In this work, the effect of reversible elastic lattice strain on the electronic properties of a) (Pr1−y Lay )0.7 Ca0.3 MnO3 (PLCMO) thin films and b) the interface layer of La0.7 Sr0.3 MnO3 (LSMO) with SrTiO3 (STO) has been determined using piezoelectric substrates. Lattice strain is known to effectively alter the electronic structure of compounds from the manganite family, since it shifts the balance of competing electronic interactions by changing bond angles and bond lengths.The PLCMO films have been repared by pulsed laser deposition (PLD) from a La0.7 Ca0.3 MnO3 (LCMO) and a Pr0.7 Ca0.3 MnO3 (PCMO) target. The metal- insulator phase boundary has been established to lie around y = 0.6. In films with y = 0.6, the piezoelectric release of tensile strain in the film plane induces a drastic reduction of the resistance, or a “colossal” elastoresistance. Resistive gauge factors as high as Γ = 1000 have been found. Consistent with the transport results, the release of tensile strain leads to an increase in both the Curie temperature and the magnetisation. The coexistence of the ferromagnetic metallic (FMM) and charge ordered insulating (COI) phases in PLCMO has been found to be strongly affected by the reversible substrate strain. Both the magnetisation and the resistance data in controlled strain states demonstrate a strong suppression of the ferromagnetic double exchange interaction by tensile strain. [La0.7 Sr0.3 MnO/SrTiO3 ] superlattices have been deposited on STO and piezoelectric PMN-PT (001) (PbMg1/3 Nb2/3 O3 )0.72 (PbTiO3 )0.28 substrates by PLD. X-ray reflectivity (XRR) measurements show clear Kiessig fringes as well as the larger interference maxima caused by the superlattice, giving qualitative proof of a well defined superlattice structure with sharp interfaces on both substrates. With decreasing LSMO layer thickness d, the samples show a sharp decrease of the Curie temperature, accompanied by a decrease of the saturation magnetisation and an increase of the coercive field around d = 5 nm. Reversible strain measurements on thicker superlattices (d = 16.7 nm) reveal a behaviour of the magnetisation similar to that of single thick films of LSMO. When d is decreased, the strain induced relative change in magnetisation ∆M/M0 shows a behaviour comparable to PLCMO thin films. This has been attributed to the increased volume fraction of the LSMO inter- face layer with STO, which displays a reduced magnetic order and phase-separated tendencies. From the absolute change in magnetisation per interface, the thickness of the so-called magnetically “dead” layer of the LSMO has been estimated to lie between 1.3 nm and 1.7 nm in the superlattices grown on PMN-PT.

Magnetische Formgedächtnsilegierungen haben die besondere Fähigkeit, sich im äußeren Magnetfeld zu verformen. Dies geschieht aufgrund von Zwillingsgrenzenbewegung in der martensitischen Tieftemperaturphase. Da der Effekt bislang an Einkristallen untersucht wurde, ist es das Ziel dieser Arbeit, den Effekt an polykristallinen Proben nachzuweisen. Dafür wurden Proben nach dem Prinzip der gerichteten Erstarrung präpariert. Deren Kristallstruktur wurde durch geeignete Zusammensetzung und Wärmebehandlung einphasig eingestellt. Mechanisches Training und weitere Wärmebehandlungen ermöglichten schließlich die Demonstration der magnetischen Dehnung von ca. 1 % an polykristallinen Proben. Durch zusätzliche Einkopplung akustischer Wellen konnte die Dehnung auf 2,2 % gesteigert werden.

Magnetic shape memory (MSM) materials are a new class of smart materials which exhibit shape deformation under the influence of an external magnetic field. They are interesting for various types of applications, including actuators, displacement/force sensors, and motion dampers. Due to the huge strain and the magnetic field-driven nature, MSM materials show definite advantages over other smart materials, e.g. conventional thermal shape memory materials, in terms of displacement and speed. The principle behind the magnetic field induced strain (MFIS) is the strong coupling between magnetization and lattice structure. The investigation of both static and dynamic magnetic domain structures in MSM materials is a key step in optimizing the properties for future possible devices. In this work, optical polarization microscopy is applied to investigate the twin boundary and magnetic domain wall motion in bulk NiMnGa single crystals. Surface magnetic domain patterns on adjacent sides of bulk crystals are revealed for the first time providing comprehensive information about the domain arrangement inside the bulk and at the twin boundary. The tilting of the easy axis with respect to the sample surface determines the preferable domain size and leads to spike domain formation on the surface. Out-of-plane surface domains extend into the bulk within a single variant, while a twin boundary mirrors the domain pattern from adjacent variants. Furthermore, magnetic domain evolution during twin boundary motion is observed. The partial absence of domain wall motion throughout the process contradicts currently proposed models. The magnetic state alternates along a moving twin boundary. With the abrupt nucleation of the second variant this leads to the formation of sections of magnetically highly charged head-on domain structures at the twin boundaries. On the other hand, a dynamic actuation experimental setup, which is capable to provide high magnetic fields in a wide range of frequency, was developed in the course of this study. The observation of reversible twin boundary motion up to 600 Hz exhibits the dependence of strain, hysteresis, and twin boundary velocity on the actuation speed. MFIS increases with frequency, while the onset field is similar in all observed cases. Twin boundary mobility enhancement by fast twin boundary motion is proposed to explain the increase in MFIS. The twin boundary velocity is shown to be inversely proportional to the twin boundary density. No limit of twin boundary velocity is observed in the investigated frequency range.

Durch erstmalige Anwendung des η-Umlaufverfahrens auf mechanisch ermüdete Ni-Polykristalle konnten – aufgrund konstanter Meßbedingungen und konstanter Eindringtiefe der Röntgenstrahlung – genauere Eigenspannungsmessungen durchgeführt werden als bisher. Ein speziell entwickeltes Auswertungsverfahren, das auf die Eigenspannungsanalyse von η-Umlaufmessungen abgestimmt ist, wird als „sin2η-Methode“ detailliert vorgestellt. Die experimentellen Resultate unterstreichen die Vorteile, die sich – besonders im Falle sehr geringer Eigenspannungen und wenn auch der dehnungsfreie Ausgangszustand (d0 -Wert) gemessen werden konnte – aus der Nutzung dieser Methode ergeben. Mit Hilfe des magnetischen Barkhausen-Rauschens durchgeführte Vergleichsuntersuchungen stehen im Einklang mit den röntgenographischen Messungen. / When applying the η-rotation-method to mechanically fatigued Ni polycrystals the first time, stress measurements with higher accuracy could be realized due to the constant penetration depth of the X-rays and because of constant measuring conditions. A specifically developed procedure, tuned to analyze residual stresses from η-rotation measurements, will be presented in detail as „sin2η-method“. The experimental results emphasize the benefits of using this method – especially in the case of very low residual stresses and if the initial strain-free state (d0 -value) was obtained from reference measurements. Comparative investigations by means of the magnetic Barkhausen noise agree with the X-ray measurements.