  In this study, a preparation route of Co-Cu alloys with soft magnetic
properties by high-pressure torsion deformation is introduced. Nanocrystalline,
supersaturated single-phase microstructures are obtained after deformation of
Co-Cu alloys, which are prepared from an initial powder mixture with
Co-contents above 70 wt.%. Isochronal annealing treatments up to 400{\deg}C
further reveal a remarkable microstructural stability. Only at 600{\deg}C, the
supersaturated phase decomposes into two fcc-phases. The coercivity, measured
by SQUID as a function of annealing temperature, remains significantly below
the value for bulk-Co in all states investigated. In order to understand the
measured magnetic properties in detail, a quantitative analysis of the magnetic
microstructure is carried out by magnetic force microscopy and correlated to
the observed changes in coercivity. Our results show that the rising coercivity
can be explained by a magnetic hardening effect occurring in context with
spinodal decomposition.
