Baoshan Zhang

Preparation and properties of a novel iron-coated carbon fiber

Abstract A novel iron-coated carbon fiber has been fabricated by electrochemical method. Magnetic properties, complex permeability, and complex permitivity have been measured. It is found that the sample has a low coercive force. By doping a few fibers to general absorbers (carbonyl iron), the complex permeability and permitivity of composites achieved a rise without the obvious increase of filling density. This result shows that the doping of fibers to general absorber will effectively decrease the thickness of the microwave absorption layer.

Influence of the magnetic field annealing on the extrinsic damping of FeCoB soft magnetic films

In order to investigate the high-frequency damping properties of the ferromagnetic film for the electromagnetic shielding applications, FeCoB films with the thickness of 200 nm were fabricated by the rf magnetron sputtering and subsequently annealed with a static in-plane magnetic field of 2000 Oe under a series of temperatures. The annealed films exhibit excellent soft magnetic and microwave loss properties. The local resonance from the multiphase microstructure and two-magnon scattering at anisotropy inhomogeneities make an additional contribution to the extrinsic damping of the films. As a result, the film annealed at 450 °C achieves a high full width at half maximum Δf up to 6 GHz for the imaginary part of the frequency-dependent permeability. These films could be novel candidates for the electromagnetic shielding applications.

Left-handed materials made of dilute ferromagnetic wire arrays with gyrotropic tensors

In this article, we theoretically investigate properties of normally incident microwave propagation in dilute metallic ferromagnetic wire array (DMFWA) slabs with gyrotropic tensors. It is found that DMFWA may become left-handed materials (LHM) in a narrow frequency range below the plasma frequency, even the real part of the corresponding inversion permeability element is positive. Comparing with the isotropic materials having same diagonal permeability elements, due to existence of off-diagonal elements of the gyrotropic permeability tensor, the DMFWA becomes a LHM in a higher frequency range depended on its geometric structure. In addition, E, H, and wave vector Re(k) of microwave propagating in the DMFWA form an approximately left-handed triplet of vectors.

Inhomogeneous exciting field dependence of permeability and microwave properties of trilayer ferromagnetic films with in-plane uniaxial anisotropy

Based upon the Landau–Lifshitz equation and Maxwells equations, the permeability and microwave properties of trilayer ferromagnetic films with in-plane uniaxial anisotropy are investigated simultaneously. It is found that, due to coexistence of interlayer exchange coupling and the gyrotropic permeability tensor in the film, the permeability, especially the magnetic parameter 1/ν = μyy−μxy2/μxx, related directly to properties of the normally incident microwave propagation in the film, may be influenced significantly by inhomogeneous exciting fields applied in the layer magnetic moments. In turn, properties of normally incident microwave propagation in the film may be modified strongly; e.g., giant modification of absorption peaks, such as the number, frequency position, shape, and so on, are predicated.

Thin magnetic coating for low-frequency broadband microwave absorption

Planar anisotropy carbonyl-iron (PACI) particles were prepared from 3 μm diameter sphere-shaped carbonyl-iron materials by high-energy ball milling technique. And the oriented planar anisotropy carbonyl-iron (OPACI) composite was fabricated using rotational orientation method. The OPACI composite possessed much higher real and imaginary permeability and relatively lower real and imaginary permittivity than other absorbers in 1–18 GHz frequency range. The susceptibility versus frequency curves have been fitted quite well using Kittel equation and Debye equation. Compared with natural resonance, the wall displacement susceptibility plays a dominating role for the high real and imaginary permeability. For OPACI composite, the minimum reflection loss (RL) can be as low as −44 dB at 1.8 GHz with the thickness of 2.3 mm, −7.3 dB at 1 GHz, −20 dB at 2 GHz, −5.4 dB at 8 GHz, and the bandwidth can reach 1.9 GHz for the good performance (RL < −10 dB) in the frequency ranges of 1.2–3.1 GHz, indicating a promising ab...

Broadband left-handed materials made of thin soft ferromagnetic films with in-plane uniaxial anisotropy

We study theoretically characteristics of normally incident E-polarized microwave propagation in soft ferromagnetic films with in-plane uniaxial anisotropy, and find that due to the existence of the gyrotropic permeability tensor, the soft ferromagnetic film may become a left-handed material over a broad frequency band. Furthermore, the bandwidth and the frequency positions can be easily modified by the external magnetic field. Finally, the energy loss of the microwave propagating in the film is investigated.

High-frequency magnetodielectric response in yttrium iron garnet at room temperature

Magnetic and dielectric properties of Yttrium Iron Garnet are measured over a frequency ranging from 0.5 GHz to 10 GHz with a magnetic field applied parallel to the propagation direction of the microwave. At the same time, the magnetodielectric phenomena are detected quantitatively. The maximum amplitude of the magnetodielectric coefficient is acquired at the ferromagnetic resonance frequency, and the value is up to 1.2% with the magnetic field of 1500 Oe applied. The phenomena have been explained by the Faradays electromagnetic induction of the precession of the magnetic moments in the electromagnetic field at the ferromagnetic resonance frequency.

Tunable storage states' transition in slotted ferromagnetic nanorings

In magnetic random access memory, free layers are used to store data bits as “0” or “1.” For free layers with slotted nanoring structures, the magnetization configuration of counterclockwise vortex state can be defined as “0” state, while clockwise vortex state as “1” state. It is important to have a controllable and stable state transition process to make sure that “0” state can switch to “1” state and vice versa. Up to now, it has seldom been reported that the transition process is heavily affected by the static anisotropy field Hk-stat of free layers. A sufficient Hk-stat will substantially reduce the probability of successful state transition. To increase the accuracy of writing data, the free layers must be prepared with a low anisotropy field. In this paper, we present a rotational sputtering method, which can finish the desired isotropic film preparation, and thus realize a stable state transition.

Hysteretic Behavior of the Dynamic Permeability in FeCoB Thin Films

The microwave permeability in a soft magnetic FeCoB film as a function of the external magnetic field is measured in the frequency range from 0.2 to 8 GHz. When the field applied parallel to the orientation of the magnetization varies from 90 to -90 Oe, the permeability spectrum shows hysteretic behavior. Double magnetic resonance peaks come forth and evolve in the imaginary part of the permeability when the field approaches the coercive field of the sample. This phenomenon is attributed to the formation of domains, the resonance frequency of the reversed domains being different than that of the non-reversed domains. Both peak frequencies and peak amplitudes are interpreted quantitatively using simple considerations based on the Landau-Lifshitz equation and the knowledge of the static magnetic properties of the film. The ratio of the reversed magnetic moments to the non-reversed ones is estimated by fitting curves of the permeability spectrum.

Thickness effects on microwave properties and ferromagnetic resonance of Fe-Co-B magnetic thin films

In order to apply Fe-Co-B thin films for high-frequency magnetic devices, the thickness effects on the magnetic properties and microwave properties of these films have been studied. The complex permeability of these thin films has been measured at the frequency of 2GHz with cavity. The real part of the permeability μ/ of the thin films increases with the decease of the thickness of the thin films because the decrease of the thickness of the thin films leads to the increase of the natural resonance frequency. The imaginary part of the permeability μ// of the thin films exhibits a minimal value at 150nm thickness, which is due to the simultaneous effect of the natural resonance and eddy current effect. The FMR line width broadening can be observed with the increase of the thickness of the thin films from the FMR investigation, which supports the research results above.