# Articles

#### 2021

• R. Aeschlimann, M. N. Grisolia, G. Sanchez-Santolino, J. Varignon, F. Choueikani, R. Mattana, V. Garcia, S. Fusil, T. Fröhlich, M. Braden, B. Delley, M. Varela, P. Ohresser, J. Santamaria, A. Barthélémy, C. Piamonteze, M. Bibes, X-ray absorption and x-ray magnetic circular dichroism in bulk and thin films of ferrimagnetic GdTiO3. Physical Review Materials. 5, 014407 (2021).
Résumé : Perovskite rare-earth titanates are prototypical Mott insulators in which ${\mathrm{Ti}}^{3+}$ ions with $3{d}^{1}$ electronic configuration exhibit ferromagnetic or antiferromagnetic spin order, depending on the rare-earth size. This peculiar magnetic behavior has, however, been barely studied with element-specific probes, either in bulk or in thin films. The recent finding of fingerprints of ferromagnetism in two-dimensional electron gases at oxide interfaces involving rare-earth titanates has produced a surge of the interest in these complex materials. Harnessing the interfacial magnetic states in these heterostructures calls for a better understanding of their insufficiently explored magnetic states in bulk and especially in thin film form. In this paper, we combine high-resolution transmission electron microscopy with x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD) to determine the structural, electronic, and magnetic structure of $\mathrm{GdTi}{\mathrm{O}}_{3}$ in bulk and thin film form. In both cases, we find that the sample surface is strongly overoxidized but a few nm below, Ti is mostly 3+ and shows a large XMCD. We provide evidence for the ferrimagnetic nature of $\mathrm{GdTi}{\mathrm{O}}_{3}$ with antialigned Gd and Ti sublattices and show that, just as in antiferromagnetic $\mathrm{LaTi}{\mathrm{O}}_{3}$ or ferromagnetic $\mathrm{YTi}{\mathrm{O}}_{3}$, Ti carries no orbital moment.

• A. Barthelemy, N. Bergeal, M. Bibes, A. Caviglia, R. Citro, M. Cuoco, A. Kalaboukhov, B. Kalisky, A. Perroni, J. Santamaria, D. Stornaiuolo, M. Salluzzo, Quasi-two-dimensional electron gas at the oxide interfaces for topological quantum physics. Europhysics Letters. 133, 17001 (2021).

• P. Brus, V. Zatko, M. Galbiati, F. Godel, S. Collin, B. Servet, S. Xavier, R. Aubry, P. Garabedian, M. - B. Martin, B. Dlubak, P. Seneor, O. Bezencenet, Large‐Scale‐Compatible Stabilization of a 2D Semiconductor Platform toward Discrete Components. Advanced Electronic Materials. 7, 2001109 (2021).
Résumé : Atomically thin 2D semiconductors are strong candidates for the tetrad of size‐weight‐power‐and‐cost (SWAP‐C) reduction for many devices. This work focuses on the evaluation of a large‐scale compatib...

• B. Chen, N. Gauquelin, R. J. Green, J. H. Lee, C. Piamonteze, M. Spreitzer, D. Jannis, J. Verbeeck, M. Bibes, M. Huijben, G. Rijnders, G. Koster, Spatially Controlled Octahedral Rotations and Metal–Insulator Transitions in Nickelate Superlattices. Nano Letters. 21, 1295 (2021).
Résumé : The properties of correlated oxides can be manipulated by forming short-period superlattices since the layer thicknesses are comparable with the typical length scales of the involved correlations and interface effects. Herein, we studied the metal–insulator transitions (MITs) in tetragonal NdNiO3/SrTiO3 superlattices by controlling the NdNiO3 layer thickness, n in the unit cell, spanning the length scale of the interfacial octahedral coupling. Scanning transmission electron microscopy reveals a crossover from a modulated octahedral superstructure at n = 8 to a uniform nontilt pattern at n = 4, accompanied by a drastically weakened insulating ground state. Upon further reducing n the predominant dimensionality effect continuously raises the MIT temperature, while leaving the antiferromagnetic transition temperature unaltered down to n = 2. Remarkably, the MIT can be enhanced by imposing a sufficiently large strain even with strongly suppressed octahedral rotations. Our results demonstrate the relevance for the control of oxide functionalities at reduced dimensions.

• M. Cosset-Chéneau, L. Vila, G. Zahnd, D. Gusakova, V. T. Pham, C. Grèzes, X. Waintal, A. Marty, H. Jaffrès, J. - P. Attané, Measurement of the Spin Absorption Anisotropy in Lateral Spin Valves. Physical Review Letters. 126, 027201 (2021).
Résumé : The spin absorption process in a ferromagnetic material depends on the spin orientation relative to the magnetization. Using a ferromagnet to absorb the pure spin current created within a lateral spin valve, we evidence and quantify a sizable orientation dependence of the spin absorption in Co, CoFe, and NiFe. These experiments allow us to determine the spin-mixing conductance, an elusive but fundamental parameter of the spin-dependent transport. We show that the obtained values cannot be understood within a model considering only the Larmor, transverse decoherence, and spin diffusion lengths, and rather suggest that the spin-mixing conductance is actually limited by the Sharvin conductance.

• D. Céspedes‐Berrocal, H. Damas, S. Petit‐Watelot, D. Maccariello, P. Tang, A. Arriola‐Córdova, P. Vallobra, Y. Xu, J. - L. Bello, E. Martin, S. Migot, J. Ghanbaja, S. Zhang, M. Hehn, S. Mangin, C. Panagopoulos, V. Cros, A. Fert, J. - C. Rojas‐Sánchez, Current‐Induced Spin Torques on Single GdFeCo Magnetic Layers. Advanced Materials. 33, 2007047 (2021).
Résumé : A new platform is proposed for spintronics. GdFeCo/Cu bilayers are found to be 20 times more efficient than Pt layers to generate spin currents from charge currents. It is also shown that these spin ...

• J. Ding, C. Liu, V. Kalappattil, Y. Zhang, O. Mosendz, U. Erugu, R. Yu, J. Tian, A. DeMann, S. B. Field, X. Yang, H. Ding, J. Tang, B. Terris, A. Fert, H. Chen, M. Wu, Switching of a Magnet by Spin-Orbit Torque from a Topological Dirac Semimetal. Advanced Materials. 33, 2005909 (2021).
Résumé : Room-temperature switching of magnetization in a CoFeB thin film by a spin-orbit torque from a neighboring topological Dirac semimetal α-Sn thin film is reported. The α-Sn film is grown by sputtering...

• M. Drong, T. Fördös, H. Jaffrès, J. J. Peřina, K. Postava, P. Ciompa, J. Pištora, H. - J. Drouhin, Spin-VCSELs with Local Optical Anisotropies: Toward Terahertz Polarization Modulation. Physical Review Applied. 15, 014041 (2021).
Résumé : Putting a different spin on photonics: Spin-injected vertical-cavity surface-emitting lasers (Spin-VCSELs) with strong anisotropies are important in particular for ultrafast and terahertz applications, but the field is being held back in part because of lack of sufficiently predictive physical models. This study uses temporal coupled-mode theory with the transfer-matrix formalism to make progress by extending the spin-flip model. Additionally, the authors propose a spin-VCSEL with an anisotropic intracavity grating. This approach points to engineering solutions for ultrafast optical communication and compact terahertz sources.

• N. Figueiredo-Prestes, J. Zarpellon, D. da Silva Costa, I. Mazzaro, P. C. de Camargo, A. J. A. de Oliveira, C. Deranlot, J. - M. George, D. H. Mosca, Thermal Stability of Ultrathin Co/Pt Multilayers. The Journal of Physical Chemistry C. 125, 4885 (2021).
Résumé : Multilayered films composed of five repetitions of Co (0.6 nm)/Pt (0.8 nm) bilayers grown in a Pt buffer layer have shown reversible structural and magnetic behaviors from room temperature to approximately 490 K during heating processes at rates of 2 K/min. The thermal evolution of the Co/Pt interfaces was monitored by X-ray diffraction and reflectivity, as well as magnetometry measurements performed in situ. Phenomenological evaluations and Monte Carlo simulations were used to determine the temperature onsets of the thermally activated diffusion processes, interfacial degradation, and loss of perpendicular magnetic anisotropy (PMA). Above this temperature, the non-linear behavior for the thermal expansion coefficient and the perpendicular saturation magnetization indicate the loss of integrity at interfaces. Our results are consistent with the degradation of Co/Pt interfaces and consequent loss of PMA driven by defect-mediated atomic migration from 500 to 750 K. Above 500 K, the atomic migration leads to deterioration of chemical modulation of the Co/Pt interfaces and the formation of a CoXPt1–X solid solution with low saturation magnetization. Below 490 K, Co/Pt multilayer films exhibit an effective linear thermal expansion coefficient of about (6.4 ± 0.6) × 10–6 K–1 and a remanence to saturation magnetization ratio of 0.87.

• A. Finco, A. Haykal, R. Tanos, F. Fabre, S. Chouaieb, W. Akhtar, I. Robert-Philip, W. Legrand, F. Ajejas, K. Bouzehouane, N. Reyren, T. Devolder, J. - P. Adam, J. - V. Kim, V. Cros, V. Jacques, Imaging non-collinear antiferromagnetic textures via single spin relaxometry. Nature Communications. 12, 767 (2021).
Résumé : In this manuscript, Finco et al demonstrate the use of a quantum magnetometer based on a single NV centre for all-optical imaging of antiferromagnetic (AFM) spin textures. By exploiting variations of the NV spin relaxation rate, they succeed in imaging AFM domain walls and skyrmions.

• S. Fragkos, L. Baringthon, P. Tsipas, E. Xenogiannopoulou, P. le Fèvre, P. Kumar, H. Okuno, N. Reyren, A. Lemaitre, G. Patriarche, J. - M. George, A. Dimoulas, Topological surface states in epitaxial (SnBi2Te4)n (Bi2Te3)m natural van der Waals superlattices. Physical Review Materials. 5, 014203 (2021).
Résumé : Topological insulators are good candidates for charge to spin conversion with high efficiency due to their spin-polarized topological surface states (TSSs). In this work, we provide experimental evidence for two-dimensional (2D) TSSs in ${(\mathrm{Sn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4})}_{n} {({\mathrm{Bi}}_{2}{\mathrm{Te}}_{3})}_{m}$ natural van der Waals superlattices grown by molecular beam epitaxy using angle resolved photoelectron spectroscopy and magnetotransport. While the TSSs overlap with bulk conduction band (BCB) states at the Fermi energy, it is shown that by increasing the Sn composition, the influence of BCB states is reduced and becomes minimum for $\mathrm{Sn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$. The latter compound, found to be in the form of septuplet layers, shows weak antilocalization effect with a prefactor $\ensuremath{\alpha}\ensuremath{\sim}--0.41$, indicating that the TSSs and the bulk behave as one 2D channel in which magnetotransport properties are influenced by large spin-orbit coupling.

• T. Guillet, A. Marty, C. Vergnaud, M. Jamet, C. Zucchetti, G. Isella, Q. Barbedienne, H. Jaffrès, N. Reyren, J. - M. George, A. Fert, Large Rashba unidirectional magnetoresistance in the Fe/Ge(111) interface states. Physical Review B. 103, 064411 (2021).
Résumé : The structure inversion asymmetry at surfaces and interfaces gives rise to the Rashba spin-orbit interaction (SOI), that breaks the spin degeneracy of surface or interface states. Hence, when an electric current runs through a surface or interface, this Rashba effect generates an effective magnetic field acting on the electron spin. This provides an additional tool to manipulate the spin state in materials such as Si and Ge possessing inversion symmetry in their bulk form. The existence of Rashba states could be demonstrated by photoemission spectroscopy at the interface between different metals and Ge(111) and by spin-charge conversion experiments at the Fe/Ge(111) interface even though it is made of two light elements. In this paper, we identify the fingerprint of the Rashba states at the Fe/Ge(111) interface by magnetotransport measurements in the form of a large unidirectional magnetoresistance of up to $0.1%$. From its temperature dependence, we find that the Rashba energy splitting is larger than in pure Ge(111) subsurface states.

• A. S. Jenkins, L. S. E. Alvarez, S. Memshawy, P. Bortolotti, V. Cros, P. P. Freitas, R. Ferreira, Electrical characterisation of higher order spin wave modes in vortex-based magnetic tunnel junctions. Communications Physics. 4, 107 (2021).
Résumé : The control and manipulation of spin waves holds promise for miniaturized radio-frequency spintronic devices. The authors demonstrate electrical access to the dynamics of magnetic vortices in confined geometries: a new avenue of research for future applications exploiting super high frequency behaviour for microwave communications and computing applications.

• A. Johansson, B. Göbel, J. Henk, M. Bibes, I. Mertig, Spin and orbital Edelstein effects in a two-dimensional electron gas: Theory and application to SrTiO3 interfaces. Physical Review Research. 3, 013275 (2021).
Résumé : The authors study the electrically induced magnetization at SrTiO${}_{3}$ interfaces that originates from nonequilibrium spin and orbital moments.

• M. Jotta Garcia, J. Moulin, S. Wittrock, S. Tsunegi, K. Yakushiji, A. Fukushima, H. Kubota, S. Yuasa, U. Ebels, M. Pannetier-Lecoeur, C. Fermon, R. Lebrun, P. Bortolotti, A. Solignac, V. Cros, Spin–torque dynamics for noise reduction in vortex-based sensors. Applied Physics Letters. 118, 122401 (2021).
Résumé : The performance of magnetoresistive sensors is today mainly limited by their 1/f low-frequency noise. Here, we study this noise component in vortex-based TMR sensors. We compare the noise level in different magnetization configurations of the device, i.e., vortex state or uniform parallel or antiparallel states. We find that the vortex state is at least an order of magnitude noisier than the uniform states. Nevertheless, by activating the spin-transfer-induced dynamics of the vortex configuration, we observe a reduction of the 1/f noise, close to the values measured in the AP state, as the vortex core has a lower probability of pinning into defect sites. Additionally, by driving the dynamics of the vortex core by a non-resonant rf field or current, we demonstrate that the 1/f noise can be further decreased. The ability to reduce the 1/f low-frequency noise in vortex-based devices by leveraging their spin-transfer dynamics thus enhances their applicability in the magnetic sensors' landscape.

• M. Lachheb, Q. Zhu, S. Fusil, Q. Wu, C. Carrétéro, A. Vecchiola, M. Bibes, D. Martinotti, C. Mathieu, C. Lubin, A. Pancotti, X. Li-Bourrelier, A. Gloter, B. Dkhil, V. Garcia, N. Barrett, Surface and bulk ferroelectric phase transition in super-tetragonal BiFe03 thin films. Physical Review Materials. 5, 024410 (2021).
Résumé : 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 (&gt;50%) 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.

• R. Mattana, N. Locatelli, V. Cros, dans Magnetism and Accelerator-Based Light Sources (2021; http://link.springer.com/chapter/10.1007/978-3-030-64623-3_5)p. 131-163.
Résumé : Having access to the electronic and magnetic properties of spintronic systems is of crucial importance in view of their future technological developments. Our purpose in this chapter is to elaborate...

• L. D. N. Mouafo, F. Godel, L. Simon, Y. J. Dappe, W. Baaziz, U. N. Noumbé, E. Lorchat, M. - B. Martin, S. Berciaud, B. Doudin, O. Ersen, B. Dlubak, P. Seneor, J. - F. Dayen, 0D/2D Heterostructures Vertical Single Electron Transistor. Advanced Functional Materials. 31, 2008255 (2021).
Résumé : The concept of 0D–2D vertical single electron transistors is unveiled. It allows to combine the large Coulomb energy of nanoclusters with the electronic capabilities of a two‐dimensional channel, act...

• M. Och, M. - B. Martin, B. Dlubak, P. Seneor, C. Mattevi, Synthesis of emerging 2D layered magnetic materials. Nanoscale. 13, 2157 (2021).
Résumé : van der Waals atomically thin magnetic materials have been recently discovered. They have attracted enormous attention as they present unique magnetic properties, holding potential to tailor spin-based device properties and enable next generation data storage and communication devices. To fully understand the magnetism in two-dimensions, the synthesis of 2D materials over large areas with precise thickness control has to be accomplished. Here, we review the recent advancements in the synthesis of these materials spanning from metal halides, transition metal dichalcogenides, metal phosphosulphides, to ternary metal tellurides. We initially discuss the emerging device concepts based on magnetic van der Waals materials including what has been achieved with graphene. We then review the state of the art of the synthesis of these materials and we discuss the potential routes to achieve the synthesis of wafer-scale atomically thin magnetic materials. We discuss the synthetic achievements in relation to the structural characteristics of the materials and we scrutinise the physical properties of the precursors in relation to the synthesis conditions. We highlight the challenges related to the synthesis of 2D magnets and we provide a perspective for possible advancement of available synthesis methods to respond to the need for scalable production and high materials quality.

• T. - E. Park, L. Peng, J. Liang, A. Hallal, F. S. Yasin, X. Zhang, K. M. Song, S. J. Kim, K. Kim, M. Weigand, G. Schütz, S. Finizio, J. Raabe, K. Garcia, J. Xia, Y. Zhou, M. Ezawa, X. Liu, J. Chang, H. C. Koo, Y. D. Kim, M. Chshiev, A. Fert, H. Yang, X. Yu, S. Woo, Néel-type skyrmions and their current-induced motion in van der Waals ferromagnet-based heterostructures. Physical Review B. 103, 104410 (2021).

• S. Prosandeev, J. Grollier, D. Talbayev, B. Dkhil, L. Bellaiche, Ultrafast Neuromorphic Dynamics Using Hidden Phases in the Prototype of Relaxor Ferroelectrics. Physical Review Letters. 126, 027602 (2021).
Résumé : Materials possessing multiple states are promising to emulate synaptic and neuronic behaviors. Their operation frequency, typically in or below the GHz range, however, limits the speed of neuromorphic computing. Ultrafast THz electric field excitation has been employed to induce nonequilibrium states of matter, called hidden phases in oxides. One may wonder if there are systems for which THz pulses can generate neuronic and synaptic behavior, via the creation of hidden phases. Using atomistic simulations, we discover that relaxor ferroelectrics can emulate all the key neuronic and memristive synaptic features. Their occurrence originates from the activation of many hidden phases of polarization order, resulting from the response of nanoregions to THz pulses. Such phases further possess different dielectric constants, which is also promising for memcapacitor devices.

• D. Sanz‐Hernández, M. Massouras, N. Reyren, N. Rougemaille, V. Schánilec, K. Bouzehouane, M. Hehn, B. Canals, D. Querlioz, J. Grollier, F. Montaigne, D. Lacour, Tunable Stochasticity in an Artificial Spin Network. Advanced Materials. 33, 2008135 (2021).
Résumé : The motion of magnetic domain‐walls within artificial spin networks leads to a tunable stochastic response of the metamaterial, which can be tailored through an external field and local lattice modif...

• A. F. Scarioni, C. Barton, H. Corte-León, S. Sievers, X. Hu, F. Ajejas, W. Legrand, N. Reyren, V. Cros, O. Kazakova, H. W. Schumacher, Thermoelectric Signature of Individual Skyrmions. Physical Review Letters. 126, 077202 (2021).
Résumé : We experimentally study the thermoelectrical signature of individual skyrmions in chiral $\mathrm{Pt}/\mathrm{Co}/\mathrm{Ru}$ multilayers. Using a combination of controlled nucleation, single skyrmion annihilation, and magnetic field dependent measurements the thermoelectric signature of individual skyrmions is characterized. The observed signature is explained by the anomalous Nernst effect of the skyrmion's spin structure. Possible topological contributions to the observed thermoelectrical signature are discussed. Such thermoelectrical characterization allows for noninvasive detection and counting of skyrmions and enables fundamental studies of topological thermoelectric effects on the nanoscale.

• C. Toulouse, J. Fischer, S. Farokhipoor, L. Yedra, F. Carlà, A. Jarnac, E. Elkaim, P. Fertey, J. - N. Audinot, T. Wirtz, B. Noheda, V. Garcia, S. Fusil, I. P. Alonso, M. Guennou, J. Kreisel, Patterning enhanced tetragonality in BiFeO3 thin films with effective negative pressure by helium implantation. Physical Review Materials. 5, 024404 (2021).
Résumé : Helium implantation in epitaxial thin films is a way to control the out-of-plane deformation independently from the in-plane strain controlled by epitaxy. In particular, implantation by means of a helium microscope allows for local implantation and patterning down to the nanometer resolution, which is of interest for device applications. We present here a study of bismuth ferrite (${\mathrm{BiFeO}}_{3}$) films where strain was patterned locally by helium implantation. Our combined Raman, x-ray diffraction, and transmission electron microscopy (TEM) study shows that the implantation causes an elongation of the ${\mathrm{BiFeO}}_{3}$ unit cell and ultimately a transition towards the so-called supertetragonal polymorph via states with mixed phases. In addition, TEM reveals the onset of amorphization at a threshold dose that does not seem to impede the overall increase in tetragonality. The phase transition from the R-like to T-like ${\mathrm{BiFeO}}_{3}$ appears as first-order in character, with regions of phase coexistence and abrupt changes in lattice parameters.

• P. R. Whelan, B. Zhou, O. Bezencenet, A. Shivayogimath, N. Mishra, Q. Shen, B. S. Jessen, I. Pasternak, D. M. A. Mackenzie, J. Ji, C. Sun, P. Seneor, B. Dlubak, B. Luo, F. W. Østerberg, D. Huang, H. Shi, D. Luo, M. Wang, R. S. Ruoff, B. R. Conran, C. McAleese, C. Huyghebaert, S. Brems, T. J. Booth, I. Napal, W. Strupinski, D. H. Petersen, S. Forti, C. Coletti, A. Jouvray, K. B. K. Teo, A. Centeno, A. Zurutuza, P. Legagneux, P. U. Jepsen, . Bøggild, Case studies of electrical characterisation of graphene by terahertz time-domain spectroscopy. 2D Materials. 8, 022003 (2021).

• S. Wittrock, P. Talatchian, M. Romera, M. Garcia Jotta, M. - C. Cyrille, R. Ferreira, R. Lebrun, P. Bortolotti, U. Ebels, J. Grollier, V. Cros, Flicker and random telegraph noise between gyrotropic and dynamic C-state of a vortex based spin torque nano oscillator. AIP Advances. 11, 035042 (2021).
Résumé : Vortex based spin torque nano oscillators (STVOs) can present more complex dynamics than the spin torque induced gyrotropic (G) motion of the vortex core. The respective dynamic modes and the transition between them can be controlled by experimental parameters such as the applied dc current. An interesting behavior is the stochastic transition from the G-to a dynamic C-state occurring for large current densities. Moreover, the C-state oscillations exhibit a constant active magnetic volume. We present noise measurements in the different dynamic states that allow accessing specific properties of the stochastic transition, such as the characteristic state transition frequency. Furthermore, we confirm, as theoretically predicted, an increase of flicker noise with Idc2 when the oscillation volume remains constant with the current. These results bring insight into the potential optimization of noise properties sought for many potential rf applications with spin torque oscillators. Furthermore, the investigated stochastic characteristics open up new potentialities, for instance in the emerging field of neuromorphic computing schemes.

• S. Wittrock, P. Talatchian, M. Romera, S. Menshawy, M. Jotta Garcia, M. - C. Cyrille, R. Ferreira, R. Lebrun, P. Bortolotti, U. Ebels, J. Grollier, V. Cros, Beyond the gyrotropic motion: Dynamic C-state in vortex spin torque oscillators. Applied Physics Letters. 118, 012404 (2021).
Résumé : In the present study, we investigate a dynamical mode beyond the gyrotropic (G) motion of a magnetic vortex core in a confined magnetic disk of a nano-pillar spin torque nano-oscillator (STNO). It is characterized by the in-plane circular precession associated with a C-shaped magnetization distribution. We show a transition between G- and C-state modes, which is found to be stochastic in a current-controllable range. Supporting our experimental findings with micromagnetic simulations, we believe that the results provide further opportunities for the dynamic and stochastic control of STNOs, which could be interesting to be implemented, for example, in neuromorphic networks.

• M. - W. Yoo, J. Tornos, A. Sander, L. - F. Lin, N. Mohanta, A. Peralta, D. Sanchez-Manzano, F. Gallego, D. Haskel, J. W. Freeland, D. J. Keavney, Y. Choi, J. Strempfer, X. Wang, M. Cabero, H. B. Vasili, M. Valvidares, G. Sanchez-Santolino, J. M. Gonzalez-Calbet, A. Rivera, C. Leon, S. Rosenkranz, M. Bibes, A. Barthelemy, A. Anane, E. Dagotto, S. Okamoto, S. G. E. te Velthuis, J. Santamaria, J. E. Villegas, Large intrinsic anomalous Hall effect in SrIrO3 induced by magnetic proximity effect. Nature Communications. 12, 3283 (2021).
Résumé : The anomalous Hall effect (AHE) occurs in ferromagnets caused by intrinsic and extrinsic mechanisms. Here, Yoo et al. report large anomalous Hall conductivity and Hall angle at the interface between a ferromagnet La0.7Sr0.3MnO3 and a semimetallic SrIrO3, due to the interplay between correlated physics and topological phenomena.

• V. Zatko, S. Dubois, F. Godel, C. Carrétéro, A. Sander, S. Collin, M. Galbiati, J. Peiro, F. Panciera, G. Patriarche, P. Brus, B. Servet, J. - C. Charlier, M. - B. Martin, B. Dlubak, P. Seneor, Band-Gap Landscape Engineering in Large-Scale 2D Semiconductor van der Waals Heterostructures. ACS Nano. 15, 7279 (2021).
Résumé : We present a growth process relying on pulsed laser deposition for the elaboration of complex van der Waals heterostructures on large scales, at a 400 °C CMOS-compatible temperature. Illustratively, we define a multilayer quantum well geometry through successive in situ growths, leading to WSe2 being encapsulated into WS2 layers. The structural constitution of the quantum well geometry is confirmed by Raman spectroscopy combined with transmission electron microscopy. The large-scale high homogeneity of the resulting 2D van der Waals heterostructure is also validated by macro- and microscale Raman mappings. We illustrate the benefit of this integrative in situ approach by showing the structural preservation of even the most fragile 2D layers once encapsulated in a van der Waals heterostructure. Finally, we fabricate a vertical tunneling device based on these large-scale layers and discuss the clear signature of electronic transport controlled by the quantum well configuration with ab initio calculations in support. The flexibility of this direct growth approach, with multilayer stacks being built in a single run, allows for the definition of complex 2D heterostructures barely accessible with usual exfoliation or transfer techniques of 2D materials. Reminiscent of the III–V semiconductors’ successful exploitation, our approach unlocks virtually infinite combinations of large 2D material families in any complex van der Waals heterostructure design.

#### 2020

• F. Abreu Araujo, M. Riou, J. Torrejon, S. Tsunegi, D. Querlioz, K. Yakushiji, A. Fukushima, H. Kubota, S. Yuasa, M. - D. Stiles, J. Grollier, Role of non-linear data processing on speech recognition task in the framework of reservoir computing. Scientific Reports. 10, 328 (2020).
Résumé : The reservoir computing neural network architecture is widely used to test hardware systems for neuromorphic computing. One of the preferred tasks for bench-marking such devices is automatic speech recognition. This task requires acoustic transformations from sound waveforms with varying amplitudes to frequency domain maps that can be seen as feature extraction techniques. Depending on the conversion method, these transformations sometimes obscure the contribution of the neuromorphic hardware to the overall speech recognition performance. Here, we quantify and separate the contributions of the acoustic transformations and the neuromorphic hardware to the speech recognition success rate. We show that the non-linearity in the acoustic transformation plays a critical role in feature extraction. We compute the gain in word success rate provided by a reservoir computing device compared to the acoustic transformation only, and show that it is an appropriate bench-mark for comparing different hardware. Finally, we experimentally and numerically quantify the impact of the different acoustic transformations for neuromorphic hardware based on magnetic nano-oscillators.

• C. Back, V. Cros, H. Ebert, K. Everschor-Sitte, A. Fert, M. Garst, T. Ma, S. Mankovsky, T. L. Monchesky, M. Mostovoy, N. Nagaosa, S. S. P. Parkin, C. Pfleiderer, N. Reyren, A. Rosch, Y. Taguchi, Y. Tokura, K. von Bergmann, J. Zang, The 2020 skyrmionics roadmap. Journal of Physics D: Applied Physics. 53, 363001 (2020).

• L. Baldrati, C. Schmitt, O. Gomonay, R. Lebrun, R. Ramos, E. Saitoh, J. Sinova, M. Kläui, Efficient Spin Torques in Antiferromagnetic CoO/Pt Quantified by Comparing Field- and Current-Induced Switching. Physical Review Letters. 125, 077201 (2020).
Résumé : We achieve current-induced switching in collinear insulating antiferromagnetic $\mathrm{CoO}/\mathrm{Pt}$, with fourfold in-plane magnetic anisotropy. This is measured electrically by spin Hall magnetoresistance and confirmed by the magnetic field-induced spin-flop transition of the CoO layer. By applying current pulses and magnetic fields, we quantify the efficiency of the acting current-induced torques and estimate a current-field equivalence ratio of $4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}\text{ }\text{ }\mathrm{T}\text{ }{\mathrm{A}}^{\ensuremath{-}1}\text{ }{\mathrm{m}}^{2}$. The N\'eel vector final state $(\mathbit{n}\ensuremath{\perp}\mathbit{j})$ is in line with a thermomagnetoelastic switching mechanism for a negative magnetoelastic constant of the CoO.

• Y. Bo, P. Zhang, Y. Zhang, J. Song, S. Li, X. Liu, Spiking dynamic behaviors of NbO2 memristive neurons: A model study. Journal of Applied Physics. 127, 245101 (2020).
Résumé : Neuromorphic computing based on spikes has broad prospects in energy-efficient computation. Memristive neuron in this study is composed of two volatile memristors that have been shown to exhibit rich biological neuronal dynamics. Here, we show spiking dynamic behaviors of NbO2 memristive neurons by a detailed simulation study. With a DC input voltage, the operation windows of both periodic oscillation and neuron-like action potential spikes are recognized in the resistance–voltage phase diagrams of NbO2 memristive neurons. With a voltage pulse as the input, the periodic oscillation region can be classified into three subregions including the spike-OFF, spike-ON, and meta-spike transition regions. When the memristive neuron operates in the meta-spike transition region, it can regulate the “ON” and “OFF” states of the oscillation circuit by changing the ending time of the input pulse. It implies that both the input signal and the output signal determine the state of the circuit. The demonstration of a phase matching method provides a useful way for controlling “ON” and “OFF” states of the periodic oscillation behavior of the memristive neuron. Moreover, the effect of the circuit parameters on the peak-to-valley amplitude of the output spikes with action potential is investigated. A stable and controllable waveform output can be regulated by changing the capacitance, incorporating a series resistor, and customizing the active memristor. All these results provide a reliable reference for implementing memristive neurons in neuromorphic computing.

• J. Bréhin, F. Trier, L. M. Vicente Arche, P. Hemme, P. Noël, M. Cosset-Chéneau, J. - P. Attané, L. Vila, A. Sander, Y. Gallais, A. Sacuto, B. Dkhil, V. Garcia, S. Fusil, A. Barthélémy, M. Cazayous, M. Bibes, Switchable two-dimensional electron gas based on ferroelectric Ca:SrTiO3. Physical Review Materials. 4, 041002 (2020).
Résumé : Ferroelectric materials possess electric dipoles adding up to a macroscopic polarization that is switchable by an electric field. Most ferroelectrics are insulators but some are wide bandgap semiconductors that by doping can be turned into metallic conductors. If doping is restricted to a thin slab near the material surface, the conducting region may harbor a two-dimensional electron gas (2DEG). The 2DEG can then be affected by ferroelectric polarization switching, and may even retain ferroelectric properties coexisting with the conducting behavior. The paper by Br\'ehin $e\phantom{\rule{0}{0ex}}t$ $a\phantom{\rule{0}{0ex}}l$ reports indications of this behavior in a 2DEG at the surface of ferroelectric Ca-SrTiO${}_{3}$.

• F. Cadiz, D. Lagarde, B. Tao, J. Frougier, B. Xu, X. Devaux, S. Migot, Z. G. Wang, X. F. Han, J. - M. George, H. Carrere, A. Balocchi, T. Amand, X. Marie, B. Urbaszek, H. Jaffrès, Y. Lu, P. Renucci, Electrical detection of light helicity using a quantum-dot-based hybrid device at zero magnetic field. Physical Review Materials. 4, 124603 (2020).
Résumé : Photon helicity-dependent photocurrent is measured at zero magnetic field on a device based on an ensemble of (In,Ga)As/GaAs quantum dots that are embedded into a GaAs-based $p$-$i$-$n$ diode. This device can be also operated as a spin light-emitting diode to emit the light with a circular polarization up to 20% at zero magnetic field. The demonstration of the multifunctional capabilities of the device gives valuable insights into the spin relaxation of the electrons in the quantum dots and paves the way for future spin-optoelectronic applications.

• D. Chaudy, O. Llopis, B. Marcilhac, Y. Lemaitre, O. d'Allivy Kelly, J. - M. Hode, J. - M. Lesage, A Low Phase Noise All Cryogenic Microwave Oscillator Based on a Superconductor Resonator. IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control. 67, 2750-2756 (2020).

• J. - Y. Chauleau, T. Chirac, S. Fusil, V. Garcia, W. Akhtar, J. Tranchida, P. Thibaudeau, I. Gross, C. Blouzon, A. Finco, M. Bibes, B. Dkhil, D. D. Khalyavin, P. Manuel, V. Jacques, N. Jaouen, M. Viret, Electric and antiferromagnetic chiral textures at multiferroic domain walls. Nature Materials. 19, 386-390 (2020).
Résumé : Chiral electric and magnetic structures are observed at domain walls in thin films of the room-temperature multiferroic BiFeO3.

• B. Chen, N. Gauquelin, D. Jannis, D. M. Cunha, U. Halisdemir, C. Piamonteze, J. H. Lee, J. Belhadi, F. Eltes, S. Abel, Z. Jovanović, M. Spreitzer, J. Fompeyrine, J. Verbeeck, M. Bibes, M. Huijben, G. Rijnders, Gn Koster, Strain‐Engineered Metal‐to‐Insulator Transition and Orbital Polarization in Nickelate Superlattices Integrated on Silicon. Advanced Materials. 32, 2004995 (2020).
Résumé : Strain engineering is well demonstrated for perovskite oxides, mostly by growing epitaxial films onto an isostructural oxide substrate having a different lattice constant. The joint effects of epitax...

• D. Crété, Y. Lemaître, B. Marcilhac, E. Recoba-Pawlowski, J. Trastoy, C. Ulysse, Optimal SQUID Loop Size in Arrays of HTS SQUIDs. Journal of Physics: Conference Series. 1559, 012012 (2020).

• T. H. Dang, Q. Barbedienne, D. Q. To, E. Rongione, N. Reyren, F. Godel, S. Collin, J. M. George, H. Jaffrès, Anomalous Hall effect in 3d/5d multilayers mediated by interface scattering and nonlocal spin conductivity. Physical Review B. 102, 144405 (2020).
Résumé : We have evidenced unconventional anomalous Hall effects (AHEs) in $3d/5d$ (${\mathrm{Co}0.2\mathrm{nm}/\mathrm{Ni}0.6\mathrm{nm})}_{N}$ multilayers grown on a thin Pt layer or thin Au:W alloys with perpendicular magnetic anisotropy (PMA) properties. The inversion of AHEs observed with one Pt series is explained by considering the opposite sign of the effective spin-orbit coupling of Pt compared to Co/Ni combined with peculiar specular electronic reflections. Using 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 of opposite sign. 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 (with the number of repetition layers $N&lt;17$). The spin-Hall angle in Pt is found to be larger than the one previously measured by complementary spin-pumping inverse spin-Hall effect experiments in a geometry of current perpendicular to the plane. Whereas magnetic proximity effects cannot explain the effect, spin-current leakage and spin-orbit assisted electron scattering at Pt/(Co,Ni) interfaces fit the experiments. We also extract the main relevant electronic transport parameters governing the overall effects in current-in-plane (CIP) currents and demonstrate, in particular, that the specularity/nonspecularity in the electronic diffusion processes play an essential role to explain the observed results.

• T. H. Dang, J. Hawecker, E. Rongione, G. B. Flores, D. Q. To, J. C. Rojas-Sanchez, H. Nong, J. Mangeney, J. Tignon, F. Godel, S. Collin, P. Seneor, M. Bibes, A. Fert, M. Anane, J. - M. George, L. Vila, M. Cosset-Cheneau, D. Dolfi, R. Lebrun, P. Bortolotti, K. Belashchenko, S. Dhillon, H. Jaffrès, Ultrafast spin-currents and charge conversion at 3d-5d interfaces probed by time-domain terahertz spectroscopy. Applied Physics Reviews. 7, 041409 (2020).
Résumé : 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 one 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.

• M. W. Daniels, A. Madhavan, P. Talatchian, A. Mizrahi, M. - D. Stiles, Energy-Efficient Stochastic Computing with Superparamagnetic Tunnel Junctions. Physical Review Applied. 13, 034016 (2020).
Résumé : Most computing schemes that employ superparamagnetic tunnel junctions control them with analog currents, leading to substantial Ohmic losses and requiring digital-to-analog converters. Here the authors forego current control and embed these junctions in digital circuits to produce programmable randomness, which leads to a neural network that can recognize handwritten digits at only 150 nJ per inference. This energy efficiency is made possible by the general insight that, while nanodevices provide useful dynamics for innovative computing, understanding their integration with digital logic systems is crucial to developing viable applications.

• V. E. Demidov, S. Urazhdin, A. Anane, V. Cros, S. O. Demokritov, Spin–orbit-torque magnonics. Journal of Applied Physics. 127, 170901 (2020).
Résumé : The field of magnonics, which utilizes propagating spin waves for nanoscale transmission and processing of information, has been significantly advanced by the advent of the spin–orbit torque. The l...
Mots-clés : Magnonics.

• D. Di Nuzzo, R. Mizuta, K. Nakanishi, M. - B. Martin, A. I. Aria, R. Weatherup, R. H. Friend, S. Hofmann, J. Alexander-Webber, Graphene-passivated nickel as an efficient hole-injecting electrode for large area organic semiconductor devices. Applied Physics Letters. 116, 163301 (2020).
Résumé : Efficient injection of charge from metal electrodes into semiconductors is of paramount importance to obtain high performance optoelectronic devices. The quality of the interface between the electr...

• B. Dieny, I. L. Prejbeanu, K. Garello, P. Gambardella, P. Freitas, R. Lehndorff, W. Raberg, U. Ebels, S. O. Demokritov, J. Akerman, A. Deac, P. Pirro, C. Adelmann, A. Anane, A. V. Chumak, A. Hirohata, S. Mangin, S. O. Valenzuela, M. C. Onbaşlı, M. d’Aquino, G. Prenat, G. Finocchio, L. Lopez-Diaz, R. Chantrell, O. Chubykalo-Fesenko, P. Bortolotti, Opportunities and challenges for spintronics in the microelectronics industry. Nature Electronics. 3, 446 (2020).
Résumé : This Review Article examines the potential of spintronics in four key areas of application —memories, sensors, microwave devices, and logic devices — and discusses the challenges that need be addressed in order to integrate spintronic materials and functionalities into mainstream microelectronic platforms.

• M. Drong, T. Fördös, H. Jaffrès, J. P. Jr, K. Postava, J. Pištora, H. - J. Drouhin, Local and mean-field approaches for modeling semiconductor spin-lasers. Journal of Optics. 22, 055001 (2020).

• A. Dyrdał, J. Barnaś, A. Fert, Spin-Momentum-Locking Inhomogeneities as a Source of Bilinear Magnetoresistance in Topological Insulators. Physical Review Letters. 124, 046802 (2020).
Résumé : A new model for bilinear magnetoresistance, so called because it is linear in both the electric and the magnetic field, proposes another origin mechanism that accounts for recent contradictory experimental observations.

• D. M. Evans, V. Garcia, D. Meier, M. Bibes, Domains and domain walls in multiferroics. Physical Sciences Reviews. 5, 20190067 (2020).
Résumé : Multiferroics are materials combining several ferroic orders, such as ferroelectricity, ferro- (or antiferro-) magnetism, ferroelasticity and ferrotoroidicity. They are of interest both from a fundamental perspective, as they have multiple (coupled) non-linear functional responses providing a veritable myriad of correlated phenomena, and because of the opportunity to apply these functionalities for new device applications. One application is, for instance, in non-volatile memory, which has led to special attention being devoted to ferroelectric and magnetic multiferroics. The vision is to combine the low writing power of ferroelectric information with the easy, non-volatile reading of magnetic information to give a “best of both worlds” computer memory. For this to be realised, the two ferroic orders need to be intimately linked via the magnetoelectric effect. The magnetoelectric coupling – the way polarization and magnetization interact – is manifested by the formation and interactions of domains and domain walls, and so to understand how to engineer future devices one must first understand the interactions of domains and domain walls. In this article, we provide a short introduction to the domain formation in ferroelectrics and ferromagnets, as well as different microscopy techniques that enable the visualization of such domains. We then review the recent research on multiferroic domains and domain walls, including their manipulation and intriguing properties, such as enhanced conductivity and anomalous magnetic order. Finally, we discuss future perspectives concerning the field of multiferroic domain walls and emergent topological structures such as ferroelectric vortices and skyrmions.

• K. Fallon, S. Hughes, K. Zeissler, W. Legrand, F. Ajejas, D. Maccariello, S. McFadzean, W. Smith, D. McGrouther, S. Collin, N. Reyren, V. Cros, C. H. Marrows, S. McVitie, Controlled Individual Skyrmion Nucleation at Artificial Defects Formed by Ion Irradiation. Small. 16, 1907450 (2020).
Résumé : Nanoscale artificial defects in magnetic multilayer films with perpendicular magnetic anisotropy (PMA) and Dzyaloshinskii–Moriya interaction (DMI), made in a controllable manner using a focused ion b...

• M. Galbiati, V. Zatko, F. Godel, P. Hirschauer, A. Vecchiola, K. Bouzehouane, S. Collin, B. Servet, A. Cantarero, F. Petroff, M. - B. Martin, B. Dlubak, P. Seneor, Very Long Term Stabilization of a 2D Magnet down to the Monolayer for Device Integration. ACS Applied Electronic Materials. 2, 3508 (2020).
Résumé : 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.

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