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In this article, we present a numerical study on stabilization and eigenmodes of the so-called skyrmion chiral spin texture in nanometric dots. The first aim of this study is to identify the appropriate multilayer in a set of Pt/Co/MgO structures with different Co thicknesses that have been previously experimentally characterized. Stabilization occurs if the energy favoring skyrmions is greater than the geometric mean of the exchange and anisotropy energies. Both the energy favoring skyrmions and the anisotropy contribution depend on the Co thickness. The appropriate multilayer is obtained for a specific Co thickness. MuMax simulations are used to calculate the precise static magnetization configuration for the experimental parameters, allowing us select the appropriate structure. Moreover, in view of experimental study of skyrmion dynamics by means of Brillouin light scattering, the eigenfrequency, eigenmode profile, and spectral density are calculated for different dot sizes. Finally, the optimal dot size that allows for a feasible experiment is obtained.

Vibrating sample magnetometery (VSM) combined with Brillouin light scattering (BLS) were used to investigate the Dzyaloshinskii-Moriya interaction (iDMI), the perpendicular magnetic anisotropy (PMA) and the damping in ${\mathrm{Co}}_{0.5}{\mathrm{Fe}}_{0.5}$-based systems grown by sputtering on $\mathrm{Si}/{\mathrm{SiO}}_{2}$ substrates, using Pt or Cu buffer layers and Hf capping layer of variable thickness $(0\ensuremath{\le}{t}_{\mathrm{Hf}}\ensuremath{\le}2\phantom{\rule{0.28em}{0ex}}\mathrm{nm})$. VSM measurements revealed a significant decrease of the areal magnetic moment at saturation as ${t}_{\mathrm{Hf}}$ increases, most likely due to the increase of the magnetic dead layer thickness at CoFe/Hf interface up to $0.73\ifmmode\pm\else\textpm\fi{}0.06\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$ for ${t}_{\mathrm{Hf}}=2\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$. Damping measurements allowed concluding to the weaker spin pumping contribution by the CoFe/Hf interface compared to Pt/CoFe. The analysis of the ${t}_{\mathrm{Hf}}$ dependence of damping within a model considering the contribution of each interface allowed us to determine the spin diffusion length of Hf, found to be 1.7 nm. The Hf thickness dependence of iDMI and PMA constants revealed significant (weak) contribution of the CoFe/Hf interface to PMA (iDMI). The surface iDMI constant of CoFe/Hf interface was estimated to be $(\ensuremath{-}0.37\ifmmode\pm\else\textpm\fi{}0.12)\ifmmode\times\else\texttimes\fi{}{10}^{--7}\phantom{\rule{0.28em}{0ex}}\mathrm{erg}/\mathrm{cm}$. This suggests that the iDMI sign of Hf and Pt are opposite, and an additive behavior can be obtained when they are placed in opposite sides of the CoFe layer, allowing thus to strengthen the iDMI, needed for some applications. Quadratic correlations between PMA and iDMI constants and linear dependence of damping versus PMA constant were obtained confirming their relationship with the spin-orbit coupling.

We present experimental and numerical results on the magnetization reversal induced by spin–orbit torques of micronic disks of a ferromagnetic multilayer with perpendicular magnetic anisotropy on top of a Pt track: Pt (6 nm)/[Co(0.2 nm)/Ni(0.6 nm)] × 5/Al(5 nm). The current induced magnetization switching process is probed by anomalous Hall effect measurements and Kerr microscopy. The electrical characterization reveals the critical current for the complete reversal to be about 3×1011 A/m2, and Kerr microscopy uncovers a deterministic nucleation that depends on current and field polarity. Through the use of experimental switching phase diagrams coupled to micromagnetic simulations, we evaluated the field-like to damping-like torque ratio to be 0.73 ± 0.05, which is in good agreement with experimental values observed by second harmonic measurements. These measurements emphasize an unexpectedly large field-like contribution in this relatively thick Co/Ni multilayer (4 nm). In light of these experiments and simulations, we discuss the key parameters needed to understand the magnetization reversal, namely, the field and damping-like torques and the Dzyaloshinskii–Moriya interaction.

In ${\mathrm{Ta/(Co,Fe)B/HfO}}_{2}$ stacks, a gate voltage drives, in a nonvolatile way, the system from an underoxidized state exhibiting in-plane anisotropy (IPA) to an optimum oxidation level resulting in perpendicular anisotropy (PMA) and further into an overoxidized state with IPA. The $\mathrm{IPA}\phantom{\rule{0.1em}{0ex}}\ensuremath{\rightarrow}\phantom{\rule{0.1em}{0ex}}\mathrm{PMA}$ regime is found to be significantly faster than the $\mathrm{PMA}\phantom{\rule{0.1em}{0ex}}\ensuremath{\rightarrow}\phantom{\rule{0.1em}{0ex}}\mathrm{IPA}$ regime, whereas only the latter shows full reversibility under the same gate voltages. The effective damping parameter also shows a marked dependence with gate voltage in the $\mathrm{IPA}\phantom{\rule{0.1em}{0ex}}\ensuremath{\rightarrow}\phantom{\rule{0.1em}{0ex}}\mathrm{PMA}$ regime, going from 0.029 to 0.012, and only a modest increase to 0.014 in the $\mathrm{PMA}\phantom{\rule{0.1em}{0ex}}\ensuremath{\rightarrow}\phantom{\rule{0.1em}{0ex}}\mathrm{IPA}$ regime. The existence of two magnetoionic regimes has been linked to a difference in the chemical environment of the anchoring points of oxygen species added to underoxidized or overoxidized layers. Our results show that multiple magnetoionic regimes can exist in a single device and that their characterization is of great importance for the design of high-performance spintronics devices.

Magnetic skyrmions are localized chiral spin textures, which offer great promise to store and process information at the nanoscale. In the presence of asymmetric exchange interactions, their chirality, which governs their dynamics, is generally considered as an intrinsic parameter set during the sample deposition. In this work, we experimentally demonstrate that a gate voltage can control this key parameter. We probe the chirality of skyrmions and chiral domain walls by observing the direction of their current-induced motion and show that a gate voltage can reverse it. This local and dynamical reversal of the chirality is due to a sign inversion of the interfacial Dzyaloshinskii-Moriya interaction that we attribute to ionic migration of oxygen under gate voltage. Micromagnetic simulations show that the chirality reversal is a continuous transformation, in which the skyrmion is conserved. This control of chirality with 2 - 3 V gate voltage can be used for skyrmion-based logic devices, yielding new functionalities., Comment: 4 figures

International audience; Magnetic skyrmions are deemed to be the forerunners of novel spintronic memory and logic devices. While their observation and their current-driven motion at room temperature have been demonstrated, certain issues regarding their nucleation, stability, pinning and skyrmion Hall effect still need to be overcome to realise functional devices. Here we demonstrate that focused He+-ion-irradiation can be used to create and guide skyrmions in racetracks. We show that the weakening of the perpendicular magnetic anisotropy and Dzyaloshinskii-Moriya interaction in the track defined by ion-irradiation leads to the formation of stable isolated skyrmions. Current-driven skyrmion motion experiments and simulations reveal that the skyrmions move along the irradiated track, resulting in a suppression of the skyrmion Hall effect, and that the skyrmion velocity and confinement can be enhanced by tuning the magnetic properties. These results open up a new path to nucleate and guide skyrmions in racetrack devices.

International audience; Perpendicular magnetic anisotropy (PMA), spin pumping induced damping and interfacial Dzyaloshinskii-Moriya interaction (iDMI), which are spin-orbit coupling-related phenomena of utmost importance for applications, were experimentally investigated in as grown and 225°C annealed CoFeB/PtOx, CoFeB/TaOx and Ta/CoFeB/TaOx systems by means of vibrating sample magnetometry, microstrip ferromagnetic resonance and Brillouin light scattering techniques. By varying Co8Fe72B20 (CoFeB) thickness in the range 0.8-10 nm, the effect of Ta buffer layer on anisotropy and damping was first studied, where a large surface magnetic anisotropy (Ks=2.1±0.16 erg/cm2) was measured in the unbuffered CoFeB/TaOx(0.8nm) system most likely due to their higher roughness induced by the substrate. Ks degrades significantly for CoFeB film thickness below 2 nm where spontaneous perpendicular magnetization was found to be impossible without Ta buffer layer. PMA, iDMI and damping of as-deposited and 225°C annealed CoFeB(1.5 nm)/PtOx systems were measured as a function of PtOx thickness in the range 0.7-1.6 nm. Their strong dependence versus the PtOx thickness was attributed to the decrease of the magnetic dead layer as PtOx thickness increases. Linear dependence of damping versus PMA constant was obtained confirming their relation with the spin orbit coupling. Moreover, annealing increases PMA and the effective mixing conductance probably due to the enhancement of the CoFeB crystal structure and interfaces.

We report on propagating spin-wave spectroscopy measurements carried out on coplanar nano-antenna devices made from a Si/SiO$_2$/Ru(5nm)/Co(20)/Pt(5nm) film. The measurements were analyzed in detail by employing newly developed theoretical modeling and de-embedding procedures. The magnetic parameters of the film were determined by complementary Brillouin light scattering and ferromagnetic resonance measurements. The propagating spin wave signals could be accounted for quantitatively for the range of externally applied magnetic fields investigated in this study: 130-1500 Oe.

We have studied the magnetism of a cobalt ultrathin film deposited on different two-dimensional (2D) materials, namely graphene, h-BN, and ${\mathrm{WSe}}_{2}$ by the Brillouin light scattering technique. The studied samples are prepared by a pick-up method of large flakes deposited on ${\mathrm{SiO}}_{2}$ and the subsequent physical vapor deposition of metal layers, in a similar way to what is done to make spintronic devices out of such materials. Compared to the reference layer $(\mathrm{Co}/{\mathrm{SiO}}_{2})$, the perpendicular magnetic anisotropy is enhanced in the Co/2D systems, although less than what could be expected on single crystal samples. This result is quantitatively discussed by comparison with ab initio calculations in the case of the Co/graphene interface. We also measure an increase of the magnetic damping and a small Dzyaloshinskii-Moriya interaction in such samples which are discussed with respect to the recent literature.

Microstrip line ferromagnetic resonance (MS-FMR) and Brillouin light scattering (BLS) in the Damon-Eshbach geometry were used to investigate the perpendicular magnetic anisotropy (PMA), the damping and the interfacial Dzyaloshinskii-Moriya (iDMI) interaction in ferromagnetic $(\mathrm{FM})/\mathrm{Ta}{\mathrm{O}}_{x}\text{\ensuremath{-}}\mathrm{based}$ systems as a function of the ferromagnetic ($\mathrm{FM}=\mathrm{Co}$ or ${\mathrm{Co}}_{8}{\mathrm{Fe}}_{72}{\mathrm{B}}_{20}$) and the $\mathrm{Ta}{\mathrm{O}}_{x}$ thicknesses (oxidation level). The analysis of the experimental FMR and BLS data has shown that the effective magnetization, the Gilbert damping parameter \ensuremath{\alpha} and the iDMI are inversely proportional to the CoFeB and the Co films thickness. The BLS investigation of the iDMI variation versus the $\mathrm{Ta}{\mathrm{O}}_{x}$ thickness and oxidation level reveals a contribution of $\mathrm{FM}/\mathrm{Ta}{\mathrm{O}}_{x}$ mediated by the presence of a Rashba field at this interface. Finally, we evidenced a correlation between iDMI and PMA by varying the Cu spacer thickness in the $\mathrm{Pt}/\mathrm{Cu}/\mathrm{Co}/\mathrm{Ta}{\mathrm{O}}_{x}$ system and we showed that both PMA and iDMI are localized at the first atomic monolayers of the Pt/Co interface. The observed non-linear dependence of PMA versus iDMI constant is attributed to similar interface orbital hybridizations involved in both quantities.

The perpendicular magnetic anisotropy (PMA) and the interfacial Dzyaloshinskii-Moriya interaction (iDMI) are investigated in as grown and 300 °C annealed Co-based ultrathin systems. For this, Co films of various thicknesses (0.8 nm ⩽ t Co ⩽ 5.7 nm) were deposited by magnetron sputtering on thermally oxidized Si substrates using Pt, W, Ir, Ti, Ru and MgO buffer or/and capping layers. X-ray diffraction was used to investigate their structural properties and vibrating sample magnetometry (VSM) was used to determine the magnetic dead layer thickness and the magnetization at saturation (M s). VSM revealed that the M s for the Pt and the Ir buffered and capped films is the largest. Microstrip line ferromagnetic resonance (MS-FMR), used to extract the gyromagnetic ratio of the thicker Co films, revealed the existence of a second order PMA term, which is thickness dependent. Brillouin light scattering (BLS) in the Damon-Eshbach configuration was used to investigate the thickness dependence of the iDMI effective constant from the spin wave vector dependence of the frequency difference between Stokes and anti-Stokes lines. BLS and MS-FMR techniques were combined to measure the spin wave frequency variation as a function of the in-plane applied magnetic field (where the second order PMA contribution vanishes). The thickness dependence of the effective magnetization was then deduced and used to investigate PMA. For all the systems, PMA results from interface and volume contributions that we determined. The largest interface PMA constants were obtained for Pt- and Ir-based systems due to the electron hybridization of Co with these heavy metals having high spin orbit coupling. Annealing at 300 °C increases both the interface PMA and iDMI for the Pt/Co/MgO most probably due to de-mixing of interpenetrating oxygen atoms from the Co layer and the formation of a sharp Co/O interface.

Tuning the Dzyaloshinskii-Moriya interaction (DMI) using electric (E)-fields in magnetic devices has opened up new perspectives for controlling the stabilization of chiral spin structures. Recent efforts have used voltage-induced charge redistribution at magnetic/oxides interfaces to modulate the DMI. This approach is attractive for active devices but tends to be volatile, making it energy-demanding. Here we demonstrate nonvolatile E-field manipulation of the DMI by ionic-liquid gating of Pt/Co/HfO2 ultra thin films. The E-field effect on the DMI is linked to the migration of oxygen species from the HfO2 layer into the Co and Pt layers and subsequent anchoring. This effect permanently changes the properties of the material, showing that E-fields can be used not only for local gating in devices but also as a material design tool for post growth tuning of the DMI.

International audience

The manipulation of magnetization with interfacial modification using various spin-orbit coupling phenomena has been recently revisited due to its scientific and technological potential for next-generation memory devices. Herein, we experimentally and theoretically demonstrate the interfacial Dzyaloshinskii–Moriya interaction characteristics penetrating through a MgO dielectric layer inserted between the Pt and CoFeSiB. The inserted MgO layer seems to function as a chiral exchange interaction mediator of the interfacial Dzyaloshinskii–Moriya interaction from the heavy metal atoms to ferromagnet ones. The potential physical mechanism of the anti-symmetric exchange is based on the tunneling-like behavior of conduction electrons through the semi-conductor-like ultrathin MgO. Such behavior can be correlated with the oscillations of the indirect exchange coupling of the Ruderman–Kittel–Kasuya–Yosida type. From the theoretical demonstration, we could provide approximate estimation and show qualitative trends peculiar to the system under investigation., The mechanism of the interfacial Dzyaloshinskii-Moriya interaction in heavy metal-ferromagnet heterostructures is debated. Here, the authors show the oscillating behaviour of the interaction as a function of the MgO spacer layer thickness, supporting the interlayer exchange coupling mechanism of the Ruderman-Kittel-Kasuya-Yosida type.

Assemblies of vertical multiwall carbon nanotubes, (VCNTs), have been synthesized by coupling dewetting of cobalt or nickel ultrathin layers and plasma enhanced chemical vapor deposition. Electronic microscopies revealed well defined micrometer length nanotubes with inner radius of 3-4 nm and outer radius of 8-9 nm. Similar structural qualities have been revealed by Raman measurements. Dynamic behaviour of these VCNTs assemblies have been studied by means of Brillouin light scattering technique. The measured inelastic light scattering from VCNTs is attributed to bending vibrations of the nanotubes. The observed frequencies on both assemblies, considered as dense effective media, are compatible with an effective Young modulus of 850 GPa.

We report the results of the experimental investigation of magnonic Bloch modes in the presence of asymmetric dispersion induced by the interfacial Dzyaloshinskii-Moriya interaction by means of Brillouin light-scattering technique. It was realized in a specially designed ultrathin CoFeB/Pt periodic structure consisting of an array of rectangular nanostrips separated by half-etched grooves. The proposed theory based on the coupled mode approach explains the major features observed experimentally, such as Brillouin zone folding and the asymmetry of the magnonic band-gap points in the reciprocal $k$ space.

Electric control of magnetism is a prerequisite for efficient and low power spintronic devices. More specifically, in heavy metal/ ferromagnet/ insulator heterostructures, voltage gating has been shown to locally and dynamically tune magnetic properties like interface anisotropy and saturation magnetization. However, its effect on interfacial Dzyaloshinskii-Moriya Interaction (DMI), which is crucial for the stability of magnetic skyrmions, has been challenging to achieve and has not been reported yet for ultrathin films. Here, we demonstrate 130% variation of DMI with electric field in Ta/FeCoB/TaOx trilayers through Brillouin Light Spectroscopy (BLS). Using polar- Magneto-Optical-Kerr-Effect microscopy, we further show a monotonic variation of DMI and skyrmionic bubble size with electric field, with an unprecedented efficiency. We anticipate through our observations that a sign reversal of DMI with electric field is possible, leading to a chirality switch. This dynamic manipulation of DMI establishes an additional degree of control to engineer programmable skyrmion based memory or logic devices., Comment: 17 pages, 4 figures

Morphological, magnetic and elastic properties of 5 nm-thick Co 49 Pt 51 films, sputtered on glass substrates, with 20 nm-thick Ta (seed) and Pt (buffer) layers were studied as function of the deposition temperature T d ranging between room temperature and 350° C. Atomic and magnetic force microscopy, vibrating sample magnetometer and Brillouin light scattering techniques were used to investigate the root mean square (RMS) roughness, the magnetic domain configuration, the coercive field (H c ), the perpendicular magnetic anisotropy (PMA), and the dynamic magnetic and elastic properties of the films with T d . The results show that surface uniformity was enhanced since the RMS roughness decreases with T d while magnetic domains typical of films with high PMA are observed. H c and PMA are found to sensibly increase with T d . The dynamic magnetization behavior is characterized by magnetic modes related with the co-existence of hard and soft magnetic areas within the samples. The elastic properties of the stack were first analyzed by means of a model describing the main variation of the elastic wave frequencies within the frame of weighted average thickness, density, Young’s modulus and Poisson coefficient of all the layers constituting the stacks. However, while H c and PMA keep increasing with T d , a more precise experimental analysis of the mechanical behavior shows that the group velocity starts increasing and finally decreases with T d , suggesting that knowledge of the influence of T d on the mechanical properties of each individual layer composing the stack is required to obtain a more accurate analysis.

We performed numerical simulations to determine the effect of the most influential operating parameters on the performance of a radio frequency (RF) induction-heating system in which RF magnetic fields inductively heat metal foils to grow graphene. The thermal efficiency of the system depends on the geometry as well as on the materials’ electrical conductivity and skin depth. The process is simulated under specific graphene and two-dimensional (2D) materials growth conditions using finite elements software in order to predict the transient temperature and magnetic field distribution during standard graphene and 2D materials growth conditions. The proposed model considers different coil Helmholtz-like geometries and 11 metal foils, including Ag, Au, Cu, Ni, Co, Pd, Pt, Rh, Ir, Mo, and W. In each case, an optimal window of process variables ensuring a temperature range of 1035 °C–1084 °C or 700 °C–750 °C suitable for graphene and MoS2 growth, respectively, was found. Temperature gradients calculated from the simulated profiles between the edge and the center of the substrate showed a thermal uniformity of less than ∼2% for coinage metals like Au, Ag, and Cu and up to 7% for Pd. Model validation was performed for graphene growth on copper. Due to its limited heat conductivity, good heating uniformity was obtained. As a consequence, full coverage of monolayer graphene on copper with few defects and a grain domain size of ∼2 µm was obtained. The substrate temperature reached ∼1035 °C from ambient after only ∼90 s, in excellent agreement with model predictions. This allows for improved process efficiency in terms of fast, localized, homogeneous, and precise heating with energy saving. Due to these advantages, inductive heating has great potential for large-scale and rapid manufacturing of graphene and 2D materials.

Brillouin light scattering (BLS) combined with vibrating sample magnetometry (VSM) have been used to investigate the Dzyaloshinskii–Moriya interaction (DMI) in Pt-buffered Co2FeAl0.5Si0.5 ultrathin films of various thicknesses. VSM measurements of the Co2FeAl0.5Si0.5 (CFAS) thickness dependence of the saturation magnetic moment per unit area revealed a magnetization at saturation of 1286 emu/cm3 and 0.31 nm magnetic dead layer. Furthermore, thickest films ( $t_{{\text {CFAS}}} >1$ nm) are in-plane magnetized, while the thinner ones are perpendicular magnetized. BLS measurements, in the Damon–Eshbach geometry, under an in-plane applied magnetic field revealed the non-reciprocity of the spin waves (SWs) propagating in opposite directions (Stokes and anti-Stokes lines) due to the Pt-induced DMI. Stokes and anti-Stokes lines’ frequency mismatch varies linearly as function of the SW vector allowing the determination of the effective DMI constant. Its thickness dependence leads to determine a value of −0.42 pJ/m for the DMI interface constant, which is significantly lower than that of Pt/Co/AlO x films. Moreover, BLS measurements revealed that the effective magnetization varies linearly with the reciprocal effective CFAS thickness due to the perpendicular interface anisotropy, estimated to be 0.49 mJ/m2, which reinforces the perpendicular magnetization easy axis.

We study the optical and magneto-optical properties of low-concentration arrays of ultrathin nickel nanowires by means of Brillouin spectroscopy of thermal magnons. Brillouin spectroscopy in such quasitransparent magneto-optical structures is dominated by the Bragg phase synchronism mechanism. At variance with ultrathin cobalt nanowire arrays, the Stokes/anti-Stokes scattering pattern is practically symmetrical. This feature is attributed to their peculiar optical properties.

Biodegradable poly(ester-urethane)-based scaffolds with an elastomeric character offer special mechanical properties by reducing the mismatch of Young’s modulus between rigid thermoplastic scaffolds classically used, and soft and dynamic tissues to regenerate (skin, tendons, muscles). In this study, porous scaffolds had been elaborated by using a high internal phase emulsion process with variable pores size (150–1800 μm), a porosity of 85% and sufficient strength making them suitable for cell culture, tissue formation and therefore various tissue engineering applications. The scaffold mechanical properties were evaluated at different frequencies. Analysis by conventional quasi-static mechanical tests (0.1 Hz) demonstrated that the scaffolds exhibited an elastomeric character with an effective Young modulus of 165 kPa. In addition, the determination of dynamical viscoelastic parameters was assessed at higher frequencies (MHz–GHz) by the ultrasonic pulse echo method (10 MHz) and micro-Brillouin inelastic light scattering (13 GHz) analysis. Our measurements revealed a strong increased of the Young modulus with increasing frequency and a linear correlation (log-log scale) between stiffer hypersonic (MHz–GHz) modulus and softer low-frequency one. The slope α = 0.194 is characterizing the viscoelastic behavior of this polymer. In addition, micro-Raman was coupled to the micro-Brillouin to determine at the same location the chemical properties of the polymer.

We study the influence of He+ irradiation induced interface intermixing on magnetic domain wall (DW) dynamics in W-CoFeB (0.6 nm)-MgO ultrathin films, which exhibit high perpendicular magnetic anisotropy and large Dzyaloshinskii-Moriya interaction (DMI) values. Whereas the pristine films exhibit strong DW pinning, we observe a large increase in the DW velocity in the creep regime upon He+ irradiation, which is attributed to the reduction of pinning centers induced by interface intermixing. Asymmetric in-plane field-driven domain expansion experiments show that the DMI value is slightly reduced upon irradiation, and a direct relationship between DMI and interface anisotropy is demonstrated. Our findings provide insights into the material design and interface control for DW dynamics, as well as for DMI, enabling the development of high-performance spintronic devices based on ultrathin magnetic layers.

International audience; The possibility to tune the Dzyaloshinskii Moriya interaction (DMI) by electric (E) field gating in ultra-thin magnetic materials has opened new perspectives in terms of controlling the stabilization of chiral spin structures. Most recent efforts have used voltage-induced charge redistribution at the interface between a metal and an oxide to modulate DMI. This approach is attractive for active devices but it tends to be volatile, making it energy demanding, and it is limited by Coulomb screening in the metal. Here we have demonstrated the non-volatile E-field manipulation of DMI by ionic liquid gating of Pt/Co/HfO2 ultra-thin films. The E-field effect on DMI scales with the E-field exposure time and is proposed to be linked to the migration and subsequent anchoring of oxygen species from the HfO2 layer into the Co and Pt layers. This effect permanently changes the properties of the material showing that E-fields can not only be used for local gating in devices but also as a highly scalable materials design tool for post-growth tuning of DMI.

An assembly of randomly organized cobalt nanoparticles were obtained by solid-state dewetting of a 3 nm-thick cobalt layer. Vibrating sample magnetometry and Brillouin light scattering techniques were used to investigate both their static and dynamic behaviors with respect to the initial native cobalt layer. The measurements obtained from the assembly of the obtained nanodots were analyzed by means of shape anisotropy contribution. The Brillouin spectra revealed an unusual reversed Stokes/anti-Stokes line height asymmetry comparing to that observed on the native layer. The effective optical properties of the nanodots, combined with the relation between mean field and inside-dot field allowed explaining the observed reversed height asymmetry. The assembly of nanodots behave as an effective magnetic and optical medium where these properties can be tuned by the elaboration process.

Bimetallic one-dimensional (1-D) cobalt-palladium magnetic nanowires encapsuled by carbon nanotubes were synthesized on silicon substrate using plasma enhanced chemical vapor deposition technique. After the deposition of the catalyst, the growth of nanotubes takes place in two stages. The first is a thermal pretreatment to transform continuous nanometer bimetallic thick film into isolated and uniformly distributed nanoparticles over the entire surface of the substrate. The second step results in the growth of nanotubes perpendicular to the substrate by the addition of carbon atoms on the insulated metal nanoparticles. While growing the nanotubes at given thermochemical conditions, a Co-Pd eutectic is thought to diffuse inside the cavity of the nanotube along a length of few hundreds of nanometers as determined by high resolution, spatially resolved Electron Energy Loss Spectroscopy (EELS), and energy filtered elemental mapping. The magnetic anisotropy along the nanotube directions is observed. Ferromagnetic or superparamagnetic-like behavior of the filled nanotubes was measured through local magneto-optical Kerr effect or global superconducting quantum interference device measurements, respectively. Information on the magnetism of filled nanotubes at different scales is pointed out and discussed.

Correlation between interfacial Dzyaloshinskii–Moriya interaction (iDMI), perpendicular magnetic anisotropy (PMA) and spin pumping-induced damping was investigated in CoFeB-based systems grown by sputtering on Si substrates, using Pt, Ta, Cu, W and MgO capping layers. Vibrating sample magnetometer, Brillouin light scattering (BLS) and broadband ferromagnetic resonance techniques were combined for this aim. The CoFeB thickness dependence of iDMI and PMA constants, in CoFeB/X (where X = Pt, Cu/Pt, Ta/Pt or W/Al), revealed that only the CoFeB/Pt system presents a measurable iDMI and that the interfacial PMA is mostly similar except for the Ta/CoFeB/Ta/Pt system. Therefore, no clear correlation between the above-mentioned interfacially-driven and spin-orbit coupling related quantities was observed due to their different origins in our systems. An efficient sample design involving various spacer layers of variable thicknesses in Ta/CoFeB(1.5 nm)/Y/Pt (where Y = Cu, Ta, MgO) allowed evidence of a linear correlation between iDMI, PMA constants and the effective spin mixing conductance. The linear dependence, which could result from the narrow variation range of PMA and/or iDMI, is attributed to the similar interface orbital hybridizations involved in PMA, iDMI and spin pumping-induced damping.

Since metals are high-surface energy materials, it is difficult to stabilize metal nanoparticles on conductive substrates, on the other side, producing nanoparticles on low surface energy oxides is hindered by high temperature activated phenomena such as diffusion on the surface as well as in the bulk. In this work, we present a simple and efficient approach to produce controlled cobalt (Co) magnetic nanoparticles by solid state dewetting of sub 3 nm metal films deposited on conductive and low surface energy titanium silicon nitride (TiSiN) barriers. For this purpose, ultrathin Co films with thickness of 1, 2 and 3 nm were first deposited on the top of the TiSiN layer; then Co films were submitted to different variations of dewetting protocols. Solid state dewetting process will be analyzed from kinetic and thermodynamic points of view. Structural and magnetic characterizations of the patterned nanoparticles showed that the size, the density and the structure of the generated nanoparticles are well controlled using TiSiN barrier and plasma treatment. The magnetic measurements showed relatively high coercivity and confirmed that dewetting parameters can be used to tune the design of nanoparticles and their size distribution on TiSiN substrates, thereby providing a promising tool to control their coercive field.

International audience; The interfacial Dzyaloshinskii-Moriya Interaction (DMI) plays a crucial role in chiral domain wall (DW) motion, favoring fast DW velocities. We explore the effect of interface disorder on DMI and DW dynamics in perpendicular magnetized Ta/CoFeB/MgO thin films. Light He + irradiation has been used to gently engineer interface intermixing on a scale of 0.1 nm. We demonstrate that a slight modification of the Ta/CoFeB interface leads to an increase of the DMI value accompanied by an enhancement of DW velocity in the flow regime. Using micromagnetic simulations based on granular structures, we show that the enhancement of DW velocity is mainly related to an increase in the distribution of magnetic parameters related to the interface. We further infer that the DMI modulation is related to the asymmetric disorder induced by irradiation leading to alloying with the Ta buffer layer. Understanding the role of disorder is therefore crucial for the design of future devices where post-growth interface alloying can be used to finely tune the DMI.

International audience; In tissue engineering, porous biodegradable scaffolds are developed with morphological, chemical and mechanical properties to promote cell response. Therefore, the scaffold characterization at a (sub)micrometer and (bio)molecular level is paramount sincecells are sensitive to the chemical signals, the rigidity, and the spatial structuring of their microenvironment. In addition to the analysis at room temperature by conventional quasi-static (0.1–45 Hz) mechanical tests, the ultrasonic (10 MHz) and μ-Brillouin inelastic light scattering (13 GHz) were used in this study to assess the dynamical viscoelastic parameters at different frequencies of elastomeric scaffolds. Time-temperature superposition principle was used to increase the high frequency interval (100 MHz–100 THz) of Brillouin experiments providing a mean to analyse the viscoelastic behavior with the fractional derivative viscoelastic model. Moreover, the μ-Raman analysis carried out simultaneously during the μ-Brillouin experiment, gave the local chemical composition.

We present a complete calculation of spin waves in magnetic stacks, including the interfacial Dzyaloshinskii-Moriya interaction (iDMI) as a boundary condition, and discuss the influence of interlayer coupling and magnetic anisotropy on the spin-wave nonreciprocity. We show that the usual simple approach relating the iDMI strength to the slope of the spin-wave nonreciprocity versus the spin-wave number, in the case of a thin ferromagnetic film in contact with a heavy metal exhibiting a strong-orbit coupling, is not valid in many more complex structures. In the case of stacks made of identical magnetic layers, an analytical method allowed us to check that the effect of iDMI on the entire structure is captured by including the total thickness of the ferromagnetic layers. The experimental data obtained for some systems by means of Brillouin light scattering are analyzed through the presented models.

The interfacial Dzyaloshinskii-Moriya interaction (IDMI) is investigated in Co2FeAl (CFA) ultrathin films of various thicknesses (0.8 nm ≤ tCFA ≤ 2nm) grown by sputtering on Si substrates, using Pt, W, Ir, and MgO buffer or/and capping layers. Vibrating sample magnetometry reveals that the magnetization at saturation (Ms) for the Pt- and Ir-buffered films is higher than the usual Ms of CFA due to the proximity-induced magnetization (PIM) in Ir and Pt estimated to be 19% and 27%, respectively. The presence of PIM in these materials is confirmed using x-ray resonant magnetic reflectivity. Moreover, while no PIM is induced in W, higher PIM is obtained with Pt when it is used as a buffer layer rather than a capping layer. Brillouin light scattering in the Damon-Eshbach geometry is used to investigate the thickness dependences of the IDMI constants from the spin-wave nonreciprocity and the perpendicular anisotropy field versus the annealing temperature. The IDMI sign is found to be negative for Pt / CFA and Ir / CFA, while it is positive for W / CFA. The thickness dependence of the effective IDMI constant for stacks involving Pt and W shows the existence of two regimes similar to that of the perpendicular anisotropy constant due to the degradation of the interfaces as the CFA thickness approaches a critical thickness. The surface IDMI and anisotropy constants of each stack are determined for the thickest samples where a linear thickness dependence of the effective IDMI constant and the effective magnetization are observed. The interface anisotropy and IDMI constants investigated for the Pt / CFA / MgO system show different trends with the annealing temperature. The decrease of the IDMI constant with increasing annealing temperature is probably due to the electronic structure changes at the interfaces, while the increase of the interface anisotropy constant is coherent with the interface quality and disorder enhancement.

25 nm to 50 nm Co2FeAl (CFA) thick wire arrays with varying widths and spacing have been patterned from continuous CFA films deposited on MgO(001) using e-beam lithography and Ar ion milling. Magneto-optical Kerr effect, transverse bias initial inverse susceptibility and torque measurements reveal that the in-plane magnetic anisotropy of the wires is dominantly monitored by a uniaxial term, in contrast with the continuous films where it is governed by the superposition of a fourfold term and of a smaller uniaxial term. The microstrip ferromagnetic resonance spectra performed using a magnetic field H, applied in the plane of the studied sample along various directions, or perpendicularly to this plane, gave us access to various quantized modes originating from the patterning. In addition, Brillouin light scattering also exhibits quantized modes. A large part of the experimental observations can be quantitatively interpreted as resulting from the demagnetizing terms induced by the geometrical patterning. However, the presented model, simply built on the effect of the demagnetizing field, is not able to give account of all the quantized modes present in the resonance spectra. When H is parallel to the wires, a more complete description is used: it considers the wave-vector quantization induced by the patterning. For the magnetic modes concerned by both approaches, the correspondence between the 2 models is easily established. When H is not parallel to the wires quantitative descriptions of the behavior of the field dependence of the observed modes still can often be performed. Finally, in all the studied patterned samples, the uniform magnetic mode, termed “film mode”, relative to the parent continuous film is observed by ferromagnetic resonance: such a behavior, which has been reported previously, remains to be completely interpreted.

Magnetic skyrmions are chiral spin structures with a whirling configuration. Their topological properties, nanometer size and the fact that they can be moved by small current densities have opened a new paradigm for the manipulation of magnetisation at the nanoscale. To date, chiral skyrmion structures have been experimentally demonstrated only in bulk materials and in epitaxial ultrathin films and under external magnetic field or at low temperature. Here, we report on the observation of stable skyrmions in sputtered ultrathin Pt/Co/MgO nanostructures, at room temperature and zero applied magnetic field. We use high lateral resolution X-ray magnetic circular dichroism microscopy to image their chiral N\'eel internal structure which we explain as due to the large strength of the Dzyaloshinskii-Moriya interaction as revealed by spin wave spectroscopy measurements. Our results are substantiated by micromagnetic simulations and numerical models, which allow the identification of the physical mechanisms governing the size and stability of the skyrmions., Comment: Submitted version. Extended version to appear in Nature Nanotechnology

Co2FeAl (CFA) thin films of various thicknesses (1 nm $\le d \le 10$ nm) have been grown on (001) MgO single-crystal substrates and annealed at different temperatures ( $T_{a}$ ). Their magnetic properties have been studied by vibrating sample magnetometry, broadband microstrip ferromagnetic resonance (MS-FMR), and Brillouin light scattering (BLS). The effective magnetization, deduced from the fit of MS-FMR and BLS measurements, linearly decreases with the inverse thickness ( $1/d$ ) for all the annealing temperatures. This allows quantifying perpendicular surface anisotropy constants of 1.46, 1.71, and 1.29 erg/cm2 for $T_{a} = 215$ °C, 265 °C, and 315 °C, respectively. The perpendicular magnetization is achieved for the CFA films below a critical thickness that increases with increasing $T_{a}$ . Moreover, the fourfold anisotropy fields, measured in the in-plane magnetized films, increase with $T_{a}$ and decrease with respect to the CFA inverse thickness.

The analytical theory of the dipole-exchange spin-wave excitations of ferromagnetic cylinders of elliptical cross section with the bias magnetic field perpendicular to the axis of the cylinder is presented. Explicit analytical expressions for the frequency spectrum and the magnetization spatial distribution are derived. Their evolution as a function of cross-section's aspect ratios has been investigated. The developed formalism allows successful interpretation of experimental Brillouin Light Scattering spectra obtained previously for Ni nano-wires both qualitatively and quantitatively. It also provides a tool for comprehensive analysis of phenomena, in which spatial distribution and polarization of high-frequency magnetization play crucial role.

$\mathrm{C}{\mathrm{o}}_{2}\mathrm{FeAl}$ (CFA) ultrathin films, of various thicknesses $(0.9\phantom{\rule{0.28em}{0ex}}\mathrm{nm}\ensuremath{\le}{t}_{\mathrm{CFA}}\ensuremath{\le}1.8\phantom{\rule{4pt}{0ex}}\mathrm{nm})$, have been grown by sputtering on Si substrates, using Ir as a buffer layer. The magnetic properties of these structures have been studied by vibrating sample magnetometry (VSM), miscrostrip ferromagnetic resonance (MS-FMR), and Brillouin light scattering (BLS) in the Damon-Eshbach geometry. VSM characterizations show that films are mostly in-plane magnetized and the saturating field perpendicular to the film plane increases with decreasing CFA thickness suggesting the existence of a perpendicular interface anisotropy. The presence of a magnetic dead layer of 0.44 nm has been detected by VSM. The MS-FMR with the magnetic field applied perpendicularly to the film plane has been used to determine the gyromagnetic factor. The BLS measurements reveal a pronounced nonreciprocal spin wave propagation, due to the interfacial Dzyaloshinskii-Moriya interaction (DMI) induced by the Ir interface with CFA, which increases with decreasing CFA thickness. The DMI sign has been found to be the same (negative) as that of Pt/Co, in contrast to the ab initio calculation on Ir/Co, where it is found to be positive. The thickness dependence of the effective DMI constant shows the existence of two regimes similarly to that of the perpendicular anisotropy constant. The surface DMI constant ${D}_{s}$ was estimated to be $\ensuremath{-}0.37\phantom{\rule{0.16em}{0ex}}\mathrm{pJ}/\mathrm{m}$ for the thickest samples, where a linear thickness dependence of the effective DMI constant has been observed.

The Dzyaloshinskii-Moriya interaction has been shown to stabilise Nèel domain walls in magnetic thin films, allowing high domain wall velocities driven by spin current effects. The interfacial Dzyaloshinskii-Moriya interaction (IDMI) occurs at the interface between ferromagnetic and heavy metal layers with strong spin-orbit coupling, but details of the interaction remain to be understood and the role of proximity induced magnetism (PIM) in the heavy metal is unknown. Here IDMI and PIM are reported in Pt determined as a function of Au and Ir spacer layers in Pt/Co/Au,Ir/Pt. Both interactions are found to be sensitive to sub-nanometre changes in the spacer thickness, correlating over sub-monolayer spacer thicknesses, but not for thicker spacers where IDMI continues to change even after PIM is lost.

Interface Dzyaloshinskii-Moriya interactions (iDMIs) in interlayer exchange coupled (IEC) Pt/${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$(1.12 nm)/Ru/${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$(1.12 nm) systems have been studied theoretically and experimentally. A vibrating sample magnetometer has been used to measure their magnetization at saturation and their interlayer exchange coupling constants. The latter are found to be of an antiferromagnetic nature for the investigated Ru range thickness ($0.5\text{--}1\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$). Their dynamic magnetic properties were studied using the Brillouin light scattering (BLS) technique. The BLS measurements reveal pronounced nonreciprocal spin-wave propagation. In contrast to the calculations for symmetrical IEC CoFeB layers, this experimental nonreciprocity is Ru thickness and thus coupling strength dependent. Therefore, to explain the experimental behavior, a theoretical model based on the perpendicular interface anisotropy difference between the bottom and top CoFeB layers has been developed. We show that the Ru thickness dependence of the spin-wave nonreciprocity is well reproduced by considering a constant iDMI and different perpendicular interfacial anisotropy fields between the top and bottom CoFeB layers. This anisotropy difference has been confirmed by the investigation of the CoFeB thickness dependence of the effective magnetization of Pt/CoFeB/Ru and Ru/CoFeB/MgO individual layers, where a linear behavior has been observed.

Nature Nanotechnology 11, 449–454 (2016); published online 25 January 2016; corrected after print 18 July 2017 In the version of this Article originally published, the Dzyaloshinskii–Moriya interaction parameter, D, should have been multiplied by √3. The vertical scale of Fig. 2 has been updated accordingly as has the following sentence concerning D values: “For 5 monolayers (ML) of Co, equivalent to a total Co thickness of 1 nm, the ab initio calculations predict = 4.

We have used Brillouin Light Scattering spectroscopy to independently determine the in-plane Magneto-Crystalline Anisotropy and the Dzyaloshinskii-Moriya Interaction (DMI) in out-of-plane magnetized Au/Co/W(110). We found that the DMI strength is 2-3 times larger along the bcc$[\bar{1}10]$ than along the bcc$[001]$ direction. We use analytical considerations to illustrate the relationship between the crystal symmetry of the stack and the anisotropy of microscopic DMI. Such an anisotropic DMI is the first step to realize isolated elliptical skyrmions or anti-skyrmions in thin film systems with $C_{2v}$ symmetry., Revised and extended version