Jianguo Wan

Giant magnetoelectric effect of a hybrid of magnetostrictive and piezoelectric composites

A magnetoelectric (ME) hybrid structure is constructed by an efficient coupling between magnetostrictive Terfenol-D (Tb0.27-0.30Dy0.73-0.70Fe1.90-1.95)/epoxy and piezoelectric lead–zirconate–titanate [Pb(Zr0.52Ti0.48)O3]/epoxy composites. Significant ME effect produced by the piezoelectric k31 coupling with the longitudinal vibration of the magnetostrictive component over a wide range of frequency is observed. It is revealed that the ME effect can be enhanced by applying an optimized magnetic bias. A magnetoelectric voltage coefficient as high as 8700 mV/cm Oe is recorded at the resonance frequency of 59.2 kHz for the structure with an optimized magnetic bias of 0.7 kOe. Our measurement confirms that the eddy current loss remains negligibly small at an operating frequency as high as ∼200 kHz, predicting very promising applications of the present ME structure.

High-yield synthesis of uniform Ag nanowires with high aspect ratios by introducing the long-chain PVP in an improved polyol process

Polyvinyl pyrrolidone (PVP) with different molecular weights was used as capping agent to synthesize silver nanowires through a polyol process. The results indicated that the yields and aspect ratios of silver nanowires were controlled by the chain length of PVP and increased with increasing the molecular weight (MW) of PVP. When the long-chain PVP-K90 (MW= 800,000) was used, the product was uniform in size and was dominated by nanowires with high aspect ratios. The growth mechanism of the nanowires was studied. It is proposed that the chemical adsorption of Ag+ on the PVP chains at the initial stage promotes the growth of Ag nanowires.

Resonance magnetoelectric effect in bulk composites of lead zirconate titanate and nickel ferrite

Magnetoelectric (ME) Pb(Zr0.52Ti0.48)O3 (PZT)–NiFe2O4 bulk composites with various PZT volume fractions were prepared. The ME coupling coefficient, impedance, and flux density versus the frequency were measured. The magnetic bias and PZT volume fraction in the composites are investigated to optimize the ME output. It is revealed that ME resonances are caused by electromechanical resonance in the piezoelectric phase and magnetomechanical resonance in the magnetostrictive phase. Maximum magnetoelectric voltage coefficient resonance values of 6.7 V/cm Oe at 132.6 kHz and 6.12 V/cm Oe at 427.2 kHz for the composite with optimized PZT volume fraction of 0.55 were obtained at optimized magnetic bias of 0.6 kOe.

Electric-field-induced magnetization in Pb(Zr,Ti)O3/Terfenol-D composite structures

We report the electric-field-induced magnetization (EIM) in the composite structures made by combining Pb(Zr0.52Ti0.48)O3 (PZT) and Tb0.30Dy0.7Fe2 (Terfenol-D). The results showed that the EIM could be generated in the composite structures due to the efficient stress-mediated electromagnetic coupling interaction between the piezoelectric PZT and magnetostrictive Terfenol-D. This EIM effect depended significantly on the driving electric field frequency and was highly sensitive to the dc magnetic bias, which exhibited a promising potential in the low-level dc magnetic field detecting application.

Numerical modeling of magnetoelectric effect in a composite structure

The mechanical coupling effect in a magnetoelectric (ME) composite structure in which a magnetostrictive component is bonded with a piezoelectric one is simulated by numerical technique, focusing on an optimization of the magnetoelectric coupling output. The simulation starts from an experimentally developed ME composite structure and takes into account the mechanical coupling mechanism between the two components. A numerical optimization algorithm is developed, predicting a significant enhancement of the ME output by optimizing the component dimension. This algorithm can also be used for optimum design of other ME composite structures in terms of the largest ME output.

Magnetoelectric resonance-bandwidth broadening of Terfenol-D/epoxy-Pb(Zr,Ti)O3 bilayers in parallel and series connections

The bandwidth of magnetoelectric (ME) resonance for a combined structure where several Terfenol-D/epoxy-Pb(Zr,Ti)O3 bilayers are connected in parallel and series was measured. A relationship between the bilayer length and resonance frequency was used to design the combined structure, in which the bilayered components of different lengths were connected in the parallel and series forms, respectively. The measured giant ME effect of the combined structure showed a much wider frequency response to external field than a single bilayered structure, and the ME effect far from the resonance ranges was enhanced significantly too. A qualitative analysis based on the equivalent circuit concept was presented to explain these effects.

Two-dimensional universal conductance fluctuations and the electron-phonon interaction of surface states in Bi2Te2Se microflakes

The universal conductance fluctuations (UCFs), one of the most important manifestations of mesoscopic electronic interference, have not yet been demonstrated for the two-dimensional surface state of topological insulators (TIs). Even if one delicately suppresses the bulk conductance by improving the quality of TI crystals, the fluctuation of the bulk conductance still keeps competitive and difficult to be separated from the desired UCFs of surface carriers. Here we report on the experimental evidence of the UCFs of the two-dimensional surface state in the bulk insulating Bi2Te2Se microflakes. The solely-B⊥-dependent UCF is achieved and its temperature dependence is investigated. The surface transport is further revealed by weak antilocalizations. Such survived UCFs of the surface states result from the limited dephasing length of the bulk carriers in ternary crystals. The electron-phonon interaction is addressed as a secondary source of the surface state dephasing based on the temperature-dependent scaling behavior.

Scaling dopant states in a semiconducting nanostructure by chemically resolved electron energy-loss spectroscopy: a case study on Co-doped ZnO.

Dilute dopant introduces foreign states to the electronic structures of host semiconductors and imparts intriguing properties to the materials. Identifying and positioning the dopant states are of crucial importance for seeking the underlying mechanism in the doped systems. However, such determination has still been challenging, particularly for individual nanostructured materials, due to the lack of the spectroscopic probe that possesses both nanometer spatial resolution and chemical resolution. Here, we shall demonstrate the successful scaling of dopant states of individual semiconducting nanostructures by chemically resolved electron energy-loss spectroscopy (EELS), taking the individual Co-doped ZnO nanorods as an example. Guided by the Co dopant spatial distribution mapped by the core-loss EELS technique, chemical resolution is achieved in the accumulated valence electron energy-loss spectra. Three Co dopant states are successfully identified and positioned in the host ZnO bands. Furthermore, the electron extension degrees of the Co dopant states are assessed on the basis of the multiple-atom effect. The above experimental inputs optimize the density functional theoretical calculations, which generates the corrected full electronic structures of (Zn,Co)O dilute magnetic semiconductors. These results give a carrier-mediated interpretation for the room-temperature ferromagnetism of Co-doped ZnO nanostructures based on a recent theory.

Structures and magnetic properties of SinMn(n=1–15) clusters

The structure, electronic, magnetic properties of Si(n)Mn clusters up to n=15 are systematically investigated using the density functional theory within the generalized gradient approximation. In the most stable configurations of Si(n)Mn clusters, the equilibrium site of Mn atom gradually moves from convex, to a surface, and to a concave site as the number of Si atoms varying from 1 to 15. Starting from n=11, the Mn atom completely falls into the center of the Si outer frame, forming Mn-encapsulated Si cages. Maximum peaks of second-order energy difference are found at n=6, 8, 10, and 12, indicating that these clusters possess relatively higher stability. The electronic structures and magnetic properties of Si(n)Mn clusters are discussed. The magnetic moment of Si(n)Mn clusters mainly is located on Mn atom. The 3d electrons in Mn atom play a dominant role in the determination of the magnetism of Mn atom in Si(n)Mn clusters. Furthermore, the moment of Mn atom in Si(n)Mn clusters exhibits oscillatory behavior and are quenched at n>7 except for n=12, mainly due to the charge transfer, strong hybridization between Mn 4s, 3d, 4p and Si 3s, 3p states.

Effect of magnetic bias field on magnetoelectric coupling in magnetoelectric composites

The effect of dc magnetic bias field on the magnetoelectric coupling of a two-component magnetoelectric composite structure is investigated numerically using the finite-element method, in which the nonlinear magnetostress coupling for the magnetostrictive component is taken into account. It is shown that the magnetostress coupling coefficient increases first and then falls down with increasing of the bias field, and this behavior is argued to be responsible for the dependence of magnetoelectric yield on the bias field. The numerical modeling using the ANSYS5.5 finite element algorithm for the Tb0.3Dy0.7Fe1.9-epoxy/Pb(Zr0.52Ti0.48)O3-epoxy composite structure gives fairly consistent results with the experiments.