Coexistence of superconductivity and ferromagnetism in the d-band metal ZrZn2

It has generally been believed that, within the context of the Bardeen-Coop er-Schrieffer (BCS) theory of superconductivity, the conduction electrons i n a metal cannot be both ferromagnetically ordered and superconducting(1,2) . Even when the superconductivity has been interpreted as arising from magn etic mediation of the paired electrons, it was thought that the superconduc ting state occurs in the paramagnetic phase(3,4). Here we report the observ ation of superconductivity in the ferromagnetically ordered phase of the d- electron compound ZrZn2. The specific heat anomaly associated with the supe rconducting transition in this material appears to be absent, and the super conducting state is very sensitive to defects, occurring only in very pure samples. Under hydrostatic pressure superconductivity and ferromagnetism di sappear at the same pressure, so the ferromagnetic state appears to be a pr erequisite for superconductivity. When combined with the recent observation of superconductivity in UGe2 (ref. 4), our results suggest that metallic f erromagnets may universally become superconducting when the magnetization i s small.