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Magnetism in Azurite Studied by Muon Spin Rotation (a status report)

Magnetism in Azurite Studied by Muon Spin Rotation (a status report) M. Kraken, S. Süllow and F.J. Litterst IPKM, TU Braunschweig A.U.B. Wolter, IFW Dresden B. Wolf and M. Lang, Phys. Inst. Univ. Frankfurt a.M. Ch. Baines and H. Luetkens, PSI, Villigen (Switzerland). HFI-NQI 2010, CERN.

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Magnetism in Azurite Studied by Muon Spin Rotation (a status report)

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  1. Magnetism in Azurite Studied by Muon Spin Rotation (a status report) M. Kraken, S. Süllow and F.J. Litterst IPKM, TU Braunschweig A.U.B. Wolter, IFW Dresden B. Wolf and M. Lang, Phys. Inst. Univ. Frankfurt a.M. Ch. Baines and H. Luetkens, PSI, Villigen (Switzerland) HFI-NQI 2010, CERN

  2. use as pigment (typical during Renaissance) Albrecht Altdorfer ca. 1513 Emperor Maximilian‘s triumph

  3. Azurite Cu3(CO3)2(OH)2 model for a Heisenberg S=1/2 chain dimer mono-mer Belokoneva et al., PCM 2001 Various coupling scenarios Frustration? is there magnetic order? role of inter-chain coupling? or anisotropy? (DM)

  4. K. Rule et al

  5. coupling within dimers inter-chain? dimer-dimer! coupling of monomers our µSR Kikuchi et al., PRL 2005 Earlier µSR data: Kikuchi et al, Prog. Theor. Phys. Suppl. 2005 indications for static order below TN

  6. µ+SR: • magnetically ordered fractions • spontaneous muon spin rotation frequencies in ordered state • inhomogeneous and homogeneous broadening • critical slowing-down • Samples: single-crystal and powder (same as used for neutron experiments, C. Gibson et al, PRB 81, 2010)

  7. From recent neutron scattering (C. Gibson et al): Cu2+ monomer moment ~ 0.68 µB Cu2+ in dimer~ 0.25 µB location of µ+? probably close to O, not yet clarified

  8. from µSR in transverse magnetic field, polycrystalline sample magnetic fraction TN strongly broadened short range order above TN

  9. Zero field µSR Above about 6 K: mainly Gaussian damping due to static nuclear moments Below about 6 K: one fast + one slowly damped signal: electronic spin fluctuations

  10. damping of slowly damped signal above TN for polycrystal λT=γμ2<ΔB2>τS (µs-1) ~(T-TN)-0.9 critical behavior? Probably not! Short-range order above TN! T(K) TN

  11. much weaker T-dependence! slowing-down of spinon excitations? T dependence of weakly damped signal above TN for single crystal

  12. (at least) 2 “spontaneously“ rotating signals below TN For single crystal, not observed in polycrystal

  13. about 20% of signal close to monomer? 4 β≈0.28(5) neutrons: β≈0.12?? extinctions effects! (Rule) f(MHz) about 80% of signal 2 close to dimer? T(K) 1 2 TN=1.8 K spontaneous muon spin rotation frequencies static order!

  14. Transverse damping of µ+ close to still another transition, see also neutron (Rule et al) and elastic data (Cong et al 2009)

  15. Summary: • polycrystal: short range order above TN=1.8K • damping above TN : <ΔB2> τS ~ (T-TN)-0.9, critical? Probably not! • τS are on the order of tens to hundreds of ns (from “LF decoupling experiments“): dynamic sro • static magnetic order below TN ≈ 1.9K, strongly broadened • single crystal: well defined TN=1.8 K • damping above TN: much weaker T dependence than for polycrystal, spinons? • at least 2 spontaneous muon spin rotation frequencies (different sites), magnetization curve • despite muon sites are not yet identified: ordered Cu moments can only be on the order of tenths of µB (see neutron data) • indications for a further magnetic change below 0.5K • reason for 3D order still not clarified • No “simple“ model system!

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