* Corresponding author. Tel.: #55-11-8186877; fax: #55- 11-8186984. E-mail address: hercilio@macbeth.if.usp.br (H.R. Rechen- berg). Journal of Magnetism and Magnetic Materials 226}230 (2001) 1930}1932 Mo K ssbauer and magnetic characterisation of carbon-coated small iron particles H.R. Rechenberg*, J.A.H. Coaquira, C. Marquina, B. Garcm H a-Landa, M.R. Ibarra, A.M. Benito, W. Maser, E. Mun  oz, M.T. Martm H nez Instituto de Fisica, Universidade de Sa J o Paulo, C.P. 66318, 05315-970 Sa J o Paulo, Brazil DFMC and ICMA, Universidad de Zaragoza-CSIC, Zaragoza, Spain Instituto de Carboqun & mica CSIC, P.O.Box 589, Zaragoza, Spain Abstract Carbon-coated Fe particles have been produced by the Kra K tschmer}Hu!mann carbon-arc discharge method. Soot, collarette and cathode samples have been separately characterised by Mo K ssbauer spectroscopy and magnetisation measurements in the temperature range 4.2}300K. From the analysis of the results the content of the di!erent Fe phases and Fe-carbon solid solutions in our samples has been determined, as well as an estimation of particle sizes, which were found to be approximately 9 and 13nm for the soot and collarette samples, respectively.  2001 Elsevier Science B.V. All rights reserved. Keywords: Nanoparticles; Mo K ssbauer spectroscopy; Magnetisation curves The discovery and production of new carbon nanos- tructures like Fullerenes [1,2], carbon nanotubes [2], bucky onions [3], polyhedral graphitic particles [4], as well as amorphous carbon opened new possibilities of encapsulating metallic nanocrystals. From the applica- tions point of view the possibility of producing carbon- coated magnetic particles will avoid disadvantages such as oxidation, etc., which is a very important aspect to be taken into account when working with ferromagnetic materials. In this work the Kra K tschmer}Hu!mann [2] carbon- arc discharge method has been used with the idea to produce carbon-coated ferromagnetic particles. The starting material placed in the furnace anode was a bar composed of a mixture of graphite and 16.5 wt% Fe. The electrode was evaporated during 2 min by an arc current of 100A (which corresponds to an applied voltage of 42V) in a constant He pressure of 660 mbar. Three di!erentproductswereobtained:thesootcollectedatthe furnace chamber wall, the deposit grown in the cathode, and a carbonaceous spongy material surrounding it, hereinafter called collarette. The magnetic characterisa- tionoftheobtainedproductswasachievedbymagnetisa- tionmeasurementsaswellasMo K ssbauer spectroscopy,in the temperature range 4.2}300K. Mo K ssbauer spectra of collarette and soot samples mea- suredat4.2KareshowninFig.1.Thespectrawere "tted with the following components: (a) a magnetic sextet (B  "34.1}34.4T) accompanied by a less intense one (B  "31.1}31.5T), assigned to -Fe; (b) a 25.1T sextet characteristic of Fe  C; (c) a single line and a weak doub- let, originating from -Fe. These three phases are usually found in similarly prepared Fe}C composites (e.g. Ref. [5]), but in our case the presence of a satellite sextet and doublet, respectively, in -Fe and -Fe, indicate that the amount of C in solution in the latter phases is larger than is usually found. The total fraction of ferromagnetic material (- Fe#Fe  C) was 78% in the collarette and 67% in the soot samples. In contrast, the cathode powder contained a surprisingly low ((10%) fraction of nonmagnetic 0304-8853/01/$-see front matter  2001 Elsevier Science B.V. All rights reserved. PII:S0304-8853(00)00667-3