Heyne, Lutz
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Analytical description for current-induced vortex core displacement
In this article, an analytical model for current-induced vortex core displacement is developed. By using this model, one can solve the equations of motion analytically to determine the effects of the adiabatic and nonadiabatic spin-torque terms. The final displacement direction of the vortex core due to the two torque terms mirrors their relative strengths. The resulting vortex core displacement direction combined with the amplitude of the displacement is thus a measure for both torque terms.
Manipulation of Magnetic Domain Walls and Vortices by Current Injection
Die vorherrschenden Magnetisierungskonfigurationen in Mikrometer kleinen weichmagnetischen Drähten und Scheiben sind Domänenwände und Vortices. Diese Arbeit beschäftigt sich mit der Manipulation solcher Konfigurationen mithilfe spinpolarisierter Ströme. Für die Untersuchung der lithografisch hergestellten Strukturen werden Röntgen-Photoemissions-Elektronen-Mikroskope verwendet. Unter Ausnutzung des zirkularen Dichroismus kann die Magnetisierung in der Probe hoch aufgelöst abgebildet werden. Die Experimente teilen sich in zwei Gruppen: Untersuchungen der strominduzierten Domänenwandbewegung in magnetischen Drähten und Untersuchungen zu strominduzierten Vortexkernverschiebungen in magnetischen Scheiben. Erstere beschäftigen sich unter anderen mit systematischen Studien zu den kritischen Stromdichten, die erforderlich sind, um Domänenwände zu verschieben. Des Weiteren werden die strominduzierten Umwandlungen des Domänenwandtypes sowie die Abhängigkeit der Domänenwandgeschwindigkeit von der Stromdichte untersucht. Diese Studien beziehen sich auf das Material Permalloy (Ni80Fe20). Die Ergebnisse erlauben Rückschlüsse bezüglisch des nicht-adiabatischen Spintransfer Terms. Dieser Beitrag und der mit ihm assoziierte nicht-adiabatische Parameter wird gegenwärtig kontrovers diskutiert. Die Untersuchungen zeigen unter anderem, dass der nicht-adiabatische Parameter größer als die Dämpfungskonstante ist. Die Studien zur strominduzierten Domänenwandverschiebung werden vervollständigt durch Untersuchungen an anderen Materialien, wie zum Beispiel bei der Multilagenschicht Pt/CoFeB/Pt, in dem die Magnetisierung aus der Probenebene hinaus zeigt. In diesem System wird der dominante Einfluss des Oersted Feldes auf die magnetischen Domänenkonfiguration nachgewiesen. Dieser ermöglicht alternativ zu dem Spintransfer eine gezielte Manipulation der Spinkonfiguation über das Oersted Feld. Die Untersuchungen der strominduzierten Vortexkernverschiebung in magnetischen Scheiben basieren auf einer neuenMessmethode, die im Rahmen der Doktorarbeit entwickelt wurde. Diese erlaubt es, während der Strominjektion Bilder der magnetischen Konfiguration aufzunehmen. Unter Ausnutzung der speziellen Vortextopologie gelingt so erstmals eine Trennung der verschiedenen Effekte, über die der Strom auf die Spinstruktur wirkt. Dadurch lassen sich die relativen Stärken des adiabatischen und nicht-adiabatischen Spin-Transfer Terms sowie des Oersted Feldes bestimmen. Insbesondere gelingt auf diese Weise eine Bestimmung des nicht-adiabatischen Parameters zu 0.15. Die experimentellen Ergebnisse werden ergänzt durch analytische Rechnungen sowie mikromagnetische Simulationen, die einen besseren Vergleich von Theorie und experimentellen Ergebnissen gestatten und somit zu einem besseren Verständnis beitragen. Die im Verlauf dieser Arbeit erhaltenen Ergebnisse zeigen deutlich die wichtige Rolle des nicht-adiabatischen Term für den Spintransfer zwischen injizierten Strom und lokaler Magnetisierung.
Domain-Wall Depinning Assisted by Pure Spin Currents
We study the depinning of domain walls by pure diffusive spin currents in a nonlocal spin valve structure based on two ferromagnetic Permalloy elements with copper as the nonmagnetic spin conduit. The injected spin current is absorbed by the second Permalloy structure with a domain wall, and from the dependence of the wall depinning field on the spin current density we find an efficiency of 6 x 10 -14 T/(A/m²), which is more than an order of magnitude larger than for conventional current induced domain-wall motion. Theoretically we find that this high efficiency arises from the surface torques exerted by the absorbed spin current that lead to efficient depinning.
Current-induced vortex dynamics and pinning potentials probed by homodyne detection
Using a homodyne detection scheme, we show that we can determine the polarity and chirality of a magnetic vortex in an asymmetric magnetic disk as well as the resonance frequency and phase shift of the dynamic vortex gyration excited by a spin-polarized current. From systematic phase measurements, we deduce the relative contributions of the spin torque and the Oersted field, which is found to dominate the excitation. Local pinning sites in the disk lead to an increased resonance frequency and a reduced amplitude. This allows us to draw a map of the pinning sites and thus to characterize the full potential in the disk.
In situ contacting and current-injection into samples in photoemission electron microscopes
Studying the interaction of spin-polarized currents with the magnetization configuration is of high interest due to the possible applications and the novel physics involved. High-resolution magnetic imaging is one of the key techniques necessary for a better understanding of these effects. Here, we present an extension to a magnetic microscope that allows for in situ current injection into the structure investigated, and furthermore for the study of current induced magnetization changes during pulsed current injection. The developed setup is highly flexible and can be used for a wide range of investigations. Examples of current-induced domain wall motion and vortex core displacements measured using this setup are presented.
Imaging of Domain Wall Inertia in Permalloy Half-Ring Nanowires by Time-Resolved Photoemission Electron Microscopy
Using photoemission electron microscopy, we image the dynamics of a field pulse excited domain wall in a Permalloy nanowire. We find a delay in the onset of the wall motion with respect to the excitation and an oscillatory relaxation of the domain wall back to its equilibrium position, defined by an external magnetic field. The origin of both of these inertia effects is the transfer of energy between energy reservoirs. By imaging the distribution of the exchange energy in the wall spin structure, we determine these reservoirs, which are the basis of the domain wall mass concept.
Magnetic-field-induced domain-wall motion in permalloy nanowires with modified Gilbert damping
Domain wall (DW) depinning and motion in the viscous regime induced by magnetic fields, are investigated in planar permalloy nanowires in which the Gilbert damping α is tuned in the range 0.008–0.26 by doping with Ho. Real time, spatially resolved magneto-optic Kerr effect measurements yield depinning field distributions and DW mobilities. Depinning occurs at discrete values of the field which are correlated with different metastable DW states and changed by the doping. For α<0.033, the DW mobilities are smaller than expected while for α≥0.033, there is agreement between the measured DW mobilities and those predicted by the standard one-dimensional model of field-induced DW motion. Micromagnetic simulations indicate that this is because as α increases, the DW spin structure becomes increasingly rigid. Only when the damping is large can the DW be approximated as a pointlike quasiparticle that exhibits the simple translational motion predicted in the viscous regime. When the damping is small, the DW spin structure undergoes periodic distortions that lead to a velocity reduction. We therefore show that Ho doping of permalloy nanowires enables engineering of the DW depinning and mobility, as well as the extent of the viscous regime.
Current-induced motion and pinning of domain walls in spin-valve nanowires studied by XMCD-PEEM
Very large average velocities, up to 600 m/s, have been found for domain-wall motion driven by 3-ns-long pulses of electric current in zero magnetic field in the NiFe layer of 200-nm-wide NiFe/Cu/Co nanowires. For longer pulses, the domain-wall motion is strongly hindered by pinning potentials. Dipolar interactions between the NiFe and Co layers caused by anisotropy inhomogeneities have been identified as the most important among the different potential sources of DW pinning. The domain-wall velocities increase with current density, but a substantial drop is observed at current densities above 4×1011 A/m2.
Spatially resolved measurements of the ferromagnetic phase transition by ac-susceptibility investigations with x-ray photoelectron emission microscope
Spatially resolved ac susceptibility measurements on epitaxial Fe films are performed as a function of temperature using a conventional soft-x-ray photoelectron emission microscope. A magnetic contrast is observed at sample locations where the magnetic film undergoes a para/ferromagnetic phase transition. Due to the wedge structure of the Fe film and the thickness dependence of the Curie temperature the spatial extend of the phase transition region and the correlation length can be estimated.
Direct determination of large Spin-Torque nonadiabaticity in Vortex Core Dynamics
We use a pump-probe photoemission electron microscopy technique to image the displacement of vortex cores in Permalloy discs due to the spin-torque effect during current pulse injection. Exploiting the distinctly different symmetries of the spin torques and the Oersted-field torque with respect to the vortex spin structure we determine the torques unambiguously, and we quantify the amplitude of the strongly debated nonadiabatic spin torque. The nonadiabaticity parameter is found to be β=0.15±0.07, which is more than an order of magnitude larger than the damping constant α, pointing to strong nonadiabatic transport across the high magnetization gradient vortex spin structures.