High- Tc cuprates, iron pnictides, organic BEDT and TMTSF, alkali-doped C60, 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. Sr2RuO4 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 Sr2RuO4 emerges from a surrounding Fermi-liquid metallic state. Using muon spin relaxation and magnetic susceptibility measurements, we demonstrate here that (Sr,Ca)2RuO4 has a ground state with static magnetic order over nearly the entire range of (Ca, Sr) substitution, with spin-glass behaviour in Sr1.5Ca0.5RuO4 and Ca1.5Sr0.5RuO4. The resulting new magnetic phase diagram establishes the proximity of superconductivity in Sr2RuO4 to competing static magnetic order.
a, Phase diagrams for (Ca,Sr)2RuO4 and Sr2(Ru,Ti)O4, based on the present results of Λ(T) in ZF- μSR (closed red symbols) and 1/T1 in LF- μSR (open red symbols), and the peak temperature (closed blue symbols) and the irreversibility onset temperature (open blue symbols) of the magnetic susceptibility χ. Static magnetic order develops in the coloured region. The blue diamonds represent the susceptibility results obtained for the present specimens used in μSR, the blue circles denote points from Minakata and Maeno16 and the blue triangles are from Nakatsuji and colleagues10. The slanted-stripe colouring indicates regions involving phase separation (see Supplementary Information SC). SG denotes spin glass and I-SDW indicates incommensurate spin density wave. b, ZF- μSR relaxation rate Λ(T→0) (left axis) and the LF- μSR decoupling field Ho, both of which are indicators of a static random field proportional to the average ordered moment size. A scale for the moment size is given by using the absolute value 0.3 Bohr magnetons per Ru obtained in the y = 0.09 (Ru, Ti) system from the Bragg peak intensity of neutron scattering17 (middle axis). c, Weiss temperatures ΘW (left axis) and the peak susceptibility values (right axis), demonstrating strong antiferromagnetic spin correlations in the Sr-rich side of the (Ca, Sr) system and in the (Ru, Ti) system. A strong tendency towards ferromagnetic correlations can be seen by the peaking of χ and the reduction of ΘW at x = 0.5.