New magnetic phase diagram of (Sr,Ca)2RuO4

High- T c cuprates, iron pnictides, organic BEDT and TMTSF, alkali-doped C 60 , and heavy-fermion systems have superconducting states adjacent to competing states exhibiting static antiferromagnetic or spin density wave order. This feature has promoted pictures for their superconducting pairing mediated by spin fluctuations. Sr 2 RuO 4 is another unconventional superconductor which almost certainly has a p-wave pairing. The absence of known signatures of static magnetism in the Sr-rich side of the (Ca, Sr) substitution space, however, has led to a prevailing view that the superconducting state in Sr 2 RuO 4 emerges from a surrounding Fermi-liquid metallic state. Using muon spin relaxation and magnetic susceptibility measurements, we demonstrate here that (Sr,Ca) 2 RuO 4 has a ground state with static magnetic order over nearly the entire range of (Ca, Sr) substitution, with spin-glass behaviour in Sr 1.5 Ca 0.5 RuO 4 and Ca 1.5 Sr 0.5 RuO 4 . The resulting new magnetic phase diagram establishes the proximity of superconductivity in Sr 2 RuO 4 to competing static magnetic order. In most unconventional superconductors, the superconducting phase is adjacent to a phase with some type of magnetic order. However, this is not a universal feature. For example, no magnetic order has so far been observed in Sr 2 RuO 4 . Now, low-energy muon relaxation experiments show the presence of a static magnetic order for this material, suggesting that this feature may in fact be universal.