arXiv subjects

S. Collin

Publications and source records attributed to S. Collin.

At least 19 recordsLinked to original sources

arXiv2025

Orbital torque and efficient magnetization switching using ultrathin Co|Al light-metal interfaces: Experiments and modeling

The emergence of the orbital degree of freedom in modern orbitronics offers a promising alternative to heavy metals for the efficient control of magnetization. In this context, identifying interfaces that exhibit orbital-momentum locking and an orbital Rashba-Edelstein response to an external electric field is of primary importance. In this work, we experimentally investigate the Co/Al system and extend the study to Co/Pt/Al structures. We show that inserting ultrathin Pt layers between Co and Al can significantly modify the orbital properties, highlighting the critical role of Co/Al orbital bonding in generating orbital polarization. We further model the orbital response of these systems using semi-phenomenological approaches and linear-response theory within the framework of density-functional theory.

cond-mat.mtrl-sci↗

arXiv2025

Quantitative analysis of vectorial torques in thin 3d Co ferromagnet using orbital-spin conversion

Recent findings in orbitronics pointed out large current-induced torques originating, in the current understanding, from incident orbital currents. These are generated by orbital Rashba-Edelstein effect (OREE) produced at the interface between some light metal and oxides films e.g. by naturally oxidized copper layer (Cu*). In the present work, by using second harmonic Hall techniques, we determine the ratio of orbital vs spin currents exerting torques on thin transition metals Co ferromagnet in systems using an orbit-to-spin Pt converter as interlayer with Cu*. Our results quantifying damping like torques show that both orbital and spin currents are enhanced in these systems. Moreover, the experimental determination of the decoherence length in a sample series with varying Co thickness clearly demonstrates the interfacial generation of the orbital currents in Cu* by Orbital Rashba-Edelstein effects (REE) leading to subsequent magnetic torque in Co over a typical lengthscale of several nanometers

cond-mat.mtrl-sci↗

arXiv2024

Alternative harmonic detection approach for quantitative determination of spin and orbital torques

In this study, the spin-orbit torque (SOT) in light metal oxide systems is investigated using an experimental approach based on harmonic Hall voltage techniques in out-of-plane (OOP) angular geometry for samples with in-plane magnetic anisotropy. In parallel, an analytical derivation of this alternative OOP harmonic Hall detection geometry has been developed, followed by experimental validation to extract SOT effective fields. In addition, to accurately quantifying SOT, this method allows complete characterization of thermoelectric effects, opening promising avenues for accurate SOT characterization in related systems. In particular, this study corroborates the critical role of naturally oxidized copper interfaced with metallic Cu in the generation of orbital current in Co(2)|Pt(4)|CuOx(3), demonstrating a two-fold increase in damping-like torques compared to a reference sample with an oxidized Al capping layer. These findings offer promising directions for future research on the application aspect of non-equilibrium orbital angular momentum.

cond-mat.mtrl-sci↗

arXiv2023

Quantifying the large contribution from orbital Rashba effect to the effective damping-like torque on magnetization

The generation of large spin currents, and the associated spin torques, which are at the heart of modern spintronics, have long been achieved by charge-to-spin conversion mechanisms, i.e. the spin Hall effect and/or the Rashba effect, intrinsically linked to a strong spin-orbit coupling. Recently, a novel path has been predicted and observed for achieving significant current-induced torques originating from light elements, hence possessing a weak spin-orbit interaction. These findings point out to the potential involvement of the orbital counterpart of electrons, namely the orbital Hall and orbital Rashba effects. In this study, we aim at quantifying these orbital-related contributions to the effective torques acting on a thin Co layer in different systems. First, in Pt|Co|Cu|AlOx stacking, we demonstrate a comparable torque strength coming from the conversion due to the orbital Rashba effect at the Cu|AlOx interface and the one from the effective spin Hall effect in bottom Pt|Co system. Secondly, in order to amplify the orbital-to-spin conversion, we investigate the impact of an intermediate Pt layer in Co|Pt|Cu|CuOx. From the Pt thickness dependence of the effective torques determined by harmonic Hall measurements complemented by spin Hall magneto-resistance and THz spectroscopy experiments, we demonstrate that a large orbital Rashba effect is present at the Cu|CuOx interface, leading to a twofold enhancement of the net torques on Co for the optimal Pt thickness. Our findings not only demonstrate the crucial role that orbital currents can play in low-dimensional systems with weak spin-orbit coupling, but also reveal that they enable more energy efficient manipulation of magnetization in spintronic devices.

cond-mat.mtrl-sci↗

arXiv2023

X-ray holography of skyrmionic cocoons in aperiodic magnetic multilayers

The development and characterization of three-dimensional (3D) topological magnetic textures has become an important topic in modern magnetism both for fundamental and technological perspectives. Among the novel 3D spin textures, skyrmionic cocoons have been successfully stabilized in magnetic multilayers having a variable thickness of the ferromagnet in the vertical direction of the stack. These ellipsoidal 3D magnetic textures remain vertically confined in a fraction of the total thickness while coexisting with fully columnar skyrmions. Here, we use X-ray holography with about 15 nm lateral resolution to investigate how their properties depend on the field and temperature. We observe circular objects with different amplitude of contrast which evidences the presence of different 3D objects located in various vertical parts of the multilayer. Moreover, we witness during out-of-plane cycling an attractive interaction between cocoons located at various heights, mainly due to the stray field, which impacts their horizontal positioning. The X-ray holography measurements also allow to determine the size of the cocoons at remanence which, at room temperature, possess diameter close to 100 nm in average. Combining this transmission technique with magnetic force microscopy and micromagnetic simulations gives a precise insight into the 3D distribution of the magnetization which demonstrate the 3D nature of skyrmionic cocoons.

cond-mat.mes-hall↗

arXiv2022

Large interfacial Rashba interaction and giant spin-orbit torques in atomically thin metallic heterostructures

The ability of spin-orbit interactions to convert charge current into spin current, most often in the bulk of heavy metal thin films, has been the hallmark of spintronics in the last decade. In this study, we demonstrate how the insertion of light metal element interface profoundly affects both the nature of spin-orbit torque and its efficiency in terms of damping-like ($H_{\text{DL}}$) and field-like ($H_{\text{FL}}$) effective fields in ultrathin Co ferromagnet. Indeed, we measure unexpectedly large $H_{\text{FL}}$/$H_{\text{DL}}$ ratio ($\sim$2.5) upon inserting a 1.4 nm thin Al layer in Pt|Co|Al|Pt as compared to a similar stacking including Cu instead of Al. From our modelling, these results strongly evidence the presence of large Rashba interaction at Co|Al interface producing a giant $H_{\text{FL}}$, which was not expected from a metallic interface. The occurrence of such enhanced torques from an interfacial origin is further validated by demonstrating current-induced magnetization reversal showing a significant decrease of the critical current for switching.

cond-mat.mes-hall↗

arXiv2022

Spintronic THz emitters based on transition metals and semi-metals/Pt multilayers

Spintronic terahertz (THz) emitters (STE) based on the inverse spin Hall effect in ferromagnetic/heavy metal (FM/HM) heterostructures have become important sources for THz pulse generation. The design, materials and control of these interfaces at the nanometer level has become vital to engineer their THz emission properties.In this work, we present studies of the optimization of such structures through a multi-pronged approach, taking advantage of material and interface engineering to enhance the THz spintronic emission. This includes: the application of multi-stacks of HM/FM junctions and their application to trilayer structures, the use of spin-sinks to simultaneously enhance the THz emitted fields and reduce the use of thick Pt layers to reduce optical absorption, and the use of semi-metals to increase the spin polarization and thus the THz emission. Through these approaches, significant enhancements of the THz field can be achieved. Importantly, taking into account the optical absorption permits to elucidate novel phenomena such as the relation between the spin diffusion length and the spin-sink using THz spectroscopy, as well as possibly distinguishing between self and interface spin-to-charge conversion in semi-metals.

cond-mat.mtrl-sci↗

arXiv2020

Ultrafast spin-currents and charge conversion at 3d-5d interfaces probed by time-domain terahertz spectroscopy

Spintronic structures are extensively investigated for their spin orbit torque properties, required for magnetic commutation functionalities. Current progress in these materials is dependent on the interface engineering for the optimization of spin transmission. Here, we advance the analysis of ultrafast spin-charge conversion phenomena at ferromagnetic-transition metal interfaces due to their inverse spin-Hall effect properties. In particular the intrinsic inverse spin Hall effect of Pt-based systems and extrinsic inverse spin-Hall effect of Au:W and Au:Ta in NiFe/Au:(W,Ta) bilayers are investigated. The spin-charge conversion is probed by complementary techniques -- ultrafast THz time domain spectroscopy in the dynamic regime for THz pulse emission and ferromagnetic resonance spin-pumping measurements in the GHz regime in the steady state -- to determine the role played by the material properties, resistivities, spin transmission at metallic interfaces and spin-flip rates. These measurements show the correspondence between the THz time domain spectroscopy and ferromagnetic spin-pumping for the different set of samples in term of the spin mixing conductance. The latter quantity is a critical parameter, determining the strength of the THz emission from spintronic interfaces. This is further supported by ab-initio calculations, simulations and analysis of the spin-diffusion and spin relaxation of carriers within the multilayers in the time domain, permitting to determine the main trends and the role of spin transmission at interfaces. This work illustrates that time domain spectroscopy for spin-based THz emission is a powerful technique to probe spin-dynamics at active spintronic interfaces and to extract key material properties for spin-charge conversion.

physics.app-ph↗

arXiv2020

Tailored flux pinning in superconductor/ferromagnet multilayers with engineered magnetic domain morphology from stripes to skyrmions

Superconductor/Ferromagnet (S/F) hybrid systems show interesting magneto-transport behaviors that result from the transfer of properties between both constituents. For instance, magnetic memory can be transferred from the F into the S through the pinning of superconducting vortices by the ferromagnetic textures. The ability to tailor this type of induced behavior is important to broaden its range of applications. Here we show that engineering the F magnetization reversal allows tuning the strength of the vortex pinning (and memory) effects, as well as the field range in which they appear. This is done by using magnetic multilayers in which Co thin films are combined with different heavy metals (Ru, Ir, Pt). By choosing the materials, thicknesses, and stacking order of the layers, we can design the characteristic domain size and morphology, from out-of-plane magnetized stripe domains to much smaller magnetic skyrmions. These changes strongly affect the magneto-transport properties. The underlying mechanisms are identified by comparing the experimental results to a magnetic pinning model.

cond-mat.supr-con↗

arXiv2019

Unconventional anomalous Hall effect in 3d/5d multilayers mediated by the nonlocal spin-conductivity

We evidenced unconventionnal Anomalous Hall Effects (AHE) in 3d/5d (Co0.2nm/Ni0.6nm)N multilayers grown on a thin Pt layer or thin Au:W alloy. The inversion observed on AHE originates from the opposite sign of the spin-orbit coupling of Pt compared to Ni. Via advanced simulations methods for the description of the spin-current profiles based on the spin-dependent Boltzmann formalism, we extracted the spin Hall angle (SHA) of Pt and (Co/Ni) as well as the relevant transport parameters. The extracted SHA for Pt, +20%, is opposite to the one of (Co/Ni), giving rise to an effective AHE inversion for thin (Co/Ni) multilayers (N < 17). The spin Hall angle in Pt is found to be larger than the one previously measured in combined spin-pumping inverse spin-Hall effect experiments in a geometry of current perpendicular to plane. Whereas magnetic proximity effects cannot explain the effect, spin-current leakage and anisotropic electron scattering at Pt/(Co,Ni) interfaces fit the experiments.

cond-mat.mes-hall↗

arXiv2019

Quantitative imaging of hybrid chiral spin textures in magnetic multilayer systems by Lorentz microscopy

Chiral magnetic textures in ultrathin perpendicularly magnetised multilayer film stacks with an interfacial Dzyaloshinskii-Moriya interaction have been the focus of much research recently. The chirality associated with the broken inversion symmetry at the interface between an ultrathin ferromagnetic layer and a heavy metal with large spin-orbit coupling supports homochiral N\'eel domain walls and hedgehog (N\'eel) skyrmions. Under spin-orbit torques these N\'eel type magnetic structures are predicted, and have been measured, to move at high velocities. However recent studies have indicated that some multilayered systems may possess a more complex hybrid domain wall configuration, due to the competition between interfacial DMI and interlayer dipolar fields. These twisted textures are expected to have thickness dependent N\'eel and Bloch contributions to the domain or skyrmion walls. In this work, we use the methods of Lorentz microscopy to measure quantitatively for the first time experimentally both; i) the contributions of the N\'eel and Bloch contributions and ii) their spatial spin variation at high resolution. These are compared with modelled and simulated structures which are in excellent agreement with our experimental results. Our quantitative analysis provides powerful direct evidence of the Bloch wall component which exists in these hybrid walls and will be significant when exploiting such phenomena in spintronic applications.

cond-mat.mtrl-sci↗

arXiv2017

A transmission electron microscope study of N\'eel skyrmion magnetic textures in multilayer thin film systems with large interfacial chiral interaction

Skyrmions in ultrathin ferromagnetic metal (FM)/heavy metal (HM) multilayer systems produced by conventional sputtering methods have recently generated huge interest due to their applications in the field of spintronics. The sandwich structure with two correctly-chosen heavy metal layers provides an additive interfacial exchange interaction which promotes domain wall or skyrmion spin textures that are N\'eel in character and with a fixed chirality. Lorentz transmission electron microscopy (TEM) is a high resolution method ideally suited to quantitatively image such chiral magnetic configurations. When allied with physical and chemical TEM analysis of both planar and cross-sectional samples, key length scales such as grain size and the chiral variation of the magnetisation variation have been identified and measured. We present data showing the importance of the grain size (mostly < 10nm) measured from direct imaging and its potential role in describing observed behaviour of isolated skyrmions (diameter < 100nm). In the latter the region in which the magnetization rotates is measured to be around 30 nm. Such quantitative information on the multiscale magnetisation variations in the system is key to understanding and exploiting the behaviour of skyrmions for future device applications.

cond-mat.mtrl-sci↗

arXiv2017

Chirality in magnetic multilayers probed by the symmetry and the amplitude of dichroism in X-ray resonant magnetic scattering

Chirality in condensed matter is now a topic of the utmost importance because of its significant role in the understanding and mastering of a large variety of new fundamental physicals mechanisms. Versatile experimental approaches, capable to reveal easily the exact winding of order parameters are therefore essential. Here we report X-ray resonant magnetic scattering (XRMS) as a straightforward tool to identify directly the properties of chiral magnetic systems. We show that it can straight-forwardly and unambiguously determine the main characteristics of chiral magnetic distributions: i.e. its chiral nature, the quantitative winding sense (clockwise or counter-clockwise) and its type (N\'eel/cycloidal or Bloch/helical). This method is model-independent, does not require a-priori knowledge of magnetic parameters and can be applied to any system with magnetic domains ranging from few nanometers (wavelength limited) to several microns. By using prototypical multilayers with tailored magnetic chiralities based on the Co|Pt interface we illustrate the strength of this method.

cond-mat.mtrl-sci↗

arXiv2017

Large enhancement of the spin Hall effect in Au by scattering with side-jump on Ta impurities

We present measurements of the Spin Hall Effect (SHE) in AuW and AuTa alloys for a large range of W or Ta concentrations by combining experiments on lateral spin valves and Ferromagnetic-Resonance/spin pumping technique. The main result is the identification of a large enhancement of the Spin Hall Angle (SHA) by the side-jump mechanism on Ta impurities, with a SHA as high as + 0.5 (i.e $50\%$) for about 10\% of Ta. In contrast the SHA in AuW does not exceed + 0.15 and can be explained by intrinsic SHE of the alloy without significant extrinsic contribution from skew or side-jump scattering by W impurities. The AuTa alloys, as they combine a very large SHA with a moderate resistivity (smaller than $85\,\mu\Omega.cm$), are promising for spintronic devices exploiting the SHE.

cond-mat.mtrl-sci↗

arXiv2015

Perpendicular magnetization reversal in Pt/[Co/Ni]$_3$/Al multilayers via the Spin Hall Effect of Pt

We experimentally investigate the current-induced magnetization reversal in Pt/[Co/Ni]$_3$/Al multilayers combining the anomalous Hall effect and magneto-optical Kerr effect techniques in crossbar geometry. The magnetization reversal occurs through nucleation and propagation of a domain of opposite polarity for a current density of the order of 0.3 TA/m$^2$. In these experiments we demonstrate a full control of each stage: i)the {\O}rsted field controls the domain nucleation and ii) domain-wall propagation occurs by spin torque from the Pt spin Hall effect. This scenario requires an in-plane magnetic field to tune the domain wall center orientation along the current for efficient domain wall propagation. Indeed, as nucleated, domain walls are chiral and N\'eel like due to the interfacial Dzyaloshinskii-Moriya interaction.

cond-mat.mtrl-sci↗

arXiv2014

Mapping the radiative and the apparent non-radiative local density of states in the near field of a metallic nanoantenna

We present a novel method to extract the various contributions to the photonic local density of states from near-field fluorescence maps. The approach is based on the simultaneous mapping of the fluorescence intensity and decay rate, and on the rigorous application of the reciprocity theorem. It allows us to separate the contributions of the radiative and the apparent non-radiative local density of states to the change in the decay rate. The apparent non-radiative contribution accounts for losses due to radiation out of the detection solid angle and to absorption in the environment. Data analysis relies on a new analytical calculation, and does not require the use of numerical simulations. One of the most relevant applications of the method is the characterization of nanostructures aimed at maximizing the number of photons emitted in the detection solid angle, which is a crucial issue in modern nanophotonics.

physics.optics↗

arXiv2013

Broadband and efficient diffraction

Surface topography dictates the deterministic functionality of diffraction by a surface. In order to maximize the efficiency with which a diffractive optical component, such as a grating or a diffractive lens, directs light into a chosen order of diffraction, it is necessary that it be "blazed". The efficiency of most diffractive optical components reported so far varies with the wavelength, and blazing is achieved only at a specific nominal energy, the blaze wavelength. The existence of spurious light in undesirable orders represents a severe limitation that prevents using diffractive components in broadband systems. Here we experimentally demonstrate that broadband blazing over almost one octave can be achieved by combining advanced optical design strategies and artificial dielectric materials that offer dispersion chromatism much stronger than those of conventional bulk materials. The possibility of maintaining an efficient funneling of the energy into a specific order over a broad spectral range may empower advanced research to achieve greater control over the propagation of light, leading to more compact, efficient and versatile optical components.

physics.optics↗

arXiv2013

Effect of annealing on the superconducting properties of a-Nb(x)Si(1-x) thin films

a-Nb(x)Si(1-x) thin films with thicknesses down to 25 Å have been structurally characterized by TEM (Transmission Electron Microscopy) measurements. As-deposited or annealed films are shown to be continuous and homogeneous in composition and thickness, up to an annealing temperature of 500°C. We have carried out low temperature transport measurements on these films close to the superconductor-to-insulator transition (SIT), and shown a qualitative difference between the effect of annealing or composition, and a reduction of the film thickness on the superconducting properties of a-NbSi. These results question the pertinence of the sheet resistance R_square as the relevant parameter to describe the SIT.

cond-mat.supr-con↗