arXiv subjects

C. K. Ong

Publications and source records attributed to C. K. Ong.

31 records · Page 2Linked to original sources

arXiv2003

Improvement of dielectric loss of doped Ba0.5Sr0.5TiO3 thin films for tunable microwave devices

Al2O3-Ba0.5Sr0.5TiO3 (Al2O3-BST) thin films, with different Al2O3 contents, were deposited on (100) LaAlO3 substrate by pulsed laser deposition (PLD) technique. The Al2O3-BST films was demosnstrated to be a suitable systems to fabricate ferroelectric thin films with low dielectric loss and higher figure of merit for tunable microwave devices. Pure BST thin films were also fabricated for comparison purpose. The films' structure and morphology were analyzed by X-ray diffractiopn and scanning electron microscopy, respectively; nad showed that the surface roughness for the Al2O3-BST films increased with the Al2O3 content. Apart from that, the broadening in the intensity peak in XRD result indicating the grain size of the Al2O3-BST films reduced with the increasing of Al2O3 dopant. We measured the dielctric properties of Al2O3-BST films with a home-made non-destructive dual resonator method at frequency ~ 7.7 GHZ. The effect of doped Al2O3 into BST thin films significantly reduced the dielectric constant, dielectric loss and tunability compare to pure BST thin film. Our result shows the figure of merit (K), used to compare the films with varied dielectric properties, increased with the Al2O3 content. Therefore Al2O3-BST films show the potential to be exploited in tunable microwave devices.

cond-mat.mtrl-sci↗

arXiv2003

Amplification of evanescent waves in a lossy left-handed material slab

We carry out finite-difference time-domain (FDTD) simulations, with a specially-designed boundary condition, on pure evanescent waves interacting with a lossy left-handed material (LHM) slab. Our results provide the first full-wave numerical evidence for the amplification of evanescent waves inside a LHM slab of finite absorption. The amplification is due to the interactions between the evanescent waves and the coupled surface polaritons at the two surfaces of the LHM slab and the physical process can be described by a simple model.

cond-mat.mtrl-sci↗

arXiv2003

Structural and electrical properties of tantalum nitride thin films fabricated by using reactive radio-frequency magnetron sputtering

TaN thin film is an attractive interlayer as well as a diffusion barrier layer in [FeN/TaN](n) multilayers for the application as potential write-head materials in high-density magnetic recording. We synthesized two series of TaN films on glass and Si substrates by using reactive radio-frequency sputtering under 5-mtorr Ar/N-2 processing pressure with varied N-2 partial pressure, and carried out systematic characterization analyses of the films. We observed clear changes of phases in the films from metallic bcc Ta to a mixture of bcc Ta(N) and hexagonal Ta2N, then sequentially to fcc TaN and a mixture of TaN with N-rich phases when the N2 partial pressure increased from 0.0% to 30%. The changes were associated with changes in the grain shapes as well as in the preferred crystalline orientation of the films from bcc Ta [100] to [110], then to random and finally to fcc TaN [111], correspondingly. They were also associated with a change in film resistivity from metallic to semiconductor-like behavior in the range of 77-295 K. The films showed a typical polycrystalline textured structure with small, crystallized domains and irregular grain shapes. Clear preferred (111) stacks parallel to the substrate surface with embedded amorphous regions were observed in the film. TaN film with [ 111]-preferred orientation and a resistivity of 6.0 m Omega cm was obtained at 25% N-2 partial pressure, which may be suitable for the interlayer in [FeN/TaN](n) multilayers.

cond-mat.mtrl-sci↗

arXiv2003

Magnetic anisotropy and magnetoresistance of sputtered [(FeTaN)/(TaN)](n) multilayers

We studied the in-plane magnetic anisotropy of rf (radio frequency) sputtered [(FeTaN)/(TaN)](n) multilayers synthesized on Si substrates. In the multilayers where n=5, the FeTaN thickness is fixed at 30 nm and the thickness of TaN, t(TaN), is varied from 0 to 6.0 nm, we observed a clear trend that, with increasing t(TaN), the values of coercivity, grain size, and amplitude of maximum magnetoresistance (MR) of the samples all decrease first and then increase after reaching a minimum when t(TaN) is around 2.0-4.0 nm. This trend is also associated with an evolution of in-plane magnetic anisotropy, where the multilayers change from uniaxial anisotropy to biaxial at t(TaN) around 4.0 nm and above. We attribute the phenomena to the interlayer coupling effect of FeTaN films as a function of the coupling layer (TaN) thickness, rather than to the thickness dependence observed in single-layered FeTaN films, where the direction of easy axis switches 90degrees when the film is thicker than 300 nm. The in-plane anisotropy of the [(FeTaN)/(TaN)](n) multilayers also shows signs of oscillation when the number of coupling layers varies. The MR effects observed are mainly due to anisotropy MR (AMR), while the grain size and exchange coupling may also contribute to the change of maximum MR ratios in the multilayers with changing t(TaN).

cond-mat.mtrl-sci↗

arXiv2003

Evolution of In-Plane Magnetic Anisotropy In Sputtered FeTaN/TaN/FeTaN Sandwich Films

FeTaN/TaN/FeTaN sandwich films, FeTaN/TaN and TaN/FeTaN bilayers were synthesized by using RF magnetron sputtering. The magnetic properties, crystalline structures, microstructures and surface morphologies of the as-deposited samples were characterized using angle-resolved M-H loop tracer, VSM, XRD, TEM, AES and AFM. An evolution of the in-plane anisotropy was observed with the changing thickness of the nonmagnetic TaN interlayer in the FeTaN/TaN/FeTaN sandwiches, such as the easy-hard axis switching and the appearing of biaxial anisotropy. It is ascribed to three possible mechanisms, which are interlayer magnetic coupling, stress, and interface roughness, respectively. Interlayer coupling and stress anisotropies may be the major reasons to cause the easy-hard axis switching in the sandwiches. Whereas, magnetostatic and interface anisotropies may be the major reasons to cause biaxial anisotropy in the sandwiches, in which magnetostatic anisotropy is the dominant one.

cond-mat.mtrl-sci↗

arXiv2003

Pulsed Laser Deposition of LaAlO3-Ba0.5Sr0.5TiO3 Thin Films for Tunable Device Applications

LaAlO3-Ba0.5Sr0.5TiO3 (LAO-BST) thin films, with different LAO contents, were deposited on Pt/Ti/SiO2/Si and (100) LaAlO3 substrate, via a pulsed laser deposition (PLD). Phase composition and microstructure of the LAO-BST thin films were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The LAO-BST thin films were solid solution of LAO nd BST, with lattice constant decreasing with increasing LAO content. The composition of (LAO)x(BST)1-x, estimated from the lattice constant of the thin films derived from the targets of 1/6, 2/6 and 3/6 LAO, was 0.143, 0.278, 0.476, respectively. The grain size of the thin films decreased as a result of the incorporation of LAO into BST. Dielectric properties of the LAO-BST thin films on Pt/Ti/SiO2/Si were measured at low frequency (100 kHz), while those on LaAlO3 substrate were characterized at high frequency (~7.9 GHz). The dielectric constants of the film derived from pure BST target and those from the targets with 1/6, 2/6 and 3/6 area ratio of LAO, on Pt/Ti/SiO2/Si substrate, were 772, 514, 395 and 282, with a dielectric loss of 0.096, 0.023, 0.024 and 0.025, and a dielectric tunability of 65%, 53%, 43% and 14%, respectively. The dielectric constant of the LAO-BST films on LaAlO3 substrate were 2436, 2148, 2018, 1805, with a maximum dielectric tunability of 11%, 13%, 10% and 8% at a maximum applied voltage of 2.4 kV (~9.2 kV/cm).

cond-mat.mtrl-sci↗

arXiv2002

Magneto- and Electrotransport Properties of Nd0.67Sr0.33Mn1-xFexO3 (x = 0.00, 0.05)

Simultaneous studies on the magnetic and electrotransport properties of Nd_{0.67} Sr_{0.33} Mn_{1-x} Fe_x O_3 polycrystalline bulk and epitaxial thin films (x = 0.00 and 0.05) have been carried out. A magnetoresistance (MR) as high as ~ 33% is observed for Nd_{0.67} Sr_{0.33} MnO_3 bulk at the metal-insulator transition (MIT) temperature (T_p) of 273 K in a magnetic field (H) of 10 kOe. Fe substitution at Mn sites reduces the ferromagnetic (FM) ordering temperature (T_c) and leads to an overall increase in MR. MR as high as ~ 90% is observed for thin film (x = 0.05) at H = 10 kOe with T_p = 100 K while the corresponding bulk has a MR of only ~ 43%. Low field MR in epitaxial thin films also display a pronounced anisotropy effect compared to bulk materials.

cond-mat.mtrl-sci↗

arXiv2002

Determinations of upper critical field in the c-a plane

Within a continuous Ginzburg-Landau model for layered superconductors, two procedures are proposed to determine the upper critical field parallel to the $c$-$a$ crystal plane with an angle $θ$ tilted from the c-axis. For an intrinsically layered superconductor, the upper critical fields for $θ\to 90^\circ$, as determined by the two procedures, are consistent with each other and in reasonably good agreement with those determined by similar procedures suited for the parallel upper critical field ($θ= 90^\circ$). The profile of the order parameter obtained at $B_{c2}$ is Gaussian-like, indicating the plausibility of the procedures proposed.

cond-mat.supr-con↗

arXiv2002

Properties of parallel upper critical field within Continuous Ginzburg-Landau model

In this paper, we employ a continuous Ginzburg-Landau model to study the behaviors of the parallel upper critical field of an intrinsically-layered superconductor. Near $T_c$ where the order parameter is nearly homogeneous, the parallel upper critical field is found to vary as $(1-T/T_c)^{1/2}$. With a well-localized order parameter, the same field temperature dependence holds over the whole temperature range. The profile of the order parameter at the parallel upper critical field may be of a Gaussian type, which is consistent with the usual linear Ginzburg-Landau theory. In addition, the influences of the unit cell dimension and the average effective masses on the parallel upper critical field and the associated order parameter are also addressed.

cond-mat.supr-con↗

arXiv2001

Reversible Structural Transition in Epitaxial Manganite Film

A reversible structural transition of an epitaxial La2/3Sr1/3MnO3 film deposited on the LaAlO3 substrate has been investigated by means of in situ high-resolution transmission electron microscopy (HREM) and electron diffraction, combined with image and diffraction calculations. We observe that the crystallographic symmetry of the film can be lowered via electron beam irradiation, leading to a rhombohedral-monoclinic transition. This transition can be attributed to the cooperating effect of the mismatch stress and the irradiation-induced thermal-stress.

cond-mat.str-el↗

arXiv2000

Q-Factor Measurement of Nonlinear Superconducting Resonators

A novel method, which combined a multi-bandwidth measurement and an extrapolation procedure, is proposed for extracting the loaded Q-factor (Q_{L}) with improved accuracy from non-Lorentzian resonances of nonlinear superconducting resonators.

cond-mat.supr-con↗

arXiv2000

Hysteresis measurement of anomalous microwave surface resistance in superconducting thin films

The anomalous decrease in microwave surface resistance, R_{s}, of superconducting YBa_{2}Cu_{3}O_{7-d} (YBCO) thin films in the presence of a low dc magnetic field is studied using a microstrip resonator technique. We have done a dc field hysteresis measurement of R_{s} to study the effects of vortex penetration on the anomalous effect. It is shown that the anomaly happens at a field level far below the low critical field, H_{c1,strip}, of the superconducting microstrip and vortex (Abrikosov) penetration would eliminate the anomalous effect observed at low field. This implies that the anomalous effect is not contributed by vortices.

cond-mat.supr-con↗

arXiv1999

Anomalous microwave response of high-temperature superconducting thin-film microstrip resonator in weak dc magnetic fields

We have studied an anomalous microwave (mw) response of superconducting YBa_{2}Cu_{3}O_{7-delta} (YBCO) microstrip resonators in the presence of a weak dc magnetic field, H_{dc}. The surface resistance (R_{s}) and reactance (X_{s}) show a correlated non-monotonic behaviour as a function of H_{dc}. R_{s} and X_{s} were found to initially decrease with elevated H_{dc} and then increase after H_{dc} reaches a crossover field, H_{c}, which is independent of the amplitude and frequency of the input mw signal within the measurements. The frequency dependence of R_{s} is almost linear at fixed H_{dc} with different magnitudes ( H_{c}). The impedance plane analysis demonstrates that r_{H}, which is defined as the ratio of the change in R_{s}(H_{dc}) and that in X_{s}(H_{dc}), is about 0.6 at H_{dc} H_{c}. The H_{dc} dependence of the surface impedance is qualitatively independent of the orientation of H_{dc}.

cond-mat.supr-con↗