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Electrodeposited Fe-Co-Ni alloys are promising candidates for the writer in ultrahigh-density magnetic recording systems. In this paper, the mechanical properties of these materials have been studied by stress measurements as a function of temperature and nanoindentation. The alloy films (∼1-μm-thick) were electrodeposited onto Cu/Ti/Si substrates. Three distinct structural regions were observed: FCC, mixed FCC+BCC, and BCC phases as Fe content in the alloy increased. The internal stress of Fe-Co-Ni films increases with decreasing current density, as reported for Ni-Fe films. The hardness of the as-deposited films was around 4.8 GPa; after annealing to 400°C hardness was reduced to 3.6 GPa. The coefficient of thermal expansion CTE was ∼10-11 × 10-6°C-1, comparable to typical values of bulk Ni78Fe22 (CTE -12 × 10-1°C-1). Reduced Young's modulus E/(1 - ν2) changes from 150 to 180 Gpa, corresponding to the structural change from BCC to FCC. The ratio H/E is smaller than other materials investigated for write heads, indicating a lower resistance to deformation. H/E further decreases after annealing.