena, it is possible to obtain new magnetic metamaterials (in particular, those possessing simultaneously negative effective dielectric permittivity and magnetic permeability) tunable in a broad frequency range.     

Microwave properties of planar hexagonal ferrites

A large number of samples of hexagonal ferrite, with planar anisotropy incorporating various metal ions, were synthesized and tested for FMR (ferromagnetic resonance). Measurements were made at room temperature in transmission cavities at frequencies between 10 and 35 Gc/s on magnetically oriented polycrystalline compounds. Of the possible crystal forms for hexagonal ferrite, those designated asW, Y, andZwere investigated. TheYcompounds were found to have the most useful microwave properties. The resonance line width was found to vary between 100 and 1200 Oe, and the anisotropy field varied from zero to 40000 Oe. TheWandZcompounds have planar or uniaxial anisotropy, depending on the composition. The region of zero anisotropy was determined. The magnetically oriented polycrystalline materials resemble single crystals in having easy and hard directions of magnetization which require different magnetic field values for FMR. Qualitative information about how well the crystallites are aligned with each other can be obtained from the examination of two FMR absorption curves made with the sample turned so that its easy and hard directions, respectively, are aligned with the magnetic field. Curves are shown to illustrate the difference between nonoriented, partially oriented, and well-oriented materials.     

Nonreciprocal waves in absorbing multilayer systems

The propagation of naturally polarized light through a Fabry-Perot resonator with a layer of cholesteric liquid crystal is considered. In the presence of absorption, this system exhibits nonreciprocity even for naturally polarized light, which implies the existence of a new type of nonreciprocal reflection. An analysis of the peculiarities of this nonreciprocity shows that strongly nonreciprocal multilayer systems can be used as optical diodes or optical insulators (transmitting signals via an optical communication channel in one direction and blocking signals propagating in the opposite direction) or as systems for the accumulation of light energy.     

Design of drop-in microstrip circulator

The operation of a ferrite drop-in circulator is theoretically predicted and experimentally confirmed. The cavity model is applied both to the ferrite disc and the dielectric region directly below the ferrite disc. The standing-wave condition required by the dielectric cavity plus the two ferrite circulation conditions determine uniquely three design parameters for the circulator: the magnitude biasing field, the radius of the ferrite disc, and the dielectric constant of the dielectric sleeve surrounding the ferrite disc. In addition, the characteristic impedance of the microstrip feeder lines was purposely designed for 50 Ω in order to eliminate the need for impedance transformers. We found that for a thin dielectric substrate with dielectric constant smaller than that of the ferrite, the circulation frequency can be slightly above FMR resonance. Three circulators were fabricated using design parameters calculated by our theory. The measurements are in reasonable agreement with predicted performance of the device     

A study on the behavior of laminated electromagnetic wave absorber

Since ferrites are electromagnetic wave absorbing materials of composition-dependent resonance type, useful frequency ranges are typically narrow. In the present study, laminated electromagnetic wave absorbers composed of ferrite-ferrite or ferrite-dielectric layers were constructed to improve electromagnetic wave absorbing properties. When ferrites of different composition and thickness were layered, center frequency of the laminated ferrite could be changed over a range of values lying between the center frequencies of each ferrite. Compared with monolithic ferrites, ferrite-dielectric laminates exhibited broader frequency ranges with 20 dB or more attenuation     

Spin Wave Resonances in GaMnAs

Ferromagnetic resonance (FMR) measurements were carried out on a series of GaMnAs thin films with different thicknesses grown by low temperature molecular beam epitaxy. Clear spin wave resonances were observed in addition to FMR when the applied magnetic field was perpendicular to the film plane. The spin wave spectra show a nearly linear dependence of the resonance mode positions on the mode number, suggesting that the magnetization profile of the GaMnAs films is not uniform in the growth direction. A first-order analysis of these effects is presented along with the experimental data.     

Ferromagnetic Resonance Investigation of Hexaferrite Nanoparticles Prepared by Sol-Gel Auto-Combustion Method

In this study, the magnetic and electromagnetic wave-absorbing properties of barium and strontium ferrite nanopowders prepared by a sol-gel technique were investigated. To study the structural characteristics of hexaferrites, X-ray diffraction analysis was used. Investigation of the morphologies of nanoparticles was carried out by field emission scanning electron microscopy. A vibrating sample magnetometer was used in order to examine the magnetic properties of synthesized hexaferrites at room temperature. Ferromagnetic resonance (FMR) was used to investigate ferromagnetic resonance of the powders. Experimental results indicated that the materials had hexagonal structures with desirable magnetic properties. A low-field absorption signal was observed with the same phase as the FMR absorption in barium hexaferrites.     

Efficient tunability and circuit model of nested-U nanoresonators in optical metasurfaces

Tunable metasurfaces can be employed to physically or mechanically engineer and control electromagnetic wave properties like reflection and transmission and their associated spectral characteristics like resonance frequency. Here, we propose highly tunable and sensitive metasurfaces composed of an array of a nested double U-shaped (NDU) nanoresonators on elastic polydimethylsiloxane substrate, operating in infrared region. The mechanical deformation varies the spaces between the coupled resonator elements which in turn leads to corresponding variations in the equivalent capacitance and inductance between the U-shaped elements causing efficient tunability. In addition to the higher signal strengths, it is also reported that the resonant frequency of the proposed metasurface exhibits substantial spectral shift. The observed remarkable trends are adequately verified by the developed equivalent circuit model for the proposed NDU-structure.     

Polarization Independent Dual-band Metamaterial Based Radar Absorbing Structure (RAS) for MillimeterWave Applications

The EM analysis of multi-layered metamaterial based radar absorbing structure (RAS) with dual-band characteristics in millimeter wave frequency regime has been carried out in this paper using transmission line transfer matrix (TLTM) method for TE and TM polarizations. The proposed metamaterial-based RAS exhibits dual-band characteristics at centre frequencies 120 GHz and 175 GHz with very low power reflection. It absorbs more than 90% power of incidence wave over the frequency range from 111-131 GHz at first resonance and from 164.5-185 GHz at second resonance without metal backing plate, which is desirable for stealth applications. It also showed very low (< 1.6%) transmission over the frequency of interest for both TE and TM polarizations. The proposed metamaterial-RAS has potential applications in the design of multi-band sensor systems and RCS reduction in millimeter wave frequency regime.     

Ferromagnetic and spin wave resonances in thin layer of expanded austenite phase

Four samples of austenite coatings deposited by reactive magnetron sputtering on silicon substrate at four different temperatures and pressures were investigated by ferromagnetic resonance (FMR) method at room temperature. The expanded austenite phase S (γ _N ) layers with thickness in the 160–273 nm range and concentration of magnetic atoms: 72 % Fe, 18 % Cr and 10 % Ni, were obtained. The coatings with nanometric size grains were strongly textured and grown mostly in [100] direction, perpendicular to the sample surface. Intense FMR spectra were recorded at various angles between the static magnetic field direction and the sample surface. A strong magnetic anisotropy of the main uniform FMR mode was observed and the effective magnetization 4πM _eff determined. Spin wave resonance (SWR) modes were observed in all investigated samples in out-of-plane geometry of the magnetic field. The resonance fields of SWR modes in our samples varied linearly with the spin wave mode number. The value of the effective magnon stiffness constant was determined assuming a parabolic shape of the magnetization variation across the sample thickness.     

Tunable dual-band nearly perfect absorption based on a compound metallic grating

Traditional metallic gratings and novel metamaterials are two basic kinds of candidates for perfect absorption. Comparatively speaking, metallic grating is the preferred choice for the same absorption effect because it is structurally simpler and more convenient to fabricate. However, to date, most of the perfect absorption effects achieved based on metamaterials are also available using an metallic grating except the tunable dual(multi)-band perfect absorption. To fill this gap, in this paper, by adding subgrooves on the rear surface as well as inside the grating slits to a free-standing metallic grating, tunable dual-band perfect absorption is also obtained for the first time. The grooves inside the slits is to tune the frequency of the Cavity Mode(CM) resonance which enhances the transmission and suppresses the reflectance simultaneously. The grooves on the rear surface give rise to the phase resonance which not only suppresses the transmission but also reinforces the reflectance depression effect. Thus, when the phase resonance and the frequency tunable CM resonance occur together, transmission and reflection can be suppressed simultaneously, dual-band nearly perfect absorption with tunable frequencies is obtained. To our knowledge, this perfect absorption phenomenon is achieved for the first time in a designed metallic grating structure.     

High transmission efficiency for surface plasmon resonance by use of a dielectric grating

The efficiency of the transmission of surface plasmon waves by use of a dielectric diffraction grating is discussed. The Kretschmann device allows us to obtain a surface plasmon resonance that consists of an absorption peak in the reflection spectrum. When surface plasmon resonance occurs, the TM-polarization mode of the incident electromagnetic wave is neither transmitted nor reflected. The procedure to transform an 4bsorption peak into a transmission peak is described. Transmittivity of 68% is obtained for a simple structure that consists of a thin-film layer of Ag coated on a volume diffraction grating and embedded between two dielectric media. The results presented herein were obtained by numerical simulations that were carried out by use of an algorithm based on the rigorous coupled-wave theory.     

Nonreciprocity in magnetoelectric crystals

We show here that antiferromagnetic crystals, and crystals with expected simultaneous ferroelectric and antiferromagnetic or antiferroelectric and antiferromagnetic properties, which display the magnetoelectric effect, will possess the fundamental property of nonreciprocity based solely upon this effect when exposed to electromagnetic waves. Proof is presented of this phenomenon by enlisting the anisotropic reaction theorem. Defining measures of reciprocity from this theorem, it is demonstrated which crystals have nonreciprocal behavior based upon the magnetoelectric effect and which crystals may be expected to show nonreciprocal behavior based upon the magnetoelectric effect     

Field effect transistor circulators

Field-effect-transistor circulators that are compatible with monolithic microwave integrated circuit technology are described. These passive nonreciprocal components are formed with inductors, capacitors, transmission line sections, and microwave transistors, and all can be integrated on a single IC chip the size of a pinhead. The operating principle, power-handling capability, and noise performance of these components are discussed for both narrowband and wideband applications, and examples of their practical implementation are given. The transistor-based 2-, 3-, and 4-port passive nonreciprocal components described offer an alternative to ferrite miniature circulators with potential advantages of small size and lower cost in production volume     

Linear array arrangement using composite right-/left-handed transmission lines for magnetic resonance imaging

Array coils for magnetic resonance imaging have been used to improve field uniformity, improve signal-to-noise ratios, and increase imaging speed. Alternative radio frequency (RF) coils that use metamaterials, such as loop or microstrip coils, have recently been proposed and are expected to provide better performance than the traditional RF array coils. Transmission lines (TLs) based on metamaterials are known as composite right- and left-handed (CRLH) TLs, which are artificially created by adding inductances and capacitances to a common TL. CRLH TLs have a zero-order resonance mode, wherein wave propagation is independent of the TL's electrical length. Decoupling between array elements is important for obtaining the benefits of parallel imaging. In this study, we analyze the decoupling properties between two CRLH TLs. In addition, we design a linear array of four CRLH TLs to obtain a uniform magnetic (|B₁|)-field in the axial- and longitudinal-direction at 7T for the corresponding frequency of 300 MHz.