Circularly-polarized extreme UV and X-ray radiation provides valuable access to the structural, electronic and magnetic properties of materials. To date, this capability was available only at largescale X-ray facilities such as synchrotrons. Here we demonstrate the first bright, phase-matched, extreme UV circularly-polarized high harmonics and use this new light source for magnetic circular dichroism measurements at the M-shell absorption edges of Co. We show that phase matching of circularly-polarized harmonics is unique and robust, producing a photon flux comparable to the linearly polarized high harmonic sources that have been used very successfully for ultrafast element-selective magneto-optic experiments. This work thus represents a critical advance that makes possible element-specific imaging and spectroscopy of multiple elements simultaneously in magnetic and other chiral media with very high spatial and temporal resolution, using tabletop-scale setups. IntroductionCircularly polarized radiation in the extreme ultraviolet (EUV) and soft X-ray spectral regions has proven to be extremely useful for investigating chirality-sensitive light-matter interactions. It enables studies of chiral molecules using photoelectron circular dichroism 1 , ultrafast molecular decay dynamics 2 , the direct measurement of quantum phases (e.g. Berry's phase and pseudo-spin) in graphene and topological insulators 3-5 and reconstruction of band structure and modal phases in solids 6 . For magnetic materials, circularly polarized soft x-rays are particularly useful for X-ray Magnetic Circular Dichroism (XMCD) spectroscopy 7 . XMCD enables element-selective probing as well as coherent imaging and holography of magnetic structures with nanometer resolution [8][9][10] . Moreover, it can also be used to extract detailed information about the magnetic state by distinguishing between the spin and orbital magnetic moments of each element. Thus, time-resolved XMCD can probe the element-specific dynamics of the spin and orbital moments when interacting with the electronic and phononic degrees of freedom in a material [11][12][13][14] . However, the time resolution available to date for XMCD has been > 100 fs, limited by the pulse duration and timing jitter of synchrotron pulses [15][16][17] . To date it has not been possible to probe spin dynamics of multiple elements simultaneously within the same sample, because the photon energy must be tuned across the various absorption edges at the large-scale facilities where these experiments are currently performed.2 Table-top soft x-ray sources based on high harmonic upconversion of femtosecond laser pulses represent a viable alternative to large-scale sources for many applications, due to their unique ability to generate bright, broadband, ultrashort and coherent light with an energy spectrum reaching into the keV region 18 . High harmonic generation (HHG) not only enables coherent imaging of nanometer structures with a spatial resolution approaching the diffraction limit 19 , but also accesses...

Proposals for novel spin--orbitronic logic 1 and memory devices 2 are often predicated on assumptions as to how materials with large spin--orbit coupling interact with ferromagnets when in contact. Such interactions give rise to a host of novel phenomena, such as spin--orbit torques 3,4 , chiral spin--structures 5,6 and chiral spin--torques 7,8 . These chiral properties are related to the anti--symmetric exchange, also referred to as the interfacial Dzyaloshinskii--Moriya interaction (DMI) 9,10 . For numerous phenomena, the relative strengths of the symmetric Heisenberg exchange and the DMI is of great importance. Here, we use optical spin--wave spectroscopy (Brillouin light scattering) to directly determine the DMI vector ! D for a series of Ni 80 Fe 20 /Pt samples, and then compare the nearest--neighbor DMI coupling energy with the independently measured Heisenberg exchange integral. We find that the Ni 80 Fe 20 --thickness--dependencies of both the microscopic symmetric--and antisymmetric--exchange are identical, consistent with the notion that the basic mechanisms of the DMI and Heisenberg exchange essentially share the same underlying physics, as was originally proposed by Moriya 11 . While of significant fundamental importance, this result also leads us to a deeper understanding of DMI and how it could be optimized for spin--orbitronic applications.Recent experimental results have demonstrated how the interplay of symmetric (Heisenberg) exchange and anti--symmetric (DMI) exchange together with anisotropy can give rise to a variety of magnetostatic phenomena, such as magnetic skyrmion lattices 12 , spiral spin structures 13 and chiral domain walls 14 . In bilayer materials with a sufficiently thin, perpendicular magnetized ferromagnet (FM) adjacent to a metal with large spin--orbit coupling in the conduction band, a large DMI favors Néel domain walls with a fixed chirality 15 as opposed to Bloch walls. The combination of a chiral domain wall structure and spin--orbit torque can give rise to fast current induced domain wall motion 3 . The direction and the speed are both dependent on the sign and the strength of the DMI and the spin--orbit torque 8,7 . Moreover, theory for a Rashba model predicts that the interfacial spin--orbit torque is proportional to the ratio of symmetric and anti--symmetric exchange 16 . Thus, direct determination of both the DMI and Heisenberg exchange is crucial for the understanding of the underlying physics in such materials systems and a better understanding of the spin--orbit torques.To date, direct measurements of anti--symmetric exchange are limited to exotic measurement techniques that can only be applied to a few highly specialized sample systems. For example, the DMI constant has been measured via synchrotron--based X--ray scattering interferometry for the weak ferromagnet FeBO 3 17 , by spin--polarized electron energy loss spectroscopy for an atomic bilayer of Fe on W(110) 18 and by spin--polarized scanning tunneling microscopy for atomic monolayer Mn on W(110) 5 .Until...

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Magnetic damping is of critical importance for devices that seek to exploit the electronic spin degree of freedom, as damping strongly a ects the energy required and speed at which a device can operate. However, theory has struggled to quantitatively predict the damping, even in common ferromagnetic materials 1-3 . This presents a challenge for a broad range of applications in spintronics 4 and spin-orbitronics that depend on materials and structures with ultra-low damping 5,6 . It is believed that achieving ultra-low damping in metallic ferromagnets is limited by the scattering of magnons by the conduction electrons. However, we report on a binary alloy of cobalt and iron that overcomes this obstacle and exhibits a damping parameter approaching 10 −4 , which is comparable to values reported only for ferrimagnetic insulators 7,8 . We explain this phenomenon by a unique feature of the band structure in this system: the density of states exhibits a sharp minimum at the Fermi level at the same alloy concentration at which the minimum in the magnetic damping is found. This discovery provides both a significant fundamental understanding of damping mechanisms and a test of the theoretical predictions proposed by Mankovsky and colleagues 3 .In recent decades, several theoretical approaches have attempted to quantitatively predict magnetic damping in metallic systems. One of the early promising theories was that of Kambersky, who introduced the so-called breathing Fermi-surface model 9-11 . More recently, Gilmore and Stiles 2 as well as Thonig et al. 12 demonstrated a generalized torque correlation model that includes both intraband (conductivity-like) and interband (resistivity-like) transitions. The use of scattering theory to describe damping was later applied by Brataas et al. 13 and Liu et al. 14 to describe damping in transition metals. A numerical realization of a linear response damping model was implemented by Mankovsky 3 for Ni-Co, Ni-Fe, Fe-V and Co-Fe alloys. For the Co-Fe alloy, these calculations predict a minimum intrinsic damping of α int ≈ 0.0005 at a Co-concentration of 10% to 20%, but was not experimentally observed 15 .Underlying this theoretical work is the goal of achieving new systems with ultra-low damping that are required in many magnonic and spin-orbitronics applications 7,8 . Ferrimagnetic insulators such as yttrium-iron-garnet (YIG) have long been the workhorse for these investigations, because YIG films as thin as 25 nm have experimental damping parameters as low as 0.9 × 10 −4 (ref. 16). Such ultra-low damping can be achieved in insulating ferrimagnets in part due to the absence of conduction electrons-and, therefore, the suppression of magnon-electron scattering. However, insulators cannot be used in most spintronic and spin-orbitronic applications, where a charge current through the magnetic material is required, nor is the requirement of growth on gadolinium gallium garnet templates compatible with spintronics and complementary metal-oxide semiconductor (CMOS) fabrication processes. One...