The thesis is devoted to the study of in-chain impurities in spin 1/2 antiferromagnetic Heisenberg chains (S=1/2 aHC's)---a model which accompanies the research on magnetism since the early days of quantum theory and which is one of the few integrable spin systems. With respect to impurities it is special insofar as an impurity perturbs the system strongly due to its topology: there is no way around the defect. To what extend the one-dimensional picture stays a good basis for the description of real materials even if the chains are disturbed by in-chain impurities is an interesting question which is addressed in this work. For this purpose, Cu Nuclear Magnetic Resonance (NMR) measurements on the cuprate spin chain compounds SrCuO2 and Sr2CuO3 intentionally doped with nickel (Ni), zinc (Zn) and palladium (Pd) are presented. These materials are well known to be among the best realizations of the S=1/2 aHC model and their large exchange coupling constants allow the investigation of the low-energy dynamics within experimentally easily feasible temperatures. NMR provides the unique ability to study the static and dynamic magnetic properties of the spin chains locally which is important since randomly placed impurities break the translational invariance. Because copper is the magnetically active ion in those materials and the copper nuclear spin is most directly coupled to its electron spin, the NMR measurements have been performed on the copper site. The measurements show in all cases that there are changes in the results of these measurements as compared to the pure compounds which indicate the opening of gaps in the excitation spectra of the spin chains and the emergence of oscillations of the local susceptibility close to the impurities. These experimental observations are compared to theoretical predictions to clarify if and to what extend the already proposed model for these doped systems---the finite spin chain---is suitable to predict the behavior of real materials. Thereby, each impurity shows peculiarities. While Zn and Pd are know to be spin 0 impurities, it is not clear if Ni carries spin 1. To shed some light on this issue is another scope of this work. For Zn impurities, there are indications that they avoid to occupy copper sites, other than in the layered cuprate compounds. Also this matter is considered.

Obwohl ultrahohe Magnetfelder jenseits von 100 T mittels spezieller Methoden experimentell nutzbar wurden, sind Magnetisierungsmessungen in diesen Feldern noch immer sehr kompliziert. Magnetfelder im Megagauß-Bereich lassen sich heute allein durch die semidestruktive Single-Turn-Coil (STC)-Methode und durch die vollständig destruktive Flußkompression erzeugen. In beiden Fällen sind die gepulsten Felder auf eine Dauer von wenigen Mikrosekunden und ein nutzbares Feldvolumen von wenigen Kubikzentimetern beschränkt. Dadurch bedingt sind nur solche Methoden für Magnetisierungsmessungen anwendbar, die sich an diese ungünstigen Randbedingungen anpassen lassen. Die Faraday-Rotation kann für optisch transparente Proben sinnvoll sein, jedoch ist die induktive Messung mittels kompensierter Pick-up-Spulen die einzige direkte Methode. Die vorliegende Arbeit beschreibt die Entwicklung und Anwendung eines solchen Meßsystems in einem STC-Feldgenerator. Im Hinblick auf das Meßsystem hat sich die sehr gute geometrische Kompensation der Pick-up-Spulen als wesentlich herausgestellt. Wichtig sind daneben aber auch eine hohe Bandbreite bzw. eine unverzerrte Sprungantwort. Als äußerst effektiv hat sich insbesondere die umfassende elektromagnetische Abschirmung des Meßsystems erwiesen, so daß die vom STC-Generator ausgehenden Störungen weitgehend unterdrückt werden. Besondere, für Magnetisierungsmessungen ungünstige Eigenschaften des STC-Systems, werden dargestellt und diskutiert. Magnetisierungsmessungen bei tiefen Temperaturen an den van Vleck-Paramagneten TmPO4 und TmPO4 ergaben Anomalien bei rund 30 T bzw. 50 T für ein Feld parallel zur tetragonalen Achse. Dieser Effekt resultiert aus einer Überkreuzung der beiden niedrigsten Energieniveaus in starken Magnetfeldern und führt zu einem scharfen Sprung in der Magnetisierung. Temperaturabhängige Messungen belegen den wesentlichen Einfluß des magnetokalorischen Effekts in kurzen gepulsten Feldern. Pulverproben der intermetallischen Verbindungen RMn2Ge2 (R = Gd, Tb, Dy, Ho, Er, Y) wurden bei Helium-Temperaturen in Feldern bis zu 150 T untersucht. Abhängig vom Selten-Erd-Ion wurden in allen Verbindungen ein oder mehrere feldinduzierte metamagnetische Phasenübergänge beobachtet. Sie können auf eine Änderung der magnetischen Struktur des Mn-Untersystems zurückgeführt werden. Die kritischen Felder für den Übergang zur ferromagnetischen Phase betragen zwischen 90 T und 118 T (YMn2Ge2: 55 T). Detaillierte Rechnungen werden im Rahmen eines erweiterten Molekularfeld-Modells durchgeführt. / Although ultrahigh magnetic fields in excess of 100 T have become available by specialized techniques, magnetization measurements in these fields are still a challenging task. Nowadays, magnetic fields in the megagauss range can be produced only by the semidestructive single-turn coil (STC) technique and by the fully destructive flux compression. In both cases these pulsed fields are limited to microsecond pulse duration and a usable field volume of not more than a few cubic centimeters. As a result, only those methods can be used for magnetization measurements, which can be adapted to these difficult boundary conditions. The Faraday rotation can be suitable for optically transparent samples, but the only direct technique is the induction method based on compensated pick-up coils. The present work deals with the development of such a measurement system and its application to various materials in fields up to 150 T produced by the STC. Regarding the measurement system, geometrically very well compensated pick-up coils turned out to be absolutely necessary as well as a large bandwidth and an appropriate transient response. In particular a comprehensive electromagnetic shielding, which makes use of a wire metal, proved to be very effective in suppressing disturbances produced by the STC system. Besides, some inherent characteristics and limitations of the STC generator with respect to magnetization measurements are revealed and discussed. Magnetization measurements were performed on the van Vleck paramagnets TmPO4 and PrVO4, and anomalies were found at about 30 T and 50 T, respectively, at low temperatures for a field along the tetragonal axis. This effect is due to a crossing of the lowest energy levels in high magnetic fields and results in a sharp jump in magnetization. Temperature dependent measurements provide evidence that the magnetocaloric effect plays an essential role in short pulse fields produced by the STC. Powder samples of the intermetallic compounds RMn2Ge2 (R = Gd, Tb, Dy, Ho, Er, Y) have been investigated at liquid helium temperature up to 150 T. Depending on the rare-earth ion one or more field-induced metamagnetic phase transitions have been observed in all compounds, and they are found to be associated with a change in the magnetic structure of the Mn subsystem. The phase transitions to the ferromagnetic phase occur between 90 T and 118 T (YMn2Ge2: 55 T). Detailed calculations are performed using an extended molecular field model.

Hoch manganhaltige Eisenbasislegierungen sind bei Raumtemperatur austenitisch und antiferromagnetisch (afm). Dabei besteht die Besonderheit, dass sich durch Legierung die afm Übergangstemperatur (Néeltemperatur) so einstellen lässt, dass sie nahe Raumtemperatur liegt. FeMn-Basislegierungen zeigen in Abhängigkeit von der Zusammensetzung Transformation Induced Plasticity (TRIP) und/oder Twinning Induced Plasticity (TWIP), d.h. die niedrige Stapelfehlerenergie dieser Legierungen führt zu verformungsinduzierter, metastabiler Phasenbildung (TRIP) bzw. zur Bildung von Verformungszwillingen (TWIP) und dadurch zu außerordentlich hoher Duktilität bei gleichzeitig hoher Verfestigung. Darüber hinaus haben FeMn-Basislegierungen einen ausgeprägten Magnetovolumeneffekt und magnetoelastischen Effekt durch magnetische Ordnung. Daher sind die untersuchten FeMnNiCr-Basislegierungen auch prototypisch für afm Élinvarlegierungen. Da Élinvar jedoch für invariable Elastizität steht, bedingt eine Anwendung als temperaturkompensierte Konstantmodullegierungen die Glättung der ausgeprägten magnetischen Anomalien, die industriell noch in keiner Anwendung realisiert wurde. Der Vorteil dies für eine Anwendung zu erreichen, läge in der Unempfindlichkeit feinmechanischer Bauelemente, gegenüber magnetischen Feldern, die bei den industriell verfügbaren ferromagnetischen Élinvarlegierungen nicht gewährleistet ist. Mit Bezug zu feinmechanischen Schwingsystemen spielen dabei neben der Einstellung der magnetoelastischen Eigenschaften die Prozessierbarkeit, Kaltumformbarkeit und Festigkeit sowie deren wechselseitige Beeinflussung eine große Rolle. Die vorliegende Arbeit befasst sich daher mit der Anwendbarkeit der untersuchten FeMnNiCr-Legierungen. Dabei wurden grundlegende Untersuchungen zur Plastizität durchgeführt, die die mechanische Zwillingsbildung in diesen Legierungen charakterisiert und ein Modell der mechanischen Zwillingsbildung bei kleinen plastischen Dehnungen vorschlägt, das eine Abschätzung der Stapelfehlerenergie erlaubt. Die Untersuchung des Antiferromagnetismus umgeformter Proben zeigt das Auftreten thermoremanenter Magnetisierung (TRM), deren Größe mit dem Umformgrad der untersuchten Proben skaliert. Sie wird den durch Umformdefekte erzeugten unkompensierten Momenten in der afm Spinstruktur zugeschrieben. Diese werden durch eine magnetische Feldkühlung magnetisiert und koppeln durch Austauschwechselwirkung an die umgebende antiferromagnetische Matrix unterhalb der Néeltemperatur. Das komplexe thermomagnetische Verhalten der unkompensierten Momente wird experimentell beschrieben und phänomenologisch gedeutet. Die Weiterentwicklung und Bewertung technischer, ausscheidbarer FeMnNiCrBe- und FeMnNiCr(Ti, Al)-Legierungen wird mit Bezug zu den grundlegenden Untersuchungen dargestellt. Es wird gezeigt, dass die neu entwickelten ausscheidbaren FeMnNiCr(Ti, Al)-Legierungen eine vielversprechende Ausgangsbasis darstellen, afm Élinvarlegierungen technisch umzusetzen. / High manganese iron-base alloys are austenitic and antiferromagnetic (afm) at room temperature. By further alloying it is possible to tune the afm transition temperature (Néel temperature) near room temperature. FeMn-base alloys show extraordinary strain hardening as well as ductility because of Transformation Induced Plasticity (TRIP) and/or Twinning Induced Plasticty (TWIP), i.e. in dependence on composition the generally low stacking fault energy in these alloys allows for the mechanically induced formation of metastable phases (TRIP) or deformation twinning (TWIP). Furthermore, magnetic order causes distinct magnetovolume and magnetoelastic effects in these afm FeMn-base alloys. The investigated FeMnNiCr-base alloys are therefore prototypic for afm Élinvar alloys. However, as Élinvar is meant for invariant elasticity, an application as temperature compensated alloy with constant elastic modulus requires the smoothing of the pronounced magnetic anomalies, that is not industrially available yet. The advantage of afm Élinvar alloys in precision mechanics applications, would be their impassiveness with respect to magnetic fields that is not achievable by their ferromagnetic counterparts. For precision components like mechanic oscillators not only the tuning of the magnetoelastic properties but also the processing, cold formability and mechanical properties as well as their interplay have strong influence. Therefore this work addresses the applicability of the studied FeMnNiCr alloys. Elementary investigations on plasticity characterise the occurrence of TWIP in these alloys and propose a modell for deformation twinning at low plastic strains that allows for an estimation of the stacking fault energy. The investigations on the antiferromagnetism of deformed samples show the appearance of thermoremanent magnetisation (TRM). Its magnitude scales with the degree of deformation. The TRM is therefore attributed to uncompensated moments in the afm spin structure due to deformation induced defects. These are magnetised by a magnetic field cooling and couple to the afm matrix by exchange interaction below the Néel temperature. The complex thermomagnetic behaviour of the uncompensated moments is experimentally described and phenomenologically explained. The further development and assessment of engineering-grade pecipitable FeMnNiCrBe and FeMnNiCr(Ti, Al) alloys is presented in relation to the aforementioned elementary investigations. It is shown that the newly developped precipitable FeMnNiCr(Ti, Al) alloys are good candidates for afm Élinvar alloys in application.

In this thesis multidomain states in magnetically ordered systems with competing long-range and short range interactions are under consideration. In particular, in antiferromagnetically coupled multilayers with perpendicular anisotropy unusual multidomain textures can be stabilized due to a close competition between long-range demagnetization fields and short-range interlayer exchange coupling. These spatially inhomogeneous magnetic textures of regular multidomain configurations and irregular networks of topological defects as well as complex magnetization reversal processes are described in the frame of the phenomenological theory of magnetic domains. Using a modified model of stripe domains it is theoretically shown that the competition between dipolar coupling and antiferromagnetic interlayer exchange coupling causes an instability of ferromagnetically ordered multidomain states and results in three possible ground states: ferromagnetic multidomain state, antiferromagnetic homogeneous and antiferromagnetic multidomain states. The presented theory allows qualitatively to define the area of existence for each of these states depending on geometrical and material parameters of multilayers. In antiferromagnetically coupled superlattices with perpendicular anisotropy an applied magnetic bias field stabilizes specific multidomain states, so-called metamagnetic domains. A phenomenological theory developed in this thesis allows to derive the equilibrium sizes of metamagnetic stripe and bubble domains as functions of the antiferromagnetic exchange, the magnetic bias field, and the geometrical parameters of the multilayer. The magnetic phase diagram includes three different types of metamagnetic domain states, namely multidomains in the surface layer and in internal layers, and also mixed multidomain states may arise. Qualitative and quantitative analysis of step-like magnetization reversal shows a good agreement between the theory and experiment. Analytical equations have been derived for the stray field components of these multidomain states in perpendicular multilayer systems. In particular, closed expressions for stray fields in the case of ferromagnetic and antiferromagnetic stripes are presented. The theoretical approach provides a basis for the analysis of magnetic force microscopy (MFM) images from this class of nanomagnetic systems. Peculiarities of the MFM contrast have been calculated for realistic tip models. These characteristic features in the MFM signals can be employed for the investigations of the different multidomain modes. The methods developed for stripe-like magnetic domains are employed to calculate magnetization processes in twinned microstructures of ferromagnetic shape-memory materials. The remarkable phenomenon of giant magnetic field induced strain transformations in such ferromagnetic shape memory alloys as Ni-Mn-Ga, Ni-Mn-Al, or Fe-Pd arises as an interplay of two physical effects: (i) A martensitic transition creating competing phases, i.e. crystallographic domains or variants, which are crystallographically equivalent but have different orientation. (ii) High uniaxial magnetocrystalline anisotropy that pins the magnetization vectors along certain directions of these martensite variants. Then, an applied magnetic field can drive a microstructural transformation by which the martensitic twins, i.e. the different crystallographic domains, are redistributed in the martensitic state. Within the phenomenological (micromagnetic) theory the equilibrium parameters of multivariant stripe patterns have been derived as functions of the applied field for an extended single-crystalline plate. The calculated magnetic phase diagram allows to give a detailed description of the magnetic field-driven martensitic twin rearrangement in single crystals of magnetic shape-memory alloys. The analysis reveals the crucial role of preformed twins and of the dipolar stray-field energy for the magnetic-field driven transformation process in magnetic shape-memory materials. This work has been done in close collaboration with a group of experimentalists from Institute of Metallic Materials of IFW Dresden, Germany and San Jose Research Center of Hitachi Global Storage Technologies, United States. Comparisons between theoretical and experimental data from this cooperation are presented throughout this thesis as vital part of my work on these different subjects.

The thesis is devoted to the study of in-chain impurities in spin 1/2 antiferromagnetic Heisenberg chains (S=1/2 aHC's)---a model which accompanies the research on magnetism since the early days of quantum theory and which is one of the few integrable spin systems. With respect to impurities it is special insofar as an impurity perturbs the system strongly due to its topology: there is no way around the defect. To what extend the one-dimensional picture stays a good basis for the description of real materials even if the chains are disturbed by in-chain impurities is an interesting question which is addressed in this work. For this purpose, Cu Nuclear Magnetic Resonance (NMR) measurements on the cuprate spin chain compounds SrCuO2 and Sr2CuO3 intentionally doped with nickel (Ni), zinc (Zn) and palladium (Pd) are presented. These materials are well known to be among the best realizations of the S=1/2 aHC model and their large exchange coupling constants allow the investigation of the low-energy dynamics within experimentally easily feasible temperatures. NMR provides the unique ability to study the static and dynamic magnetic properties of the spin chains locally which is important since randomly placed impurities break the translational invariance. Because copper is the magnetically active ion in those materials and the copper nuclear spin is most directly coupled to its electron spin, the NMR measurements have been performed on the copper site. The measurements show in all cases that there are changes in the results of these measurements as compared to the pure compounds which indicate the opening of gaps in the excitation spectra of the spin chains and the emergence of oscillations of the local susceptibility close to the impurities. These experimental observations are compared to theoretical predictions to clarify if and to what extend the already proposed model for these doped systems---the finite spin chain---is suitable to predict the behavior of real materials. Thereby, each impurity shows peculiarities. While Zn and Pd are know to be spin 0 impurities, it is not clear if Ni carries spin 1. To shed some light on this issue is another scope of this work. For Zn impurities, there are indications that they avoid to occupy copper sites, other than in the layered cuprate compounds. Also this matter is considered.

Nutzbare Energie ist ein knappes Gut. Aus ökonomischen und ökologischen Gründen wird die effiziente und nachhaltige Nutzung der verfügbaren Energie angestrebt. Wird Energie in elektrischer Form bereitgestellt oder transportiert, kommt der Minimierung der Verluste an elektrotechnischen Anlagen oder Bauelementen eine grundlegende Bedeutung zu. So werden beispielsweise Transformatorenkomponenten und Verstärkerelemente aus weichmagnetischen Werkstoffen mit geringem Ummagnetisierungsverlust gefertigt. In dieser Arbeit wird der Zusammenhang zwischen der magnetischen Mikrostruktur und dem magnetischen Ummagnetisierungsverlust nanokristalliner Ringbandkerne untersucht. Der Einfluss von Stärke und Lage einer induzierten Anisotropie wird anhand induktiver Hysteresemessung und simultaner Beobachtung magnetischer Domänen mit stroboskopischer Kerrmikroskopie charakterisiert.

In this thesis multidomain states in magnetically ordered systems with competing long-range and short range interactions are under consideration. In particular, in antiferromagnetically coupled multilayers with perpendicular anisotropy unusual multidomain textures can be stabilized due to a close competition between long-range demagnetization fields and short-range interlayer exchange coupling. These spatially inhomogeneous magnetic textures of regular multidomain configurations and irregular networks of topological defects as well as complex magnetization reversal processes are described in the frame of the phenomenological theory of magnetic domains. Using a modified model of stripe domains it is theoretically shown that the competition between dipolar coupling and antiferromagnetic interlayer exchange coupling causes an instability of ferromagnetically ordered multidomain states and results in three possible ground states: ferromagnetic multidomain state, antiferromagnetic homogeneous and antiferromagnetic multidomain states. The presented theory allows qualitatively to define the area of existence for each of these states depending on geometrical and material parameters of multilayers. In antiferromagnetically coupled superlattices with perpendicular anisotropy an applied magnetic bias field stabilizes specific multidomain states, so-called metamagnetic domains. A phenomenological theory developed in this thesis allows to derive the equilibrium sizes of metamagnetic stripe and bubble domains as functions of the antiferromagnetic exchange, the magnetic bias field, and the geometrical parameters of the multilayer. The magnetic phase diagram includes three different types of metamagnetic domain states, namely multidomains in the surface layer and in internal layers, and also mixed multidomain states may arise. Qualitative and quantitative analysis of step-like magnetization reversal shows a good agreement between the theory and experiment. Analytical equations have been derived for the stray field components of these multidomain states in perpendicular multilayer systems. In particular, closed expressions for stray fields in the case of ferromagnetic and antiferromagnetic stripes are presented. The theoretical approach provides a basis for the analysis of magnetic force microscopy (MFM) images from this class of nanomagnetic systems. Peculiarities of the MFM contrast have been calculated for realistic tip models. These characteristic features in the MFM signals can be employed for the investigations of the different multidomain modes. The methods developed for stripe-like magnetic domains are employed to calculate magnetization processes in twinned microstructures of ferromagnetic shape-memory materials. The remarkable phenomenon of giant magnetic field induced strain transformations in such ferromagnetic shape memory alloys as Ni-Mn-Ga, Ni-Mn-Al, or Fe-Pd arises as an interplay of two physical effects: (i) A martensitic transition creating competing phases, i.e. crystallographic domains or variants, which are crystallographically equivalent but have different orientation. (ii) High uniaxial magnetocrystalline anisotropy that pins the magnetization vectors along certain directions of these martensite variants. Then, an applied magnetic field can drive a microstructural transformation by which the martensitic twins, i.e. the different crystallographic domains, are redistributed in the martensitic state. Within the phenomenological (micromagnetic) theory the equilibrium parameters of multivariant stripe patterns have been derived as functions of the applied field for an extended single-crystalline plate. The calculated magnetic phase diagram allows to give a detailed description of the magnetic field-driven martensitic twin rearrangement in single crystals of magnetic shape-memory alloys. The analysis reveals the crucial role of preformed twins and of the dipolar stray-field energy for the magnetic-field driven transformation process in magnetic shape-memory materials. This work has been done in close collaboration with a group of experimentalists from Institute of Metallic Materials of IFW Dresden, Germany and San Jose Research Center of Hitachi Global Storage Technologies, United States. Comparisons between theoretical and experimental data from this cooperation are presented throughout this thesis as vital part of my work on these different subjects.

In dieser Doktorarbeit wird die Magnetisierungsdynamik von strukturierten, weichmagnetischen Einzelschichten aus Permalloy (Ni81Fe19) mittels stroboskopischer Kerrmikroskopie untersucht. Die Dicke der in unterschiedliche Formen (Kreise, Quadrate und Rechtecke) strukturierten magnetischen Schicht beträgt 50 nm bzw. 160 nm. Durch die Verwendung eines gepulsten Lasers als Beleuchtungsquelle wird eine Zeitauflösung erreicht, die unter 25 ps liegt. Parallel zu den Laserpulsen wird die Probe mit gepulsten Magnetfeldern angeregt und die Reaktion der Magnetisierung auf diese schnellen Magnetfeldänderungen wird untersucht. Diese Reaktion der Magnetisierung unterscheidet sich deutlich von einer Anregung mit quasistatischen Magnetfeldern. Durch die stroboskopische Beobachtungsweise sind nur reversible Prozesse sichtbar, irreversible Prozesse werden durch die Mittelung über mehrere Millionen Anregungs- und Beobachtungspulse nicht abgebildet. Dies wird bei der Anregung eines Vortex in einer Kreisscheibe deutlich, bei dem die Magnetisierungsrichtung im Vortexkern durch das gepulste Magnetfeld teilweise geschaltet wird. Dadurch ändert sich der Drehsinn der spiralförmigen Relaxationsbewegung des Vortex, was zu einer Überlagerung der beiden Bewegungen während der Beobachtung führt. Desweiteren wird eine Vervielfältigung von Vortex-Antivortex Paaren in Stachelwänden durch hochfrequente Felder gezeigt. Diese Vervielfältigung führt zur Erzeugung eines neuen, metastabilen Zustandes mit geringerem Stachelabstand. Mit steigender Frequenz des Feldes fällt der Stachelabstand bis auf 30 % des Ausgangswertes. Ab einer Grenzfrequenz, die durch die ferromagnetische Resonanz gegeben ist, kann die Magnetisierung dem Feld nicht mehr folgen und die Wandstruktur ist vergleichbar mit der im quasistatischen Grenzfall. Auch in dickeren Elementen wird diese Erzeugung beobachtet, wo sie zu einer irreversiblen Wandtransformation von der asymmetrischen Blochwand zur Stachelwand führt. Bei der Pulsanregung eines Landau-Domänenzustandes in einem Quadrat kommt es zur Bildung von sichelartigen Domänen an den Ecken des Quadrates. Die Entstehung dieser Domänen geschieht relativ schnell innerhalb einer Nanosekunde. Während der Relaxation der Magnetisierung lösen sich diese neu entstandenen Domänen durch Wandverschiebung wieder auf. Die Auflösung der Domänen geschieht deutlich langsamer als die Entstehung, was durch die unterschiedlichen Mechanismen, die bei der Entstehung (Magnetisierungsdrehung) und Auflösung (Wandverschiebung) der Domänen involviert sind, begründet werden kann. Außerdem kommt es zu einer inkohärenten Drehung der Magnetisierung in der Domäne mit antiparalleler Ausrichtung der Magnetisierung bezüglich des Pulsfeldes. Diese Drehung der Magnetisierung, lateral abwechselnd nach links und rechts, wird durch eine leichte Abweichung der Magnetisierung von einer perfekt homogenen Ausrichtung begünstigt.

Consolidating a microscopic understanding of magnetic properties is crucial for a rational design of magnetic materials with tailored characteristics. The interplay of 3d and 4f magnetism in rare-earth transition metal antimonides is an ideal platform to search for such complex behavior. Here the synthesis, crystal growth, structure, and complex magnetic properties are reported of the new compound Pr3Fe3Sb7 as studied by magnetization and electrical transport measurements in static and pulsed magnetic fields up to 56 T, powder neutron diffraction, and Mößbauer spectroscopy. On cooling without external magnetic field, Pr3Fe3Sb7 shows spontaneous magnetization, indicating a symmetry breaking without a compensating domain structure. The Fe substructure exhibits noncollinear ferromagnetic order below the Curie temperature TC ≈ 380 K. Two spin orientations exist, which approximately align along the Fe–Fe bond directions, one parallel to the ab plane and a second one with the moments canting away from the c axis. The Pr substructure orders below 40 K, leading to a spin-reorientation transition (SRT) of the iron substructure. In low fields, the Fe and Pr magnetic moments order antiparallel to each other, which gives rise to a magnetization antiparallel to the external field. At 1.4 K, the magnetization approaches saturation above 40 T. The compound exhibits metallic resistivity along the c axis, with a small anomaly at the SRT.

This thesis presents a comprehensive study of the magnetic properties and of quantum phase transitions (QPTs) of two different systems which have been investigated by means of low-temperature magnetization measurements. The systems are the heavy-fermion Yb(Rh1-xCox)2Si2 (metallic) and the quantum magnet NiCl2-4SC(NH2)2 (insulator). Although they are very different materials, they share two common properties: magnetism and QPTs. Magnetism originates in Yb(Rh1-xCox)2Si2 from the trivalent state of the Yb3+ ions with effective spin S = 1=2. In NiCl2-4SC(NH2)2, the magnetic Ni2+ ions have spin S = 1. These magnetic ions are located on a body-centered tetragonal lattice in both systems and, in this study, the QPTs are induced by an external magnetic field. In Yb(Rh1-xCox)2Si2 the evolution of magnetism from itinerant in slightly Co-doped YbRh2Si2 to local in YbCo2Si2 is examined analyzing the magnetic moment versus chemical pressure x phase diagram in high-quality single crystals, which indicates a continuous change of dominating energy scale from the Kondo to the RKKY one. The physics of the antiferromagnet YbCo2Si2 can be completely understood. On the other hand, the physics of pure and slightly Co-containing YbRh2Si2 is much more complex, due to the itinerant character of magnetism and the vicinity of the system to an unconventional quantum critical point (QCP). The field-induced AFM QCP in Yb(Rh0.93Co0.07)2Si2 and in pure YbRh2Si2 under a pressure of 1.5GPa is characterized by means of the magnetic Grüneisen ratio. The final part of this thesis describes quantum criticality near the field-induced QCP in NiCl2-4SC(NH2)2 . These results will be compared to the theory of QPTs in Ising and XY antiferromagnets. Since the XY -AFM ordering can be described as BEC of magnons by mapping the spin-1 system into a gas of hardcore bosons, the temperature dependence of the magnetization for a BEC is analytically derived and compared to the results just below the critical field. The remarkable agreement between the BEC theory and experiments in this quantum magnet is one of the most prominent examples of the concept of universality.