Competition Between Antiferromagnetism and Ferromagnetism in Sr ₂RuO ₄ Probed by Mn and Co Doping

Abstract

Spin-triplet superconductivity in Sr ₂RuO ₄ has attracted enormous interest. Like other unconventional superconductors, superconductivity in Sr ₂RuO ₄ is in close proximity to magnetic instability. Undoped Sr ₂RuO ₄ exhibits incommensurate antiferromagnetic (AFM) fluctuations, which can evolve into static, short-range AFM order via Ti doping. Moreover, weak ferromagnetic (FM) coupling in Sr ₂RuO ₄ has also been suggested by NMR/neutron scattering experiments and studies on Ca _2−xSr _xRuO ₄ and Sr _2−yLa _yRuO ₄, implying orbital dependent magnetism. We report bulk static, short-range FM order in Sr ₂RuO ₄ triggered by <2% Co doping, showing superconductivity in Sr ₂RuO ₄ is much closer to FM instability than previously reported in Ca _2−xSr _xRuO ₄. We also find Mn doping can effectively establish incommensurate AFM order, with T _N ~ 50 K for 3% Mn doping. These new results place Sr ₂RuO ₄ 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 Sr ₂RuO ₄.