The metastable magnetic alloy Co10Cu90has been prepared by the conventional melt-spinning technique. The rapid solidification process results in an extended solubility of Co in Cu although some Co particles already have precipitated during quenching. In comparison with conventional ageing methods, Joule heating could also be adopted to fabricate granular Co-Cu alloys with even finer Co particles. The finest size of 4.0nm has been achieved for Co10Cu90alloy after annealing with a currentI= 4 A for 1 rnin. Single-domain behaviour results in a value of coercivity in excess of 40.0 kAm−1for the sample annealed withI= 9.5 A. Below the size of 6.0 nm, Co particles exhibit superparamagnetic behaviour at room temperature, which has been investigated using a superconducting quantum interference device magnetometer. A unique change inMswith annealing current or temperature was discovered.