Dandan Wen

Magnetic and high frequency properties of nanogranular CoFe-TiO2 films

A series of soft magnetic nanogranular CoFe-TiO2 films with different thicknesses were fabricated on Si substrates by co-sputtering CoFe and TiO2, and the magnetic and high frequency properties were studied in details. X-ray diffraction analysis confirmed that the films were nanocrystalline/amorphous composites. A saturation magnetization as high as 21 kGs and a ferromagnetic resonance (FMR) frequency above 3 GHz were obtained. It was found that the coercivity decreased steeply with the increase of film thickness and its minimum value was about 6 Oe. In addition, the variations of FMR peak broadness of the imaginary part and damping factor were also presented and discussed as functions of film thickness.

Large In-Plane Uniaxial Magnetic Anisotropy in the Ferromagnetic/Ferroelectric Heterostructures

Amorphous FeSiBC film with large magnetostriction and small magnetocrystalline anisotropy was deposited on the poled (110)-oriented Pb(Mg,Nb)O3-PbTiO3 crystal substrate to form ferromagnetic/ferroelectric (FM/FE) heterostructures. It is shown that large in-plane uniaxial magnetic anisotropy (IPUMA)(>250 Oe) can be induced upon applying in situ magnetic biasing field along the [001] direction during deposition. The IPUMA is tenfold of normally magnetic-field-induced anisotropy of FeSiBC/SiO2/Si with the same biasing magnetic field, therefore dramatically pushing the FM resonance frequency from 1 to 4.2 GHz. Our investigation shows that the FE domain of the poled Pb(Mg,Nb)O3-PbTiO3 crystal can imprint into the FeSiBC layer and provide negative magnetic stress anisotropy upon applying biasing field along the [001] direction, but positive magnetic stress anisotropy upon applying biasing field along the [110] direction.

Memorizing UV exposure energy in resistance — A smart patch based on conductive polymer

We have successfully demonstrated a smart UV patch to memorize the accumulative energy from the exposure to UV light over a period of time. The sensing principle is based on changes in the electrical resistance of a composite made of photo-acid generator triphenyl sulfonium triflate (TST), poly aniline emeraldine (PANI-EB), polyethylene glycol (PEG) and carbon nanotubes (CNTs). The sheet resistance of the composite can drop five orders of magnitude within 30 minutes of exposure to the UV light using an LED light source of 10Mw/cm2. The photo-induced protonation process can release protons from photo acid generators. With the help of the polyethylene glycol and CNTs, protons can diffuse across the film and cause the doping of polyaniline emeraldine base into a conductive polyaniline salt state to increase the conductivity.

Theoretical investigation of magnetoelectric surface acoustic wave characteristics of ZnO/Metglas layered composite

The surface acoustic wave properties of piezoelectric/magnetostrictive layered structures consisting of insulating ZnO and metallic Metglas with giant Δ E effect were studied based on a stable scattering matrix method. Only the first Rayleigh mode was found with phase velocity between 2200 m/s and 2650 m/s, and the maximum electro-mechanical coupling coefficient about 1%. It was found that the center frequency of ZnO/Metglas is highly sensitive on the change of magnetic field, up to 440 MHz/Oe. However, there is a cutoff Young’s modulus of Metglas for different designs of SAW, below which the Rayleigh mode will disappear. For a magnetoelectric SAW design with the center frequency of 335 MHz and covering a full magnetic field range from -1.4 to +1.4 Oe, the frequency sensitivity is 212 MHz/Oe, equivalent to a magnetic field sensitivity of 5 × 10−12 Tesla. Unlike conventional magnetoelectric bulk laminates or film stacks, the detection of frequency shift instead of electrical charge allows not only shrinkag...

A wireless smart UV accumulation patch based on conductive polymer and CNT composites

Accumulative ultraviolet (UV) radiation from sunlight could be harmful to human skin and a reliable detection scheme is desirable as part of the protection strategy. This article reports the fabrication of an all passive, battery-free, irreversible, wireless smart UV patch for recording the amount of UV energy accumulation by resistance change. An ultrahigh sensitivity, with 5 orders of magnitude in resistance drop within 2 hours of exposure to sunlight, has been successfully demonstrated. The photo-induced protonation of a conductive polymer is assisted by carbon nanotube (CNT) composites for improved detection sensitivity. More importantly, the whole sensor has been successfully fabricated and optimized on a patterned flexible substrate using a laser printing technique for the electrodes and a subsequent dip-coating method has been utilized to deposit the sensing composite. As such, this novel UV accumulation sensor along with its proposed sensing scheme could be further developed for potential commercial applications.

Self-assembled 1-3D Pb(ZrTi)O3-NiFe2O4 magnetoelectric nanocomposite films with narrow FMR linewidth

Ferrites are critical for nonreciprocal RF/microwave devices. In the current work, epitaxial NiFe2O4 nanowires were self-assembled in Pb(ZrTi)O3 matrix on a (001)-oriented MgAl2O4 single-crystal substrate by a 90° off-axis RF magnetron sputtering method. The influences of substrate temperature, argon-over-oxygen ratio and RF power on the growth of NiFe2O4 nanowires have been addressed. Owing to the excellent lattice match between the NFO phase and the PZT matrix, very low ferromagnetic resonance (FMR) linewidth of 139 Oe was obtained at 8.969 GHz, even better than the single-layer epitaxial NiFe2O4 film, therefore making them suitable for RF/microwave nonreciprocal devices.

Tailoring of the soft magnetic property and uniaxial anisotropy of magnetostrictive films by interlayer

Laminated amorphous FeSiBC films with various spacer layers, including Cu, Co0.45Cu0.55, Co0.8Cu0.2, and CoFe, were prepared in order to study the effect of interface structure and magnetic exchange interaction on the magnetic softness and uniaxial anisotropy of multilayered film. It is found that laminating FeSiBC film with thin nonmagnetic or weak magnetic spacers yields much lower coercivity and higher remanent magnetization than those with magnetic spacers. Optimal films with the desired properties of Hc ∼ 1.5 Oe, Mr/Ms = 95%, and Hk ∼ 16 Oe were obtained. Therefore, it is confirmed that the exchange interaction constant of spacer layer plays a more important role than that of interface structure. Furthermore, laminating FeSiBC with nonmagnetic layers only slightly changes magnetostrictive coefficient.