Spin dynamics in mesoscopic size magnetic systems: A {sup 1}HNMR study in rings of iron (III) ions
- Department of Chemistry, University of Florence, Firenze (Italy)
- Ames Laboratory and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011 (United States)
- Department of Chemistry, University of Modena, Modena (Italy)
Two magnetic molecular clusters containing almost coplanar rings of iron (III) ions with spinS=5/2 have been investigated by {sup 1}H NMR and relaxation measurements. The first system, which will be referred to as Fe6, is a molecule of general formula [NaFe{sub 6}(OCH{sub 3}){sub 12}(C{sub 17}O{sub 4}H{sub 15}){sub 6}]{sup +}ClO{sub 4}{sup {minus}} {ital or} [NaFe{sub 6}(OCH{sub 3}){sub 12}(C{sub 15}H{sub 11}O{sub 2}){sub 6}]{sup +}ClO{sub 4}{sup {minus}} {ital or} [LiFe{sub 6}(OCH{sub 3}){sub 12}(C{sub 15}H{sub 11}O{sub 2}){sub 6}]{sup +}ClO{sub 4}{sup {minus}} while the second type of ring, denoted Fe10, corresponds to the molecule [Fe{sub 10}(OCH{sub 3}){sub 20}(C{sub 2}H{sub 2}O{sub 2}Cl){sub 10}]. The {sup 1}H NMR linewidth is broadened by the nuclear dipolar interaction and by the dipolar coupling of the protons with the iron (III) paramagnetic moment. It is found that the nuclear spin-lattice relaxation rate, T{sub 1}{sup {minus}1}, of the proton is a sensitive probe of the Fe spin dynamics. In both clusters, T{sub 1}{sup {minus}1} decreases with decreasing temperatures from room temperature, goes through a peak just below about 30 K in Fe6 and 10 K in Fe10, and it drops exponentially to very small values at helium temperature. The temperature dependence of the relaxation rate is discussed in terms of the fluctuations of the local spins within the allowed total spin configurations in the framework of the weak collision theory to describe the nuclear relaxation. We use the calculated energy levels for the Fe6 ring based on a Heisenberg Hamiltonian and the value of J obtained from the fit of the magnetic susceptibility to describe semiquantitatively the behavior of T{sub 1}{sup {minus}1} vs T. The exponential drop of T{sub 1}{sup {minus}1} at low temperature is consistent with a nonmagnetic singlet ground state separated by an energy gap from the first excited triplet state. (Abstract Truncated)
- Research Organization:
- Ames National Laboratory
- DOE Contract Number:
- W-7405-ENG-82
- OSTI ID:
- 544500
- Journal Information:
- Physical Review, B: Condensed Matter, Vol. 55, Issue 21; Other Information: PBD: Jun 1997
- Country of Publication:
- United States
- Language:
- English
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