A spectroscopic comparison between several high-symmetry S = 10 Mn 12 single-molecule magnets

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

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

4. Note the fine structures Hard-plane rotations for d-Mn12-acetate f = 51.3 GHz T = 15 K arXiv/cond-mat/0404390 Data for h-Mn12-acetate in: S. Hill et al., PRL 90, 217204 (2003)

5. Determination of transverse crystal-field interactions in d-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, 217204 (2003) f Identical to h-Mn12-Ac Hard-plane (xy-plane) rotations

6. Determination of transverse crystal-field interactions in d-Mn12-Ac f 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/0404390 Incompatible with the crystallographic symmetry! HC and HE incommensurate!

7. Disorder lowers the symmetry of the molecules = del Barco et al., arXiv/cond-mat/0404390 E. del Barco et al., PRL 91, 047203 (2003) S. Hill et al., PRL 90, 217204 (2003)

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

9. Spin Hamiltonian parameters for Mn12-tBuAc B//c Bc Hard plane rotations 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. b9 does not appear until 2.5 degrees of rotation a10 vanishes in the first degree of rotation Rotation away from the hard plane q Phys. Rev. B 70, 094429 (2004)

11. Compare Mn12-tBuAc and Mn12-Ac HFEPR spectra Disorder! E-strain simulations, no disorder Clean 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, 054426 (2004) Mn12-Ac: Takahashi et al., Phys. Rev. B 70, 094429 (2004) del Barco et al., arXiv/cond-mat/0404390 Hill et al., Phys. Rev. Lett. 90, 217204 (2003)

13. = del Barco et al., arXiv/cond-mat/0404390