The Formation of High-Coercivity, Oriented, Nanophase Cobalt Precipitates in Al 2 O 3 Single Crystals by Ion Implantatio
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implantation to induce highly anisotropic magnetic properties and to increase the Co-particle coercivity up to a value of-1.3 kG. EXPERIMENT The implantation of Co+ ions into (0001)-oriented cx-A120
3 single
crystals was carried out
at a temperature of -302 K, an implant energy of 140 keV, and a dose of 8.0 x 1016 ions/cm 2 . An average implanted-Co-ion depth of -50 nm below the A120 3 surface was calculated using the TRIM code. In order to form precipitates of the implanted species, subsequent thermal treatments were carried out for 2 h in a slightly reducing atmosphere of Ar + 4%H 2 using a tube furnace at T = 1100 'C. After 2 h at this temperature, the samples were quenched to room temperature. The implantation of Pt+ ions subsequent to the formation of metallic Co precipitates in A120 3 was carried out at a sample temperature of -302 K and an implant energy of 230 keV. Pt+ implant doses ranging from 10 to 1016 ions/cm 2 were used in order to investigate the effect of Pt doping on the magnetic properties of Co particles in A12 0 3 . The Pt-ion implantation depth was calculated using the TRIM code in order to superimpose the Pt-implant depth with that of the metallic Co precipitates. The metallic Co precipitates in single-crystal A12 0 3 were characterized prior to and after Pt-ion implantation by using Rutherford backscattering (RBS)/channeling spectroscopy, x-ray diffraction, transmission electron microscopy (TEM), and magnetic circular dichroism (MCD). MCD is a magneto-optical technique that measures the difference in absorption between rightand left-circularly polarized light. MCD measurements were made using a 10 kG electromagnet and utilized photoelastic-polarization modulation to investigate the magneto-optical effects as a function of wavelength and applied magnetic field with both the polarization of the light and the applied field oriented perpendicular to the specimen surface [5]. The anisotropic magnetic properties of the precipitate system were determined using a superconducting quantum interference device (SQUID) magnetometer. The magnetization and magnetic anisotropy of the Co precipitates both prior to and after Pt implantation were investigated, and magnetization curves were measured at -297 K and 77 K with the applied magnetic field oriented parallel or perpendicular to the plane containing the implanted particles. Complementary chemical analysis in the precipitate region was carried out during the TEM examination by using energy-dispersive spectroscopy (EDS), and x-ray diffraction measurements were performed using a rotating-anode Cu Kei x-ray source, a vertically focusing LiF monochromator, and a Huber four-circle diffractometer. RESULTS AND DISCUSSION TEM and X-ray Analysis of Co Particles in A120 3 TEM micrographs illustrating both plan-view (a) and cross-sectional (b) images of metallic Co particles formed by ion implantation and thermal processing are shown in Fig. 1. Moir6 fringes can be seen in the cross-sectional view and provide an indication of the degree of crystallographic alignm
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