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The magnetoelectric properties of exchange-coupled Ni/CoFeB-based composite multiferroic microstructures are investigated. The strength and sign of the magnetoelastic effect are found to be strongly correlated with the ratio between the thicknesses of two magnetostrictive materials. In cases where the thickness ratio deviates significantly from one, the magnetoelastic behavior of the multiferroic microstructures is dominated by the thicker layer, which contributes more strongly to the observed magnetoelastic effect. More symmetric structures with a thickness ratio equal to one show an emergent interfacial behavior which cannot be accounted for simply by summing up the magnetoelastic effects occurring in the two constituent layers. This aspect is clearly visible in the case of ultrathin bilayers, where the exchange coupling drastically affects the magnetic behavior of the Ni layer, making the Ni/CoFeB bilayer a promising next-generation synthetic magnetic system entirely. This study demonstrates the richness and high tunability of composite multiferroic systems based on coupled magnetic bilayers compared to their single magnetic layer counterparts. Furthermore, because of the compatibility of CoFeB with present magnetic tunnel junction-based spintronic technologies, the reported findings are expected to be of great interest for the development of ultralow-power magnetoelectric memory devices.

Topological insulators have emerged as an important material class for efficient spin-charge interconversion. Most topological insulators considered to date are binary or ternary compounds, with the exception of $\ensuremath{\alpha}\ensuremath{-}\mathrm{Sn}$. Here we report a comprehensive characterization of the growth, magnetotransport properties, and current-induced spin-orbit torques of $\ensuremath{\alpha}\ensuremath{-}\mathrm{Sn}$ and $\ensuremath{\beta}\ensuremath{-}\mathrm{Sn}$-based ferromagnetic heterostructures. We show that $\ensuremath{\alpha}\ensuremath{-}\mathrm{Sn}$ grown with a Bi surfactant on CdTe(001) promotes large spin-orbit torques in a ferromagnetic FeCo layer at room temperature, comparable to Pt, whereas $\ensuremath{\alpha}\ensuremath{-}\mathrm{Sn}$ grown without Bi surfactant and the nontopological phase, $\ensuremath{\beta}\ensuremath{-}\mathrm{Sn}$, induce lower torques. The dampinglike and fieldlike spin-orbit torque efficiency in $\ensuremath{\alpha}\ensuremath{-}\mathrm{Sn}$ with Bi are 0.12 and 0.18, respectively. Further, we show that $\ensuremath{\alpha}\ensuremath{-}\mathrm{Sn}$ grown with and without Bi presents a spin-Hall-like magnetoresistance comparable to that found in heavy metal/ferromagnet bilayers. Our work demonstrates direct and efficient charge-to-spin conversion in $\ensuremath{\alpha}\ensuremath{-}\mathrm{Sn}$ ferromagnetic heterostructures, showing that $\ensuremath{\alpha}\ensuremath{-}\mathrm{Sn}$ is a promising material for current-induced magnetization control in spintronic devices.

International audience; Many questions are still open regarding the physical mechanisms behind the magnetic switching in Gd-Fe-Co alloys by single optical pulses. Phenomenological models suggest a femtosecond scale exchange relaxation between sublattice magnetization as the driving mechanism for switching. The recent observation of thermally induced switching in Gd-Fe-Co by using both several picosecond optical laser pulse as well as electric current pulses has questioned this previous understanding. This has raised the question of whether or not the same switching mechanics are acting at the femtosecond and picosecond scales. In this work, we aim at filling this gap in the understanding of the switching mechanisms behind thermal single-pulse switching. To that end, we have studied experimentally thermal single-pulse switching in Gd-Fe-Co alloys, for a wide range of system parameters, such as composition, laser power, and pulse duration. We provide a quantitative description of the switching dynamics using atomistic spin dynamics methods with excellent agreement between the model and our experiments across a wide range of parameters and timescales, ranging from femtoseconds to picoseconds. Furthermore, we find distinct element-specific damping parameters as a key ingredient for switching with long picosecond pulses and argue that switching with pulse durations as long as 15 ps is possible due to a low damping constant of Gd. Our findings can be easily extended to speed up dynamics in other contexts where ferrimagnetic Gd-Fe-Co alloys have been already demonstrated to show fast and energy-efficient processes, e.g., domain-wall motion in a track and spin-orbit torque switching in spintronics devices.

Spin transport via magnon diffusion in magnetic insulators is important for a broad range of spin-based phenomena and devices. However, the absence of the magnon equivalent of an electric force is a bottleneck. In this work, we demonstrate the controlled generation of magnon drift currents in yttrium iron garnet/platinum heterostructures. By performing electrical injection and detection of incoherent magnons, we find magnon drift currents that stem from the interfacial Dzyaloshinskii-Moriya interaction. We can further control the magnon drift by the orientation of the magnetic field. The drift current changes the magnon propagation length by up to $\pm$ 6 % relative to diffusion. We generalize the magnonic spin transport theory to include a finite drift velocity resulting from any inversion asymmetric interaction, and obtain results consistent with our experiments., 6 pages, 3 figures

We report on the combination of current-induced spin-orbit torques and giant magnetoresistance in a single device to achieve all-electrical write and read out of the magnetization. The device consists of perpendicularly magnetized TbCo and Co layers separated by a Pt or Cu spacer. Current injection through such layers exerts spin-orbit torques and switches the magnetization of the Co layer while the TbCo magnetization remains fixed. Subsequent current injection of lower amplitude senses the relative orientation of the magnetization of the Co and TbCo layers, which results in two distinct resistance levels for parallel and antiparallel alignment due to the current-in-plane giant magnetoresistance effect. We further show that the giant magnetoresistance of devices including a single TbCo/spacer/Co trilayer can be improved from 0.02% to 6% by using a Cu spacer instead of Pt. This type of devices offers an alternative route to a two terminal spintronic memory that can be fabricated with moderate effort., Comment: 18 pages, 5 figures

Domain walls in magnetic thin films display a complex dynamical response when subject to an external drive. It is claimed that different dynamic regimes are correlated with the domain-wall roughness, i.e., with the fluctuations of domain-wall position due to the inherent disorder in the system. Therefore, key to understanding the dynamics of domain walls is to have a statistically meaningful measure of the domain-wall roughness. Here we present a thorough study of the roughness parameters, i.e., roughness exponent and roughness amplitude, for domain walls in a ferrimagnetic GdFeCo thin film in the creep regime. Histograms of roughness parameters are constructed with more than 40 independent realizations under the same experimental conditions, and the average values and standard deviations are compared in different conditions. We found that the most prominent feature of the obtained distributions is their large standard deviations, which is a signature of large fluctuations. We show that even if the roughness parameters for a particular domain wall are well known, these parameters are not necessarily representative of the underlying physics of the system. In the low field limit, within the creep regime of domain-wall motion, we found the average roughness exponent and roughness amplitude to be around 0.75 and 0.45 μm2, respectively. When an in-plane magnetic field is applied we observed that, even though the distributions are wide, changes in the mean values of roughness parameters can be identified; the roughness exponent decreasing to values around 0.72 while the roughness amplitude increases to 0.65 μm2. Our results call for a careful consideration of statistical averaging over different domains walls when reporting roughness exponents. Fil: Jordán Ringgold, Daniel. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro. Archivo Histórico del Centro Atómico Bariloche e Instituto Balseiro | Universidad Nacional de Cuyo. Instituto Balseiro. Archivo Histórico del Centro Atómico Bariloche e Instituto Balseiro; Argentina Fil: Albornoz, Lucas Javier. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche | Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche; Argentina Fil: Gorchon, Jon. Lawrence Berkeley National Laboratory; Estados Unidos Fil: Lambert, Charles Henri. University of California at Berkeley; Estados Unidos Fil: Salahuddin, Sayeef. University of California at Berkeley; Estados Unidos Fil: Bokor, Jeffrey. University of California at Berkeley; Estados Unidos. Lawrence Berkeley National Laboratory; Estados Unidos Fil: Curiale, Carlos Javier. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche | Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche; Argentina Fil: Bustingorry, Sebastian. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche | Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche; Argentina

Metal trifluoroacetates (TFAs) are commonplace precursors for bulk and nanostructured metal fluorides and oxides obtained usually through their thermal decomposition. Very little, however, is known about the atomistic mechanism of such a thermal conversion, or even the crystal structure of the precursors. In this study, we detail the structural evolution of Fe(TFA)3 upon its thermal decomposition into rhombohedral iron(III) fluoride. Several distinct structural motifs have been identified in the temperature range of 250–350 °C. In particular, Fe(TFA)3, Fe2F(TFA)5, and FeF(TFA)2 are composed of infinite chains. In addition, several volatile molecular species with ring structure—FenFn(TFA)2n (n = 6–10)—have been characterized both by single-crystal X-ray diffraction and by mass spectrometry. Further conversion of TFA groups into fluoride ions leads to a layered FeF2(TFA). All these compounds feature bridging TFA ligands and octahedral fluoro- or oxo-coordination of Fe. The retention of the +3 oxidation state is confirmed by 57Fe Mössbauer spectroscopy. Magnetization measurements point to the ferrimagnetic ordering in FeF2(TFA) at T ≤ 150 K, unlike Fe(TFA)3, which remains paramagnetic at as low as 10 K. This study highlights the potential of metal TFAs for the rational synthesis of molecular and solid-state hybrid compounds by their versatile thermal decomposition into fully inorganic materials., Chemistry of Materials, 32 (6), ISSN:0897-4756

International audience; Spin-transfer-driven oscillations of a transverse domain wall confined to a perpendicular spin-valve structure are investigated using a one-dimensional model. The stack consists of a polarizer, nonmagnetic spacer, soft free layer, and pinned magnetic layer. It is found that the domain-wall oscillation frequency is a nonmonotonic, highly asymmetrical function of applied electric current, showing a strong dependence on the current direction and the relative strengths of the interfacial and bulk spin-transfer torques. Micromagnetic analysis reveals that the surprising and atypical oscillator response is due to an interplay between the interfacial spin-transfer torque, the bulk spin-transfer torque, the exchange torque, and the damping torque. The underlying physical and material responses are examined, including the important role of the domain-wall twist. The competitions between the involved torques under different operating conditions suggest that the oscillator could serve as a model system to investigate magnetic and spintronic phenomena at the nanoscale. The observed current-dependent twisting of the free-layer magnetization about the axis of precession may further be found interesting for investigations of the interaction between spin-polarized current and chiral spin structures.

This article describes a novel approach which allows for the mutual determination of a ferromagnetic thin film's static and dynamic magnetic behaviours in the presence of an external thermodynamic stimulus. Using a combination of polarized neutron reflectometry (PNR) and ferromagnetic resonance (FMR) techniques, it is shown that information such as magnetic depth profiles and magnetization dynamics can be obtained for a ferromagnetic film in both transient and static states in the presence of a hydrogen gas atmosphere. Presented here are the proposed scheme, the instrumentation concept and the first experimental results obtained from implementing a custom-made PNR with an in situ FMR sample chamber on the PLATYPUS time-of-flight reflectometer beamline at the Australian Centre for Neutron Scattering (ANSTO).

Target Skyrmions (TSks) are extended topological spin textures with a constant chirality where the rotation of the z component of the magnetization is larger than π. TSks have topological charge 1 or 0, if the z component of the magnetization Mz goes through a rotation of nπ where n is an odd or even integer, respectively. TSks with a rotation of the z component of up to 4π have been imaged via high spatial resolution element-specific x-ray imaging. The TSks were generated by weakly coupling 30 nm thin Permalloy (Ni80Fe20, PY) disks with a 1 μm diameter to asymmetric (Ir 1 nm/Co 1.5 nm/Pt 1 nm) × 7 multilayers that exhibit Dzyaloshinskii-Moriya interaction. The PY disks stabilize the TSks in the multilayers due to reduced stray field energy and enforced circular symmetry from pinning of domain walls at the edges of the disks. Upon applying external magnetic fields, it is the skyrmion core at the center that ensures stability of the TSk, whereas the collapse of the extended structures in the TSk does not depend on the topological charge.

International audience; The magnetic and magneto-caloric properties of (Gd1−xTbx) alloy thin films and [Gd/Tb] multilayers have been investigated. All the samples had a total thickness of 100 nm and were deposited by DC sputtering. By changing the composition of the system, the ordering temperature (Tc) and the magnetic entropy changes of the compounds could be tuned. The results show that the Tc of the alloy thin films is close to 270 K for x = 0.2. Moreover, creating [Gd/Tb] multilayers with the same thickness and concentration of the studied Gd80Tb20 alloy film enables to strongly increase the relative cooling power, and reach values twice as big as from the corresponding alloys. These results suggest that nanostructuring of [Gd/Tb] multilayers may be a promising route to tailor the magnetocaloric response of materials.

Intense current pulses are often required to operate microelectronic and spintronic devices. Notably, strong current pulses have been shown to induce magnetoresistance changes attributed to domain reorientation in antiferromagnet/heavy metal bilayers and non-centrosymmetric antiferromagnets. In such cases, nonmagnetic resistivity changes may dominate over signatures of antiferromagnetic switching. We report systematic measurements of the current-induced changes of the transverse and longitudinal resistance of Pt and Pt/NiO layers deposited on insulating substrates, namely, Si/SiO 2, Si/Si 3 N 4, and Al 2 O 3. We identify the range of pulse amplitude and length that can be used without affecting the resistance and show that it increases with the device size and thermal diffusivity of the substrate. No significant difference is observed in the resistive response of Pt and NiO/Pt devices, thus precluding evidence on the switching of antiferromagnetic domains in NiO. The variation of the transverse resistance is associated to a thermally activated process in Pt that decays following a double exponential law with characteristic timescales of a few minutes to hours. We use a Wheatstone bridge model to discriminate between positive and negative resistance changes, highlighting competing annealing and electromigration effects. Depending on the training of the devices, the transverse resistance can either increase or decrease between current pulses. Furthermore, we elucidate the origin of the nonmonotonic resistance baseline, which we attribute to training effects combined with the asymmetric distribution of the current. These results provide insight into the origin of current-induced resistance changes in metal layers and a guide to minimize nonmagnetic artifacts in switching experiments of antiferromagnets. © 2020 Author(s). ISSN:0021-8979 ISSN:1089-7550

We report a method to model, simulate and predict high-level nuclear waste (HLW) thermal behavior over short and long time-scales. The approach relies on an inverse method using homogenized thermal fluxes at the HLW package surfaces during the vitrification process. The thermal simulation can then be used as input in subsequent structural analyses. This allows us to compare for two different vitrification processes the developed thermal stress build-up investigated inside the glass block. The first process is the original experimental vitrification process, without the presence of the radionuclides. The second one is a simulation made possible by the proposed inverse method, where the cooling and solidification for a HLW integrate the long-term heat source of the radionuclides.

International audience; Current induced magnetization manipulation in a spin valve structure where the free layer is a magnetic hard/soft composite structure is studied using micromagnetic simulations. In this structure where the hard layers has strong perpendicular magnetic anisotropy, a domain wall can be nucleated in the soft layer due to the spin transfer torque effect. Depending on the magnetic properties of the layers and the current intensity the domain wall can induce the free layer reversal or be pinned by the hard layer. For these non-uniform magnetic configurations both bulk and interface spin transfer torques need to be considered. The potential reduction of the critical current observed in this geometry is of potential technological interest.

We report on spin-orbit torque measurements in ferrimagnetic ${\mathrm{Gd}}_{21}{({\mathrm{Fe}}_{90}{\mathrm{Co}}_{10})}_{79}$ films with bulk perpendicular magnetic anisotropy and thicknesses up to $30\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$. The dampinglike and fieldlike torques have been quantified using second harmonic Hall voltage detection. Both torques show an inverse linear dependence on the thickness that is indicative of the interfacial nature of the torques. The spin-Hall angle remains constant over the thickness range of 10 to $30\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$ ${\mathrm{Gd}}_{21}{({\mathrm{Fe}}_{90}{\mathrm{Co}}_{10})}_{79}$. Remarkably, we find that this interfacial torque is able to switch a $30\text{\ensuremath{-}}\mathrm{nm}$-thick ${\mathrm{Gd}}_{21}{({\mathrm{Fe}}_{90}{\mathrm{Co}}_{10})}_{79}$ film with a reasonably large thermal stability of $\ensuremath{\approx}100\phantom{\rule{0.16em}{0ex}}{k}_{B}T$.

In a number of recent experiments, it has been shown that femtosecond laser pulses can control magnetization on picosecond timescales, which is at least an order of magnitude faster compared to conventional magnetization dynamics. Among these demonstrations, one material system (GdFeCo ferromagnetic films) is particularly interesting, as deterministic toggle-switching of the magnetic order has been achieved without the need of any symmetry breaking magnetic field. This phenomenon is often referred to as all optical switching (AOS). However, so far, GdFeCo remains the only material system where such deterministic switching has been observed. When extended to ferromagnetic systems, which are of greater interest in many technological applications, only a partial effect can be achieved, which in turn requires repeated laser pulses for full switching. However, such repeated pulsing is not only energy hungry, it also negates the speed advantage of AOS. Motivated by this problem, we have developed a general method for single-shot, picosecond timescale, complete all optical switching of ferromagnetic materials. We demonstrate that in exchange-coupled layers of Co/Pt and GdFeCo, single shot, switching of the ferromagnetic Co/Pt layer is achieved within 7 picoseconds after irradiation by a femtosecond laser pulse. We believe that this approach will greatly expand the range of materials and applications for ultrafast magnetic switching., 11 pages, 3 figures, supplementary materials

International audience; Spin-polarized charge transfer at the interface between a ferromagnetic (FM)metal and a molecule can lead to ferromagnetic coupling and to a high spinpolarization at room temperature. The magnetic properties of these interfacescan not only alter those of the ferromagnet but can also stabilize molecularspin chains with interesting opportunities toward quantum computing. Withthe aim to enhance an organic spintronic device’s functionality, external controlover this spin polarization may thus be achieved by altering the ferromagnet/molecule interface’s magnetic properties. To do so, the magnetoelectricproperties of an underlying ferroelectric/ferromagnetic interface are utilized.Switching the ferroelectric polarization state of a PbZr0.2Ti0.8O3 (PZT) bottomlayer within a PZT/Co/FePc-based (Pc - phthalocyanine) device alters the X-raymagnetic circular dichroism of the Fe site within the phthalocyanine moleculartop layer. Thus, how to electrically alter the magnetic properties of an interfacewith high spin polarization at room temperature is demonstrated. Thisexpands electrical control over spin-polarized FM/molecule interfaces, which isfirst demonstrated using ferroelectric molecules, to all molecular classes.

We report the magnetic response of Au/GdFeCo bilayers to optical irradiation of the Au surface. For bilayers with Au thickness greater than 50 nm, the great majority of energy is absorbed by the Au electrons, creating an initial temperature differential of thousands of Kelvin between the Au and GdFeCo layers. The resulting electronic heat currents between the Au and GdFeCo layers last for several picoseconds with energy flux in excess of 2 TW m-2, and provide sufficient heating to the GdFeCo electrons to induce deterministic reversal of the magnetic moment., Comment: 17 pages, 5 figures

© 2017 American Physical Society. We report the magnetic response of Co/Pt multilayers to picosecond electrical heating. Using photoconductive Auston switches, we generate electrical pulses with 5.5 ps duration and hundreds of pico-Joules to pass through Co/Pt multilayers. The electrical pulse heats the electrons in the Co/Pt multilayers and causes an ultrafast reduction in the magnetic moment. A comparison between optical and electrically induced demagnetization of the Co/Pt multilayers reveals significantly different dynamics for optical vs electrical heating. We attribute the disparate dynamics to the dependence of the electron-phonon interaction on the average energy and the total number of initially excited electrons.

Magnetic switching is induced in 10 ps by electrical current pulses., The field of spintronics involves the study of both spin and charge transport in solid-state devices. Ultrafast magnetism involves the use of femtosecond laser pulses to manipulate magnetic order on subpicosecond time scales. We unite these phenomena by using picosecond charge current pulses to rapidly excite conduction electrons in magnetic metals. We observe deterministic, repeatable ultrafast reversal of the magnetization of a GdFeCo thin film with a single sub–10-ps electrical pulse. The magnetization reverses in ~10 ps, which is more than one order of magnitude faster than any other electrically controlled magnetic switching, and demonstrates a fundamentally new electrical switching mechanism that does not require spin-polarized currents or spin-transfer/orbit torques. The energy density required for switching is low, projecting to only 4 fJ needed to switch a (20 nm)3 cell. This discovery introduces a new field of research into ultrafast charge current–driven spintronic phenomena and devices.

Ultrafast magnetization switching at picosecond and sub-picosecond time scales has tremendous technological potential but still poses numerous questions regarding the underlying quantum mechanical phenomena, including the roles of and interactions between the electrons, spins, and phonons (lattice). At the nanometer-scale dimensions relevant for modern applications, these phenomena become increasingly more pronounced. Until now, helicity-independent all-optical switching (HI-AOS) has been largely limited to amorphous Gd-Fe-Co alloys, for which scaling was challenging due to their relatively low anisotropies. In this work, we demonstrate HI-AOS in amorphous GdCo and scale it to nanometer dimensions while still maintaining uniform out-of-plane magnetization. Single shot HI-AOS is demonstrated in these patterned samples down to a minimum optically detectable magnetic dot size of 200 nm. The ultrafast switching behavior was also confirmed using time-resolved magneto-optic Kerr effect measurements and found to settle to its opposite magnetization state at faster rates for smaller dot diameters, passing a threshold of 75% magnetization reversal within approximately 2 ps for a 200 nm dot compared to approximately 40 ps for a 15 μm pattern. The size dependence of the ultrafast switching is explained in terms of the electron-phonon and spin-lattice interactions.

We investigated the magnetic and magnetocaloric properties of Gd100-xCox ( x = 40 to 56) thin films fabricated by the sputtering technique. Under an applied field change &Delta, H = 20 &nbsp, kOe , the magnetic entropy change ( &Delta, S m ) decreases from 2.64 Jkg&minus, 1K&minus, 1 for x = 44 to about 1.27 Jkg&minus, 1 for x = 56. Increasing the Co concentration from x = 40 to 56 shifts the Curie temperature of Gd100-xCox ( x = 40 to 56) thin films from 180 K toward 337 K. Moreover, we extracted the values of critical parameters Tc, &beta, &gamma, and &delta, by using the modified Arrott plot methods. The results indicate the presence of a long-range ferromagnetic order. More importantly, we showed that the relative cooling power (RCP), which is a key parameter in magnetic refrigeration applications, is strongly enhanced by changing the Co concentration in the Gd100-xCox thin films. Our findings help pave the way toward the enhancement of the magnetocaloric effect in magnetic thin films.

International audience; To understand the necessary condition for the observation of all-optical helicity-dependent switching (AO-HDS) of magnetization in thin films, we investigated ferromagnetic Co/Pt and Co/Ni multilayers as well as ferrimagnetic TbCo alloys as a function of magnetic layer compositions and thicknesses. We show that both ferro-and ferrimagnets with high saturation magnetization show AO-HDS if their magnetic thickness is strongly reduced below a material-dependent threshold thickness. By taking into account the demagnetizing energy and the domain wall energy, we are able to define a criterion to predict whether AO-HDS or thermal demagnetization (TD) will be observed. This criterion for the observation of AO-HDS is that the equilibrium size of magnetic domains forming during the cooling process should be larger than the laser spot size. From these results we anticipate that more magnetic materials are expected to show AO-HDS. However, the effect of the optical pulses' helicity is hidden by the formation of small magnetic domains during the cooling process.

Magnetization manipulation in the absence of an external magnetic field is a topic of great interest, since many novel physical phenomena need to be understood and promising new applications can be imagined. Cutting-edge experiments have shown the capability to switch the magnetization of magnetic thin films using ultrashort polarized laser pulses. In 2007, it was first observed that the magnetization switching for GdFeCo alloy thin films was helicity-dependent and later helicity-independent switching was also demonstrated on the same material. Recently, all-optical switching has also been discovered for a much larger variety of magnetic materials (ferrimagnetic, ferromagnetic films and granular nanostructures), where the theoretical models explaining the switching in GdFeCo films do not appear to apply, thus questioning the uniqueness of the microscopic origin of all-optical switching. Here, we show that two different all-optical switching mechanisms can be distinguished; a "single pulse" switching and a "cumulative" switching process whose rich microscopic origin is discussed. We demonstrate that the latter is a two-step mechanism; a heat-driven demagnetization followed by a helicity-dependent remagnetization. This is achieved by an all-electrical and time-dependent investigation of the all-optical switching in ferrimagnetic and ferromagnetic Hall crosses via the anomalous Hall effect, enabling to probe the all-optical switching on different timescales., Comment: 1 page, LaTeX; classified reference numbers

Author(s): Streubel, Robert; Lambert, Charles-Henri; Kent, Noah; Ercius, Peter; N'Diaye, Alpha T; Ophus, Colin; Salahuddin, Sayeef; Fischer, Peter

Inversion symmetry breaking has become a vital research area in modern magnetism with phenomena including the Rashba effect, spin Hall effect, and the Dzyaloshinskii-Moriya interaction (DMI)-a vector spin exchange. The latter one may stabilize chiral spin textures with topologically nontrivial properties, such as Skyrmions. So far, chiral spin textures have mainly been studied in helimagnets and thin ferromagnets with heavy-element capping. Here, the concept of chirality driven by interfacial DMI is generalized to complex multicomponent systems and demonstrated on the example of chiral ferrimagnetism in amorphous GdCo films. Utilizing Lorentz microscopy and X-ray magnetic circular dichroism spectroscopy, and tailoring thickness, capping, and rare-earth composition, reveal that 2 nm thick GdCo films preserve ferrimagnetism and stabilize chiral domain walls. The type of chiral domain walls depends on the rare-earth composition/saturation magnetization, enabling a possible temperature control of the intrinsic properties of ferrimagnetic domain walls.

International audience; Magnetic refrigeration based on the magneto-caloric effect is one of the best alternatives to compete with vapor-compression technology. The viability of a magnetic refrigeration system for magnetic cooling can be tested by exploiting the materials in various forms, ranging from bulk to nanostructured materials. In order to achieve a wide refrigerating temperature range in magnetic refrigeration, we study in this paper a 100 nm-thick Gd-Co alloys-based multilayer stack. The stack is made of four individual Gd-Co alloy layers with different values of concentration and Curie temperature (TC). A magnetic entropy change associated with the second-order magnetic phase transition was determined from the magnetic isotherms. Moreover, the relative cooling power (RCP) of the studied Gd-Co-based multilayer is enhanced compared to the one of bulk Gd, and reaches a value of 200 J/kg. Such an enhancement of the RCP is not due to an enhanced maximum variation of entropy, but this is due to a much broader magnetic entropy peak. This study demonstrates the potential of nanostructured Gd-Co multilayer stack for magnetic cooling applications.

International audience; Devices studies are often augmented by separately conducted materials science studies to achieve better insight into device operation [1–4]. In one such materials science technique called X-ray absorption spectroscopy (XAS), the energy of soft X-ray photons is tuned so as to drive core level excitations of a specific atomic species within the sample (see Figure 1a). The resulting absorption spectrum yields information on the quantity and charge state of these atoms present, on their chemical environment and their resulting electronic/magnetic properties. When applied using the brilliance of a synchrotron facility, this technique is sensitive to minute populations of atoms, even when buried within a heterostructure. Resolving the properties of these atoms within a device built from this heterostruc-ture is in turn interpreted as providing insight into the device's performance. [2,5] As an important refinement, operando studies [6–12] alter the device's state (e.g., Materials science and device studies have, when implemented jointly as " operando " studies, better revealed the causal link between the properties of the device's materials and its operation, with applications ranging from gas sensing to information and energy technologies. Here, as a further step that maximizes this causal link, the paper focuses on the electronic properties of those atoms that drive a device's operation by using it to read out the materials property. It is demonstrated how this method can reveal insight into the operation of a macroscale, industrial-grade microelectronic device on the atomic level. A magnetic tunnel junction's (MTJ's) current, which involves charge transport across different atomic species and interfaces, is measured while these atoms absorb soft X-rays with synchrotron-grade brilliance. X-ray absorption is found to affect magnetotransport when the photon energy and linear polarization are tuned to excite FeO bonds parallel to the MTJ's interfaces. This explicit link between the device's spintronic performance and these FeO bonds, although predicted, challenges conventional wisdom on their detrimental spintronic impact. The technique opens interdisciplinary possibilities to directly probe the role of different atomic species on device operation, and shall considerably simplify the materials science iterations within device research.

We present an experimental study of all-optical helicity-dependent switching (AO-HDS) of ferromagnetic Pt/Co/Pt heterostructures with perpendicular magnetic anisotropy. The sample is patterned into a Hall cross and the AO-HDS is measured via the anomalous Hall effect. This all-electrical probing of the magnetization during AO-HDS enables a statistical quantification of the switching ratio for different laser parameters, such as the threshold power to achieve AO-HDS and the exposure time needed to reach complete switching at a given laser power. We find that the AO-HDS is a cumulative process, a certain number of optical pulses is needed to obtain a full and reproducible helicity-dependent switching. The deterministic switching of the ferromagnetic Pt/Co/Pt Hall cross provides a full “opto-spintronic device,” where the remanent magnetization can be all-optically and reproducibly written and erased without the need of an external magnetic field.

International audience; The spin-torque switching of metallic nanopillar spin valves showing strong perpendicular anisotropy are studied. The magnetic states of the layers depend on extrinsic parameters such as the magnetic field and the dc current applied to the device. A state diagram presents a comprehensive graph of the role of those parameters on the spin-valve magnetic response. After explaining how state diagrams can be built and the different possible representation, experimental state diagrams are studied for perpendicular devices and the influence of lateral size, temperature, and field orientation are shown. An analytical model of a purely uniaxial system is presented. It is shown that this simple model does not properly reflect the experimental results, whereas if the symmetry is broken a qualitative agreement is obtained. Finally, the possible origins of the symmetry break are discussed in light of an analytical model and numerical simulations.