Atom Probe Studies of Interfaces in Metallic Multilayers
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ATOM PROBE STUDIES OF INTERFACES IN METALLIC MULTILAYERS ALFRED CEREZO, JONATHON M. HYDE, MARK G. HETHERINGTONt AND AMANDA K. PETFORD-LONG Department of Materials, University of Oxford, Parks Road, Oxford OXI 3PH, U.K. ABSTRACT The atom probe field-ion microscope has been used to study the diffusion at interfaces in metallic multilayers deposited directly onto field-ion specimens and to develop models for the solid state reactions occuring at the atomic-scale in multilayer systems. Results are presented for the low temperature annealing of a Co-Ni multilayer. Intermixing over about 2 atomic planes is found even in as-deposited samples, extending to mnm after heating at 300'C for 1 hour. Using atom probe results from bulk alloys, a Monte Carlo simulation has been developed for the Fe-Cr system, in which a miscibility gap exists, and is being used in an attempt to model the behaviour of interfaces in Fe-Cr multilayers. Preliminary results are presented, showing that interfaces which are initially mixed over 10 atomic planes become sharper by an 'interface spinodal' reaction. INTRODUCTION Multilayer films (MLF) are an exciting new class of materials, in which composite films are produced by alternate deposition of two different elements or compounds. The electronic and mechanical effects which arise in the multilayer configuration can produce a material with properties not found in bulk materials. For example, MLF with suitable magnetic properties are being developed for applications as permanent magnet materials and high density magnetic storage media. In all applications for which MLF are being considered, the parameter with the greatest influence on the properties of interest is the quality of the interfaces between adjacent layers [1], and even atomic-scale variations between specimens are likely to result in important differences in final properties. The ultra-high spatial resolution of the atom probe field-ion microscope [2, 31 provides a powerful method by which information can be gained on the chemistry of interfaces in MLF. In this technique, field evaporation is used to remove atoms from the surface of a specimen in the form of a sharp needle point, with end-radius about 100nm. The evaporated atoms from a region about 2nm in diameter on the specimen surface pass through a selection aperture into a time-of-flight mass spectrometer. The high field sensitivity of field evaporation makes this technique highly surface sensitive, resulting in a composition-depth profile with 2nm lateral resolution, and atomic layer depth resolution. In the more recently developed position-sensitive atom probe (POSAP) [4, 51, both the chemical identity and source position is obtained for atoms field-evaporated from a region 20nm in diameter on the specimen surface. Although the technique has lower mass resolution than the conventional atom probe, it is capable of reconstructing, in three dimensions, the chemical variations originally present in the sample, and so permitting analysis of the interface structure with sub-nanometre (bu
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