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We report on the magnetic properties of Dy atoms adsorbed on the (001) surface of SrTiO3. X-ray magnetic circular dichroism reveals slow relaxation of the Dy magnetization on a time scale of about 800 s at 2.5 K, unusually associated with an easy-plane magnetic anisotropy. We attribute these properties to Dy atoms occupying hollow adsorption sites on the TiO2-terminated surface. Conversely, Ho atoms adsorbed on the same surface show paramagnetic behavior down to 2.5 K. With the help of atomic multiplet simulations and first-principles calculations, we establish that Dy populates also the top-O and bridge sites on the coexisting SrO-terminated surface. A simple magnetization relaxation model predicts these two sites to have an even longer magnetization lifetime than the hollow site. Moreover, the adsorption of Dy on the insulating SrTiO3 crystal leads, regardless of the surface termination, to the formation of a spin-polarized two-dimensional electron gas of Ti 3d(xy) character, together with an antiferromagnetic Dy-Ti coupling. Our findings support the feasibility of tuning the magnetic properties of the rare-earth atoms by acting on the substrate electronic gas with electric fields., ACS Nano, 16 (7), ISSN:1936-0851, ISSN:1936-086X

Ferrimagnets are the magnetic materials with the fastest current-induced dynamics reported so far. Among them, rare-earth transition-metal (RE-TM) alloys offer a fertile playground for studying the behavior of multi-sublattice systems with tunable composition and magnetic interactions. This review provides a survey of the magnetic dynamics excited by current-induced spin-orbit torques (SOTs) in RE-TM ferrimagnets coupled to heavy-metal layers. It summarizes the magnetic properties of RE-TM alloys and discusses how interfacial SOTs result in efficient magnetization switching and fast domain-wall motion close to the magnetization and angular momentum compensation points. Recent work shows that the switching is a multiphase process affected by significant stochastic fluctuations. However, strong SOTs results in fast and deterministic sub-ns switching with minimal energy dissipation. In addition, the RE and TM magnetizations can respond asynchronously to SOTs during the reversal. This asynchronous dynamics pinpoints the different strength of the SOTs acting on the two sublattices and challenges the usual assumption of rigid inter-sublattice antiferromagnetic coupling. Overall, the ability to tailor the timescale and reversal mode of RE-TM alloys allows for optimizing the speed of ferrimagnetic spintronic devices and provides insight into the current-induced transfer of angular momentum in systems with synergistic ferromagnetic and antiferromagnetic interactions., Advanced Materials Interfaces, 9 (36), ISSN:2196-7350

Measurements of the transverse Hall resistance are widely used to investigate electron transport, magnetization phenomena, and topological quantum states. Owing to the difficulty of probing transient changes of the transverse resistance, the vast majority of Hall effect experiments are carried out in stationary conditions using either dc or ac. Here we present an approach to perform time-resolved measurements of the transient Hall resistance during current-pulse injection with sub-nanosecond temporal resolution. We apply this technique to investigate in real-time the magnetization reversal caused by spin-orbit torques in ferrimagnetic GdFeCo dots. Single-shot Hall effect measurements show that the current-induced switching of GdFeCo is widely distributed in time and characterized by significant activation delays, which limit the total switching speed despite the high domain-wall velocity typical of ferrimagnets. Our method applies to a broad range of current-induced phenomena and can be combined with non-electrical excitations to perform pump-probe Hall effect measurements., Nature Communications, 12 (1), ISSN:2041-1723

Electron paramagnetic resonance (EPR) spectroscopy is widely used to characterize paramagnetic complexes. Recently, EPR combined with scanning tunneling microscopy (STM) achieved single-spin sensitivity with sub-angstrom spatial resolution. The excitation mechanism of EPR in STM, however, is broadly debated, raising concerns about widespread application of this technique. We present an extensive experimental study and modeling of EPR-STM of Fe and hydrogenated Ti atoms on a MgO surface. Our results support a piezoelectric coupling mechanism, in which the EPR species oscillate adiabatically in the inhomogeneous magnetic field of the STM tip. An analysis based on Bloch equations combined with atomic-multiplet calculations identifies different EPR driving forces. Specifically, transverse magnetic field gradients drive the spin-1/2 hydrogenated Ti, whereas longitudinal magnetic field gradients drive the spin-2 Fe. Also, our results highlight the potential of piezoelectric coupling to induce electric dipole moments, thereby broadening the scope of EPR-STM to nonpolar species and nonlinear excitation schemes., Science Advances, 6 (40), ISSN:2375-2548

Combining electron paramagnetic resonance (EPR) with scanning tunneling microscopy (STM) enables detailed insight into the interactions and magnetic properties of single atoms on surfaces. A requirement for EPR-STM is the efficient coupling of microwave excitations to the tunnel junction. Here, we achieve a coupling efficiency of the order of unity by using a radio-frequency antenna placed parallel to the STM tip, which we interpret using a simple capacitive-coupling model. We further demonstrate the possibility to perform EPR-STM routinely above 4 K using amplitude as well as frequency modulation of the radio-frequency excitation. We directly compare different acquisition modes on hydrogenated Ti atoms and highlight the advantages of frequency and magnetic-field sweeps as well as amplitude and frequency modulation in order to maximize the EPR signal. The possibility to tune the microwave-excitation scheme and to perform EPR-STM at relatively high temperature and high power opens this technique to a broad range of experiments, ranging from pulsed EPR spectroscopy to coherent spin manipulation of single-atom ensembles., Physical Review Research, 2 (1), ISSN:2643-1564

In the field of spintronics, the potential of applications exploiting magnetic materials are explored such as computation and data storage. Particularly important for such applications are the different interactions such as the Ruderman–Kittel–Kasuya–Yosida interaction and Dzyaloshinskii–Moriya interaction. Until now, these interactions have been mainly exploited in vertical stacks but little attention has been given to mechanisms that couple laterally patterned regions. In this doctoral work, it is shown how magnetic patterning techniques are a powerful tool to demonstrate new functionalities in lateral spintronic devices, with the focus on the development of two novel coupling mechanisms, namely lateral chiral coupling and lateral exchange coupling. With the detailed characterization of these couplings, new tools in the spintronics device design kit are provided for future applications. The first coupling mechanism addressed is lateral chiral coupling. Here, the lateral chiral coupling strength in a ferromagnetic Co-based system was characterized by measuring the asymmetric domain wall motion. The chiral coupling mechanism is extended to a CoB-based system, which is interesting because of the low domain wall pinning. With this dramatic reduction of the domain wall pinning, a spontaneous domain wall motion is observed in absence of any applied magnetic field. It is shown that the effective chiral coupling strength can be tuned by modifying the device geometry. In particular, by decreasing the domain wall track width, the effective chiral coupling strength increases. An analytical model combined with micromagnetic simulations provided the quantitative basis for the experiments. The simulated stopping fields had a good match to the experimental results. Furthermore, the existence of chiral coupling in a ferrimagnetic CoBGd based system was demonstrated with the realization of a Boolean NOT gate function. The second coupling mechanism that was developed is lateral exchange coupling. This was realized by modifying the compensation temperature in a ferrimagnetic GdCo-based system via both oxidation and He+ irradiation methods. The lateral exchange coupling strength was determined from asymmetric domain wall propagation experiments at different temperatures, with a maximum in the exchange bias of 2.5 T. Furthermore, the evolution of the electrical current heating induced exchange bias in a Hall bar at different temperatures is explored. Kerr microscopy of the asymmetric domain wall motion in microscopic tracks provided a better understanding of domain wall dynamics at different applied magnetic fields and the interfacial origin of the lateral exchange coupling at room temperature. Other patterns such as a single square divided into two regions and checkerboard patterns with their dimensions ranging from a few hundreds of nm to a few µm were imaged via magnetic force microscopy and nitrogen-vacancy microscopy for further quantitative understanding of the domain wall profiles by measuring the stray field across the domain wall regions. In addition, spin orbit torques were combined with lateral exchange coupling to realize reliable switching of exchange bias in laterally coupled devices. In order to manipulate the above couplings after device fabrication, an electrolyte GdOₓ was included, so that the coupling could be adjusted by applying a gate voltage. For lateral chiral coupling, successful turning on and off of the coupling was successfully demonstrated in ferromagnetic Co-based nanostructures by changing the anisotropy from out-of-plane to in-plane and vice versa. For lateral exchange coupling, preliminary results of the manipulation of the compensation temperature are presented using electrolyte-based gating, with the potential for further manipulation of lateral exchange coupling with different patterns such as domain wall tracks and checkerboard. In summary, lateral chiral and exchange couplings in ferromagnetic and ferrimagnetic based systems have been developed and characterized. Furthermore, a voltage control design has been implemented to the devices to realize manipulation of couplings after device fabrication. These open the possibilities for new spintronic designs such as reprogrammable domain wall based Boolean logic devices.

In 2021, roughly 1.72 trillion photos were taken every day, 6 million tweets were posted on twitter every minute, 143 billion mobile apps were downloaded. If just social media and smartphone users generate such staggering numbers, it is not hard to imagine why 90% of the world’s data was created just in the last two years. The technological solution to this ever expanding thirst for data storage is to make data storage devices more energy efficient, reliable, faster and smaller. A novel method to write data in magnetic materials with electric currents is to use spin-orbit torques (SOTs), which were pioneered in heavy metal/ferromagnet bilayer systems. Recent studies show sub-nanosecond timescale writing of data in such devices and large reading voltages by tunneling magnetoresistance (TMR). Although ferromagnets remain one of the most reliable non-volatile memory storage materials, antiferromagnets have recently been investigated for their potential to be much faster, more energy efficient, highly resilient to external stimuli and even exhibit multi-level memory, which could empower analog-like brain inspired computing. However, information in antiferromagnets is hard to detect and antiferromagnetic ordering is hard to reorient with external fields making current induced changes to antiferromagnetism important. In this thesis we explore the use of antiferromagnets both as a data storage material and as a source of spin currents instead of heavy metals. In the first part of this thesis, we explore the use of the collinear antiferromagnets for field-free current-induced switching of a perpendicular ferromagnet. The need for an external magnetic field is a hurdle for SOT-based memories. PtMn being an antiferromagnet can induce an exchange bias when in contact with a ferromagnet such as a [Co/Ni] multilayer while simultaneously functioning as a spin current source. The exchange bias couples the PtMn and [Co/Ni] at the interface, thus emulating an effective magnetic field, which facilitates memristive field free switching. X-ray photoemission electron microscopy reveals that the switching occurs by gradual expansion and sequential switching of pre-existing blocked magnetic domains as a result of current pulses. The PtMn layer has a granular texture, with average domains sizes smaller than 100 nm, whereas the ferromagnetic domains in Co/Ni are typically larger than 200 nm.This implies that, although the AFM critically affects the switching characteristics through the exchange bias, there are no spatial correlations between the AFM and FM domains. The blocked domains and the granular distribution of exchange bias constrain the origin as well as the displacement of the domain walls, thus leading to highly reproducible switching patterns as a function of the applied current pulses. Secondly, we explore the non collinear topological Weyl antiferromagnet Mn3Sn as an information storage medium and elucidate the mechanism of current induced switching in this antiferromagnet. Despite it’s small magnetic moment of 2kA/m this material exhibits a large anomalous Hall effect (AHE) of 1μΩcm, making the detection of the antiferromagnetic order viable. Extensive optimization efforts were undertaken to realise polycrystalline and epitaxial samples of Mn3Sn suitable for magnetotransport. We demonstrate SOT switching of the magnetic octupole vector in Mn3Sn as a function of pulse shape, magnetic field, temperature, and time. We discover that the switching behavior can be either bistable or tristable depending on the temporal structure of the current pulses. Time-resolved Hall effect measurements performed during the current pulsing reveal that Mn3Sn switching proceeds via a two-step demagnetization-remagnetization process caused by self- heating over a timescale of tens of ns followed by cooling in the presence of spin-orbit torques. Our results shed light on the switching dynamics of Mn3Sn and prove the existence of extrinsic limits on its switching speed. This sets a precendent for the first time-resolved measurement of switching in a topological antiferromagnet. Additionally, epitaxial growth of Mn3Sn in different orientations sets the stage for future experiments that explore the exotic nature of the material and as a source of spin-currents. The third topic is dedicated to the exploration of the non-collinear ferromagnet FeMnGe, which is known to have an incommensurate spin structure. Magnetometry shows a large magnetic moment of 300 kA/m below an ordering temperature of 220 K and 30 kA/m at room temperature. X-ray magnetic circular dichroism reveals ferromagnetic coupling of the Fe and Mn sublattices at low temperature and the Mn exhibits weakly ferromagnetic properties at room temperature. AHE and magnetoresistance measurement correlate well with the magnetometry up to room temperature but show that a second phase transition exists at 465 K which is only reached by Joule heating. A large linear negative magnetoresistance due to magnon-magnetoresistance is maximised at the two transition temperatures indicative of magnetic ordering. We discover a higher order anomalous Hall effect which can be separated based on angular symmetry and exhibits a field dependence hinting at the presence of unconventional magnetic ordering and spin textures in the FeMnGe thin films.

Antiferromagnetic materials are receiving widespread attention because they promise to enable smaller, faster, and more resilient data processing units compared to state-of- the-art ferromagnets. The possibility to control the orientation of the magnetic domains in antiferromagnets by either electrical or optical means is revolutionizing the field of magnetism and opening a wealth of applications in spintronics. Outstanding open questions bear on both fundamental problems (switching mechanism) as well as on applied aspects (amplitude of the readout signal). One of the great unknowns in the field of antiferromagnetism is the structure of domain walls, which is key to both of these aspects. In particular, chiral walls are required for current-controlled magnetisation switching. The limited knowledge about antiferromagnetic domain walls as well as the switching mechanism is mainly due to the difficulty of imaging antiferromagnets. The main focus of this thesis is to address the aforementioned problems by imaging antiferromagnets using nanoscale scanning diamond magnetometry. We will elaborate throughout this thesis on the question of what information about the spin structure can be inferred when measuring the magnetic stray field on the surface of an antiferromagnet. Related to that, we discuss current-induced effects in antiferromagnets and to what extent those can be detected using nanoscale diamond magnetometry. We first calibrate our technique by determining the internal structure of a domain wall in a ferrimagnetic insulator Tm3Fe5O12 and estimate its magnetisation. Ferrimagnets, being in a broader sense between ferro- and antiferromagnets, are detectable by common magnetic probes, which we use to confirm our methodology. We reveal that the domain walls in Tm3Fe5O12 have a left Néel character indicating the presence of interfacial Dzyaloshinskii-Moriya interaction. These results open the possibility to stabilize chiral spin textures in centrosymmetric magnetic insulators important for spintronic applications. We then extend these insights and methods to the archetypical antiferromagnet Cr2O3. Here we present, to the best of our knowledge, the first experimental observation of a domain wall in a pure, intrinsic antiferromagnet. We reveal that the intrinsic domain wall structure is Bloch-like and can become Néel-like if sufficient in-plane magnetic anisotropy is present. Our experimental observation is significant because the theory does not predict any preference for Bloch or Néel domain wall structure. Finally, we combine nanoscale scanning diamond magnetometry with electrical pulsing and resistance measurements to shed light on the switching mechanism and readout signal in the metallic antiferromagnet CuMnAs. We find that, besides the known reorientation of the Néel vector, the domain pattern fragments upon injection of current pulses. This provides an explanation for the recently-observed unipolar high-resistive switching signals in CuMnAs, and their relaxation and demonstrates a novel memristive effect. Besides these observations, we show that nanoscale scanning diamond magnetometry can be successfully applied to map the domain structure of in-plane antiferromagnets. Our results point out directions for future research in the field of antiferromagnetic spintronics and provide a new methodology and concepts relevant for the quantum sensing and imaging communities.

Nanostructured magnets are essential materials in numerous key applications, such as energy conversion, automotive industry, data storage and medical imaging. These developing technologies address critical climate change and sustainability issues and demand better magnetic materials. In order to design better magnets, it is crucial to understand the connection between the magnetic performance and nanoscale microstructure. Particularly important are grain and grain-boundary architecture, where complex spin structures may form due to competing magnetic interactions. So far, technological limitations in imaging nanoscopic spin structures have hindered progress in the field. In this thesis, Lorentz transmission electron microscopy (LTEM) and off-axis electron holography are combined with micromagnetic simulations to characterize complex spin structures and understand their connection to the nanoscale microstructure and macroscopic magnetic performance. Complex spin textures arise in chiral magnets due to the competition of the Heisenberg exchange and Dzyaloshinskii-Moriya interactions, which favor collinearity and curling of neighboring spins, respectively. Particle-like magnetic skyrmions form if the symmetry is broken by an external magnetic field or internal perpendicular magnetic anisotropy. Skyrmions have topologically non-trivial spin winding, meaning that they cannot be unwound without a phase transition. This stability makes them appealing for future spintronic applications, such as data storage. It is therefore important to understand how skyrmions behave in the presence of defects. In this thesis, it is numerically predicted that in chiral magnetic films without defects skyrmion lattices undergo a first-order phase transition in which they invert their magnetization. The inversion is induced by an external magnetic field and mediated by a temporary formation of antiskyrmions, which have an opposite spin winding to that of skyrmions. In the presence of defects, the inversion is replaced by a second-order phase transition of defect-induced melting, where the skyrmion lattice is gradually annihilated around the defects. This is experimentally observed in chiral FeGe thin films, where the coexistence of original and inverted skyrmion lattices is also found. These results signify the importance of defect analysis for the development of future spintronic materials. Contrary to what was previously believed, it is shown in this thesis that complex spin structures, such as topologically non-trivial domain walls and magnetic vortices, can also form in Sm(Co,Fe,Cu,Zr) permanent magnets due to their complex microstructure, which comprises SmCo5 cells with Cu-rich Sm2Co17 cell walls and a Zr-rich Z phase. These spin structures play a crucial role in the magnetization reversal by inducing spin curling and domain-wall nucleation where the Z phase intersects the cells and cell walls. The Z-phase thickness should therefore be minimized to increase the coercivity. It is further demonstrated by in situ heating experiments that the magnetization reversal in Sm(Co,Fe,Cu,Zr) magnets is dominated by domain-wall pinning at Sm2Co17 cell walls at low temperatures and domain-wall nucleation at high temperatures. The transition temperature between pinning and nucleation, as well as the coercivity, increase with increasing the slope of the Cu gradient in the cell walls. However, the coercivity also decreases with increasing the total Cu concentration in the cell walls. The Cu gradient should therefore be carefully designed to maximize the coercivity. These findings give clear guidelines on how to improve the performance of Sm(Co,Fe,Cu,Zr) magnets and reveal the complexity of magnetic structures and processes in permanent magnets. Magnetic imaging is generally performed in the highly specialized LTEM mode, where the objective lens is turned off to protect the sample from its magnetic field and replaced by a Lorentz lens below the sample. In this thesis, it is shown that such a modification is not required for permanent magnets because their magnetic state is not affected by the objective lens due to their high coercivity. Specifically, the domain-wall width parameter and the magnetic induction-thickness product are measured by conventional and Lorentz TEM with similar precision and accuracy. This demonstrates that magnetic imaging experiments on permanent magnets are accessible to a much wider scientific community.

We live in a society defined by constant growth. This is exemplified by information technologies which, over a time period of only a few decades, have transformed our way of life. This rapid expansion has relied heavily on there being ”plenty of room at the bottom”. However, as we are faced with the physical limitations of continuous miniaturization, there is a need for a new paradigm to support the growing computing power requirements of our society. The field of spintronics provides us with several opportunities to increase the functionality of electronic devices in a sustainable way. After the discovery of the giant magnetoresistance and the tunneling magnetoresistance, which have revolutionized data storage technologies, scientists have continued to unravel novel effects and propose new applications that exploit the interplay of charge and spin degrees of freedom in electronic devices. In the last decade, a growing number of ways to measure and manipulate the magnetic state, which is used to store data, have arisen. One common thread in modern spintronics is the importance of interfaces. In this thesis, we deal with a specific type of interface, that is both well-known, and often overlooked: ferromagnet/oxide interfaces. Specifically, we explore two main areas: firstly, magnetotransport and current-induced SOTs, and secondly, the tuning of magnetic anisotropies to induce chiral effects. To investigate the resulting physical phenomena, both parts of this work have involved the development of new fabrication techniques as well as data acquisition and analysis methodologies. To investigate the magnetotransport of ferromagnet/oxide interfaces, we study model systems consisting of Fe/oxide heterostructures epitaxially grown on MgO(001). Our investigation reveals that current-induced effects that are typical of heavy metal/ ferromagnet bilayers, such as spin-orbit torques and unidirectional magnetoresistance, also exist in Fe/oxide systems. In addition, the crystallinity of the Fe is imprinted onto these effects, resulting in a dependence on the orientation of the current relative to the crystal lattice. Our results show that charge-spin conversion effects do not require the presence of heavy metals as the source of spin currents. Moreover, they show that subtle differences in the morphology and chemical composition of the interfaces can produce sizable changes of the spin-orbit torques and unidirectional magnetoresistance. Besides playing a role in the magnetotransport properties, oxide interfaces can also influence the magnetic anisotropy of ferromagnets. In this thesis, we exploit the interface-mediated anisotropy in Pt/Co/AlOx to induce what we coined ”lateral chiral coupling”. This coupling requires nanoscale ontrol over the magnetic anisotropy as it makes use of the Dzyaloshinskii–Moriya interaction, which has a very short range. For this purpose, we combined state-of-the-art fabrication techniques to develop a new process, which opened the door to the fabrication of laterally coupled in-plane and out-of-plane magnetized regions of the same layer. The effect of the lateral chiral coupling is manifold, leading to the exchange bias of adjacent in-plane and out-of-plane magnetized regions and the antiferromagnetic coupling of out-of-plane nanomagnets mediated by an in-plane spacer region. Moreover, the lateral coupling enables field-free magnetization switching using spin-orbit torques, as well as domain nucleation sites with reconfigurable energy barriers. The results of this thesis highlight the potential of ferromagnet/oxide interfaces to tune the magnetic ground state of singlelayer ferromagnets as well as their magnetotransport properties and current-induced spin-orbit torques.

Local effects in electrical charge transport are not only of fundamental interest in solid state physics and material science, but are of significant interest also for application in novel electronic devices. The aim of this thesis was to build a scanning tunneling potentiometry (STP) setup, capable of measuring the local electrochemical poten- tial (ECP) distribution with high spatial resolution in current biased samples. More specifically this entails the study of the local transport properties of graphene. STP is based on scanning tunneling microscopy (STM) and therefore inherits the ability to acquire the sample topography with atomic resolution. Therefore, STP offers a unique method to directly relate electrical transport properties as given by the local ECP to the morphological features of the sample such as grain boundaries, defects, and changes in composition. In the first part of the thesis we perform a complete analysis of the performance and noise levels for STP including different measurement modes. Therefore, we can provide a thorough understanding of the method and illustrate a way forward to reach the ultimate noise level. We show that the dc method achieves a lower noise than the ac method, but is susceptible to flicker noise as well as offsets in the electronics. We implement experimentally both the dc and ac STP detection schemes and measure the local ECP distribution in epitaxial graphene. Due to the presence of flicker noise the ac and dc modes perform comparably well. We discuss the influence of thermal effects on the noise performance of STP. The second part of the thesis is focused on the local charge transport properties in graphene, in particular, on topographical features such as the interface between mono- and bilayer graphene, substrate steps and pits. We demonstrate that the insides of pits formed on the surface of epitaxial graphene on SiC are not covered in graphene, and thus present a discontinuity of graphene with the exact shape of the pit. The pits act as strong scattering regions for the electrons resulting in a dipolar potential drop in the ECP in their vicinity. The measured dipole magnitudes are much larger than what is expected for ohmic (diffusive) transport. We show that the dipole mag- nitudes correspond to the quasi-ballistic transport regime where the mean free path is comparable to the pit dimensions. The results presented in this thesis show that local variations in the ECP can be studied on the nm scale with STP. Moreover, the measured effects demonstrate that epitaxial graphene is a very interesting model system for studying quasi-ballistic transport and has great promise in extending the investigations into the near-ballistic or even ballistic regime through control of the growth process, gating or intercalation.

In my thesis, I have strengthened the understanding of the vibrational dynamics of materials in response to stimulation by intense laser pulses. Vibrations of the crystal structure can be used to manipulate a material's properties in the electronic ground state on timescales less than a picosecond by changing the average ionic lattice that the electrons experience. In order to leverage this technique, the quantized vibrations of the lattice, called optical phonons, have to be excited coherently by intense pulses in the terahertz and mid-infrared spectral region. Excitation mechanisms for optical phonons have been investigated theoretically on a phenomenological level for several decades, but due to the high computational costs, a quantitative description within the density functional theory framework has only become possible in recent years. I further developed and used this toolkit to explain experimental discoveries that were not previously understood and to predict new phenomena arising from coherent optical phonon excitation. Here, I present three major research projects that I have completed on this topic: In the first project, I proposed a novel nonlinear phonon coupling mechanism that enhances the controllability over structural distortions induced in the crystal lattice. In the second project, I created a general framework that captures the different stimulated Raman scattering mechanisms for the excitation of Raman-active phonons in insulators. In particular, I proposed a novel scattering mechanism that significantly increases the efficiency of the excitation over established methods. In the third project I showed that magnetism arises from temporally varying electric polarization. I have demonstrated this phenomenon with the example of circularly polarized phonons that produce a magnetic moment, which interacts with an external magnetic field or the magnetic order of the material itself. I conclude the thesis by giving a perspective for future research directions. I suggest that new discoveries in the field will be led by experiments carried out at free-electron laser facilities and by developments in the first-principles description of coupled electron- and spin-phonon dynamics. Understanding the time-dependent physics of phonons may in turn lead to new paradigms in designing materials with greater functionality in the static case.

Today's society relies on efficient ways to write and read digital information due to the ever increasing need of efficient data processing and storage. One possible approach to address this problem is to use magnetic media as non-volatile memory. Progress in thin-film preparation techniques during the last decades enabled the fabrication of nanometer-thick magnetic structures, in which interfacial effects dominate the electrical and magnetic properties. Such effects offer novel functionalities in electronic devices using the spin degree of freedom to carry and store information. This objective is at the core of the research field spintronics. One of the major challenges in spintronics is to find efficient ways of controlling the magnetization in heterostrucutures by purely electrical means. A promising approach is to use the current-induced spin-orbit torques (SOTs) in heavy metal/ferromagnet bilayers, which offer a variety of ways to manipulate the magnetization. Recent experiments demonstrated current-induced magnetization reversal down to the sub-nanosecond timescale, suggesting that SOTs in heavy metal/ferromagnet bilayers can outperform other methods for magnetization manipulation. In this thesis, we explore SOTs in two new classes of materials that are interesting from both a fundamental and an applied perspective. We focus on single layers of a non-centrosymmetric system where intrinsic SOTs may arise due to distinct properties linked to the crystal structure. Potentially new SOT symmetries may improve the performance of magnetization manipulation in spintronic devices. Combining this system with an additional Pt layer, we seek to optimize the SOT efficiency, since Pt is a material known to induce large conventional SOTs. We shall employ this aspect of Pt further in ultrathin Pt-based heterostructures with a magnetic insulator, our second material class. Magnetic insulators have great potential for additional functionalities based on their low magnetization damping. Therefore, we concentrate on the magnetic properties and the influence of interfacial SOTs on magnetic insulators in these heterostructures. First, we focus on the investigation of SOTs in inversion-asymmetric crystals of PtMnSb, a half-metallic Heusler compound. We characterize the structural and magnetic properties of PtMnSb with thickness ranging from 7.5 to 50~nm. X-ray diffraction and transmission electron microscopy provide evidence of crystalline quality and the saturation magnetization is close to the bulk value as determined with a superconducting quantum interference device. Using magnetotransport measurements at different applied fields, we characterize the magnetic anisotropy and reveal a dominating 4-fold in-plane symmetry. For the SOT investigation, we employ the harmonic Hall method, which allows us to identify four different SOTs in PtMnSb single layers with varying magnitudes up to 1 mT per 10^7 A/cm2 of injected current density. In addition to the intrinsic counterpart of the conventional field-like and damping-like SOTs generally found in heavy metal/ferromagnetic bilayers, we discover two more SOTs, which depend on higher order magnetization components. Using different crystal directions for current injection, we conclude that the SOT effective fields have Rashba symmetry. Our results suggest that shear strain strongly affects the magnitude of the SOTs, leading to a reduction of the effective fields towards the thicker films. By combining PtMnSb to bilayers with Pt in the thickness range from 2 to 9 nm, we are able to tune the SOT efficiency of the total damping-like SOT to a maximal value for 3-nm-thick Pt. We discuss the Pt thickness dependence of all SOT contributions, giving a comprehensive picture of the interplay of intrinsic SOTs in PtMnSb with the conventional SOTs due to Pt. The magnitudes of the total SOTs may be further enhanced by reducing the partially defective interface in our samples. Second, we concentrate on yttrium iron garnet (YIG) as a prominent representative of magnetic insulators. In particular, we report on the structure, magnetization, magnetic anisotropy, and domain morphology of ultrathin YIG/Pt films with thickness ranging from 3 to 90 nm. We find that the saturation magnetization is close to the bulk value in the thickest films and decreases towards low thickness with a strong reduction below 10 nm. We characterize the magnetic anisotropy by measuring the transverse spin Hall magnetoresistance as a function of applied field. Our results reveal strong easy plane anisotropy fields of the order of 50 to 100 mT, which add to the demagnetizing field, as well as weaker in-plane uniaxial anisotropy ranging from 10 to 100 microtesla. The in-plane easy axis direction changes with thickness, but presents also significant fluctuations among samples with the same thickness grown on the same substrate. X-ray photoelectron emission microscopy reveals the formation of zig-zag magnetic domains in YIG films thicker than 10 nm, which have dimensions larger than several 100 micrometer and are separated by Néel-type domain walls. Smaller domains characterized by interspersed elongated features are found in YIG films thinner than 10 nm. For the investigation of SOTs in YIG/Pt, we develop a technique with injected current densities in the range of 10^5~A/cm2, which is two orders of magnitude lower than conventional methods. We find effective fields in YIG/Pt that are dominated by the Oersted field with a small opposing contribution due to the field-like SOT in Pt. The overall magnitude of the current-induced field can be significantly enhanced by replacing Pt with W due to constructive overlay of the Oersted field and the larger field-like SOT in W compared to Pt. The effective field changes in an unusual manner as a function of YIG thickness in YIG/Pt bilayers, which we ascribe to pinning sites in thick YIG consistent with the previous magnetic domain analysis. Finally, we demonstrate prototype current-assisted in-plane magnetization switching for YIG/Pt. Our results are a reciprocal approach to most YIG/Pt experiments, which are based on spin-to-charge conversion using the spin-pumping technique.

The strong interplay between structural, electronic and chemical properties renders oxide materials very appealing in the hunt for novel technological designs. Outstanding examples include phenomena like colossal magnetoresistance and the appearance of a two-dimensional electron system (2DES) at the interface between two band insulators. Ferroic materials attract even more attention because they have a remanent state, i.e. a domain state, that is controllable with an external field. Different technological applications, for example the use in nonvolatile random-access memories for data storage, require specific domain states. As a result, the area of domain engineering in ferroic thin films is in continuous expansion and high structural quality is a persistent requirement. The advances in growth technology of ferroic oxide layers, approaching the same atomic precision attained in semiconductor heterostructures, are promoted by the employment of techniques that can monitor the structural development of the oxide while it is growing. The information about the structure, however, does not reveal the domain architecture. Nonlinear optical phenomena, and in particular second harmonic generation (SHG), possess the innate capability of detecting ordering phenomena, but their implementation to characterize single- and multi-layer ferroic systems is only at the early stages. In this work, SHG is utilized to discriminate the effects that strain, external fields and interfaces have on the film domain architecture at the nanoscale, beyond resolution limit. First, a correlation between the local distribution of the polarization, the symmetry of the compound and the nonlinear optical response is found. Then, the probing laser is coupled inside the growing chamber with the purpose of detecting in real time the emergence of the ferroelectric order. This paves the way for a complete comprehension of the mechanisms behind the development of the ferroic properties and might therefore assist the quest of integrating these complex materials into next-generation electronic devices.