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Tunable, Low-Energy Muons as Local Probes of Magnetism

Tunable, Low-Energy Muons as Local Probes of Magnetism. Toni Shiroka Laboratory for Muon-Spin Spectroscopy, Paul Scherrer Institut, Villigen, SWITZERLAND. Ferrocarbon NEST Meeting Marrakech, 16 February 2007. A light proton or a heavy electron?. What is a muon?.

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Tunable, Low-Energy Muons as Local Probes of Magnetism

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  1. Tunable, Low-Energy Muonsas Local Probes of Magnetism Toni Shiroka Laboratory for Muon-Spin Spectroscopy, Paul Scherrer Institut, Villigen, SWITZERLAND Ferrocarbon NEST Meeting Marrakech, 16 February 2007

  2. A light proton or a heavy electron? What is a muon? Muon’s ID card

  3. Maximal parity violation in the weak interaction decay of pions  ~100% spin-polarised muon. 2 body decay  monoener-getic, 4.12 MeV muons • Anisotropic distribution of decay positrons  the stati-stical average direction of muon ensemble is known. 3 body decay  Polyenerg. e+ S • The muon keeps initial polari-zation. Its spin evolution depends sensitively on the spatial and dynamical features of muon environment How to use muons to study matter?

  4. Schematic TF µSR experiment Basics of the µSR technique µSR – Muon Spin Rotation / Relaxation / Resonance • Transverse Field µSR: excellent probe of magnetic fields: =   B

  5. Basics of the µSR technique µSR – Muon Spin Rotation / Relaxation / Resonance • Transverse Field µSR: excellent probe of magnetic fields: =   B • Longitudinal (Zero) Field µSR: very sensitive to weak internal magnetism and useful for studying dynamics • ZF-µSR offers unique advantages Schematic LF (ZF) µSR experiment

  6. Key advantanges of the µSR • Extremely high sensitivity: single particle detection • Large magnetic moment: μμ = 3.18 μp = 8.89 μn • Full polarization in zero field, independent of temperature • No restrictions in choice of materials to be studied • Muons are a purely magnetic probe (S = ½, no quadrupolar splitting, etc.) • No perturbation of the system (unlike spin probes in EPR) • No special isotope is needed (as in NMR, Mössbauer) • Independent determination of magnetic moment and of magnetic volume fraction

  7. Muons as a probe of magnetism - I • Muons are microscopic probes, sensitive to short range and/or local ordering (both FM and AFM) • Muons can easily sense very weak effects • Unique measurements also in zero applied field Other features: • Interstitial probes complementary to NMR • Number of implanted muons << number of atoms  low sample damage

  8. Zero-field µSR in an organic ferromagnet vs. temperature S. J. Blundell, et al., Euro-phys. Lett.31 (1995) 573 Muons as a probe of magnetism - II • Muon spin rotation is particularly suitable for: • Small moment magnetism • Random magnetism (e.g. spin glasses) • Short range effects (where neutron scatt. can’t help) • Muons can sense very broad lines (up to ~ 100 MHz) and measure short relaxation times (down to ~ 10 ns)

  9. Non magnetic, but with strongly magnetic impurities Uniformly weakly magnetic ? • Susceptibility: Provides average information, hence same response for very different situations! • MuSR: Provides local information, hence can distinguish between the two situations.  False claims of room temperature organic ferromagnetism! Still not convinced?

  10. Extending µSR technique to new domains • Low energy muons  Study of thin films, surfaces, interfaces, nanomaterials, etc. • Pulsed environments  Experiments otherwise impossible or very difficult • High magnetic fields  Study matter in different High pressure, etc. conditions (not only STP) • …

  11. “Surface Muons”from + decay at rest (~ 4 MeV). Suitable for bulk condensed matter studies: but no depth resolution bulk thin films, multi- layers, etc. Low-energy µ+ (LEM) at PSI: E < 30 keV • Allow depth-dependent µSR investigations ( ~1 – 300 nm) • Extend the use of µSR to new objects of investigations • New magnetic/spin probe for thin films, multi-layers, surface boundaries, buried layers, etc. Why we need low-energy muons?

  12. Principle of the moderator technique Surface Muons (~ 4 MeV) Epithermal muons (~ 15 eV) Al Ar Production of eV muons by moderation Rare gas solids are good muon moderators due to: • Large band gap (~ 14 eV for Ar) • Low phonon excitation energies (~ 5 meV) Energy: 10 ± 5 eV Efficiency: 10-4 - 10-5

  13. Apparatus: transport & energy tuning … Overview of the experimental apparatus K. Träger, et al., Physica B,290, (2000) 684

  14. … and the real one! The intrinsically low LE-µ+ gene-ration efficiency requires a high intensity 4-MeV “surface” µ+ beam The µE4 beam line at PSI, specifi-cally designed to deliver the highest intensity of 2·108 µ+/s on moderator target (3x3cm2), produces up to 20.000 LE- µ+/s !

  15. superconductor B(z) l z • Magnetic field profile B(z) • Characteristic supercond. lengths l, x Depth dependent LE-µSR studies Bext n(z,Em) Muon implantation profile for a particular muon energy E  SR experiment  magnetic field probability distribution p(B) sensed by the muons      n(z) dz = p(B) dB B = B(z)

  16. Examples of low-energy µSR studies • Magnetism: • Interlayer exchange coupling in multilayers, superparamagnetism in mass selected nanoclusters, magnetic ordering in buried, strained/stressed films, surface vs. bulk magnetism in LaCoO3 • Superconductivity (near surface) • Non-local effects, isotope effects, vortices across surface, vortex motion and pattern formation in 2D • Interplay/Coexistence of Magnetism/Superconductivity • YBCO/SRO superlattices, Fe/Pb multilayers, YBCO/PBCO/YBCO multilayers, Spin glass transition/sc in LSCO meander films, Surface magnetism/superconductivity in La1.9Ce0.1CuO4, search for spontaneous magnetization at the surface of YBCO110 • Dimensional or surface effects • Surface polymer dynamics, Finite size effects in spin glass freezing, • Surface vs bulk magnetism in LaCoO3 • Hydrogen states and dynamics in semiconductors and dieletrics • Low k-materials (nanoporous silica), hydrogen states in semiconductor and insulating films • Basics of LE-µSR • Implantation studies, behavior at surfaces, diffusion at interfaces, muon moderation studies

  17. Conclusions • Polarized muons are excellent probes of matter, especially of its magnetic properties. • Extension of the current µSR technique capabilities (e.g. to low-energy µSR), gives access to new physics domains, e.g.: thin films, interfaces, multilayers, nanomaterials, etc. • The European Ferrocarbon project can avail itself of two facilities in Europe: ISIS and PSI, to better understand the occurrence and behaviour of organic magnetism.

  18. Acknowledgements • People: • S. Cottrell, G.H. Eaton, P.J.C. King • ISIS - Rutherford Appleton Lab., UK • T. Lancaster • University of Oxford, UK • C. Bucci, R. De Renzi, G. Guidi, M. Riccò • Università di Parma, Italy • E. Morenzoni, Th. Prokscha, R. Scheuermann • Paul Scherrer Institut, Switzerland • Funding: • NMI3 (EU project), INFM and CNR

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