<p>In recent years, correlated magnetic phenomena have emerged as a unique resource for enabling alternative computing, memory, and sensing applications. This has led to the exploration of novel magnetic hybrid platforms with the promise of improved figures of merit over the state-of-the-art. In this dissertation, we delve into several example platforms where magnets interact with various other degrees of freedom, resulting in enhanced figures of merit and/or the emergence of novel functionalities.</p><p>First, we investigate the possibility of utilizing the collective resonant mode of nanomagnets to enhance the electric field sensitivity of quantum spin defects. While quantum systems have garnered significant attention in recent years for their extraordinary potential in information processing, their potential in the field of quantum sensing remains yet to be fully explored. Quantum systems, with their inherent fragility to external signals, can be harnessed as powerful tools to develop highly efficient sensors. In this dissertation, we explore the potential of a specific type of quantum sensor, namely the quantum spin defects as an electric field sensor, when integrated with a nanomagnet/piezoelectric composite multiferroic. This integration yields at least an order of magnitude enhancement in sensitivity, presenting a promising avenue for quantum sensing applications.</p><p>Next, we shift our focus towards harnessing magnetic correlation in the emerging class of atomically thin magnets known as van der Waals magnets. These magnets provide distinctive opportunities for controlling and exploiting magnetic correlations. Specifically, these platforms allow for tunable magnetic interactions by twisting two vertically adjacent layers of the magnet, features that are unique to van der Waals materials. By capitalizing on such twist degrees of freedom, we demonstrate the creation of twist-tunable nanoscale magnetic ground states. This capability opens up avenues for applications such as high-density memories and magnon crystals.</p><p>Interestingly, the same material platform also allows for exploiting magnetic correlation by controlling the local electrical environment. We uncover the symmetry-allowed spin-charge coupling mechanisms in the heterostructures of such magnets, a prediction that has received experimental support. Utilizing such an understanding, we propose a setup for the electrical generation of magnons. Magnons—the elementary excitation of spin waves—have garnered a lot of attention these days due to their potential to couple various diverse physical systems and in the field of low dissipation computing. Our findings offer a potential pathway towards the realization of magnon-based spintronic devices.</p>

This dissertation presents a substantial contribution to a new field of material science, the investigation of the magnetic properties of 3D curved surfaces, achieved by using a self-assembled geometrical transformation of an initially planar membrane. Essential magnetic properties of thin films can be modified by the process of transforming them from a 2D planar film to a 3D curved surface. By investigating and controlling the reasons that influence the properties, it is possible to improve the functionality of existing devices in addition to laying the foundation for the future development of microelectronic devices based on curved magnetic structures. To accomplish this, it is necessary both to fabricate high-quality 3D curved objects and to establish reliable characterization methods based on commonly available technology. The primary objective of this dissertation is to develop techniques for characterizing the static and dynamic magnetic properties of self-assembled rolled 3D geometries. The second objective is to examine the origin of shape-, size- and strain/curvature-induced effects. The developed approach based on anisotropic magnetoresistance (AMR) measurement can quantitatively define the rolling-induced static magnetic changes, namely the induced magnetoelastic anisotropy, thus eliminating the need for microscopic imaging to characterize the structures. The interpretation of the AMR signal obtained on curved stripes is enabled by simultaneous visualization of the domain patterns and micromagnetic simulations. The developed approach is used to examine the effect of sign and magnitude of curvature on the induced anisotropies by altering the rolling direction and diameter of the 'Swiss-roll'. Furthermore, a time-averaged imaging technique based on conventional microscopies (magnetic force microscopy and Kerr microscopy) offers a novel strategy for investigating nanoscale periodic domain wall oscillations and hence dynamic magnetic characteristics of flat and curved structures. This method exploits the benefit of a position-dependent dwell time of periodically oscillating DWs and can determine the trajectory and amplitude of DW oscillation with sub-100 nm resolution. The uniqueness of this technique resides in the ease of the imaging procedure, unlike other DW dynamics imaging methods. The combined understanding of rolling-induced anisotropy and imaging DW oscillation is utilized to examine the dependence of DW dynamics on external stimuli and the structure's physical properties, such as lateral size, film thickness, and curvature-induced anisotropy. The presented methods and fundamental studies help to comprehend the rapidly expanding field of 3-dimensional nanomagnetism and advance high-performance magneto-electronic devices based on self-assembly rolling.

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Generating, miniaturizing and controlling spin waves on the nanometer scale is of great interest for magnonics. For instance, this holds the prospect of exploring wave-based logic concepts and reduced Joule heating, by avoiding charge transport, in spin-wave circuitry. In this work, a novel approach is for the first time confirmed experimentally, which allows confining spin-wave transport to nanometre-wide channels defined by magnetic domain walls. This is investigated for different domain wall types( 90deg and180deg Néel walls) in two material systems of polycrystalline Ni81Fe19 and epitaxial Fe. The study covers the thermal, linear and non-linear regime utilizing micro- focused Brillouin light scattering microscopy complemented by micromagnetic simulations. An initially linear dispersion dominated by dipolar interactions is found for the guided spin waves. These are transversally confined to sub-wavelength wide beams with a well-defined wave vector along the domain wall channel. In the non-linear regime, higher harmonic generation of additional spin-wave beams at the sides of the domain wall channel is observed. Furthermore, the possibility to shift the position of the domain wall over several microns by small magnetic fields is demonstrated, while maintaining its spin-wave channeling functionality. Additionally, spin-wave transmittance along domain walls, which change direction at the edges of the structure as well as between interconnected walls of identical and different type is studied. Characterization of spin-wave transmission through interconnected domain walls is an important step towards the development of magnonic circuitry based on domain wall(-networks). With respect to developing flexible and scalable spin-wave sources, the second part of this thesis addresses auto-oscillations in spin Hall oscillators (based on a Pt / Ni81Fe19 bilayer) of tapered nanowire geometry. In these systems, a simultaneous formation of two separate spin-wave bullets of distinct localization and frequency has been indicated. This spin-wave bullet formation is con- firmed experimentally and investigated for different driving currents. Subsequently, control over these bullets by injecting external microwave signals of varying frequency and power is demon- strated, switching the oscillator into single-mode operation. Three synchronized auto-oscillatory states are observed, which can be selected by the frequency of the externally imprinted signal. This synchronization results in linewidth reduction and frequency-locking of the individual bullet modes. Simultaneously the bullet-amplitude is amplified and is found to scale as P2/3 with the injected microwave power P. This amplification and control over position and frequency of the spin-wave bullets is promising for the development of microwave amplifiers/detectors and spin- wave sources on the nanoscale based on spin Hall oscillators.:1 Introduction 1 2 Theoretical background 4 2.1 Energy density of thin film ferromagnets and domain(wall) formation 2.2 Magnetizationdynamicsinthinfilmferromagnets 11 2.2.1 Spin-wavedispersioninthelinearregime 13 2.2.2 Magnetizationdynamicsinthenon-linearregime 17 2.3 SpinHallOscillators 21 2.3.1 Spin Hall effect and spin transfer torque in a ferromagnet/heavy-metal bi- layersystem 21 2.3.2 Characteristics of magnetization auto-oscillations 25 2.3.3 Improvement of monochromaticity, coherence and output power by injec- tionlocking 28 3 Materials and Methods 31 3.1 ElectronBeamLithography,EBL 31 3.2 Ni81Fe19 microstructures 32 3.3 Femicrostructures 34 3.4 TaperedspinHalloscillators 35 3.5 Micro-focused Brillouin Light Scattering Spectroscopy, μBLS 36 3.5.1 μBLSspatialresolution 40 4 Experimental results 43 4.1 Spin-wave dynamics in multi-domain magnetic configurations 43 4.1.1 Spin-wave dynamics of 180◦ Néel walls in rectangular elements 44 4.1.2 Spin-wave dynamics of 90◦ Néel walls in square elements 63 4.1.3 Spin-wave dynamics of interconnected Néel walls in Fe wires 76 4.2 Auto-oscillationintaperedwiregeometries 88 4.2.1 Initial static magnetic configuration and effective field 89 4.2.2 Thermally excited dynamics and spectral properties 91 4.2.3 Direct microwave excitation of spin-wave dynamics 93 4.2.4 Auto-oscillatoryresponse 96 4.2.5 Microwaveamplificationandinjectionlocking 104 5 Summary and outlook 114 Own publications 118 Bibliography 120 Acknowledgement 141 A Appendix 143 A.1 Splitting process in magnetic domains confined by domain walls 143 A.2 reconfigurable remanent states in square structures stabilized by local ion irradiation 144 A.3 Domain wall displacements induced by a scanning laser beam 145 A.4 Magnetic Force Microscopy investigation of the domain wall type and width 147 A.5 Micromagnetic simulations: problem definition and analysis 149 A.6 Current dependence of auto-oscillations in the tapered SHO 152 A.7 Fabrication of Ni81Fe19 microstructures for spin waves in domain walls 153

In dieser Arbeit werden die Auswirkungen epitaktischer Dehnung auf die Eigenschaften ferromagnetischer und ferroelektrischer Perowskitschichten untersucht. Dazu wird der biaxiale Dehnungszustand einer Schicht reversibel verändert, indem einkristalline piezoelektrische Pb(Mg1/3Nb2/3)0.72Ti0.28O3 (001) Substrate (PMN-PT) verwendet werden. Ergänzt werden die Messungen mit dieser “dynamischen” Methode durch Untersuchungen an statisch gedehnten Schichten, gewachsen auf LaAlxSc1-xO3-Pufferschichten mit gezielt abgestimmter Gitterfehlpassung. Drei verschiedene Materialsysteme werden studiert: die ferromagnetischen Oxide La0.8Sr0.2CoO3 und SrRuO3 und das ferroelektrische Pb(Zr,Ti)O3. Für La0.8Sr0.2CoO3 wird ein dehnungsinduzierter Übergang von der bekannten ferromagnetischen Phase zu einer magnetisch weniger geordneten, spinglasartigen Phase nachgewiesen. Es ergeben sich keine Hinweise auf eine Beeinflussung des Co-Spinzustandes. In epitaktischen SrRuO3-Schichten bewirkt eine Zugdehnung einen strukturellen Phasenübergang von der orthorhombischen Bulk-Phase zu einer out-of-plane orientierten tetragonalen Phase. Die leichte Richtung liegt in der Ebene. Reversible Dehnungsmessungen zeigen einen deutlichen Einfluss auf die ferromagnetische Ordnungstemperatur und deuten auf eine geringe Veränderung des magnetischen Moments hin. Der Dehnungseffekt auf die elektrischen Transporteigenschaften wird bestimmt. Pb(Zr,Ti)O3 wird als ferroelektrisches Standardmaterial genutzt, um erstmalig den Einfluss biaxialer Dehnung auf das ferroelektrische Schaltverhalten dünner Schichten zu untersuchen. Für kleine elektrische Felder zeigen die Messungen das typische Verhalten einer gepinnten Domänenwandbewegung. Hier wird der Schaltvorgang unter Piezokompression stark beschleunigt. Werden an die elektrischen Kontakte größere elektrische Felder angelegt, geht die Domänenwandbewegung in das Depinning-Regime über. Die Schaltkinetik wird in diesem Bereich unter Piezokompression leicht verlangsamt. / In this work, the effect of epitaxial strain on the properties of ferromagnetic and ferroelectric perovskite thin films is studied. Single-crystalline piezoelectric Pb(Mg1/3Nb2/3)0.72Ti0.28O3 (001) substrates (PMN-PT) are utilized to reversibly change the biaxial strain state of the films. The measurements performed by this “dynamic” approach are complemented by studying statically strained films grown on LaAlxSc1-xO3 buffer layers with deliberately tuned lattice misfit. Three different material systems are investigated: the ferromagnetic oxides La0.8Sr0.2CoO3 and SrRuO3 and the ferroelectric compound Pb(Zr,Ti)O3. In case of La0.8Sr0.2CoO3 a strain-induced transition from the known ferromagnetic phase to a magnetically less ordered spinglas-like phase is observed. No indications for an effect on the Co spin state are found. In epitaxial SrRuO3 films tensile strain is causing a structural phase transition from the bulk-like orthorhombic structure to an out-of-plane oriented tetragonal phase. The magnetic easy axis is in the film plane. Reversible strain experiments show a significant effect on the ferromagnetic ordering temperature and point to a small change of the magnetic moment. The strain effect on the electric transport properties is also determined. Pb(Zr,Ti)O3 as a standard ferroelectric material is used to study the influence of biaxial strain on the ferroelectric switching behaviour of thin films for the first time. At small electric fields the measurements reveal the typical signs of creep-like domain wall motion caused by wall pinning. In this regime the switching process is accelerated strongly under piezo-compression. For higher electric fields a transition of the domain wall motion to the depinning regime is observed. Here, the switching kinetics is slowed down moderately by compressive strain.

In dieser Arbeit werden die Auswirkungen epitaktischer Dehnung auf die Eigenschaften ferromagnetischer und ferroelektrischer Perowskitschichten untersucht. Dazu wird der biaxiale Dehnungszustand einer Schicht reversibel verändert, indem einkristalline piezoelektrische Pb(Mg1/3Nb2/3)0.72Ti0.28O3 (001) Substrate (PMN-PT) verwendet werden. Ergänzt werden die Messungen mit dieser “dynamischen” Methode durch Untersuchungen an statisch gedehnten Schichten, gewachsen auf LaAlxSc1-xO3-Pufferschichten mit gezielt abgestimmter Gitterfehlpassung. Drei verschiedene Materialsysteme werden studiert: die ferromagnetischen Oxide La0.8Sr0.2CoO3 und SrRuO3 und das ferroelektrische Pb(Zr,Ti)O3. Für La0.8Sr0.2CoO3 wird ein dehnungsinduzierter Übergang von der bekannten ferromagnetischen Phase zu einer magnetisch weniger geordneten, spinglasartigen Phase nachgewiesen. Es ergeben sich keine Hinweise auf eine Beeinflussung des Co-Spinzustandes. In epitaktischen SrRuO3-Schichten bewirkt eine Zugdehnung einen strukturellen Phasenübergang von der orthorhombischen Bulk-Phase zu einer out-of-plane orientierten tetragonalen Phase. Die leichte Richtung liegt in der Ebene. Reversible Dehnungsmessungen zeigen einen deutlichen Einfluss auf die ferromagnetische Ordnungstemperatur und deuten auf eine geringe Veränderung des magnetischen Moments hin. Der Dehnungseffekt auf die elektrischen Transporteigenschaften wird bestimmt. Pb(Zr,Ti)O3 wird als ferroelektrisches Standardmaterial genutzt, um erstmalig den Einfluss biaxialer Dehnung auf das ferroelektrische Schaltverhalten dünner Schichten zu untersuchen. Für kleine elektrische Felder zeigen die Messungen das typische Verhalten einer gepinnten Domänenwandbewegung. Hier wird der Schaltvorgang unter Piezokompression stark beschleunigt. Werden an die elektrischen Kontakte größere elektrische Felder angelegt, geht die Domänenwandbewegung in das Depinning-Regime über. Die Schaltkinetik wird in diesem Bereich unter Piezokompression leicht verlangsamt.:1 Einführung 1.1 Motivation 1.2 Methodik 1.3 Übersicht 2 Probenherstellung und -charakterisierung 2.1 Gepulste Laserdeposition 2.1.1 Prinzip 2.1.2 Aufbau 2.1.3 RHEED 2.1.4 Optimierung des Schichtwachstums 2.1.5 Targets 2.1.6 Substrate 2.2 Röntgendiffraktion 2.2.1 Röntgenmethoden 2.2.2 Röntgenreflektometrie 2.3 SQUID-Magnetometrie 2.4 Rasterkraftmikroskopie 2.5 Transportmessungen 2.6 Elektrische Polarisationsmessungen 3 PMN-PT 3.1 PMN-PT als piezoelektrisches Dünnschicht-Substrat 3.2 PMN-PT als Piezoaktuator 3.3 Temperaturabhängigkeit der Piezodehnung 3.4 Dehnungsübertragung in die Schicht 4 Puffersysteme 4.1 Motivation 4.2 LaAlxSc1−xO3 4.3 BaxSr1−xTiO3 5 Dehnungseinfluss auf ferromagnetische Filme - La0.8Sr0.2CoO3 5.1 Grundlagen zu La1−xSrxCoO3 5.1.1 Struktur 5.1.2 Spinzustand 5.1.3 Magnetische Wechselwirkungen / Doppelaustausch 5.1.4 Phasendiagramm / magnetische Phasenseparation 5.2 Messungen 5.2.1 Gitter- und Mikrostruktur 5.2.2 Curie-Temperatur 5.2.3 Magnetoelastischer Effekt 5.2.4 Magnetisierungsschleifen 5.2.5 elektrischer Transport 5.3 Zusammenfassung und Ausblick 6 Dehnungseinfluss auf ferromagnetische Filme - SrRuO3 6.1 Grundlagen zu SrRuO3 6.1.1 Struktur 6.1.2 Magnetismus 6.1.3 Elektrischer Transport 6.2 Messungen 6.2.1 Gitter- und Mikrostruktur 6.2.2 Magnetismus 6.2.3 Elektrischer Transport 6.3 Zusammenfassung und Ausblick 7 Dehnungseinfluss auf ferroelektrische Filme - PbZr1−xTixO3 7.1 Grundlagen 7.1.1 PbZr1−xTixO3 7.1.2 Elektrische Polarisation 7.1.3 Koerzitivfeld 7.1.4 Domänendynamik 7.2 Messungen 7.2.1 Gitterstruktur 7.2.2 Standardcharakterisierung: Dehnungseinfluss auf die remanente Polarisation Pr und das Koerzitivfeld EC 7.2.2.1 Statische Messungen 7.2.2.2 Dehnungsmessungen 7.2.3 PUND-Messungen: Dehnungseinfluss auf die charakteristische Schaltzeit tsw 7.3 Zusammenfassung und Ausblick 8 Zusammenfassung / In this work, the effect of epitaxial strain on the properties of ferromagnetic and ferroelectric perovskite thin films is studied. Single-crystalline piezoelectric Pb(Mg1/3Nb2/3)0.72Ti0.28O3 (001) substrates (PMN-PT) are utilized to reversibly change the biaxial strain state of the films. The measurements performed by this “dynamic” approach are complemented by studying statically strained films grown on LaAlxSc1-xO3 buffer layers with deliberately tuned lattice misfit. Three different material systems are investigated: the ferromagnetic oxides La0.8Sr0.2CoO3 and SrRuO3 and the ferroelectric compound Pb(Zr,Ti)O3. In case of La0.8Sr0.2CoO3 a strain-induced transition from the known ferromagnetic phase to a magnetically less ordered spinglas-like phase is observed. No indications for an effect on the Co spin state are found. In epitaxial SrRuO3 films tensile strain is causing a structural phase transition from the bulk-like orthorhombic structure to an out-of-plane oriented tetragonal phase. The magnetic easy axis is in the film plane. Reversible strain experiments show a significant effect on the ferromagnetic ordering temperature and point to a small change of the magnetic moment. The strain effect on the electric transport properties is also determined. Pb(Zr,Ti)O3 as a standard ferroelectric material is used to study the influence of biaxial strain on the ferroelectric switching behaviour of thin films for the first time. At small electric fields the measurements reveal the typical signs of creep-like domain wall motion caused by wall pinning. In this regime the switching process is accelerated strongly under piezo-compression. For higher electric fields a transition of the domain wall motion to the depinning regime is observed. Here, the switching kinetics is slowed down moderately by compressive strain.:1 Einführung 1.1 Motivation 1.2 Methodik 1.3 Übersicht 2 Probenherstellung und -charakterisierung 2.1 Gepulste Laserdeposition 2.1.1 Prinzip 2.1.2 Aufbau 2.1.3 RHEED 2.1.4 Optimierung des Schichtwachstums 2.1.5 Targets 2.1.6 Substrate 2.2 Röntgendiffraktion 2.2.1 Röntgenmethoden 2.2.2 Röntgenreflektometrie 2.3 SQUID-Magnetometrie 2.4 Rasterkraftmikroskopie 2.5 Transportmessungen 2.6 Elektrische Polarisationsmessungen 3 PMN-PT 3.1 PMN-PT als piezoelektrisches Dünnschicht-Substrat 3.2 PMN-PT als Piezoaktuator 3.3 Temperaturabhängigkeit der Piezodehnung 3.4 Dehnungsübertragung in die Schicht 4 Puffersysteme 4.1 Motivation 4.2 LaAlxSc1−xO3 4.3 BaxSr1−xTiO3 5 Dehnungseinfluss auf ferromagnetische Filme - La0.8Sr0.2CoO3 5.1 Grundlagen zu La1−xSrxCoO3 5.1.1 Struktur 5.1.2 Spinzustand 5.1.3 Magnetische Wechselwirkungen / Doppelaustausch 5.1.4 Phasendiagramm / magnetische Phasenseparation 5.2 Messungen 5.2.1 Gitter- und Mikrostruktur 5.2.2 Curie-Temperatur 5.2.3 Magnetoelastischer Effekt 5.2.4 Magnetisierungsschleifen 5.2.5 elektrischer Transport 5.3 Zusammenfassung und Ausblick 6 Dehnungseinfluss auf ferromagnetische Filme - SrRuO3 6.1 Grundlagen zu SrRuO3 6.1.1 Struktur 6.1.2 Magnetismus 6.1.3 Elektrischer Transport 6.2 Messungen 6.2.1 Gitter- und Mikrostruktur 6.2.2 Magnetismus 6.2.3 Elektrischer Transport 6.3 Zusammenfassung und Ausblick 7 Dehnungseinfluss auf ferroelektrische Filme - PbZr1−xTixO3 7.1 Grundlagen 7.1.1 PbZr1−xTixO3 7.1.2 Elektrische Polarisation 7.1.3 Koerzitivfeld 7.1.4 Domänendynamik 7.2 Messungen 7.2.1 Gitterstruktur 7.2.2 Standardcharakterisierung: Dehnungseinfluss auf die remanente Polarisation Pr und das Koerzitivfeld EC 7.2.2.1 Statische Messungen 7.2.2.2 Dehnungsmessungen 7.2.3 PUND-Messungen: Dehnungseinfluss auf die charakteristische Schaltzeit tsw 7.3 Zusammenfassung und Ausblick 8 Zusammenfassung