Skyrmions at room temperature From magnetic thin films to magnetic multilayers
Year: 2015

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  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  The existence of skyrmion spin configuration has been predicted theoretically about thirty years ago  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  or FeGe  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  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*(S1xS2) where D12 is the DM vector  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  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  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  Details about the growth conditions and the characterization of their magnetic properties are presented in the supplementary materials  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  Fig. 1. Interfacial Dzyaloshinskii Moriya interaction (DMI) in asymmetric magnetic multilayers.  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.  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  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 m0H 38 mT, the magnetic domains favored by the field extend