Formation of Ferromagnetic FePt Nanoparticles by Ion Implantation
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Formation of Ferromagnetic FePt Nanoparticles by Ion Implantation C. W. White, S. P. Withrow, J. D. Budai, L. A. Boatner, K. D. Sorge, J. R. Thompson, K. S. Beaty1, and A. Meldrum1 Oak Ridge National Laboratory, Oak Ridge, TN 37831 1 The University of Alberta, Edmonton, Alberta, Canada ABSTRACT Oriented ferromagnetic FePt nanoparticles with the face-centered tetragonal L10 structure were produced in Al2O3 single crystal hosts by ion implantation and annealing. Both the orientation and particle-size of the FePt particles depend strongly on the implantation conditions. The magnetic coercivities are extremely high, reaching values in excess of 20 kOe for Pt concentrations of ~45% in the FePt alloy. Ferromagnetic FePt nanoparticles were also produced in amorphous SiO2 by ion implantation and annealing. INTRODUCTION Materials with a high magnetocrystalline anisotropy have recently generated considerable interest since these materials are excellent candidates for ultrahigh-density magnetic data storage applications [1]. Intermetallic, chemically ordered alloys of FePt and CoPt exist in the face centered tetragonal (fct) L10 structure. This structure consists of alternating planes of Fe (Co) atoms and Pt atoms stacked along the direction of the crystal, and this arrangement gives rise to a very high magnetocrystalline anisotropy. Consequently, high magnetic coercivities should be achievable in nanoparticles of these materials even down to very small grain sizes. Nanoparticles of FePt have previously been synthesized chemically as a superlattice of monodispersed spherical particles [2] and FePt nanoparticles have been incorporated into amorphous Al2O3 [3, 4], SiO2 [5], Be2O3 [6], and Si3N4 [7]. In this work, we have used ion implantation followed by thermal annealing to produce oriented FexPt1-x nanoparticles embedded in the near surface region of single-crystal Al2O3 [8]. The nanoparticles are ferromagnetic and can exhibit an extremely high magnetic coercivity (>20 kOe) depending on the Fe/Pt ratio. The formation of ferromagnetic FexPt1-x nanoparticles in amorphous SiO2 is also demonstrated. EXPERIMENTAL PROCEDURE Single crystals of c-axis-oriented (0001) Al2O3 were used in this work. The sequential implantation of Fe (350 keV) and Pt (910 keV) ions was used to produce overlapping concentration profiles at a projected range of ~170 nm. The Fe dose was fixed at 1 × 1017/cm2 and the Pt dose was chosen to produce the desired Pt atomic fraction x [where x = Pt/(Fe + Pt)] in the alloy. Samples were implanted either at high temperature (550°C for Fe and 500°C for Pt) or at a lower temperature (200°C for Fe and Pt). Implantation at elevated temperatures was used to try to maintain crystallinity in the matrix during implantation. Implanted samples were annealed at 1100°C for 2 hours in flowing Ar + 4% H2 followed by slow cooling. The samples were characterized by Rutherford backscattering (RBS)-ion channeling (2.3 MeV He+), by x-ray diffraction (Cu Kα radiation using both four-circle and powder diffractometers), and by transmissio
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