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Magnetic Properties of Carbonado Diamonds

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Magnetic Properties of Carbonado Diamonds ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 300 (2006) 368–372 www.elsevier.com/locate/jmmm Magnetic properties of carbonado diamonds C.B. Fitzgerald, M. VenkatesanÃ, A.P. Douvalis, J.M.D. Coey Physics Department, Trinity College Dublin, Dublin 2, Ireland Received 26 April 2005; received in revised form 9 May 2005 Available online 16 June 2005 Abstract Carbonados are porous polycrystalline diamonds of uncertain origin, which are found at locations in Bahia (NE Brazil) and in the Central African Republic. Their combination of extreme hardness and toughness is ideal for drilling and cutting tools. A variety of elements including Fe, Ni, Cu and Ag are associated with the diamonds. Following a suggestion that Mn- doped diamond may be intrinsically ferromagnetic, we have analysed six samples from Brazil which show ferromagnetic moments of up to 3 A m2 kgÀ1, which is reduced by acid leaching. X-ray diffraction on the crushed powder showed a cubic diamond structure, with no secondary phases visible. Scanning electron microscopy shows an inhomogeneous distribution of heavy metals and Mo¨ssbauer spectroscopy indicates that iron is present as native iron. We find no evidence from the carbonados for the existence of transition-metal doped ferromagnetic carbon. The intrinsic diamagnetic susceptibility of carbonados is À4.5 Â 10À9 m3 kgÀ1, similar to the accepted value for diamond,À5 Â 10À9 m3 kgÀ1. r 2005 Elsevier B.V. All rights reserved. PACS: 75.30.Cr; 75.25.Ha Keywords: Susceptibility; Curie temperature; X-ray diffraction; Diamond Carbonados are polycrystalline diamond aggre- Brazilian and CAR deposits may actually consti- gates, between 2.6 and 3.8 Ga old, found only in tute a single source as the continents were then alluvial deposits in Brazil and the Central African unseparated in the Precambrian. Their name is due Republic (CAR). Though they have no value as to their grey-black colour, which develops when gems they are ideal industrial abrasives as their diamond crystallites are sintered in a region of random crystallographic orientation makes them temperature and pressure where there is a highly resistant to crack propagation. While tendency for the diamond to transform to graphite theories surrounding their origin abound, the [1]. Carbonados are irregularly shaped and porous (approximately 10% void space), with crystallite ÃCorresponding author. Tel.: +353 1 6082171; sizes ranging from 0.5 to 500 mm. fax: +353 1 6711759. An extensive review discusses the possible E-mail address: [email protected] (M. Venkatesan). origins of carbonados [2]. It has been suggested 0304-8853/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2005.05.019 ARTICLE IN PRESS C.B. Fitzgerald et al. / Journal of Magnetism and Magnetic Materials 300 (2006) 368–372 369 that they formed when crustal organic material subducted to the upper mantle and converted to diamond, consistent with their typically crustal selection of inclusion minerals (e.g. orthoclase, florencite, kaolinite) [3,4]. However, carbonados are absent from other well-known subduction zones. Although carbonados are similar in many ways to kimberlites, a mantle origin is considered to be unlikely given the significant amounts of helium within the samples and their high porosity, with trapped organic material inside, in addition to lacking significant mineral inclusions [5,6]. Kaminsky suggested that carbonado was formed (a) by irradiation of carbonaceous materials by associated U or Th [7], while possible extraterres- trial origins include the idea that they originated outside the solar system as ejecta from a collapsing red giant star [8], or are the result of a meteoritic impact [9,10]. More than a dozen different mineral inclusions have been reported including haematite, magnetite, chromite, rutile, calcite and quartz [11–14].In addition, a number of native metals and alloy inclusions have been documented including Fe, Fe–Mn, Fe–Ni, Ni, Cr, Ti and SiC [14,15].Native iron occurs sparingly terrestrially, but is common in meteorites. In the native state, it is highly unstable in (b) the oxidizing conditions in rocks of the upper crust and in the earth’s atmosphere and so iron usually appears in oxides. Terrestrial native iron is either a primary magmatic constituent or a secondary product formed by reduction of iron compounds by assimilated carbonaceous material [16].Whether the magnetic inclusions are within the carbonado matrix [17], or reside almost exclusively in open pores at the diamond surface [18],isdisputed,but has implications for carbonado genesis. Interest in the magnetic properties of doped diamond has been stimulated on the one hand by the prediction by Dietl et al. [19], based on mean field calculations of hole-mediated exchange, that (c) diamond, a wide-gap semiconductor with Fig. 1. Carbonado diamonds from Brazil: (a) many carbonados Eg ¼ 5:4 eV, should become a ferromagnet with a (b and c) scanning electron micrograph images showing metallic high Curie temperature, TC 480 K when doped inclusions in the carbon matrix (bright spots). with 5 at% Mn, and on the other hand by the observation of superconductivity in boron-doped crystalline quality, or to produce bulk doped bulk [20] and thin film [21] specimens. As it is synthetic diamond, we chose to examine the difficult to obtain doped diamond films of good magnetic properties of natural carbonados in ARTICLE IN PRESS 370 C.B. Fitzgerald et al. / Journal of Magnetism and Magnetic Materials 300 (2006) 368–372 order to identify the origin of the ferromagnetism, concentrated in the pores. Metallic inclusions are to see if we can find any evidence that these seen as bright spots in the diamond in the images in materials may be intrinsically ferromagnetic semi- Fig. 1b. Typical sizes are in the range 0.5–5 mm. The conductors. main impurities identified by EDAX were iron, The carbonados examined were of Brazilian potassium, calcium, silicon, aluminium and sul- origin [22]. Masses were in the range 3–20 mg. phur, and less commonly copper, cobalt, zinc, Some are illustrated in Fig. 1. They are different chlorine and titanium. Results of 14 analyses are shades of grey in colour. X-ray diffraction analysis summarized in Table 1. While many of these was carried out on the crushed powder and impurities have been widely reported previously, showed a cubic diamond structure with (1 1 1) cobalt has not. For some of the inclusions elemental ( and (2 2 0) reflections and a ¼ 3:56 A. No other maps show that Fe is associated with Cr, Mn, Ni phases are visible. Many, but not all, samples and Co. In others, Co appears as an isolated respond to a small permanent magnet. metallic inclusion. Other alloy inclusions include Morphology and composition were studied by Cu–Zn and Si–Ti. After acid treatment, most of the scanning electron microscopy (SEM) and energy metallic inclusions seen by SEM are eliminated. dispersive X-ray analysis (EDAX). Magnetization Magnetization curves for four samples at room was measured using a vibrating-sample magnet- temperature are shown in Fig. 2a. They show a ometer (VSM) or a superconducting quantum ferromagnetic signal, which corresponds to a interference device (SQUID) magnetometer. spontaneous magnetization Ms in the range Mo¨ssbauer spectra were taken using a constant 0.25–3 A m2 kgÀ1 which is superposed on a dia- acceleration spectrometer, with a source of 57Co in magnetic background, partly due to the carbonado Rh. In order to remove any associated phases, we and partly due to the gelatine sample holder. After treated the diamonds in aqua-regia solution for the acid treatment, the magnetic signal is signifi- 24 h. The samples were then rinsed twice in water cantly reduced for most of the samples—Fig. 2b.It and then in acetone. is clear that much of the magnetization was SEM images and EDAX mapping show that the associated with surface or open pore material, distribution of heavy metals is inhomogeneous, rather than the bulk of the carbonado matrix. Table 1 EDAX analyses of 14 different carbonado samples before acid treatment 1234567891011121314 C 98.91 98.31 98.79 39.84 58.39 47.29 64.49 62.29 70.25 88.21 76.13 76.83 55.34 92.70 Al 0.42 0.52 0.41 0.30 0.54 0.32 2.71 0.56 — 0.29 0.29 0.21 0.01 0.26 Si 0.05 0.06 0.04 0.08 0.27 0.88 1.77 0.90 —————— P — 0.16 0.04 — —————————— S — 0.03 0.07 — 1.66 0.88 — 9.59 —————— Cl — — — 0.08 1.43 0.68 0.39 ————— 1.41 0.35 K — 0.02 — — 0.02 0.09 3.19 0.58 —————— Ca — 0.23 0.11 0.09 1.44 3.01 ———————— Ti — — — 0.21 — — 0.87 0.23 —————— Cr———————— 5.94 ————— Mn — — — 0.11 ———— 0.64 ————— Fe 0.63 0.67 0.57 13.33 0.75 1.10 7.02 17.38 20.80 10.14 5.55 12.47 7.16 0.15 Co 0.02 0.01 — — —————————— Ni — — — 34.80 — 0.15 — — 2.38 ————— Cu————18.33 0.20 — 0.20 — 0.43 0.25 0.16 0.04 — Zn———— 3.85 ———— 0.89 ———— Ag————————————— 6.48 Ba———— 0.69 4.15 ———————— ARTICLE IN PRESS C.B. Fitzgerald et al. / Journal of Magnetism and Magnetic Materials 300 (2006) 368–372 371 4 0.20 Car3 - 300 K 3 0.15 Car3 - 2 K 2 0.10 ) 0.05 ) 1 -1 -1 kg 2 kg 2 0 0.00 (Am (Am -0.05 σ σ -1 Car1 -0.10 -2 Car4 Car5 -0.15 -3 Car6 -0.20 -4 -1.0 -0.5 0.0 0.5 1.0 -1.0 -0.5 0.0 0.5 1.0 µ oH (T) µ (a) oH (T) Fig.
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