K. Garcia

Additive interfacial chiral interaction in multilayers for stabilization of small individual skyrmions at room temperature

Facing the ever-growing demand for data storage will most probably require a new paradigm. Nanoscale magnetic skyrmions are anticipated to solve this issue as they are arguably the smallest spin textures in magnetic thin films in nature. We designed cobalt-based multilayered thin films in which the cobalt layer is sandwiched between two heavy metals and so provides additive interfacial Dzyaloshinskii-Moriya interactions (DMIs), which reach a value close to 2 mJ m(-2) in the case of the Ir|Co|Pt asymmetric multilayers. Using a magnetization-sensitive scanning X-ray transmission microscopy technique, we imaged small magnetic domains at very low fields in these multilayers. The study of their behaviour in a perpendicular magnetic field allows us to conclude that they are actually magnetic skyrmions stabilized by the large DMI. This discovery of stable sub-100 nm individual skyrmions at room temperature in a technologically relevant material opens the way for device applications in the near future.Facing the ever-growing demand for data storage will most probably require a new paradigm. Magnetic skyrmions are anticipated to solve this issue as they are arguably the smallest spin textures in magnetic thin films in nature. We designed cobalt-based multilayered thin films where the cobalt layer is sandwiched between two heavy metals providing additive interfacial Dzyaloshinskii-Moriya interactions, which reach about 2 mJ/m in the case of the Ir|Co|Pt multilayers. Using a magnetization-sensitive scanning x-ray transmission microscopy technique, we imaged magnetic bubble-like domains in these multilayers. The study of their behavior in magnetic field allows us to conclude that they are actually magnetic skyrmions stabilized by the Dzyaloshinskii-Moriya interaction. This discovery of stable skyrmions at room temperature in a technologically relevant material opens the way for device applications in a near future. A major societal challenge is related to the continually increasing quantity of information to process and store. The hard disk drives, in which information is encoded magnetically, allow nowadays the storage of zettabytes (10) of information, but this technology should soon reach its limits. An up-and-coming avenue has been opened by the discovery of magnetic skyrmions [1], i.e. spin windings that can be localized within a diameter of a few nanometers and can move like particles [2]. These magnetic solitons, remarkably robust against defects due to the topology of their magnetic texture [3], are promising for being the ultimate magnetic bits to carry and store information. The topology of the skyrmions also appears to further underlie other important features such as their current-induced motion induced by small dc currents that is crucial for real applications but also the existence of a specific component in Hall Effect [4-6] that can be used advantageously for an electrical read-out of the information carried by nano-scale skyrmions. We proposed recently that these skyrmions could be used in future storage devices and information processing [2]. The existence of skyrmion spin configuration has been predicted theoretically about thirty years ago [1] but it was only recently that skyrmion lattices have been observed in crystals with noncentrosymmetric lattices, e.g. B20 crystallographic structure in MnSi [7-9], FeCoSi [10] or FeGe [5] crystals. In 2011, skyrmions have also been identified in single ultrathin ferromagnetic films with out-of-plane magnetization (Fe and FePd) deposited on a heavy metal substrate such as Ir(1 1 1) [11, 12]. Thin magnetic films appear to be more compatible with technological developments, though the observation of skyrmions in thin films has been limited up to now to low temperature and also needs, in some cases, the presence of a large applied magnetic field [12]. The study of these new magnetic phases associated with chiral interactions has generated a very strong interest in the community of solid state physics and magnetic systems with reduced dimensions. The magnetic skyrmions are magnetic solitons that are, in most cases, induced by chiral interactions of the Dzyaloshinskii-Moriya (DM) type, which result from spin-orbit effects in the absence of inversion symmetry. The DM Interaction (DMI) between two neighboring atomic spins S1 and S2 can be expressed as: HDMI = D12·(S1×S2) where D12 is the DM vector (Fig. 1A). In this work, we will focus on ultrathin magnetic systems in which the DMI results from the breaking of the inversion symmetry at the interface of a magnetic layer and can be large at the interface with a heavy metal having a large spin-orbit coupling [11]. In such case, the DM vector D12 is perpendicular to the r(S1)−r(S2) line (r(·) being the position vector) and gives rise to cycloidal skyrmionic configurations, which have a given chirality, also called hedgehog skyrmion (see Fig. 1B). Our main goal is to extend the already-observed generation of interfaceinduced skyrmions from single magnetic films [11, 12] to multilayers by stacking layers of magnetic metals and nonmagnetic heavy metals (e.g. Pt and Ir) so as to induce DMI at all the magnetic interfaces (Fig.1A). The advantages of such innovative multilayered systems are twofold. Firstly we anticipate that thermal stability of skyrmions can be greatly improved, simply because of the increase of their magnetic volume, as it turns out for our samples that the same magnetic texture extends vertically throughout the multilayer. Secondly, the choice of two different heavy metals A and B sandwiching each magnetic layer potentially allows tuning the amplitude of interfacial chiral interaction, notably to increase it drastically if the two heavy materials induce interfacial chiral interactions of opposite symmetries and parallel D when they are on opposite sides of the magnetic layer. As we will see, our multilayers present bubble-like local configurations characterized by a reversed magnetization in the center [15]. The first important conclusion that can be draught is that the presence of these bubble-like configurations cannot be accounted for by dipolar interaction but only by the large DMI of our asymmetric multilayered systems. We will then show that the size of the bubble-like configurations and its field dependence is consistent with micromagnetic simulations of skyrmionic states induced by large DMI. This stabilization in multilayered films and at room temperature of individual magnetic skyrmions induced by chiral interactions represents the most important result of this work. Given the important features of magnetic skyrmions associated to their topological nature compared to other magnetic configurations (size, easier current-induced propagation, smaller sensitivity to defects, ...), these advances represent a definite breakthrough in the research on single skyrmion towards the potential development of skyrmion-based devices. Multilayers with additive chiral interaction at interfaces with heavy metal layers The prototype of the multilayered systems in which we aim to investigate the magnetic configuration is presented in Fig. 1A. The samples grown by sputtering deposition are stacks of 10 Ir|Co|Pt trilayers, each trilayer being composed of a 0.6 nm thick Co layer sandwiched between 1 nm of Ir and 1 nm of Pt: Pt10|Co0.6|Pt1|{Ir1|Co0.6|Pt1}10|Pt3. The choice of the two heavy materials i.e. Pt and Ir, has been guided by recent experiments of asymmetric domain wall propagation [16, 17] and recent ab initio predictions of opposite DMI for Co on Ir and Co on Pt [18], which corresponds to additive DMI at the two interfaces of the Co layers sandwiched between Ir and Pt. In addition to these Ir|Co|Pt asymmetric multilayers, we also prepared reference samples of Pt|Co|Pt with symmetric interfaces, in which one can expect a cancellation, at least partial, of the interfacial DMI [19]. Details about the growth conditions and the characterization of their magnetic properties are presented in the supplementary materials [14]. From SQUID measurements on our multilayers, we deduce a magnetization at saturation of 0.96 ± 0.10 MA/m (1.6 ± 0.2 MA/m) and an effective anisotropy of 0.17 ± 0.04 MJ/m (0.25 ± 0.07 MJ/m) for the Ir|Co|Pt (Pt|Co|Pt) system. These magnetic parameters have been used for micromagnetic simulations of their magnetic configuration [20, 21]. We present here simulations for the case of Co layers completely decoupled. The case taking into account the proximity-induced moment [22, 23] in Pt and Ir is detailed in the supplementary materials and lead to even larger estimation of the DMI amplitude [14]. Fig. 1. Interfacial Dzyaloshinskii-Moriya interaction (DMI) in asymmetric magnetic multilayers. (A) The DMI for two magnetic atoms close to an atom with large spin-orbit coupling in the Fert-Levy picture [13]. Zoom on a single trilayer composed of a magnetic layer (gray) sandwiched between two different heavy metals A (blue) and B (green) that induce the same chirality (same orientation of D) when A is below and B above the magnetic layer, and finally on an asymmetric multilayer made of several repetition of the trilayer. (B) Sketch of an isolated hedgehog skyrmion stabilized by interfacial chiral interaction in a magnetic thin film. (C-F) 800×800 nm out-of-plane magnetization (mz) map obtained by Scanning Transmission X-ray Microscopy (STXM) on a {Ir|Co|Pt}10 multilayer at room temperature for applied out-of-plane magnetic fields of 8 (C), 38 (D), 73 (E) and 83 mT (F). (G) Experimental dichroic signal through a magnetic circular domain (skyrmion) as observed at 22 mT (black dots). The corresponding STXM 360×360 nm image is in inset. The blue dashed curve is the magnetization profile of an ideal 30 nm-radius skyrmion and the red curve derives from the model described in the text. (H) Same type of data at 58 mT and corresponding simulation of 20 nm-radius skyrmion. The dichroic signal is not reversed due to the limited resolution of the STXM [14]. Magnetization mapping in asymetric multilayers: Bubbles or skyrmions? In order to map the distribution of the vertical component of the magnetization in our Ir|Co|Pt multilayers and to follow its evolution as a function of the external perpendicular field, we have performed Scanning Transmission X-ray Microscopy (STXM) experiments on samples grown on Si3N4 membranes by measuring the dichroic signal at Co L3-edge [14]. In Fig. 1C-F, we display a series of images obtained at different perpendicular field values. After saturation at large negative field, we observe a domain configuration (Fig. 1C) at low positive field that combines some worm-shape domains together with other domains having almost a circular shape. Note that the magnetic contrast that we detect by STXM is an averaged value of the magnetic configuration throughout the entire multilayer. When the field is increased to μ0H⊥ = 38 mT, the magnetic domains favored by the field extend (Fig. 1D). Before re

The nature of domain walls in ultrathin ferromagnets revealed by scanning nanomagnetometry.

The capacity to propagate magnetic domain walls with spin-polarized currents underpins several schemes for information storage and processing using spintronic devices. A key question involves the internal structure of the domain walls, which governs their response to certain current-driven torques such as the spin Hall effect. Here we show that magnetic microscopy based on a single nitrogen-vacancy defect in diamond can provide a direct determination of the internal wall structure in ultrathin ferromagnetic films under ambient conditions. We find pure Bloch walls in Ta/CoFeB(1 nm)/MgO, while left-handed Néel walls are observed in Pt/Co(0.6 nm)/AlOx. The latter indicates the presence of a sizable interfacial Dzyaloshinskii-Moriya interaction, which has strong bearing on the feasibility of exploiting novel chiral states such as skyrmions for information technologies.

Room-Temperature Current-Induced Generation and Motion of sub-100 nm Skyrmions

Magnetic skyrmions are nanoscale windings of the spin configuration that hold great promise for technology due to their topology-related properties and extremely reduced sizes. After the recent observation at room temperature of sub-100 nm skyrmions stabilized by interfacial chiral interaction in magnetic multilayers, several pending questions remain to be solved, notably about the means to nucleate individual compact skyrmions or the exact nature of their motion. In this study, a method leading to the formation of magnetic skyrmions in a micrometer-sized track using homogeneous current injection is evidenced. Spin-transfer-induced motion of these small electrical-current-generated skyrmions is then demonstrated and the role of the out-of-plane magnetic field in the stabilization of the moving skyrmions is also analyzed. The results of these experimental observations of spin torque induced motion are compared to micromagnetic simulations reproducing a granular type, nonuniform magnetic multilayer in order to address the particularly important role of the magnetic inhomogeneities on the current-induced motion of sub-100 nm skyrmions for which the material grains size is comparable to the skyrmion diameter.

Real-space imaging of non-collinear antiferromagnetic order with a single-spin magnetometer

Although ferromagnets have many applications, their large magnetization and the resulting energy cost for switching magnetic moments bring into question their suitability for reliable low-power spintronic devices. Non-collinear antiferromagnetic systems do not suffer from this problem, and often have extra functionalities: non-collinear spin order may break space-inversion symmetry and thus allow electric-field control of magnetism, or may produce emergent spin–orbit effects that enable efficient spin–charge interconversion. To harness these traits for next-generation spintronics, the nanoscale control and imaging capabilities that are now routine for ferromagnets must be developed for antiferromagnetic systems. Here, using a non-invasive, scanning single-spin magnetometer based on a nitrogen–vacancy defect in diamond, we demonstrate real-space visualization of non-collinear antiferromagnetic order in a magnetic thin film at room temperature. We image the spin cycloid of a multiferroic bismuth ferrite (BiFeO3) thin film and extract a period of about 70 nanometres, consistent with values determined by macroscopic diffraction. In addition, we take advantage of the magnetoelectric coupling present in BiFeO3 to manipulate the cycloid propagation direction by an electric field. Besides highlighting the potential of nitrogen–vacancy magnetometry for imaging complex antiferromagnetic orders at the nanoscale, these results demonstrate how BiFeO3 can be used in the design of reconfigurable nanoscale spin textures.

Electrical detection of single magnetic skyrmions in metallic multilayers at room temperature

Magnetic skyrmions are topologically protected whirling spin textures that can be stabilized in magnetic materials by an asymmetric exchange interaction between neighbouring spins that imposes a fixed chirality. Their small size, together with the robustness against external perturbations, make magnetic skyrmions potential storage bits in a novel generation of memory and logic devices. To this aim, their contribution to the electrical transport properties of a device must be characterized—however, the existing demonstrations are limited to low temperatures and mainly in magnetic materials with a B20 crystal structure. Here we combine concomitant magnetic force microscopy and Hall resistivity measurements to demonstrate the electrical detection of sub-100 nm skyrmions in a multilayered thin film at room temperature. Furthermore, we detect and analyse the Hall signal of a single skyrmion, which indicates that it arises from the anomalous Hall effect with a negligible contribution from the topological Hall effect.Single magnetic skyrmions are electrically detected in magnetic multilayers at room temperature, and their main contribution to the signal, which is enhanced for tracks approaching the size of the skyrmions, comes from the anomalous—rather than topological—Hall effect.

Performance analysis of MgO-based perpendicularly magnetized tunnel junctions

We studied state of the art perpendicularly magnetized tunnel junctions to identify performance improvement opportunities. The free layer has both a low damping and a large anisotropy. Conversely, the perpendicular remanence of the reference layer requires its encapsulation and its coupling with the hard layer. The weak pinning and low damping of the reference layer may make it prone to fluctuations induced by spin-torque. The combined optimization of the interface anisotropies on both sides of the MgO, together with the reproducibility of the interlayer exchange coupling are the main material challenges for our type of magnetic tunnel junctions.

Effect of Ta Insertion in Reference Layers of MTJs With Perpendicular Anisotropy

We analyze the opportunity of inserting a Ta layer between the polarizing section and the high anisotropy section in the reference subsystem of CoFeB-based tunnel junctions with perpendicular magnetic anisotropy. Using vector network analyzer ferromagnetic resonance, polar Kerr magnetometry, and magnetization dynamics modeling, we deduce the strength of the ferromagnetic interlayer exchange coupling energy J through various thickness of tantalum that impacts on the overall performance of the tunnel junctions. J culminates at 0.44 mJ/m2 through 3 Å of Ta, but the Co/Pt properties are then suboptimal. Our methodology can be used to rationally find the performance optimum in the reference layers of perpendicularly magnetized tunnel junctions.

Measuring the Magnetic Moment Density in Patterned Ultrathin Ferromagnets with Submicrometer Resolution

We present a new approach to infer the surface density of magnetic moments

Direct measurement of interfacial Dzyaloshinskii-Moriya interaction in X vertical bar CoFeB vertical bar MgO heterostructures with a scanning NV magnetometer (X=Ta, TaN, and W)

I_s

A transmission electron microscope study of Néel skyrmion magnetic textures in multilayer thin film systems with large interfacial chiral interaction

in ultrathin ferromagnetic films with perpendicular anisotropy. It relies on quantitative stray field measurements with an atomic-size magnetometer based on the nitrogen-vacancy center in diamond. The method is applied to microstructures patterned in a 1-nm-thick film of CoFeB. We report measurements of