Browsing by Author "Hallak, Fadi el"

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Magnetic anisotropy of molecular nanomagnets
(2009) Hallak, Fadi el; Dressel, Martin (Prof. Dr.)
Magnetic anisotropy lies at the heart of the molecular magnetism field of research. It is the magnetic anisotropy which leads to slow relaxing magnetic moments in single molecule magnets. Furthermore, it plays a vital role in the process of macroscopic quantum tunneling of the magnetization. Consequently, the understanding of the magnetic anisotropy in molecular magnetic clusters can lead to a better understanding of the underlying physics of the magnetic and quantum behavior of molecular nanomagnets. In this Thesis, we have aimed at studying the magnetic anisotropy in diverse molecular magnetic systems. We have demonstrated the necessity of employing complementary techniques in studying the magnetic anisotropy in molecular magnets, where the information extracted from a single method may in some cases lead not only to an incomplete, but rather to an erroneous picture. Spectroscopy remains to date the most powerful method to investigate anisotropy in such systems. We have used frequency domain magnetic resonance to spectroscopically probe the spin fine structure of the ground state in molecular nanomagnets. However, not all systems are spectroscopically active, and spectroscopy may fail in systems with unfavorable spin-lattice relaxation times, large zero field splittings, or excessive dipolar broadening. Therefore, we have developed a torque magnetometry setup to either complement or replace spectroscopy in some systems. Finally, we combined both methods, thus creating a novel torque detected broad band ESR technique. In addition, we have investigated the interplay between the isotropic exchange interaction (J) and the single ion anisotropy (D), and how does that affect the magnetic as well as the quantum mechanical properties of the system. In the strong exchange limit (J>D), the giant spin model can nicely describe the properties of the system at low temperatures. This was demonstrated by using our newly developed torque detected ESR method to investigate an Fe4 system. Studying a family of Mn based compounds, we have seen that the anisotropy barrier is strongly modified in the intermediate exchange limit (J≈D), where the excited spin states play a crucial role in the magnetization relaxation by mixing into the spin ground state. On the other extreme of the coupling scheme, a dominant anisotropy term (D>J) leads to an unconventional energy scheme in an Fe(II) dimer. The system shows a field induced spin state crossing from the S=0 to the S=4 levels which is accompanied by a giant step in the magnetization. Moreover, the theoretical analysis demonstrated that this molecule is a very promising candidate for the direct observation of coherent tunneling in molecular nanomagnets. Finally, we performed high field torque magnetometric investigations to quantify the anisotropy in a molecular Dy3 triangle. This study has contributed to the understanding of magnetic anisotropy in lanthanide-based molecular magnets, which may lead to the development of improved single molecule magnets.