Stephen Hill NHMFL and Florida State University, Physics

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Stephen HillNHMFL and Florida State University,
Physics
EPR Studies of Heavy Atom Molecule-Based Magnets

• Outline of talk
• Idea behind the title of this talk
• Nice recent example Radical Ferromagnet
• Mononuclear nanomagnets based on Lanthanide ions
• CW and pulsed EPR studies of Ho system
• Coherent quantum tunneling dynamics

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Stephen HillNHMFL and Florida State University,
Physics
EPR Studies of Heavy Atom Molecule-Based Magnets
In collaboration with Radical
Ferromagnets Steven Winter and Richard
Oakley, U. Waterloo Saiti Datta and Alexey
Kovalev (NHMFL Postdocs) Holmium
polyoxometallate Saiti Datta and Sanhita
Ghosh (FSU/NHMFL postdoc/student) Eugenio
Coronado and Salvador Cardona-Serra, U. Valencia,
Spain Enrique del Barco, U. Central Florida
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Heavy Atom Radical Ferromagnets
Record Tc 17K Hc 0.15 T
Oakley et al., JACS 130, 14791 (2008) JACS 131,
7112 (2009)
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Radicals well known to EPR spectroscopists
Tryptophan (Trp) radical in azurin, an electron
transfer protein
S. Stoll, D. Britt UC Davis

• g tensor characteristic of microenvironment .
• Compare to electronic structure calculations.
• Crucial for systems with small g anisotropy
  (tryptophans, tetra-pyrroles, e.g.,
  chloro-phylls, and organic photovoltaic materials)

Stoll et al., JACS 132, 11812 (2010) JACS 131,
1986 (2009).
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Heavy Atom Radical Ferromagnets
Record Tc 17K Hc 0.15 T
9.0
8.7
8.4
Resonance field (tesla)
1 HA 0.8 T 2 HA 0.45 T
8.1
7.8
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Heavy Atom Radical Ferromagnets
Record Tc 17K Hc 0.15 T
Hubbard Hamiltonian with spin-orbit (s) and
hopping (h) perturbations
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Mononuclear Lanthanide Single Molecule Magnets
Hunds rule coupling for Ho3 L 6, S 2, J
8 5I8 Axial ligand-field mJ 5 I 7/2
nuclear spin (100)
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Mononuclear Lanthanide Molecular Nanomagnets
Based on Polyoxometalates
Ln(W5O18)29- (LnIII Tb, Dy, Ho, Er, Tm, and
Yb)
D4d
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Mononuclear Lanthanide Molecular Nanomagnets
Based on Polyoxometalates
Er3 compound
Er3 and Ho3 Exhibit some SMM characteristics
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Mononuclear Lanthanide Molecular Nanomagnets
Based on Polyoxometalates
Fits to cmT NMR
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Mononuclear Lanthanide Molecular Nanomagnets
Based on Polyoxometalates
Hunds rule coupling for Ho3 L 6, S 2, J
8 5I8
gJ 5/4
Ground state mJ 4
Ho3 Xe4f10
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Mononuclear Lanthanide Molecular Nanomagnets
Based on Polyoxometalates
Hunds rule coupling for Ho3 L 6, S 2, J
8 5I8
gJ 5/4

• Other relevant details
• 100 I 7/2 nuclear spin
• Strong hyperfine coupling
• Dilution HoxY1-x(W5O18)29-
• Na charge compensation
• H2O solvent

Ho3 Xe4f10
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High(ish) frequency EPR of HoxY1-x(W5O18)29- (x
0.25)
Broad 8 line spectrum due to strong hyperfine
coupling to Ho nucleus, I 7/2
B//c
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High(ish) frequency EPR of HoxY1-x(W5O18)29- (x
0.25)

• Nominally (strongly) forbidden transitions mJ
  -4 ? 4, DmI 0

• This suggests mixing (tunneling) of mJ states (no
  EPR for f gt 100 GHz)

Next excited level at least 20-30 cm-1 above
1 K 21 GHz 1 cm-1 30 GHz
B//c
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Angle-dependence HoxY1-x(W5O18)29- single
crystal (x 0.25)

• Indicative of strong anisotropy associated with J
  8 ground state
• Note hyperfine splitting also exhibits
  significant anisotropy

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Full Matrix Analysis of the Angle-dependence
gz 1.06 A 835 MHz (0.0278 cm-1)

• Simulations assume isotropic g
• data do not constrain gxy so well
• Free ion g 1.25

D 0.600 cm-1, B04 6.94 10-3 cm-1, B06
-4.88 10-5 cm-1
Ligand field parameters from AlDamen et al.,
Inorg. Chem. 48, 3467 (2009)
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Standard CW X-band EPR of HoxY1-x(W5O18)29- (x
0.25)
Multi-frequency studies does D4d
parameterization hold water?
f 9.5 GHz
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Standard CW X-band EPR of HoxY1-x(W5O18)29- (x
0.25)
Multi-frequency studies does D4d
parameterization hold water?
f 9.5 GHz
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Standard CW X-band EPR of HoxY1-x(W5O18)29- (x
0.25)
D4d symmetry approximate ? natural to add
9 GHz tunneling gap - D
f 9.5 GHz
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Standard CW X-band EPR of HoxY1-x(W5O18)29- (x
0.25)
Standard B1 ? B0 configuration
Parallel mode (B1//B0)
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Pulsed X-band EPR of HoxY1-x(W5O18)29- (x
0.25)
Rabi oscillations remarkably long T2
T1 1 ms T2 140 ns
T 4.8 K
Hahn echo sequence
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Pulsed X-band EPR of HoxY1-x(W5O18)29- (x
0.25)
Rabi oscillations remarkably long T2
Cr7Ni (S 1) 0.2mg/mL, T2 300 ns _at_ 5K Ardavan
et al., PRL 98, 057201 (2007) Fe4 0.5g/mL, 95
GHz and B 0 Schlegel et al., PRL 101, 147203
(2008) Fe8 240 GHz and 4.6 T (kBT 11.5
K) Takahashi et al., PRL 102, 087603 (2009)
Fe4
S 5
T 4.8 K
T2 140 ns
Fe8 S 10
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Pulsed X-band EPR of HoxY1-x(W5O18)29- (x
0.25)
Echo-detected spectrum is T2 weighted
Spectrum also sensitive to pulse sequence
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Pulsed X-band EPR of HoxY1-x(W5O18)29- (x
0.25)
Competing anisotropies (TUNNELING) ? no
longer obvious what is parallel/perpendicular
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Pulsed X-band EPR of HoxY1-x(W5O18)29- (x
0.25)
Cancelation resonances ? significant reduction in
decoherence
Note excitation bandwidth Comparable to linewidth
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Pulsed X-band EPR of HoxY1-x(W5O18)29- (x
0.25)
COHERENT QUANTUM TUNNELING
Note excitation bandwidth Comparable to linewidth
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Pulsed X-band EPR of HoxY1-x(W5O18)29- (x 0.1)

• Sample not perfectly aligned shift to consistent
  with simulations
• Cancelation resonances now stronger than the
  standard ones!!
• T2 factor of two larger for cancelation resonances

Impurity in cavity
T2 200 ns
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Pulsed X-band EPR concentration dependence
Electron-Spin-Echo- Envelope-Modulation (ESEEM)
1.2 ms
ESEEM frequency Consistent with Coupling to
protons