Preparation and characterisation of magnetoresistive materials

This thesis is a study of the structural, magnetic and electrical properties of giant magnetoresistive (GMR) and related materials. Magnetoresistance is the phenomenon whereby the electrical resistance of a sample changes upon the application of a magnetic field. The research described here concentrates particularly on 'granular' GMR materials, which comprise grains of a magnetic element or alloy embedded in a non-magnetic metallic matrix. Materials were made either in powder form by mechanical alloying, or as thin films by means of pulsed laser deposition. They were characterised using a variety of techniques including Mossbauer spectroscopy and x-ray diffraction. The main investigations were centred on two ternary systems: one containing iron, copper and silver, and the other iron, cobalt and silver. In each case a series of samples was prepared and the systematic change in properties with composition was analysed. In the ternary Fe-Cu-Ag mechanically alloyed system, the resulting structure was found to be highly dependent on composition. Under equilibrium conditions the three elements are immiscible, but with mechanical alloying it was possible to produce both single phase crystalline and structurally disordered alloys. All alloys were metastable and decomposed gradually on heating. The Fe-Co-Ag system is different in that iron and cobalt are naturally miscible. In this case the structure comprised Fe-Co grains embedded in a silver matrix, thus producing a promising material for GMR applications. The variation in structure and magnetoresistance with composition and preparation conditions was investigated, both for the mechanically alloyed powders and for thin films produced by pulsed laser deposition. The largest MR for the mechanically alloyed samples was 8.2% in 9 T at 10 K in (Fe0.15Co0.85)30Ag70, but larger values of up to 41% at 4.2 K and 4.7 T were obtained in Co30Ag70 thin films.