Electronic structure, spin couplings, and hyperfine properties of nanoscale molecular magnets

Z. Zeng, Diana Guenzburger, D. E. Ellis
1999 Physical Review B (Condensed Matter)  
First-principles self-consistent spin-polarized electronic structure calculations were performed for the nanoscale magnetic molecules Mn 12 O 12 (CH 3 COO) 16 (H 2 O) 4 and Fe 11 O 6 (OH) 6 (O 2 CPh) 15 . The numerical Discrete Variational method was employed, within Density Functional theory. Charges and magnetic moments were obtained for the atoms, as well as Density of States diagrams, and charge and spin density maps. For Mn 12 O 12 (CH 3 COO) 16 (H 2 O) 4 , values of the Heisenberg
more » ... parameters J were derived from the calculations; Mössbauer hyperfine parameters were calculated for Fe 11 O 6 (OH) 6 (O 2 CPh) 15 and compared to reported experimental values. Key-words: Nanoscale; Molecular magnets. CBPF-NF-056/98 -2photosystem II of bacterial photosynthesis [6],[7],[8]. Furthermore, many interesting magnetic properties have been demonstrated for this molecule, which crystalizes in a tetragonal structure as first determined by Lis in 1980 [9]. AC susceptibility, magnetization and EPR measurements led to the conclusion that each molecule has a total spin S=10 [10], [11] ; this large spin combined with a large easy-axis anisotropy leads to superparamagnetic behavior at low temperature , with very long relaxation time which results in pronounced hysteresis [12] . The spins of the individual magnetic ions are coupled strongly together in each molecular unit , which behaves as one small magnet; on the other hand, the magnetic interactions between units are practically negligible, due mainly to the crown of ligands surrounding and isolating the metal-oxo CBPF-NF-056/98 -3core. These properties make such system a potential candidate for molecular-sizeunits data storage devices. Many experimental measurements have been reported for this molecule, such as proton NMR and muon spin rotation [13], neutron diffraction with [14] or without [15] an applied magnetic field, magnetic circular dichroism [16] and high-frequency EPR [17]. A very exciting discovery has been the observation of steps in the hysteresis loop of the magnetization in a powdered sample [18] or single crystals [19] of Mn 12 at low temperatures. This was interpreted as a manifestation of quantum tunnelling in a macroscopic property. The nanoscale molecule Fe 11 also forms crystals of well-defined structure [20], with the molecules containing a Fe-O core surrounded by the organic ligands. This structure forms a model for ferritin. There are three crystallographically different sites for the Fe ions, all in the formal oxidation state +3. These molecular aggregates have similar magnetic properties to Mn 12 in the sense that the individual Fe spins within each molecule couple together strongly resulting in magnetic order . Moreover, Mössbauer spectroscopy measurements of the hyperfine parameters are available [20]. The results of the electronic structure calculations provide new insight into these nanoscale molecular magnets, and will add to the experimental data to provide a more complete understanding of their properties. A similar theoretical study has been reported for the mesoscopic cluster [Fe(OMe) 2 (O 2 CCH 2 Cl)] 10 , a molecular antiferromagnet denominated "ferric wheel" due to its circular arrangement [21] . We have employed the spin-polarized Discrete Variational method (DVM) [22] of Density Functional theory (DFT) [23] to obtain energy levels, charge and spin densities for Mn 12 and Fe 11 . Charges and magnetic moments on the ions are also reported. Hyperfine parameters are calculated for Fe 11 and compared to experimental values. Finally, the Heisenberg spin-coupling parameters J are obtained from the calculations for Mn 12 , employing the magnetic transition state concept [24] . In Section II we describe briefly the method employed, in Section III we report the results for Mn 12 , in Section IV the results for Fe 11 , and in Section V we briefly state our main conclusions. CBPF-NF-056/98 -5-Mn(1) +3.056 +0.2396 Mn(2) −3.889 +0.0871 Mn(3) −4.039 Heisenberg Exchange parameters (cm -1 ) J 12 − − − −94.3 J 13 −50.1 J 23 −70.8
doi:10.1103/physrevb.59.6927 fatcat:m2vj7h2zabhcnlec246rhl3moa