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Register a new userTime-resolved imaging of OE rsted field induced magnetization dynamics in cylindrical magnetic nanowires
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Added by
Michael Sch\\\"obitz
Publication date
2021
fields
Physics
and research's language is
English
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Recent studies in three dimensional spintronics propose that the OE rsted field plays a significant role in cylindrical nanowires. However, there is no direct report of its impact on magnetic textures. Here, we use time-resolved scanning transmission X-ray microscopy to image the dynamic response of magnetization in cylindrical Co$_{30}$Ni$_{70}$ nanowires subjected to nanosecond OE rsted field pulses. We observe the tilting of longitudinally magnetized domains towards the azimuthal OE rsted field direction and create a robust model to reproduce the differential magnetic contrasts and extract the angle of tilt. Further, we report the compression and expansion, or breathing, of a Bloch-point domain wall that occurs when weak pulses with opposite sign are applied. We expect that this work lays the foundation for and provides an incentive to further studying complex and fascinating magnetization dynamics in nanowires, especially the predicted ultra-fast domain wall motion and associated spin wave emissions.
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A wire that conducts an electric current will give rise to circular magnetic field (the {O}rsted magnetic field) that is easily calculated using the Maxwell-Ampere equation. For wires with diameters in the macroscopic scale, this is an established physical law that has been demonstrated for two centuries. The Maxwell-Ampere equation is based on the argument that the induction of {O}rsted magnetic field is only a result of the displacement of charge. An alternative derivation of the {O}rsted magnetic field in conductors was suggested in [J. Mag. Mag. Mat. 504, 166660 (2020)] (will be called the current magnetization hypothesis (CMH) thereupon), which proposes that the {O}rsted magnetic field results from a two-body interaction. The present work establishes computationally, using simplified wire models, that the CMH reproduces the results of the Maxwell-Ampere equation for wires with a square cross section. Thus, CMH is proposed as a microscopic theory of magnetic induction, in contrast to the Maxwellian continuum theory of magnetic induction. I demonstrate that CMH could resolve the apparent contradiction between the observed induced magnetic field and that predicted by the Maxwell-Ampere equation in nanowires, as was reported in [Phys. Rev. B 99, 014436 (2019)]. The CMH shows that a possible reason for such contradiction is the presence of non-conductive surface layers in conductors.
The reversal of the magnetization under the influence of a field pulse has been previously predicted to be an incoherent process with several competing phenomena such as domain wall relaxation, spin wave-mediated instability regions, and vortex-core mediated reversal dynamics. However, there has been no study on the direct observation of the switching process with the aid of a microwave signal input. We report a time-resolved imaging study of magnetization reversal in patterned magnetic structures under the influence of a field pulse with microwave assistance. The microwave frequency is varied to demonstrate the effect of resonant microwave-assisted switching. We observe that the switching process is dominated by spin wave dynamics generated as a result of magnetic instabilities in the structures, and identify the frequencies that are most dominant in magnetization reversal.
The nature of magnetization reversal in an isolated cylindrical nanomagnet has been studied employing time-resolved magnetoresistance measurement. We find that the reversal mode is highly stochastic, occurring either by multimode or single-step switching. Intriguingly, the stochasticity was found to depend on the alignment of the driving magnetic field to the long axis of the nanowires, where predominantly multimode switching gives way to single-step switching behavior as the field direction is rotated from parallel to transverse with respect to the nanowire axis.
We report several procedures for the robust nucleation of magnetic domain walls in cylindrical permalloy nanowires. Specific features of the magnetic force microscopy contrast of such soft wires are discussed, with a view to avoid the misinterpretation of the magnetization states. The domain walls could be moved under quasistatic magnetic fields in the range 0.1--10 mT.
Recently magnetic storage and magnetic memory have shifted towards the use of magnetic thin films with perpendicular magnetic anisotropy (PMA). Understanding the magnetic damping in these materials is crucial, but normal Ferromagnetic Resonance (FMR) measurements face some limitations. The desire to quantify the damping in materials with PMA has resulted in the adoption of Time-Resolved Magneto-optical Kerr Effect (TR-MOKE) measurements. In this paper, we discuss the angle and field dependent signals in TR-MOKE, and utilize a numerical algorithm based on the Landau-Lifshitz-Gilbert (LLG) equation to provide information on the optimal conditions to run TR-MOKE measurements.
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