Docstoc

Apr07

Document Sample
Apr07 Powered By Docstoc
					Magnetic Properties of C60 Polymers


            Antonis N. Andriotis
     Institute of Electronic Structure and Laser, (IESL),
 Foundation for Research and Technology – Hellas, (FORTH),
      P.O.Box 1527, 71110 Heraklio, Crete, Greece

                 andriot@iesl.forth.gr



            Lexington, KY, 07 April 2004



 Work supported by the EU-GROWTH research project
  AMMARE (G5RD-CT-2001-00478) “New Nanoscale
  Materials for Advanced Magnetic Storage Devices”
                      Collaborators

                   Prof. Madhu Menon
       (Center for Computational Sciences and Dpt. of Physics
         and Astronomy, Univ. of Kentucky, Lexington, KY)



                Dr. R. Michael Sheetz
(Center for Computational Sciences, Univ. of Kentucky, Lexington, KY)




           Prof. Leonid Chernozatonskii
                  (Institute of Biochemical Physics,
                Russian Academy of Sciences, Moscow)
Outline
    • Computational methods
       used
    • Magnetism in C-based
       materials
      - Some (representative)
       experimental results
      - Proposed theoretical
       models
    • Ferromagnetism of Rh-
       C60 : Present work
    • Features of C-based FMs
    • Conclusions
   Computational methods used
• Tight Binding Molecular Dynamics
  (TBMD)
  Orthogonal
  Non-orthogonal [Menon-Subbaswamy, PR B50, 11577
  (1994)]
  Real space and k-space calculations
• Ab initio methods (at B3LYP level of
  approximation) (Gaussian 98)
 Magnetism in C-based materials
• Some (Representative) Experimental results
            Experimental results
Magnetism in carbon-based materials
  Rh-C60
                   • Pressure polymerized fullerenes ; Rh-
                     C60 at 1,020-1,050 K [T.L.Makarova et al
                      (Nature, 413, 716 (2001)]
                   • Activated Carbon Fibers (ACFs)
                      [Shibayama et al, PRL 84, 1744 (2000)]
                   • Fluorinated graphite [Panich et al JPCS
           P-nnn
                      62, 959 (2001)]
                   • Carbon foam [Umemoto et al PRB 64,
                      193409 (2001)]
                   • TDAE- C60 B.Narymbetov            et al, Nature, 407, 883
                      (2000); T.Sato et al, PRB 55, 11052 (1997) ]
                   • p-nitrophenyl-nitronyl-nitroxide
         Experimental results
    Magnetism in carbon-based materials
Rh-C60
           •   Pressure polymerized fullerines ; Rh-C60 at
               1,020-1,050 K [T.L.Makarova et al (Nature, 413, 716 (2001)]
           •   Activated Carbon Fibers (ACFs) [Shibayama et al, PRL
               84, 1744 (2000)] (Mean size 30 Å and one spin per ACF leads to
               inter-spin distance of 30 Å.) Exchange interaction between
               localized non-bonding -electrons mediated by conducting
               -electrons similar to the s-d interaction. Charge transfer
               interaction between edge states and the -bands favors the
               development of the conduction-electron-mediated
               interaction network.
           •   Fluorinated graphite [Panich et al JPCS 62, 959 (2001)]
               Paramagnetism is caused by local moments of dangling
               bonds the latter resulting from the reaction of a F and a C atom
               which converts an sp2 to an sp3 bond. Strong exchange between
               these moments may be mediated by a C-C bond.
           •   Carbon foam [Umemoto et al PRB 64, 193409 (2001)]
   Experimental results
Magnetism in TDAE- C60
   • TDAE-C60 [Tc = 16 K ] [ B.Narymbetov                 et
     al, Nature, 407, 883 (2000); T.Sato et al, PRB 55,
     11052 (1997) ] (donor-acceptor type material)
   • Key feature for FM-ism to appear :
     Mutual orientation of adjacent
     fullerene molecules
   • Existence of two phases (FM, PM)
     associated with two possible bonding
     configurations (6-6 , 5-6)
        FMism appears when
          Magn. Energy > Config. Energy.
     - Resonance structure effect ?
       TDAE-C60 : Properties
• It is a donor-acceptor type magnetic material
• Monoclinic structure
• Unusually short distances between neighbouring
  C60 molecules
• -form Ferromagnetic ;
  ’-form : Paramagnetic ; ’-form turns to -form
  by annealing
• T(Curie)=16 K
• No band-FM-ism; no SPM-ism; it is a nearly
  isotropic Heisenberg ferromagnet.
 Experimental results
Magnetism in p-nnn
           • p-nitrophenyl-
             nitronyl-nitroxide
           • p-nitrophenyl-
             nitronyl-nitroxide :
             1st made purely
             organic FM
      Experimental results
    Magnetism in p-nnn
K.Awaga and Y.Maruyama, CPL 158, 556 (1989)

                      • p-nitrophenyl-
                        nitronyl-nitroxide
                      • It exhibits a quinoid
                        resonance structure
                      • Dipole-dipole interact.
                      • Spin polarization
                        effect is probably
                        enhanced by n-
                        exchange interaction
                         (n=el-ns in nitro group)
           Experimental results (1)
 Magnetic polymerized fullerenes
• In pressure polymerized fullerenes the magnetic
  phase appears when fullerene cages are about
  to break down (formation of graphitized
  fullerenes)
• From a set of samples prepared at nominally
  identical conditions only part of samples show
  magnetism ; the determination of the structural
  difference between magnetic and non-magnetic
  samples has not been achieved by conventional
  characterization methods.
          Experimental results (2)
 Magnetic polymerized fullerenes
• Samples are polymeric and crystalline
• Samples contain impurities. However,
  magnetism is not influenced by them
• Samples gave no indication of
  superparamagnetism
          Experimental results (3)
 Magnetic polymerized fullerenes
• The magnetic phase appears as islands in a
  non magnetic matrix forming stripe
  domains as well as corrugated domain
  patterns
• Sample-magnetism is determined by
  preparation conditions
 Magnetism in C-based materials
• Some (Representative) Experimental results
• Theory : Proposed models for :
  - Mechanisms leading to unpaired electrons
  - FM coupling among unpaired electrons
            Mechanisms leading to
             unpaired electrons


THEORY
          Mechanisms leading to FM
         coupling of unpaired electrons
 Theory : Mechanisms leading to unpaired
electrons (1a) [M.Hjort and S.Stafstrom, PR B61, 14089 (2000)]
                                • Vacancies in graphite
                                  create new states below
                                  EF . These states
                                  contribute an extra -
                                  electron localized at the
                                  vacancy.
                                • For non-interacting
                                  vacancies this extra
                                  electron gives rise to an
                                   unpaired spin
Theory : Mechanisms leading to unpaired
electrons (1b) [A.A.El-Barbary et al, PR B68, 144107 (2003)]
                               • Formation of
                                 pentagon-one dangling
                                 bond (5-1db)
Theory : Mechanisms leading to unpaired
             electrons (2)
                    • Edge carbon atoms
                       [K.Harigaya, CPL 340, 123 (2001)]
                    • Shift of one layer relative to
                       the neighbouring ones necessary
                       for bulk magnetic appearance
                    • Defect sites (or active side
                      atoms A,B,A’,B’) and
                      interlayer coupling
                      affect magnetic properties
Theory : Mechanisms leading to unpaired
             electrons (3)




                    • Stone-Wales defects         [Kim
                      et al, PR B68, 125420 (2003)]
                    • Tetrapod (Negative
                      gaussian curvature) [Park et
                      al PRL, 91, 237204 (2003)]
Stone-Wales defect
         • A 90 degrees local
           bond rotation in a
           graphitic network
           leads to the formation
           of two heptagons and
           two pentagons
         • Static (dynamic)
           activation barrier for
           formation 8-12 (3.6)
           eV in SWCNs
Magnetism in C-based Materials
                         Synopsis (1a)
• Theory : Origin of unpaired spins
 - Disorder C-atoms (vacancies)
 Localized spins in (disordered) C-materials are most
  probably due to - electrons localized at the vacancy.
 - Edge states (on zig-zag edge-carbon atoms)
 [Yoshiwaza et al, Carbon 32, 1517 (1994); Fujita et al JPSJ 65, 1920 (1996)]
 The localized spins are considered to originate from the
 nonbonding edge states of the -electron.
Magnetism in C-based Materials
                     Synopsis (1b)
• Theory : Origin of FM coupling
• Exchange interaction between localized non-bonding -
  electrons mediated by conducting -electrons similar to the
  s-d interaction. Charge transfer interaction between
  edge states and the -bands favors the development of
  the conduction-electron-mediated interaction network
  (case of activated carbon fibers).
• Strong exchange between magnetic moments may be
  mediated by a C-C bond (case of fluorinated graphite).
       Geometric frustration upon
           polymerization
Avoiding frustration
                         • Avoid geometrical
                           frustration by stress-
                           driven bond selection
                         • Applied anisotropic
Frustrated C60 polymer
                           stress selects the
                           directions of bonding
                           [L.Marques et al, PR B68, 193408
                           (2003)]
Geometric frustration and FM-ism




                        •   M.J.Harris et al, PRL 79, 2554 (1997)
                        •   H.Tsunetsugu and Y. Motome, PR B68, 060405
                            (2003)

                        •   The presence of local Ising
                            anisotropy leads to a
                            geometrically frustrated g.s. In
                            the presence of a magnetic field,
                            magnetic order develops
   Pyrochlore lattice
Geometric frustration and FM-ism
       [O.Tchernyshyov et al PRL, 88, 067203 (2002)]

• The geometrically frustrated system is
  revealed by the vast degeneracy of its g.s.
• It undergoes a magnetic Jahn-Teller (“spin-
  Teller”) distortion
• The lifting of frustration may be achieved
  through a coupling between spin and lattice
  degrees of freedom (elastic versus magnetic
  energy)
 Magnetism in C-based materials
• Some (Representative) Experimental results
• Theory : Proposed models leading to :
  - Mechanisms leading to unpaired electrons
  - FM coupling among unpaired electrons
• Our results
              Polymeric fullerenes
     Orthorombic          Tetragonal           Rombohedral




     T<650 K , P~1-9GPa     T>650 K ; P~2GPa   T~1000-1100 K ; P=6GPa


• Tetragonal and Rombohedral polymeres are 2D
  (along <110> and <111> directions respectively) ;
  interlayer coupling of van der Waals type as in
  graphite. The orthorombic phase is 3D.
  Methods for synthesizing C60-
            polymers
• Photochemical methods
  They lead to a distorted fcc structure with
  isotropic polymerization along all nn directions
• High Pressure – High Temperature (HP-HT)
  They lead to ordered structures
• All polymers exhibit the same polymeric
  bond : 2+2 cycloaddition
      2+2 66/66 cycloaddition
                                    1   2




                                    3   4




• In the 1-4 atoms (all with sp3 bonds) of the cycloaddition
  bond there is accumulation of negative charge leading to a
  higher occupancy of the pz orbital (z-axis perpendicular
  to plane of 4-atoms).
• Defects in C60 molecules do not change this feature
• Defects in C60 molecules accumulate positive charge
          2+2 cycloaddition
      S.Okada and A.Oshiyama, PR B68, 235402 (2003)

• In C60(65)-polymer :
  a=9.19 A ; c=24.5 A ; r(C-C)=1.56 A
  metallic, non-magnetic ; E(ABC)  E(ECB)
  E(per atom) = 0.702-0.719 eV
• In C60(66)-polymer :
                        r(C-C)=1.64 A
  semiconductor ; indirect gap 0.5
  E(ABC)=E(ACB) ; E(per atom)= 0.428 eV
           C60 versus Rh-C60

• In C60-polymers the distance between the
  C60 molecules is ~9.1-9.2 A. In non
  polymerized systems the distance is ~10 A.
• Polymerized C60 is a geometrically
  frustrated system.
                C60 versus Rh-C60

          C60                               Rh-C60
                                [ “66/66” 2+2 cycloaddition bonds
- Band Gap : 2.3-2.5eV              1.64 Å]
  (1.6 eV Xu-Scuseria PRL 74,    [Xu-Scuseria PRL 74, 274 (1995)]
  274 (1995)                    - Band Gap 1.0 eV
- Diameter : 7.1 Å              - Diameter of C60 7.1 Å
                                - d(C60 -C60 )= 9.17
                                  (9.2exp ) Å (in plane)
                                - d(C60 -C60 ) = 9.8 Å
                                  (interplane)
                                - Inclusion of “56” bonding
                                  makes material less stable
                                  but with vanishing gap
Various 2D and 1D polymeric C60 -based
structures [ A.N.Andriotis et al, PRL 90, 026801 (2003) ]


                                • (a), (b), (c ) contain sp3
                                  and sp2 bonded C-atoms
                                • (d) contains only sp2
                                  bonded C-atoms
  2D rhombohedral C60 (Rh-C60 ) with
               vacancies




• Vacancies appear among carbon atoms colored in red in
  the above picture; sp3 bonded C-atoms (resulting from the
  2+2-cycloaddition mechanism at the C60-interfaces) are
  shown in green.
    Turning on e-e correlations
Values of soth (in eV) for different structures
considered in this work (TBMD calculation).


                        • Structure          No
                                            vacancy
                        •   (a)Rd-C 60      0.85
                        •   (b)Tetragonal   1.00
                        •   (c)Linear A     0.70
                        •   (d)Linear B     0.80
    Turning on e-e correlations
Values of soth (in eV) for different structures
          considered in this work.


                        • Structure        No       With
                                          vacancy
                          vacancy
                        • (a)Rd-C 60      0.85
                          0.03
                        • (b)Tetragonal    1.00
                          0.20
                        • (c)Linear A     0.70
                          0.10
                        • (d)Linear B     0.80
Magnetic Moment of Rd-C60

•   SQUID magnetometry on Rh-C60 :
•   Sample-mass  3.2 mgr
•   Msat 0.2 G  1.29 B / 2C60
•   (in high quality samples it reaches 1B / C60 )
•    <  >  0.01 B per atom
•   U  10 eV (exp)
•   so = U < >/2  0.05 eV
         Band Structure Results
               (TBMD calculation)
• Energies along the -M (-X) direction for the
  hexagonal (square) structures. In particular,
• For Rh-C60 the band energies (in eV) along the -M
  direction have the following dispersion (setting EF =
  0.00 and with degeneracies in parenthesis):
         -0.20(1), 0.00(2), 0.20(1), 1.26(1), 1.35(1).
• For the square-C60 the corresponding dispersion
  structure in the -X direction is :
        -0.08(1), 0.02(1), 0.12(1), 1.30(1), 2.19(1);
         No degeneracy
Band Structure of Rh-C60
            • Band energies along the -
              M
               direction
            • Band at EF is doubly
              degenerate
Band Structure of defect free C60
                 • (Top) :
                   S.Okada and S.Saito PR B57,
                   4039 (1997).
                   (Energies measured with respect to
                   top of valence band).

                 • (Bottom) :
                   A.V.Okotrub et al JCP, 115,
                   5637 (2001)
   Band Structure of defect free C60
          S.Okada and A.Oshiyama, PR B68, 235402 (2003)




• The C60(65)-polymer is :
  metallic, non-magnetic ;
  E(per atom) = 0.702-0.719 eV
• The C60(66)-polymer is:
  semiconductor ; indirect gap
   0.5 E(per atom)= 0.428 eV
  C60 - C60 dimer
(Ab initio calculations)
             • atoms participating in the
                C60 - C60 coupling :
               - are all sp3
               - accumulate 0.5 |e| (in pz)
               - not affected by vacancy
             • atoms surrounding each
                vacancy (3 per vacancy) :
               - loose 0.5 |e|
              - Electric dipole moment
                2.264 Debye per C60
              - are related with the
                appearance of non-zero m.m.
  C60 - C60 dimer
(Ab initio calculations)
               Atoms participating in
               the C60 - C60 coupling
               accumulate 0.5 |e|
               mainly in the pz orbital

              Thus, in Rh-C60
              moments favor FM
              alignment while in
              tetragonal C60 this is
              more difficult to be
              achieved
Atomic charges from Natural population
               analysis
            Atomic Charges From Natural Population Analysis


CC60 Hexagonal With Vacancy


                   31         37
                                                                                   • Atoms in red box (40,
             34

             48
                        42 ++

                        52
                                    45

                                    53 ++
                                                                                     44, 60, 62) participate
              47

                   51
                        57

                              59
                                    58
                                                                                     in 2+2 cycloaddition
                                                                                   • Atoms in blue boxes
                   49         56         54           36

                        50                    43 +          24

                              44         40             29 +



                        67
                              62

                                   69
                                         60

                                              78
                                                   61

                                                           63      68
                                                                         75

                                                                              84
                                                                                     surround vacancies
                        74         77 ++ 86                        76         91
                                                           73
                                                                         85
                                        92 ++
                                                           82      93


                                                           87
                                                                    99
                                                   81


                                                   88

                                              98           83

                                                           94 ++


                                                101                103
Charge-wave along C60-dimer ?
             Synopsis (2)
    Characteristic features of Rh-C60
• It has high symmetry
• It has a degenerate ground state
• It exhibits defects
• It develops intra-molecular charge transfer and large
  electric dipole moment
• It has flat bands at EF
             Synopsis (2)
    Characteristic features of Rh-C60
• It has high symmetry
• It has a degenerate ground state
• It exhibits defects
• It develops intra-molecular charge transfer and large
  electric dipole moment
• It has flat bands at EF
• These appear in one way or the other as basic
  properties of non-metallic FM-nets
Rh-C60 : I-V characteristics
                  Symmetric bias




                   Asymmetric bias
Electron DOS of Rh-C60
  PARENTHESIS : Comparison
   between other organic or non-
  metallic Ferromagnets and Rh-C60
        Common features
1. Defects
2. Degenerate ground state
3. Charge transfer
      Common features (1a)
• Defects    • Hexaborites (CaB6)
                [B6 vacancies]
             • Oxides of rocksalt structure
               [cationic vacancies (CaO) ]
             • Rh-C60 [vacancies]
             • FM-TDAE- C60 [Orientation
               changes]
             • Substitution of cations in oxides
               (Co : ZnO, TiO2) [substitution
               changes]
    Polycrystalline Co-substituted ZnO
        A.S.Risbud et al, PR B68, 205202 (2003)

• Unit cells used :
• Zn(16)O(16) : Non-magnetic
• Zn(15)Co(1)O(16) : FM and AF states isoenergetic(FM<AF)
• Zn(14)Co(2)O(16) : FM gets a bit better than AF (60meV)
• Zn(13)Co(2)O(16) : Zn-Vacancy (p-type doping)
  stabilizes the FM state
• Zn(14)Co(2)O(15) : O-vacancy (n-doping) stabilizes the
  AF state
• Exp. Results in agreement with theoretical ones
 Defects as a mean to transform insulating
     nonmagnetic compounds to FMs
• Hexaborites [D.P.Young et al, Nature (London) 397, 412
  (1999); R.Monnier and B.Delley, PRL 87, 15720 (2001)]
   (B6 vacancies in a superlattice can lead to a FM ground state even at
  very low concentration; B6 is ionic (+2) thus, a B6 vacancy needs
  two electrons for charge compensation.)
• Co substituted TiO2               [Y.Matsumoto et al, Science 291,
  854 (2001)]
  (Tetravalent Ti is substituted by a divalent Co. In this case two holes
  (in oxygen 2p-band) are needed for charge compensation for every
  Ti-substitution by Co.)
• Co substituted ZnO [K.Ueda et al                APL 79, 988 (2001)]
 Defects as a mean to transform insulating
     nonmagnetic compounds to FMs
         Oxides with rock salt structure ( CaO)
                [I.S.Elfimov et al, PRL, 89, 216403 (2002)]
• Prerequisites for forming FM states
      Dilute (Ca-)vacancies (in CaO) lead to FM ground state.
     For a magnetic state to occur in the defected material :
      (i) the g.s. of the charge compensating MO must be degenerate;
      (ii) the local impurity potential should be large enough to quite strongly
           bind the charge to its n.n. atoms (formation of magn. moment).
 Possible mechanism leading to FM-tism
   e-e correlations (e.g., on site Cb interaction) lead to a high-spin
    g.s. for the charge compensating MO.
   superexchange and/or double-exchange may lead to a FM
    coupling of formed magn. moments.
      Common features (1b)
• Defects     The defect nature of fullerene
              magnetism draws an analogy
              with the magnetism of
              graphite, hexaborides, oxides
              of rocksalt structure and doped
              semiconductors.

              In divalent hexaborides, defects
              are becoming donors (Hall,
              TEP results ) [Fisk et al, Physika
              B,312-313, 808 (2002)]
       Common features (2)
• Degenerate ground   • Hubbard models
  state                 exhibiting flat bands
                      • In charge transfer systems
                        charge compensating MO
                        must be degenerate (e.g. in
                        CaO with dilute Ca-
                        vacancies and other oxides
                        of rocksalt structure)
                      • Rh-C60
Mielke-Tasaki’s flat-band FMsm
• H.Tasaki, Prog. Theor. Phys. 99, 489 (1998)
• A.Mielke, J. of Phys., A24, L73 (and 3371)
  (1991)
• Hubbard models having highly degenerate
  ground states may be FM
       Common features (3)
• Charge transfer   • p-nitrophenyl-nitronyl-
                      nitroxide (1st made purely
                      organic FM)
                    • Hexaborites, Co substituted
                      TiO2 , Co substituted ZnO,
                      CaO with Ca-vacancies
                    • McConnell-II model
                    • Rh-C60
 Magnetism in C-based materials
• Some (Representative) Experimental results
• Theory : Proposed models for :
  - Mechanisms leading to unpaired electrons
  - FM coupling among unpaired electrons
• Our results
• Searching for new FCU
Searching for new FCU
    Searching for new FCU




• Charge transfer (Mc Connell II)
           McConnell Model II
[H.M.McConnell, Proc. R.A.Welch Found. Chem. Res. 11, 144 (1967)]

                                 • In charge transfer
                                   complexes of alternating
                                   donor and acceptor mol-es
                                   a FM alignment can occur
                                   if the neutral donor or
                                   acceptor mol-e has a
                                   triplet g.s.
                                 • Critics : Too simple to
                                   account for observed FM ;
                                   difficult to be applied
                                 • Breslow’s extensions
                                 • Defects
                                 • Spin polarization
                                     (Ovchinnikov, Mc Connell I)
    Searching for new FCU




• Charge transfer (Mc Connell II)
• Defects
Defects as a mean to transform insulating
    nonmagnetic compounds to FMs


Defects lead to m.m.-formation as a result of
 molecular Hund’s rule coupling
The energetics are determined by KE and
 symmetry rather than exchange interaction
High-symmetry systems are good candidates
 for this type of FM-tism
          FM-coupling
Superexchange and/or double-
exchange may lead to a FM coupling
of defect-formed magnetic moments.

Critical concentration (of defects)
necessary for establishing the
(exchange) interaction network
      Searching for new FCU




• Charge transfer (Mc Connell II)
• Defects
• Spin polarization (Ovchinnikov, Mc Connell I)
         Ovchinnikov’s model
    [A.A.Ovchinnikov, Theoretica Chimica Acta, 47, 297 (1978)]




• Ovchinnikov’s Spin polarization model
  starred atoms having large positive spin density induce
  small negative spin density on the un-starred atoms
• McConnell-I model [FM coupling between atoms carrying
  opposite spin density (1963) ]
• Other effective exchange pathways
  Effective exchange pathways
• Dipole-dipole interactions
• p-nitrophenyl-nitronyl-nitroxide
• Use of (transition) metals as FM-coupler
  between organic free radicals
• (between monoradicals in single crystals
  Hydrogen bonding )
• Resonance structure effect ??
Exchange pathway
        • dipole-dipole
          interactions between
          nitrophenyl nitronyl
          nitroxides
        • Enhanced spin-
          polarization
              Synopsis (3)
        Searching for new FCU
• Charge and/or spin transfer associated with
  “defects in general” can lead to exchange
  interaction pathways. Examples :
   - vacancies / defects
   - variation of bonding configuration
    [associated either with orientation changes (TDAE-C60) or
    changes in the geometry of surrounding atoms (edge C-states)]
  - non-Kekule structures (“starred” and “unstarred”
    atoms)
                     Synopsis (4)
     Factors influencing
        FM-Coupling

• High symmetry
• Degenerate ground state
• Defect-concentration
  greater than a critical
  value (percolation
  threshold)
• e-e correlations
• (Resonance structure ?)
                     Synopsis (5)
     Factors influencing
        FM-Coupling                 Features of Rh-C60

• High symmetry             • high symmetry
• Degenerate ground state   • degenerate ground
• Defect-concentration        state
  greater than a critical   • Exhibits flat bands at
  value (percolation          EF
  threshold)
                            • Exhibits defects
• e-e correlations
                            • intramol-ar charge
• (Resonance structure ?)
                              transfer and large
                              electric dipole moment
            CONCLUSION
Possible scenario for FM-ism of Rh-C60
            CONCLUSION
Possible scenario for FM-ism of Rh-C60
• The 2+2 cycloaddition bonds and defects     Reminiscent of :
  break the chains (symmetry) of alternate   McConnell-
                                               Breslow-
  single-double C-C bonds ;                    Ovchinnikov
  this leads to charge and spin transfer.      (starred-unstarred)
            CONCLUSION
Possible scenario for FM-ism of Rh-C60
• The 2+2 cycloaddition bonds and defects     Reminiscent of :
  break the chains (symmetry) of alternate   McConnell-
                                               Breslow-
  single-double C-C bonds ;                    Ovchinnikov
  this leads to charge and spin transfer.      (starred-unstarred)

• The high symmetry provides the             McConnell-Tasaki-
  degenerate g.s. and appropriate excited     Mielke
  states.
            CONCLUSION
Possible scenario for FM-ism of Rh-C60
• The 2+2 cycloaddition bonds and defects     Reminiscent of :
  break the chains (symmetry) of alternate   McConnell-
                                               Breslow-
  single-double C-C bonds ;                    Ovchinnikov
  this leads to charge and spin transfer.      (starred-unstarred)

• The high symmetry provides the             McConnell-Tasaki-
  degenerate g.s. and appropriate excited     Mielke
  states.
• e-e correlations contribute to energy      lead to triplet g.s.

  spectrum (splittings)
            CONCLUSION
Possible scenario for FM-ism of Rh-C60
• The 2+2 cycloaddition bonds and defects     Reminiscent of :
  break the chains (symmetry) of alternate   McConnell-
                                               Breslow-
  single-double C-C bonds ;                    Ovchinnikov
  this leads to charge and spin transfer.      (starred-unstarred)
• The high symmetry provides the
  degenerate g.s. and appropriate excited    McConnell-Tasaki-
                                              Mielke
  states.
• e-e correlations contribute to energy      lead to triplet g.s.
  spectrum (splittings)
• Dipole-dipole interactions developing
  an exchange pathway seems possible;        effective exchange
                                               pathway
  s-d type of interaction not ruled out
• Possibility of resonant-charge-states
Magnetic Properties of C60 Polymers


            Antonis N. Andriotis
     Institute of Electronic Structure and Laser, (IESL),
 Foundation for Research and Technology – Hellas, (FORTH),
      P.O.Box 1527, 71110 Heraklio, Crete, Greece

                 andriot@iesl.forth.gr



         Heraklion, Crete, 25 November 2003



 Work supported by the EU-GROWTH research project
  AMMARE (G5RD-CT-2001-00478) “New Nanoscale
  Materials for Advanced Magnetic Storage Devices”

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:4
posted:12/3/2011
language:English
pages:82