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 kg1, 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 4.5 109 m3 kg1, similar to the accepted value for diamond,5 109 m3 kg1. 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

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 kg1 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 -0.05

-1 σ (Am σ (Am 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. 3. Carbonado 3 measured at 300 and 2 K. 4 1 2 3 3 of carbonado is deduced as 4.5(1) 4 9 3 1 2 10 m kg . This is similar to the accepted value 5 9 3 1 6 for diamond, 5 10 m kg . We ﬁnd no )

-1 1 evidence for superconductivity. kg 2 0 The magnetization of sample 1 is almost unaffected by the acid treatment and so it was

σ (Am -1 chosen for Curie temperature measurements. The

-2 magnetization was measured as a function of temperature, data below 300 K being taken in the -3 SQUID magnetometer, and a high-temperature -4 furnace attachment used to measure up to 800 K. -1.0 -0.5 0.0 0.5 1.0 Data indicate that the Curie temperature, TC, µ (b) oH (T) exceeds 800 K. They are consistent with metallic iron, for which Tc is 1043 K. Fig. 2. Magnetization curves for carbonados (a) before and (b) Definite confirmation that the ferromagnetism after acid treatment. of the carbonados is really due to the inclusions of ferromagnetic iron is provided by the Mo¨ssbauer In order to measure the diamagnetic suscept- spectrum of carbonado 6, shown in Fig. 4. The ibility accurately, we took an acid-treated sample spectrum is poorly defined because of the small with very little ferromagnetic signal, and placed it size of the absorber, and the small iron content. symmetrically between two gel caps in a straw for Nevertheless, a least-squares fit to a lorentzian the SQUID measurement. A background measure- sextet with the linewidth constrained as 0.3 mm s1 ment with no sample was made for comparison. gives a hyperfine field Bhf ¼ 32 ð1Þ T, an isomer Data are shown in Fig. 3. The smaller (in shift ¼ 0:00 ð5Þ mm s1 relative to iron metal, and a magnitude) susceptibility at 2 K compared with quadrupole shift D ¼ 0:0ð1Þ mm s1. Their para- 300 K is attributed to the presence of some para- meters are those of metallic iron. magnetic impurities in the diamond. Assuming a Our results are largely consistent with the recent Curie law, with an effective moment of O3 mB, the work of Kletetschka et al. [18] who examined the concentration is o5 ppm. The main contribution magnetic properties of a series of 20 samples from to the susceptibility is therefore deduced from the the CAR. They found evidence of ferromagne- 300 K data. The room-temperature susceptibility tism in room-temperature measurements, with a ARTICLE IN PRESS

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100.10 References 100.05 [1] H.T. Hall, Science 169 (1970). 100.00 [2] P.J. Heaney, E.P. Vicenzi, S. De, Elements 1 (2005) 85. 99.95 [3] D.N. Robinson, Miner. Sci. Eng. 10 (1978) 55. 99.90 [4] H. Kamioka, K. Shibata, I. Kajizuke, T. Ohta, Geochem. J. 30 (1996) 189. 99.85 [5] F.V. Kaminski, S.I. Kirikilitsa, G.K. Eremenko, I.A. 99.80 Polkanov, A.J. Khrenov, Dokl. Akad. Nauk SSSR 249 (1979) 443. 99.75 [6] S.E. Haggerty, Proceedings of the Fifth NIRIM Interna- Relative Transmission (%) 99.70 tional Symposium on Advanced Materials, 1998 pp. 39–42. 99.65 [7] F.V. Kaminski, Dokl. Akad. Nauk SSSR 291 (1987) 439. -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 [8] S.A. Haggerty, EOS Trans. AGU Supp. S143, 1996. Velocity (mms-1) [9] J.V. Smith, J.B. Dawson, Geology 13 (1985) 342. [10] S. De, P.J. Heaney, R.B. Hargraves, E.P. Vicenzi, P.T. Fig. 4. Mo¨ssbauer spectrum at room temperature for sample Taylor, Earth Planet, Sci. Lett. 164 (1998) 421. car1. [11] L.F. Trueb, E.C. de Wys, Am. Miner. 56 (1971) 1252. [12] L.F. Trueb, W.C. Butterman, Am. Miner. 54 (1969) 412. 4 saturation magnetization in the range 10 – [13] L.F. Trueb, E.C. de Wys, Science 165 (1969) 799. 102 Am2 kg1. From the variation of the ferro- [14] A.I. Gorshkov, S.V. Titkov, A.M. Pleshakov, A.V. magnetic signal on acid leaching, they concluded Sivtsov, L.V. Bershov, Geol. Ore Deposit 38 (1996) 114. [15] S. De, P.J. Heaney, Eos 77 (1996). that the ferromagnetism was mostly associated [16] H. Klein, Manual of Mineralogy, 19th ed., Wiley, with magnetic carriers at the surface and in open New York. pores, rather than in the bulk of the material. [17] S. De, P.J. Heaney, R.B. Hargraves, E.P. Vicenzi, P.T. In conclusion, we ﬁnd no evidence from the Taylor, Earth Planet, Sci. Lett. 164 (1998) 421. carbonados for the existence of transition-metal- [18] G. Kletetschka, P.T. Taylor, P.J. Wasilewski, H.G.M. Hill, Earth Planet, Sci. Lett. 181 (2000) 279. doped ferromagnetic carbon. Instead, the ferro- [19] T. Dietl, et al., Phys. Rev. B 63 (2001) 195205. magnetism is associated with inclusions of metallic [20] E.A. Ekimov, V.A. Sidorov, E.D. Bauer, N.N. Mel’nik, iron, iron-based alloys or cobalt. N.J. Curro, J. Thompson, S.M. Stishov, Nature 428 (2004) 542. This work was supported by Science Founda- [21] Y. Takano, et al., Appl. Phy. Lett. 85 (2004) 2851. [22] Samples were supplied by Shannon Abrasives Ltd., tion Ireland. The authors are grateful to Plamen Shannon, Co. Clare, Ireland. Stamenov for advice and discussions.