Diese Arbeit untersucht die vielfältigen Wechselwirkungen zwischen elektronischen und strukturellen Eigenschaften in Perovskit-Oxiden und in einem molekularen Kristall. Optische Anregung mit ultrakurzen Lichtimpulsen verändert die elektronische Struktur und die Dynamik der damit verbundenen reversiblen Gitterveränderung wird mit zeitaufgelöster Femtosekunden Röntgenbeugung direkt aufgezeichnet. Eine Nanostruktur aus metallischen und ferromagnetischen Strontium Ruthenat (SRO) und dielektrischen Strontium Titanat Schichten dient als Modellsystem, um optisch induzierten Druck auf einer subpikosekunden Zeitskala zu untersuchen. In der ferromagnetischen Phase zeigen phononischer und magnetostriktiver Druck eine vergleichbare ultraschnelle Dynamik und eine ähnliche Größe unterschiedlichen Vorzeichens. Die Amplitude des magnetischen Drucks folgt dem Quadrat der temperaturabhängigen Magnetisierung. In einem weiteren Doppelschichtsystem komprimiert der sich ultraschnell aufbauende phononische Druck in SRO benachbarte ferroelektrische Blei Zirkonat Titanat Schichten. Dies reduziert die tetragonale Verzerrung von bis zu 2 Prozent innerhalb 1.5 Pikosekunden und koppelt an die ferroelektrische "weiche Mode", beziehungsweise an die Ionenverschiebung innerhalb der Einheitszelle. Damit verbunden wird die makroskopische Polarisation bis zu 100 Prozent reduziert; aufgrund der Anharmonizität der Kopplung mit einer Verzögerung von 500 Femtosekunden. Femtosekunden Photoanregung von Chromophoren in einem molekularen Kristall induziert eine Änderung des Diopolmomentes durch intramolekularen Ladungstransfer. Die Änderung der gestreuten Röntgenintensität weist auf eine Molekül-Rotationsbewegung in der Umgebung angeregte Dipole hin, welche der 10 Pikosekunden Dynamik des Ladungstransfer folgt. Die transienten Röntgenstreusignale werden vollständig von der kollektiven Solvatation bestimmt und verdecken lokale, intramolekulare Strukturänderungen. / This thesis investigates the manifold couplings between electronic and structural properties in crystalline Perovskite oxides and a polar molecular crystal. Ultrashort optical excitation changes the electronic structure and the dynamics of the connected reversible lattice rearrangement is imaged in real time by femtosecond X-ray scattering experiments. An epitaxially grown superlattice consisting of alternating nanolayers of metallic and ferromagnetic strontium ruthenate (SRO) and dielectric strontium titanate serves as a model system to study optically generated stress. In the ferromagnetic phase, phonon-mediated and magnetostrictive stress in SRO display similar sub-picosecond dynamics, similar strengths but opposite sign and different excitation spectra. The amplitude of the magnetic component follows the temperature dependent magnetization square, whereas the strength of phononic stress is determined by the amount of deposited energy only. The ultrafast, phonon-mediated stress in SRO compresses ferroelectric nanolayers of lead zirconate titanate in a further superlattice system. This change of tetragonal distortion of the ferroelectric layer reaches up to 2 percent within 1.5 picoseconds and couples to the ferroelectric soft mode, or ion displacement within the unit cell. As a result, the macroscopic polarization is reduced by up to 100 percent with a 500 femtosecond delay that is due to final elongation time of the two anharmonically coupled modes. Femtosecond photoexcitation of organic chromophores in a molecular, polar crystal induces strong changes of the electronic dipole moment via intramolecular charge transfer. Ultrafast changes of transmitted X-ray intensity evidence an angular rotation of molecules around excited dipoles following the 10 picosecond kinetics of the charge transfer reaction. Transient X-ray scattering is governed by solvation, masking changes of the chromophore''s molecular structure.

This thesis presents the NMR measurements on the single crystals LiMnPO4 and Li0.9FeAs. Therefore, the thesis is divided into two separated sections. The first part reports on the competitive next generation cathode material LiMnPO4 with a stable reversible capacity up to 145 mAh/g and a rather flat discharge voltage curve at 4.1 V. For the basic understanding of the material the magnetic properties have been investigated by a Li and P NMR study in the paramagnetic and antiferromagnetic phase. LiMnPO4 shows a strong anisotropy of the dipolar hyperfine coupling due to the strong local magnetic moments at the Mn site. The corresponding dipole tensor of the Li- and P-nuclei is fully determined by orientation and temperature dependent NMR experiments and compared to the calculated values from crystal structure data. Deviations of the experimentally determined values from the theoretical ones are discussed in terms of Mn disorder which could have an impact on the mobility of the Li ions. The disorder is corroborated by diffuse x-ray diffraction experiments which indicate a shift of the heavy elements in the lattice, namely the Mn atoms. Furthermore, the spin arrangement in the relative strong field of 7.0494 T in the antiferromagnetic state is understood by the NMR measurements. In order to obtain parameters of the Li ion diffusion in LiMnPO4 measurements of the spin lattice relaxation rate were performed. Due to the strong dipolar coupling between the Li-nuclei and the magnetic moments at the Mn site it is difficult to extract parameters which can characterize the diffusive behavior of the Li ions. The second section reports on the AC/DC susceptibility and NMR/NQR studies on Li deficit samples labeled as Li0.9FeAs. LiFeAs belongs to the family of the superconducting Pnictides which are discovered in 2008 by H. Hosono et al. In recent studies the stoichiometric compound reveals triplet superconductivity below Tc ∼ 18 K which demands ferromagnetic coupling of the electrons in the Cooper pairs. In Li0.9FeAs the Li deficit acts like hole doping which suppresses the superconductivity. Then ferromagnetism can arise which is very interesting because of the vicinity to the triplet superconductivity. With the microscopic methods NMR/NQR on the Li and As nuclei, it was investigated where the ferromagnetism can be located in Li0.9FeAs. Recent susceptibility, ESR and µSR studies reveal an internal field due to the ferromagnetism. In contrast, the internal field could not be used to perform zero field NMR measurements. Possible reasons for this discrepancy are discussed. In addition, the automatic insitu AC susceptibility technique by using the NMR radio frequency circuit has been tested by a reference compound Co2TiGa which shows itinerant ferromagnetism. Similar curves are observed for Li0.9FeAs which indicate the existence of itinerant magnetic moments in Li0.9FeAs. Furthermore, in order to determine the size of the dipolar contribution from the magnetic moments of the Fe the dipolar hyperfine coupling tensor was calculated from the crystal structure data. The comparison of the experimental and calculated hyperfine coupling elements reveals transferred hyperfine fields in LiFeAs.

The focus of the work is on the epitaxial growth of Mn5Ge3 layers on Ge (001) via ultra-fast solid-state reaction between Mn and Ge using millisecond range FLA at the ambient pressure in continuous N2 flow. Epitaxial Mn5Ge3 layers were obtained both on Ge (001) and Ge (111) substrates by optimizing the fabrication parameters, Mn thickness (30 nm), FLA energy density (100-110 Jcm-2) and FLA duration time. The epitaxial relationship between the alloy film and substrate is the (100) plane of Mn5Ge3 along [001] direction parallel with the [100] direction of Ge (001) plane. It is notable that the hexagonal c axis of Mn5Ge3 on Ge (001) is parallel to the film surface plane, while the reported Mn5Ge3’s c axis on Ge (111) tends to be perpendicular to the film plane. In fact, using ultrafast-SPE the c-axis of Mn5Ge3 is always parallel to the sample surface. Mn5Ge3 films exhibit ferromagnetism which is demonstrated by the anomalous Hall effect up to the TC = 283±5 K. The films exhibit their in-plane magnetic easy axis along the hexagonal c-axis independent of the Mn5Ge3 film thickness. This provides a new avenue for the fabrication of Ge-based spin-injectors fully compatible with industrial CMOS technology. The deeper understanding of the magnetic, structural and electrical properties of (100) epitaxial Mn5Ge3 grown on Ge (001) are presented by utilizing DFT calculation (by our collaborator M. Birowska) and various experimental methods. The Mn atoms in Mn5Ge3 occupy two distinct Wyckoff positions with fourfold (Mn1) and sixfold (Mn2) multiplicity. During cooling down to 100 K the Mn5Ge3 unit-cell shows remarkable structural deformation. The nearest distance d3 between Mn2-Mn2 atoms in the hexagonal a-b plane is shortened much faster than the nearest distance d1 between Mn1-Mn1 atoms along hexagonal c axis. The DFT calculations show that below critical distance d3 < 3.002 Å, the Mn2 atoms are AFM coupled while for d3 > 3.002 Å the coupling is FM. The FM coupling between Mn1 atoms weakly depends on the atomic distance d1. Moreover, there is a transition from collinear to noncollinear spin configuration at about 70±5 K. Simultaneously, at low temperature, the angular dependent magnetoresistance shows a switching from multi-fold component to twofold symmetry. The combination of different experimental techniques with theoretical calculations enabled us to conclude that the switching between non-collinear and collinear spin configurations and the variation of anisotropic magnetoresistance in Mn5Ge3 is due to the strain induced change of the magnetic coupling between Mn2-Mn2 atoms. Finally, the effects of strain on the structural and magnetic properties of epitaxial Mn5Ge3 on Ge (111) substrate by applying ms-range FLA are investigated. The X-ray diffraction results demonstrate that during FLA process the formation of nonmagnetic secondary phases of MnxGey is fully suppressed and the in-plane tensile strain is enhanced. The temperature dependent magnetization indicates that after FLA the Curie temperature of Mn5Ge3 increases from 283±5 K to above 400 K. Further Monte Carlo simulations manifest that the change of the strain in Mn5Ge3 during ms-range FLA modifies the distance between adjacent Mn atoms in the hexagonal basal plane, which provokes the different ferromagnetic interaction between them. Consequently, the significant increase of Curie temperature is observed. This provides a good way to improve the Curie temperature of Mn5Ge3 which is promising to realize room-temperature operated Ge based spin-injectors.

Half-metallic ferromagnets (HMFs), with fully spin-polarized conduction electrons, are prime candidates for optimizing spintronic devices. Many Heusler compounds (a class of ternary and quaternary intermetallics) are predicted to be HMFs, in particular Co$_{2}YZ$ (where $Y$ is usually another transition metal, and $Z$ is an s-p element). Crystal structure is controlled by thermodynamics to a large extent. Ideally, one should be able to control and optimize properties which are of interest by appropriately "tuning" the structure (e.g. annealing), but first one must understand the structure and its relation to observed physical properties. A local structural probe technique such as nuclear magnetic resonance (NMR) is an essential tool for identifying and quantifying the various atomic-scale orderings. Different Heusler structure types and antisite disorders affect the material's physical properties. In this thesis, order-disorder phenomena in both bulk and thin film samples of Co$_2$Mn$_{1-x}$Si$_x$ and Co$_2$Mn$_{1-x}$Fe$_x$Si have been systematically studied using NMR. Though it is the films which are directly implemented in actual devices, studying bulk samples as model systems provides invaluable information regarding the material properties. The evolution of local atomic structure in numerous thin films has been shown to depend greatly on preparation parameters, including post-deposition annealing temperature, and specific stoichiometry. For Co$_2$MnSi films, the ideal post-annealing temperature for promoting the $L2_1$ atomic structure was found; the threshold temperature above which structure continues to become higher-ordered in the bulk, but where too much interdiffusion at the buffer interface occurs, degrading the smooth interfaces necessary for high magnetoresistance ratios. NMR also adds evidence that Co$_2$Mn$_x$Si$_{0.88}$ ($x>$1) electrodes in magnetic tunnel junctions have highest tunneling magneto-resistance because the excess Mn suppresses the formation of detrimental Co$_{Mn}$ antisites. A systematic investigation of several thermal and magnetic properties, including Sommerfeld coefficients, Debye temperatures, saturation magnetic moments, spin-wave stiffness, and magnon specific heat coefficient, were measured for selected Co$_2$-based ternary and quaternary Heusler compounds. Obtained values were compared with theoretical ones calculated using electronic band structure methods. It has been systematically shown that adding a magnon term to the specific heat has a negligible effect on the electronic contribution in all cases.

Die vorliegende Arbeit befasst sich mit dem ferromagnetischen Kondo-Gitter-Modell (s-d-, s-f-Modell) für Filmstrukturen. Die Spin-Fermion-Wechselwirkung des Modells kommt in Materialien vor, in denen lokalisierte Spins mit beweglichen Ladungsträgern wechselwirken, wie etwa in (verdünnten) magnetischen Halbleitern, Manganaten, oder Seltene-Erd-Verbindungen. Die durch die Ladungsträger vermittelte, indirekte Wechselwirkung zwischen den lokalisierten Spins reicht von der langreichweitigen, oszillierenden RKKY-Austauschwechselwirkung im Falle schwacher Kopplung bis zur kurzreichweitigen Doppelaustausch-Wechselwirkung bei starker Spin-Fermion-Kopplung. Beide Grenzfälle werden in dieser Arbeit durch die Abbildung des Problems auf ein effektives Heisenberg-Modell erfasst. Der Einfluss von reduzierter Translationssymmetrie auf die effektive Austauschwechselwirkung und auf die magnetischen Eigenschaften des ferromagnetischen Kondo-Gitter-Modells wird untersucht. Curie-Temperaturen werden für verschiedene Parameterkonstellationen berechnet. Die Auswirkungen von Ladungstransfer und von Gitter-Relaxation auf die magnetische Oberflächenstabilität werden betrachtet. Die Diskussion bezieht sich auf die Modifizierungen der Zustandsdichte und der kinetischen Energie im dimensionsreduzierten Fall, da die effektiven Austauschintegrale eng mit diesen Größen verknüpft sind. Die Bedeutung von Spinwellen für den Magnetismus dünner Filme und an der Oberfläche wird gezeigt. Die Interlagen-Austauschkopplung stellt ein besonders interessantes und wichtiges Beispiel der indirekten Wechselwirkung zwischen lokalisierten Momenten dar. Im Rahmen einer RKKY-Behandlung wird die Kopplung zwischen Monolagen in dünnen Filmen untersucht. Sie wird entscheidend durch die Art der ebenen und senkrechten Ladungsträgerdispersion bestimmt und ist jenseits eines kritischen Wertes der Fermi-Energie stark unterdrückt. Schließlich wird die temperaturabhängige magnetische Stabilität von interlagen-gekoppelten dünnen Filmen behandelt und die Bedingungen für einen temperaturgetriebenen magnetischen Reorientierungsübergang werden diskutiert. / This thesis is concerned with the ferromagnetic Kondo lattice (s-d, s-f) model for film geometry. The spin-fermion interaction of this model refers to materials in which localized spins interact with mobile charge carriers like in (dilute) magnetic semiconductors, manganites, or rare-earth compounds. The carrier-mediated, indirect interaction between the localized spins comprises the long-range, oscillatory RKKY exchange interaction in the weak-coupling case and the short-range double-exchange interaction for strong spin-fermion coupling. Both limits are recovered in this work by mapping the problem onto an effective Heisenberg model. The influence of reduced translational symmetry on the effective exchange interaction and on the magnetic properties of the ferromagnetic Kondo lattice model is investigated. Curie temperatures are obtained for different parameter constellations. The consequences of charge transfer and of lattice relaxation on the magnetic stability at the surface are considered. Since the effective exchange integrals are closely related to the electronic structure in terms of the density of states and of the kinetic energy, the discussion is based on the modifications of these quantities in the dimensionally-reduced case. The important role of spin waves for thin film and surface magnetism is demonstrated. Interlayer exchange coupling represents a particularly interesting and important manifestation of the indirect interaction among localized magnetic moments. The coupling between monatomic layers in thin films is studied in the framework of an RKKY approach. It is decisively determined by the type of in-plane and perpendicular dispersion of the charge carriers and is strongly suppressed above a critical value of the Fermi energy. Finally, the temperature-dependent magnetic stability of thin interlayer-coupled films is addressed and the conditions for a temperature-driven magnetic reorientation transition are discussed.

Die vorliegende Arbeit beschäftigt sich mit der mikroskopischen Modellierung von strominduziertem Schalten der Magnetisierung in magnetischen Tunnelstrukturen. Die Tunnelstruktur besteht aus zwei durch einen nichtmagnetischen Isolator voneinander getrennten Ferromagneten und einem Paramagneten, der als Elektronenreservoir dient. Die Ferromagnete werden beide durch das Hubbard-Modell beschrieben. Durch Anlegen einer Spannung verschieben sich die chemischen Potentiale auf beiden Seiten des Isolators, wodurch ein endlicher Tunnelstrom entsteht. Dieser wird im Rahmen des Modells durch eine Hybridisierung zwischen benachbarten Schichten simuliert. Das Modell muss im Nichtgleichgewicht gelöst werden, da aufgrund der unterschiedlichen chemischen Potentiale thermodynamisches Gleichgewicht nicht angenommen werden darf. Daher wird zur analytischen Auswertung der Keldysh-Formalismus verwendet, der eine Erweiterung der Viel-Teilchen-Theorie ins Nichtgleichgewicht darstellt. Da es sich beim Hubbard-Modell um ein nicht exakt lösbares Viel-Teilchen-Modell handelt, wurde in der Arbeit eine approximative Lösung, der sogenannte Nichtgleichgewichtsspektraldichteansatz, entwickelt. Dieser beruht auf einer Hochenergieentwicklung der retardierten Greenfunktion mit Hilfe der exakt berechenbaren Spektralmomente. Die numerischen Resultate stimmen qualitativ mit dem Experiment überein. Insbesondere gelingt es, das Hystereseverhalten der Magnetisierung des freien Ferromagneten in Abhängigkeit der angelegten Spannung korrekt zu reproduzieren. Es kann somit allein durch Anlegen einer Spannung kontrolliert zwischen paralleler und antiparalleler Ausrichtung der Magnetisierungen geschaltet werden. Dieses Phänomen ist anhand der entsprechenden Quasiteilchenzustandsdichten erklärbar. Weiterhin wird das Verhalten der kritischen Spannung systematisch in Form von Phasendiagrammen dargestellt und diskutiert. / This thesis is concerned with the microscopic modelling of current-induced switching of magnetization in magnetic tunnel junctions. The tunnel junction consists of two ferromagnets which are divided by a nonmagnetic insulator and a paramagnet, which acts as an electron reservoir. The ferromagnets are both described by the Hubbard model. By applying a voltage the chemical potentials on both sides of the insulator are shifted which results in a finite tunneling current. Within the model the current is simulated by a hybridization between neighbouring regions. The model has to be solved in non-equilibrium since thermal equilibrium requires a constant chemical potential for the whole system, which is not the case due to the voltage. Thus the Keldysh formalism will be used for evaluating the model. Since the Hubbard model is not exactly solvable one needs approximations. In this work a non-equilbrium spectral density approach is developed. It is based on a high-energy expansion of the retarded Green''s function and takes interactions beyond the mean field level into account. The numerical results of the theory are in qualitative agreement with experiments. It will be shown that it is possible to correctly get the hysterisis behaviour of the magnetization of the free ferromagnet in dependence on the applied voltage. Thus the relative alignment of the two magnetizations can be switched just by applying an electric field. This can be explained with the corresponding quasiparticle densities of state. Furthermore the behaviour of the critical voltage will be discussed systematically by calculating phase diagrams of the tunnel junction.

In der vorliegenden Arbeit werden durch den inversen piezoelektrischen Effekt kontrolliert Dehnungen in Lanthanmanganatschichten eingebracht und ihr Einfluss auf die Eigenschaften der Schichten untersucht. Dazu wird im ersten Teil der Arbeit ein Zweischichtsystem bestehend aus einer Manganatschicht aus La0,7Sr0,3MnO3, La0,8Ca0,2MnO3 oder La0,7Ce0,3MnO3 und einer piezoelektrischen Schicht aus PbZr0,52Ti0,48O3 untersucht. Der epitaktisch auf Einkristallsubstraten abgeschiedene Aufbau entspricht einer Feldeffekt-Transistor-Struktur. Neben den Effekten der Dehnung auf den elektrischen Widerstand der Manganatschicht wird auch der elektrische Feldeffekt untersucht. Durch mechanische Klemmung des Substrats können nur kleine Dehnungen in die Manganatschichten eingebracht werden. Um größere und homogene Dehnungen steuerbar in Manganatschichten einzubringen, werden im zweiten Teil der Arbeit La0,7Sr0,3MnO3 - Schichten auf piezoelektrischen Einkristallsubstraten der Verbindung (1-x)Pb(Mg1/3Nb2/3)O3 - xPbTiO3 mit x = 0,28 epitaktisch abgeschieden. Der Einfluss von mechanischen Dehnungen von bis zu 0,1% auf den elektrischen Transport, die ferromagnetische Übergangstemperatur und die Magnetisierung kann so eingehend untersucht werden. Es wird ein außergewöhnlich großer Einfluss von Dehnungen auf die Eigenschaften von La0,7Sr0,3MnO3 gefunden. / In this work, strain arising from the inverse piezoelectric effect is induced into lanthanum manganite thin films in order to change and control their properties. In the first part of this work, manganite films of the compositions La0.7Sr0.3MnO3, La0.8Ca0.2MnO3 or La0.7Ce0.3MnO3 are combined with a piezoelectric layer of the composition PbZr0.52Ti0.48O3 in a bilayer system. This structure is grown epitaxially on single crystal substrates and corresponds to a field-effect transistor setup. Besides effects of strain on the electrical resistance of the manganite layers, field effects are observed. Due to clamping of the substrate, only small strains can be induced to the manganite films. In order to apply larger and homogeneous controllable strain to the manganite layers, thin films of La0.7Sr0.3MnO3 are grown epitaxially on piezoelectric single crystal substrates of the composition (1-x)Pb(Mg1/3Nb2/3)O3 - xPbTiO3, x = 0.28. Strain levels up to 0.1% are reached. The influence of the strain on electrical transport, ferromagnetic transition temperature and magnetization is analyzed. A remarkably large influence of the strain on the properties of La0.7Sr0.3MnO3 is found.

Die Arbeit befasst sich mit der Elektronenspinresonanz (ESR) stark korrelierter Elektronensysteme mit ferromagnetischen Wechselwirkungen. Es wurden dafür Messungen an den Kondogitter-Systemen CeRuPO und CeOsPO, der Dotierungsreihe CeFeAs1-xPxO, den niederdimensionalen frustrierten Quadratgittern AA’VO(PO4)2 sowie in dem schwach ferromagnetischen Metall Nb1-yFe2+y durchgeführt. Alle Verbindungen zeigen entweder eine ferromagnetische Ordnung oder befinden sich in der Nähe einer ferromagnetischen Instabilität, die die Eigenschaften des stark korrelierten Systems beeinflusst.

Spin-polarized two-dimensional electron states (2DESs) at surfaces and interfaces of magnetically active materials attract immense interest because of the idea of exploiting fermion spins rather than charge in next generation electronics. Applying angle-resolved photoelectron spectroscopy, we show that the silicon surface of GdRh2Si2 bears two distinct 2DESs, one being a Shockley surface state, and the other a Dirac surface resonance. Both are subject to strong exchange interaction with the ordered 4f-moments lying underneath the Si-Rh-Si trilayer. The spin degeneracy of the Shockley state breaks down below ~90 K, and the splitting of the resulting subbands saturates upon cooling at values as high as ~185 meV. The spin splitting of the Dirac state becomes clearly visible around ~60 K, reaching a maximum of ~70 meV. An abrupt increase of surface magnetization at around the same temperature suggests that the Dirac state contributes significantly to the magnetic properties at the Si surface. We also show the possibility to tune the properties of 2DESs by depositing alkali metal atoms. The unique temperature-dependent ferromagnetic properties of the Si-terminated surface in GdRh2Si2 could be exploited when combined with functional adlayers deposited on top for which novel phenomena related to magnetism can be anticipated.

Half-metallic ferromagnets (HMFs), with fully spin-polarized conduction electrons, are prime candidates for optimizing spintronic devices. Many Heusler compounds (a class of ternary and quaternary intermetallics) are predicted to be HMFs, in particular Co$_{2}YZ$ (where $Y$ is usually another transition metal, and $Z$ is an s-p element). Crystal structure is controlled by thermodynamics to a large extent. Ideally, one should be able to control and optimize properties which are of interest by appropriately "tuning" the structure (e.g. annealing), but first one must understand the structure and its relation to observed physical properties. A local structural probe technique such as nuclear magnetic resonance (NMR) is an essential tool for identifying and quantifying the various atomic-scale orderings. Different Heusler structure types and antisite disorders affect the material's physical properties. In this thesis, order-disorder phenomena in both bulk and thin film samples of Co$_2$Mn$_{1-x}$Si$_x$ and Co$_2$Mn$_{1-x}$Fe$_x$Si have been systematically studied using NMR. Though it is the films which are directly implemented in actual devices, studying bulk samples as model systems provides invaluable information regarding the material properties. The evolution of local atomic structure in numerous thin films has been shown to depend greatly on preparation parameters, including post-deposition annealing temperature, and specific stoichiometry. For Co$_2$MnSi films, the ideal post-annealing temperature for promoting the $L2_1$ atomic structure was found; the threshold temperature above which structure continues to become higher-ordered in the bulk, but where too much interdiffusion at the buffer interface occurs, degrading the smooth interfaces necessary for high magnetoresistance ratios. NMR also adds evidence that Co$_2$Mn$_x$Si$_{0.88}$ ($x>$1) electrodes in magnetic tunnel junctions have highest tunneling magneto-resistance because the excess Mn suppresses the formation of detrimental Co$_{Mn}$ antisites. A systematic investigation of several thermal and magnetic properties, including Sommerfeld coefficients, Debye temperatures, saturation magnetic moments, spin-wave stiffness, and magnon specific heat coefficient, were measured for selected Co$_2$-based ternary and quaternary Heusler compounds. Obtained values were compared with theoretical ones calculated using electronic band structure methods. It has been systematically shown that adding a magnon term to the specific heat has a negligible effect on the electronic contribution in all cases.