A Correction to this paper has been published: https://doi.org/10.1038/s41467-021-21915-9

Graphite intercalation compounds (GICs) are a group of layered materials that are suitable as monochromators for cold neutrons. KC24 is a particularly interesting compound in this regard as it features a large c-axis lattice spacing of 8.74 Å, high reflectivity, and the possibility to produce large crystals with mosaicity that matches the beam divergence of cold neutron guides. GICs can be synthesized with different levels of intercalation, known as the stage of the compounds. Each stage displays a specific d-spacing. Impure GIC-monochromators containing multiple stages produce mixing of neutron wavelengths, which complicates data analysis on neutron reflectometers. We discuss the implications of GIC crystal purity and stage contamination for neutron reflectometry and show how GIC crystals can be characterized by time-of-flight neutron diffraction providing an efficient and quantifiable measure of the reflected wavelength spectrum. This allows taking into account multiple wavelength contaminations and ascertains the robustness of reflectometry measurements.

Edge roughness in magnetic stripe arrays is an important structural parameter with a sizable impact on magnetic domains, domain correlation, pinning and propagation of domain walls in micro- and nanostripes proposed for data storage and logic devices. We have investigated two different stripe arrays fabricated by lithographic methods from permalloy Fe22Ni78, one with rough edges and another one with smooth edges. Performing polarized neutron reflectivity and polarized neutron off-specular scattering both patterns yield specular reflectivity curves with pronounced Kiessig fringes and well resolved Bragg bands in the off-specular regime due to the lateral periodicity. When applying a saturating magnetic field perpendicular to the stripes, a significant diffuse scattering can be detected close to the total reflection edge where it is enhanced by the Yoneda effect. This diffuse magnetic scattering can be attributed to inhomogeneities of the magnetic induction in the empty space between the stripes. In contrast, the sample with much less edge roughness does not exhibit off-specular diffuse scattering. Thus polarized neutron scattering from stripe patterns is a suitable tool for quantitative characterization of inhomogeneous magnetic flux distribution in the interspace between magnetic stripes invoked by their edge roughness.

Traditionally, neutron scattering is an essential method for the analysis of spin structures and spin excitations in bulk materials. Over the last 30 years, polarized neutron scattering in terms of reflectometry has also contributed largely to the analysis of magnetic thin films and magnetic multilayers. More recently it has been shown that polarized neutron reflectivity is, in addition, a suitable tool for the study of thin films laterally patterned with magnetic stripes or islands. We provide a brief overview of the fundamental properties of polarized neutron reflectivity, considering different domain states, domain fluctuations, and different domain sizes with respect to the neutron coherence volume. The discussion is exemplified by a set of simulated reflectivities assuming either complete polarization and polarization analysis, or a reduced form of polarized neutron reflectivity without polarization analysis. Furthermore, we emphasize the importance of the neutron coherence volume for the interpretation of specular and off-specular intensity maps, in particular when studying laterally non-homogeneous magnetic films. Finally, experimental results, fits, and simulations are shown for specular and off-specular scattering from a magnetic film that has been lithographically patterned into a periodic stripe array. These experiments demonstrate the different and mutually complementary information that can be gained when orienting the stripe array parallel or perpendicular to the scattering plane.

Engineering the magnetic properties (Gilbert damping, saturation magnetization, exchange stiffness, and magnetic anisotropy) of multicomponent magnetic compounds plays a key role in fundamental magnetism and its applications. Here, we perform a systematic study of (Ni81Fe19)(100-x)Gd-x films with x = 0%, 5%, 9%, and 13% using ferromagnetic resonance (FMR), element-specific time-resolved x-ray magnetic resonance, and femtosecond time-resolved magneto-optical pump-probe techniques. The comparative analysis of field and time domain FMR methods, with complimentary information extracted from the dynamics of high-frequency exchange magnons in ferromagnetic thin films, is used to investigate the dependence of Gilbert damping on the Gd concentration.

We measure the transient strain profile in a nanoscale multilayer system composed of Yttrium, Holmium and Niobium after laser excitation using ultrafast X-ray diffraction. The strain propagation through each layer is determined by transient changes of the material-specific Bragg angles. We experimentally derive the exponentially decreasing stress profile driving the strain wave and show that it closely matches the optical penetration depth. Below the Neel temperature of Ho, the optical excitation triggers negative thermal expansion, which is induced by a quasi-instantaneous contractive stress, and a second contractive stress contribution rising on a 12 ps timescale. These two timescales have recently been measured for the spin-disordering in Ho [Rettig et al, PRL 116, 257202 (2016)]. As a consequence we observe an unconventional bipolar strain pulse with an inverted sign travelling through the heterostructure.

A major step to implement antiferromagnetic Mn2Au in spintronics is to understand the exchange bias (EB) mechanism in Mn2Au/ferromagnet bilayers, which can provide an opportunity for reading the magnetic state of Mn2Au via TMR. A very high bulk Néel temperature of Mn2Au (>1500 K) renders its magnetic structure very robust. For this reason, generating an EB by field cooling (FC) in high-quality epitaxial Mn2Au/ferromagnet bilayers is a challenging task. We succeeded in inducing an EB in such systems by tuning the growth conditions of Mn2Au. Our results indicate that the EB effect increases with decreasing atomic order of the Mn2Au films and can reach EB fields up to 200 Oe. This large EB effect reveals the importance of disorder and interfacial pinning in Mn2Au for its appearance.

In the field of antiferromagnetic (AFM) spintronics, information about the N\'eel vector, AFM domain sizes, and spin-flop fields is a prerequisite for device applications but is not available easily. We have investigated AFM domains and spin-flop-induced changes of domain patterns in ${\mathrm{Mn}}_{2}\mathrm{Au}$(001) epitaxial thin films by x-ray magnetic linear dichroism photoemission electron microscopy (PEEM) using magnetic fields up to 70 T. As-prepared ${\mathrm{Mn}}_{2}\mathrm{Au}$ films exhibit AFM domains with an average size $\ensuremath{\le}1\ensuremath{\mu}\mathrm{m}$. Application of a 30 T field, exceeding the spin-flop field, along a magnetocrystalline easy axis dramatically increases the AFM domain size with N\'eel vectors perpendicular to the applied field direction. The width of N\'eel-type domain walls (DW) is below the spatial resolution of the PEEM and therefore can only be estimated from an analysis of the DW profile to be smaller than 80 nm. Furthermore, using the values for the DW width and the spin-flop field, we evaluate an in-plane anisotropy constant ranging between 1 and $17\phantom{\rule{0.28em}{0ex}}\mathrm{eV}/\mathrm{f}.\mathrm{u}.$.

Mn$_2$Au is an important antiferromagnetic (AF) material for spintronics applications. Due to its very high N\'eel temperature of about 1500 K, some of the basic properties are difficult to explore, such as the AF susceptibility and the exchange constants. Experimental determination of these properties is further complicated in thin films by unavoidable presence of uncompensated and quasiloose spins on antisites and at interfaces. Using x-ray magnetic circular dichroism (XMCD), we have measured the spin and orbital contribution to the susceptibility in the direction perpendicular to the in-plane magnetic moments of a Mn$_2$Au(001) film and in fields up to 8 T. By performing these measurements at a low temperature of 7 K and at room temperature, we were able to separate the loose spin contribution from the susceptibility of AF coupled spins. The value of the AF exchange constant obtained with this method for a 10 nm thick Mn$_2$Au(001) film equals to (24 $\pm$ 5) meV.