1. A spectroscopic comparison between several high-symmetry S = 10 Mn12 single-molecule magnets
S. Hill, N. Anderson, A. Wilson, S. Takahashi, and J. Lawrence
Department of Physics, University of Florida, Gainesville
N. Chakov, M. Murugesu, and G. Christou
Department of Chemistry, University of Florida, Gainesville
M. North, and N. Dalal
Department of Chemistry and Biochemistry, Florida State University, Tallahassee
High-field/frequency EPR methodology
Determination of transverse crystal field parameters
Emphasis on Mn12-acetate and solvent disorder
The new Mn12-tBuAc and Mn12-BrAc complexes
Summary and conclusions
Supported by: NSF, Research Corporation, & University of Florida

2. Single-crystal, high-field/frequency EPR
field//z
z, S4-axis Hz
Magnetic dipole transitions (Dms = ±1) - note frequency scale!
First of all, these terms are not so small:
How on earth are we going to measure tiny transverse terms?

3. Single-crystal, high-field/frequency EPR
Rotate field in xy-plane and look for symmetry effects s
z, S4-axis
Hxy
In high-field limit (gmBB > DS), ms represents spin- projection along the applied field-axis

4. Hard-plane rotations for d-Mn12-acetate
f = 51.3 GHz
T = 15 K
Note the fine structures
arXiv/cond-mat/
Data for h-Mn12-acetate in: S. Hill et al., PRL 90, (2003)

5. Determination of transverse crystal-field interactions in d-Mn12-Ac
Identical to h-Mn12-Ac
Four-fold line shifts due to a quartic transverse interaction in HT
Previously inferred from neutron studies
Mirebeau et al., PRL 83, 628 (1999)
B44 is the only free parameter in our fit
S. Hill et al., PRL 90, (2003)
Hard-plane (xy-plane) rotations f

6. Determination of transverse crystal-field interactions in d-Mn12-Ac
Identical to h-Mn12-Ac
Two-fold line shifts associated with the high- and low-field shoulders due to a quadratic transverse interaction in HT
del Barco et al., arXiv/cond-mat/ f
Incompatible with the crystallographic symmetry!
HC and HE incommensurate!

7. Disorder lowers the symmetry of the molecules=
del Barco et al., arXiv/cond-mat/
E. del Barco et al.,
PRL 91, (2003)
S. Hill et al., PRL 90,
(2003)

8. Mn12-Ac Mn12-tBuAc Phys. Rev. B 70, 054426 (2004)
[Mn12O12(O2CMe)16(H2O)4]·2MeCO2H·4H2O vs. [Mn12O12(O2CCH2But)16(MeOH)4]·MeOH
Phys. Rev. B 70, (2004)
Synthesis:
[Mn12O12(O2CMe)16(H2O)4] RCO2H [Mn12O12(O2CR)16(H2O)4] + 16 MeCO2H
CH2Cl2
Mn12-Ac
Mn12-tBuAc
Less solvent of crystallization
Bulky R group: well separated molecules
Well aligned

9. Spectroscopists Hamiltonian: Physicists Hamiltonian:
Spin Hamiltonian parameters for Mn12-tBuAc
Hard plane
rotations
B//c
Bc
Spectroscopists Hamiltonian:
Ĥ = DŜz2 + B40Ô40 + B44Ô44
Physicists Hamiltonian:
Ĥ = D´Ŝz2 + BŜz4 + C(Ŝ+4 + Ŝ-4)
g// = 2; g = 1.94
D, B40, g// from easy axis data
B44 from hard plane rotations
g from perpendicular data
(unpublished)

10. Rotation away from the hard planeq
b9 does not appear until 2.5 degrees of rotation
a10 vanishes in the first degree of rotation
Phys. Rev. B 70,
(2004)

11. Compare Mn12-tBuAc and Mn12-Ac HFEPR spectra
simulations, no disorder
Clean
Disorder!
E-strain
Mn12-Ac: discrete easy-axis tilting

12. Summary and conclusions
Considerable body of experimental data in support of Cornia's solvent disorder model in both h-Mn12-Ac and d-Mn12-Ac.
We have now found a nice Mn12-BrAc and Mn12-tBuAc systems which do not exhibit the disorder found in Mn12-Ac.
Mn12-BrAc: Petukhov et al., Phys. Rev. B 70, (2004)
Mn12-Ac: Takahashi et al., Phys. Rev. B 70, (2004)
del Barco et al., arXiv/cond-mat/
Hill et al., Phys. Rev. Lett. 90, (2003)

13. =
del Barco et al., arXiv/cond-mat/