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

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

Heavy Atom Radical Ferromagnets Record: T c = 17K H c = 0.15 T Oakley et al., JACS 130, (2008); JACS 131, 7112 (2009)

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, (2010); JACS 131, 1986 (2009). Radicals well known to EPR spectroscopists

Heavy Atom Radical Ferromagnets Most importantly: huge (record) coercive field (1.4 kOe at 2 K) Resonance field (tesla) Record: T c = 17K H c = 0.15 T 1: H A = 0.8 T 2: H A = 0.45 T

Heavy Atom Radical Ferromagnets Record: T c = 17K H c = 0.15 T Hubbard Hamiltonian with spin-orbit (s) and hopping (h) perturbations

Ishikawa et al., Mononuclear Lanthanide Single Molecule Magnets Hund’s rule coupling for Ho 3+ : L = 6, S = 2, J = 8; 5 I 8 Axial ligand-field: m J = ±5 I = 7/2 nuclear spin (100%)

Mononuclear Lanthanide Molecular Nanomagnets Based on Polyoxometalates Mononuclear Lanthanide Molecular Nanomagnets Based on Polyoxometalates [Ln(W 5 O 18 ) 2 ] 9- (Ln III = Tb, Dy, Ho, Er, Tm, and Yb) ~D 4d AlDamen et al.,

Er 3+ and Ho 3+ Exhibit some SMM characteristics Er 3+ compound Mononuclear Lanthanide Molecular Nanomagnets Based on Polyoxometalates Mononuclear Lanthanide Molecular Nanomagnets Based on Polyoxometalates

AlDamen et al., D 4d (   ≠ 45 o ) Fits to  m T & NMR Mononuclear Lanthanide Molecular Nanomagnets Based on Polyoxometalates Mononuclear Lanthanide Molecular Nanomagnets Based on Polyoxometalates

Ho 3+ : [Xe]4f 10 Ground state: m J = ±4 AlDamen et al., Hund’s rule coupling for Ho 3+ : L = 6, S = 2, J = 8; 5 I 8 D = cm  1, B 0 4 = 6.94 ×10  3 cm  1, B 0 6 =  4.88 ×10  5 cm  1 g J = 5/4 Mononuclear Lanthanide Molecular Nanomagnets Based on Polyoxometalates Mononuclear Lanthanide Molecular Nanomagnets Based on Polyoxometalates

Ho 3+ : [Xe]4f 10 AlDamen et al., Hund’s rule coupling for Ho 3+ : L = 6, S = 2, J = 8; 5 I 8 D = cm  1, B 0 4 = 6.94 ×10  3 cm  1, B 0 6 =  4.88 ×10  5 cm  1 g J = 5/4 Other relevant details: 100% I = 7/2 nuclear spin100% I = 7/2 nuclear spin Strong hyperfine couplingStrong hyperfine coupling Dilution: [Ho x Y 1-x (W 5 O 18 ) 2 ] 9-Dilution: [Ho x Y 1-x (W 5 O 18 ) 2 ] 9- Na + charge compensationNa + charge compensation H 2 O solventH 2 O solvent Mononuclear Lanthanide Molecular Nanomagnets Based on Polyoxometalates Mononuclear Lanthanide Molecular Nanomagnets Based on Polyoxometalates

B//c Broad 8 line spectrum due to strong hyperfine coupling to Ho nucleus, I = 7/2 High(ish) frequency EPR of [Ho x Y 1-x (W 5 O 18 ) 2 ] 9- ( x = 0.25 )

B//c 1 K = 21 GHz 1 cm -1 = 30 GHz Nominally (strongly) forbidden transitions: m J =  4  +4,  m I = 0Nominally (strongly) forbidden transitions: m J =  4  +4,  m I = 0 Next excited level at least cm -1 above This suggests mixing (tunneling) of m J states (no EPR for f > 100 GHz)This suggests mixing (tunneling) of m J states (no EPR for f > 100 GHz) High(ish) frequency EPR of [Ho x Y 1-x (W 5 O 18 ) 2 ] 9- ( x = 0.25 )

Indicative of strong anisotropy associated with J = 8 ground state Note: hyperfine splitting also exhibits significant anisotropy Angle-dependence: [Ho x Y 1-x (W 5 O 18 ) 2 ] 9- single crystal ( x = 0.25 )

D = cm  1, B 0 4 = 6.94 ×10  3 cm  1, B 0 6 =  4.88 ×10  5 cm  1 Full Matrix Analysis of the Angle-dependence Ligand field parameters from: AlDamen et al., Inorg. Chem. 48, 3467 (2009) g z = 1.06 A = 835 MHz ( cm -1 ) Simulations assume isotropic g data do not constrain g xy so well Free ion g = 1.25

Standard CW X-band EPR of [Ho x Y 1-x (W 5 O 18 ) 2 ] 9- ( x = 0.25 ) Multi-frequency studies: does D 4d parameterization hold water? f ~ 9.5 GHz

Standard CW X-band EPR of [Ho x Y 1-x (W 5 O 18 ) 2 ] 9- ( x = 0.25 ) Multi-frequency studies: does D 4d parameterization hold water? f ~ 9.5 GHz

Standard CW X-band EPR of [Ho x Y 1-x (W 5 O 18 ) 2 ] 9- ( x = 0.25 ) f ~ 9.5 GHz D 4d symmetry approximate → natural to add: ~9 GHz tunneling gap -    ≠ 45 o

Standard B 1  B 0 configuration Parallel mode (B 1 //B 0 ) Standard CW X-band EPR of [Ho x Y 1-x (W 5 O 18 ) 2 ] 9- ( x = 0.25 )

T 1 ~ 1  s T 2 ~ 140 ns Pulsed X-band EPR of [Ho x Y 1-x (W 5 O 18 ) 2 ] 9- ( x = 0.25 ) T = 4.8 K Hahn echo sequence Rabi oscillations: remarkably long T 2

T = 4.8 K T 2 ~ 140 ns Pulsed X-band EPR of [Ho x Y 1-x (W 5 O 18 ) 2 ] 9- ( x = 0.25 ) Rabi oscillations: remarkably long T 2 Fe 8 S = 10 Fe 4 S = 5 Cr 7 Ni (S = 1): 0.2mg/mL, T 2 ~300 5K Ardavan et al., PRL 98, (2007) Fe 4 : 0.5g/mL, 95 GHz and B = 0 Schlegel et al., PRL 101, (2008) Fe 8 : 240 GHz and 4.6 T (k B T ~ 11.5 K) Takahashi et al., PRL 102, (2009)

Echo-detected spectrum is T 2 weighted Spectrumalsosensitive to pulse sequence Pulsed X-band EPR of [Ho x Y 1-x (W 5 O 18 ) 2 ] 9- ( x = 0.25 )

 m I = 0  m I = ±1 Competing anisotropies (TUNNELING): → no longer obvious what is parallel/perpendicular Pulsed X-band EPR of [Ho x Y 1-x (W 5 O 18 ) 2 ] 9- ( x = 0.25 )

Cancelation resonances → significant reduction in decoherence 1 Mohammady et al., Phys. Rev. Lett. 105, (2010) Bi (I = 9/2) in Si 1 Note: excitation bandwidth Comparable to linewidth

Pulsed X-band EPR of [Ho x Y 1-x (W 5 O 18 ) 2 ] 9- ( x = 0.25 ) COHERENT QUANTUM TUNNELING Note: excitation bandwidth Comparable to linewidth

Pulsed X-band EPR of [Ho x Y 1-x (W 5 O 18 ) 2 ] 9- ( x = 0.1 ) Impurity in cavity Sample not perfectly aligned; shift to consistent with simulationsSample not perfectly aligned; shift to consistent with simulations Cancelation resonances now stronger than the standard ones!!Cancelation resonances now stronger than the standard ones!! T 2 factor of two larger for cancelation resonancesT 2 factor of two larger for cancelation resonances T 2 ~ 200 ns

Electron-Spin-Echo- Envelope-Modulation (ESEEM) 1.2  s Pulsed X-band EPR: concentration dependence ESEEM frequency Consistent with Coupling to protons