Ferrromagnetic microstructure on metal and its characterization for magnetoresistance
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Transition-metal granular solids:
We have systematically investigated the magnetotransport properties of five binary metallic systems (Co-Ag, Fe-Ag, Fe-Cu, Fe-Au, and Fe-Pt). Except for Fe-Pt, which forms a solid-solution in sputtering synthesis, all other systems exhibit giant magnetoresistance (GMR) effects and are immiscible or almost immiscible alloys. Using a combination of structural analysis involving transmission electron microscopy, x-ray diffraction, and magnetic characterization, we have revealed the microstructure for some of the granular alloys where the magnetic particles are embedded in a nonmagnetic matrix. Across the whole particle volume fraction range (0≤ x≤ 100%), a universal peak of GMR at x= 0.15∼ 0.25 is uncovered in every granular system. The suppression of GMR in the high-x region is attributed to the percolation effect, which also causes a concurrent reduction in magnetic coercivity. Magnetic particle size, a variable under thermal annealing, is another important factor affecting GMR. We have observed correlations between transport properties and particle size, supporting the view that magnetic scattering occurs predominantly on the surface of the particles. Quantitatively, both the field and temperature dependence of the GMR effect can be well accounted for by a modified exchange interaction model. Spin-wave excitation and electron-phonon interaction at finite temperatures are detrimental to GMR. Superparamagnetism and large magnetic anisotropy impede the saturation process of GMR, but have no effect on the saturated value of GMR.