R. C. Doole

Spectroscopic electron tomography.

The elemental mapping techniques in analytical transmission electron microscopy (TEM), energy filtered imaging (EFTEM) and EDX-mapping, are shown to provide new routes for tomographic reconstructions of 3D chemical maps on the nanoscale. The inelastic scattering does not only provide chemical sensitivity but also improves the linear projection relationship between mass density and image intensity, which often fails in bright field TEM of crystalline materials due to diffraction contrast. Instrumental requirements and artefact sources within the contrast formation mechanisms and within the numerical reconstruction are assessed.

Determination of the isothermal nucleation and growth parameters for the crystallization of thin Ge2Sb2Te5 films

The isothermal crystallization of thin amorphous Ge2Sb2Te5 films, sandwiched between Si3N4 dielectric layers, was followed in real time using in situ transmission electron microscopy. A temperature-dependent incubation time is observed. After this incubation time, the crystallization is found to follow Johnson–Mehl–Avrami–Kolmogorov (JMAK) transformation kinetics. The JMAK parameters were determined, as well as the individual nucleation and growth parameters. The relationships between the JMAK parameters and the nucleation and growth parameters were tested and found to be valid. Nucleation was found to occur at the interfaces with the dielectric layers. The average grain size after crystallization did not show appreciable temperature dependence in the temperature range investigated.

Structural studies of Ag nanocrystals embedded in amorphous Al2O3 grown by pulsed laser deposition

Thin films consisting of layers of Ag nanocrystals (NCs) embedded in amorphous Al2O3 were grown by pulsed laser deposition. High-resolution electron microscopy was used to characterize the structure of the films. The growth kinetics of the NCs were studied by varying the Ag content of the films between 0.8 and 12.4×1015 atoms cm-2 which produced NCs with average diameters of between 1.1 and 9.6 nm. At low Ag content the NCs have a spherical in-plane shape with a narrow size distribution but they become more elongated with a broader size distribution as the Ag content increases due to coarsening and coalescence of the NCs. Underneath each layer of NCs there is a continuous layer of what is believed to be Ag implanted into the amorphous Al2O3 due to the high kinetic energy, of the order of 100 eV, of the Ag species produced during laser ablation.

IN SITU STUDIES OF THE CRYSTALLIZATION KINETICS IN SB-GE FILMS

The crystallization process in SbxGe1−x alloy films has been observed during in situ annealing in a transmission electron microscope. Results are presented for two films with x=0.89 (89 at. % Sb) and x=0.71 (71 at. % Sb), which lie on either side of the eutectic composition (x=0.85). In the former films radial crystals are observed to grow rapidly from discrete nuclei, whereas in the latter films the crystallization process occurs through a near‐planar front. In addition, quantitative data obtained from these experiments show that the Sb0.89Ge0.11 films have a higher activation energy for crystal growth and a lower temperature for the nucleation of crystals. Significant differences are observed between the crystallization processes for the two films studied, with the Sb0.89Ge0.11 film showing better potential for development as an ultrafast optical phase‐change storage medium.

Magnetic behavior of Fe:Al2O3 nanocomposite films produced by pulsed laser deposition

Structured nanocomposite films consisting of five Fe layers embedded in an amorphous Al2O3 matrix (Fe:Al2O3) have been grown by sequential pulsed laser deposition. The formation of well isolated quasispherical nanocrystals is observed for samples with Fe content per layer close to 6.5×1015 atoms/cm2. Increasing the Fe content leads first to the formation of elongated nanocrystals and then to quasicontinuous layers. The evolution in the shape and size of the nanocrystals is reflected in the magnetic behavior of these systems. A crossover from a low temperature ferromagnetic regime to a high temperature superparamagnetic regime is observed at a temperature of 23 K in the samples containing isolated quasi-spherical nanocrystals. In this case, a reduced moment per Fe atom (1.4 μB/atom) with respect to the value for α-Fe (2.2 μB/atom) is estimated. This behavior is attributed to the presence of a Fe-oxide surface shell on the nanocrystals. The large values of the estimated effective magnetic anisotropy (1.4×10...

THICKNESS AND GRAIN-SIZE DEPENDENCE OF THE COERCIVITY IN PERMALLOY THIN FILMS

This paper reports the effect of thickness and grain size on the coercivity of Ni81Fe19 permalloy thin films (2.5–30 nm) sputtered on glass substrates for their application in magnetoresistive sensors. Coercivity was systematically investigated as a function of underlayer materials, thickness, and substrate temperature. Lateral grain size of the sputtered films was investigated. The grain size reduced very quickly in the thinner films. It was also found that the coercivity of the films with very small lateral grain size is much lower than those with a normal grain size. The lowest coercivity (Hc=0.8 Oe) was observed in 7.5 nm thick film and having a grain size of 4 nm. When an underlayer is used, its crystallinity affects the lateral grain size in the permalloy and correlates with the observed coercivity variation [P. Galtier, R. Jerome, and T. Valet, Mater. Res. Soc. Symp. Proc. 313, 417 (1993).] It was also observed that the coercivity of the permalloy is dependent on the nature of the underlayer. It wa...

Improved Foucault imaging of magnetic domains with a modified 400 kV transmission electron microscope

Modifications have been made to a 400 kV side‐entry transmission electron microscope fitted with a low‐field objective pole piece, in order to position apertures close to the back focal plane of the objective lens, as there is no direct access to the required position in the column. The modifications have facilitated developments in the imaging of magnetic domain structure in magnetic materials using the Foucault technique, for which the correct positioning of the objective aperture is crucial. All usual transmission electron microscopy facilities are retained, allowing a full range of specimen holders and imaging modes to be used along with TV recording and electron energy loss spectroscopy. Some initial results are presented from Fe/Cr and Co/Pt multilayer films, for which the Lorentz deflection angle is very small, and for which the Fresnel imaging mode is of limited use as the grain structure contrast masks the magnetic contrast. Initial results are also presented from NdFeB permanent magnet material ...

Studies on the effect of Cobalt and Titanium substitution in NdFeB thin films for magnetic recording media

We have investigated as series of substituted NdFeB thin films (NdFeBX where X = Co, Cu, Ti, Zr partially replacing Fe) of different thickness. The perpendicular coercivity has a maximum of 2700 Oe for a thickness of 200 nm after annealing at 625°C for 15 mins. It is found that coercivity can be controlled by varying the thickness of the films. We have observed that the grain size is controlled to less than 25 nm in the substituted films.

The effects of annealing on magnetic domain structure and interface profile in sputtered Fe/Cr multilayer films

Abstract Changes in the microstructure and magnetic domain structure of sputtered Fe/Cr multilayers were recorded during in situ heating in an electron microscope. The films showed different behaviour for different Cr spacer layer thicknesses. Three-dimensional chemical information from position sensitive atom probe microanalysis showed improved Fe/Cr interface smoothness after annealing.

Influence of the relaxation state on the crystallization kinetics of Sb-rich SbGe amorphous films

Abstract The influence of the stress relaxation state on the crystallization process has been studied in amorphous Sb0.87Ge0.13 films. Relaxed amorphous areas produced by laser irradiation have been crystallized by isothermal annealing during in situ observation in a transmission electron microscope. The results show that the relaxed material crystallizes at a lower temperature than the as-grown material, and has a higher crystal growth velocity and a lower activation energy for crystal growth. The differences in the crystallization kinetics are in the nucleation process rather than in the growth process.