[en] A unified approach combining polarized neutron and resonant x-ray magnetic reflectometry has been applied to determine the magnetic structure in an (Fe(35 (angstrom))/Gd(50 (angstrom)))₅ multilayer as a function of temperature and magnetic field. Simultaneous self-consistent refinement of neutron and x-ray data made it possible to resolve the element-specific magnetization profile in the multilayer with unprecedented accuracy. It is shown that the small number of bilayer periods together with the asymmetric termination (Fe-top, Gd-bottom) lead to unique low-temperature magnetic phases characterized by significant twisting of Fe and Gd magnetic moments and nonuniform distribution of vectorial magnetization within Gd layers. A twisted magnetic state was found to be stable at small magnetic fields and at a low temperature of 20 K, which is well below the compensation temperature of this artificial ferrimagnetic system.

[en] Manganese ferrite (MnFe₂O₄) is a well-known magnetic material widely used in electronics for many years. It is well established that its magnetic behavior is strongly influenced by local structural properties of Mn ions, which are distributed between crystallographically inequivalent tetrahedral and octahedral sites in the unit cell. In order to understand and be able to tune properties of these structures, it is necessary to have detailed site-specific structural information on the system. Here we report on the application of diffraction-anomalous fine structure (DAFS) spectroscopy to resolve site-specific Mn local structures in manganese ferrite films. The DAFS measurements were done at undulator beamline 4-ID-D of the Advanced Photon Source at Argonne National Laboratory. The DAFS spectra (Fig. 1) were measured at several Bragg reflections in the vicinity of the Mn absorption K-edge, having probed separately contributions from tetrahedrally and octahedrally coordinated Mn sites. The DAFS data analysis done with an iterative Kramers-Kroenig algorithm made it possible to solve separately the local structure around different inequivalent Mn sites in the unit cell. The reliability of the data treatment was checked carefully, and it was showed that the site-specific structural parameters obtained with DAFS allow us to describe fluorescence EXAFS spectrum measured independently. Fig. 2 shows individual site contributions to the imaginary part of the resonant scattering amplitude obtained from the treatment of the data of Fig. 1. The analysis of the refined site-specific absorption spectra was done using EXAFS methods based on theoretical standards. We provided direct evidence for the tetrahedral Mn-O bond distance being increased relative to the corresponding Fe-O distance in bulk manganese ferrites. The first coordination shell number was found to be reduced significantly for Mn atoms at these sites. This finding is consistent with the well-known tendency of Mn to be tetrahedrally coordinated in these compounds.

[en] A new method is presented for the direct determination of the coupling angle in magnetic exchange multilayers. Strong anomalies in the off-specular neutron spin-flip scattering intensity at critical angles give a new tool, which is tested with Fe/Cr multilayers. In addition, a complete two-dimensional data analysis of specular reflection and off-specular scattering has been employed to verify atomic spin correlation in Fe/Cr multilayers, a typical system showing the GMR effect

[en] Different types of roughness in magnetic multilayers can be distinguished with off-specular neutron scattering. We report on roughness originating from magnetic domains in Fe/Cr multilayers and on structural roughness being created at the Fe-Cr interfaces. The influence of these types of roughness on the GMR-effect is outlined

[en] We describe a novel method for phase recovery in neutron reflectometry which is based on using Gd reference layer of known thickness. Deposition of Gd reference layer at the top of unknown nanostructure allows us to reconstruct the complex reflection coefficient from the structure and to solve phase problem for magnetic nanostructures under investigation. This method makes it possible to use direct model-independent approaches (for example, Gelfand-Levitan-Marchenko approach) to obtain the scattering potential. (paper)

[en] We have studied the influence Cr and Pd spacers on structure and magnetic ordering in Fe/Gd superlattices. The insertion of Cr and Pd spacers in Fe/Gd has been found to cause structural changes in Gd layers. In particular, an additional fcc Gd phase appears in Fe/Cr/Gd along with the main hcp Gd phase, while there is amorphous Gd structure in Fe/Pd/Gd. By combining SQUID magnetometry and polarized neutron reflectometry we have shown that interlayer Fe-Gd exchange coupling through Pd spacers is much stronger than through Cr one. (paper)

No abstract available

[en] Peculiarities of the formation of a neutron enhanced standing wave in the structure with a thin highly absorbing layer of gadolinium are considered in the article. An analogue of the poisoning effect well known in reactor physics was found. The effect is stronger for the Nb/Gd/Nb system. Despite of this effect, for a Nb/Gd bilayer and a Nb/Gd/Nb trilayer placed between Al₂O₃ substrate and Cu layer, it is shown theoretically and experimentally that one order of magnitude enhancement of neutron density is possible in the vicinity of the Gd layer. This enhancement makes it possible to study domain formation in the Gd layer under transition of the Nb layer(s) into the superconducting state (cryptoferromagnetic phase). (paper)

[en] Static and dynamic magnetic properties of a [Fe(35 Å)/Gd(50 Å)]₁₂ superlattice are investigated experimentally in the temperature range 5–295 K using SQUID magnetometery and the ferromagnetic resonance (FMR) technique at frequencies 7–38 GHz. The obtained magnetization curves and FMR spectra are analysed theoretically using numerical simulation on the basis of the effective field model. At every given temperature, both static and resonance experimental data can be approximated well within the proposed model. However, a considerable temperature dependence of the effective field parameter in gadolinium layers has to be taken into account to achieve reasonable agreement with the experimental data in the entire temperature range studied. To describe the peculiarities of experimental FMR spectra, a non-local diffusion-type absorption term in Landau–Lifshitz equations is considered in addition to the Gilbert damping term. Possible reasons for the observed effects are discussed. (paper)

[en] The element- and site-specificity of X-ray resonant magnetic scattering (XRMS) makes it an ideal tool for furthering our understanding of complex magnetic systems. In the hard X-rays, XRMS is readily applied to most antiferromagnets where the relatively weak resonant magnetic scattering (10^-2-10^-6I_c) is separated in reciprocal space from the stronger, Bragg charge scattered intensity, I_c. In ferro(ferri)-magnetic materials, however, such separation does not occur and measurements of resonant magnetic scattering in the presence of strong charge scattering are quite challenging. We discuss the use of charge-magnetic interference resonant scattering for studies of ferromagnetic (FM) crystals and layered films. We review the challenges and opportunities afforded by this approach, particularly when using circularly polarized X-rays. We illustrate current capabilities at the Advanced Photon Source with studies aimed at probing site-specific magnetism in ferromagnetic crystals, and interfacial magnetism in films. (authors)