Microstructural evolution and correlated magnetic domain configuration of nanoparticles embedded in a single crystal of Cu₇₅-Ni₂₀-Fe₅ alloy

We have investigated the microstructural evolution and magnetic domain configurations in nano-scale Fe-Ni rich precipitates formed in a single-crystal specimen of Cu-20at%Ni-5at%Fe alloy on isothermal annealing at 873 K and 973 K, using a combination of transmission electron microscopy (TEM), electron backscattering diffraction, field-emission scanning electron microscopy (FE-SEM), and laser-based photoemission electron microscopy (laser-PEEM). The TEM and FE-SEM observations showed that small, spherical solute-rich particles formed randomly in the initial stage of the precipitation, but on isothermal annealing, cubic, rectangular, plate-shaped and rod-shaped precipitates appeared and aligned along the <1 0 0 > directions in the copper-rich matrix. Laser-PEEM was applied to single-crystal specimens of the alloy and allowed direct observations of magnetic domain configurations in individual ferromagnetic particles at the nanometer scale. This revealed that cubic particles of size approximately 50-60 nm consist of single magnetic domains, but particles of size 100 nm have a closed spin structure (e.g. vortex or multiple domains).