Bernd Rellinghaus

Structure and Magnetic Properties of L10-Ordered Fe–Pt Alloys and Nanoparticles

Publisher Summary Fe–Pt alloys are known to exhibit high coercivity due to high magnetocrystalline anisotropy (MCA) of the L 1 0 FePt phase. The main intrinsic magnetic properties of this itinerant-electron ferromagnet are reported to be T c ＝750 K (the Curie temperature), Js ＝ 1.43 T (the spontaneous magnetisation at room temperature), and K 1 ＝ 6.6MJ/m 3 (the firstmagnetic anisotropy constant at room temperature). L 1 0 -based Fe–Pt alloys have a huge potential for a variety of applications, most importantly, in the field of magnetic recording and specialized permanent magnet applications. Because of the unique combination of excellent intrinsic magnetic properties and good corrosion resistance, L 1 0 -based Fe–Pt thin films and nanoparticles are promising candidates for ultrahigh-density magnetic storage media. FePt nanoparticles are also considered for such applications as contrast agents in magnetic resonance imaging for bio-medical applications or for catalysis. The structure and, consequently, the magnetic properties of the Fe–Pt alloys differ depending on the approach used for their preparation, especially when such aspects as the size dependence of the chemical ordering in very small nanoparticles or the influence of a lattice misfit between a film and a substrate start to play a major role.

Local Magnetic Suppression of Topological Surface States in Bi2Te3 Nanowires

Locally induced, magnetic order on the surface of a topological insulator nanowire could enable room-temperature topological quantum devices. Here we report on the realization of selective magnetic control over topological surface states on a single facet of a rectangular Bi2Te3 nanowire via a magnetic insulating Fe3O4 substrate. Low-temperature magnetotransport studies provide evidence for local time-reversal symmetry breaking and for enhanced gapping of the interfacial 1D energy spectrum by perpendicular magnetic-field components, leaving the remaining nanowire facets unaffected. Our results open up great opportunities for development of dissipation-less electronics and spintronics.

Electron beam induced dehydrogenation of MgH2 studied by VEELS

Nanosized or nanoconfined hydrides are promising materials for solid-state hydrogen storage. Most of these hydrides, however, degrade fast during the structural characterization utilizing transmission electron microscopy (TEM) upon the irradiation with the imaging electron beam due to radiolysis. We use ball-milled MgH2 as a reference material for in-situ TEM experiments under low-dose conditions to study and quantitatively understand the electron beam-induced dehydrogenation. For this, valence electron energy loss spectroscopy (VEELS) measurements are conducted in a monochromated FEI Titan3 80–300 microscope. From observing the plasmonic absorptions it is found that MgH2 successively converts into Mg upon electron irradiation. The temporal evolution of the spectra is analyzed quantitatively to determine the thickness-dependent, characteristic electron doses for electron energies of both 80 and 300 keV. The measured electron doses can be quantitatively explained by the inelastic scattering of the incident high-energy electrons by the MgH2 plasmon. The obtained insights are also relevant for the TEM characterization of other hydrides.

Synthesis process, size and composition effects of spherical Fe3O4 and FeO@Fe3O4 core/shell nanoparticles

In this work, we investigate the size, composition and magnetic behavior of a series of iron oxide nanoparticles prepared by means of high temperature decomposition of an iron oleate precursor. Different synthesis conditions, such as gas atmosphere, precursor ratio and heating rate were tested to obtain a direct correlation between the final sample structure and the varied parameter. The synthesis products were characterized by X-ray diffraction, transmission electron microscopy and small-angle X-ray scattering, respectively. We studied six samples with rather narrow size distribution and mean diameters from 8 nm to 16 nm. The particles with diameter below 11 nm were found to be spinel-type, monocrystalline, and their magnetic response can be ascribed to a single domain framework. On the other hand, two-phase core–shell FeO@Fe3O4 of mean sizes of 15 nm and 16 nm were obtained by increasing the amount of oleic acid and the heating rate. The magnetic behavior of these samples exhibits interesting interface features, related to the exchange coupling phenomenon between the FeO and Fe3O4. We discuss how the different synthesis conditions may lead to the presence of this FeO phase, and how the core–shell configuration and other structural features affect the macroscopic magnetic behavior.

Self-organized double-wall oxide nanotube layers on glass-forming Ti-Zr-Si(-Nb) alloys

In this work, we report for the first time on the use of melt spun glass-forming alloys - Ti75Zr10Si15 (TZS) and Ti60Zr10Si15Nb15 (TZSN) - as substrates for the growth of anodic oxide nanotube layers. Upon their anodization in ethylene glycol based electrolytes, highly ordered nanotube layers were achieved. In comparison to TiO2 nanotube layers grown on Ti foils, under the same conditions for reference, smaller diameter nanotubes (~116nm for TZS and ~90nm for TZSN) and shorter nanotubes (~11.5μm and ~6.5μm for TZS and TZSN, respectively) were obtained for both amorphous alloys. Furthermore, TEM and STEM studies, coupled with EDX analysis, revealed a double-wall structure of the as-grown amorphous oxide nanotubes with Ti species being enriched in the inner wall, and Si species in the outer wall, whereby Zr and Nb species were homogeneously distributed.