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Electromagnetic interference shielding composite materials can be designed with properties that are tailored for specific applications. We have explored the magnetic properties of Ni-coated carbon fibers embedded in a polymer matrix through an extrusion process. This method can produce large, pliable flat sheets, which can be easily bent and shaped. Microscopy imaging shows that the extrusion process preferentially orders most of the fibers with their long axis along the extrusion direction. The measured samples reached 90% of the saturation magnetization value at a field of approximately 800 Oe along the extrusion direction, much lower than needed for other directions (in-plane perpendicular to the extrusion or perpendicular to the foil surface). A field on the order of 6 kOe must be applied to obtain the full saturation of the magnetization in all three directions. The absorption characteristics were measured, with zero external applied field to determine the shielding effectiveness of the composite materials for multiple orientations with respect to the TE10 mode of an S-band waveguide. The nickel-coated carbon fiber composite materials exhibit dramatic orientation-dependent shielding effectiveness. The most effective orientation has shielding effectiveness of up to 45 dB at 4.5 GHz, which is comparable to multi-walled carbon nanotube composites with similar volumetric filling fractions. [ABSTRACT FROM AUTHOR]

We demonstrate the nonlinear generation of spin-wave edge modes with half the frequency of the applied oscillating field in a Co25Fe75 ferromagnetic stripe through micromagnetic simulations and experiments. The generation of half-frequency modes depends on the simultaneous presence of resonances near both the driving frequency and the half-frequency in different regions of the material. The half-frequency generation occurs in a system that is thin enough that typical three-magnon decay would not be allowed in a ferromagnetic resonance experiment in an extended film. We find that a limited range of driving frequencies will produce a half-frequency for a given set of system parameters. This range can be tuned by the strength of the oscillating field and the strength of the static external field. Our experimental results agree well with the findings from the simulations. [ABSTRACT FROM AUTHOR]

The effective microwave permittivity of a composite comprising anisotropic particles suspended in a liquid (also known as an artificial Kerr material) is calculated using a numerical scheme. The results are compared to those calculated using analytic effective medium expressions. Several composite materials are predicted to have permittivity that can be tuned over a greater range than liquid crystals, which are currently used in many tunable microwave devices such as phase-shifters and modulators. Estimates for the static electric field required to tune such materials are provided and show that such materials are realistic. [ABSTRACT FROM AUTHOR]

Waves in magnetic systems often show nonreciprocal propagation, where the properties of a wave depends on whether it is travelling in a forward or reversed direction. This fundamental property is used in devices such as isolators, which often use a transverse electric (TE) polarized wave. In this paper, we study propagation of electromagnetic waves in small microstrip waveguides where the wave propagates at an arbitrary angle with respect to the magnetization direction. The angle allows mixtures of TE and transverse magnetic (TM) modes. This has distinct benefits, allowing the nonreciprocity typically found in TE modes, but also allowing propagation of low frequency TM modes, which do not have a cut-off frequency. Our calculations, using Barium hexaferrite as the magnetic material, show significant nonreciprocity and isolation are possible. Differences in attenuation above 75 dB/cm are found for these mixed-polarization waves for propagation angles of about 40°. We study nonreciprocal propagation as a function of propagation angle and other material parameters for both primarily TM and primarily TE modes. [ABSTRACT FROM AUTHOR]

Recent work studied surface plasmon resonances in structured materials by the method of attenuated total reflection using a prism on top of a metallic grating. That calculation considered Transverse Magnetic polarized radiation, involved an expansion in 121 Fourier modes, and found a number of interesting features. Many of these features were attributed to localized plasmons or other factors, which arise from a discrete structure. We use a simple effective medium theory to address the same problem, and find many of the same reflection features observed in the more complex calculation, indicating that localization is not an important factor. We also evaluate the possibility of using some of the new features in the reflection spectrum for bio-sensing and find that the sensitivity of the system to small changes in relative permittivity is increased compared to some standard methods. [ABSTRACT FROM AUTHOR]

We perform a theoretical study of the nonreciprocal reflection of high-frequency microwave radiation from ferromagnetic films with thin overlayers. Reflection from metallic ferromagnetic films is always near unity and shows no nonreciprocity. In contrast, reflection from a structure which has a dielectric overlayer on top of a film composed of insulated ferromagnetic nanoparticles or nanostructures can show significant nonreciprocity in the 75-80 GHz frequency range, a very high value. This can be important for devices such as isolators or circulators. [ABSTRACT FROM AUTHOR]

We demonstrate an on-wafer liquid crystal phase shifter which has a tunable 0-300°/cm phase shift at 110 GHz. The results show no dispersion over the entire frequency range indicating a tunable 'true time delay' of up to 2.5 ps/cm at all frequencies. The inherent losses in the liquid crystal are small, less than 1 dB/cm over the range of 1-110 GHz. The full tunability is achieved using small voltages, close to 10 V. We anticipate that one could achieve a phase shift of 600°/cm at 220 GHz. [ABSTRACT FROM AUTHOR]

There is need for non-reciprocal devices such as circulators and isolators. Although such devices are common at frequencies below 10 GHz, there is a lack of compact, low-weight, devices at higher microwave frequencies. This paper examines the non-reciprocal behavior associated with attenuated total reflection (ATR) for multi-layered dielectric and magnetic structures. Non-reciprocal behaviors produced by ATR have been explored for semi-infinite magnetic materials. This paper focuses on ATR behavior with magnetic films of finite thickness, from thick layers of around 3 cm to thin layers of about 1 µm. The results show significant non-reciprocity even for magnetic layers less than 0.1 cm thick, with reflection loss differences of more than 30 dB between positive and negative signal propagation. Results are presented for yttrium iron garnet and M type barium hexagonal ferrites. The two materials allow nonreciprocal behavior at different frequencies, 5-20 GHz for the garnet and 45-80 GHz for the hexagonal ferrite. [ABSTRACT FROM AUTHOR]

We explore the response of a magnetic bilayer to a driving microwave field using micromagnetic simulations. The bilayer consists of 8 nm of a material with a high uniaxial anisotropy and 56 nm of a material with a lower uniaxial anisotropy. The width and length of the structure is 100 × 100 square microns. A small applied field, opposite to the magnetization, switches most of the lower anisotropy material but not the higher anisotropy material, forming a domain wall between the two materials. We evaluate the frequencies of the magnetic eigenmodes for the entire system using Fourier analysis and then drive the structure with an oscillating magnetic field at each of the eigenfrequencies. When the oscillating microwave field is added, the static switching field required to align both layers is decreased compared to the undriven case. With a driving field strength of 120 Oe the switching field is reduced by about 40%, from 1.12 kOe for the undriven case to 0.55 Oe for the driven case. [ABSTRACT FROM AUTHOR]

We explore a new geometry allowing effective excitation of the lowest antisymmetric standing spin wave mode in ferromagnetic metallic films with symmetrical boundary conditions. The approach is based on the use of a coplanar waveguide with the ferromagnetic film, Permalloy (Py), playing the role of the signal line. In addition, we study a signal line which is a sandwich of Py inside two nonmagnetic metallic films. We find that the thickness and conductivity of the metal films can significantly alter the amount of absorption, at ferromagnetic resonance, between the symmetric and antisymmetric spin wave modes. The experimental results are supported by numerical calculations indicating the origin of the strength of the absorption. [ABSTRACT FROM AUTHOR]

Recently there has been interest in small, planar, high frequency (10–50 GHz) signal processing devices which are based on metallic ferromagnets. This paper, in contrast, presents theoretical results for devices utilizing a hexagonal ferrite. The notch filter results show attenuation of frequencies in the 40–60 GHz range with applied fields in the 1–5 kOe range. For geometries similar to the ultrasmall metallic devices, the transmission loss at the notch can be from 20 to 140 dB/cm depending on the thickness of the hexagonal ferrite film and the inclusion of dielectric spacers. The phase shifter results show phase shifts of up to 360°, with losses below 2 dB/cm. These results are obtained for devices using thin films around 1 μm in thickness for the hexagonal ferrite, and for reasonable linewidths below 200 Oe. [ABSTRACT FROM AUTHOR]

We present a theoretical investigation of the thermal hysteresis of iron dots exchange-coupled to an antiferromagnetic substrate. We consider a temperature interval bounded by the Néel temperature of the substrate, and we calculate the heating and cooling curves in the presence of an external field oriented opposite to the interface exchange field. The thermal hysteresis is due to the temperature variation of the interface field and the influence of the geometrical shapes and sizes of the dots on the magnetic states and switching mechanisms. We show that Fe dots on an uncompensated NiO substrate exhibit large thermal hysteresis at room temperature, and external fields of a few kOe. The width of the hysteresis loops depends on the dimensions of the ferromagnetic dot, and can be significant for dots elongated in the direction of the interface field. [ABSTRACT FROM AUTHOR]

We study small thin-layer magnetic notch filters which operate in the 5–40 GHz range. Past theoretical work has concentrated on a structure where the magnetic film was right next to one of the conductive films in a waveguide. Here we present a theoretical model, which investigates the properties of a waveguide with two dielectric films and one magnetic film placed between two outer conductive layers. The results show this more general structure produces a deeper attenuation and a narrower peak compared to the earlier structure. The additional attenuation varies from 0 to 30 dB/cm, depending on the thickness and position of the magnetic film. This article also examines the reflection of the guided waves as they enter the notch filter. The results from an effective medium calculation show that a signal experiences the largest return losses near the ferromagnetic resonance frequency of the magnetic film, with typical losses below -4 dB. The return loss can be reduced significantly if the linewidth in the ferromagnetic film is increased. The effective medium results are compared to an experiment that measured return losses in a microstrip device. The experiment had maximum return losses between -4 and -8 dB. [ABSTRACT FROM AUTHOR]

We study the performance of a high frequency band stop filter which can work in both the microwave and infrared regions using an antiferromagnet film as the active material. We find that the band stop frequencies can be adjusted by changing the applied magnetic field or by changing the temperature. Also, the attenuation of the wave at a band stop frequency can be controlled by varying the physical parameters such as the thickness of the antiferromagnetic film or the thickness of the dielectric spacer. In addition, the attenuation is strongly dependent on the anisotropy field in the antiferromagnet. Further, the band stop filter using antiferromagnet does not have the constraints due to skin depth and conductivity losses as many of the antiferromagnets are insulators. [ABSTRACT FROM AUTHOR]

Presents the theory of a high frequency magnetic tunable filter and phase shifter. Identification of the theory; Information on the numerical results and applications to the theory; Details on the conclusion.

Presents a study which investigated the localization of electronic states of semiconductor superlattices. Boundaries of wave function; Equation probabilities of electrons; Properties of the ten-well system; Approximation of real superlattices.

Presents a study that investigated the magnetic excitations in epitaxial Fe(38 Å)/Cr(13 Å)/Fe(38 Å) thin film sandwiches using Brillouin light scattering (BLS) in the backscattering configuration. Experimental setup for BLS measurements in the backscattering configuration; Backscattering geometry for the Briillouini light-scattering experiment on sandwich films; Experimental results and magnetization curve connections.

Modern, high frequency, microwave devices for communications technologies can be made with thin ferromagnetic films with narrow microwave resonance linewidths. Recently, there has been interest in magnetostrictive materials where the material constants can change substantially with stresses and applied magnetic fields. We report the development of single crystal thin (20 nm thick) magnetostrictive films of Fe1-xGax (x=0.20 FeGa(A), 0.23 FeGa(B), 0.28 FeGa(C) on GaAs(001) substrates and on their use in prototype microwave devices. These Galfenol films have a narrower linewidth than any previously reported similar thin films. We fabricate and characterize novel microstrip-based monolithic microwave devices using Galfenol thin films as an active element. We find a number of important features: (1) There is a large absorption (up to 30 dB/cm) at the resonance frequency. (2) The linewidth of the device is narrow ~1.5GHz. (3) The saturation magnetization of the samples decreases with the increase in Ga contents. (4) The cubic anisotropy is close to zero (~0.06 kOe for FeGa(A)) and becomes negative for higher concentration of Ga content in the samples, and (5) the damping increases with increase in Ga concentration. [ABSTRACT FROM AUTHOR]

Recently, coated nanoshells combined with magnetic nanoparticles and cancer-cell-specific antibodies have been used to develop a multifunctional platform for simultaneously diagnosing and treating cancer, via magnetic resonance imaging and photothermal therapy. For this application, core-shell nano-spheres with a low resonance frequency (low GHz range) in a strong applied field are required. In this aspect, ferromagnetic resonance experiment is an important tool for determining the dynamic properties of nano-materials. Magnetic field dependence of resonance frequency (fr) and linewidths (Δf and ΔH) for both the ZnO coated and uncoated CoFe2O4 hollow spheres are studied using a vector network analyzer. As compared to uncoated CoFe2O4 hollow sphere, ZnO coated CoFe2O4 showed reduced resonance frequency, larger Δf and ΔH, reduced gyromagnetic ratio and effective fields. The experimental results are confirmed with the effective medium theory. [ABSTRACT FROM AUTHOR]

We constructed a series of microstrip and co-planar microwave waveguides. These structures use metallic ferromagnets and therefore exhibit strongly frequency-dependent attenuation and phase-shift effects. The lines have maximum attenuation peaks occurring at the ferromagnetic resonance frequency, which increases with applied magnetic field. Such properties are used in band-stop filters. The devices used monocrystalline Fe films grown by Molecular Beam Epitaxy and polycrystalline sputtered permalloy films. For our devices that incorporated Fe the band-stop frequencies ranged from 10-20 GHz for applied fields up to only 80 kA/m (1000 Oersted). For devices using permalloy, the band-stop frequency was in the 5-10 GHz range for applied fields less than 80 kA/m. The maximum power attenuation was about 100 dB/cm, much larger than the previously reported values of 4 dB/cm. The resonance condition also affects the phase of the transmitted wave, strongly changing phase above and below the resonance frequency. The result is a phase-shifter that is tunable with applied magnetic field. We observed phase changes of over 360 [degrees]/cm with an applied field of less than 40 kA/m. Index Terms--Coplanar waveguide, ferromagnetic resonance, filter, microstrip, phase shifter.

We studied the magnetic field dependence of the resonance frequency (fr) and frequency linewidth (Δf) of iron oxide nanoparticles (Fe3O4) with different particle sizes spin-coated on a coplanar waveguide. Using a vector network analyzer, we find that the resonance frequency increases with an increase in applied field for all particle sizes, while Δf decreases with the increase in the particle size. We have also carried out a theoretical study using the power absorbed by the different regions of the coplanar waveguide and found that the results are in accordance with the experiment. [ABSTRACT FROM AUTHOR]

To suppress noise in electronic devices at gigahertz frequencies, the signal attenuation in the pass-band frequency region must be minimized to enhance the signal integrity without distortion. We designed, fabricated, and evaluated two noise suppression microstructures, one using nickel nanorod arrays in a porous anodic aluminum oxide dielectric and a second structure based on a Ni80Fe20 (Permalloy) film in a microstrip waveguide geometry. Both noise suppression structures function as tunable devices in the microwave range. The devices show signal attenuation at the resonance frequency which is increased more than -20 dB compared with a microstrip line without any magnetic material. At ferromagnetic resonance the power loss in the nanorod structure was 15% higher than that of a Permalloy based device. In the pass-band frequency region the signal attenuation/insertion loss of the nanorod arrays was about 2 dB higher than that of the Permalloy film. It is observed that to get good noise suppression, the signal distortion due to the signal phase shift must also be minimized. The differential phase shift of the Ni nanorod device was ∼20% smaller than that of the Permalloy film device. We also designed noise suppressors using Ni nanorods by changing aspect ratios (rod lengths of 5–50 μm and diameter of 200 nm). The noise suppression frequency and the magnitude of signal attenuation in the noise suppressor were increased as the Py element became thicker. [ABSTRACT FROM AUTHOR]

We determined the exchange anisotropy and rotational anisotropy of IrMn(7 nm)/Fe(t=3–20 nm)/IrMn(7 nm) exchange-biased structures using conventional ferromagnetic resonance (FMR) and network analyzer FMR (NA-FMR). Compared to single Fe layer films of identical thickness, we observe an isotropic downward shift and an angular variation of the FMR resonance field in the multilayer structures. The isotropic shift originates from the rotational anisotropy, while the angular variation originates from the exchange anisotropy. Both exchange anisotropy and rotational anisotropy increase with decreasing Fe thickness in the exchange-biased structures. The isotropic downward shift of the resonance field translates to an upward shift of the resonance frequency, and can be used to boost the operational frequency of microwave devices (bandpass/stop filters) by several gigahertz. [ABSTRACT FROM AUTHOR]

We report on the dynamic properties of Permalloy nanostrips at gagahertz frequencies. The thickness of the strips is 100 nm, strip width is 300 nm, strip spacing is 1 μm, and length is 0.3–100 μm; aspect ratios are 1:1, 1:2, 1:3, 1:5, 1:10, and 1:333. The dynamic behavior was studied by network analyzer ferromagnetic resonance (FMR) using Permalloy strips on a coplanar waveguide in flip-chip geometry. The FMR mode frequencies (fr) can be controlled by the aspect ratio as well as by the applied magnetic field (H). In longer strips (1:10 and 1:333), the excitation frequencies show a soft mode behavior (Heff=990 Oe) when the field is along the hard axis. However, along the easy axis (along the strip length), fr increases with applied field. At a field of 3 kOe, fr values are almost independent of aspect ratio along the easy axis except for the 1:1 strip. Along the hard axis, the frequencies are strongly dependent upon the aspect ratio. We also observed that the frequency linewidths of the strips are dependent on the aspect ratio. [ABSTRACT FROM AUTHOR]