Yulan Jing

Influence of microstructure on permeability dispersion and power loss of NiZn ferrite

Permeability spectra of NiZn ferrite with different microstructures had been resolved into contributions of domain wall resonance and spin rotation relaxation. The fitting results of permeability dispersion revealed the relationships among domain wall resonance, spin rotation relaxation mechanisms, and microstructures. Pcv (power loss) was analyzed by dividing Pcv into Ph (hysteresis loss) and Pe+Pr (eddy current loss and residual loss) from the frequency dependence of the power loss. When excited under large flux density, sample with larger average grain size and closed pores could obtain lower Pcv. This could be attributed to the fact that closed pores were not easy to block domain wall movement and grain boundaries became the predominant domain wall pinning factor. However, for the low induction condition, sample with small grain size had better performance on Pcv due to the fact that influence of the closed pores was more significant. With increasing frequency, Pe+Pr gradually became the predominant f...

A tunable hybrid metamaterial absorber based on vanadium oxide films

A tunable hybrid metamaterial absorber (MA) in the microwave band was designed, fabricated and characterized. The hybrid MA was realized by incorporating a VO2 film into the conventional resonant MA. By thermally triggering the insulator–metal phase transition of the VO2 film, the impedance match condition was broken and a deep amplitude modulation of about 63.3% to the electromagnetic wave absorption was achieved. A moderate blue-shift of the resonance frequency was observed which is promising for practical applications. This VO2-based MA exhibits many advantages such as strong tunability, frequency agility, simple fabrication and ease of scaling to the terahertz band.

Low-loss NiCuZn ferrite with matching permeability and permittivity by two-step sintering process

Magneto-dielectric materials with matched permeability and permittivity are promising candidates as loading materials to reduce the physical dimensions of low-frequency antennas. In this study, NiCuZn ferrites were prepared by both the traditional final-stage sintering process and the two-step sintering process to obtain the magneto-dielectric materials. It was found that the samples sintered by the two-step sintering process tended to obtain microstructure with dense, homogeneous, and small average grain size, which was favorable to obtain low magnetic and dielectric losses. The sample sintered by the two-step sintering process with high temperature 950 °C and holding temperature 900 °C could obtain almost equal permeability and permittivity of around 11.8. And the magnetic and dielectric loss tangents were lower than 0.015 in a frequency range from 10 to 100 MHz. These properties make the material useful to the design of miniaturized antennas.

Changing and reversing the exchange bias in a current-in-plane spin valve by means of an electric current

The authors show that a current flowing in a direction not perpendicular to the layer planes of an exchange-biased spin valve systematically changes the exchange bias. The direction of the exchange bias can be completely reversed when a large enough current is applied. This effect occurs only when the direction of the initial sweeping magnetic field is antiparallel to the exchange-bias field. The effect is attributed to the recently predicted current-induced torque in an antiferromagnet and provides evidence to support the prediction that the critical current is smaller in an antiferromagnet than the typical value for current switching in a ferromagnet.

Microstructure and magnetic properties of Ni-Zn ferrites doped with MnO2

To improve the performance of Ni-Zn ferrites for power field use, the influence of MnO2 additive on the properties of Ni-Zn ferrites was investigated by the conventional powder metallurgy. The results show that MnO2 does not form a visible second phase in the doping mass fraction range of (0-2.0%). The average grain size, sintering density and real permeability gradually decrease with the increase of the MnO2 content. And the DC resistivity continuously increases with the increase of MnO2 content. The saturation magnetization (magnetic moment in unit mass) first increases slightly when mass fraction of MnO2 is less than 0.4% MnO2, and then gradually decreases with increasing the MnO2 mass fraction due to the exchange interaction of the cations. When the excitation frequency is less than 1 MHz, the power loss (P(subscript cv)) continuously increases with increasing the MnO2 content due to the decrease of average grain size. However, when the excitation frequency exceeds 1 MHz, eddy current loss gradually becomes the predominant contribution to P(subscript cv). And the sample with a higher resistivity favors a lower P(subscript cv), except for the sample with 2.0% MnO2. The sample without additive has the best P(subscript cv) when worked at frequencies less than 1 MHz; and the sample with 1.6% MnO2 additive has the best P(subscript cv) when worked at frequencies higher than 1 MHz.

Effects of

The influences of Nb2O5 addition on the microstructure and magnetic properties, especially DC-bias-superposition characteristic of the low-temperature-fired NiCuZn ferrites, were investigated. It was found that Nb ions entered into the lattice when Nb2O5 content was less than 0.3 wt%, after that, a second phase, bismuth iron (zinc or nickel) niobium oxide appeared. The second phase consumed some sintering aid Bi2O3 and had obvious influences on sintered density, grain size and magnetic properties, such as permeability of the ferrites. All of the sintered density, saturation magnetic flux density and average grain size gradually decreased with Nb2O5 content. Initial permeability first increased, and then continuously decreased with Nb2O5 content. The sample with 0.1 wt% Nb2O5 displayed the highest initial permeability, which was mainly due to the decrease of magnetocrystalline anisotropy constant. Under relatively low superposition magnetic field H ≤ 375 A/m, a higher initial permeability favored to obtain a higher incremental permeability. However, when the superposition magnetic field continued to increase, some samples with a lower initial permeability displayed a higher incremental permeability. The normalized incremental permeability was mainly determined by the coercivity of the samples. A higher coercivity favored the attainment of a better DC-bias-superposition characteristic on normalized permeability. So the sample with 0.5 wt% Nb2O5, which displayed the largest coercivity, had the best performance. Possible mechanisms contributing to the above results were discussed in detail.

{\hbox {Nb}}_{2}{\hbox {O}}_{5}

Low-temperature-fired (Ni0.4-xMgxCu0.2Zn0.4)Fe1.96O4 ferrites with x=0, 0.1, 0.2, 0.3, and 0.4, have been synthesized by the conventional mixed oxide method and investigated their electromagnetic properties. Initial permeability is observed to increase with increase in Mg substitutions. At the same time, sintering density, magnetic flux density, remanence, coercivity, and Snoeks product decrease with increase in Mg substitutions. Furthermore, microstructures and permittivity spectra have no obvious changes with Mg substitutions. Possible mechanisms contributing to the above results are discussed.

on DC-Bias-Superposition Characteristic of the Low-Temperature-Fired NiCuZn Ferrites

High quality VO2 thin films have been fabricated on silicon substrates using magnetron sputtering by introducing Al2O3 thin films as a buffer. The ultrathin Al2O3 deposited by plasma-assisted atomic layer deposition leads to a greatly improved crystallinity and textures in VO2 films. Dramatic change in electrical resistivity (4 orders of magnitude) and a small thermal hysteresis loop (~4 K) are obtained across the metal–insulator phase transition (MIT). Remarkably, by applying perpendicular voltage to a VO2/Al2O3 based metal/VO2/semiconductor device, electrically driven MIT switching characteristics have been observed with a tiny tunneling leakage current of ~10 μA. These results show that an electric field alone is sufficient to trigger the MIT, and the realization of VO2 based ultrafast electrical switching devices on a silicon substrate is possible.

Electromagnetic Properties of Mg-Substituted NiCuZn Ferrites for Multilayer Chip Inductors Applications

Effects of symmetry reduction on the magnon band gaps (MBGs) in two-dimensional (2D) magnonic crystals (MCs) were investigated by solving the Landau–Lifshitz equation with the plane wave method. The symmetry reduction is achieved by introducing additional scatterers into each unit cell or by reorienting noncircular scatterers. The numerical results show that the MBGs in a square lattice can be improved by introducing additional scatterers or rotating the square rods. In honeycomb and triangular lattices, the MBGs can be improved only by introducing additional scatterers, however, rotating the square rods is an invalid way to increase the MBGs.

Tuning the phase transitions of VO2 thin films on silicon substrates using ultrathin Al2O3 as buffer layers

Recently, resonant metamaterial absorbers (MAs) at microwave and terahertz (THz) bands have attracted much attention due to the advantages such as high absorption, low density, and thin thickness [1–6]. The MA generally composed of a metamaterial layer and a metal plate layer separated by a dielectric spacer. With this kind of novel device, unity absorptivity can be realized by matching the impedance of MA to free space. Besides that, wide-angle, polarization insensitive and even multi-bands/wide-band absorption can be achieved through properly device designing [7-11]. Furthermore, our previous investigations on MA show that the absorber traps the incident electronmagnetic (EM) wave into some specific spots of the devices, and then converts it into heat [9, 12]. All these features make MAs very useful in areas such as EM detector/imager, anti-electromagnetic interference, stealth technology, phase imaging, spectroscopy and thermal emission.