Strain Anisotropies in Core/Shell Magnetic Nanostructures
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Strain Anisotropies in Core/Shell Magnetic Nanostructures Georgia C. Papaefthymiou Department of Physics, Villanova University, Villanova, PA 19085 ABSTRACT The magnetic properties of nanosized iron-oxo molecular clusters have been investigated via M`ssbauer spectroscopy and compared to those of silica coated iron-oxide nanoparticles. The clusters, prepared by controlled hydrolytic iron polymerization reactions, contain a ~ 1.2 nm diameter magnetic core of spin-coupled iron ions surrounded by a shell of benzoate ligands. The nanoparticles, prepared via sol-gel synthesis, contain a ~ 4.0 nm average diameter (-Fe2O3 core coated by a shell of SiO2. Both systems exhibit magnetic bistability at low temperatures with estimated magnetic anisotropy constants of Keff = 0.63x105 J/m3 for the clusters and Keff = 0.55 x105 J/m3 for the particles. The similar values of Keff indicate that these two systems experience similar degrees of strain at the core/shell interface. This is further supported by the values of the quadrupole splitting, )EQ=0.77 mm/s for the clusters and )EQ=0.75 mm/s for the particles, pointing to same degree of distortion from pure octahedral or tetrahedral symmetry at the iron coordination sites for either system. Implications of these observations for the surface atomic structure of (-Fe2O3 nanoparticles are discussed. INTRODUCTION It has been widely observed that values of magnetic anisotropies in small particles exceed by two orders of magnitude those of their corresponding bulk magnetocrystalline anisotropies [1]. This has been attributed to the dominating effect of surface strain anisotropies in small particles. In this respect, core/shell magnetic nanostructures are of special interest as surface strain anisotropy on the magnetic core can be altered through core/shell interface microstructure manipulation. Given the important technological applications of small magnetic particles in magnetic memory storage [2] and biotechnology [3], better understanding and greater control of surface strain at the atomic level is desired. Small core/shell particles, of ~3-10 nm diameter, are produced in finite particle size distributions leading to broad X-ray diffraction patterns. This makes detailed structural characterization of the particle core/shell interface difficult. In contrast, nanosize core/shell molecular complexes, of ~ 1-2 nm diameter, can be synthesized as monodispersed, identical, chemical structures which are amenable to X-ray structural characterization [4]. Such clusters lie at the molecular/solid boundary and exhibit collective magnetic phenomena [5]. Comparative studies of related systems may shed light at core/shell interface structural characteristics in small particles. We have examined the electronic and magnetic properties of two iron based, nanoscale core/shell structures derived by self-assembly, co-precipitation techniques. The first structure is a nanosized iron-oxo molecular complex of the form Fe16MnO10(OH)10/(O2CPh)20 [1], and the second is (-Fe2O3/SiO2 [2] nanoparticles. A comparative
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