The realization of fully reconfigurable, voltage-controlled, and programmable on-chip magnonic devices is essential to fully harness the potential of spin waves for signal processing, logic and neuromorphic computing. Yet, existing demonstrations of electrical tuning of magnonic responses are either volatile, current-driven and thus energy-inefficient, or rely on local strain modification limiting their scalability for wafer-scale integration. Here, we address this challenge using a BiFeO3/La0.67Sr0.33MnO3 multiferroic thin film heterostructure. We show that ferroelectric domain engineering in BiFeO3 enables deterministic tuning of the magnon dispersion of La0.67Sr0.33MnO3, producing frequency shifts up to ∼ 150 MHz and allowing reconfigurable waveguiding. Micro-focused Brillouin light scattering directly images these effects, revealing electrically defined magnonic waveguides and spatially programmable dispersion. Compared to conventional approaches, this method provides non-volatile and reversible control. Furthermore, using an inverse-design simulation code, we demonstrate the capability of our platform to perform advanced magnonic functions such as frequency demultiplexing. Our results open a new avenue for using magnetoelectric heterostructures for magnonic logic, with further applicability to reservoir and neuromorphic computing and AI driven magnonic devices.

The ability to process time series at low energy cost is critical for many applications. Recurrent neural networks, which can perform such tasks, are computationally expensive when implemented in software on conventional computers. Here we propose to implement a recurrent neural network in hardware using spintronic oscillators as dynamical neurons. Using numerical simulations, we build a multilayer network and demonstrate that we can use back-propagation through time and standard machine-learning tools to train this network. Leveraging the transient dynamics of the spintronic oscillators, we solve the sequential digits classification task with 89.83% +/- 2.91% accuracy, as good as the equivalent software network. We devise guidelines on how to choose the time constant of the oscillators as well as hyperparameters of the network to adapt to different input time scales.

As CMOS technologies face challenges in dimensional and voltage scaling, the demand for novel logic devices has never been greater, with spin-based devices offering scaling potential, at the cost of significantly high switching energies. Alternatively, magnetoelectric materials are predicted to enable low-power magnetization control, a solution with limited device-level results. Here, we demonstrate voltage-based magnetization switching and reading in nanodevices at room temperature, enabled by exchange coupling between multiferroic BiFeO 3 and ferromagnetic CoFe, for writing, and spin-to-charge current conversion between CoFe and Pt, for reading. We show that, upon the electrical switching of the BiFeO 3 , the magnetization of the CoFe can be reversed, giving rise to different voltage outputs. Through additional microscopy techniques, magnetization reversal is linked with the polarization state and antiferromagnetic cycloid propagation direction in the BiFeO 3 . This study constitutes the building block for magnetoelectric spin-orbit logic, opening a new avenue for low-power beyond-CMOS technologies.

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.

Room-temperature stabilization of skyrmions in magnetic multilayered systems results from a fine balance between several magnetic interactions, namely, symmetric and antisymmetric exchange, dipolar interaction and perpendicular magnetic anisotropy as well as, in most cases, Zeeman through an applied external field. Such field-driven stabilization approach is, however, not compatible with most of the anticipated skyrmion based applications, e.g., skyrmion memories and logic or neuromorphic computing, which motivates a reduction or a cancellation of field requirements. Here, we present a method to stabilize at room-temperature and zero-field, a densely packed skyrmion phase in ferromagnetic multilayers with moderate number of repetitions. To this aim, we finely tune the multilayer parameters to stabilize a dense skyrmion phase. Then, relying on the interlayer electronic coupling to an adjacent bias magnetic layer with strong perpendicular magnetic anisotropy and uniform magnetization, we demonstrate the stabilization of sub-60 nm diameter skyrmions at zero-field with adjustable skyrmion density.

The discovery of graphene has opened novel exciting opportunities in terms of functionalities and performances for spintronics devices. To date, it is mainly graphene properties for efficient in-plane spin transport which have been put forward. [1] We will present here experimental results concerning integration of graphene and other 2D Materials in vertical Magnetic Tunnel Junctions (MTJ), with strong technological potential. [2]We will show that a thin graphene passivation layer, directly integrated by low temperature catalyzed chemical vapor deposition (CVD), [3] allows to preserve a highly surface sensitive spin current polarizer/analyzer behavior. Characterizations of complete spin valves making use of graphene grown by CVD will be presented. The graphene layer prevents the oxidation of ferromagnets, unlocking in turn the exploration of spin filtering phenomena at graphene/ferromagnet interfaces, [4] as well as the introduction of novel ambient/wet processes for spintronics such as atomic layer deposition (ALD) and electro-grafting. [5] We will discuss the measured experimental spin signals in light of bulk band structure spin filtering effect as usually observed with MgO, but also highlight the role of interfacial hybridization (a.k.a. spinterface) for spin selection with ab-initio calculations in support. [6] The interfacial hybridization potential for spintronics will be underlined by the discussion of graphene/insulating FM based MTJ devices towards gated 2D spin sources. [7] We will further discuss the various observed spin filtering effects by analyzing results with other 2D materials (such as h-BN and ${\mathrm {WS}}_{2}$) integrated in MTJ devices. [8] Finally, we will expand the discussion to a novel pulsed laser deposition (PLD) approach for the definition of complex van der Waals heterostructures of 2D materials in MTJs. [9] This PLD growth approach unlocks the association in heterostructure of wide families of multifunctional 2D materials, including the most delicate ones. The different presented experiments unveil promising approaches for the quantum engineering of multifunctional 2D materials heterostructures for spintronics

With the deceleration of dimensional and voltage scaling in CMOS technologies, the demand for novel logic devices has never been greater. While spin-based devices present a major opportunity towards favorable scaling, switching energies are still orders of magnitude above the expected values. Alternatively, magnetoelectric materials are predicted to enable low-power control of magnetization, a relatively unexplored pathway with sparse results at a device level. Here, we demonstrate voltage-based magnetization switching and reading in nanodevices at room temperature, enabled by exchange coupling between multiferroic BiFeO$_3$ and ferromagnetic CoFe, for the writing, and spin-to-charge current conversion between CoFe and Pt, for the reading. Unlike other current-based spintronic devices, magnetization writing is driven solely by voltage pulses. We show that, upon electrical switching of the BiFeO$_3$, the magnetization of the CoFe can be reversed, giving rise to different voltage outputs. The voltage-induced switching is supported through a combination of piezoresponse, magnetic force microscopy, and scanning nitrogen-vacancy magnetometry, where magnetization reversal is linked with the polarization state and antiferromagnetic cycloid propagation direction in the BiFeO$_3$. This study constitutes the building block for magnetoelectric spin-orbit logic, as well as a new avenue for low-power beyond-CMOS technologies.

Skyrmions are topological magnetic solitons that exhibit a rich variety of dynamics, such as breathing and gyration, which can involve collective behavior in arrangements like skyrmion lattices. However, such localized excitations typically lie in the gap of the spin wave spectrum and do not couple to propagating modes. By combining magnetic force microscopy, broadband ferromagnetic resonance, and micromagnetics simulations, we show that in thin-film multilayers of [Pt/FeCoB/AlOx]20 a high-frequency (>12 GHz) mode accompanies the skyrmion lattice phase, which involves the coherent precession of the skyrmion cores that results in the generation of 50–80 nm wavelength spin waves flowing into the uniformly magnetized background. This observation is made possible by a Gilbert damping constant of ∼0.02, which is nearly an order of magnitude lower than in similar ultrathin materials. The simulations also reveal a complex three-dimensional spin structure of the skyrmion cores, which plays a key role for spin wave generation.

Three-dimensional spin textures emerge as promising quasi-particles for encoding information in future spintronic devices. The third dimension provides more malleability regarding their properties and more flexibility for potential applications. However, the stabilization and characterization of such quasi-particles in easily implementable systems remain a work in progress. Here we observe a three-dimensional magnetic texture that sits in the interior of magnetic thin films aperiodic multilayers and possesses a characteristic ellipsoidal shape. Interestingly, these objects that we call skyrmionic cocoons can coexist with more standard tubular skyrmions going through all the multilayer as evidenced by the existence of two very different contrasts in room temperature magnetic force microscopy. The presence of these novel skyrmionic textures as well as the understanding of their layer resolved chiral and topological properties have been investigated by micromagnetic simulations. Finally, we show that the skyrmionic cocoons can be electrically detected using magneto-transport measurements.

We report on the growth of a ferromagnetic cobalt electrode by atomic layer deposition (ALD) and demonstrate it as a functional spin source in complete magnetic tunnel junctions (MTJs). Using an in situ protocol, we integrate a reference tunnel barrier on top of the ALD cobalt spin source stabilizing its metallic nature and allowing further characterization. The cobalt layer, grown in mbar conditions with chemical precursors, is assessed to be metallic and ferromagnetic using both x-ray photoelectron spectroscopy and superconducting quantum interference device magnetometry measurements. Atomic force microscopy tapping and conductive tip mode analyses reveal a very flat film with low roughness (0.2 nm RMS) with a high homogeneity of surface conductivity matching the best reference samples grown by sputtering. We finally evaluate its behavior in full MTJ spin valves, using a reference spin analyzer to highlight that the ALD grown layer is, indeed, spin polarized and can act as a functional spintronics electrode. This result opens the perspective of exploiting the benefits of ALD (such as the wide area low-cost process, extreme conformality, layer by layer growth of heterostructures, area selectivity, etc.) for spintronics applications.

We report on large spin-filtering effects in epitaxial graphene-based spin valves, strongly enhanced in our specific multilayer case. Our results were obtained by the effective association of chemical vapor deposited (CVD) multilayer graphene with a high quality epitaxial Ni(111) ferromagnetic spin source. We highlight that the Ni(111) spin source electrode crystallinity and metallic state are preserved and stabilized by multilayer graphene CVD growth. Complete nanometric spin valve junctions are fabricated using a local probe indentation process, and spin properties are extracted from the graphene-protected ferromagnetic electrode through the use of a reference Al2O3/Co spin analyzer. Strikingly, spin-transport measurements in these structures give rise to large negative tunnel magneto-resistance TMR = -160%, pointing to a particularly large spin polarization for the Ni(111)/Gr interface PNi/Gr, evaluated up to -98%. We then discuss an emerging physical picture of graphene-ferromagnet systems, sustained both by experimental data and ab initio calculations, intimately combining efficient spin filtering effects arising (i) from the bulk band structure of the graphene layers purifying the extracted spin direction, (ii) from the hybridization effects modulating the amplitude of spin polarized scattering states over the first few graphene layers at the interface, and (iii) from the epitaxial interfacial matching of the graphene layers with the spin-polarized Ni surface selecting well-defined spin polarized channels. Importantly, these main spin selection effects are shown to be either cooperating or competing, explaining why our transport results were not observed before. Overall, this study unveils a path to harness the full potential of low Resitance.Area (RA) graphene interfaces in efficient spin-based devices.

Multiferroics offer an elegant means to implement voltage control and on the fly reconfigurability in microscopic, nanoscaled systems based on ferromagnetic materials. These properties are particularly interesting for the field of magnonics, where spin waves are used to perform advanced logical or analogue functions. Recently, the emergence of nanomagnonics is expected to eventually lead to the large-scale integration of magnonic devices. However, a compact voltage-controlled, on demand reconfigurable magnonic system has yet to be shown. Here, we introduce the combination of multiferroics with ferromagnets in a fully epitaxial heterostructure to achieve such voltage-controlled and reconfigurable magnonic systems. Imprinting a remnant electrical polarization in thin multiferroic BiFeO3 with a periodicity of 500 nm yields a modulation of the effective magnetic field in the micrometer-scale, ferromagnetic La2/3Sr1/3MnO3 magnonic waveguide. We evidence the magnetoelectric coupling by characterizing the spin wave propagation spectrum in this artificial, voltage induced, magnonic crystal and demonstrate the occurrence of a robust magnonic band gap with >20 dB rejection.

We present an experimental study of vortex dynamics in magnetic nanocontacts based on pseudo spin valves comprising the Co_2MnGe Heusler compound. The films were grown by molecular beam epitaxy, where precise stoichiometry control and tailored stacking order allowed us to define the bottom ferromagnetic layer as the reference layer, with minimal coupling between the free and reference layers. 20-nm diameter nanocontacts were fabricated using a nano-indentation technique, leading to self-sustained gyration of the vortex generated by spin-transfer torques above a certain current threshold. By combining frequency- and time-domain measurements, we show that different types of spin-transfer induced dynamics related to different modes associated to the magnetic vortex configuration can be observed, such as mode hopping, mode coexistence and mode extinction appear in addition to the usual gyration mode.

The spectral signatures of magnetic skyrmions under microwave field excitation are of fundamental interest and can be an asset for high frequency applications. These topological solitons can be tailored in multilayered thin films, but the experimental observation of their spin wave dynamics remains elusive, in particular due to large damping. Here, we study Pt/FeCoB/AlO$_x$ multilayers hosting dense and robust skyrmion lattices at room temperature with Gilbert damping of $\sim 0.02$. We use magnetic force microscopy to characterise their static magnetic phases and broadband ferromagnetic resonance to probe their high frequency response. Micromagnetic simulations reproduce the experiments with accuracy and allow us to identify distinct resonant modes detected in the skyrmion lattice phase. Low ($<$ 2 GHz) and intermediate frequency ($2-8$ GHz) modes involve excitations localised to skyrmion edges in conjunction with precession of the uniform background magnetisation, while a high frequency ($>$ 12 GHz) mode corresponds to in-phase skyrmion core precession emitting spin waves into uniform background with wavelengths in the 50--80 nm range commensurate with the lattice structure. These findings could be instrumental in the investigation of room temperature wave scattering and the implementation of novel microwave processing schemes in reconfigurable arrays of solitons.

The temperature-dependent ferroelectric properties of super-tetragonal ${\mathrm{BiFeO}}_{3}$ are investigated using surface-sensitive low-energy electron microscopy (LEEM). We use epitaxial oxide ${\mathrm{BiFeO}}_{3}/{\mathrm{Ca}}_{0.96}{\mathrm{Ce}}_{0.04}{\mathrm{MnO}}_{3}$ bilayers grown by pulsed laser deposition on ${\mathrm{YAlO}}_{3}$ substrates. Ferroelectric, micrometer-scale domains are written by piezoresponse force microscopy and subsequently observed by LEEM from room temperature up to about 950 K. Kelvin probe force microscopy and LEEM spectroscopy reveal that the surface potential is efficiently (g50%) screened by adsorbates that are only released after annealing above 873 $\ifmmode\pm\else\textpm\fi{}$ 50 K in ultrahigh vacuum. The surface structure and chemistry of the ferroelectric thin films are analyzed using scanning transmission electron microscopy, electron energy loss spectroscopy, and x-ray photoelectron spectroscopy, discarding the occurrence of a putative ``skin layer'' effect. While its magnetic and structural transitions were reported in the literature, the true, ferroelectric Curie temperature of super-tetragonal ${\mathrm{BiFeO}}_{3}$ has not been determined so far. Here, we measure a Curie temperature of 930 $\ifmmode\pm\else\textpm\fi{}$ 30 K for the super-tetragonal ${\mathrm{BiFeO}}_{3}$ surface and corroborate it with volume-sensitive, temperature-dependent x-ray diffraction measurements. These results suggest that LEEM can be used as a powerful tool to probe surface charge and ferroelectric transitions in ultrathin films.

We present and fully characterize a flow cell dedicated to imaging in liquid at the nanoscale. Its use as a routine sample environment for soft X-ray spectromicroscopy is demonstrated, in particular through the spectral analysis of inorganic particles in water. The care taken in delineating the fluidic pathways and the precision associated with pressure actuation ensure the efficiency of fluid renewal under the beam, which in turn guarantees a successful utilization of this microfluidic tool for in situ kinetic studies. The assembly of the described flow cell necessitates no sophisticated microfabrication and can be easily implemented in any laboratory. Furthermore, the design principles we relied on are transposable to all microscopies involving strongly absorbed radiation (e.g. X-ray, electron), as well as to all kinds of X-ray diffraction/scattering techniques.

2D materials have recently demonstrated a strong potential for spintronic applications. This has been further reinforced by the discovery of ferromagnetic 2D layers. Nevertheless, the fragility of many 2D magnetic materials to ambient conditions has so far hindered their faster characterization and integration into devices. We report here on a simple large-scale method that allows to stabilize strongly air sensitive materials, such as CrBr3, down to the monolayer limit with ultrathin barriers grown by atomic layer deposition (ALD). We focus on MgO as a passivation layer to additionally serve as tunnel spin injection barrier for spintronic applications. We develop a special removable combined protection-encapsulation stack to better preserve 2D material and MgO barrier qualities during device fabrication. This scheme allows to observe 2D ferromagnet stability over one year of air exposure and to demonstrate CrBr3 successful integration into vertical devices. Overall, these results highlight an efficient way to handle these materials in ambient conditions, unlocking possibilities to fasten their advanced characterization and ease their integration into devices.

Room-temperature skyrmions in ferromagnetic films and multilayers show promise for encoding information bits in new computing technologies. Despite recent progress, ferromagnetic order generates dipolar fields that prevent ultrasmall skyrmion sizes, and allows a transverse deflection of moving skyrmions that hinders their efficient manipulation. Antiferromagnetic skyrmions shall lift these limitations. Here we demonstrate that room-temperature antiferromagnetic skyrmions can be stabilized in synthetic antiferromagnets (SAFs), in which perpendicular magnetic anisotropy, antiferromagnetic coupling and chiral order can be adjusted concurrently. Utilizing interlayer electronic coupling to an adjacent bias layer, we demonstrate that spin-spiral states obtained in a SAF with vanishing perpendicular magnetic anisotropy can be turned into isolated antiferromagnetic skyrmions. We also provide model-based estimates of skyrmion size and stability, showing that room-temperature antiferromagnetic skyrmions below 10 nm in radius can be anticipated in further optimized SAFs. Antiferromagnetic skyrmions in SAFs may thus solve major issues associated with ferromagnetic skyrmions for low-power spintronic devices.

We report on spin transport in WS2-based 2D-magnetic tunnel junctions (2D-MTJs), unveiling a band structure spin filtering effect specific to the transition metal dichalcogenides (TMDCs) family. WS2 mono-, bi-, and trilayers are derived by a chemical vapor deposition process and further characterized by Raman spectroscopy, atomic force microscopy (AFM), and photoluminescence spectroscopy. The WS2 layers are then integrated in complete Co/Al2O3/WS2/Co MTJ hybrid spin-valve structures. We make use of a tunnel Co/Al2O3 spin analyzer to probe the extracted spin-polarized current from the WS2/Co interface and its evolution as a function of WS2 layer thicknesses. For monolayer WS2, our technological approach enables the extraction of the largest spin signal reported for a TMDC-based spin valve, corresponding to a spin polarization of PCo/WS2 = 12%. Interestingly, for bi- and trilayer WS2, the spin signal is reversed, which indicates a switch in the mechanism of interfacial spin extraction. With the support of ab initio calculations, we propose a model to address the experimentally measured inversion of the spin polarization based on the change in the WS2 band structure while going from monolayer (direct bandgap) to bilayer (indirect bandgap). These experiments illustrate the rich potential of the families of semiconducting 2D materials for the control of spin currents in 2D-MTJs.