Lutz Heyne

Current-induced vortex nucleation and annihilation in vortex domain walls

We report observations of the effect of electrical currents on the propagation and spin structure of vortex walls in NiFe wires. We find that magnetic vortices are nucleated and annihilated due to the spin torque effect. The velocity is found to be directly correlated with these transformations and decreases with increasing number of vortices. The transformations are observed in wide elements, while in narrower structures the propagation of single vortex walls prevails.

Tunable steady-state domain wall oscillator with perpendicular magnetic anisotropy

We theoretically study domain wall oscillations upon the injection of a dc current through a geometrically constrained wire with perpendicular magnetic anisotropy. The frequency spectrum of the oscillation can be tuned by the injected current density and additionally by the application of an external magnetic field. Our analytical calculations are supported by micromagnetic simulations based on the Landau–Lifshitz–Gilbert equation. The simple concept of our localized steady-state oscillator might prove useful as a nanoscale microwave generator with possible applications in telecommunications or for rf-assisted writing in magnetic hard drives.

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 6×10{-14}  T/(A/m{2}), 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.

Direct observation of high velocity current induced domain wall motion

We study fast vortex wall propagation in Permalloy wires induced by 3 ns short current pulses with sub 100 ps rise time using high resolution magnetic imaging at zero field. We find a constant domain wall displacement after each current pulse as well as current induced domain wall structure changes, even at these very short timescales. The domain wall velocities are found to be above 100 m/s and independent of the domain wall spin structure. Comparison to experiments with longer pulses points to the pulse shape as the origin of the high velocities.

Direct imaging of current-induced domain wall motion in CoFeB structures

By direct x-ray photoemission electron microscopy imaging, we probe current-induced domain wall motion in 20nm thick CoFeB wires. We observe transverse walls for all wire widths up to 1500nm as a consequence of the small saturation magnetization of the material. High critical current densities above 1×1012A∕m2 for wall displacement due to the spin transfer torque effect are found. The critical current densities jc increase further with decreasing wire width indicating that jc is governed by extrinsic pinning due to edge defects. In addition to wall displacements, we observe wall transformations to energetically favorable wall types due to heating. Owing to the high Curie temperature though, the sample temperature stays below the Curie temperature even for the highest current densities where structural damage sets in.

Selective domain wall depinning by localized Oersted fields and Joule heating

Using low temperature magnetoresistance measurements, the possibility to selectively move a domain wall locally by applying current pulses through a Au nanowire adjacent to a permalloy element is studied. We find that the domain wall depinning field is drastically modified with increasing current density due to the Joule heating and the Oersted field of the current, and controlled motion due to the Oersted field without any externally applied fields is achieved. By placing the domain wall at various distances from the Au wire, we determine the range of the Joule heating and the Oersted field and both effects can be separated.

Reversible switching between bidomain states by injection of current pulses in a magnetic wire with out-of-plane magnetization

The influence of current pulses on the domain structure of a 2μm wide wire composed of a soft out-of-plane magnetized magnetic material is studied by high spatial resolution nonintrusive magnetic imaging. The injection of current pulses (1012A∕m2) leads to stable magnetic states composed of two domains with opposite magnetization direction separated by a domain wall parallel to the wire. The direction of the magnetization in the domains is reversed back and forth by applying successive current pulses with opposite polarity. The formation and control of the domain states by the current is attributed to the effect of the Oersted field, which is calculated to be large enough to induce the switching.

Vortex domain wall chirality rectification due to the interaction with end domain spin structures in permalloy nanowires

The interaction of vortex domain walls with the end domain spin structure present at the rectangular end of a ferromagnetic nanowire is investigated using Lorentz transmission electron microscopy. When vortex walls are moved with short field pulses towards the wire end an end vortex is formed, whose chirality is independent of the original vortex wall chirality but is determined by the spin configuration of the end domain. This acts as a domain wall chirality “rectifier,” which could be useful for applications based on domain walls. The observed chirality transformations are reproduced by micromagnetic simulations showing a complex reversal mechanism.

Concepts for Domain Wall Motion in Nanoscale Ferromagnetic Elements due to Spin Torque and in Particular Oersted Fields

Herein, different concepts for domain wall propagation based on currents and fields that could potentially be used in magnetic data storage devices based on domains and domain walls are reviewed. By direct imaging, we show that vortex and transverse walls can be displaced using currents due to the spin transfer torque effect. For the case of field-induced wall motion, particular attention is paid to the influence of localized fields and local heating on the depinning and propagation of domain walls. Using an Au nanowire adjacent to a permalloy structure with a domain wall, the depinning field of the wall, when current pulses are injected into the Au nanowire, was studied. The current pulse drastically modified the depinning field, which depended on the interplay between the externally applied field direction and polarity of the current, leading subsequently to an Oersted field and heating of the permalloy at the interface with the Au wire. Placing the domain wall at various distances from the Au wire and studying different wall propagation directions, the range of Joule heating and Oersted field was determined; both effects could be separated. Approaches beyond conventional field- and current-induced wall displacement are briefly discussed.

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.