The main goal of the present work was the synthesis of alkaline-earth nitridomanganates (AExMnyNz) with extended anionic structures and the characterization of their electronic and magnetic properties. Up to now, only compounds with isolated nitridomanganate anions have been reported in the discussed ternary systems. A systematic exploratory synthesis, employing high-temperature treatment of AE nitrides and Mn under controlled N2 pressure, yielded more than ten new nitridomanganates. Their crystal structures contain anionic building blocks of different dimensionalities, ranging from isolated species to three-dimensional frameworks. In general, the formation of Mn-rich compositions was found to be driven by the emergence of Mn-Mn interactions, which creates a link between nitridometalates and transition-metal-rich binary nitrides. The obtained nitridomanganates display a plethora of interesting phenomena, such as large spin-orbit coupling, magnetic frustration, quenching of magnetism due to Mn-Mn interactions, and metal-insulator transition.

Molecule-based magnets (molecular magnets) have attracted much interest in recent decades both from an experimental and from a theoretical point of view, not only because of their interesting physical effects, but also because of their potential applications: e.g., molecular spintronics, quantum computing, high density information storage, and nanomedicine. Molecular magnets are at the very bottom of the possible size of nanomagnets. On reducing the size of objects down to the nanoscale, the coexistence of classical properties and quantum properties in these systems may be observed. In additional, molecular magnets exist with structural variability and permit selective substitution of the ligands in order to alter their magnetic properties. Therefore, these characteristics make such molecules suitable candidates for studying molecular magnetism. They can be used as model systems for a detailed understanding of interplay between structural and magnetic properties of them in order to optimize desired magnetic properties. This thesis considers the investigation of magnetic properties of several new transition metal molecular compounds via different experimental techniques (continuous wave electron spin resonance (CW ESR), pulse ESR, high-field/high-frequency ESR (HF-ESR) and static magnetization techniques). The first studied compounds were mono- and trinuclear Cu(II)-(oxamato, oxamidato)/bis(oxamidato) type compounds. First, all components of the g-tensor and the tensors of onsite ACu and transferred AN HF interactions of mononuclear Cu(II)- bis(oxamidato) compounds have been determined from CW ESR measurements at 10 GHz and at room temperature and pulse ELDOR detected NMR measurements at 35 GHz and at 20 K. The spin density distributions of the mononuclear compounds have been calculated from the experimentally obtained HF tensors. The magnetic exchange constants J of their corresponding trinuclear compounds were determined from susceptibility measurements versus temperature. Our discussion of the spin density distribution of the mononuclear compounds together with the results of the magnetic characterization of their corresponding trinuclear compounds show that the spin population of the mononuclear compounds is in interplay with the J values of their corresponding trinuclear compounds. The second studied compounds were polynuclear Cu(II)-(bis)oxamato compounds with ferrocene and ferrocenium ligands. The magnetic properties of these compounds were studied by susceptibility measurements versus temperature to determine J values. In addition, the ESR technique is used to investigate the magnetic properties of the studied compounds because they contain two different magnetic ions and because only the ESR technique can selectively excite different electron spin species. These studies together with geometries of the ferrocenium ligands determined by crystallographic studies indicate that the magnetic interaction between a central Cu(II) and a Fe(III) ions changed from the antiferromagnetic coupling to the ferromagnetic coupling when a stronger distortion of the axial symmetry in the feroccenium cation exists. Therefore, the degree of the distortion of the feroccenium cation is a control parameter for the sign of the interaction between the central Cu(II) ion and the Fe(III) spins of the studied compounds. The last two studied molecular magnets were a binuclear Ni(II) compound (Ni(II)-dimer) and a cube-like tetranuclear compound with a [Fe4O4]-cube core (Fe4-cube). HF-ESR measurements enabled us to determine the g-factor, the sign, and the absolute value of the magnetic anisotropy parameters. Using this information together with static magnetization measurements, the J value and the magnetic ground state of the studied compounds have been determined. In Ni(II)-dimer, two Ni(II) ions, each having a spin S = 1, are coupled antiferromagnetically that leads to a ground state with total spin Stot = 0. An easy plane magnetic anisotropy with a preferable direction for each Ni(II) ion is found. For Fe4-cube, a ground state with total spin Stot = 8 has been determined. The analysis of the frequency dependence and temperature dependence of HF-ESR lines reveals an easy axis magnetic anisotropy (Dcube = -22 GHz (-1 K)) corresponding to an energy barrier of U = 64 K for the thermal relaxation of the magnetization. These results indicate that Fe4-cube is favorable to show single molecular magnet (SMM) behavior.

The main goal of the present work was the synthesis of alkaline-earth nitridomanganates (AExMnyNz) with extended anionic structures and the characterization of their electronic and magnetic properties. Up to now, only compounds with isolated nitridomanganate anions have been reported in the discussed ternary systems. A systematic exploratory synthesis, employing high-temperature treatment of AE nitrides and Mn under controlled N2 pressure, yielded more than ten new nitridomanganates. Their crystal structures contain anionic building blocks of different dimensionalities, ranging from isolated species to three-dimensional frameworks. In general, the formation of Mn-rich compositions was found to be driven by the emergence of Mn-Mn interactions, which creates a link between nitridometalates and transition-metal-rich binary nitrides. The obtained nitridomanganates display a plethora of interesting phenomena, such as large spin-orbit coupling, magnetic frustration, quenching of magnetism due to Mn-Mn interactions, and metal-insulator transition.

In diesem Beitrag wird der Neuaufbau eines Versuchsstandes beschrieben, der zur Untersuchung magnetischer Eigenschaften von ferromagnetischen Werkstoffen dient. Angefangen bei der Notwendigkeit eines solchen Versuchsstandes werden der Aufbau, die Komponenten sowie die Besonderheiten beschrieben und erste Messergebnisse nach der Inbetriebnahme gezeigt. / This paper describes the new construction of a test bench used to investigate magnetic properties of ferromagnetic materials. Starting with the necessity of a test bench of this kind, the setup, the components and the special features are described and the first measurement results are shown after commissioning.

No description available.

The research in low-dimensional (low-D) quantum spin systems has become an arduous challenge for the condensed matter physics community during the last years. In systems with low dimensional magnetic interactions the exchange coupling is restricted to dimensions lower than the full three-D exhibited by the bulk real material. The remarkable interest in this field is fueled by a continuous stream of striking discoveries like superconductivity, quantum liquid and spin gap states, chiral phases, etc, derived from the strong effect of quantum fluctuations on the macroscopic properties of the system and the competition between electronic and magnetic degrees of freedom. The main goal of the current studies is to reach a broad understanding of the mechanisms that participate in the formation of those novel ground states as well as the characteristic dependence with respect to relevant physical parameters. In this thesis we present the results of an Electron Spin Resonance (ESR)-based study on different quasi-1D spin systems, exemplifying the realization of 1D-magnetic spin-chains typically containing transition metal oxides such as Cu2+ or V4+. The local sensitivity of the ESR technique has been considered useful in exploring magnetic excitation energies, dominant mechanisms of exchange interactions, spin fluctuations and the dimensionality of the electron spin system, among others. Aside from ESR other experimental results, e.g., magnetization and nuclear magnetic resonance besides some theoretical approaches were especially helpful in achieving a proper understanding and modeling of those low-D spin systems. This thesis is organized into two parts: The first three chapters are devoted to the basic knowledge of the subject. The first chapter is about magnetic exchange interactions between spin moments and the effect of the crystal field potential and the external magnetic field. The second chapter is a short introduction on exchange interactions in a 1D-spin chain, and the third chapter is devoted to ESR basics and the elucidation of dynamic magnetic properties from the absorption spectrum parameters. The second part deals with the experimental results. In the fourth chapter we start with the magnetization results from the zero-dimensional endohedral fullerene Dy3N@C80. This system is seemingly ESR “silent” at the frequency of X-band experiments. The fifth chapter shows an unexpected temperature dependence of the anisotropy in the homometallic ferrimagnet Na2Cu5Si4O14 containing alternating dimer-trimer units in the zig-zag Cu-O chains. In the sixth chapter different magnetic species in the layer structure of vanadium oxide nanotubes (VOx-NT) have been identified, confirming earlier magnetization measurements. Moreover the superparamagnetic-like nature of the Li-doped VOx-NT samples was found to justify its ferromagnetic character at particular Li concentration on the room temperature scale. In the seventh chapter the Li2ZrCuO4 system is presented as a unique model to study the influence of additional interactions on frustrated magnetism. The eighth chapter highlights the magnetic properties of the pyrocompound Cu2As2O7. The results suggest significant spin fluctuations below TN. The thesis closes with the summary and the list of references.

Molecule-based magnets (molecular magnets) have attracted much interest in recent decades both from an experimental and from a theoretical point of view, not only because of their interesting physical effects, but also because of their potential applications: e.g., molecular spintronics, quantum computing, high density information storage, and nanomedicine. Molecular magnets are at the very bottom of the possible size of nanomagnets. On reducing the size of objects down to the nanoscale, the coexistence of classical properties and quantum properties in these systems may be observed. In additional, molecular magnets exist with structural variability and permit selective substitution of the ligands in order to alter their magnetic properties. Therefore, these characteristics make such molecules suitable candidates for studying molecular magnetism. They can be used as model systems for a detailed understanding of interplay between structural and magnetic properties of them in order to optimize desired magnetic properties. This thesis considers the investigation of magnetic properties of several new transition metal molecular compounds via different experimental techniques (continuous wave electron spin resonance (CW ESR), pulse ESR, high-field/high-frequency ESR (HF-ESR) and static magnetization techniques). The first studied compounds were mono- and trinuclear Cu(II)-(oxamato, oxamidato)/bis(oxamidato) type compounds. First, all components of the g-tensor and the tensors of onsite ACu and transferred AN HF interactions of mononuclear Cu(II)- bis(oxamidato) compounds have been determined from CW ESR measurements at 10 GHz and at room temperature and pulse ELDOR detected NMR measurements at 35 GHz and at 20 K. The spin density distributions of the mononuclear compounds have been calculated from the experimentally obtained HF tensors. The magnetic exchange constants J of their corresponding trinuclear compounds were determined from susceptibility measurements versus temperature. Our discussion of the spin density distribution of the mononuclear compounds together with the results of the magnetic characterization of their corresponding trinuclear compounds show that the spin population of the mononuclear compounds is in interplay with the J values of their corresponding trinuclear compounds. The second studied compounds were polynuclear Cu(II)-(bis)oxamato compounds with ferrocene and ferrocenium ligands. The magnetic properties of these compounds were studied by susceptibility measurements versus temperature to determine J values. In addition, the ESR technique is used to investigate the magnetic properties of the studied compounds because they contain two different magnetic ions and because only the ESR technique can selectively excite different electron spin species. These studies together with geometries of the ferrocenium ligands determined by crystallographic studies indicate that the magnetic interaction between a central Cu(II) and a Fe(III) ions changed from the antiferromagnetic coupling to the ferromagnetic coupling when a stronger distortion of the axial symmetry in the feroccenium cation exists. Therefore, the degree of the distortion of the feroccenium cation is a control parameter for the sign of the interaction between the central Cu(II) ion and the Fe(III) spins of the studied compounds. The last two studied molecular magnets were a binuclear Ni(II) compound (Ni(II)-dimer) and a cube-like tetranuclear compound with a [Fe4O4]-cube core (Fe4-cube). HF-ESR measurements enabled us to determine the g-factor, the sign, and the absolute value of the magnetic anisotropy parameters. Using this information together with static magnetization measurements, the J value and the magnetic ground state of the studied compounds have been determined. In Ni(II)-dimer, two Ni(II) ions, each having a spin S = 1, are coupled antiferromagnetically that leads to a ground state with total spin Stot = 0. An easy plane magnetic anisotropy with a preferable direction for each Ni(II) ion is found. For Fe4-cube, a ground state with total spin Stot = 8 has been determined. The analysis of the frequency dependence and temperature dependence of HF-ESR lines reveals an easy axis magnetic anisotropy (Dcube = -22 GHz (-1 K)) corresponding to an energy barrier of U = 64 K for the thermal relaxation of the magnetization. These results indicate that Fe4-cube is favorable to show single molecular magnet (SMM) behavior.

In diesem Beitrag werden die magnetischen Eigenschaften von ferromagnetischen Proben, welche mittels des neuen 3D-Multimaterialdruckverfahrens (3DMMD) hergestellt wurden, mit herkömmlichen Magnetkreismaterialien verglichen. Dazu wird zunächst die Technologie des Druckverfahrens sowie das Messprinzip und der Versuchsstand beschrieben. Im Weiteren wird ein Überblick über die Materialentwicklung gegeben und die Messergebnisse diskutiert. Es folgt die Betrachtung relevanter Einflüsse bei der Herstellung von Magnetkreisen sowie der Vergleich der Messergebnisse verschiedener Materialien. / In this paper, the magnetic properties of ferromagnetic samples produced using the new 3D multi-material printing process (3DMMD) are compared with conventional magnetic circuit materials. First the technology of the printing process as well as the measuring principle and the test bench are described. Furthermore, an overview of the material development is given and the measurement results are discussed. This is followed by the consideration of relevant influences in the production of magnetic circuits and the comparison of the measurement results of different materials.

Der Fokus dieser Dissertation liegt auf den gemischt-metallischen Dysprosium-Nitrid-Clusterfullerenen. Durch die Inklusion von bis zu drei Lanthanoiden mit unvollständig gefüllten 4f-Orbitalen weisen diese Clusterfullerene eine Vielzahl interessanter magnetischer Eigenschaften auf. Die magnetische Charakterisierung der Nitrid-Clusterfullerene DyxSc3-xN@C80-Ih (x = 1 - 3) zeigte bereits 2014 den Einfluss der Stöchiometrie auf das magnetische Verhalten und stufte diese Verbindungen als Einzelmolekülmagnete ein. Im Rahmen dieser Arbeit wurde das Zusammenspiel zwischen den strukturellen Eigenschaften und dem magnetischen Verhalten der Clusterfullerene untersucht. Der Fokus lag auf der Synthese und der magnetischen Charakterisierung von Clusterfullerenen mit unterschiedlicher Kohlenstoffkäfiggröße bzw. -isomerie, unterschiedlicher Clusterzusammensetzung bzw. Cluster-bildender Metalle und dem Einfluss des nichtmetallischen Zentralatoms des Clusters. Die Dysprosium-Nitrid-Clusterfullerene wurden über ein modifiziertes Krätschmer-Huffman-Verfahren und unter Verwendung der trimetallischen Nitridtemplatmethode synthetisiert und anschließend mittels HPLC fraktioniert und massenspektrometrisch analysiert. Die magnetische Charakterisierung der Clusterfullerene gelang mittels DC-SQUID-Magnetometrie. Die im Rahmen dieser Arbeit untersuchten Fullerene konnten als Einzelmolekülmagnete identifiziert werden. Das magnetische Verhalten der Nitrid-Clusterfullerene wird hauptsächlich durch den Cluster M3N und weniger durch den diamagnetischen Kohlenstoffkäfig bestimmt. Jedoch wurde für DySc2N@C80-D5h und Dy2ScN@C80-D5h eine verringerte Lebensdauer der Magnetisierung im Vergleich zu ihren Analoga mit Ih-Kohlenstoffkäfigsymmetrie beobachtet. Stärkeren Einfluss hat die Kohlenstoffkäfiggröße. Für DySc2N@C68, Dy2ScN@C84 und Dy2ScN@C88 wurde eine deutliche Abnahme der Remanenz, der Blocktemperaturen und der Relaxationzeiten festgestellt. Als Ursache werden die veränderten Dy-N-Bindungslängen diskutiert. Die Clusterfullerene Dy2MN@C80-Ih und DyM2N@C80-Ih (M = Gd, Er, Lu) enthalten neben Dysprosium ein weiteres Lanthanoid im Cluster. Das zweite Lanthanoid M ruft eine erhebliche Änderungen der magnetischen Eigenschaften hervor. Die paramagnetischen Metalle Gd und Er wirken sich stark negativ auf die magnetische Remanenz aus. Für Dy2LuN@C80-Ih und DyLu2N@C80-Ih wurde ein ähnliches magnetisches Verhalten wie für DyxSc3-xN@C80-Ih (x = 1, 2) verzeichnet. Durch die Verdünnung des Fullerens DyLu2N@C80-Ih mit der diamagnetischen Verbindung Lu3N@C80-Ih wurde zusätzlich eine Erhöhung der Hysterese der Magnetisierung im untersuchten Temperaturbereich registriert. Der Einfluss der nichtmetallischen Clusterspezies auf die magnetischen Eigenschaften wurde anhand der Carbid-Clusterfullerene Dy2TiC@C80-Ih, -D5h und Dy2TiC2@C80-Ih untersucht. Obwohl die Fullerene Dy2TiC@C80-Ih bzw. -D5h sich nur durch die isoelektronische Ti-C-Clustereinheit von den Nitrid-Clusterfullerenen unterscheiden, ist deren Remanenz nur halb so groß. Ein weiteres Kohlenstoffatom im Cluster, wie in Dy2TiC2@C80-Ih, ruft eine weitere Abnahme der Hysterese der Magnetisierung hervor. Die veränderte Bindungssituation der Carbid-Cluster wird als Ursache für das beobachtete magnetische Verhalten herangezogen.

Since the last few decades frustrated spin systems have attracted much interest. These studies are motivated by the rich variety of their unusual magnetic properties and potential applications. In this thesis, excitation spectra of the weakly coupled dimer system Ba3Cr2O8, the spin-1/2 chain material with distorted diamond structure Cu3(CO3)2(OH)2 (natural mineral azurite), and the quasi-twodimensional antiferromagnet with triangle spin structure Cs2CuBr4 have been studied by means of high-field electron spin resonance. Two pairs of gapped modes corresponding to transitions from a spin-singlet ground state to the first excited triplet state with zero-field energy gaps, of 19.1 and 27 K were observed in Ba3Cr2O8. The observation of ground-state excitations clearly indicates the presence of a non-secular term allowing these transitions. Our findings are of crucial importance for the interpretation of the field-induced transitions in this material (with critical fields Hc1 = 12.5 T and Hc2 = 23.6 T) in terms of the magnon Bose-Einstein condensation. The natural mineral azurite, Cu3(CO3)2(OH)2, has been studied in magnetic fields up to 50 T, revealing several modes not observed previously. Based on the obtained data, all three critical fields were identified. A substantial zero-field energy gap, Δ = 9.6 K, has been observed in Cs2CuBr4 above the ordering temperature. It is argued that contrary to the case for the isostructural Cs2CuCl4, the size of the gap can not be explained solely by the uniform Dzyaloshinskii-Moriya interaction, but it is rather the result of the geometrical frustration stabilizing the spin-disordered state in Cs2CuBr4 in the close vicinity of the quantum phase transition between a spiral magnetically ordered state and a 2D quantum spin liquid.