Phillip Wohlhüter

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

Spin configurations in Co2FeAl0.4Si0.6 Heusler alloy thin film elements

We determine experimentally the spin structure of half-metallic Co2FeAl0.4Si0.6 Heusler alloy elements using magnetic microscopy. Following magnetic saturation, the dominant magnetic states consist of quasi-uniform configurations, where a strong influence from the magnetocrystalline anisotropy is visible. Heating experiments show the stability of the spin configuration of domain walls in confined geometries up to 800 K. The switching temperature for the transition from transverse to vortex walls in ring elements is found to increase with ring width, an effect attributed to structural changes and consequent changes in magnetic anisotropy, which start to occur in the narrower elements at lower temperatures.

Micromagnetic simulation of exchange coupled ferri-/ferromagnetic composite in bit patterned media

Ferri-/ferromagnetic exchange coupled composites are promising candidates for bit patterned media because of the ability to control the magnetic properties of the ferrimagnet by its composition. A micromagnetic model for the bilayer system is presented where we also incorporate the microstructural features of both layers. Micromagnetic finite element simulations are performed to investigate the magnetization reversal behaviour of such media. By adding the exchange coupled ferrimagnet to the ferromagnet, the switching field could be reduced by up to 40% and also the switching field distribution is narrowed. To reach these significant improvements, an interface exchange coupling strength of 2 mJ/m2 is required.

Nanoscale switch for vortex polarization mediated by Bloch core formation in magnetic hybrid systems

Vortices are fundamental magnetic topological structures characterized by a curling magnetization around a highly stable nanometric core. The control of the polarization of this core and its gyration is key to the utilization of vortices in technological applications. So far polarization control has been achieved in single-material structures using magnetic fields, spin-polarized currents or spin waves. Here we demonstrate local control of the vortex core orientation in hybrid structures where the vortex in an in-plane Permalloy film coexists with out-of-plane maze domains in a Co/Pd multilayer. The vortex core reverses its polarization on crossing a maze domain boundary. This reversal is mediated by a pair of magnetic singularities, known as Bloch points, and leads to the transient formation of a three-dimensional magnetization structure: a Bloch core. The interaction between vortex and domain wall thus acts as a nanoscale switch for the vortex core polarization.

Control of the magnetization in pre-patterned half-metallic La0.7Sr0.3MnO3 nanostructures

The evolution of the magnetization configurations in highly spin polarized La0:7Sr0:3MnO3 (LSMO) thin film elements (20-60 nm in thickness) as a function of external magnetic field and temperature is studied by direct magnetic imaging using x-ray magnetic circular dichroism photoemission electron microscopy. The sample structuring is done via a pre-patterning process using a Cr mask layer. The LSMO grows amorphous on the Cr layer for the 20 nm thick film but polycrystalline at larger thicknesses. Temperature dependent studies allow for a direct comparison of the properties of the strained and unstrained LSMO regions on a single sample and show that the polycrystalline areas exhibit a higher TC compared to the epitaxial areas. The single crystalline areas are largely magnetically decoupled from the matrix. The magnetic switching between domain states and domain wall spin structures is determined for LSMO ring elements of varying size and thickness. We find that the magnetic field values required to depin domain walls or to nucleate domains increase with decreasing ring width due to the increasing role of shape anisotropy and edge defects. Both transverse and vortex domain walls are stable spin configurations at room temperature and at zero field. In particular, we demonstrate that the desired domain wall type can be selected by applying an appropriate field sequence.

Control of the gyration dynamics of magnetic vortices by the magnetoelastic effect

The influence of a strain-induced uniaxial magnetoelastic anisotropy on the magnetic vortex core dynamics in microstructured magnetostrictive Co40Fe40B20 elements was investigated with time-resolved scanning transmission x-ray microscopy. The measurements revealed a monotonically decreasing eigenfrequency of the vortex core gyration with the increasing magnetoelastic anisotropy, which follows closely the predictions from micromagnetic modeling.

Switching field distribution of exchange coupled ferri-/ferromagnetic composite bit patterned media

We investigate the switching field distribution and the resulting bit error rate of exchange coupled ferri-/ferromagnetic bilayer island arrays by micromagnetic simulations. Using islands with varying microstructure and anisotropic properties, the intrinsic switching field distribution is computed. The dipolar contribution to the switching field distribution is obtained separately by using a model of a triangular patterned island array resembling 1.4 Tb/in2 bit patterned media. Both contributions are computed for different thicknesses of the soft exchange coupled ferrimagnet and also for ferromagnetic single phase FePt islands. A bit patterned media with a bilayer structure of FeGd( 5 nm)/FePt( 5 nm) shows a bit error rate of 10−4 with a write field of 1.16 T.

Observation of the out-of-plane magnetization in a mesoscopic ferromagnetic structure superjacent to a superconductor

The geometry of magnetic flux penetration in a high temperature superconductor at a buried interface was imaged using element-specific x-ray excited luminescence. We performed low temperature observation of the flux penetration in YBa2Cu3O7–δ (YBCO) at a buried interface by imaging of the perpendicular magnetization component in square Permalloy (Py) mesostructures patterned superjacent to a YBCO film. Element specific imaging below the critical temperature of YBCO reveals a cross-like geometry of the perpendicular magnetization component which is decorated by regions of alternating out-of-plane magnetization at the edges of the patterned Py structures. The cross structure can be attributed to the geometry of flux penetration originating from the superconductor and is reproduced using micromagnetic simulations. Our experimental method opens up possibilities for the investigation of flux penetration in superconductors at the nanoscale.The geometry of magnetic flux penetration in a high temperature superconductor at a buried interface was imaged using element-specific x-ray excited luminescence. We performed low temperature observation of the flux penetration in YBa2Cu3O7–δ (YBCO) at a buried interface by imaging of the perpendicular magnetization component in square Permalloy (Py) mesostructures patterned superjacent to a YBCO film. Element specific imaging below the critical temperature of YBCO reveals a cross-like geometry of the perpendicular magnetization component which is decorated by regions of alternating out-of-plane magnetization at the edges of the patterned Py structures. The cross structure can be attributed to the geometry of flux penetration originating from the superconductor and is reproduced using micromagnetic simulations. Our experimental method opens up possibilities for the investigation of flux penetration in superconductors at the nanoscale.

Tunable magnetic vortex resonance in a potential well

We use frequency-resolved x-ray microscopy to fully characterize the potential well of a magnetic vortex in a soft ferromagnetic permalloy square. The vortex core is excited with magnetic broadband pulses and simultaneously displaced with a static magnetic field. We observe a frequency increase (blueshift) in the gyrotropic mode of the vortex core with increasing bias field. Supported by micromagnetic simulations, we show that this frequency increase is accompanied by internal deformation of the vortex core. The ability to modify the inner structure of the vortex core provides a mechanism to control the dynamics of magnetic vortices.

Vortex motion in amorphous ferrimagnetic thin film elements

Amorphous Fe64Gd36 thin film square elements are investigated by imaging in the Fresnel mode of a transmission electron microscope (TEM). The equilibrium state without an applied field shows the well-known four-domain flux closure pattern with in-plane magnetization. However, the vortex is displaced from the center of the square element and the domain walls are curved. In a reference measurement of a thin Ni81Fe19 element, the vortex core is perfectly centered and the domain walls straight. When an increasing external field is applied in-plane, the vortex core can be moved. While this motion of the vortex core is linear in NiFe elements, in the ferrimagnetic FeGd squares the vortex core moves by sudden jumps. Micromagnetic simulations show that the asymmetry of the domain patterns as well as the vortex core pinning and depinning can be attributed to random anisotropy and a patchy microstructure in amorphous films.