Jyh Shen Tsay

Magnetic phase diagram of ultrathin Co/Si(111) film studied by surface magneto-optic Kerr effect

A magnetic phase diagram of the ultrathin Co/Si(111) film deposited at 300 K has been established by the surface magneto-optic Kerr effect technique. The temperature, where ferromagnetism vanishes, increases from 375 to 625 K as the coverage of the Co film increases from 3.5 to 16 monolayers. A quantitative calculation of the normalized Auger signal of CoSi2 shows that the calculated values lie between the experimental measured Auger signals before and after ferromagnetism vanishes for films with coverage between 3.5 and 9.1 monolayers. For samples with higher coverage, the experimental data are smaller than that by calculation. This may be qualitatively explained by Co atoms escaping from the CoSi2 phase to diffuse into the Si substrate. The disappearance of ferromagnetism is mainly attributed to silicide formation.

Microscopic interfacial structures and magnetic properties of ultrathin Co∕Si(111) films

The relation between magnetic properties and microscopic structure for a metal/semiconductor system is described. Cobalt films on a CoSi interface possess an in-plane easy axis of magnetization as the result of magnetocrystalline anisotropy of the Co∕CoSi interface. On a Si(111)-7×7 surface, direct evidence for the formation of CoSi2 compounds at the interface was found by the appearance of doubled spot defects in scanning tunneling microscopic images. The interfacial effects cause the easy axis of magnetization of a Co∕Si interface to be canted out of plane.

Diffusion and alloy formation of Co ultrathin films on Pt(111)

The kinetics of the interdiffusion of Co adatoms on a Pt(111) surface was studied by Auger electron spectroscopy at various temperatures with Co coverage near 1 ML. After annealing, the Co concentration decreases dramatically before it approaches a constant value. The Co–Pt alloy is confined in the few topmost layers. Activation energies and pre‐exponential diffusion coefficients are obtained from an Arrhenius plot. The values are 0.90 eV and 6.6×10−11 cm2/s, respectively. They are independent on the coverage in this ultrathin system. After annealing the 1 ML Co ultrathin film on a Pt(111) surface at 600 K, Co atoms diffuse into the first two monolayers only. Both the topmost and the second layers contain 50% of Co atoms. In addition, ultraviolet photoelectron spectroscopy was used to monitor the alloy formation of Co/Pt(111) in the annealing process.

Structure evolution for annealing Co ultrathin films on Pt(111)

Abstract Low-energy electron diffraction (LEED), Auger electron spectroscopy (AES) and ultraviolet photelectron spectroscopy (UPS) were used to study the thermal evolution of the structure of cobalt ultrathin films on a Pt(111) surface at different coverages. The phase diagram of surface structures was obtained from LEED observation. Several structural phases of this system were observed, i.e. pseudo-(1 × 1), incoherent epitaxy, extra p(2 × 2), rotational incoherent epitaxy, and alloying (1 × 1) structures. The coverage and temperature ranges were examined for each structure. Alloy formation was verified by AES. The changes in valence-band structures during phase transitions were studied by UPS.

Rotated incommensurate domains of Co ultrathin films on Pt(111)

Abstract Low energy electron diffraction (LEED) and Auger electron spectroscopy were used to study the initial growth of cobalt ultrathin films on a Pt(111) surface at room and low temperatures. The films show incoherent epitaxy at room temperature. Unrotated and rotated incommensurate Co domains with two equivalent angles of rotation, +4.9° and −4.9°, with respect to the aligned substrate, are observed by LEED for 2 monolayers of Co Pt (111) after applying an annealing treatment. From a calculation of the mismatch for the corrugated surface, we confirm this reorientation angle. The evolution of the LEED satellite pattern for the deposition at 140 K is the same as for the deposition at room temperature, but a faint (2 × 2) LEED pattern was observed for higher coverages. UV photoelectron spectroscopy was used to monitor the evolution of the density of electronic states during deposition.

Spin reorientation transitions and structures of electrodeposited Ni/Cu(100) ultrathin films with and without Pb additives

Magnetic properties and surface structures of Ni/Cu(100) ultrathin films are studied by means of magneto-optical Kerr effect and in situ scanning tunneling microscopy in combination with cyclic voltammetry. At the initial stage of Ni deposition on a Cu(100) electrode, nickel atoms attach onto the steps and the surface shows single atomic steps corresponding to a layer-by-layer growth. For thicker Ni/Cu(100) films, nanometer-size clusters are randomly distributed on the surface showing a three-dimensional island growth. For thinner Ni layers in the coherent region, the magnetic anisotropy energy of the Cl-electrolyte/Ni interface is small. The reduction of squareness of the hysteresis loops is related to the inhomogeneous growth of the Ni layers. For thicker Ni layers in the incoherent region, the negative value of interface anisotropy for the Cl-electrolyte/Ni interface has a strong impact on perpendicular magnetic anisotropy and plays an important role on the reduction of the Ni thickness for spin reorientation transition in the electrolyte condition. By adding Pb additives, the deposition of a Pb wetting layer causes a defaceting phenomenon and the hydrogen evolution reaction is reduced. As the Ni thickness increases, the growth of Ni changes from layer-by-layer to quasi-two-dimensional islands with a flat top layer. With a Pb additive, the spin reorientation transitions of the Ni/Cu(100) system are not significantly influenced. However, due to the change of the growth mode by Pb atoms as a surfactant, the squareness of the hysteresis loops is enhanced for all the Ni thicknesses.

Cr-Doped ZnO Prepared by Electrochemical Deposition

This study demonstrated the preparation of a Cr-doped ZnO wurtzite structure without any impurity phases (metallic Zn, Cr, Zn(OH) 2 , ZnCrO 4 , etc.) via electrodeposition. The surface morphology, lattice structure, Cr content, chemical binding characteristics, and optical properties of the deposits were examined by field-emission-scanning electron microscopy, X-ray diffraction, inductive coupled plasma mass spectroscopy, X-ray photoelectron spectroscopy, UV-visible spectroscopy, and photoluminescence, respectively. Cr-doped ZnO in the shape of hexangular columns appears when the applied potential is equal to or more positive than -1.2 V SSCE . The thickness of the deposits was within the range of 1.07-2.25 μm. Cr was in its trivalent state in the ZnO lattice. Both the high concentration of Cr ions in baths and the more negative applied potential impede the formation of the ZnO(002) plane. The redshift of the bandgap of the deposits from 3.31 to 3.18 eV occurs after the introduction of Cr impurity into the ZnO lattice. The photoluminescence results show both UV and visible light emissions from the electrodeposited specimens.

Magnetic properties and microstructure of ultrathin Co∕Si(111) films

The magnetic properties and microstructure of ultrathin Co films grown on a Si(111)-7×7 surface were investigated. The experimental results observed by surface magneto-optic Kerr effect (SMOKE) and scanning tunneling microscopy show that the surface morphological evolution of x ML (monolayer) Co∕Si(111) films is strongly related to their magnetic properties. Due to the formation of a CoSi2 layer, no magnetic signal could be detected by SMOKE for x=2.1. Both longitudinal and polar hysteresis loops appear for 4.2–8.5 ML Co∕Si(111) films because of their rougher surfaces. When the Co thickness is increased to 11 ML, a magnetic hysteresis loop only occurs in the longitudinal configuration, which can be attributed to the contribution of volume anisotropy. After annealing an 11 ML Co∕Si(111) film at 400 and 500K, the surface becomes rougher, inducing magnetic anisotropy on the polar configuration. When the annealing temperature was increased to 600K, however, the Co could react with Si to form a nonmagnetic cob...

Magnetic properties of ultrathin Co/Ge (111) film with oxygen surfactant

Magnetic properties of ultrathin Co∕Ge(111) films with oxygen surfactant have been investigated using surface magneto-optic Kerr effect technique. As the oxygen exposure increases, their magnetic properties could be significantly modified. As the thickness of Co films increases to above 6 ML (monolayer), pure cobalt islands start to accumulate on the surface and the amount of oxygen on the surface layers increases with increasing oxygen exposure time. Series experiments of different sequences of oxygen exposure and Co deposition have been performed. From the results of slight chemical shift and depth profiling measurements, one can conclude that oxygen plays a role as a surfactant. The adsorbed oxygen influences the electronic density of states of Co and leads to the changes of the magnetic properties. The appearance of the O∕Co∕Ge interface could modify the stress anisotropy, and as a result the coercivity of 30 ML Co∕Ge(111) increases from 730to810Oe with 500L of oxygen exposure.

Giant enhancement of magneto-optical response and increase in perpendicular magnetic anisotropy of ultrathin Co/Pt(111) films upon thermal annealing

The annealing effects on the magnetic properties, crystallographic structure, and alloy formation, were studied for Co/Pt (111) ultrathin films at coverages up to 6.6 ML, using in situ magneto-optical Kerr effect, low energy electron diffraction, Auger electron spectroscopy, and ultraviolet photoelectron spectroscopy. After the postdeposition annealing in the temperature range of 500–800 K, a significant increase in perpendicular magnetic anisotropy at high coverages and a large enhancement of magneto-optical response with the value about 200%–300% of that before annealing for all coverages investigated are observed. Both findings are shown to be correlated to the formation of a kind of Co–Pt interface (surface) alloy. This is expected to be mainly attributed to the transfer of strong spin-orbit coupling of the Pt into the Co magnetic orbital due to the hybridization during interface alloy formation.