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Magnetization reversal and domain wall dynamics in Pt/Co/AlOx trilayers have been tuned by He+ ion irradiation. Fluences up to 1.5x10$^{15}$ ions/cm$^2$ strongly decrease the perpendicular magnetic anisotropy (PMA), without affecting neither the spontaneous magnetization nor the strength of the Dzyaloshinskii-Moriya interaction (DMI). This confirms the robustness of the DMI interaction against interfacial chemical intermixing, already predicted by theory. In parallel with the decrease of the PMA in the irradiated samples, a strong decrease of the depinning field is observed. This allows the domain walls to reach large maximum velocities with lower magnetic fields with respect to those needed for the pristine films. Decoupling PMA from DMI can therefore be beneficial for the design of low energy devices based on domain wall dynamics. When the samples are irradiated with larger He+ fluences, the magnetization gets close to the out-of-plane/in-plane reorientation transition where 100nm size magnetic skyrmions are stabilized. We observe that as the He+ fluence increases, the skyrmion size decreases while these magnetic textures become more stable against the application of an external magnetic field., Comment: 10 pages, 4 figures

International audience; We experimentally investigate the range of the Dzyaloshinskii-Moriya interaction (DMI) occurring at magnetic interfaces within metallic heterostructures. To this aim we perform Brillouin light scattering spectroscopy on a set of Co-based, asymmetric metallic heterostructures, incorporating atomically thin continuous films obtained by room-temperature sputtering, and of identical orientation and quality. We thus access to the intrinsic dependence of the interfacial DMI and other magnetic interactions on the thickness of the non-magnetic layer adjacent to Co, which is chosen among Pt, Ru and Au. Notably, we observe that a robust DMI is already generated by as few as two atomic planes of Pt, and that interfacial DMI can be efficiently suppressed by a dusting of Ru equivalent to a single atomic plane coverage. These results point directly towards a mechanism where DMI is generated within the two first atomic planes away from the interface, in agreement with first-principles calculations. This locally generated DMI is however likely to be modulated by more distant atoms in the case of strain effects. The short-range aspect of the interfacial DMI opens up the synthesis of dense magnetic multilayers, allowing for a strong interfacial DMI even with very thin layers, which can be further tuned by strain engineering.

International audience; Here, we present magneto-optical studies of a surface domain wall (DW) shape in as-cast and heat-treated glass-coated microwires (MW). Technique optimized for the study of thin magnetic cylinders is used for local mapping of the magnetization process on wire's circumference. Consequently, the shape of the surface DW can be visualized at an arbitrary point along the wire's length. Samples undergo sequential heat treatment, and DW shape in the same sample before and after heat treatment is compared. Variation in DW tilting angle and length is observed as a result of annealing.; Nous présentons une étude par magnéto-optique de la forme des parois magnétiques dans des microfils magnétiques gainés de verre, tant dans l'état brut de fabrication qu'après traitement thermique. La technique magnéto-optique utilisée permet de remonter au profil d'aimantation à la surface des microfils. La position de la paroi peut ainsi être visualisée à toute position sur cette surface. Les échantillons ont subi une série de recuits thermiques, et les positions des parois avant et après chaque traitement ont été comparées. Nous avons observé une variation de l'angle d'inclinaison de la paroi, et de sa longueur selon l'axe du fil, suite aux recuits.

International audience; This chapter explains what topology brings to the physics of magnetic textures in real space, from a physicist's point of view. The statics of magnetic textures is first discussed, and the distinction is made between a topological defect and a topologically stable-or protected-structure, also called topological soliton. The attention is then turned towards the physically observable consequences of topology, mainly in the dynamics of magnetic textures. Connection with experimental observations and applications is made, from early works at the beginnings of magnetic data storage up to the recent work on magnetic skyrmions, which triggered a revival of interest in topology as applied to magnetism.

An analytical model for the domain wall structure in ultrathin films with perpendicular easy axis and interfacial Dzyaloshinskii-Moriya interaction, submitted to an arbitrary in-plane magnetic field, is presented. Its solution is simplified to the numerical minimization of an analytic function of just one variable. The model predictions are compared to numerical micromagnetic simulations, using parameters of existing samples, revealing a very good agreement. Remaining differences are analyzed, and partly corrected. Differences with the predictions of the simplest model, usually found in the literature, in which only the domain wall moment's in-plane orientation can vary, are exemplified. The model allows accurate computations, as a function of in-plane field module and orientation, of the domain wall tension and width, quantities controlling the creep motion of domain walls in such films., Comment: 13 pages, 15 figures

We study field-driven domain wall (DW) velocities in asymmetric multilayer stacks with perpendicular magnetic anisotropy and Dzyaloshinskii-Moriya interaction (DMI), both experimentally and by micromagnetic simulations. Using magneto-optical Kerr microscopy under intense and nanoseconds-long fields, we show that DWs in these films propagate at velocities up to hundreds of m/s and that, instead of the expected decrease of velocity after the Walker field, a long plateau with constant velocity is observed, before breakdown. Both the maximum speed and the field extent of the velocity plateau strongly depend on the values of the spontaneous magnetization and the DMI strength, as well as on the magnetic anisotropy. Micromagnetic simulations reproduce these features in sufficiently wide strips, even for perfect samples. A physical model explaining the microscopic origin of the velocity plateau is proposed., 10 pages, 9 figures

Magnetic properties of highly magnetostrictive amorphous glass-coated microwires are strongly correlated to the presence of a glass coating that introduces a spatially inhomogeneous stress field distribution. We investigate the influence of mechanical stresses on the inclination of magnetic domain walls in magnetic microwires. Magneto-optical Kerr effect imaging is used to compare the tilted orientation of the domain wall shape in as-cast and annealed microwires. Angular dependencies of magnetization loops measured by alternating gradient field magnetometry reveal that the change of domain wall tilting with annealing is related to the decrease of magnetic anisotropy with axial orientation. Finally, micromagnetic simulations are used to show that sufficiently high uniaxial magnetic anisotropy gives rise to the presence of observed charged domain walls with tilted orientation.

The nonplanar sample surface is a crucial feature that must be taken into account for a good interpretation of magneto-optical observations of magnetic cylindrical microwires. This is due to the fact that a curved topography gives rise to a spatial distribution of local planes of incidence as a function of position on the circumference of a cylinder. Analytical calculations of the magnetic contrast of magnetic cylinders spontaneously magnetized in the axial direction reveal that an axial magnetization reversal produces a characteristic black-and-white magneto-optical contrast and that it does not correspond to two oppositely magnetized domains, as would be concluded in the case of planar samples. Optical and magneto-optical observations of thin cylindrical FeSiB wires support this model. Calculations are also performed for the cases of circular and polar magnetizations.

This article reviews static and dynamic interfacial effects in magnetism, focusing on interfacially-driven magnetic effects and phenomena associated with spin-orbit coupling and intrinsic symmetry breaking at interfaces. It provides a historical background and literature survey, but focuses on recent progress, identifying the most exciting new scientific results and pointing to promising future research directions. It starts with an introduction and overview of how basic magnetic properties are affected by interfaces, then turns to a discussion of charge and spin transport through and near interfaces and how these can be used to control the properties of the magnetic layer. Important concepts include spin accumulation, spin currents, spin transfer torque, and spin pumping. An overview is provided to the current state of knowledge and existing review literature on interfacial effects such as exchange bias, exchange spring magnets, spin Hall effect, oxide heterostructures, and topological insulators. The article highlights recent discoveries of interface-induced magnetism and non-collinear spin textures, non-linear dynamics including spin torque transfer and magnetization reversal induced by interfaces, and interfacial effects in ultrafast magnetization processes., 193 pages, including 28 figures inserted after text and references. Paper accepted in Reviews of Modern Physics

Observing jumping domain walls Domain walls, which separate regions of opposite magnetization in a ferromagnet, have rich dynamics that are difficult to characterize in small samples. Tetienne et al. imaged the magnetization of a thin ferromagnetic wire and observed the jumping of a domain wall between different positions along the wire. They used a scanning magnetic microscope based on a defect in diamond. The laser light needed to operate the microscope also enabled the control of the domain wall motion by causing local heating, which made the illuminated position more likely to contain a domain wall. Science , this issue p. 1366

Amorphous glass-coated microwires are well known by high domain wall velocities that reach up to 15 km/s. Previous works have shown that such fast domain wall dynamics can be manipulated by properly selected conditions of thermal annealing. Along with the magnetic anisotropy, the domain wall velocity is strongly influenced by surface shell of domains too. Here, the magneto-optical study of the surface reversal process in microwire is carried out by magneto-optical Kerr effect (MOKE) microscopy. It is shown that the nonplanar surface of the cylindrical samples gives rise to a complex MOKE. Trapping the domain wall in a potential, well confirmed the inclined structure of the domain wall. Such inclination can be found partially responsible for apparent high domain wall velocities measured by Sixtus-Tonks method in microwires.

The application of a neutron microbeam to the investigation of an internal magnetic microstructure has been demonstrated experimentally. The object of investigation is an amorphous magnetic wire 190 μm in diameter, with the axial and circular magnetic domains formed inside it. A narrow neutron microbeam with a width of 2.6 μm, which is formed by a three-layer film (a planar waveguide) serves as the probe for scanning with a high spatial resolution. The method of investigation is the neutron spin precession upon transmission through the sample. © 2014 Pleiades Publishing, Ltd., This study was supported by the IMAMINE 2010-09T project (France).

Magnetic skyrmions are topologically stable spin configurations, which usually originate from chiral interactions known as Dzyaloshinskii-Moriya interactions. Skyrmion lattices were initially observed in bulk non-centrosymmetric crystals, but have more recently been noted in ultrathin films, where their existence is explained by interfacial Dzyaloshinskii-Moriya interactions induced by the proximity to an adjacent layer with strong spin-orbit coupling. Skyrmions are promising candidates as information carriers for future information-processing devices due to their small size (down to a few nanometres) and to the very small current densities needed to displace skyrmion lattices. However, any practical application will probably require the creation, manipulation and detection of isolated skyrmions in magnetic thin-film nanostructures. Here, we demonstrate by numerical investigations that an isolated skyrmion can be a stable configuration in a nanostructure, can be locally nucleated by injection of spin-polarized current, and can be displaced by current-induced spin torques, even in the presence of large defects.

The high frequency magnetic susceptibility of glass-coated microwires with effective circumferential anisotropy has been modeled by micromagnetics. In their amorphous and magnetically soft metallic nucleus, the negative magnetostriction induces a continuous and non-uniform vortex-like structure, with an axially magnetized core coupled to a circumferentially magnetized outer shell. The model is a one-dimensional radial micromagnetic model where exchange, magnetoelastic and magnetostatic energies are considered. The microwave permeability under AC (axial and circumferential) fields is found to be expressed as a sum over a set of eigenmodes, each with a characteristic resonance frequency. In the framework of this model, a sum rule on the frequency-dependent permeability is shown to hold. It provides a figure of merit and guidelines for the design of high permeability materials for applications in the microwave range. © 2012 Elsevier B.V.

The microwave permeability of flake-shaped particles is investigated using a shorted microstrip setup. Several characterizations using two distinct field configurations, with and without the presence of a static field, are performed to provide information about the magnetization dynamics of this new material. A first insight is provided through the framework of Kittel's resonance, but additional features are also found and discussed.

The Dzyaloshinskii-Moriya Interaction (DMI) has recently attracted considerable interest owing to its fundamental role in the stabilization of chiral spin textures in ultrathin ferromagnets, which are interesting candidates for novel spintronic technologies. Here, the authors present an experimental study of the DMI strength that is induced in nanometer-thick CoFeB ferromagnetic thin films in contact with different nonmagnetic metal underlayers. They use a novel technique for noninvasive high-sensitivity sensing of magnetic field: a scanning nanomagnetometer based on the magnetic response of a single nitrogen-vacancy defect in diamond. The magnetic domain walls are mapped, the spin structure and type of domain wall determined, and the DMI strength extracted. Importantly, the authors find local variation of the DMI constant, which clearly suggests that local field mapping techniques are extremely important to study this physics.

A path method is implemented in order to precisely and generally describe the collapse of isolated skyrmions in a Co/Pt(111) monolayer, on the basis of atomic scale simulations. Two collapse mechanisms with different energy barriers are found. The most obvious path, featuring a homogeneous shrinking gives the largest energy, whereas the lowest energy barrier is shown to comply with the outcome of Langevin dynamics under a destabilizing field of 0.25 T, with a lifetime of 20~ns at around 80~K. For this lowest energy barrier path, skyrmion destabilization occurs much before any topology change, suggesting that topology plays a minor role in the skyrmion stability. On the contrary, an important role appears devoted to the Dzyaloshinskii-Moriya interaction, establishing a route towards improved skyrmion stability., Comment: 9 pages, 6 figures

The dynamics of magnetic domain walls in ultrathin strip-patterned Pt/Co/AlOx samples with perpendicular easy axis has been studied experimentally under an easy-axis field, superposed to a hard-axis field oriented along the strip. The easy-axis field is large so that the domain walls move well beyond the creep regime. A chiral effect is observed where the domain wall velocity shows a monotonous and surprisingly large variation with an in-plane field. A micromagnetic analysis, combining analytic, one-dimensional, and two-dimensional simulations with structural disorder, shows that this behavior can be reproduced with a Dzyaloshinskii-Moriya interaction of the interfacial type, with due consideration of the dynamics of the tilt degree of freedom of the domain wall. The estimated effective value of this interaction ($D\ensuremath{\approx}\ensuremath{-}2.2\phantom{\rule{0.28em}{0ex}}\mathrm{mJ}/{\mathrm{m}}^{2}$ for a 0.6 nm Co thickness) is consistent with values obtained by other techniques. It is also shown, by micromagnetic analysis, that several modes and characteristic times occur in the dynamics of the tilt of such domain walls.

Controlling the position of a magnetic domain wall with electric current may allow for new types of non-volatile memory and logic devices. To be practical, however, the threshold current density necessary for domain wall motion must be reduced below present values. Intrinsic pinning due to magnetic anisotropy, as recently observed in perpendicularly magnetized Co/Ni nanowires, has been shown to give rise to an intrinsic current threshold J(th)(0). Here, we show that domain wall motion can be induced at current densities 40% below J(th)(0) when an external magnetic field of the order of the domain wall pinning field is applied. We observe that the velocity of the domain wall motion is the vector sum of current- and field-induced velocities, and that the domain wall can be driven against the direction of a magnetic field as large as 2,000 Oe, even at currents below J(th)(0). We show that this counterintuitive phenomenon is triggered by Walker breakdown, and that the additive velocities provide a unique way of simultaneously determining the spin polarization of current and the Gilbert damping constant.

We study soft magnetic bilayers having orthogonal, in-plane easy axes. The layers are thicker than the Bloch wall width linked to the anisotropy, so that a helical magnetization with a large angle exists across the sample thickness. The magnetic domains structure has been investigated at both sample surfaces, using magneto-optical microscopy. The domain structure is found to be similar to that of double films with biquadratic coupling. Two kinds of domain walls are identified, namely with a 90° and 180° rotation of the average magnetization. The detailed structure and energy of these walls are studied by micromagnetic calculations.

The static magnetization profile of glass-coated microwires with effective circular anisotropy is investigated using micromagnetics. In this family of microwires, the ferromagnetic nucleus with an amorphous character presents a magnetic structure composed of an inner region with axial domains and an outer region with circular domains, due to magnetoelastic anisotropy. A one-dimensional micromagnetic model is developed, taking into account both the exchange and magnetoelastic anisotropy energies, and solved quasi analytically. The total energy, magnetization profiles and magnetization curves are investigated as a function of radius and anisotropy constant of the nucleus. This work represents a fundamental study of the magnetization process in these amorphous microwires and provides guidelines for the production of microwires with tailored magnetic properties. En passant, the nucleation problem in an infinite cylinder, introduced by W.F. Brown, is revisited.

This paper investigates the non steady-state displacement of magnetic domain walls in a nanostrip submitted to a time-dependent spin-polarized current flowing along the nanostrip. First, numerical micromagnetic simulations show that a domain wall can move under application of a current pulse, and that the displacement resulting from a conversion of the domain wall structure is quantized. The numerical findings are subsequently explained in the framework of simplified analytic models, namely the 1D model and the point-core vortex model. We then introduce the concept of an angle linked to the magnetization of a general domain wall, and show that it allows understanding the transient phenomena quite generally. Simple analytic formulas are derived and compared to experiments. For this, charts are given for the key parameters of the domain wall mechanics, as obtained from numerical micromagnetic simulations. We finally discuss the limitations of this work, by looking at the influence of temperature elevation under current, presence of a non-adiabatic term, and of disorder.

Ballistic electron emision microscopy spectra have been measured at room temperature on Au films deposited on the Si(111)-H surface by different procedures. In order to analyze them in detail, we propose a fully analytical description of these spectra, directly based on the phase-space model of Bell and Kaiser. This allows fitting experimental data over a wide voltage range, comprising the threshold and the quasi-linear regions. Two main independent parameters are extracted from the fits, namely the effective Schottky barrier height and hot-electron transmission of the sample. These show a clear variation with sample preparation conditions.

Spin waves in perpendicularly magnetized ${\text{Pt/Co/AlO}}_{x}/\text{Pt}$ ultrathin films with varying Co thicknesses (0.6--1.2 nm) have been studied with Brillouin light spectroscopy in the Damon-Eshbach geometry. The measurements reveal a pronounced nonreciprocal propagation, which increases with decreasing Co thickness. This nonreciprocity, attributed to an interfacial Dzyaloshinskii-Moriya interaction (DMI), is significantly stronger than asymmetries resulting from surface anisotropies for such modes. Results are consistent with an interfacial DMI constant ${D}_{\mathrm{s}}=\ensuremath{-}1.7\ifmmode\pm\else\textpm\fi{}0.11\phantom{\rule{0.28em}{0ex}}\text{pJ}$/m, which favors left-handed chiral spin structures. This suggests that such films below 1 nm in thickness should support chiral states such as skyrmions at room temperature.

In the field of neutron scattering, the need for micro-sized (1-50 μm) thermal or cold neutron beams has recently appeared, typically in the field of neutron imaging to probe samples with a high spatial resolution. We discuss various possibilities of producing such micro-sized neutron beams. The advantages and drawbacks of the different techniques are discussed. We show that reflective optics offers the most flexible way of producing tiny neutron beams together with an enhanced signal to background ratio. The use of such micro beams is illustrated by the study of micrometric diameter magnetic wires., This research project has been supported by the RTRA Triangle de la Physique (project IMAMINE 2010-090 T) and by the European Commission under the 7th Framework Program through the ‘Research Infrastructures’ action of the ‘Capacities’ Program, NMI3-II Grant number 283883 on Neutron Imaging.

We propose a short overview of a few selected issues of magnetism in reduced dimensions, which are the most relevant to set the background for more specialized contributions to the present Special Issue. Magnetic anisotropy in reduced dimensions is discussed, on a theoretical basis, then with experimental reports and views from surface to single-atom anisotropy. Then conventional magnetization states are reviewed, including macrospins, single domains, multidomains, and domain walls in stripes. Dipolar coupling is examined for lateral interactions in arrays, and for interlayer interactions in films and dots. Finally thermally-assisted magnetization reversal and superparamagnetism are presented. For each topic we sought a balance between well established knowledge and recent developments., 13 pages. Part of a Special Issue of the C. R. Physique devoted to spinelectronics (2005)

The wall structure phase diagram in nano-strips is established by numerical calculations, exhibiting a hitherto unknown wall type, the asymmetric transverse wall. The diagram of the wall-width parameter is obtained both from a one-dimensional fit of the wall structure and from the domain wall motion velocity under field, the latter being more relevant to experiments. These two estimates show some differences, which are understood using the definition of the effective wall width proposed by Thiele.

The requirements for imaging magnetic vortices properly by magnetic force microscopy are discussed. Special attention is devoted to the influence of the tip on the sample magnetic configuration. It is shown that one can obtain a quantitative understanding of the experiments using micromagnetic simulations within a general framework for reversible tip-induced perturbations. Finally, preliminary results obtained with specially prepared 'nanowire tips' are presented.

Magnetic force microscopy (MFM) studies concerning electrodeposited nanowires are reviewed. It is shown that Co nanowires with 35 nm diameter can exhibit single domain states, opening the way to their potential use as MFM probes. Besides, special attention is devoted to the MFM imaging process of soft magnetic samples. Different observations proving the tip influence when analyzing patterned permalloy elements are presented. Experimental results and numerical simulations using a general framework for reversible perturbations are shown to agree quantitatively.

We create mesoscopic point and line defects by scanning probe lithography to control the magnetization reversal process in Pt/Co/Pt ultra thin film devices. The domain wall propagation near the defects is studied by the Kerr microscopy and extraordinary Hall effect measurements. The observed domain wall pinning is used to block and channel the domain expansion and to create artificial domain patterns.

The nucleation of reversed magnetic domains in Pt/Co/AlO(x) microstructures with perpendicular anisotropy was studied experimentally in the presence of an in-plane magnetic field. For large enough in-plane field, nucleation was observed preferentially at an edge of the sample normal to this field. The position at which nucleation takes place was observed to depend in a chiral way on the initial magnetization and applied field directions. A quantitative explanation of these results is proposed, based on the existence of a sizable Dzyaloshinskii-Moriya interaction in this sample. Another consequence of this interaction is that the energy of domain walls can become negative for in-plane fields smaller than the effective anisotropy field.

The present manuscript represents the third part of a series of studies about a continuous micromagnetic model for amorphous microwires with non-uniform magnetic structure (Torrejon et al., J. Magn. Magn. Mater. 323 (2011) 283; Torrejon et al., J. Magn. Magn. Mater. 333 (2013) 144). Here we compare the predictions of this model with experiments, and show the validity of this approach when a uniform magnetic structure in the microwire cannot be considered. The analyzed microwires exhibit ultrasoft magnetic behaviour and negative magnetostriction, with a non-uniform magnetic structure composed of an axially magnetized inner core exchange-coupled with a circumferentially magnetized outer shell. The static properties were obtained by magnetometry. The high frequency response, axial permeability, was measured from a conventional single coil permeameter connected to a network analyzer. The microwave response is strongly affected by skin effect, which therefore needs to be taken into account for comparison with theory. The validity of the continuous model is proved through the experimental dependence of the permeability on axial static field. Finally, the efficient dynamic magnetization is evaluated from the imaginary component of permeability. © 2014 Elsevier B.V.

The recent observation of current-induced domain wall (DW) motion with large velocity in ultrathin magnetic wires has opened new opportunities for spintronic devices. However, there is still no consensus on the underlying mechanisms of DW motion. Key to this debate is the DW structure, which can be of Bloch or N\'eel type, and dramatically affects the efficiency of the different proposed mechanisms. To date, most experiments aiming to address this question have relied on deducing the DW structure and chirality from its motion under additional in-plane applied fields, which is indirect and involves strong assumptions on its dynamics. Here we introduce a general method enabling direct, in situ, determination of the DW structure in ultrathin ferromagnets. It relies on local measurements of the stray field distribution above the DW using a scanning nanomagnetometer based on the Nitrogen-Vacancy defect in diamond. We first apply the method to a Ta/Co40Fe40B20(1 nm)/MgO magnetic wire and find clear signature of pure Bloch DWs. In contrast, we observe left-handed N\'eel DWs in a Pt/Co(0.6 nm)/AlOx wire, providing direct evidence for the presence of a sizable Dzyaloshinskii-Moriya interaction (DMI) at the Pt/Co interface. This method offers a new path for exploring interfacial DMI in ultrathin ferromagnets and elucidating the physics of DW motion under current., Comment: Main text and Supplementary Information, 33 pages and 12 figures

The magnetization dynamics is studied on 3D micromagnetic structures and in the aperiodic time regime. Experiments are compared with various micromagnetic simulations. The model system chosen is a vertical Bloch line inside a bubble garnet epitaxial film. It is shown that the Slonczewski equations describe with remarkable accuracy the experimental phenomena, qualitatively and quantitatively. The damping constant derived from the adjustment of all data is distinctly higher than the value derived from ferromagnetic resonance. Models for this discrepancy are discussed.

Micromagnetic simulations have been performed in order to obtain deeper insight into the domain structures within multilayer films, as they are expected to differ from those of single films. These 2D calculations have been done in the case of multilayers exhibiting a moderate perpendicular anisotropy, with no indirect exchange coupling between the magnetic layers, where a “weak stripe" domain structure develops. First, these results are compared quantitatively to the very detailed experimental data available in the literature on the (Co/Au)N system. More generally, the nucleation of a stripe pattern in multilayers is discussed as a function of the magnetic parameters and the number of magnetic layers in the stack. Compared to a single film, two main differences appear in the equilibrium domain period and the magnetization profiles. The physical origin of these effects is discussed.

The propagation of a single magnetic Bloch domain wall (DW) in a mesoscopic wire is studied by use of the extraordinary Hall effect. Whereas a straight DW propagates freely in a wire with constant lateral width, the propagation in a Hall cross reveals that both DW shape and DW velocity vary strongly as a function of DW position. These results give the first experimental evidence that the dynamical properties of a magnetic DW in a patterned structure are strongly affected by the device geometry. This has to be taken into account for future applications to spin electronic devices.

We report on current-induced domain-wall propagation in a patterned GaMnAs microwire with perpendicular magnetization. A slowing down of the propagation velocity has been found when the moving domain wall extends over only half of the width of the wire. This slowing down is related to the elongation of a longitudinal wall along the axis of the wire. By using an energy balance argument, the expected theoretical dependence of the velocity change has been calculated and compared with the experimental results. According to this, the energy associated with the longitudinal domain wall is found to change when a current passes through the wire. These results provide possible evidence of transverse spin effects when a current flows parallel to a magnetic domain wall.

Thin-film ferromagnetic disks present a vortex spin structure whose dynamics, added to the small size (~10 nm) of their core, earned them intensive study. Here we use a scanning nitrogen-vacancy (NV) center microscope to quantitatively map the stray magnetic field above a 1 micron-diameter disk of permalloy, unambiguously revealing the vortex core. Analysis of both probe-to-sample distance and tip motion effects through stroboscopic measurements, allows us to compare directly our quantitative images to micromagnetic simulations of an ideal structure. Slight perturbations with respect to the perfect vortex structure are clearly detected either due to an applied in-plane magnetic field or imperfections of the magnetic structures. This work demonstrates the potential of scanning NV microscopy to map tiny stray field variations from nanostructures, providing a nanoscale, non-perturbative detection of their magnetic texture., 5 pages, 4 figures

We propose a geometrical method to explore the problem of coherent magnetization rotation, for an arbitrary anisotropy and in three dimensions. This method is a nontrivial generalization of the astroid construction which is well known in two dimensions. Specific features to the three-dimensional (3D) problem are highlighted. In order to establish a connection with the thermal and quantum theories of magnetization relaxation, the local curvatures of the potential are also evaluated geometrically. As an application of the method to a real 3D problem, the determination of the effective anisotropy constants from 3D switching field measurements is discussed and first results shown.