Interfacial Effects and Inhomogeneous Magnetic States in Magnetic Heterostructures

Many interfacial effects occurring in magnetic heterostructures are related to emergence of inhomogeneous magnetic states. In these states, a magnetic order and a direction of magnetization change significantly on the scale of a few nanometers along the thickness of the heterostructures. This PhD dissertation covers interfacial effects and inhomogeneous magnetic states in three different types of magnetic heterostructures. First, we investigated an unusual exchange bias effect between pinned and unpinned uncompensated magnetization in antiferromagnetic FeMn, which is in proximity with diamagnetic Cu. Using a specially designed antiferromagnet-ferromagnet spin valves, it was shown that even a 110-kOe magnetic field cannot reverse the pinned uncompensated magnetization at the Cu/FeMn interface. The experimental results indicate that the pinning is induced by stress developed in the FeMn/Cu multilayers due to strong thermal contraction of Cu. Second, we studied magnetic behavior of heterostructures composed of two immensely different ferromagnetic material: rare-earth Gd and permalloy, which are coupled antiferromagnetically. Magnetic field causes an appearance of in-plane domain walls and states in which magnetization is not collinear to a magnetic field. Using a combination of magnetometry and magnetotransport measurements, it was investigated how these inhomogeneous magnetic states evolve at different magnetic fields and temperatures. Fitting experimental dependences to the proposed micromagnetic model allowed obtaining microscopic parameters of the Py/Gd heterostructures. It was also discovered that microdisks composed of the Py/Gd multilayers demonstrate unusual thermal behavior: the microdot magnetization can be transformed from a single domain state to a vortex state in a constant magnetic field by changing the temperature. Moreover, for some temperature ranges, the change in temperature is the only possibility to nucleate the vortex in the disks. Third, we considered special superconductor/ferromagnet heterostructures, for which, the inhomogeneous magnetic states are constructed artificially to generate odd-triplet superconductivity. It was shown that the suppression of the critical temperature observed for the inhomogeneous magnetic states, which was reported previously by other groups, was not due to the odd-triplet superconductivity. Additionally, kinetic inductance measurements revealed that a slight change in the magnetic field affects the superconducting condensate much stronger than possible generation of the odd-triplet pairing

Voltage-controlled magnetism enabled by resistive switching

The discovery of new mechanisms of controlling magnetic properties by electric fields or currents furthers the fundamental understanding of magnetism and has important implications for practical use. Here, we present a novel approach of utilizing resistive switching to control magnetic anisotropy. We study a ferromagnetic oxide that exhibits an electrically triggered metal-to-insulator phase transition producing a volatile resistive switching. This switching occurs in a characteristic spatial pattern: the formation of a transverse insulating barrier inside a metallic matrix resulting in an unusual ferromagnetic/paramagnetic/ferromagnetic configuration. We found that the formation of this voltage-driven paramagnetic insulating barrier is accompanied by the emergence of a strong uniaxial magnetic anisotropy that overpowers the intrinsic material anisotropy. Our results demonstrate that resistive switching is an effective tool for manipulating magnetic properties. Because resistive switching can be induced in a very broad range of materials, our findings could enable a new class of voltage-controlled magnetism systems

Coupled Macrospins: Mode Dynamics in Symmetric and Asymmetric Vertices

We report the microwave response of symmetric and asymmetric threefold clusters with nearly contacting segments that can serve as the node in a Kagome artificial spin ice lattice. The structures are patterned on a coplanar waveguide and consist of elongated and nearly-contacting ellipses with uniform thickness. Branches of the ferromagnetic resonance spectra display mode softening that correlates well with the calculations, whereas agreement between the measured and simulated static magnetization is more qualitative

Spin Vortex Resonance in Non-planar Ferromagnetic Dots

In planar structures, the vortex resonance frequency changes little as a function of an in-plane magnetic field as long as the vortex state persists. Altering the topography of the element leads to a vastly different dynamic response that arises due to the local vortex core confinement effect. In this work, we studied the magnetic excitations in non-planar ferromagnetic dots using a broadband microwave spectroscopy technique. Two distinct resonance frequency ranges were observed depending on the position of the vortex core controllable by applying a relatively small magnetic field. The micromagnetic simulations are in qualitative agreement with the. experimental results

Topological spin memory of antiferromagnetically coupled skyrmion pairs in Co/Gd/Pt multilayers

Antiferromagnetically (AFM) coupled skyrmions offer potential advantages for spintronic devices, including reduced dipolar fields that may enable smaller skyrmion sizes and a reduction of the skyrmion Hall effect. However, the topological stability of AFM-coupled skyrmions subjected to dramatic spin deformation through low-temperature cycling has not been investigated. Here we report the discovery of a topological spin memory effect for AFM-coupled skyrmion pairs in [Co/Gd/Pt]10 multilayered films. Photoemission electron microscopy imaging shows that bubble skyrmions in the multilayer that are stable at room temperature evolve into complex in-plane spin textures as the temperature is lowered and reform completely when the sample is warmed back up. Simulations demonstrate that Dzyaloshinskii-Moriya interactions play a key role in this spin memory effect, and furthermore reveal that the topological charge is preserved throughout the dramatic spin texture rearrangement and recovery. These results highlight a key aspect of topological protection—the preservation of the topological properties under continuous deformation—and also provide a promising avenue for information encryption and recovery

Detection of uncompensated magnetization at the interface of an epitaxial antiferromagnetic insulator

We have probed directly the temperature and magnetic field dependence of pinned uncompensated magnetization at the interface of antiferromagnetic FeF2 with Cu, using FeF2-Cu-Co spin valves. Electrons polarized by the Co layer are scattered by the pinned uncompensated moments at the FeF2-Cu interface giving rise to giant magnetoresistance. We determined the direction and magnitude of the pinned uncompensated magnetization at different magnetic fields and temperatures using the angular dependencies of resistance. The strong FeF2 anisotropy pins the uncompensated magnetization along the easy axis independent of the cooling field orientation. Most interestingly, magnetic fields as high as 90 kOe cannot break the pinning at the FeF2-Cu interface. This proves that the pinned interfacial magnetization is strongly coupled to the antiferromagnetic order inside the bulk FeF2 layer. Studies as a function of FeF2 crystalline orientation show that uncompensated spins are only detected in a spin valve with (110) crystal orientation, but not in valves containing FeF2(100) and FeF2(001). This observation is in agreement with symmetry-related considerations which predict the equilibrium boundary magnetization for the FeF2(110) layer

Acoustoelectric drag current in vanadium oxide films

Two different Mott insulator wires, vanadium dioxide and vanadium sesquioxide, were prepared on the piezoelectric LiNbO3 substrates. Coupling of acoustic waves propagating in LiNbO3 with free carriers in vanadium oxide gives rise to the acoustoelectric effect that manifests itself as the generation of direct electric current by the acoustic wave. According to a phenomenological model, the value of the effect strongly depends on the wires conductivity, which, for the vanadium-oxide films, changes by a few orders of magnitude. We demonstrated that this yields a significant enhancement of the direct current (DC) current generated in the wires at the metal-insulator transition temperatures. The sign of the generated DC voltage is different for excitations by surface and bulk acoustic wave modes, which may happen due to reverse wave propagation at the substrate surface. For each resonance mode, polarities of the generated DC signal are the same in both wires, despite the signs of charge carriers being different for these materials. It was shown that two complementary techniques (acoustoelectric and Hall effect measurements) yield opposite signs of charge carriers in VO2.United States Department of Energy (DOE) DE FG02 87ER-45332 FA9550-16-1-0122 FA9550-18-1-0438 Comision Nacional de Investigacion Cientifica y Tecnologica (CONICYT) CONICYT FONDECYT 1160639 CEDENNA through the Financiamiento Basal para Centros Cienticos y Tecnologicos de Excelencia-FB080

Coupled macrospins: Mode dynamics in symmetric and asymmetric vertices

We report the microwave response of symmetric and asymmetric threefold clusters with nearly contacting segments that can serve as the node in a Kagome artificial spin ice lattice. The structures are patterned on a coplanar waveguide and consist of elongated and nearly-contacting ellipses with uniform thickness. Branches of the ferromagnetic resonance spectra display mode softening that correlates well with the calculations, whereas agreement between the measured and simulated static magnetization is more qualitative

Aphasia associated with lacunar infarctions

Hintergrund: Aphasien gehören nicht zu den typischen klinischen Manifestationen lakunärer Hirninfarkte, sind jedoch im Rahmen seltener atypischer lakunärer Syndrome beschrieben. Ziel der Arbeit: Beschreibung von Aphasiemustern und betroffener Fasertrakte bei lakunären Infarkten. Material und Methoden: Fallserie von drei Patienten mit in der Magnetresonanztomographie nachgewiesenen lakunären Hirninfarkten und Aphasie. Identifikation betroffener Faserbahnen mittels Fasertraktographie der koregistrierten Schädigungsorte in Gehirnen zweier gesunder Probanden. Ergebnisse: Radiologisch waren die Lakunen, die Aphasien hervorriefen, weit lateral im Marklager der linken Hemisphäre gelegen und befanden sich im Vergleich zu der Lakune eines nichtaphasischen Kontrollpatienten weiter rostrodorsal. Klinisch fand sich trotz Aussparung des Kortex, Thalamus und weiter Teile der Basalganglien eine leichte bis moderate nichtflüssige Aphasie mit syntaktischen Defiziten. In der Fasertraktographie zeigten die aphasischen im Vergleich zum nichtaphasischen Patienten eine stärkere Affektion der Fasern des linken Fasciculus arcuatus sowie eine Beteiligung des frontostriatalen und frontalen Aslant-Trakts. Diskussion: Links lateral gelegene lakunäre Infarkte können durch Beteiligung sprachrelevanter Fasertrakte eine klinisch relevante Aphasie hervorrufen.Background: Typical lacunar syndromes do not include aphasia but aphasia has been reported in rare atypical lacunar syndromes. Objective; Description of the phenomenology and of affected fiber tracts. Material and methods: Case series of three patients with lacunar stroke as evidenced by magnetic resonance imaging. Identification of affected fiber tracts via fiber tracking from coregistered lesion sites in brains of two healthy participants. Results: The lacunar strokes that produced aphasia were located in the very lateral territory of perforating branches of the middle cerebral artery and extended along the external capsule into its most rostrodorsal aspect. Even though the cortex, thalamus and most parts of the basal ganglia were unaffected, patients exhibited a mild to moderate nonfluent aphasia with syntactic deficits. Fiber tracking revealed that in contrast to the nonaphasic control patient with a neighboring lacunar stroke, the aphasic patient strokes involved particularly fibers of the left arcuate fascicle as well as fibers of the frontostriatal and frontal aslant tracts. Conclusion: Left lateral lacunar stroke can cause clinically relevant aphasia through disruption of speech-relevant fiber tracts