Dumbbell‐like Au‐Fe3O4 nanoparticles and their single‐component counterparts, Au and Fe3O4, were compared regarding their H2O2 reduction capability. The Au‐Fe3O4 nanoparticles are catalytically more active, which is attributed to polarization effects from Au to Fe3O4. This activity can be further tuned by the size of the nanoparticles.

Hantelförmige Au‐Fe3O4‐Nanopartikel und ihre separaten Gegenstücke, Au und Fe3O4, wurden hinsichtlich ihrer Effizienz bei der H2O2‐Reduktion verglichen. Die höhere Aktivität der Au‐Fe3O4‐Nanopartikel wird auf die Polarisation von Fe3O4 durch Au zurückgeführt. Die Aktivität lässt sich außerdem noch über die Nanopartikelgröße einstellen.

We report exchange bias (EB) effect in the Au-Fe3O4 composite nanoparticle system, where one or more Fe3O4 nanoparticles are attached to an Au seed particle forming ‘dimer’ and ‘cluster’ morphologies, with the clusters showing much stronger EB in comparison with the dimers. The EB effect develops due to the presence of stress at the Au-Fe3O4 interface which leads to the generation of highly disordered, anisotropic surface spins in the Fe3O4 particle. The EB effect is lost with the removal of the interfacial stress. Our atomistic Monte Carlo studies are in excellent agreement with the experimental results. These results show a new path towards tuning EB in nanostructures, namely controllably creating interfacial stress, and opens up the possibility of tuning the anisotropic properties of biocompatible nanoparticles via a controllable exchange coupling mechanism.

Half-doped Pr 1-x Sr x CoO 3 (x=0.5) displays anomalous magnetism most notably manifest in the field-cooled magnetization versus temperature curves under different applied cooling fields.Recently, an explanation was advanced that a magnetocrystalline anisotropy transition driven by a structural transition at 120 K is the origin of this behavior. In this paper, we further elucidate the nature of the magnetic anisotropy across the low temperature phase transition in this material by means of transverse susceptibility (TS) measurements performed using a self-resonant tunnel diode oscillator. TS probes magnetic materials by means of a small radio frequency oriented transverse to a DC field which sweeps from positive to negative saturation. TS scans as a function of field clearly reveal peaks associated with the anisotropy (H K ) and switching fields (H S ). When peak position is examined as a function of temperature, around 120 K the signature of a ferromagnetic to ferromagnetic (FM-FM) phase transition is evident as a sharp feature in H K and a corresponding cusp in H S . A third TS peak (not previously observed in other classes of magnetic oxides such as manganites and spinel ferrites) is found to be correlated with the crossover field (H cr ) in the unconventional magnetization versus temperature (M(T)) behavior. 2We observe a strong temperature dependence of H cr around 120 K using this technique, which suggests the magnetic field-influenced magnetocrystalline anisotropy transition. We show the switching between the high-field magnetization state and the low-field magnetization state associated with the magnetocrystalline anisotropy transition is irreversible when the magnetic field is re-cycled. Finally, we demonstrate that the TS peak magnitude indicates easy axis switching associated with this phase transition, even in these polycrystalline samples. Our results further confirm that TS provides new insights into the magnetic behavior of complex oxides. 75.30.Gw, 75.47.Lx, 75.30.Cr PACS: