Structure Of Co Films Research Articles

Tuning the anisotropy and domain structure of Co films by variable growth conditions and seed layers

Co thin films on Si (1 0 0) substrates with different seed layers (Au or AlOX) have been deposited for variable speed of substrate rotation (Rsub) at room temperature (RT) or 200 °C. Hysteresis measurements by magneto optic Kerr effect reveals that the films show growth induced uniaxial anisotropy when prepared without the substrate rotation (i.e. Rsub = 0 rpm). The films prepared at RT show a decrease but non-vanishing uniaxial anisotropy with increase in Rsub. However, high temperature deposition along with substrate rotation leads to formation of isotropic films. The change in anisotropy has been quantitatively investigated using ferromagnetic resonance (FMR) spectroscopy. Further, domain images observed by Kerr microscopy corroborate to the change in anisotropy behavior. We find that by choosing the right combination of seed layer as well as the growth parameters, the anisotropy can be systematically tuned.

Structure and atomic interactions at theCo∕Pd(001)interface: Surface x-ray diffraction and atomic-scale simulations

Using surface x-ray diffraction and atomic-scale simulations we have studied the structure of Co films deposited on Pd(001) at room temperature by thermal (TD) and pulsed laser deposition (PLD). While for TD we find epitaxial growth of Co without sizable amount of intermixing, for PLD films substantial alloying is observed. Ab initio calculations indicate an oscillatory Co-Co interaction in the Pd layers, which is repulsive for nearest neighbors and attractive for next-nearest neighbors.

Magnetic properties of Co nanocolumns fabricated by oblique-angle deposition

The magnetic properties of columnar Co films on SiO2 substrates fabricated by oblique-angle incident thermal evaporation at room temperature were systematically examined by multiple techniques, including magnetic force microscopy (MFM), magneto-optical Kerr effect (MOKE), and scanning electron microscopy (SEM). Films with thickness ranging from 50 to 500 nm were deposited at the incident angles θ (with respect to substrate normal) from 0° to 85°. For films with thickness of ∼500 nm, the SEM shows the column tilt angle β increases as the θ angle increases and β<θ. The MFM images show that for 55°<θ<75°, stripe domains are formed and are nearly parallel to the direction of incident vapor beam. The hysteresis loops obtained from MOKE show that along the direction perpendicular to the incident vapor beam the coercivity Hc stays almost constant for all θ angles and the squareness decreases as the θ increases. This is in contrast to the increase of Hc and the increase of squareness in the direction parallel to the incident vapor beam for θ>60° . The result implies that for θ>60°, the axis parallel to the incident beam behaves more like the in-plane easy axis. These magnetic anisotropies are correlated to the angular-dependent columnar structure of Co films.

Influence of Au and Cu overlayers on the magnetic structure of Co films on W(110)

The dependence of the angular distribution of the magnetization in ultrathin epitaxial Co layers upon the thickness of Au and Cu overlayers is studied in situ by spin-polarized low-energy electron microscopy in the thickness range from 0 to 3 monolayers. Only Cu overlayers cause a peak in the coverage dependence of the perpendicular magnetic anisotropy at 1.5 ML coverage. The relation between the nanostructure of the overlayers and the anisotropy is discussed.

Effects of substrate temperature and ion irradiation on the formation and structure of Co films on NaCl(100)

The effects of substrate temperature and ion irradiation on the formation and the structure of thin Co films during vacuum deposition onto air-cleaved NaCl(100) single crystal surfaces were investigated using transmission electron microscopy (TEM). Both rise in the substrate temperature and the ion irradiation, caused considerable changes in average grain size, structure and orientation of the Co films. In particular, a well pronounced cubic (fcc) phase was established from ion irradiation, whereas the physical vapour deposited room temperature Co films without ion irradiation show a hexagonal (hcp) structure. The effects of ion irradiation during the growth of Co films on NaCl(100) substrate at room temperature were found to be identical to the effects of raising the substrate temperature to the range of 80°C–90°C during the physical vapour deposition.

Effect of surface free energy of underlayer materials on crystal growth of Co polycrystalline films

Crystallographic texture of polycrystalline Co films prepared by sputtering on a variety of underlayers has been investigated. The texture of polycrystalline Co films greatly depends on underlayer material, and this dependence is well explained by taking into account the surface free energy difference between the underlayer material (γu) and Co (γCo). When γu is larger than γCo, Co films can be grown with a well-defined crystallographic texture of hcp-Co(001), which is the lowest surface energy crystal plane in order to minimize the total surface energy. In contrast, if γu is much smaller than γCo, no remarkable underlayer effects can be observed in the crystallographic structure of Co films.

The structure of Co films on Cu(111) up to 15 ML

New structural results of the epitaxial growth of Co Cu(111) in the coverage regime 1.5–15 ML are presented as investigated by quantitative LEED and STM. In the low coverage regime there is pure fcc stacking with copper diffused on top of 30% of the domains. The growth is ideally pseudomorphic. At intermediate coverages hcp stacked domains develop on the cost of fcc domains whereby the latter seem to be stabilized by their top copper layer. At high coverages the film is predominantly hcp exhibiting the ideal lattice parameter of Co(0001).

Growth of Co films on Cu(111) studied in real space.

The atomic structure of Co films deposited on Cu(111) at room temperature is studied by imaging backscattered secondary electrons. The patterns show in-registry growth for the first two atomic layers. The next two monolayers show features of the fcc as well as the hcp structure. The hexagonal structure characteristic for bulk Co is found for subsequent layers.

Influence of crystal structure on the magnetoresistance of Co/Cr multilayers

Epitaxial Co/Cr multilayers, and single-crystal Co thin films etc. have been grown on MgO and Al2O3 substrates with Cr and Mo as buffer layers by molecular beam epitaxy technique. From the structure and magnetoresistance studies, we have found that the ferromagnetic anisotropy of resistance (AMR) is strongly influenced by the buffer layer, but with negligible effect due to the variation of the structure of Co films. The AMR of Co film on Cr buffer layer is quite small (0.1%); however, the MR of Co/Cr multilayers is almost one order larger than the AMR of Co film on Cr buffer layer. An enhancement factor of 4 for the MR in Co/Cr multilayers by the interface roughness has been observed. This suggests that the effect due to the spin dependent scattering at the interfacial regions of the superlattice is larger than that due to the spin dependent scattering in the ferromagnetic layers for the MR in the Co/Cr multilayer system.

Structure of Co films grown on Cu(111) studied by photoelectron diffraction.

The structure of epitaxial cobalt on Cu(111) single-crystal surfaces was studied by angle-resolved x-ray-photoelectron diffraction and low-energy electron diffraction. We find that initial growth of Co is in the metastable fcc phase. The first cobalt monolayer occupies the substrate fcc continuation sites and grows as a flat layer coherent to the substrate lattice. The fcc stacking sequence continues for two-layer-thick cobalt films. As the film thickness increases beyond two layers, hcp stacking faults occur as domains in mixed fcc-hcp layers. The transition from the metastable fcc stacking sequence to the bulk hcp(0001) structure is not abrupt, but occurs gradually as the film thickness increases.

Structure determination of metastable cobalt films.

The bulk atomic structure of cobalt films deposited on GaAs has been unequivocally determined to be the body-centered-cubic metastable phase. A comparison of the conversion-electron x-ray-absorption fine structure of Co films deposited on GaAs(110) with similar measurements for the stable phases of hcp Co, fcc Cu, and bcc Fe establishes the Co bcc phase with a lattice constant of 2.82\ifmmode\pm\else\textpm\fi{}0.01 A\r{}. Furthermore, this first application of the technique to single-crystal epitaxial films shows it to be a most powerful procedure for establishing their crystal structure.