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

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?

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

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

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, 217204 (2003) Hard-plane (xy-plane) rotations f

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/0404390 f Incompatible with the crystallographic symmetry! HC and HE incommensurate!

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)

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, 054426 (2004) Synthesis: [Mn12O12(O2CMe)16(H2O)4] + 16 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

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)

Rotation away from the hard plane q b9 does not appear until 2.5 degrees of rotation a10 vanishes in the first degree of rotation Phys. Rev. B 70, 094429 (2004)

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

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)

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