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 2 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 21 ) 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][5][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][8][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

We demonstrate in situ 90° electric field-induced uniform magnetization rotation in single domain submicron ferromagnetic islands grown on a ferroelectric single crystal using x-ray photoemission electron microscopy. The experimental findings are well correlated with micromagnetic simulations, showing that the reorientation occurs by the strain-induced magnetoelectric interaction between the ferromagnetic nanostructures and the ferroelectric crystal. Specifically, the ferroelectric domain structure plays a key role in determining the response of the structure to the applied electric field, resulting in three strain-induced regimes of magnetization behavior for the single domain islands.

Magnetic skyrmions are arguably the smallest stable magnetic configuration in films, and therefore could be the ultimate magnetic storage bit [1,2] . They have also triggered a wide interest due to the new fundamental phenomena related to their topology . Numerical simulations have shown that the interfacial Dzyaloshinskii-Moriya interaction (DMI) can stabilize such skyrmions in nanoscale disks or tracks for a rather large range of DMI amplitudes for which the skyrmion can either be the ground state or metastable relative to the uniform state [4,5,6] . Here, we demonstrate experimentally the presence of skyrmions in metallic multilayers structures engineered to exhibit a strong DMI interaction . In this work, beyond the study of skyrmions in thin films [3], we focus our investigation on sputtered multilayers with vertical magnetization consisting of stacks of trilayers composed of 0 .6-nm-thick Co layers sandwiched between 5d transition metal layers, namely Pt, Ir and W . Asymmetric sandwiches were designed in order to introduce additive DMI from the top and bottom interfaces of Co [2,5,6] while also obtaining a considerable perpendicular magnetic anisotropy . We will present our results on two types of metallic multilayers grown at room temperature: |Pt10|-Co0 .6|Pt1|{Co0 .6|Pt1}x10|Pt3 and |Pt10|Co0 .6|Pt1|{Ir1|Co0 .6|Pt1}x10|Pt3 (thickness in nm) . The magnetic anisotropy is determined using standard magnetometry, while the DMI amplitude is estimated by two original methods . Based on detailed mapping of the magnetization obtained using scanning transmission X-ray microscopy (STXM) combined with the XMCD effect, STXM allows the magnetic imaging of patterned structures in a non-invasive way with nanoscale resolution (<50 nm) [7] . We acquired such images at different perpendicular magnetic fields in both symmetric Pt|Co|Pt and asymmetric Pt|Co|Ir multilayers [8] . From the analysis of the magnetic domain configurations, either at zero field after demagnetization (not shown) or following the evolution of the size of bubbles with perpendicular magnetic field (Figure 1), we evaluate consistently a DMI amplitude D as large as 2 mJ/ m 2 in Pt|Co|Ir . The process for estimating the skyrmion radius is displayed in Figure 2 . A direct consequence of having such a large DMI is that the bubble-like domains that we have identified are indeed isolated magnetic skyrmions . In micromagnetic simulations, trivial bubbles (winding number equal to zero) are not stable and vanish in a few nanoseconds or less, while skyrmions (winding number equal to one) are stable . The good agreement of the size dependence as well as the stability of the bubble domains is strong evidence that these domains are, in fact, skyrmions . The value of D is further confirmed by studying the typical width of the worm domains, which are observed at zero magnetic field, indicating consistency in the analysis .In conclusion, we demonstrate the presence, at room temperature, of skyrmions stabilized by interfacial DMI in metallic multilayers, opening the...

Current induced domain wall (DW) depinning of a narrow DW in out of plane magnetized ðPt=CoÞ 3 =Pt multilayer elements is studied by magnetotransport. We find that for conventional measurements Joule heating effects conceal the real spin torque efficiency and so we use a measurement scheme at a constant sample temperature to unambiguously extract the spin torque contribution. From the variation of the depinning magnetic field with the current pulse amplitude we directly deduce the large nonadiabaticity factor in this material and we find that its amplitude is consistent with a momentum transfer mechanism. DOI: 10.1103/PhysRevLett.101.216601 PACS numbers: 72.25.Ba, 75.60.Ch, 75.75.+a The recent discovery that a spin-polarized current can displace a domain wall (DW) through the spin transfer from conduction electrons to the local magnetization [1] has opened up an alternative approach to manipulate magnetization. Current induced domain wall motion (CIDM) has been investigated experimentally so far in detail in permalloy (Py; Ni 80 Fe 20 ) nanowires characterized by wide DWs (>100 nm) where the spin of a conduction electron is expected to follow adiabatically the magnetization direction as the electron passes across the DW [1,2]. A key question that has been raised is whether the spin transfer effect contains nonadiabatic contributions due to spin relaxation or nonadiabatic transport [2][3][4][5][6]. It was predicted [3,7] that from the efficiency of the spin transfer effect, which is measured by probing the dependence of the DW propagation magnetic field on the injected current, the nonadiabaticity can be deduced. However, in Py nanowires, the complicated 2D spin structures of the DWs prevent direct comparison to 1D models and a meaningful comparison to full 2D micromagnetic simulations is only possible if the exact spin structure during pulse injection is known, which is generally not the case. In particular, the wall deformations and transformations that have been observed [8] can render the results impossible to interpret in terms of the nonadiabaticity.To obtain simple DW spin structures, out-of-plane magnetized materials with a strong uniaxial anisotropy can be used where the simple Bloch or Néel DW spin structure is more apt for an analysis using an analytical 1D model including the nonadiabatic torque terms. In addition, a larger nonadiabaticity is expected in these materials due to the larger magnetization gradient for such narrow DWs [2,4,9]. This larger nonadiabaticity may explain the high efficiency of the current induced DW depinning reported recently in such materials [10,11]. However, another major obstacle for the determination of the nonadiabaticity from the dependence of the DW depinning magnetic field on current is that Joule heating strongly affects the thermally activated DW depinning. For experiments carried out at a constant cryostat temperature, it is thus hard to extract directly the contribution from the spin transfer torque.In this Letter we probe CIDM in out-of-plane magnetized ðPt=C...

We report on the experimental realization of tetragonal Fe-Co alloys as a constituent of Fe 0:36 Co 0:64 =Pt superlattices with huge perpendicular magnetocrystalline anisotropy energy, reaching 210 eV=atom, and a saturation magnetization of 2:5 B =atom at 40 K, in qualitative agreement with theoretical predictions. At room temperature the corresponding values 150 eV=atom and 2:2 B =atom are achieved. This suggests that Fe-Co alloys with carefully chosen combinations of composition and distortion are good candidates for high-density perpendicular storage materials. DOI: 10.1103/PhysRevLett.96.037205 PACS numbers: 75.30.Gw, 75.50.Bb, 75.50.Ss The enormous increase in the recording density of hard disk drives, by more than 6 orders of magnitude during the past 50 years, has mainly been achieved by simply scaling the dimensions of the bits recorded in the storage layer [1]. However, this traditional scaling is limited by the onset of superparamagnetism. This occurs when the grain volume V in the recording medium is reduced so that the ratio of the magnetic energy per grain to the thermal energy, K u V=k B T, becomes sufficiently small to cause the recorded data to be erased by thermal fluctuations in an intolerably short time [1,2]. K u is the uniaxial magnetocrystalline anisotropy energy (MAE), i.e., the energy required for rotating the magnetization direction from an easy axis to the hard axis. Thus, high-K u materials [3] are needed to further increase the recording density. The maximum practical MAE, however, is limited by the required write field H w K u =M s , which has to be delivered by the writing head. Thus, a large value of M s , the saturation magnetization of the recording medium, will be beneficial both through decreasing H w as well as by increasing the field available in the readback process. Hence, large values of K u and M s are indispensable properties of future high-density magnetic recording materials.Recently, based on first-principles calculations, tetragonal Fe-Co alloys were proposed as promising materials that combine the desired large values of K u and M s [4]. The advantages of the suggested alloys, as compared to other materials considered for magnetic storage [3], are their about 50% larger saturation magnetization, the huge perpendicular MAE, and the possibility to tailor the MAE by changing the alloy concentration. In addition, Fe-Co alloys do not require as high deposition temperatures as, e.g., chemically ordered L1 0 FePt [5], which has received considerable attention recently. From the calculations it was found that, for certain values of the ratio c=a, between the lengths of the body-centered tetragonal (bct) crystal's c and a axes, and for specific alloy concentrations, very high values of K u 800 eV=atom can be expected. This MAE, which is larger by 3 orders of magnitude than for bcc Fe, occurs theoretically for a composition of about Fe 0:4 Co 0:6 and c=a 1:20-1:25. Also, the predicted easy axis of magnetization for the tetragonal alloy is along the c axis, which facilitat...