Microstructure and properties of giant magneto-resistant Au 80 Co 20 , Au 80 Co 10 Fe 10 , Cu 70 Ni 25 Fe 4 Mn and Cu 53 Ni 31 Fe 15 Mn

The past decade a vast amount of studies were devoted to Giant-Magneto-Resistive (GMR) materials. Most materials investigated were composed of a multi-layered structure, but also granular systems received a lot of attention. The general concept of most studies consisted of a combination of magneto-resistance and magnetization measurements for various temperatures in-between liquid He and room temperature to characterize the materials synthesized. Most micro-structural characteristics were deduced from the magnetization measurements and only scarcely a single (HR)TEM image was added to the work. The starting point of our work is the investigation of the microstructure of the materials by means of HRTEM and Analytical TEM (ATEM). Metallurgical routes for producing GMR materials have two main advantages over the commonly used sputter-deposition methods: (i) it is a more easy and a much cheaper route to produce large quantities of material due to its self-assembling nature and (ii) bulk GMR materials are obtained instead of only thin films. Of course also clear disadvantages exist of which probably the most dominant one is the large magnetic field needed to induce significant GMR effects. From a more fundamental point of view metallurgical routes are of interest for GMR since a wide variety of structures can be prepared: magnetic precipitates can be formed in a non-magnetic matrix by either nucleation and growth or by spinodal decomposition. Such precipitates have a certain shape, an average size, a size distribution and a distribution in separation distance between neighboring precipitates. The interfaces between the particles and the matrix can be either sharp or diffuse (in case of spinodal decomposition) and such interfaces can be coherent, semi-coherent or incoherent. All these factors may influence GMR, but not much fundamental knowledge of the precise role they play is available. A second interesting microstructural feature occurs if the composition of the alloy is changed such that the magnetic phase is brought from below to above the percolation limit. Up to now we studied 4 alloys: Au80Co20, Au80Co10Fe10, Cu70Ni25Fe4Mn, Cu53Ni31Fe15Mn. The first 3 are below and the 4 alloy is above the percolation limit of the magnetic phase. The last two alloys are known to exhibit spinodal decomposition where the last one will show decomposition waves along the ,100. crystal directions [1]. If in a single crystal this wave can be directed along only one of the ,100. (e.g. by applying a uniaxial strain) a bulk multi-layered structure is prepared by self-assembly. In the present paper some selected results obtained for the 4 alloys will be presented. Scripta mater. 44 (2001) 1461–1464