van der Waals ferromagnets, such as Fe5GeTe2, offer a promising platform for spintronic devices based on chiral magnetic textures, provided a significant Dzyaloshinskii-Moriya interaction (DMI) can be induced to stabilize the textures. Here, we directly measure DMI in epitaxial Fe5GeTe2 thin films using Brillouin light scattering spectroscopy and observe a consistent DMI (D = 0.04 mJ/m2) across various thicknesses. Its weak thickness dependence, combined with the nominally symmetric film interfaces, suggests a bulk origin. Although we do not determine the microscopic mechanism, our findings are compatible with ab initio calculations linking DMI to partial ordering of Fe split sites. Additionally, we find a low magnetic dissipation (α < 0.02). The observed DMI, which could be further enhanced by optimizing the Fe site ordering, combined with low dissipation, makes Fe5GeTe2 a strong candidate for exploring the dynamics of chiral magnetic textures in two-dimensional materials.

Due to their non-trivial topology, skyrmions describe deflected trajectories, which hinders their straight propagation in nanotracks and can lead to their annihilation at the track edges. This deflection is caused by a gyrotropic force proportional to the topological charge and the angular momentum density of the host film. In this article we present clear evidence of the reversal of the topological deflection angle of skyrmions with the sign of angular momentum density. We measured the skyrmion trajectories across the angular momentum compensation temperature (TAC) in GdCo thin films, a rare earth/transition metal ferrimagnetic alloy. The sample composition was used to engineer the skyrmion stability below and above the TAC. A refined comparison of their dynamical properties evidenced a reversal of the skyrmions deflection angle with the total angular momentum density. This reversal is a clear demonstration of the possibility of tuning the skyrmion deflection angle in ferrimagnetic materials and paves the way for deflection-free skyrmion devices.

The Dzyaloshinskii-Moriya interaction (DMI) and perpendicular magnetic anisotropy (PMA) were measured on four series of Co films (1-2.2 nm thick) grown on Pt or Au and covered with h-BN or Cu. Clean h-BN/Co interfaces were obtained by exfoliating h-BN and transferring it onto the Co film in situ in the ultra-high-vacuum evaporation chamber. By comparing h-BN and Cu-covered samples, the DMI induced by the Co/h-BN interface was extracted and found to be comparable in strength to that of the Pt/Co interface, one of the largest known values. The strong observed DMI despite the weak spin-orbit interaction in h-BN supports a Rashba-like origin in agreement with recent theoretical results. Upon combination of it with Pt/Co in Pt/Co/h-BN heterostructures, even stronger PMA and DMI are found which stabilizes skyrmions at room temperature and a low magnetic field.

Magnetic skyrmions are swirling magnetic textures that can be efficiently driven with spin-orbit torques with a deflected trajectory. However, pinning slows skyrmions down and alters their trajectory, which prevents a quantitative comparison to analytical models. Here, we study skyrmions driven by spin-orbit torques at room temperature in ferrimagnetic GdCo thin films, an amorphous material with low pinning. Above a sharp current depinning threshold, we observe a clearly linear velocity increase with current that extrapolates to zero and a constant deflection angle, reaching high velocities up to 200 m/s. The mobility increases and the depinning threshold current decreases with the skyrmion diameter, which we vary using an external magnetic field. An analytical model based on the Thiele equation quantitatively reproduces these findings with a single fitting parameter. This validates the linear flow regime description and shows, in particular, the important role of skyrmion size in its dynamics.

Rare-earth transition-metal ferrimagnets have two strongly coupled sublattices of distinct chemical nature, which give rise to complex and fast dynamics of great interest to spintronics. However, the dynamics of ferrimagnets remains less understood than ferromagnets. We measure the spin wave (SW) spectra of a GdFeCo film by Brillouin light scattering spectroscopy (BLS) across its compensation temperatures---temperatures at which either the sublattices' magnetizations or their angular moments cancel out, mimicking an antiferromagnet. We find two SW modes per wave vector with complex thermal dependencies, which cross at a field-dependent temperature. We develop an analytical model based on two sublattices corresponding to the rare earth and the transition metal, which reproduces quantitatively the SW spectra and their evolution with temperature and field. This validates the proposed energy and dynamical model of the ferrimagnet, and demonstrates the usefulness of BLS in the study of this promising class of materials.

We demonstrate that effects of spin-orbit coupling and inversion asymmetry exist in a single GdFeCo ferrimagnetic layer, even without a heavy metal interface. We use electric transport measurements to quantify the spin-orbit torques. We measure the Dzyaloshinskii-Moriya interaction using Brillouin light scattering measurement technique, and we observe the resulting chiral magnetic textures using x-ray PEEM microscopy. We attribute these effects to a composition variation along the thickness, that we observed by scanning transmission electron microscopy. We show that these effects can be optimized by varying the GdFeCo thickness or by combining them with interfacial effects.

In ferromagnetic materials, the rich dynamics of magnetic domain walls (DWs) under a magnetic field or current has been successfully described using the well-known q-phi analytical model. We demonstrate here that this simple unidimensional model holds for multiple-sublattice materials such as ferrimagnetic alloys or synthetic antiferromagnets (SAFs) by using effective parameters, and it is in excellent agreement with double-lattice micromagnetic simulations. We obtain analytical laws for the DW velocity and internal precession angle as a function of net magnetization for different driving forces (magnetic field, spin transfer, and spin-orbit torques) and different propagation regimes in ferrimagnetic alloys and SAFs. The model predicts that several distinctive dynamical features occur near or at the magnetic and the angular compensation points when the net magnetization or the net angular momentum of the system vanishes, and we discuss the experimental observations that have been reported for some of them. Using a higher degree-of-freedom analytical model that accounts for inter-sublattice distortions, we give analytical expressions for these distortions that agree with the micromagnetic simulations. This model shows that the DW velocity and precession rate are independent of the strength of the intersublattice exchange coupling, and it justifies the use of the simpler effective parameters model.

A multiscale approach is reported to understand the influence of the Ca and Y co-substitution in Bi-rich iron garnet thin films. Recently, it has been demonstrated that site-selective co-substitution of Ca( )(2+)and Y(3+ )in bismuth iron garnet (BIG) leads to either n- or p-type semiconductor behavior. However, the evolution of the structural and magnetic properties of bismuth iron garnet upon doping is still unknown. In the (Ca,Y):BIG, thin films grown by pulsed laser deposition onto Gd(3)Ga(5)O(12 )substrates, structural investigations confirm the epitaxial growth of doped thin films with high crystallinity and a complete strain relaxation from 20 nm above the film/substrate interface. While x-ray diffraction only evidences a single-phase garnet, the presence of secondary phase nanocrystallites, that are absent in pure BIG grown in similar conditions, is observed by aberration-corrected scanning transmission electron microscopy. These nanocrystallites form in between garnet grains as textured and poorly crystallized hematite. Despite the presence of nanoneedles, Ca and Y co-substitution preserves the giant Faraday rotation of pure BIG and maintains the Curie temperature above 590 K. Only minor energy shifts (80 meV) or small intensity changes (10%) of the Faraday rotation are observed upon doping or annealing and are mainly related to band-gap evolution and cell volume change. Furthermore, the easy magnetization axis rotates towards in the out-of-plane direction upon doping which is also promising for potential applications in spintronics.

One fundamental obstacle to efficient ferromagnetic spintronics is magnetic precession, which intrinsically limits the dynamics of magnetic textures. We experimentally demonstrate that this precession vanishes when the net angular momentum is compensated in domain walls driven by spin–orbit torque in a ferrimagnetic GdFeCo/Pt track. We use transverse in-plane fields to provide a robust and parameter-free measurement of the domain wall internal magnetisation angle, demonstrating that, at the angular compensation, the DW tilt is zero, and thus the magnetic precession that caused it is suppressed. Our results highlight the mechanism of faster and more efficient dynamics in materials with multiple spin lattices and vanishing net angular momentum, promising for high-speed, low-power spintronic applications.

We investigated spin injection by spin pumping from a spin-injector(NiFe) into a spin-sink to detect spin fluctuations in the spin-sink. By scanning the ordering-temperature of several magnetic transitions, we found that enhanced spin pumping due to spin fluctuations applies with several ordering states: ferromagnetic(Tb) and antiferromagnetic(NiO, NiFeOx, BiFeO3, exchange-biased and unbiased IrMn). Results also represent systematic experimental investigation supporting that the effect is independent of the metallic and insulating nature of the spin-sink, and is observed whether the spin current probe involves electronic or magnonic transport, facilitating advances in material characterization and engineering for spintronic applications.

We observe domain-wall (DW) motion in ferrimagnetic TbFe wires with perpendicular anisotropy under combined field and current in the creep regime. The current action on the DW is double: Joule heating and spin-transfer torque. We propose a genuinely robust analysis of velocity, separating thermal effects and spin-transfer torque, quantifying the latter as an equivalent field in the so-called one-dimensional (1D) model. Its efficiency is much larger than in transition-metal ferromagnets above room temperature. The equivalent field reveals the large polarization-to-magnetization ratio in ferrimagnets despite a vanishing Ms. The usual 1D DW model is extended to mimic creep and predicts that, in low net magnetization systems, the internal DW structure precesses with currents above a field-independent threshold, leading to two propagation regimes with different mobilities. This is another example of how the internal DW magnetization is relevant in creep. We could not detect experimentally these two regimes, possibly because of the dispersion of the data.

One fundamental obstacle to efficient ferromagnetic spintronics is magnetic precession, which intrinsically limits the dynamics of magnetic textures. It was recently shown that higher domain wall (DW) mobility (Caretta 2018; Hrabec 2018; Kim 2017; Siddiqui 2018; Yang 2015), lower topological deflection of skyrmions (Hirata 2019; Woo 2018), and lower critical currents (Bang 2016; Woo et al. 2018) could be obtained by using materials with multiple spin sub-lattices, such as antiferromagnets, ferrimagnets, or synthetic antiferromagnets. We study DWs driven by spin-orbit torque in a ferrimagnetic GdFeCo/Pt track, and we demonstrate that the DW precession fully vanishes with a record mobility at the temperature for which the net angular momentum is compensated (TAC). We use a new method based on transverse in-plane fields that reveals the internal structure of DWs and provides a robust and parameter-free measurement of TAC. Our results put in a clear light the mechanism of faster and more efficient dynamics in systems with a reduced net angular momentum and their promise for high-speed, low-power spintronics applications.

Ferrimagnetic TbFe or TbFeCo amorphous alloy thin films have been grown by coevaporation in ultrahigh vacuum. They exhibit an out-of-plane magnetic anisotropy up to their Curie temperature with a nucleation and propagation reversal mechanism suitable for current induced domain wall motion. Rutherford backscattering experiments confirmed a fine control of the Tb depth-integrated composition within the evaporation process. However, a large set of experimental techniques were used to evidence an interface related contribution in such thin films as compared to much thicker samples. In particular, scanning transmission electron microscopy experiments evidence a depth dependent composition and perturbed top and bottom interfaces with preferential oxidation and diffusion of terbium. Despite that, amorphous and homogeneous alloy film remains in a bulklike part. The composition of that bulklike part of the magnetic layer, labeled as effective composition, is biased when compared with the depth-integrated composition. The magnetic properties of the film are mostly dictated by this effective composition, which we show changes with different top and bottom interfaces.

We study the ultraslow domain-wall motion in ferromagnetic thin films driven by a weak magnetic field. Using time-resolved magneto-optical Kerr effect microscopy, we access to the statistics of the intermittent thermally activated domain-wall jumps between deep metastable states. Our observations are consistent with the existence of creep avalanches: roughly independent clusters with broad size and ignition waiting-time distributions, each one composed by a large number of spatiotemporally correlated thermally activated elementary events. Moreover, we evidence that the large-scale geometry of domain walls is better described by depinning rather than equilibrium universal exponents.

The development of reliable and highly energy efficient multiferroic nanosystems, which can function at room temperature, is key for the design of ultralow-power magnetoelectric devices. Here, we report electrically controlled magnetic domain wall motion and magnetization switching in BaTiO3/Co50Fe50 microstructures, at room temperature. The perfect one-to-one connection between the ferroelectric domain pattern of the BaTiO3 crystal and the ferromagnetic state of the CoFe microstructures, which relies on a strain-induced magnetic anisotropy modification, is the cause of the observed magnetoelectric effect. As a result, the observed electrically driven magnetization switching is highly reliable, independent of the shape and size of the microstructures. This is a key factor that makes the studied multiferroic system very promising for integration in real-world magnetoelectric devices.

30 keV Ga + irradiation-induced changes of magnetic and magneto-optical properties of sputtered Pt/Co/Pt ultrathin trilayers films have been studied as a function of the ion fluence. Out-of-plane magnetic anisotropy states with enhanced magneto-optical effects were evidenced for specific values of cobalt thickness and irradiation fluence. Results obtained after uniform or quasi-uniform focused ion beam irradiation on either out-of-plane or in-plane magnetized sputtered pristine trilayers are compared. Similar irradiation-induced magnetic changes are evidenced in quasi-uniformly focused ion beam or uniformly irradiated films, grown either by sputtering or molecular beam epitaxy. We discuss on plausible common mechanisms underlying the observed effects.

The possibility of manipulating magnetic domain walls (DWs) using electrical current is very attractive for magnetic devices that store and process non-volatile information [1]. To estimate the efficiency of current acting on a magnetic texture (by Spin Transfer Torque for instance), the relevant quantity is a drift speed $\mathrm {u}=( \mathrm {g}\mu _{B}\mathrm {P}) /$(2eMs) J where J is the current density, P its spin polarisation in the magnetic media, Ms the net magnetisation, g the Landé factor, $\mu _{B}$ the Bohr magneton, e the electron charge [2]. The analytical $\mathrm {q}- \varphi $ model of DW motion along 1D wire shows that DW motion induced just by field or just by STT exhibits 2 different DW propagation regimes [3]. For low field or low current (low u), the DW moves steadily with just a tilt of its central magnetisation. This regime is called translational regime. For stronger field or current (strong u), the DW moves with a continuous precession of its central magnetisation. This regime is called precessional regime. In both regimes, speeds are proportional to H or u. The 2 regimes are separated by a critical field (or critical current) called Walker field (or current). Since the velocity is linear with H or u, it is possible to convert a current density acting on the DW into an equivalent field Heq defined as the field necessary to induce the same macroscopic velocity as the current density. In this equivalent field approach, Heq is proportional to u, with a proportionality constant for each regime. In classical ferromagnetic materials that have been mostly studied, P and Ms have the same physical origin and thermal dependence. Therefore, for those materials, the ratio P/ Ms entering u which governs efficiency of STT is fixed. To play with P/ Ms, we focused on more exotic materials namely Rare Earth/ Transition Metal (RETM) ferrimagnetics alloys [4] in which it is possible to tune independently Ms or P by composition or temperature. Indeed, in RETM, two populations of magnetic moments are antiferromagnetically coupled: 3d TM moments are antiparallel to 5d and localised 4f RE moments. The alloys net magnetisation is the difference of moments of the 2 populations whereas spin polarisation P arises only from that of RE and TM conduction electrons. We measured amorphous ferrimagnetic TbFe alloys thin films grown by coevaporation. They exhibit perpendicular magnetic anisotropy and P and Ms have clearly different thermal dependence (Fig 1a). The propagation of DWs in TbFe microtracks was analysed using Kerr microscopy. In a first step, we measured the velocity under continuous field (without current pulses) at different temperatures. We observed a nonlinear behaviour of velocity versus field and a strong dependence with temperature (Fig 1b). This type of DW dynamic is called creep regime. In this regime, the DWM is characterised by discrete hopping of the DW between weak pinning centres acting collectively and the DW velocity is described by an Arrhenius law. The energy barrier to overcome by thermal activation depends on the applied field H weighted by a universal exponent $(\mu = -1/4)$ that describes the motion of 1D elastic system in 2D random disorder media [5]. Fig 2a illustrates our original results demonstrating Current Induced DW Motion in TbFe wires under combined field and current and Fig 1c shows DW velocities for a few current densities. Two main observations can be done. The STT-like action pushes DWs along the electrons flow and can add or substract to the field action: a signature of STT is the increase of the split between fast (up-triangle) and slow (down-triangle) DWs. Joule heating modifies the creep dynamic and makes DWMs easier in both directions: the mean speed increases. A very careful analysis of the creep velocity was performed taking into account field, current and temperature versus time. We could evaluate Joule heating and the current contribution in terms of equivalent field Heq. In Fig 2b, Heq is reported versus current density (J) and clearly presents 2 regimes. Heq is not proportional to J P/Ms over the entire range (dashed line - material parameters from Fig 1a), as expected in conventional STT [3]. Based on an extended $\mathrm {q}- \varphi 1\mathrm {D}$ model, we describe two regimes separated by a Walker-like threshold above which the CIDWM is more efficient, maybe thanks to changes of DW structure such as the creation of Néel lines [6].

Ferrimagnetic TbFe or TbFeCo amorphous alloy thin films have been grown by co-evaporation in ultra-high vacuum. They exhibit an out-of-plane magnetic anisotropy up to their Curie temperature with a nucleation and propagation reversal mechanism suitable for current induced domain wall motion. Rutherford back scattering experiments confirmed a fine control of the Tb depth-integrated composition within the evaporation process. However, a large set of experimental techniques were used to evidence an interface related contribution in such thin films as compared to much thicker samples. In particular, scanning transmission electron microscopy experiments evidence a depth dependent composition and perturbed top and bottom interfaces with preferential oxidation and diffusion of terbium. Despite of that, amorphous and homogeneous alloy film remains in a bulk-like part. The composition of that bulk-like part of the magnetic layer, labeled as effective composition, is biased when compared with the depth-integrated composition. The magnetic properties of the film are mostly dictated by this effective composition, which we show changes with different top and bottom interfaces.

Correlation between structure, electronic properties, and magnetism in CoxGd1-x thin amorphous films was investigated. The thickness averaged properties of covered thin films are consistent with those of in-depth homogeneous amorphous alloys. In spite of that, x-ray magnetic circular dichroism (XMCD) and photoemission measurements on as-grown films have shown signatures consistent with a lateral gradient composition and a Gd surfactant effect. A tendency of Gd atoms to migrate to the surface followed by an oxidation down to 8 angstrom has been evidenced. Our results further demonstrate the extreme sensitivity of the magnetic properties of ferrimagnetic alloys to their structure and local concentration.

Using an experimental setup designed to scan a submicron sized light spot and collect the photogenerated current through larger electrodes, we map the photovoltaic response in ferroelectric BiFeO3 single crystals. We study the effect produced by a unique 180{\textdegree} ferroelectric domain wall (DW) and show that the photocurrent maps are significantly affected by its presence and shape. The effect is large in its vicinity and in the Schottky barriers at the interface with the Au electrodes, but no extra photocurrent is observed when the illuminating spot touches the DW, indicating that this particular entity is not the heart of specific photo-electric properties. Using 3D modelling, we argue that the measured effect is due to the spatial distribution of internal fields which are significantly affected by the charge of the DW due to its distortion.