Andrew Harter

Synthesis, structure, and spectroscopic and magnetic characterization of [Mn12O12(O2CCH2But)16(MeOH)4]·MeOH, a Mn12 single-molecule magnet with true axial symmetry.

The synthesis and properties are reported of a rare example of a Mn(12) single-molecule magnet (SMM) in truly axial symmetry (tetragonal, I4). [Mn(12)O(12)(O(2)CCH(2)Bu(t))(16)(MeOH)(4)]·MeOH (3·MeOH) was synthesized by carboxylate substitution on [Mn(12)O(12)(O(2)CMe)(16)(H(2)O)(4)]·2MeCO(2)H·4H(2)O (1). The complex was found to possess an S = 10 ground state, as is typical for the Mn(12) family, and displayed both frequency-dependent out-of-phase AC susceptibility signals and hysteresis loops in single-crystal magnetization vs DC field sweeps. The loops also exhibited quantum tunneling of magnetization steps at periodic field values. Single-crystal, high-frequency electron paramagnetic resonance spectra on 3·MeOH using frequencies up to 360 GHz revealed perceptibly sharper signals than for 1. Moreover, careful studies as a function of the magnetic field orientation did not reveal any satellite peaks, as observed for 1, suggesting that the crystals of 3 are homogeneous and do not contain multiple Mn(12) environments. In the single-crystal (55)Mn NMR spectrum in zero applied field, three well-resolved peaks were observed, which yielded hyperfine and quadrupole splitting at three distinct sites. However, observation of a slight asymmetry in the Mn(4+) peak was detectable, suggesting a possible decrease in the local symmetry of the Mn(4+) site. Spin-lattice (T(1)) relaxation studies were performed on single crystals of 3·MeOH down to 400 mK in an effort to approach the quantum tunneling regime, and fitting of the data using multiple functions was employed. The present work and other recent studies continue to emphasize that the new generation of truly high-symmetry Mn(12) complexes are better models for thorough investigation of the physical properties of SMMs than their predecessors such as 1.

Unconventional spin glass behavior in the cubic pyrochlore Mn2Sb2O7

The new pyrochlore Mn2Sb2O7 has been synthesized via a low-temperature method to retain cubic symmetry. The resulting material has an unconventional spin glass state at TSG~41 K, in the absence of detectable site disorder or a structural phase transition. A peak in the DC susceptibility at TSG, and a characteristic shift in the real part of the AC susceptibility was noted, but with a slightly more dynamic state than predicted through the typical Mydosh spin glass parameter of ΔTSG/[TSGlog(ω)]. Time-dependent magnetization measurements show glassy behavior below 41 K. Elastic neutron scattering measurements on powder samples show significant magnetic diffuse scattering from the Mn spins beneath the transition, similar to the scattering observed in the spin liquid Tb2Ti2O7. The dynamic character of Mn2Sb2O7 is inferred to arise from the frustration inherent within the pyrochlore lattice.