Structure and Giant Magneto-Resistive Properties of Co and CoFe nano-particles in a Au matrix
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Structure and Giant Magneto-Resistive Properties of Co and CoFe nano-particles in a Au matrix B.J. Kooi, T. Vystavel, J.Th.M. De Hosson Department of Applied Physics, Materials Science Center and the Netherlands Institute for Metals Research, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
ABSTRACT This paper focuses on the relation between the microstructure and the Giant MagnetoResistive (GMR) properties of Au80Co20 and Au80Co10Fe10 granular alloys. After annealing of quenched samples large differences in structure of Co and CoFe nano-particles arise: e.g. fcc Co versus bcc CoFe, spinodal decomposition of Co versus nucleation and growth of FeCo, truncated octahedrons for Co and plates for CoFe. Particularly the shape of the precipitates and the state of coherency at the interface between magnetic particle and non-magnetic matrix turned out to have important influences on the GMR behavior. INTRODUCTION The discovery of the Giant-Magneto-Resistive (GMR) phenomenon in 1988 in Fe-Cr multilayers [1] has given an impetus to research for specific multi-layered structure. In addition, granular systems received also considerable attention since 1992 [2,3]. Most studies consisted of a combination of magneto-electrical resistance and magnetization measurements at various temperatures ranging between liquid He and room temperature to characterize the materials synthesized. Although GMR is to a large extent an interface effect, most micro-structural characteristics were deduced from the magnetization measurements and only scarcely a single high-resolution transmission electron micrograph (HRTEM) was added to the work. In contrast, our starting point is the investigation of the microstructure of granular systems by means of HRTEM so as to provide a more direct link between structure and GMR properties of various alloys. Here we report studies of two different granular alloys: Au80Co20, Au80Co10Fe10.
EXPERIMENTAL An arc furnace was used to prepare basic alloys from their pure components. The buttons were homogenized and cold-rolled to thin foils (thickness between 20 and 100 µm). Solidsolution annealing with subsequent water-quenching was used to arrive at alloys showing the least decomposition of the magnetic and non-magnetic phase as possible. For the Au80Co20 alloy it was shown that this procedure was superior to melt spinning [4]. Finally, with heat treatments at relatively low temperatures the decomposition between the magnetic and non-magnetic phase was invoked leading to suitable nano-structures. For HRTEM a JEOL 4000 EX/II and a JEOL 2010F equipped with X-Ray Energy Dispersive Spectrometry and a Gatan Imaging Filter were used. To control the GMR measurements, a Quantum Design Physical Property Measurement System (PPMS) Q-6000 was used. An AC Electrical resistance Bridge by Linear Research (LR-700) was used to determine the electrical Y8.2.1
resistance of the samples at several temperatures between 10 and 300 K for magnetic fields up to 50 or 70 kOe. Some magnetization measurements were perfo
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