Investigation of the spin dynamics of polynuclear metal complexes by NMR
Zusammenfassung der Projektergebnisse
The Spin dynamics of the cyclic iron clusters NaFe8, LiFe6 and CsFe8 was studied by nuclear magnetic resonance (NMR). The low frequency tail of the magnetic fluctuation spectrum was observed with NMR by means of the longitudinal relaxation rate T1-1. The NMR signal of the central 7Li , 23Na, 137Cs and the surrounding 1H nuclei was measured. The spin dynamics in this frequency range is determined by the molecular groundstate and inter-molecular interactions e.g. the magnetic dipole interaction. Strong magnetic fields induce level crossings (LC) for which the groundstate changes from S = 0, M = 0 to S = 1, M = - 1 and S = 2, M = - 2 etc.. The intensity of the fluctuation spectrum increases with the field strength continuously from plateau to plateau for the S = 0, 1, 2 etc. groundstate. The modification of the molecular wave functions with the strength of the magnetic field is reflected by the width of the spectrum especially around avoided LC when the wave functions are modified due to small interactions breaking the molecular symmetry. The effect strongly reduces the width of the fluctuation spectrum inducing spin-spin correlation times much longer than the inverse nuclear Larmor-frequency. Very important is the observation that not only the modification of wave functions in the groundstate but also the repulsion of two excited states influences strongly the width of the spectrum. This effect could be observed for the first excited S=0 and S=2 states in the field range between the 1st and 2nd LC. Another important effect caused by the mixing of wave functions at the LC is a static transversal component of the molecular magnetization induced by the applied magnetic field around anti-LC. The effect of the transverse magnetization cancels in the center of the ring due to the antiferromagnetic coupling of the Fe'" ions but leads to extremely large frequency shifts of several MHz for the surrounding 1H nuclei. The magneto-elastic distorted phase around the first LC of CeFe8 could be characterized by means of this effect, which shows that the CeFe8 complex itself undergoes a deformation within the field and temperature range of the distorted phase. The opening of the related energy gap was determined by measurements of T1-1.
Projektbezogene Publikationen (Auswahl)
- 2007 Dissertation, Universität Karlsruhe, Fakultät für Physik, Physikalisches Institut. Spindynamik von zyklischen Spinclusterverbindungen
Roland Leppin
- European Physical Journal B - Condensed Matter, 2007, 55, 229-236. Density-functional calculation of the quadrupole splitting in the 23Na NMR spectrum of the ferric wheel Na@Fe6(tea)6+ for various broken-symmetry states of the Heisenberg spin model
F. A. Bischoff, O. Hübner, W. Klopper, L. Schnelzer, B. Pilawa, M. Horvatic, C. Berthier
- Physical Review Letters, 2007, 99, 87201. Huge Transverse Magnetic Polarization in the Field-Induced Phase of the Antiferromagnetic Molecular Wheel CsFes8
L. Schnelzer, O. Waldmann, M. Horvatic, S.T. Ochsenbein, S. Krämer, C. Berthier, H. U. Güdel, B. Pilawa
- 2008 Dissertation, Universität Karlsruhe, Fakultät für Physik, Physikalisches Institut. Spindynamik an zyklischen Eisen-Rädern in hohen magnetischen Feldern
Lars Schnelzer
