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 phi(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.

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.

Stripe domains are studied in perpendicular magnetic anisotropy films nanostructured with a periodic thickness modulation that induces the lateral modulation of both stripe periods and in-plane magnetization. The resulting system is the 2D equivalent of a strained superlattice with properties controlled by interfacial misfit strain within the magnetic stripe structure and shape anisotropy. This allows us to observe, experimentally for the first time, the continuous structural transformation of a grain boundary in this 2D magnetic crystal in the whole angular range. The magnetization reversal process can be tailored through the effect of misfit strain due to the coupling between disclinations in the magnetic stripe pattern and domain walls in the in-plane magnetization configuration.

The knowledge of how magnetization looks inside a ferromagnet is often hindered by the limitations of the available experimental methods which are sensitive only to the surface regions or limited in spatial resolution. Here we report a vector tomographic reconstruction based on soft X-ray transmission microscopy and magnetic dichroism data, which has allowed visualizing the three-dimensional magnetization in a ferromagnetic thin film heterostructure. Different non-trivial topological textures have been resolved and the determination of their topological charge has allowed us to identify a Bloch point and a meron-like texture. Our method relies only on experimental data and might be of wide application and interest in 3D nanomagnetism.

We studied experimentally and theoretically the perpendicular anisotropy and the stripe-domain structure in both Fe x Si 1Ϫx thin films and Fe x Si 1Ϫx /Si multilayers, the latter being in the low-modulation-length regime (0.4 nmϽϽ7 nm). The experimental study was made by means of the transversely biased initial susceptibility t␤ via the magneto-optic Kerr effect. The samples under study were prepared by dc triode sputtering at T S ϭ300 K. It is found that the appearance of stripe domains is more pronounced for decreasing as x remains constant and may be caused by both the increase in effective magnetic thickness and the reduction in effective magnetization as decreases. For multilayers with ϭ0.4 nm, the observed field dependence of t␤ Ϫ1 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 t␤ Ϫ1 when stripe domains are present, and we analyze the critical occurrence of stripe domains. We calculated the so-called pseudo-uniaxial anisotropy field H Ks , associated with the stripes, in two extreme cases: exchange-driven susceptibility or magnetic free poles ͑nonzero divergence in the bulk͒. 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 loops͒ into our model, we obtain both the perpendicular anisotropy constant K N ϭ10 4 -10 5 J/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 , whose contribution is found to be ͉K N ͉ magnetoel Ϸ1.5-4.5ϫ10 4 J/m 3 .

Whereas a great deal of work is being devoted to magnetic singularities in two-dimensional (2D) systems (surfaces, interfaces, films) due to their possible applications, much less is known about their properties along the perpendicular direction. Here, we report on a pronounced asymmetry of the in-depth distribution of meronlike magnetic textures, which are magnetic singularities similar to ½ skyrmions, in magnetic layers. Meron textures are observed to be distributed in two groups defined by their topology. One of them resides almost exclusively at the top surface of the film and the other at the bottom one. This observation has been brought to light with element-specific magnetic transmission soft x-ray microscopy. Micromagnetic simulations reveal that closure domains are at the origin of this asymmetry. The result might be of general interest for controlling magnetic three-dimensional (3D) architectures.

The development of magnetic nanostructures for applications in spintronics requires methods capable of visualizing their magnetization. Soft X-ray magnetic imaging combined with circular magnetic dichroism allows nanostructures up to 100-300 nm in thickness to be probed with resolutions of 20-40 nm. Here a new iterative tomographic reconstruction method to extract the three-dimensional magnetization configuration from tomographic projections is presented. The vector field is reconstructed by using a modified algebraic reconstruction approach based on solving a set of linear equations in an iterative manner. The application of this method is illustrated with two examples (magnetic nano-disc and micro-square heterostructure) along with comparison of error in reconstructions, and convergence of the algorithm.

A novel approach to tune the ferromagnetic resonance frequency of a soft magnetic Ni 80 Fe 20 (Permalloy = Py) film with in-plane magnetic anisotropy (IMA) based on the controlled coupling to a hard magnetic NdCo x film with perpendicular magnetic anisotropy (PMA) through a non-magnetic Al spacer is studied. Using transverse magneto-optical Kerr effect (TMOKE), alternating gradient magnetometry (AGM) as well as vector network analyzer ferromagnetic resonance (VNA-FMR) spectroscopy, the influence of both Co concentration and Al spacer thickness on the static and dynamic magnetic properties of the coupled IMA/PMA system is investigated. Compared to a single Py film, two striking effects of the coupling between IMA and PMA layers can be observed in their FMR spectra. First, there is a significant increase in the zero-field resonance frequency from 1.3 GHz up to 6.6 GHz, and second, an additional frequency hysteresis occurs at low magnetic fields applied along the hard axis. The maximum frequency difference between the frequency branches for increasing and decreasing magnetic field is as high as 1 GHz, corresponding to a tunability of about 20% at external fields of typically less than ±70 mT. The origin of the observed features in the FMR spectra is discussed by means of magnetization reversal curves.The magnetic properties of thin films and multilayers exhibiting stripe domains have been investigated extensively in both experiment and theory since their discovery more than half a century ago 1 . In recent years, research results on stripe domains have triggered the prospect of employing their unique properties in future microwave, magnonic, and spintronic devices with novel functionalities. The formation of stripe domains is the result of energy minimization as well as the competition between PMA (K ⊥ ) and shape anisotropy ( 1 2 µ 0 M 2 S ), which favor out-of-plane and in-plane magnetization, respectively. The ratio Q = 2K ⊥ /µ 0 M 2 S , known as reduced anisotropy or quality factor 2 , is commonly used to describe the extent of stripe domains. For moderate (Q < 1) to weak (Q 1) PMA, the magnetization tends to lie in the plane, but above a critical film thickness d cr , a ground state with stripe domains emerges. The latter is characterized by a perpendicular magnetization component alternating between up and down within a period λ . The critical thickness d cr is typically in the range of 20 -40 nm for moderate Q value materials such as amorphous NdCo alloys 3,4 , whereas for materials like Py with small values of Q, generally larger values of d cr = 170 -300 nm are found 5-8 . Intimately linked to the presence of stripe domains is the occurrence of a pseudo-uniaxial or rotatable anisotropy 9,10 , which is the result of the in-plane magnetization being aligned along the stripe direction. The latter, however, is not fixed as it can be reoriented by applya) Electronic