Magneto-optical properties of textured La2/3_{2/3}Sr1/3_{1/3}MnO3_3 thin films integrated on silicon via a Ca2_2Nb3_3O10_{10} nanosheet layer

We demonstrate the possibility of growing textured La$_{2/3}$Sr$_{1/3}$MnO$_3$ (LSMO) thin films on silicon substrates with magneto-optical and optical properties comparable to high-quality epitaxial layers grown on bulk SrTiO$_3$ (STO). The pulsed laser deposition growth of LSMO is achieved by a two-dimensional nanosheet (NS) seed layer of Ca$_2$Nb$_3$O$_{10}$ (CNO) inducing epitaxial stabilization of LSMO films. The resulting layers possess a higher Curie temperature and a lower overall magnetization than samples of LSMO on STO. Spectra of the full permittivity tensor were calculated from optical and magneto-optical measurements. Spectral dependencies of both the diagonal and off-diagonal elements share many similarities between the LSMO/NS/Si and LSMO/STO samples. These similarities indicate comparable electronic structures of the layers and demonstrate comparable optical quality of textured LSMO on NS/Si and epitaxial LSMO on STO

Effects of Oxygen Modification on the Structural and Magnetic Properties of Highly Epitaxial La0.7Sr0.3MnO3 (LSMO) thin films

La0.7Sr0.3MnO3, a strong semi-metallic ferromagnet having robust spin polarization and magnetic transition temperature (TC) well above 300 K, has attracted significant attention as a possible candidate for a wide range of memory, spintronic, and multifunctional devices. Since varying the oxygen partial pressure during growth is likely to change the structural and other physical functionalities of La0.7Sr0.3MnO3 (LSMO) films, here we report detailed investigations on structure, along with magnetic behavior of LSMO films with same thickness (~30 nm) but synthesized at various oxygen partial pressures: 10, 30, 50, 100, 150, 200 and 250 mTorr. The observation of only (00 l) reflections without any secondary peaks in the XRD patterns confirms the high-quality synthesis of the above-mentioned films. Surface morphology of the films reveals that these films are very smooth with low roughness, the thin films synthesized at 150 mTorr having the lowest average roughness. The increasing of magnetic TC and sharpness of the magnetic phase transitions with increasing oxygen growth pressure suggests that by decreasing the oxygen growth pressure leads to oxygen deficiencies in grown films which induce oxygen inhomogeneity. Thin films grown at 150 mTorr exhibits the highest magnetization with TC = 340 K as these thin films possess the lowest roughness and might exhibit lowest oxygen vacancies and defects. Interpretation and significance of these results in the 30 nm LSMO thin films prepared at different oxygen growth pressures are also presented, along with the existence and growth pressure dependence of negative remanent magnetization (NRM) of the above-mentioned thin films

Emergent electric field control of phase transformation in oxide superlattices.

Electric fields can transform materials with respect to their structure and properties, enabling various applications ranging from batteries to spintronics. Recently electrolytic gating, which can generate large electric fields and voltage-driven ion transfer, has been identified as a powerful means to achieve electric-field-controlled phase transformations. The class of transition metal oxides provide many potential candidates that present a strong response under electrolytic gating. However, very few show a reversible structural transformation at room-temperature. Here, we report the realization of a digitally synthesized transition metal oxide that shows a reversible, electric-field-controlled transformation between distinct crystalline phases at room-temperature. In superlattices comprised of alternating one-unit-cell of SrIrO3 and La0.2Sr0.8MnO3, we find a reversible phase transformation with a 7% lattice change and dramatic modulation in chemical, electronic, magnetic and optical properties, mediated by the reversible transfer of oxygen and hydrogen ions. Strikingly, this phase transformation is absent in the constituent oxides, solid solutions and larger period superlattices. Our findings open up this class of materials for voltage-controlled functionality

Polar domain walls trigger magnetoelectric coupling

Interface physics in oxide heterostructures is pivotal in material's science. Domain walls (DWs) in ferroic systems are examples of naturally occurring interfaces, where order parameter of neighboring domains is modified and emerging properties may develop. Here we show that electric tuning of ferroelastic domain walls in SrTiO3 leads to dramatic changes of the magnetic domain structure of a neighboring magnetic layer (La1/2Sr1/2MnO3) epitaxially clamped on a SrTiO3 substrate. We show that by exploiting the resposiveness of DWs nanoregions to external stimuli, even in absence of any domain contribution, prominent and adjustable macroscopic reactions of neighboring layers can be obtained. We conclude that polar DWs, known to exist in other materials, can be used to trigger tunable responses and may lead to new ways for manipulation of interfacial emerging properties

Tunable magnetic anisotropy and magnetotransport properties of epitaxial oxide ferromagnetic heterostructures

This dissertation discusses the magnetic and magnetotransport properties of different perovskite oxide thin films. The focus lies on the ferromagnets strontium ruthenate and ruthenium-substituted lanthanum strontium manganite. Both materials are promising candidates in the view of the creation of topologically non-trivial structures, such as magnetic skyrmions. This originates from the possibility to modify the magnetic properties, such as the magnetic anisotropy, for instance by layer thickness variations and interfacial engineering. After the first observation of hump-like features that resemble a topological Hall effect in SrRuO3-SrIrO3 bilayers, several studies aimed to unravel the origin of these anomalies. If skyrmions indeed form in such SrRuO3-SrIrO3 heterostructures, the magnetic coupling between the magnetic layers will be of particular relevance, since the coupling of the skyrmions across the multilayer stack would be desirable. This question was addressed in the framework of the thesis by the artificial design and investigation of SrRuO3-SrIrO3 heterostructures. For 2 MLs thick insulating spacer, only very weak coupling between the individual magnetic SrRuO3 layers was observed, whereas no coupling was observed for thicker SrIrO3 spacers. Such magnetic decoupling of the SrRuO3 is undesirable in the view of the coupling of skyrmions across the multilayer stack. Thus, alternative perovskite oxides should be considered as spacer materials in order to achieve ferromagnetic coupling of the SrRuO3 layers. Since enhanced ferromagnetic coupling was observed when the SrRuO3 layers were separated by metallic LaNiO3 spacers, a similar heterostructure design with strong spin-orbit coupled, but metallic spacers might be of future research interest. Due to the experimental challenges in the imaging of nanosized skyrmions, Hall effect measurements are frequently used to detect fingerprints of magnetic skyrmions. When conduction electrons get scattered by skyrmions, the topological Hall effect (THE) can be detected with technically simple experimental set-ups. However, it is problematic that also other phenomena, such as multiple, parallel (anomalous) Hall channels, can generate features that resemble a topological Hall effect. This issue was emphasized by different examples within this thesis. The magnetic force microscopy (MFM) study of an ultrathin SrRuO3-SrIrO3 bilayer, capped by SrZrO3, showed that peak-like features can be observed in Hall measurements without the existence of skyrmions. The MFM investigations revealed variations of the local layer thickness and corresponding differences of the switching fields in a bare SrRuO3 thin film. These thickness variations were also seen in the trilayer and expected to lead to band structure variations of the anomalous Hall constant. Within the model of multiple anomalous Hall channels, these local variations of switching field and AHE constant can explain the THE-like features. It was demonstrated in a second study that hump-like anomalies can be generated in the Hall loops in SrRuO3-based heterostructures, when the individual SrRuO3 layers possess distinct switching fields and anomalous Hall constants. For this purpose, heterostructures with two SrRuO3 layers of different thickness and therefore with different anomalous Hall constants and coercive fields were investigated. Here, the total Hall voltage can be written as the sum of the Hall voltages of the individual layers. This emphasizes that the conclusion about the presence of skyrmions based on transport measurements only, can be faulty. This further highlights the importance of techniques that are capable to proof the existence of skyrmions, such as real space imaging. In the second part of this dissertation, ruthenium-substituted lanthanum strontium manganite (La0.67Sr0.33Mn0.95Ru0.05O3) films, grown under moderate compressive strain on LSAT(100) substrates, were investigated. A nonmonotonic dependence of the magnetic anisotropy on the layer thickness was observed and attributed to the distinct temperature dependencies of the individual contributions of the magnetic anisotropy. Finally, strong in-plane anisotropic magnetoresistance was seen in a 42.5 nm La0.67Sr0.33Mn0.95Ru0.05O3 thin film deposited on LSAT(100). This anisotropic magnetoresistance, with mirrorlike features for the two orthogonal current directions, could be related with the magnetic anisotropy. The macroscopic magnetic behavior is in good agreement with the formation of parallel magnetic stripe domains, which were observed in a magnetic force microscopy study. The preferential alignment of the magnetic stripe domains furthermore explained the current-direction dependent contribution of magnetic domain wall resistance to the overall magnetoresistance. However, the understanding of the connection between the size and orientation of the magnetic stripe domains and the structural domains still needs to be improved. In contrast to the investigated SrRuO3 films which typically show strong perpendicular magnetic anisotropy, when it is interfaced with SrIrO3 or SrZrO3, the La0.67Sr0.33Mn0.95Ru0.05O3 films under study possessed a weak magnetic anisotropy with preferential magnetization alignment perpendicular to the thin film surface in a thickness-dependent temperature range. This is promising in the view of the tailoring of the magnetic state by small distortions of the (magnetic) energetic balance by modifications of the heterostructure design. The interfacing of Ru-substituted LSMO layers with a strong spin-orbit coupling material, such as SrIrO3, is therefore a possibility that might stabilize magnetically non-trivial structures and is considered as promising research topic in the future

Optical Characterization of Interface Magnetization in Multifunctional Oxide Heterostructures

Multifunctional oxides attract much attention recently. The strong correlated electron system involves the notable properties of colossal magnetoresistance, ferroelectric tunneling and spin transport, with the coupling of electron, spin and orbital degrees of freedom. their rich functional behavior is of potential use for nanoelectronics and data storage. Particularly interesting are the mulitferroic materials, which exhibit simultaneously electric and magnetic ordering properties. Understanding the interface coupling mechanism of these two order parameters are critical to future development of high-performance spintronic devices. The goal of this dissertation is to elucidate the interfacial magnetoelectric (ME) coupling with optical characterization method -- magnetization-induced second-harmonic generation (MSHG), which is sensitive to the interface due to the broken spatial inversion symmetry. First, ME coupling at the interface of BaTiO3 (BTO)/La0.67Sr0.33MnO3 (LSMO) is observed by applying an external electric field. The voltage-dependent magnetic contrast reveals a sharp transition from ferromagnetic (FM) to antiferromagnetic (AFM) order occurring at positive voltage (applied to LSMO contact). This novel effect is attributed to interface ME coupling. Strain or ferroelectric (FE) polarization induced mechanisms do not play an important role in this system. A new mechanism is proposed -- minority spin injection -- to modulate the interface magnetization. The minority spin injection at the interface weakens the double-exchange coupling of nearby eg electrons, thereby weakening the FM ordering. Thus the dominant AFM superexchange coupling of localized t2g electrons causes the phase transition at positive voltage. The magnetic transition is shifted to higher voltage by reducing the carrier concentration of BTO. Second, a non-multiferroic heterostructure -- SrTiO3 (STO)/La0.5Ca0.5MnO3 (LCMO)/La0.67Sr0.33MnO3 (LSMO) -- is studied to elucidate further the interface ME effect. The magnetization transition is observed but shifted to negative voltage. The LSMO is pushed to higher hole doping level due to the STO layer which acts as a hole donating layer, while the LCMO interlayer at the medium doping level displays complicated CE-type AFM phase. Thus, a negative voltage is required to lower the hole doping level of LSMO to induce the FM phase. The magnetic contrast reappears at high positive voltage, indicating the occurrence of an A-type AFM phase, which is stable at high hole doping concentration. The results of this dissertation show that the interface magnetic phase of LSMO can be controlled by an applied electric field through modulation of the hole doping level

Field Effect and Magnetically Induced Capacitive Tuning in Hole Doped La1-xSRXMnO3

Electrostatic modulation of interface conduction between semiconductors and insulating oxides is the foundation of semiconductor technology. This field effect concept can be applied on complex oxides, such as high temperature superconductors and colossal magnetoresistive manganites, in order to create new electronic and magnetic phases. Competition and coexistence of multiple nanoscale phases make them exciting to study around phase transitions. This study on hole doped La1-xSrxMnO3 systems has a two-fold purpose. One is the demonstration of the field effect on La1-xSrxMnO3 (x = 0.125, 0.2, 0.3, 0.5) thin films. It is an important step towards electrostatic control of material properties; however, a challenging task because of their charge carrier densities of 0.01-1 hole/unit cell, a few orders of magnitude larger than in doped semiconductors. Control by linear dielectrics needs huge, constantly applied bias. Energy efficient tuning with low voltages requires highly polar ferroelectric. Pb(Zr0.2Ti0.8)O3 was chosen, whose remanence provides 0.5 charge carrier/unit cell on the manganite/ferroelectric interface. La1-xSrxMnO3/Pb(Zr0.2Ti0.8)O3 heterostructures were synthesized by pulsed laser epitaxy and remarkable conduction modifications were observed in the La1-xSrxMnO3. This can be a strong foundation of a new tool to research electronic oxides. The second purpose of this work is to utilize the phase separation in manganites. There has been extensive research on multiferroic materials, in which dielectric and magnetic responses are controlled by magnetic and electric field, respectively. In order to demonstrate magnetically tuned capacitance, insulating La7/8Sr1/8MnO3 was studied. Drastic capacitance change in magnetic field was shown through a phase transitions and explained in the framework of electronic phase separation. It makes this material eligible for high frequency magnetoelectric applications. Modulating charge carriers, mobility and magnetism in magnetic oxides, superconductors and superlattices has a great impact on the emerging field of oxide electronics. These compounds overcome the scaling limitations of conventional semiconductors; using low operation voltage oxide ferroelectrics lowers energy consumption. This thesis shows that changing fundamental physical properties of complex oxides on the atomic scale is possible by ferroelectric field effect. This technique is proposed as a tool to study thin films, artificially stacked structures and to induce and optimize novel phases and phenomena

Creating Ferromagnetic Insulating La0.9Ba0.1MnO3 Thin Films by Tuning Lateral Coherence Length

In this work, heteroepitaxial vertically aligned nanocomposite (VAN) La0.9Ba0.1MnO3 (LBMO)-CeO2 films are engineered to produce ferromagnetic insulating (FMI) films. From combined X-ray photoelectron spectroscopy, X-ray diffraction and electron microscopy, the elimination of the insulator-metal (I-M) transition is shown to result from the creation of very small lateral coherence lengths (with the corresponding lateral size ~ 3 nm (~ 7 u.c.)) in the LBMO matrix, achieved by engineering a high density of CeO2 nanocolumns in the matrix. The small lateral coherence length leads to a shift in the valence band maximum and reduction of the double exchange (DE) coupling. There is no "dead layer" effect at the smallest lateral coherence length achieved of ~ 3 nm. The FMI behaviour obtained by lateral dimensional tuning is independent of substrate interactions, thus intrinsic to the film itself and hence not related to film thickness. The unique properties of VAN films give the possibility for multilayer spintronic devices that can be made without interface degradation effects between the layers.Royal Academy of Engineering - CIET1819_24, the Leverhulme Trust grant RPG-2015-017, EPSRC grants EP/N004272/1, EP/T012218/1, and EP/M000524/1, the Isaac Newton Trust in Cambridge (minute 16.24(p) and RG96474). EU grant H2020-MSCA-IF-2016 (745886)-MuStMAM

Tailoring epitaxial growth and magnetism in La1-xSrxMnO3 / SrTiO3 heterostructures via temperature-driven defect engineering

Among the class of strongly-correlated oxides, La1-xSrxMnO3 $-$ a half metallic ferromagnet with a Curie temperature above room temperature $-$ has sparked a huge interest as a functional building block for memory storage and spintronic applications. In this respect, defect engineering has been in the focus of a long-standing quest for fabricating LSMO thin films with highest quality in terms of both structural and magnetic properties. Here, we discuss the correlation between structural defects, such as oxygen vacancies and impurity islands, and magnetism in La0.74Sr0.26MnO3/SrTiO3 (LSMO/STO) epitaxial heterostructures by systematic control of the growth temperature and post-deposition annealing conditions. Upon increasing the growth temperature within the 500 $-$ 700 $^{\circ}$C range, the epitaxial LSMO films experience a progressive improvement in oxygen stoichiometry, leading to enhanced magnetic characteristics. Concurrently, however, the use of a high growth temperature triggers the diffusion of impurities from the bulk of STO, which cause the creation of off-stoichiometric, dendritic-like SrMoOx islands at the film/substrate interface. As a valuable workaround, post-deposition annealing of the LSMO films grown at a relatively-low temperature of about 500 $^{\circ}$C permits to obtain high-quality epitaxy, atomically-flat surface as well as a sharp magnetic transition above room temperature and robust ferromagnetism. Furthermore, under such optimized fabrication conditions possible scenarios for the formation of the magnetic dead layer as a function of LSMO film thickness are discussed. Our findings offer effective routes to finely tailor the complex interplay between structural and magnetic properties of LSMO thin films via temperature-controlled defect engineering