Robert M. Reeve

Observation of room-temperature magnetic skyrmions and their current-driven dynamics in ultrathin metallic ferromagnets

Magnetic skyrmions are topologically protected spin textures that exhibit fascinating physical behaviours and large potential in highly energy-efficient spintronic device applications. The main obstacles so far are that skyrmions have been observed in only a few exotic materials and at low temperatures, and fast current-driven motion of individual skyrmions has not yet been achieved. Here, we report the observation of stable magnetic skyrmions at room temperature in ultrathin transition metal ferromagnets with magnetic transmission soft X-ray microscopy. We demonstrate the ability to generate stable skyrmion lattices and drive trains of individual skyrmions by short current pulses along a magnetic racetrack at speeds exceeding 100 m s(-1) as required for applications. Our findings provide experimental evidence of recent predictions and open the door to room-temperature skyrmion spintronics in robust thin-film heterostructures.

Synchronous precessional motion of multiple domain walls in a ferromagnetic nanowire by perpendicular field pulses

Magnetic storage and logic devices based on magnetic domain wall motion rely on the precise and synchronous displacement of multiple domain walls. The conventional approach using magnetic fields does not allow for the synchronous motion of multiple domains. As an alternative method, synchronous current-induced domain wall motion was studied, but the required high-current densities prevent widespread use in devices. Here we demonstrate a radically different approach: we use out-of-plane magnetic field pulses to move in-plane domains, thus combining field-induced magnetization dynamics with the ability to move neighbouring domain walls in the same direction. Micromagnetic simulations suggest that synchronous permanent displacement of multiple magnetic walls can be achieved by using transverse domain walls with identical chirality combined with regular pinning sites and an asymmetric pulse. By performing scanning transmission X-ray microscopy, we are able to experimentally demonstrate in-plane magnetized domain wall motion due to out-of-plane magnetic field pulses.

Strain-mediated electric-field control of exchange bias in a Co90Fe10/BiFeO3/SrRuO3/PMN-PT heterostructure

The electric-field (E-field) controlled exchange bias (EB) in a Co90Fe10/BiFeO3 (BFO)/SrRuO3/PMN-PT heterostructure has been investigated under different tensile strain states. The in-plane tensile strain of the BFO film is changed from +0.52% to +0.43% as a result of external E-field applied to the PMN-PT substrate. An obvious change of EB by the control of non-volatile strain has been observed. A magnetization reversal driven by E-field has been observed in the absence of magnetic field. Our results indicate that a reversible non-volatile E-field control of a ferromagnetic layer through strain modulated multiferroic BFO could be achieved at room temperature.

Magnetic domain structure of La0.7Sr0.3MnO3 thin-films probed at variable temperature with scanning electron microscopy with polarization analysis

The domain configuration of 50 nm thick La0.7Sr0.3MnO3 films has been directly investigated using scanning electron microscopy with polarization analysis (SEMPA), with magnetic contrast obtained without the requirement for prior surface preparation. The large scale domain structure reflects a primarily four-fold anisotropy, with a small uniaxial component, consistent with magneto-optic Kerr effect measurements. We also determine the domain transition profile and find it to be in agreement with previous estimates of the domain wall width in this material. The temperature dependence of the image contrast is investigated and compared to superconducting-quantum interference device magnetometry data. A faster decrease in the SEMPA contrast is revealed, which can be explained by the techniques extreme surface sensitivity, allowing us to selectively probe the surface spin polarization which due to the double exchange mechanism exhibits a distinctly different temperature dependence than the bulk magnetization.

Spin currents injected electrically and thermally from highly spin polarized Co2MnSi

We demonstrate the injection and detection of electrically and thermally generated spin currents probed in Co2MnSi/Cu lateral spin valves. Devices with different electrode separations are patterned to measure the non-local signal as a function of the electrode spacing and we determine a relatively high effective spin polarization α of Co2MnSi to be 0.63 and the spin diffusion length of Cu to be 500 nm at room temperature. The electrically generated non-local signal is measured as a function of temperature and a maximum signal is observed for a temperature of 80 K. The thermally generated non-local signal is measured as a function of current density and temperature in a second harmonic measurement detection scheme. We find different temperature dependences for the electrically and thermally generated non-local signals, which allows us to conclude that the temperature dependence of the signals is not just dominated by the transport in the Cu wire, but there is a crucial contribution from the different gener...

Electron Beam Tuning of the Magnetic Anisotropy in Co/Cu(110) Films

The 90° in-plane spin reorientation transition caused by residual gas adsorption in the Co/Cu(110) system has been investigated using Mott electron polarimetry. The system displays a 3-D growth mode, forming islands with a preferential axis of elongation along the [1-10] direction. The adsorption of CO at the step edges is found to change the net anisotropy of the system and bring about the change in the relaxed magnetization direction. By carefully limiting the duration of electron beam exposure, it has been shown that this spin rotation can also be observed using Mott polarimetry, where it manifests itself as variations in the secondary electron spin polarization. Extended exposure of the switched film to an electron beam can reverse the easy axis direction, allowing local tuning of the magnetic anisotropy in this system.

Geometrical control of pure spin current induced domain wall depinning

We investigate the pure spin-current assisted depinning of magnetic domain walls in half ring based Py/Al lateral spin valve structures. Our optimized geometry incorporating a patterned notch in the detector electrode, directly below the Al spin conduit, provides a tailored pinning potential for a transverse domain wall and allows for a precise control over the magnetization configuration and as a result the domain wall pinning. Due to the patterned notch, we are able to study the depinning field as a function of the applied external field for certain applied current densities and observe a clear asymmetry for the two opposite field directions. Micromagnetic simulations show that this can be explained by the asymmetry of the pinning potential. By direct comparison of the calculated efficiencies for different external field and spin current directions, we are able to disentangle the different contributions from the spin transfer torque, Joule heating and the Oersted field. The observed high efficiency of the pure spin current induced spin transfer torque allows for a complete depinning of the domain wall at zero external field for a charge current density of [Formula: see text] A m-2, which is attributed to the optimal control of the position of the domain wall.

Domain wall spin structures in mesoscopic Fe rings probed by high resolution SEMPA

We present a combined theoretical and experimental study of the energetic stability and accessibility of different domain wall spin configurations in mesoscopic magnetic iron rings. The evolution is investigated as a function of the width and thickness in a regime of relevance to devices, while Fe is chosen as a material due to its simple growth in combination with attractive magnetic properties including high saturation magnetization and low intrinsic anisotropy. Micromagnetic simulations are performed to predict the lowest energy states of the domain walls, which can be either the transverse or vortex wall spin structure, in good agreement with analytical models, with simulations revealing the expected low temperature configurations observable on relaxation of the magnetic structure from saturation in an external field. In the latter case, following the domain wall nucleation process, transverse domain walls are found at larger widths and thicknesses than would be expected by comparing the competing energy terms demonstrating the importance of metastability of the states. The simulations are compared to high resolution experimental images of the magnetization using scanning electron microscopy with polarization analysis to provide a phase diagram of the various spin configurations. In addition to the vortex and simple symmetric transverse domain wall, a significant range of geometries are found to exhibit highly asymmetric transverse domain walls with properties distinct from the symmetric transverse wall. Simulations of the asymmetric walls reveal an evolution of the domain wall tilting angle with ring thickness. Analysis of all the data reveals that in addition to the geometry, the influence of materials properties, defects and thermal activation all need to be taken into account in order to understand and reliably control the experimentally accessible states, as needed for devices.

Domain wall pinning in ultra-narrow electromigrated break junctions

The study of magnetic domain walls in constrained geometries is an important topic, yet when dealing with extreme nanoscale magnetic systems artefacts can often dominate the measurements and obscure the effects of intrinsic magnetic origin. In this work we study the evolution of domain wall depinning in electromigrated ferromagnetic junctions which are both initially fabricated and subsequently tailored in-situ in clean ultra-high vacuum conditions. Carefully designed Ni(80)Fe(20) (Permalloy) notched half-ring structures are fabricated and investigated as a function of constriction width by tailoring the size of the contact using controlled in-situ electromigration. It is found that the domain wall pinning strength is increased on reducing the contact size in line with a reduction of the wall energy in narrower constrictions. Furthermore, the angular dependency and symmetry of the depinning field is measured to determine the full pinning potential for a domain wall in a system with a narrow constriction.

Development of a scanning electron microscopy with polarization analysis system for magnetic imaging with ns time resolution and phase-sensitive detection

Scanning electron microscopy with polarization analysis is a powerful lab-based magnetic imaging technique offering simultaneous imaging of multiple magnetization components and a very high spatial resolution. However, one drawback of the technique is the long required acquisition time resulting from the low inherent efficiency of spin detection, which has limited the applicability of the technique to certain quasi-static measurement schemes and materials with high magnetic contrast. Here we demonstrate the ability to improve the signal-to-noise ratio for particular classes of measurements involving periodic excitation of the magnetic structure via the implementation of a digital phase-sensitive detection scheme facilitated by the integration of a time-to-digital converter to the system. The modified setup provides dynamic imaging capabilities using selected time windows and finally full time-resolved imaging with a demonstrated time resolution of better than 2 ns.X iv :1 80 3. 09 77 5v 1 [ ph ys ic s. in sde t] 2 6 M ar 2 01 8 Development of a SEMPA system for magnetic imaging with ns time resolution and phase-sensitive detection Daniel Schönke, Andreas Oelsner, Pascal Krautscheid, 3 Robert M. Reeve, a) and Mathias Kläui 3 Institut für Physik, Johannes Gutenberg Universität Mainz, 55099 Mainz, Germany Surface Concept GmbH, 55099 Mainz, Germany MAINZ Graduate School, Johannes Gutenberg Universität Mainz, 55099 Mainz, Germany