Advances in nanoscale magnetism increasingly require characterization tools providing detailed descriptions of magnetic configurations. Magnetic transmission X-ray microscopy produces element specific magnetic domain images with nanometric lateral resolution in films up to ∼100 nm thick. Here we present an imaging method using the angular dependence of magnetic contrast in a series of high resolution transmission X-ray microscopy images to obtain quantitative descriptions of the magnetization (canting angles relative to surface normal and sense). This method is applied to 55–120 nm thick ferromagnetic NdCo₅ layers (canting angles between 65° and 22°), and to a NdCo₅ film covered with permalloy. Interestingly, permalloy induces a 43° rotation of Co magnetization towards surface normal. Our method allows identifying complex topological defects (merons or ½ skyrmions) in a NdCo₅ film that are only partially replicated by the permalloy overlayer. These results open possibilities for the characterization of deeply buried magnetic topological defects, nanostructures and devices.

We study both experimentally and theoretically the driven motion of domain walls in extended amorphous magnetic films patterned with a periodic array of asymmetric holes. We find two crossed-ratchet effects of opposite sign that change the preferred sense for domain wall propagation, depending on whether a flat or a kinked wall is moving. By solving numerically a simple ${\varphi }^4$ model we show that the essential physical ingredients for this effect are quite generic and could be realized in other experimental systems involving elastic interfaces moving in multidimensional ratchet potentials.

The magnetization reversal processes in magnetic bilayers with individual uniaxial anisotropies have been studied, both theoretically and experimentally, to analyze the possible existence of inverted hysteresis loops, that is, with negative remanent magnetization ${(M}_r).\mathrm{}$ Kerr effect measurements in amorphous ${\mathrm{YCo}}_2/{\mathrm{YCo}}_2$ bilayers and alternating gradient magnetometry in polycrystalline FeNi/FeNi samples reveal that $M_r<0\mathrm{}$ can be observed for certain directions of the applied magnetic field in the sample plane. This property has also been found in CoNbZr films annealed under an applied field. Our theoretical approach shows that the behavior of these magnetic heterogeneous systems with two coupled uniaxial anisotropies can be understood in terms of two competing effective anisotropies, one biaxial (with $K_{\mathrm{biax}})$ and one uniaxial (with $K_{\mathrm{uniax}}).\mathrm{}$ In particular, a phase diagram has been deduced for the conditions on $K_{\mathrm{biax}}$ and $K_{\mathrm{uniax}}$ that can produce negative remanence. This description indicates that, under those anisotropy conditions, inverted hysteresis loops can be observed for an applied field close to the hard axis of the effective uniaxial anisotropy, when magnetization reversal is driven by rotations and not by domain nucleation and wall movement. To consider the real situation in a ${\mathrm{YCo}}_2/{\mathrm{YCo}}_2$ bilayer sample, the predictions of this phenomenological model have been further improved by micromagnetic calculations, which are in very good agreement with the magneto-optical measurements.

We studied experimentally and theoretically the perpendicular anisotropy and the stripe-domain structure in both ${\mathrm{Fe}}_x{\mathrm{Si}}_{1-x}$ thin films and ${\mathrm{Fe}}_x{\mathrm{Si}}_{1-x}/\mathrm{Si}$ multilayers, the latter being in the low-modulation-length regime $(0.4\mathrm{}\mathrm{nm}<\mathrm{}\mathrm{\lambda }<7\mathrm{}\mathrm{}\mathrm{nm}).\mathrm{}$ The experimental study was made by means of the transversely biased initial susceptibility ${\chi }_{t\beta }$ via the magneto-optic Kerr effect. The samples under study were prepared by dc triode sputtering at $T_S=300\mathrm{}\mathrm{K}.$ It is found that the appearance of stripe domains is more pronounced for decreasing λ as $x$ remains constant and may be caused by both the increase in effective magnetic thickness and the reduction in effective magnetization as λ decreases. For multilayers with $\lambda =0.4\mathrm{}\mathrm{nm},$ the observed field dependence of ${\chi }_{t\beta }^{-1}$ is similar to that found in homogeneous thin films when weak stripe-domain structures arise as a consequence of the existence of perpendicular anisotropy $K_N.$ We propose a quasistatic one-dimensional model to explain the behavior of ${\chi }_{t\beta }^{-1}$ when stripe domains are present, and we analyze the critical occurrence of stripe domains. We calculated the so-called pseudo-uniaxial anisotropy field $H_{\mathrm{Ks}},$ associated with the stripes, in two extreme cases: exchange-driven susceptibility or magnetic free poles (nonzero divergence in the bulk). The latter case agrees better with experiment. We found that perpendicular anisotropy is not exclusive of a well-defined multilayer structure; i.e., $K_N$ arises even when there are no interfaces in the volume. By setting the experimental saturation field $H_s$ (obtained by hysteresis loops) into our model, we obtain both the perpendicular anisotropy constant $K_N{=10\mathrm{}}^4–{10}^5{\mathrm{J}/\mathrm{m}}^3$ and the critical thickness $t_c$ for the occurrence of a stripe-domain structure. Some possible sources of perpendicular anisotropy are discussed, for example, the associated isotropic compressive stress σ, whose contribution is found to be $|K_N{|}_{\mathrm{magnetoel}}\approx 1.5–4.5\times {10}^4{\mathrm{J}/\mathrm{m}}^3.$

We present detailed theoretical and experimental analysis of the magneto-optic transverse Kerr effect in magnetic multilayers. The theoretical model is based upon a phenomenological permittivity tensor. From the general result, suitable only for numerical calculations, we derive several approximate analytical expressions in order to make a qualitative discussion. The theoretical predictions are compared with experimental results in Y/Co bilayers, and the good agreement found allows for an accurate determination of the magneto-optical constants of the material. Then, the theoretical model is applied to make a detailed study of interface magnetism in ${\mathrm{Y}}_{1-x}{\mathrm{Co}}_x$ alloys, and to perform numerical simulations in Co/Cu and Fe/Cu multilayers. The results in multilayers highlight the complex behavior of the magneto-optic transverse Kerr effect, in which the contributions of the individual layers are never strictly additive. This nonlinearity is found to be strongly dependent on the $3d$ magnetic metal present and could be used to probe the alignment of the layers even in a configuration of vanishing magnetic moment.