Die vorliegende Promotionsarbeit “Density functional study of the electronic and magnetic properties of selected transition metal complexes” beschäftigt sich mit dem Zusammenhang zwischen strukturellen Merkmalen sowie elektronischen und magnetischen Eigenschaften von Einzelmolekül-Magneten. Im Wesentlichen konnte dabei gezeigt werden, dass die magnetischen Eigenschaften sowohl von strukturellen Merkmalen als auch von den elektronischen Eigenschaften bestimmt werden. Des Weiteren ergab sich, dass verschiedene Kenngrößen der magnetischen Eigenschaften (im speziellen der magnetische Grundzustand S sowie die magnetische Anisotropie D) miteinander korreliert sind. Dies ist im Besonderen für eine mögliche Anwendung von Einzelmolekül-Magneten im Bereich der Datenspeicherung von Bedeutung.

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.

Cette thèse caractérise un acier Moyen Mn à 0.2C-5Mn-2.5Al qui montre un écrouissage très fort au cours de la déformation plastique dû à l’effet TRIP. Pendant TRIP, l’austénite résiduelle paramagnétique se transforme en martensite ferromagnétique sous déformation plastique, ce qui conduit à un fort écrouissage. Le taux de cet écrouissage dépend des paramètres de fabrication et surtout la température de recuit intercritique. Ces aciers ont aussi des fois le tendance de se déformer de façon hétérogène par des bandes de Lüders ou PLC.Dans cette thèse, une méthode de caractérisation de la cinétique de transformation de phase est développée sur la base des mesures de l’aimantation saturée de l’acier. La méthode magnétique est unique dans son implémentation in-situ sans aucun effet sur l’essai de traction. Une correction pour les effets de la contrainte appliquée sur l’aimantation est aussi introduite pour la première fois avec une base physique. Les résultats des mesures magnétiques ont été comparés contre des caractérisations des bandes de déformation pour montrer que la transformation de phase coïncide avec le passage des bandes de déformation. La sensibilité à la vitesse de déformation est analysée et une caractérisation de la présence et type de bande PLC est présentée en fonction de la cinétique de transformation de phase. / This thesis studies the mechanical behavior of a 0.2C-5Mn-2.5Al Medium Mn steel that exhibits a very high degree of work hardening due to transformation-induced plasticity (TRIP) during plastic deformation. During TRIP, paramagnetic retained austenite is transformed to ferromagnetic martensite with the application of plastic strain and generates a significant amount of work hardening. The rate of work hardening is seen to vary greatly depending on processing parameters—notably the intercritical annealing temperature. These steels also often deform heterogeneously through the propagation of Lüders or PLC strain bands.This research develops a method to characterize the kinetics of the TRIP effect through measurements of the samples magnetic properties. The method is novel in that it is performed in-situ with no effect on the tensile test and is able to correct for the effects of the applied stress on the magnetic properties. The results of these experiments were compared to characterizations of the strain bands to demonstrate that TRIP coincides with the passage of a Lüders or PLC band. The strain rate sensitivity of the steels is analyzed and the presence and type of PLC bands are characterized with respect to the transformation kinetics.

Předkládaná dizertační práce pojednává o problematice pohybu doménových stěn (DS) vyvolaného spinově polarizovaným proudem v magnetických nanodrátech na bázi spinového ventilu NiFe/Cu/Co. Jedná se o tzv. efekt přenosu spinového momentu. Multivrstevnatý systém NiFe/Cu/Co, kde se doménová stěna pohybuje ve vrstvě NiFe, vykazuje velmi vysokou účinnost přenosu spinového momentu, což bylo v literatuře potvrzeno na základě magnetotransportních měření. Tato práce má za cíl pozorovat stav DS během jejich pohybu, pomocí fotoelektronové mikroskopie kombinované s kruhovým magnetickým dichroismem. Tato technika využívá synchrotronové záření, které svým časovým rozlišením umožňuje sledovat dynamickou odezvu magnetizace na elektrický proud. Podstatnou částí řešení byla optimizace růstu vrstev NiFe/Cu/Co kvůli snížení magnetické dipolární interakce mezi vrstvami. V práci je také řešen způsob přípravy nanodrátů litografickými metodami. Byly provedeny dva módy měření: i) kvazistatický, tj. pozorování DS před a po injekci proudu do nanodrátu a ii) dynamické měření, kde je DS sledována během působení proudového pulzu. S využitím kvazistatickém módu byla vypracována rozsáhlá statistika pohybu DS: i) byly naměřeny jejich vysoké rychlosti přesahující 600 m/s za působení průměrné proudové hustoty nutné k posuvu doménové stěny - 5x10^11 A/m^2; ii) DS jsou v systému NiFe/Cu/Co velmi silně zachycovány dipolární interakcí mezi NiFe a Co způsobenou nehomogenitou krystalové struktury ve vrstvě Co. V dynamickém módu bylo odhaleno, že působením Oerstedovského pole kolmého na nanodráty v rovině vzorku se magnetizace ve vrstvě NiFe silně natáčí. Tento efekt přispívá k vysokým rychlostem DS pozorovaných v nanodrátech NiFe/Cu/Co.

The concept of percolated perpendicular media (PPM) for magnetic data storage is expected to surpass the areal storage density of 1 Tbit in -², which is regarded as the fundamental limit of conventional granular CoCrPt:oxide based recording media. PPM consist of a continuous ferromagnetic thin film with densely distributed defects acting as pinning sites for magnetic domain walls. In this study, practical realizations of PPM were fabricated by the deposition of [Co/Pt]8 multilayers with perpendicular magnetic anisotropy onto nanoperforated templates with various perforation diameters and periods. The structural defects given by the templates serve as pinning sites for the magnetic domain walls within the [Co/Pt]8 multilayers. Magnetometry at both the integral and the local level was employed to investigate the influence of the template on the magnetization reversal and the domain wall pinning. It was found, that magnetic domains can be pinned at the ultimate limit, between three adjacent pinning sites. The coercivity and the depinning field, which both are a measure for the strength of the magnetic domain wall pinning, were found to increase with increasing perforation diameter. The size of magnetic domains within the magnetic film appeared not to depend solely on the diameter of the nanoperforations or on the period of the template, but on the ration between diameter and period. By means of micromagnetic simulations it was found, that the presence of ferromagnetic material within the pinning site given supports the pinning of magnetic domain walls, compared to a pinning site that is solely given by a hole in the magnetic thin film. Investigation of the evolution of the magnetization in magnetic fields smaller than the coercive field revealed, that the energy barrier against thermally induced magnetization reversal is sufficiently large to provide long-term (> 10 years) stability of an arbitrary magnetization state. This could also be qualitatively supported by micromagnetic simulations. Static read/write tests with conventional hard disk recording heads revealed the possibility of imprinting bit patterns into the PPM under study. The minimum bit pitch that could be read back thereby depended on the period of the nanoperforated template.

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.

В работе представлены результаты исследования влияния состояния поверхности ленты на распределение намагниченности и магнитную проницаемость аморфного магнитомягкого сплава Co–Ni–Fe–Cr–Mn–Si–B, а также неоднородности магнитных характеристик в исходном (закаленном) состоянии, Такие исследования имеют важное значение для оптимизации магнитных характеристик и способствуют повышению качества изделий из этого сплава. Состояние поверхности ленты изменяли при помощи термообработок на воздухе в интервале температур 90–380 оС, обработкой водой и ацетоном, формированием полимерного покрытия. Исследования показали, что ленты аморфного сплава Co–Ni–Fe–Cr–Mn–Si–B в закаленном состоянии обладают существенной неоднородностью магнитных характеристик, которая носит закономерный характер и может быть связана с технологией изготовления ленты. Обработка водой приводит к окислению и гидрированию поверхности ленты и ее влияние зависит от знака константы магнитострикции (λs). В состоянии с λs<0 способствует увеличению объема доменов с ортогональной намагниченностью и остроты магнитной текстуры в плоскости ленты, определяемой отношением объемов доменов с планарной намагниченностью, ориентированной вдоль и поперек оси ленты. В состоянии с λs>0 вызывает уменьшение объема доменов с ортогональной намагниченностью и остроты магнитной текстуры. Обработка ацетоном вследствие его окисления и гидрирования содержащимися в поверхности ионами водорода и кислорода оказывает влияние на распределение намагниченности в ленте противоположное воздействию воды, которое также зависит от знака константы магнитострикции. Увеличение длительности обработки ацетоном приводит к усилению ее влияния на изменение объема доменов с ортогональной намагниченностью и ослаблению влияния на перераспределение намагниченности в плоскости ленты. Низкотемпературный отжиг в интервале температур 90 –130оС не способствует изменению максимальной магнитной проницаемости, объема доменов с ортогональной намагниченностью и знака λs, но способствует перераспределению намагниченности в плоскости ленты в результате в результате взаимодействия с атмосферным паром. Наблюдаемое сильное уменьшение максимальной магнитной проницаемости при температурах выше 180 оС связано с ростом объема доменов с ортогональной намагниченностью и снижением остроты магнитной текстуры в плоскости ленты. При 250оС обнаружено формирование поперечной магнитной анизотропии. Полимерное покрытие, формируемое на ленте в закаленном состоянии при 90 –130оС, способствует повышению максимальной магнитной проницаемости и уменьшению объема доменов с ортогональной намагниченностью во всем интервале температур. Полученные результаты находят объяснение в рамках влияния анизотропных напряжений, индуцируемых в ленте в результате взаимодействия ее поверхности с химически активными средами, магнитоактивным полимерным покрытием и анизотропной поверхностной кристаллизацией. / The paper presents the results of the influence of the ribbon surface state on the magnetization distribution and magnetic permeability of amorphous magnetically soft Co-Ni-Fe-Cr-Mn-Si-B alloy, as well as heterogeneity of magnetic characteristics in the initial (as-quenched) state. The state of the ribbon surface was changed by heat treatment in the air in the temperature range 90-380oC, treatment with water and acetone, formation of a polymer coating. The studies showed that the Co-Ni-Fe-Cr-Mn-Si-B amorphous alloy ribbons in the as-quenched state have a significant heterogeneity of magnetic characteristics, which has a natural character and can be associated with the tape manufacturing technology. The water treatment leads to oxidation and hydrogenation of the strip surface and its influence depends on the sign of the magnetostriction constant (λs). In the state with λs<0 the volume of domains with orthogonal magnetization and the sharpness of the magnetic texture in the tape plane, determined by the ratio of the volumes of domains with planar magnetization oriented along and across the tape axis, are increased. In the condition with λs>0 the volume of domains with orthogonal magnetization and the sharpness of the magnetic texture decrease. Treatment with acetone due to its oxidation and hydrogenation by hydrogen and oxygen ions contained in the surface affects the magnetization distribution in the tape opposite to that of water, which also depends on the sign of the magnetostriction constant. Increasing the duration of treatment with acetone leads to increasing its effect on the change in the volume of domains with orthogonal magnetization and to a weaker effect on the redistribution of magnetization in the tape plane. Low-temperature annealing in the temperature range 90 - 130oC does not contribute to changes in the maximum magnetic permeability, the volume of domains with orthogonal magnetization and the sign of λs but contributes to the redistribution of magnetization in the tape plane because of interaction with atmospheric vapor. The observed strong decrease in the maximum magnetic permeability at temperatures above 180oC is associated with an increase in the volume of domains with orthogonal magnetization and a decrease in the sharpness of the magnetic texture in the tape plane. The formation of transverse magnetic anisotropy was detected at 250oC. The polymer coating formed on the tape in the quenched state at 90 - 130oC increases the maximum magnetic permeability and reduces the volume of domains with orthogonal magnetization throughout the temperature range. The obtained results find an explanation in the influence of anisotropic stresses induced in the tape because of interaction of its surface with chemically active media, magnetically active polymer coating and anisotropic surface crystallization.

В работе реализован численный алгоритм для решения уравнения Фоккера-Планка, позволяющий получать значения первой и третьей гармоники ансамбля взаимодействующих частиц для различных амплитуд переменного поля. В формулы первой и третьей гармоники вводились функции, зависящие от параметра и восприимчивости Ланжевена, с неопределенными коэффициентами. Неопределенные коэффициенты находились методом наименьших квадратов. Выражения для функций приближались данными из численного решения уравнения Фоккера-Планка и затем минимизировались относительно неопределённых коэффициентов. Получившиеся формулы сравнивались с численным решением и с известными теориями. / In this work, a numerical algorithm for solving the Fokker-Planck equation, which allows to obtain the values of the first and third harmonics of the ensemble of interacting particles for different amplitudes of the alternating field, was implemented. The functions depending on the parameter and Langevin susceptibility with uncertain coefficients were introduced into the formulas for the first and third harmonics. The uncertain coefficients were found by the least-squares method. Expressions for the functions were approximated with data from the numerical solution of the Fokker-Planck equation and then minimized with respect to the uncertain coefficients. The resulting formulas were compared with the numerical solution and with known theories.

This thesis describes the theory for static and dynamic magnetic-resonance imaging using contrast agents affecting T1 relaxation time. The available acquisition methods in the specified facility of Masaryk Oncological Institute in Brno are described. The sequences for subsequent experimental measurements are selected. The used phantoms are described. Acquisition protocol for measuring is described briefly and the evaluation method for the measured data is suggested. The best acquisition sequence and a method of measurements is chosen influenced by estimation of relaxation time T1, sensitivity and signal to noise ratio. Perfusion analysis is executed and perfusion parameters are calculated. The work was supported by the European Regional Development Fund and the State Budget of the Czech Republic (RECAMO, CZ.1.05/2.1.00/03.0101).