Competition between antiferromagnetism and ferromagnetism in Sr2RuO4 probed by Mn and Co doping.

Spin-triplet superconductivity in Sr2RuO4 has attracted enormous interest. Like other unconventional superconductors, superconductivity in Sr2RuO4 is in close proximity to magnetic instability. Undoped Sr2RuO4 exhibits incommensurate antiferromagnetic (AFM) fluctuations, which can evolve into static, short-range AFM order via Ti doping. Moreover, weak ferromagnetic (FM) coupling in Sr2RuO4 has also been suggested by NMR/neutron scattering experiments and studies on Ca2-xSrxRuO4 and Sr2-yLayRuO4, implying orbital dependent magnetism. We report bulk static, short-range FM order in Sr2RuO4 triggered by <2% Co doping, showing superconductivity in Sr2RuO4 is much closer to FM instability than previously reported in Ca2-xSrxRuO4. We also find Mn doping can effectively establish incommensurate AFM order, with TN ~ 50 K for 3% Mn doping. These new results place Sr2RuO4 in a unique situation where superconductivity lies directly on the borderline of two distinct magnetic states, highlighting the important role of competing magnetic fluctuations in determining superconducting properties of Sr2RuO4.