No abstract

Inelastic neutron scattering (INS) has been used to investigate the crystal field (CF) magnetic excitations of the analogs of the most representative lanthanoid-polyoxometalate single-molecule magnet family: Na[Ln(WO)] (Ln = Nd, Tb, Ho, Er). Ab initio complete active space self-consistent field/restricted active space state interaction calculations, extended also to the Dy analog, show good agreement with the experimentally determined low-lying CF levels, with accuracy better in most cases than that reported for approaches based only on simultaneous fitting to CF models of magnetic or spectroscopic data for isostructural Ln families. In this work we demonstrate the power of a combined spectroscopic and computational approach. Inelastic neutron scattering has provided direct access to CF levels, which together with the magnetometry data, were employed to benchmark the ab initio results. The ab initio determined wave functions corresponding to the CF levels were in turn employed to assign the INS transitions allowed by selection rules and interpret the observed relative intensities of the INS peaks. Ultimately, we have been able to establish the relationship between the wave function composition of the CF split Ln ground multiplets and the experimentally measured magnetic and spectroscopic properties for the various analogs of the Na[Ln(WO)] family.

Correlating blocking temperatures with relaxation mechanisms in single-molecule magnets.

Magnetic exchange interactions within the asymmetric dimetallic compounds [hqH][Ln(hq)(NO)]·MeOH, (Ln = Er(III) and Yb(III), hqH = 8-hydroxyquinoline) have been directly probed with EPR spectroscopy and accurately modeled by spin Hamiltonian techniques. Exploitation of site selectivity via doping experiments in Y(III) and Lu(III) matrices yields simple EPR spectra corresponding to isolated Kramers doublets, allowing determination of the local magnetic properties of the individual sites within the dimetallic compounds. CASSCF-SO calculations and INS and far-IR measurements are all employed to further support the identification and modeling of the local electronic structure for each site. EPR spectra of the pure dimetallic compounds are highly featured and correspond to transitions within the lowest-lying exchange-coupled manifold, permitting determination of the highly anisotropic magnetic exchange between the lanthanide ions. We find a unique orientation for the exchange interaction, corresponding to a common elongated oxygen bridge for both isostructural analogs. This suggests a microscopic physical connection to the magnetic superexchange. These results are of fundamental importance for building and validating model microscopic Hamiltonians to understand the origins of magnetic interactions between lanthanides and how they may be controlled with chemistry.

Combining Ising-type magnetic anisotropyw ith collinear magnetic interactions in single-molecule magnets (SMMs) is as ignificant synthetic challenge.H erein we report aD y[15-MC Cu-5] (1-Dy)S MM, where aD y III ion is held in ac entral pseudo-D 5h pocket of ar igid and planar Cu 5 metallacrown (MC). Linking two Dy[15-MC Cu-5] units with as ingle hydroxideb ridge yields the doubledecker {Dy[15-MC Cu-5]} 2 (2-Dy)S MM where the anisotropya xes of the two Dy III ions are nearly collinear,resulting in magnetic relaxation times for 2-Dy that are approximately 200 000 times slower at 2Kthan for 1-Dy in zero external field. Whereas 1-Dy and the Y III-diluted Dy@2-Y analogue do not show remanence in magnetic hysteresis experiments,t he hysteresis data for 2-Dy remain open up to 6Kwithout asudden drop at zero field. In conjunction with theoretical calculations,these results demonstrate that the axial ferromagnetic Dy-Dy coupling suppresses fast quantum tunneling of magnetization (QTM). The relaxation profiles of both complexes curiously exhibit three distinct exponential regimes,a nd hold the largest effective energy barriers for any reported d-f SMMs up to 625 cm À1 .