Session A6 - Molecular Nanomagnets.
INVITED session, Monday morning, March 22
516AB, Palais des Congres
I will discuss detailed angle-dependent single-crystal EPR measurements for several variants of the Mn_12O_12 single-molecule magnet (SMM). For the widely studied Mn_12-acetate, hard-plane rotations reveal a clear four-fold variation in the EPR peak positions which may be attributed to an intrinsic fourth-order transverse anisotropy (\hatO^4_4).^1 Meanwhile, a four-fold variation of the EPR lineshapes confirms a recent model proposed by Cornia et al.,^2 wherein disorder associated with the acetic acid of crystallization induces a locally varying quadratic (rhombic) transverse anisotropy (\hatO^2_2).^1 Rotations away from the hard plane additionally indicate that the magnetic easy-axes of the disordered Mn_12O_12 SMMs are significantly tilted (a discrete distribution with a width of up to 1.7^\circ) with respect to the global 4-fold axis. These findings explain essentially all aspects of the magnetic quantum tunneling observed in Mn_12-acetate. Time permitting, I will also present recent EPR results for a dimer of Mn_4 SMMs.^3 The measured spectra display well resolved quantum transitions involving coherent superposition states of both molecules within the dimer. This behavior may be understood in terms of an isotropic superexchange coupling between pairs of SMMs, in analogy with several recently proposed quantum devices based on artificially fabricated quantum dots or clusters. These findings highlight the potential utility of supramolecular chemistry in the design of future quantum devices based on molecular nanomagnets. This work was supported by the NSF (DMR0103290 and DMR0239481) and by Research Corporation.
^1 S. Hill et al., Phys. Rev. Lett. 90, 217204 (2003).
^2 A. Cornia et al., Phys. Rev. Lett. 89, 257201 (2002).
^3 S. Hill et al., Science 302, 1015 (2003).