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24.03.2012

Nature Materials | Article


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





Journal name:

Nature Materials

Volume:

11,

Pages:

323–328

Year published:

(2012)

DOI:

doi:10.1038/nmat3236


Received


Accepted


Published online


 





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.




Figures at a glance


left


  1. Figure 1: Previously published phase diagram and crystal structure for (Sr,Ca)2RuO4.


    a, Superconducting Tcas a function of Ti content y in Sr2(Ru1−yTiy)O4 (top axis) and residual in-plane resistivity ρab0 (bottom axis)15; the inset depicts the layered perovskite structure analogous to the cuprate superconductor (La,Ba)2CuO4. b, Previously published phase diagram of Ca2−xSrxRuO4 based on magnetic susceptibility measurements, depicting the superconducting (SC) state for x = 2, paramagnetic insulator (P-I) and commensurate antiferromagnetic insulator (CAF-I) behaviour at small x, a glassy magnetic metallic (M-M) state at slightly higher x inferred from a peak in d.c. magnetization, and the ‘cluster-glass’ (C-G) region associated with the history dependence of d.c.-magnetization around x = 0.5. For 0.5<x<2.0, the ground state was presumed to be a paramagnetic metal (P-M). The in-plane conductivity and specific heat in this region exhibit Fermi liquid behaviour below TFL (refs 8, 9, 10). T0 denotes temperature for structural phase transition. c, Temperature dependence of the c-axis resistivity ρc(T) normalized to the room-temperature value, for several (Ca, Sr) systems (blue), (Ru, Ti) systems (red), and for pure Sr2RuO4 (green) plotted on the basis of published results8, 16. Red (y = 0.09,0.05) and blue arrows (x = 1.5) indicate the magnetic ordering temperature determined by μSR and magnetic susceptibility. The upturn of ρc at low temperatures can be associated with static magnetic order, whereas superconducting Sr2RuO4exhibits a sharp reduction of ρc at low temperatures.





  2. Figure 2: μSR time spectra.


    af, ZF- μSR time spectra observed in Ca2−xSrxRuO4 systems with x = 0.5 (a), x = 0.9 (b), x = 1.5 (c) and x = 0.3 (e), and in Sr2Ru1−yTiyO4 systems with y = 0.09 (d) and y = 0.03 (f). g,h, LF- μSR time spectra in the (Ca, Sr) system with x = 1.5(g) and the (Ru, Ti) system with y = 0.09 (h). The solid lines in a, c and d represent fits to the zero-field muon spin relaxation function obtained for dilute-alloy spin glasses, equation (1) (ref. 20), and the solid lines in b, e and f represent fits to a root-exponential decay20.





  3. Figure 3: Present results of μSR and χ compared with published neutron results.


    a,b, Generalized total muon spin relaxation rate observed in (Ca,Sr)2RuO4 systems (a) and in Sr2(Ru,Ti)O4 systems (b), which exhibit an increase with decreasing temperature due to slowing down of dynamic spin fluctuations above the ordering temperature TN and a subsequent build-up of static local field below TN. c, Inverse of the magnetic susceptibility χ observed in a low field (100 G) in a select subset of the (Ca, Sr) and (Ru, Ti) systems. d, Muon spin relaxation rate 1/T1 observed in LF- μSR with a moderate strength of longitudinal field in a few select systems, which exhibit clear peaks indicating onset of static magnetic order. e, Overlay of the Bragg peak intensity IB observed in elastic neutron scattering at an incommensurate wavevector (0.3, 0.3, 0) and the μSR relaxation rate Λ(T) observed in Sr2Ru0.91Ti0.09O4 (ref. 17). A linear comparison, expected for spatially inhomogeneous spin systems, shows better scaling than a quadratic comparison. The apparent difference in onset temperature of the increase is probably due to the difference in the time windows of neutron and muon measurements. f, Imaginary part of a.c.-magnetic susceptibility Imχa.c.(T) in Sr1.5Ca0.5RuO4 and Sr2Ru0.91Ti0.09O4 measured with the a.c. field perpendicular to the ab-plane.






Figure 4: New magnetic phase diagram and ordered moment size.



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.








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