Structural and magnetic properties of Co-C composite films and Co/C multilayer films

CoC composite films and Co/C multilayer films have been prepared by a method incorporating ion beam sputtering and plasma chemical vapor deposition. It has been found that the structure and magnetic properties of both the Co-C composite and the Co/C multilayer films depend strongly on the substrate temperature during deposition. The Co-C composite film deposited at room temperature is amorphous, with relatively low saturation magnetization and coercivity. On the other hand, the film deposited at 250 °C is composed of fine Co crystallites separated by amorphous C or Co-C phase. As a result, both the saturation magnetization and coercivity are increased compared with the film deposited at room temperature. When deposited at room temperature, the Co/C multilayer film exhibits good periodicity, with a period of 70 nm (Co: 40 nm, C: 30 nm) and sharp and flat Co-C interfaces. High magnetization (602 emu/cm³) and low coercivity (1.6 Oe) are obtained for such a film. However, increasing the substrate temperature to 250 °C was found to be detrimental to the magnetic properties due to the formation of cobalt carbide at the Co-C interface. Received: 11 July 2000 / Accepted: 13 July 2000 / Published online: 30 November 2000     

Investigation of ripple structures of thin polycrystalline Co films by magnetic force microscopy

Received: 14 August 1997/Accepted: 14 August 1997     

Surface electronic structure of Gd(0001) films on W(110)

Received: 1 September 1996/Accepted: 6 January 1997     

Structure and magnetic properties of Co nanoclusters fabricated by ion beam synthesis in SiO2 films

Co nanoparticles fabricated by ion beam synthesis in SiO₂ films were investigated with transmission electron microscopy and superconducting quantum interference device technique. Variation of the thermal treatment enables the formation of Co nanoclusters of different sizes ranging from 2 to 40 nm. Small nanoclusters of about 2–3 nm are amorphous, whereas clusters above 7 nm show the configuration of cubic Co nanocrystals. Measurements of magnetisation at temperatures between 2 K and 360 K reveal superparamagnetic behaviour for the small nanoclusters up to 3 nm and ferromagnetism for clusters above 7 nm. Received: 12 February 2001 / Accepted: 3 May 2001 / Published online: 27 June 2001     

In plane to out of plane magnetic reorientation transition in partially ordered FePd thin films

It is demonstrated that perpendicular magnetic anisotropy may be obtained with a room temperature growth process in ordered (FePd) alloys. Indeed, using atomic layer by atomic layer epitaxy, a partial chemical ordering into the L1₀ structure is obtained, with a corresponding intermediate perpendicular anisotropy (). These samples provide an appropriate template for the study of the magnetic reorientation from in plane to out of plane magnetization upon layer's thickness increase. VSM, transverse Kerr measurements and magnetic force microscopy have been used in order to determine the relevant magnetic parameters and the occurrence of the reorientation transition. Received 13 October 1998 and Received in final form 5 February 1999     

Simultaneous observation of atomic step and domain wall structure of ultrathin Co films by magnetic force microscopy in ultrahigh vacuum

We have applied magnetic force microscopy in ultrahigh vacuum to study the correlation between the atomic step and magnetic domain wall structure of ultrathin Co films prepared in situ on Au(111) substrates. For the first time we were able to achieve high-resolution images showing simultaneously a clear domain wall contrast and the underlying atomic step structure. Although for in-plane magnetized Co films the domain walls were found to run preferentially in a direction perpendicular to the steps, no such correlation could be observed for out-of-plane magnetized Co films. Received: 3 June 1999 / Accepted: 4 June 1999 / Published online: 29 July 1999     

Self-organized semiconductor surfaces as templates for nanostructured magnetic thin films

Spontaneous pattern formation during epitaxial growth or ion erosion of semiconductor wafers offers an elegant route towards large-area nanostructured surfaces. In homoepitaxy, kinetics may result in rather uniform three-dimensional islands. In the case of semiconductor heteroepitaxy, strain relief leads to the formation of nanofaceted three-dimensional crystallites, which may self-organize into quasiperiodic arrays. By tuning substrate miscut and film thickness, or growing superlattices, a variety of patterns with different symmetries can be obtained, as will be summarized for the model system of SiGe on Si(001). Since these self-organized nanostructure arrays cover the entire wafer on which they are grown, they can serve as large-area nanopatterned substrates for subsequent deposition of magnetic thin films. It will be demonstrated that such templates allow the study of correlations between magnetic and chemical interfacial roughness, as well as the influence of pattern symmetry on the magnetic anisotropy of thin Co films. Furthermore, shadow deposition of magnetic material onto specially faceted nanostructure arrays allows the fabrication of nanomagnet arrays and the study of their magnetic properties. Received: 31 July 2002 / Accepted: 2 October 2002 / Published online: 5 February 2003 RID="*" ID="*"Corresponding author. Fax: +43-3842/402-760, E-mail: teichert@unileoben.ac.at     

Layer-resolved microscopy of magnetic domains in multi-layered systems

Photoelectron emission microscopy in connection with magnetic circular dichroism in soft X-ray absorption can be used for the microscopic imaging of magnetic domains in layered thin film structures consisting of several magnetic layers. Due to the element-selectivity of the method, the different magnetic layers in such a structure can be imaged separately, provided that they contain different elements. This has been applied for the investigation of Co/Cu/Ni trilayers, epitaxially grown on Cu (001). The magnetic coupling between the Co and Ni layers can be directly visualized from comparing layer-resolved magnetic domain images of both layers. As a consequence of the competition between the anisotropy energies of the two magnetic layers and the magnetic coupling energy, spin-reorientation transitions between collinear and non-collinear magnetic configurations are observed. Apart from this globally observable magnetic interlayer coupling a micromagnetic coupling mechanism is also evident from the layer-resolved domain images. It is caused by magnetostatic interaction of local stray fields from domain walls. Received: 22 August 2002 / Accepted: 2 October 2002 / Published online: 5 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-345/5511-223, E-mail: kuch@mpi-halle.de     

A relation between a metallic film covering on diamond formed during growth and nanosized inclusions in HPHT as-grown diamond single crystals

One of the most important characteristics and basic phenomena during diamond growth from liquid metal catalyst solutions saturated with carbon at high temperature–high pressure (HPHT) is that there exists a thin metallic film covering on the growing diamond, through which carbon-atom clusters are delivered to the surface of the growing diamond by diffusion. A study of microstructures of such a metallic film and a relation between the thin metallic film and the inclusions trapped in HPHT as-grown diamond single crystals may be helpful to obtain high-purity diamond single crystals. It was found that both the metallic film and the HPHT as-grown diamond single crystals contain some nanostructured regions. Examination by transmission electron microscopy suggests that the microstructure of the thin metallic film is in accordance with nanosized particles contained in HPHT as-grown diamond single crystals. The nanosized particles with several to several tens of nanometers in dimension distribute homogeneously in the metallic film and in the diamond matrix. Generally, the size of the particles in the thin metallic film is relatively larger than that within the diamond matrix. Selected area electron diffraction patterns suggest that the nanosized particles in the metallic film and nanometer inclusions within the diamond are mainly composed of f.c.c. (FeNi)₂₃C₆, hexagonal graphite and cubic γ-(FeNi). The formation of the nanosized inclusions within the diamond single crystals is thought not only to relate to the growth process and rapid quenching from high temperature after diamond synthesis, but also to be associated with large amounts of defects in the diamond, because the free energy in these defect areas is very high. The critical size of carbide, γ-(FeNi)and graphite particles within the diamond matrix should decrease and not increase according to thermodynamic theory during quenching from HPHT to room temperature and ambient pressure. Received: 13 September 2001 / Accepted: 12 June 2002 / Published online: 17 December 2002 RID="*" ID="*"Corresponding author. Fax: +86-0531/295-5081; E-mail: yinlw@sdu.edu.cn     

Effect of Electrochemical Treatment in a Lithium Chloride Solution on Field Emission from Carbon Nanotubes

Carbon nanotubes （CNTs） are electrochemically treated in a lithium chloride solution at a concentration 0. 1 mol/L The field emission properties of the CNTs are investigated at different temperatures before and after the electrochemical treatment. After treatment, the turn-on voltage to produce field emission current of 10μA decreases from 4.2kV to 2. 7kV and the field emission current increases distinctly, but the stability falls off. Based on the Fowler-Nordheim plot, the values of the work function for the CNTs are calculated, which reveals that work function decreases after the electrochemical treatment. These results are attributed to the decrease of the work function of the carbon nanotubes.     

Magnetic domain structures and magnetotransport properties in Co-Ag granular thin films

The magnetic microstructures and magnetotransport properties in granular Co_xAg_1-x films with 17%≤x≤62% were studied. Magnetic force microscopy (MFM) observations showed the presence of magnetic stripe domains in as-deposited samples with x≥45% and the evolution of the magnetic domain patterns to in-plane domains with annealing. A perpendicular magnetic anisotropy as high as about 8×10⁵ ergs/cc for as-deposited Co₆₂Ag₃₈ and about 6×10⁵ ergs/cc for as-deposited Co₄₅Ag₅₅ was observed by magnetization and torque measurements. With increasing annealing temperature, the perpendicular magnetic anisotropy became negative. The origin of the perpendicular magnetic anisotropy may be attributed to a rhombohedral distortion of the cubic cell due to residual substrate-film stresses. The magnetic stripe domains are the consequence of the interplay of the indirect or direct exchange, perpendicular magnetic anisotropy and dipolar interactions. Finally, magnetoresistance (MR) curves displayed training behaviours and different shapes when measured with different configurations (parallel, transverse and perpendicular). It is proposed that the existence and the evolution of the magnetic domain structures strongly affect the magnetotransport properties due to the extra contribution of the electron scattering at the domain walls. Furthermore, an anisotropic MR also contributes to the overall MR curves. Received: 2 March 2000 / Accepted: 28 March 2000 / Published online: 23 May 2001     

Study of Fe/Mo multilayer systems by SFM and AES

Received: 13 August 1998 / Published online: 10 February 1999     

The critical thickness of silicon-germanium layers grown by liquid phase epitaxy

Silicon-germanium layers are grown from metallic solution on (100) and (111) silicon substrates. On (111) Si, coherently strained dislocation-free SiGe layers are obtained with thicknesses larger than predicted by the current models of misfit-induced strain relaxation. A comprehensive characterisation by imaging, diffraction, and analytical electron microscopy techniques is carried out to determine the critical thickness, study the onset of plastic relaxation, and explain the particular growth mechanisms leading to an unexpectedly high thickness of elastically strained SiGe layers. A vertical Ge concentration gradient and the formation of step edges on the layers, where lateral strain relaxes locally, explain the high critical thickness. The model of Matthews and Blakeslee is modified in order to match the experimental observations for solution-grown SiGe layers. Received: 29 July 1999 / Accepted: 29 July 1999 / Published online: 27 October 1999     

Formation and crystal structure of metallic inclusions in a HPHT as-grown diamond single crystal

One of the most important characteristics associated with crystal growth technology is the entrapment of inclusions by the growing crystal. Diamond single crystals prepared under high temperature-high pressure (HPHT) usually contain metallic inclusions. In the present paper, metallic inclusions in a diamond grown from a Fe-Ni-C system using the HPHT method have been, for the first time, systematically examined by transmission electron microscopy (TEM). Energy dispersive X-ray spectrometry (EDS) , combined with selected area electron diffraction (SAD) patterns, has been used to identify the chemical composition and crystal structure of the metallic inclusions. The metallic inclusions were found to be composed mainly of cubic γ-(FeNi), face-centered cubic (FeNi)₂₃C₆, ortho-rhombic Fe₃C and hexagonal Ni₃C, which may have been formed through the entrapment of molten catalyst by the growth front or through reaction of the trapped melt with contaminants in the diamond. Received: 19 June 2000 / Accepted: 21 June 2000 / Published online: 16 August 2000     

Formation of Cu nanoparticles in GaAs using MeV ion implantation followed by thermal annealing

Analytical electron microscopy, high-resolution X-ray diffraction and combined Rutherford backscattering spectrometry and channeling experiments have been used to investigate the radiation damage and the effect of post-implantation annealing on the microstructure of GaAs(100) single crystals implanted with 1.00 MeV Cu⁺ ions to a dose of ≈ 3×10¹⁶ cm^-2 at room temperature. The experiments reveal the formation of a thick and continuous amorphous layer in the as-implanted state. Annealing up to 600 °C for 60 min does not result in the complete recovery of the lattice order. The residual disorder in GaAs has been found to be mostly microtwins and stacking fault bundles. The redistribution of implanted atoms during annealing results in the formation of nano-sized Cu particles in the GaAs matrix. The X-ray diffraction result shows a cube-on-cube orientation of the Cu particles with the GaAs lattice. The depth distribution and size of the Cu particles have been determined from the experimental data. A tentative explanation for these results is presented. Received: 15 February 1999 / Accepted: 18 February 1999 / Published online: 28 April 1999     

Role of buried ultra thin interlayer silicide on the growth of Ni film on Si(100) substrate

The presence of a buried, ultra-thin amorphous interlayer in the interface of room temperature deposited Ni film with a crystalline Si(100) substrate has been observed using cross sectional transmission electron microscopy (XTEM). The electron density of the interlayer silicide is found to be 2.02 e/?³ by specular X-ray reflectivity (XRR) measurements. X-ray diffraction (XRD) is used to investigate the growth of deposited Ni film on the buried ultra-thin silicide layer. The Ni film is found to be highly textured in an Ni(111) plane. The enthalpy of formation of the Ni/Si system is calculated using Miedema’s model to explain the role of amorphous interlayer silicide on the growth of textured Ni film. The local temperature of the interlayer silicide is calculated using enthalpy of formation and the average heat capacity of Ni and Si. The local temperature is around 1042 K if the interlayer compound is Ni₃Si and the local temperature is 1389 K if the interlayer compound is Ni₂Si. The surface mobility of the further deposited Ni atoms is enhanced due to the local temperature rise of the amorphous interlayer and produced highly textured Ni film. Received: 2 March 2000 / Accepted: 28 March 2000 / Published online: 11 May 2000     

Studies of interface structures of W films grown on patterned and bare Si(100) by TEM

+ Si(100) and bare Si(100) wafers by low pressure chemical vapour deposition (LPCVD) at 230–280 °C. The films were investigated by transmission electron microscopy (TEM). The cross-sectional TEM samples of W/Si(100) exhibited a fine scale interface roughness, which was attributed to the surface preparation. Irregular W plug structures were observed depending on the predeposition procedures. It was observed that an insufficient deposition of W films on the contact surface leads to the presence of aluminium around and underneath the plugs. This was observed by energy dispersive X-ray spectrometry (EDX). A study, using conventional electron diffraction, confirmed that no silicides formed at the interfaces of W-bare Si(l00) wafers. Received: 16 December 1996/Accepted: 6 May 1997     

Electrical and magnetic properties of CoFeAlO thin films

Influences of oxygen-partial pressure and annealing on the electrical and magnetic properties of CoFeAlO thin films were systematically investigated by means of resistivity, permeability, magnetization and ferromagnetic resonance (FMR) measurements. It was found that, with increasing oxygen-partial pressure or under annealing, the electrical resistivity of the film increased and the magnetic softness decreased, which is attributed to the microstructural change of the film. Interestingly, an as-deposited Co_45.30Fe_20.65Al_19.34O_14.71 film was found to exhibit an inverted hysteresis loop with negative coercivity, and this peculiar phenomenon disappeared upon effects of oxygen-partial pressure and annealing. It was also found that the as-deposited films owned a narrow FMR line width that increased with increasing oxygen-partial pressure or under annealing.     

Nd composition dependence of microstructure and magnetic properties for gradient sputtered NdFeB films

NdFeB films with Nd compositions varied from 13.34 to 24.30 at% were deposited by DC gradient sputtering using targets Nd_12.5Fe_71.5B₁₆ and Nd. The hard magnetic properties, grain growth direction and magnetic domain structures were dramatically influenced by Nd composition. The samples with intermediate Nd concentrations exhibited optimal magnetic properties and microstructures, such as large squareness ratio over 0.9, large energy product up to 174 kJ/m³, and vertical domain structure. However, the samples with higher and lower Nd compositions showed almost isotropic loops. (0 0 l) as main X-ray diffraction peaks in the optimal Nd composition region indicated most of Nd₂Fe₁₄B grains with c-axis perpendicular to the film plane, while NdFeB grains in other region are almost random growth. The good magnetic properties can be attributed to the vertical growth of Nd₂Fe₁₄B grains.     

Effect of Co underlayer on soft magnetic properties and microstructure of FeCo thin films

High saturation magnetization soft magnetic FeCo (=Fe₆₅Co₃₅) films were prepared using a thin Co underlayer. The FeCo/Co films exhibited a well-defined in-plane uniaxial anisotropy with easy axis coercivity (H_ce) of 10 Oe and hard axis coercivity (H_ch) of 3 Oe, and a half reduction of H_c with H_ce=4.8 Oe and H_ch=1.0 Oe was obtained when the composition was adjusted to 25 at% Co. The effective permeability of the films remains flat around 250 to 800 MHz. The saturation magnetostriction was 5.2×10⁻⁵ and the intrinsic stress was 0.8 GPa in FeCo single layer, both were slightly reduced by Co underlayer. The Co underlayer changed the preferred orientation of the FeCo films from (2 0 0) to (1 1 0) but more significantly, reduced the average grain size from ∼74 to ∼8.2 nm. It also reduced the surface roughness from 2.351 to 0.751 nm. The initial stage and interface diffusion properties were examined by TEM and XPS.