Yuanqiang Song

Effect of Zn interstitials on the magnetic and transport properties of bulk Co-doped ZnO

We have demonstrated that the bound magnetic polaron model is responsible for ferromagnetism in Co?ZnO semiconductors, where the carriers are provided by the interstitial zinc (Zni). Our experiment is unique since by changing the temperature, we are able to cross the carrier concentration threshold above which a long-range ferromagnetic order is established. Consequently, the ferromagnetic order is observed at room temperature but is weakened at temperatures below 100?K. To support our conclusion we have performed a systematic investigation on the structural, magnetic and transport properties which all give consistent results in the context of our proposed two-region model, i.e. (a) a Zni layer where carriers are sufficient to couple Co ions ferromagnetically and (b) a region with little carriers that remain in a paramagnetic state.

Co doping effect on the magnetic properties of CeO2 films on Si(111) substrates

Ce1−xCoxO2−δ films with the stoichiometry of x=0,0.03,0.06,0.1,0.125 were fabricated on Si(111) substrates using O2 assisted pulse laser deposition method. While pure CeO2 film is weak paramagnetism, integration of low Co content of 3at.% introduces ferromagnetim with a giant saturation moment (Ms) of 5μB∕Co at room temperature. Based on the first principle calculation, we attribute the giant magnetic moments to the combined contributions of spin polarized Co, Ce, and O atom with the enhancement of O vacancies. Higher Co content will depress the ferromagnetism, i.e., inverse correlation between Ms and Co contents, which is qualitatively validated by the calculated magnetic moments of Ce1−xCoxO2−δ with different Co content.

Rare earth doped CaCu3Ti4O12 electronic ceramics for high frequency applications

Ca1−xRxCu3Ti4O12 (R=La, Y, Gd; x=0, 0.1, 0.2, 0.3) electronic ceramics were fabricated by conventional solid-state reaction method. The microstructure and dielectric properties as well as impedance behavior were carefully investigated. XRD results showed that the secondary phases with the general formula R2Ti2O7 existed at grain boundaries of rare earth doped ceramics, which inhibited abnormal grain growth. The dielectric constant decreased from 4 × 105 in pure CaCu3Ti4O12 (CCTO) ceramics to 2 × 103 with rare earth doping. However, all samples showed high dielectric constant in broad frequency range (<10 MHz). The cutoff frequency (f0) was remarkably shifted to higher frequency from 13 MHz (pure CCTO ceramics) to 80 MHz (Gd-doped CCTO ceramics). Meanwhile, the dielectric loss tangent rapidly decreased approximately 10 times. These improvements of dielectric properties by rare earth doping are very useful in wide frequency chip capacitor and LTCC devices.

Correlation between sputtering parameters and composition of SmCo-based films for microelectromechanical system applications

About 3.0 μm thick SmCo-based films with additives of Fe, Cu, and Zr were deposited on Si substrates. Based on a developed semiempirical theoretical model, the dependence of the film composition on the sputtering parameters was discussed. The experimental results show that the Sm concentration increases with decreasing sputtering power or increasing Ar gas pressure, which are in good agreement with the calculated results when the preferential sputtering effect is disregarded. The effect of the sputtering parameters on the film composition provides an opportunity for the same composite target to fabricate films with Sm concentration varying from 13.8 to 17.3 at. %, which is reasonable for the permanent magnetic phase transformation (Sm2Co17→SmCo7→SmCo5). Furthermore, the observed TbCu7-type film shows a better crystal texture with a low remanence ratio for the hysteresis loops measured out plane to in plane of 0.08.

Role of dopant, defect, and host oxide in the observed room temperature ferromagnetism : Co-ZnO versus Co-CeO2

A comparative study on the room temperature ferromagnetism of Co doped ZnO and CeO2 bulk samples is presented. Co–ZnO system has been observed to switch between ferromagnetic and paramagnetic by controlling the donor defects, interstitial zinc: Zni. On the other hand, Co–CeO2 is always ferromagnetic. However, ferromagnetism increases/decreases with increase/decrease in donor defects, oxygen vacancies: VO. Systemic structural, magnetic, and transport analyses reveal that the nature of donor defects and host oxide plays a vital role in establishing ferromagnetism. This study provides an insight into the underlying mechanisms that are responsible for the ferromagnetism in Co–ZnO and Co–CeO2. Moreover, the proposed mechanisms are supported by the electronic structure of magnetic impurity ions and defects.

Room temperature magnetic and dielectric properties of cobalt doped CaCu3Ti4O12 ceramics

CaCu3Ti4−xCoxO12 (x = 0, 0.2, 0.4) ceramics were prepared by a conventional solid state reaction, and the effects of cobalt doping on the room temperature magnetic and dielectric properties were investigated. Both X-ray diffraction and energy dispersive X-ray spectroscopy confirmed the presence of Cu and Co rich phase at grain boundaries of Co-doped ceramics. Scanning electron microscopy micrographs of Co-doped samples showed a striking change from regular polyhedral particle type in pure CaCu3Ti4O12 (CCTO) to sheet-like grains with certain growth orientation. Undoped CaCu3Ti4O12 is well known for its colossal dielectric constant in a broad temperature and frequency range. The dielectric constant value was slightly changed by 5 at. % and 10 at. % Co doping, whereas the second relaxation process was clearly separated in low frequency region at room temperature. A multirelaxation mechanism was proposed to be the origin of the colossal dielectric constant. In addition, the permeability spectra measurements i...

Improvement of high-frequency characteristics of Z-type hexaferrite by dysprosium doping

Z-type hexaferrite has great potential applications as anti-EMI material for magnetic devices in the GHz region. In this work, Dy-doped Z-type hexaferrites with nominal stoichiometry of Ba3Co2DyxFe24−xO41 (x = 0.0, 0.05, 0.5, 1.0) were prepared by an improved solid-state reaction method. The effects of rare earth oxide (Dy2O3) addition on the phase composition, microstructure and electromagnetic properties of the ceramics were investigated. Structure and micromorphology characterizations indicate that certain content of Dy doping will cause the emergence of the second phase Dy3Fe5O12 at the grain boundaries of the majority phase Z-type hexaferrite, due to which the straightforward result is the grain refinement during the successive sintering process. Permeability spectra measurements show that the initial permeability reaches its maximum of 17 at 300 MHz with x = 0.5, while the cutoff frequency keeps above 800 MHz. The apparent specific anisotropy field HK of Dy-doped Z-type hexaferrites decreases with x...

Effects of Fe doping and the dielectric constant on the room temperature ferromagnetism of polycrystalline CeO2 oxides

Ce1−xFexO2−δ polycrystalline samples with x=0%–8% were synthesized by the solid phase reaction method at a low sintering temperature of 1250 °C, and the effect of Fe content on the magnetic behaviors has been systemically investigated. It is found that the pure CeO2 samples are diamagnetic, while doped samples unambiguously show the existence of room temperature (RT) ferromagnetism (FM). With the increase in x, the saturation magnetization (Ms) exhibits a steep increase to a maximum Ms of 1 emu/g at x=2.7% and then drop exponentially. A small amount of Fe doping (0.9%) promotes the densification and induces a large dielectric constant (e). Our result shows that a relatively large e and a proper Fe concentration are critical to establish the magnetic polaron and provide strong RT FM in CeO2.

Characterization of charge transport and electrical properties in disordered organic semiconductors

The conventional charge transport models based on density- and field-dependent mobility, only having a non-Arrhenius temperature dependence, cannot well describe the current–voltage (J–V) characteristics of hole-only devices made from disordered organic semiconductors. In this paper, we study the charge transport in a large variety of organic semiconductors and provide evidence for the availability of our improved mobility model based on both the Arrhenius temperature dependence and the non-Arrhenius temperature dependence. It is validated with experimental data collected from hole-only devices made of different organic materials in a wide range of operating conditions. Furthermore, we calculate and analyze in detail some important electrical properties of the relevant polymers by using a numerical method, adopting the uneven discretization and Newton iteration methods to solve the coupled equations describing the space-charge-limited current. It is shown that the boundary carrier density has a significant effect on the J–V characteristics and that the numerically calculated carrier density is a decreasing function of the distance to the interface.

Application of high-temperature permanent magnets Sm(Co, Fe, Cu, Zr)Z in PPM focusing system

Abstract Rare earth permanent magnets Sm(Co, Fe, Cu, Zr)Z with outstanding performance and high-temperature thermal stability were fabricated. Optimized by Fe content and process, Sm(Co0.72Fe0.15Cu0.1Zr0.03)7.5 magnet with Br>0.75 T and Hci>1300 kA/m at 300 °C can be obtained. According to the performance data of Sm(Co0.72Fe0.15Cu0.1Zr0.03)7.5, the magnetic field along central axis BZ in periodic permanent magnet (PPM) focusing system was simulated using electromagnetic field analysis software Maxwell 2D/3D. The BZ exhibited typical cosine curve along central axis, and the peak value of BZ was high enough to meet the demand of PPM focusing system at room temperature even at 200±20 °C. Additionally, a kind of simple cooling structure for PPM focusing system was designed by setting cooling pipe between polepieces. Simulated results showed that smooth cosine curve of BZ was successfully achieved with good control of the thickness of cooling pipe.