AVS 49th International Symposium
    Molecular and Bio-Magnetism Tuesday Sessions
       Session MB+BI+OF-TuA

Invited Paper MB+BI+OF-TuA3
Density-Functional-Based Simulation of Molecular Magnets

Tuesday, November 5, 2002, 2:40 pm, Room C-205

Session: Molecular and Bio-Magnetism
Presenter: M.R. Pederson, Naval Research Laboratory
Authors: M.R. Pederson, Naval Research Laboratory
N. Bernstein, Naval Research Laboratory
T. Baruah, Georgetown University
J. Kortus, Max-Planck-Institute, Germany
Correspondent: Click to Email
Recently a class of transition-metal containing molecules have attracted significant experimental interest because they retain their magnetic orientation at relatively high temperatures and because they exhibit quantum tunneling of magnetism. These molecular magnets consist of approximately 70-200 atoms and are typically composed of 4-15 transition metal atoms which are held in place by organic ligands and anions. The fundamental figure of merit which governs these phenomena is the magnetic anisotropy which arises due to the spin-orbit interaction and other couplings between spin and spatial degrees of freedom. Recently, a quantum-mechanical method has been developed which allows for the density-functional-based determination of magnetic anisotropies in molecules and clusters.@footnote 1@ We have used this method to calculate anisotropies in several molecular magnets which include: Mn@sub 12@O@sub 12@(RCOO)@sub 16@(H@sub 2@O)@sub 4@, Fe@sub 8@O@sub 2@(OH)@sub 12@(C@sub 6@N@sub 3@H@sub 15@)@sub 6@, Co@sub 4@C@sub 5@NH@sub 4@CH@sub 2@O)@sub 4@(CH@sub 3@OH)@sub 4@Cl@sub 4@, and [Mn@sub 10@O@sub 4@(2,2’-biphenoxide)@sub 4@Br@sub 12@]@super 4-@. Our calculations show that good agreement between experiment and theory can be obtained. While the reorientation barriers and magnetic resonant tunneling fields are primarily determined from the second-order anisotropy hamiltonian,@footnote 1@ higher-order effects can change these quantities by about ten percent. Further, such effects determine tunnel splittings and play a significant role in tunneling dynamics. Currently the primary source of such splittings is an active area of investigation. We have recently suggested that vibrationally induced changes in the spin-orbit interaction will contribute to higher-order anisotropies.@footnote 2@ Further, computational results on the 4th-order magnetic anisotropy show that this interaction may provide a dominant contribution to the higher-order barriers and that it partially contributes to tunnel splittings. We discuss these calculations and compare our results to the experimental infrared work of Sushkov et al which shows that certain vibrational intensities are strongly perturbed by applied magnetic fields in the Mn@sub 12@-Acetate system.@footnote 3@ A very brief review of the computational method, NRLMOL, used in this work will be included in the talk. @FootnoteText@ @footnote 1@ M.R. Pederson and S.N. Khanna, Phys. Rev. B 60, 9566 (1999). @footnote 2@ M.R. Pederson, N. Bernstein and J. Kortus, (Cond-mate/0201353). @footnote 3@ A.B. Sushkov, B. Jones, J.L. Musfeldt, et al, Phys. Rev B 65,(2002).