Room Temperature Magnetic Skyrmions in Gradient Composition Engineered CoPt Single Layers Adam Erickson 1 Qihan Zhang 2 Hamed Vakili 3 Suvechhya Lamichhane 3 Lanxin Jia 2 Ilja Fescenko 4 Edward Schwartz 3 Sy Hwang Liou 3 Alexey A Kovalev 3 Jingsheng Chen 2 5 Abdelghani Laraoui1 3 1Department of Mechanical Materials Engineering University of Nebraska Lincoln 900 N 16th Street W342 NH Lincoln NE 68588 United States 2Department of Materials Science and Engineering National University of Singapore Block E2 05 19 5 Engineering Drive 2 Singapore 117579 Singapore 3Department of Physics and Astronomy and the Nebraska Center for Materials and Nanoscience University of Nebraska Lincoln 855 N 16th St Lincoln NE 68588 United States 4Laser Center University of Latvia Jelgavas St 3 Riga LV 1004 Latvia 5National University of Singapore Suzhou Research Institute Suzhou Jiangsu 215123 China Equal contributions Corresponding authors msecj nus edu sg alaraoui2 unl edu ABSTRACT Topologically protected magnetic skyrmions in magnetic materials are stabilized by interfacial or bulk Dzyaloshinskii Moriya interaction DMI Interfacial DMI decays with increase of the magnetic layer thickness in just a few nanometers and bulk DMI only stabilizes magnetic skyrmions at low temperatures Consequently more flexibility in manipulation of DMI is required for utilizing nanoscale skyrmions in energy efficient memory and logic devices at room temperature RT Here we demonstrate the observation of RT skyrmions stabilized by gradient DMI g DMI in composition gradient engineered CoPt single layer films by employing topological Hall effect magnetic force microscopy and nitrogen vacancy scanning magnetometry Skyrmions remain stable at a wide range of applied magnetic fields and are confirmed to be Bloch type from micromagnetic simulation and analytical magnetization reconstruction Furthermore we observe skyrmion pairs which may be explained by skyrmion antiskyrmion interactions Our findings expand the family of magnetic materials hosting RT magnetic skyrmions by tuning g DMI via gradient polarity and choice of magnetic elements KEYWORDS skyrmion topological stability Dzyaloshinskii Moriya interaction topological Hall effect nitrogen vacancy CoPt Magnetic skyrmions are topologically nontrivial vortex like quasiparticles that possess nano to microscale dimensions and high controllability through current induced spin torque 1 When stabilized at room temperature RT they could be used as memory and logic elements promising for next generation energy efficient memory and logic devices2 as well as neuromorphic computing 3 4 Dzyaloshinskii Moriya interaction DMI is an indirect and anti symmetric exchange interaction favoring the formation of magnetic skyrmions N el skyrmions Bloch skyrmions and antiskyrmions have been experimentally observed in various materials system In ultrathin ferromagnetic heavy metal FM HM multilayers interfacial DMI5 7 originates from 1 broken inversion symmetry at interfaces between the FM and HM layers with strong spin orbit coupling SOC 1 In this case N el type skyrmions have been identified not only in monolayer Fe Ir 111 and bilayer PdFe Ir 111 8 9 but also in various FM HM heterostructures composed of sub nanometer Co layers sandwiched between HM layers Ir Pt W 7 10 12 Examples include Ir Co Pt 10 and Pt Co Ta 15 multilayer structures 10 However interfacial DMI in FM HM multilayer systems vanishes by increasing the thickness of the FM layer to a few nanometers and the stability of the magnetic skyrmions relies heavily on the quality of interfaces In contrast bulk DMI induced Bloch type skyrmions in chiral magnet B20 type compounds such as MnSi 13 Fe1 xCoxSi 14 and FeGe were observed at low temperatures 15 16 However materials with bulk DMI are rare and the DMI strength lacks tunability since it is set by the crystal structure Furthermore antiskyrmions characterized by opposite winding number and anisotropic helicity can be stabilized at RT by anisotropic DMI17 in Heusler compounds Mn1 4PtSn18 19 and Mn1 4Pt0 9Pd0 1Sn 20 21 Recently the revolution of skyrmion antiskyrmion pairs was observed in B20 type FeGe 16 Very recently sizeable DMI was realized at RT by a compositional gradient engineering in single layer films consisting of FM and HM referred to as gradient DMI g DMI The resulting g DMI originates from the combined bulk magnetization asymmetry BMA and SOC and has sign and strength dependence with the magnetization gradient 22 23 Although it is believed that magnetic skyrmions could be stabilized by g DMI the direct observation of spin textures in gradient samples is still missing In addition to g DMI non equilibrium spin torque could originate from the composition gradient induced symmetry breaking In particular field free spin orbit torque magnetization self switching is reported in gradient CoTb and CoPt single layers 24 25 Magnetic skyrmions combined with spin torque extends the application of gradient magnetic single layer systems to spintronics Motivated by these findings the investigation of topological spin textures in such gradient alloy single layers through direct imaging is critical Here we use scanning nitrogen vacancy NV magnetometry in combination with magnetic force microscopy MFM and topological Hall effect THE to observe RT magnetic skyrmions in composition gradient engineered CoPt single layers for the first time Isolated skyrmions are measured by MFM and found to correlate to the field dependent topological hall signal indicating non zero topology Bloch type skyrmions were confirmed from micromagnetic simulation and analytical magnetization reconstructions extracted from NV magnetic stray field maps The isolated skyrmions remain stable at a wide range of applied magnetic fields Pairs of skyrmions were also observed which may be attributed to higher order winding number skyrmions or by skyrmion antiskyrmion interaction Of particular interest is observing controlling exotic topologically protected spin textures in composition gradient magnetic single layer systems and greatly expanding the selection range of materials for the research and application of magnetic skyrmions 1 RESULTS AND DISCUSSION Structural and magnetic properties of CoPt single layer with gradient To host topological spin textures DMI was introduced into binary CoxPt1 x films with perpendicular magnetic anisotropy PMA via a composition gradient induced bulk inversion asymmetry 22 25 In the framework of three