Controlling the Microstructure and Magnetic Properties of Ferromagnetic Nanocrystals Produced by Ion Implantation
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Controlling the Microstructure and Magnetic Properties of Ferromagnetic Nanocrystals Produced by Ion Implantation. K.S. Beaty,1 A. Meldrum,1 J.P. Franck,1 K. Sorge,2 J. R. Thompson,2 C.W. White,3 R.A. Zuhr,3 L.A. Boatner,3 S. Honda3 1 The University of Alberta, Edmonton, AB 2 The University of Tennessee, Knoxville, TN 3 Oak Ridge National Laboratory, Oak Ridge, TN ABSTRACT Ion implantation coupled with annealing is a versatile and flexible approach to creating ferromagnetic near-surface nanocomposites that represent a wide range of particle/host combinations. We have used ion implantation and thermal processing to create a layer of Co nanoparticles in a sapphire host that was subsequently irradiated with Xe, Pt, or Pb in order to systematically modify the magnetic properties of the composite. Transmission electron microscopy (reported in an accompanying paper in this volume) was used to carry out a detailed characterization of the microstructure of the resulting near-surface composites whose magnetic properties were determined using SQUID magnetometry or magnetic circular dichroism. These composites exhibit magnetic hysteresis with coercivities ranging from near zero (i.e., superparamagnetism) up to 1.2 kG – depending on the composition and microstructure. We also present the results of preliminary experiments in which we attempt to control the spatial distribution of magnetic elements within ion-implanted ferromagnetic nanocomposites. The results demonstrate methods for tailoring the magnetic properties of nanocomposites produced by ion implantation for specific applications. INTRODUCTION Ion implantation was first used to create embedded magnetic nanoclusters over a decade ago [e.g., see Ref. 1]. By injecting varying concentrations of Fe, Co, or Ni into dielectric hosts such as crystalline Al2O3 and fused SiO2, relatively soft magnetic composites were created with low coercivities and a magnetic moment per atom similar to that of bulk magnetic material. Room-temperature superparamagnetism is often reported, due to particle sizes well below that needed to prevent random thermal reorientation of the particle magnetization. Blocking temperatures have been calculated from the precipitate sizes and anisotropy constants, and seem to agree reasonably well with experimentally observed blocking temperatures obtained from field-cooled and zero-field-cooled measurements of the magnetization. Nevertheless, a range of particle sizes is often reported, and the thermal blocking temperature is accordingly distributed over a range of temperatures. Furthermore, the effects of the dielectric host material on the magnetic properties of the composite are only rarely investigated or reported. In previous ion implantation work, single-element nanoparticles (and in some cases, oxide particles) of Fe, Ni, or Co have been produced in either SiO2 glass or crystalline sapphire wafers [1]. More recently, additional motivation for research in this area has been provided by the potential for creating new materials with possible applications as
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