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URN: urn:nbn:de:bsz:25-opus-87101
URL: http://www.freidok.uni-freiburg.de/volltexte/8710/

Nehrkorn, Joscha Paul

Exploring molecular nanomagnets with spectroscopic techniques: spin exchange coupling and high anisotropy

Untersuchungen von molekularen Nanomagneten mit spektroskopischen Techniken: Austauschwechselwirkung zwischen Spins und Hohe Anisotropie

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Kurzfassung in Englisch

The spin-exchange couplings in molecular nanomagnets (MNMs) were experimentally investigated with inelastic neutron scattering (INS) and frequency-domain Fourier-Transform THz electron paramagnetic resonance (FDFT THz EPR). Furthermore we used these two spectroscopic techniques to investigate MNMs which incorporate highly anisotropic metal ions with an unquenched orbital angular momentum.

The high-spin molecule Mn18Sr was studied by INS. From a combined analysis of the INS spectra and the magnetization measurements a magnetic model for the exchange interaction was developed. The obtained low-lying excitations, which were observed experimentally, were associated to spin-wave excitations. It was therefore possible to study the effect of the application of linear spin-wave theory to a zero-dimensional cluster.
Furthermore, two Mn7disks were studied by INS. The property of INS to probe not only the energies but also the wave functions was of high importance for our analysis of the measured data. The Q dependence allowed to identify the type of the observed transitions, which was essential for the development of a magnetic model which reproduces the experimental results. Furthermore, we analyzed the distinct Q dependence of spin wave excitations of finite rings and disks.
The single-molecule magnet (SMM) Mn12wheel attracted interest due to its magnetic hysteresis which could not be explained by the usually used giant-spin approach. Controversial discussions about the right effective model followed. A deeper understanding of the underlying exchange interactions might resolve this in a natural way. Hence, we studied this molecule with INS. The high-quality data contained around a dozen magnetic peaks, which demonstrated the diverse magnetic structure of this MNM. Several magnetic models were introduced by us. Even though, we gained some insight into the magnetic properties of the Mn12wheel, we had to admit that we were not able to develop a model which reproduces the experimental data in all aspects.

Magnetization measurements on several Co4cubane molecules revealed a curvature of the magnetization which can be associated with a level crossing between two magnetic ground states. An effective model was developed, which excellently reproduced the measured magnetizations of the Co4cubane molecule studied here at low temperatures. The structure suggested to describe the magnetic properties in the slightly idealized symmetry S4. However, our analysis revealed that the magnetic properties can only be described using C2 symmetry, which is the exact symmetry of the molecule.
The magnetic properties of the molecule Co8 were studied by analyzing the measured magnetization data with an effective model valid at low temperatures. Furthermore, we measured this sample with FDFT THz EPR. Our analysis revealed that the dominant exchange interaction within the molecule has highly anisotropic parameters. However, the obtained energy spectrum was essentially isotropic. We showed that the seemingly high anisotropy was a result of the chosen local coordinate frames. Our analysis revealed that for highly anisotropic systems like CoII clusters, the choice of the local coordinate frames, which can obviously be chosen at will, have strong impact on the interpretation and the understanding of the magnetic properties.

Four members of a series of Mn2Ln2 squares (where Ln can be any rare earth ion except for promethium, scandium and lanthanum) were studied by INS and FDFT THz EPR. First of all, this experiment demonstrated that it is possible to study MNMs containing dysprosium with INS, albeit dysprosium's high absorption cross section for neutrons.
Our analysis revealed that a profound knowledge about the crystal field splittings of the rare earth ions incorporated in MNMs is essential for the understanding of the magnetic properties of the MNM.

The SMM Mn2Nd2 butterfly and the analogue Mn2La2, which did not show SMM behavior, were studied by INS and FDFT THz EPR. Magnetic models were developed which reproduced the experimental data excellently. Mn2La2 had a remarkably high energy barrier for spin reversal. However, the relaxation of the magnetization was too fast to be detected in AC susceptibility measurements. This was rationalized by us with a large tunneling probability in the ground state. In contrary to all exceptions the replacement of the diamagnetic La with Nd does not lead to a higher anisotropy, i.e a higher energy barrier. In a sort of exchange-biased mechanism the tunneling probability was reduced to almost zero in Mn2Nd2 by the weak exchange interactions between the Mn and Nd ions.

Kurzfassung in Deutsch

Die Austauschwechselwirkungen zuwischen den Spinzentren in molekularen Nanomagneten (MNMs) wurden mit inelastischer Neutronenstreuung (INS) und mit Frequenz-Domänen Fourier-Tranformierter THZ Elktronenspinresonanz (FDFT THz EPR) untersucht. Außerdem wurden diese beiden spektroskopischen Techniken verwendet um MNMs zu untersuchen, die Metallzentren mit nicht gequenschtem Bahndrehimpuls enthalten und daher hoch anisotrop sind.

SWD-Schlagwörter: Inelastische Neutronenstreuung , Elektronenspinresonanzspektroskopie , Magnetismus
Freie Schlagwörter (englisch): molecular nanomagnets , single moelcule magnets , FDFT THz EPR
PACS Klassifikation , 75.30.Gw , 75.30.Ds , 75.45.+j , 75.50.Xx
Institut: Physikalisches Institut
Fakultät: Fakultät für Mathematik und Physik
DDC-Sachgruppe: Physik
Dokumentart: Dissertation
Erstgutachter: Waldmann, Oliver (Prof. Dr.)
Sprache: Englisch
Tag der mündlichen Prüfung: 25.07.2012
Erstellungsjahr: 2012
Publikationsdatum: 15.08.2012