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The Magnetic Phase Diagram of (Sr,Ca) 2 (Ru,Ti)O 4 Revealed by m SR

The Magnetic Phase Diagram of (Sr,Ca) 2 (Ru,Ti)O 4 Revealed by m SR. Jeremy P. Carlo jeremy.carlo@nrc.gc.ca. Columbia University Canadian Neutron Beam Centre, National Research Council. June 2, 2010. Outline. Overview Correlated electron materials Magnetic order Superconductors

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The Magnetic Phase Diagram of (Sr,Ca) 2 (Ru,Ti)O 4 Revealed by m SR

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  1. The Magnetic Phase Diagram of (Sr,Ca)2(Ru,Ti)O4 Revealed by mSR Jeremy P. Carlo jeremy.carlo@nrc.gc.ca Columbia University Canadian Neutron Beam Centre, National Research Council June 2, 2010

  2. Outline • Overview • Correlated electron materials • Magnetic order • Superconductors • The SR method • Local probe of magnetism • (Sr,Ca)2RuO4 & Sr2(Ru,Ti)O4 • Superconductivity • Magnetic Phase Diagram

  3. Overview • Relation between magnetic order & superconductivity • BCS: Cooper pairs: electron-phonon interaction • High-Tc: magnetic fluctuations more important • “Canonical” cuprate phase diagram: • Parent compound: AF • Magnetic order close to SC dome

  4. Overview • Ongoing questions: • Behavior of different families of unconventional SCs? Cuprates Heavy fermion SCs Organic SCs Sr2RuO4 Fe pnictides etc. • How do magnetism / magnetic fluctuations relate? • “Normal” state behavior, M-I / structural links? • Holy Grail: • What is the comprehensive theory of unconventional superconductivity? • Present Study

  5. The SR method • Production of muons • Protons extracted from cyclotron/synchrotron • p+ low Z production target → ++ stuff • +→ ++  • parity violation: beam is spin polarized • separate out positrons, etc. • collimate / steer beam to sample Polarized muon sources: TRIUMF, Vancouver BC PSI, Switzerland ISIS, UK (pulsed) KEK, Japan (pulsed)

  6. Continuous-beam SR • Muon beam • Positive muons + • Can rotate polarization • Insert muons one at a time • Come to rest • Interstitial sites • Near anions • Along bonds

  7. Decay Asymmetry Muon spin at decay Detection: +→ e++ + e e = E / Emax normalized e+ energy

  8. e+ detector U incoming muon counter sample e+ m+ detector time D 2.5 e+ detector D

  9. e+ detector U incoming muon counter sample e+ m+ detector time D 2.5 e+ detector D U 1.7

  10. e+ detector U incoming muon counter sample e+ m+ detector time D 2.5 e+ detector D U 1.7 D 1.2

  11. e+ detector U incoming muon counter sample e+ m+ detector time D 2.5 e+ detector D U 1.7 D 1.2 D 9.0 + 106-107 more…

  12. Histograms for opposing counters asy(t) = A0 Gz(t) (+ baseline) a Total asymmetry ~0.2-0.3 Muon spin polarization function 135.5 MHz/T Represents muons in a uniform field

  13. Field configurations • ZF-SR: •  sees: field due to nearby moments • Spontaneous ordering? • Precession • Rapid relaxation T-dependence (in-plane doping) vs. out-of-plane doping T-dependence (out-of-plane doping) vs. in-plane doping Example (CuCl)LaNb2O7 La NbO6 [CuCl]+

  14. Field configurations Example • LF-SR: •  sees: skewed local field distribution • Static order • Decoupling if Happl ~ Bint • Dynamic order • No decoupling • Drift of “1/3 tail” H  initial muon spin

  15. Field configurations • wTF-SR: • Calibration of baseline (a), total asymmetry (A0) •  sees: • (mostly) applied field (paramagnetic state), • appl. + internal fields (ordered state) H  initial muon spin Determine ordered, PM fractions Example

  16. Field configurations • (strong) TF-SR: • Order induced by applied field • Metamagnetism, etc. • Vortex lattice in Type-II SC • Rlx √<B2>  1/2  ns /m* •  = penetration depth • ns /m* = superfluid density • Polyxtal samples: distribution broadened ~ Gaussian • => Gaussian rlx • => 1/2 • => sf. density H  initial muon spin J. E. Sonier, 1998 & 2007

  17. Srn+1RunO3n+1 RuO66 • Ruddlesden-Popper series • n=: SrRuO3(113) • perovskite structure • Ferromagnetic, Tc 165K • n=3: Sr4Ru3O10(4-3-10) • multi-layered structure • FM, Tc 105K • n=2: Sr3Ru2O7 (327) • quantum metamagnetism • FM, AF fluctuations • mag. ordering w/ Mn • n=1: Sr2RuO4 (214) • Unconventional SC Tc 1.5K • Spin-triplet pairing, p-wave • isostructural to LBCO, LSCO Sr

  18. (Sr,Ca)RuO3 = ‘113’ Past Work: • n= • 3-D structure • Ca/Sr substitution • SrxCa1-xRuO3 • isoelectronic doping • FM suppressed x 0.25 • Phase separation, QPT

  19. Sr2RuO4 = ‘214’ Maeno et al. 1994 MacKenzie & Maeno, 2003 Fermi surface: • n=1 • SC state (Maeno et al. 1994) • Tc up to 1.5 K • NMR: Spin-triplet pairing • TRSB – (Luke et al. 1996) distinguish between p-wave states • Incommensurate spin fluctuations q ≈ (0.6/a, 0.6/a, 0) • Normal state: 2-D Fermi liquid • Doping: • “Out-of-plane:” Ca on Sr site: SrxCa2-xRuO4 • “In-plane:” Ti on Ru site: Sr2Ru1-yTiyO4 • Small doping on either site suppresses SC Luke et al. 1996

  20. Ca2RuO4 • AF insulator, moment 1.3B • Competition between A- and B- type ordering • TN  110-150K • Ca doping induces Mott transition • Decreased bandwidth • Increased on-site Coulomb repulsion • → Increased U/W • Ru-Ru in-plane dist > Sr2RuO4 • RuO6 flattening, tilting

  21. Ca2-xSrxRuO4 Susceptibility @ 2K: • M-I transition near x=0.2 (I-II) • Near x=0.5: (II-III) • Sharp increase in susceptibility • Correlations more FM-ish • Low susc @ higher x Old Picture: Ordering at low x only Antiferro. near x=0 Susc. peak near x=0.5 Paramagnetic at higher x SC at x=2 • SR: • Rapid relaxation observed 0.2 ≤ x ≤ 1.6 • Peaks near x 0.5, 1.5 • Ordered ground state throughout! Nakatsuji & Maeno, 2000. Nakatsuji & Maeno, 2003.

  22. Sr2Ru1-yTiyO4 • y=0: SC Sr2RuO4 • <0.2% Ti doping suppresses Tc • >2.5% doping induces magnetic ground state • neutrons: Braden et al.(2002) • Incommensurate AF in y=0.09 • q (0.3, 0.3, qz) • SR: rapid relaxation with increasing y. from MacKenzie et al. 2003

  23. Experiments • Samples • (Ca2-xSrx)2RuO4x = 0.0, 0.2, 0.3, 0.5, 0.57, 0.65, 0.9, 1.0, 1.4, 1.5, 1.6, 1.8, 1.95 • Sr2(Ru1-yTiy)O4y = 0.01, 0.03, 0.05, 0.09 • single xtals from Kyoto U. (Maeno et al. or Tsukuba (Yoshida et al. • ZF- & LF-mSR: M20 (LAMPF) and/or M15 (DR) • DC Susceptibility: ZFC, FC, H ~ 50-100 G Dilution fridge 15mK < T < 10K He gas-flow cryo 1.7K < T < 300K

  24. Ca2RuO4 ZF-SR Ca2RuO4 • SR spectra: • Sum of 2 frequencies

  25. ZF-SR Temperature Scans (Ca,Sr) system

  26. ZF-SR Temperature Scans (Ru,Ti) system

  27. Edwards-Anderson order parameter Uemura “spin glass” function (Uemura, 1985): dynamic + static das “root-exponential” “Lorentzian Kubo-Toyabe” Field width as = a √Q ld = 4a2(1-Q)/n Fluctuation rate

  28. ZF Relaxation vs. Temp: Magnetic ordering! Define: Rlx = sqrt ( d2 + as2 ) Fit to: Rlx(T) = R [ 1 – (T/To)g] zoom all Ti only Ca only

  29. LF @ base temp: decoupling → static order Fit to tanh(H/Ho) Static ordering at base temp!

  30. LF temp scans: map out dynamics

  31. Comparison of ZF & LF field estimates tanh(H/Ho) R [ 1 – (T/To)g ]

  32. Adapted from Braden et al. (2002) Neutrons: Braden Muons: present study

  33. DC Susceptibility Curie-Weiss: more AF

  34. Old view: New View:

  35. Summary: (Sr,Ca)2(Ru,Ti)O4 Past: Sr2RuO4 p-wave SC Tc 1.5K, TRSB magnetic fluctuations Sr2Ru1-yTiyO4 y  0.002 suppresses SC neutrons: incommensurate AF y = 0.09 Ca2RuO4 AF insulator TN 100-150K Sr2-xCaxRuO4 M-I transition x0.2 susceptibility peak x0.5 New: Sr2-xCaxRuO4 muons : magnetic order over almost entire range x = 0: commensurate AF, gone by x = 0.2 peaks x  0.5 (FM-ish?), 1.5 (more AF) incommensurate AF / SDW ? need long-range magnetic probe! Sr2Ru1-yTiyO4 muons: rapid relaxation y ≥ 0.03 susc: large negative w → AF

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