Colloque C8, Supplkment au n0ll, Tome 50, novembre 1989. ATOM PROBE CHARACTERISATION OF Co/TRANSITION-METAL MULTILAYER. STRUCTURES.
COLLOQUE DE PHYSIQUE Colloque C8, Supplkment au n0ll, Tome 50, novembre 1989
ATOM PROBE CHARACTERISATION OF Co/TRANSITION-METAL MULTILAYER STRUCTURES A. CEREZO, M.G.
HETHERINGTON and A.K.
PETFORD-LONG
Department of Metallurgy and Science of Materials, University of Oxford, Parks Road, GB-Oxford OX1 3PH, Great-Britain
Abstract Cobaltltmnsition-metal multilayer structures grown on COfield-ion specimens have been analysed by both conventional atom probe (AP) and the position-sensitive atom probe (POSAP). The combination of the two techniques allows the chemical abruptness and the roughness of the interfaces to be studied independently, and can be supported by high resolution transmission electron microscope (HREM) studies of multilayers deposited simultaneously on flat substrates. Layered structures can be formed with sharp interfaces, with epitaxial growth having been observed, even when using only a simple thermal evaporation source. However, growth by this method seems to be somewhat inconsistent, with carbon contamination leading to the formation of interdiffused, and in the extreme even amorphous layers. With the use of electron beam sources in place of filament evaporators, this technique should provide a powerful complement to the more conventional characterisation methods.
1 - INTRODUCTION
The ability to tailor electronic properties in semiconductor multiple quantum well structures is well known. In metallic multilayer structures, with periods down to atomic dimensions, there exists the possibility of controlling magnetic and mechanical properties by suitable selection of the layer widths [l]. Of particular interest are the composite magnetic materials made up of Cdtransition-metal multilayers, which have potential applications in the field of data storage and recording. Due to the size of these structures, chemical and morphological effects on the atomic scale are likely to be of great importance in determining the magnetic properties which are observed. While structural and magnetic information on a fine scale can be obtained using electron microscopy techniques, the high-resolution analytical capability of the atom probe provides a means to gain unambiguous information on interface chemical abruptness (as distinct from any roughness that might exist) and possible grain boundary segregation effects. Metallic multilayers composed of up to hundreds of layers are typically grown using either electron beam evaporation sources, or by sputter deposition. Whilst the use of electron beam evaporators in our experiments is planned, we wished to carry out a preliminary study using simpler thermal evaporation sources. Cobalt field-ion specimens prepared by electropolishing in standard perchloric acid solutions were imaged in a VG FIMlOO atom probe and field evaporated to give a smooth endform. After imaging, each specimen was withdrawn to the auxiliary chamber of the instrument, and allowed to warm to room temperature before deposition. In order to reduce possible sources of contamination, the specimens were allowed only the minimum warm-up time necessary, found to be 15 minutes, before the growth of the multilayers. Deposition was from conventional tungsten evaporator filaments loaded with metal in the form of wire (Fe, Ni, CO)or flakes (Cr) and was monitored usinga quartz crystal oscillator. The filament assembly in the auxiliary chamber was placed below and somewhat in front of the specimen (which was rotated during the deposition) and at a distance of IOcm. Base pressure in the chamber was
