Spin-orbit effects in asymmetrically sandwiched ferromagnetic thin films

Asymmetrically sandwiched ferromagnetic thin films display a large number of spin-orbit effects, including the Dzyaloschinsii-Moriya interaction (DMI), spin-orbit torques (SOT) and magnetoresistance (MR) effects. Their concurrence promises the implementation of interesting magnetic structures like skyrmions in future memory and logic devices. The complex interplay of various effects originating from the spin-orbit coupling and their dependencies on the microstructural details of the material system mandates a holistic characterization of its properties. In this PhD thesis, a comprehensive study of the spin-orbit effects in a chromium oxide/cobalt/platinum trilayer sample series is presented. The determination of the complete micromagnetic parameter set is based on a developed measurement routine that utilizes quasistatic methods. The unambiguous quantification of all relevant constants is crucial for the modeling of the details of magnetic structures in the system. In this context the necessity of a strict distinction of magnetic objects, that are stabilized by magnetostatics or the DMI, was revealed. Furthermore, a sample layout was developed to allow for the simultaneous quantification of the magnitudes of SOTs and MR effects from nonlinear magnetotransport measurements. In conjunction with a structural characterization, the dominating dependence of the effect magnitudes on microstructural details of the systems is concluded. Precisely characterized systems establish a solid groundwork for further investigations that are needed for viable skyrmion-based devices.:1 Introduction 2 Fundamentals 2.1 Towards new devices 2.2 Spin-orbit effects 2.2.1 Spin-current sources 2.2.2 Magnetoresistanceeffects 2.2.3 Spin-orbit torques 2.2.4 Harmonic analysis 2.3 Micromagnetic model 2.3.1 Dzyaloshinskii-Moriya interaction (DMI) 2.3.2 Consequences of the DMI for magnetic structures 2.3.3 Interface-induced DMI in asymmetrically stacked ferromagnets 2.3.4 Quantification of the interface-induced DMI 2.3.5 Levy-Fert three-site model including roughness 3 The CrOx/Co/Pt sample system 3.1 Experimental techniques 3.2 Structural characterization 4 Complete micromagnetic characterization 4.1 Magnetometry 4.1.1 Static investigation 4.1.2 Ferromagnetic resonance 4.2 DMI quantification 4.2.1 Field-driven domain wall creep motion 4.2.2 Asymmetric domain growth 4.2.3 Winding pair stability 4.3 Determination of the exchange parameter 4.3.1 Generation of circular magnetic objects 4.3.2 Homochiral magnetic bubble domains 4.4 Results 5 Magnetotransport measurements 5.1 Measurement setup 5.2 Magnetoresistance effects 5.3 Spin-orbit torque quantification 5.4 Results 6 Discussion 6.1 Structural predomination of the DMI strength 6.2 Ultra-thin limit exchange parameter reduction 6.3 Magnetotransport properties 6.4 Magneticstructures in //CrOx/Co/Pttrilayers 7 Conclusion and Outlook A Appendix A.1 Calculation of the skyrmion diameter A.2 Micromagnetic simulation of the winding pair stability Bibliography Acknowledgement

The Angola Current in a Tropical Seasonal Upwelling System: Seasonal Variability in Response to Remote Equatorial and Local Forcing

In this thesis, the flow and hydrographic characteristics of the boundary circulation off Angola are investigated, with particular emphasis on the mean properties and the seasonal cycle of the Angola Current. Moored velocity observations acquired at the Angolan shelf at 11°S between 2013 and 2016 reveal a highly variable alongshore flow with velocities in the range of ±40 cm/s superimposed on a weak poleward mean current with core velocities not exceeding 8 cm/s. These measurements question the former view of a permanently poleward flowing Angola Current, which was based on results from few ship campaigns. During the observational period a mean Angola Current southward transport of 0.32±0.05 Sv is determined. Pronounced annual and semiannual oscillations are found in the alongshore current characterized by distinct baroclinic structures. In the equatorial Atlantic, annual and semiannual oscillations are associated with resonant equatorial basin-modes of the fourth baroclinic mode for the annual cycle and the second baroclinic mode for the semiannual cycle. Equatorial basin-modes represent standing modes in a zonally bounded basin being composed of equatorial Kelvin and Rossby waves. A series of shallow water model experiments differing in the basin geometry and/or the applied forcing show a structural robustness of the corresponding horizontal patterns associated with the annual and semiannual cycles. The off-equatorial lobes of the basin-modes also impact alongshore velocity at the eastern boundary off Angola. In a suite of shallow water model simulations the remote equatorial forcing related to basin-modes is isolated from the effects of local forcing. The observed annual cycle of alongshore velocity at 11°S is well reproduced by the remote equatorial forcing. For the semiannual cycle, the inclusion of local forcing improves the agreement between observed and simulated velocity oscillations at 11°S

Der Angolastrom in einem tropischen, saisonalen Auftriebsgebiet: Saisonale Variabilität hervorgerufen durch äquatorialen und lokalen Antrieb

In this thesis, the flow and hydrographic characteristics of the boundary circulation off Angola are investigated, with particular emphasis on the mean properties and the seasonal cycle of the Angola Current. Moored velocity observations acquired at the Angolan shelf at 11°S between 2013 and 2016 reveal a highly variable alongshore flow with velocities in the range of ±40 cm/s superimposed on a weak poleward mean current with core velocities not exceeding 8 cm/s. These measurements question the former view of a permanently poleward flowing Angola Current, which was based on results from few ship campaigns. During the observational period a mean Angola Current southward transport of 0.32±0.05 Sv is determined. Pronounced annual and semiannual oscillations are found in the alongshore current characterized by distinct baroclinic structures. In the equatorial Atlantic, annual and semiannual oscillations are associated with resonant equatorial basin-modes of the fourth baroclinic mode for the annual cycle and the second baroclinic mode for the semiannual cycle. Equatorial basin-modes represent standing modes in a zonally bounded basin being composed of equatorial Kelvin and Rossby waves. A series of shallow water model experiments differing in the basin geometry and/or the applied forcing show a structural robustness of the corresponding horizontal patterns associated with the annual and semiannual cycles. The off-equatorial lobes of the basin-modes also impact alongshore velocity at the eastern boundary off Angola. In a suite of shallow water model simulations the remote equatorial forcing related to basin-modes is isolated from the effects of local forcing. The observed annual cycle of alongshore velocity at 11°S is well reproduced by the remote equatorial forcing. For the semiannual cycle, the inclusion of local forcing improves the agreement between observed and simulated velocity oscillations at 11°S

The Angola Current: Flow and hydrographic characteristics as observed at 11?S

publishedVersio

Simulation des thermischen Verhaltens asynchroner Traktionsmotoren im Lastzyklus

In diesem Beitrag wird das transiente thermische Verhalten eines asynchronen Traktionsmotors mittels eines Wärmequellennetzwerkes beschrieben. Anhand dieses Ansatzes gelingt es, Erwärmungsläufe für Nennbetriebsarten sowie für Lastzyklen unter Berücksichtigung von Umrichterverlusten und temperaturabhängigen Stromwärmeverlusten innerhalb des Entwurfsprozesses zu ermitteln.The transient thermal behavior of an asynchronous traction motor is described through a thermal equivalent circuit in this paper. With this method it is possible to calculate the temperature rise for several duty types or load cycles within the design process under consideration of the converter losses and the temperature depending resistive losses

Purely Antiferromagnetic Magnetoelectric Random Access Memory

Magnetic random access memory schemes employing magnetoelectric coupling to write binary information promise outstanding energy efficiency. We propose and demonstrate a purely antiferromagnetic magnetoelectric random access memory (AF-MERAM) that offers a remarkable 50 fold reduction of the writing threshold compared to ferromagnet-based counterparts, is robust against magnetic disturbances and exhibits no ferromagnetic hysteresis losses. Using the magnetoelectric antiferromagnet Cr2O3, we demonstrate reliable isothermal switching via gate voltage pulses and all-electric readout at room temperature. As no ferromagnetic component is present in the system, the writing magnetic field does not need to be pulsed for readout, allowing permanent magnets to be used. Based on our prototypes of these novel systems, we construct a comprehensive model of the magnetoelectric selection mechanism in thin films of magnetoelectric antiferromagnets. We identify that growth induced effects lead to emergent ferrimagnetism, which is detrimental to the robustness of the storage. After pinpointing lattice misfit as the likely origin, we provide routes to enhance or mitigate this emergent ferrimagnetism as desired. Beyond memory applications, the AF-MERAM concept introduces a general all-electric interface for antiferromagnets and should find wide applicability in purely antiferromagnetic spintronics devices.Comment: Main text (4 figures) + supplementary information (7 figures