Search

Cylindrical magnetic nanowires have been proposed as a means of storing and processing information in a 3D medium, based on the motion of domain walls~(DWs). Introducing short chemical modulations in such wires would allow for reliable digital control of DWs. Here, we outline the intricate physics of the interaction of domain walls with modulations to control their motion, combining micromagnetic simulations and experimental evidence. This interaction combines a long-range moderate magnetostatic repulsion with a local energy well. The latter depends on the respective circulation sense of magnetization in the domain wall and modulation. We also show that a modulation has the ability to switch the internal circulation of a DW upon its propagation, thereby acting as a polarizing component and opening the possibility to exploit not only the position of walls, but also their internal structure.

Topology is a powerful tool for categorizing magnetization textures, highlighting specific features in both 2D systems, such as thin films or curved surfaces, and in 3D bulk systems. In the emerging field of 3D nanomagnetism within confined geometries, the contributions from both volume and surface must be considered, requiring appropriate topological analysis to obtain a complete view of the system. Here, we consider domain walls in cylindrical magnetic nanowires to illustrate the use of topological invariants. We begin with micromagnetic simulations of domain wall transformation under the stimulus of an \OErsted field, tracking bulk and surface topological signatures, and analyzing the interplay between multiple micromagnetic objects. For instance, the extensive analysis allowed us to highlight mechanisms of domain wall type conversion from topologically non-trivial to trivial states, a phenomenon disregarded in previous studies. Additionally, we provide experimental evidence of the transient states predicted to occur during the dynamical process.

Magnetic skyrmions are topological spin textures which are envisioned as nanometre scale information carriers in magnetic memory and logic devices. The recent demonstration of room temperature stabilization of skyrmions and their current induced manipulation in industry compatible ultrathin films were first steps towards the realisation of such devices. However, important challenges remain regarding the electrical detection and the low-power nucleation of skyrmions, which are required for the read and write operations. Here, we demonstrate, using operando magnetic microscopy experiments, the electrical detection of a single magnetic skyrmion in a magnetic tunnel junction (MTJ) and its nucleation and annihilation by gate voltage via voltage control of magnetic anisotropy. The nucleated skyrmion can be further manipulated by both gate voltage and external magnetic field, leading to tunable intermediate resistance states. Our results unambiguously demonstrate the readout and voltage controlled write operations in a single MTJ device, which is a major milestone for low power skyrmion based technologies.

We report on the structural, electric and magnetic properties of $(\mathrm{Ni}_x\mathrm{Co}_{1-x})\mathrm{B}$ ferromagnetic nanotubes, displaying azimuthal magnetization. The tubes are fabricated using electroless plating in polycarbonate porous templates, with lengths several tens of micrometers, diameters from 100nm to 500nm and wall thicknesses from 10nm to 80nm. The resistivity is $\sim 1.5\times10^{-6}\mathrm{\Omega/m}$, and the anisotropic magnetoresistance~(AMR) of 0.2-0.3%, one order of magnitude larger~(resp. smaller) than in the bulk material, which we attribute to the resistance at grain boundaries. We determined the azimuthal anisotropy field from M(H) AMR loops of single tubes contacted electrically. Its magnitude is around 10mT, and tends to increase with the tube wall thickness, as well as the Co content. However, surprisingly it does not dependent much on the diameter nor on the curvature., Comment: 12 pages, 10 figures, includes methods and appendix

We analyze the micromagnetics of short longitudinal modulations of a high-magnetization material in cylindrical nanowires made of a soft-magnetic material of lower magnetization such as permalloy, combining magnetic microscopy, analytical modeling, and micromagnetic simulations. The mismatch of magnetization induces curling of magnetization around the axis in the modulations, in an attempt to screen the interfacial magnetic charges. The curling angle increases with modulation length, until a plateau is reached with nearly full charge screening for a specific length scale~$\Delta_\mathrm{mod}$, larger than the dipolar exchange length of any of the two materials. The curling circulation can be switched by the Oersted field arising from a charge current with typical magnitude $10^{12} A/m^{2}$ for a diameter of $\sim$100 nm, and reaching a maximum for $\Delta_\mathrm{mod}$.

Perpendicular shape anisotropy (PSA) offers a practical solution to downscale spin-transfer torque Magnetic Random-Access Memory (STT-MRAM) beyond the sub-20 nm technology node whilst retaining thermal stability of the storage layer magnetization. However, our understanding of the thermomagnetic behavior of PSA-STT-MRAM is often indirect, relying on magnetoresistance measurements and micromagnetic modelling. Here, the magnetism of a FeCoB / NiFe PSA-STT-MRAM nano-pillar is investigated using off-axis electron holography, providing spatially resolved magnetic information as a function of temperature, which has been previously inaccessible. Magnetic induction maps reveal the micromagnetic configuration of the NiFe storage layer (60 nm high, 20 nm diameter), confirming the PSA induced by its 3:1 aspect ratio. In-situ heating demonstrates that the PSA of the FeCoB / NiFe composite storage layer is maintained up to at least 250 degrees centigrade, and direct quantitative measurements reveal the very moderate decrease of magnetic induction with temperature. Hence, this study shows explicitly that PSA provides significant stability in STT-MRAM applications that require reliable performance over a range of operating temperatures.

Speed and reliability of magnetic domain wall (DW) motion are key parameters that must be controlled to realize the full potential of DW-based magnetic devices for logic and memory applications. A major hindrance to this is extrinsic DW pinning at specific sites related to shape and material defects, which may be present even if the sample synthesis is well controlled. Understanding the origin of DW pinning and reducing it is especially desirable in electrochemically-deposited cylindrical magnetic nanowires (NWs), for which measurements of the fascinating physics predicted by theoretical computation have been inhibited by significant pinning. We experimentally investigate DW pinning in Co$_x$Ni$_{100-x}$ NWs, by applying quasistatic magnetic fields. Wire compositions were varied with $x=20,30,40$, while the microstructure was changed by annealing or varying the pH of the electrolyte for deposition. We conclude that pinning due to grain boundaries is the dominant mechanism, decreasing inversely with both the spontaneous magnetization and grain size. Second-order effects include inhomogeneities in lattice strain and the residual magnetocrystalline anisotropy. Surface roughness, dislocations and impurities are not expected to play a significant role in DW pinning in these wire samples., Comment: 10 pages, 4 figures, regular manuscript

We probe magnetic domain walls in cylindrical soft magnetic nanowires using electron holography. We detail the modelling of expected contrast for both transverse and Bloch point domain walls and provide comparison with experimental observations performed on NiCo nanowires, involving also both magnetic and electrostatic contribution to the electron holography map. This allows the fast determination of the domain wall type without the need for uneasy and time-consuming experimental removal of the electrostatic contribution. Finally, we describe and implement a new efficient algorithm for calculating the magnetic contrast., Comment: 3 pages, 2 figures

We present an experiment studying the interaction of a strongly focused 25 fs laser pulse with a tungsten nanotip, investigating the different regimes of laser-induced electron emission. We study the dependence of the electron yield with respect to the static electric field applied to the tip. Photoelectron spectra are recorded using a retarding field spectrometer and peaks separated by the photon energy are observed with a 45 % contrast. They are a clear signature of above threshold photoemission (ATP), and are confirmed by extensive spectrally resolved studies of the laser power dependence. Understanding these mechanisms opens the route to control experiment in the strong-field regime on nanoscale objects., Comment: 9 pages, 6 figures

We developed the self-assembly of epitaxial submicrometer-sized face-centered-cubic (fcc) Co(111) dots using pulsed laser deposition. The dots display atomically-fl at facets, from which the ratios of surface and interface energies for fcc Co are deduced. Zero- field magnetic structures are investigated with magnetic force and lorentz microscopies, revealing vortex-based flux-closure patterns. A good agreement is found with micromagnetic simulations.

A micromagnetic study of epitaxial micron-sized iron dots is reported through the analysis of Fresnel contrast in Lorentz Microscopy. Their use is reviewed and developed through analysis of various magnetic structures in such dots. Simple Landau configuration is used to investigate various aspects of asymmetric Bloch domain walls. The experimental width of such a complex wall is first derived and its value is discussed with the help of micromagnetic simulations. Combination of these two approaches enables us to define what is really extracted when estimating asymmetric wall width in Lorentz Microscopy. Moreover, quantitative data on the magnetization inside the dot is retrieved using phase retrieval as well as new informations on the degrees of freedom of such walls. Finally, it is shown how the existence and the propagation of a surface vortex can be characterized and monitored. This demonstrates the ability to reach a magnetic sensitivity a priori hidden in Fresnel contrast, based on an original image treatment and backed-up by the evaluation of contrasts obtained from micromagnetic simulations.

We present an electron holography experiment enabling the local and quantitative study of magnetic properties in magnetic tunnel junction. The junction was fully characterized during the switching process and each possible magnetic configuration was highlighted with magnetic induction maps. No magnetic coupling was found between the two layers. We plot a local hysteresis loop that was compared with magnetometry measurement at the macroscopic scale confirming the validity of the local method.

We investigated with XMCD-PEEM magnetic imaging the magnetization reversal processes of N\'eel caps inside Bloch walls in self-assembled, micron-sized Fe(110) dots with flux-closure magnetic state. In most cases the magnetic-dependent processes are symmetric in field, as expected. However, some dots show pronounced asymmetric behaviors. Micromagnetic simulations suggest that the geometrical features (and their asymmetry) of the dots strongly affect the switching mechanism of the N\'eel caps., Comment: Proceeding for MMM-Intermag 2010 (Washington)

The structure of domain walls delimiting magnetic bubbles in L10 FePd thin layers is described on the basis of Lorentz transmission electron microscopy (LTEM) and multiscale magnetic simulations. Images obtained by high resolution LTEM show the existence of magnetization reversal areas inside domain walls, called vertical Bloch lines (VBLs). Combining these observations and multiscale simulations on various geometries, we can identify the structure of these VBLs, notably the presence or not of magnetic singularities.

We report the use of Lorentz microscopy to observe the domain wall structure during the magnetization process in FePd thin foils. We have focused on the magnetic structure of domain walls of bubble-shaped magnetic domains near saturation. Regions are found along the domain walls where the magnetization abruptly reverses. Multiscale magnetic simulations shown that these regions are vertical Bloch lines (VBL) and the different bubble shapes observed are then related to the inner structure of the VBLs. We were thus able to probe the presence of magnetic singularities as small as Bloch points in the inner magnetization of the domain walls.

Data storage relies on the handling of two states, called bits. The market of mass storage is currently still dominated by magnetic technology, hard disk drives for the broad public and tapes for massive archiving. In these devices each bit is stored in the form of the direction of magnetization of nanosized magnetic domains, i.e. areas of ferromagnetic materials displaying a uniform magnetization. While miniaturization is the conventional way to fuel the continuous increase of device density, disruptive solutions are also sought. To these pertain in recent years many fundamental studies no longer considering the magnetic domains themselves, but the manipulation of the domain walls (DWs) that separate such domains. Concepts of storage and logic based on the propagation of DWs along lithographically-patterned stripes have been patented, while many fundamental aspects of DW propagation deeply related to condensed matter physics are still hotly debated. If one now considers magnetic dots of submicrometer dimensions, the magnetization has a tendency to curl along the outer edges of the nanostructure to close its magnetic flux and thereby reduce its magnetostatic energy. Then both domains and DWs of well-defined geometries arise, whose combined manipulation has been proposed as a multilevel magnetic storage scheme..., Comment: To appear in Elettra Sincrotrone Highlights 2008-2009

Dimensionality cross-over is a classical topic in physics. Surprisingly it has not been searched in micromagnetism, which deals with objects such as domain walls (2D) and vortices (1D). We predict by simulation a second-order transition between these two objects, with the wall length as the Landau parameter. This was conrmed experimentally based on micron-sized ux-closure dots.

Thin film alloys with perpendicular anisotropy were studied using Lorentz Transmission Electron Microscopy (LTEM). This work focuses on the configuration of domain walls and demonstrates the suitability and accuracy of LTEM for the magnetic characterisation of perpendicular magnetic anisotropy materials. Thin films of chemically ordered ($\unit{L1_0}$) FePd alloys were investigated by micro-magnetic modelling and LTEM phase retrieval approach. The different components of magnetization described by the modeling were studied on experimental images and confirmed by LTEM contrast simulation. Furthermore, quantitative measurements of magnetic induction inside the domain walls were made by using an original method to separate the electrical and magnetical contributions to the phase information. Irregularities were also observed along the domain walls which could play a major role during the magnetization processes., Comment: Ultramicroscopy (2009) In press

Off-axis electron holography was used to observe and quantify the magnetic microstructure of a perpendicular magnetic anisotropic (PMA) recording media. Thin foils of PMA materials exhibit an interesting up and down domain configuration. These domains are found to be very stable and were observed at the same time with their stray field, closing magnetic flux in the vacuum. The magnetic moment can thus be determined locally in a volume as small as few tens of cubic nanometers.

Lorentz transmission electron microscopy (LTEM) combined with in-situ magnetizing experiments is a powerful tool for the investigation of the magnetization of the reversal process at the micron scale. We have implemented this tool on a conventional transmission electron microscope (TEM) to study the exchange anisotropy of a polycrystalline Co35Fe65/NiMn bilayer. Semi-quantitative maps of the magnetic induction were obtained at different field values by the differential phase contrast (DPC) technique adapted for a TEM (SIDPC). The hysteresis loop of the bilayer has been calculated from the relative intensity of magnetic maps. The curve shows the appearance of an exchange-bias field reveals with two distinct reversal modes of the magnetization: the first path corresponds to a reversal by wall propagation when the applied field is parallel to the anisotropy direction whereas the second is a reversal by coherent rotation of magnetic moments when the field is applied antiparallel to unidirectional anisotropy direction.

While magnetic hysteresis usually considers magnetic domains, the switching of the core of magnetic vortices has recently become an active topic. We considered Bloch domain walls, which are known to display at the surface of thin films flux-closure features called N\'eel caps. We demonstrated the controlled switching of these caps under a magnetic field, occurring via the propagation of a surface vortex. For this we considered flux-closure states in elongated micron-sized dots, so that only the central domain wall can be addressed, while domains remain unaffected., Comment: 4 pages, 3 figures

Many years of measurements obtained using French meteor radars at Garchy (latitude 47 N) and Montpazier (latitude 44 N) are used to show the existence of an 8 hour oscillation. Some examples of the structure of this wave are displayed and compared with measurements performed at Saskatoon (latitude 52 N) and Budrio (latitude 45 N). This wave can be interpreted as the solar driven terdiurnal tide, or as the result of the nonlinear interaction between the diurnal and semidiurnal tides. Both hypotheses are tested with numerical models. Incidentally, the possible existence of a 24 hour wave resulting from this interaction is also studied.