* Corresponding author. Tel.: #54-221-424-6062; fax: #54- 221-425-2006. E-mail address: mercader@venus."sica.unlp.edu.ar (R.C. Mercader). Journal of Magnetism and Magnetic Materials 226}230 (2001) 1513}1515 Magnetic behavior of nanosized cupric oxide R.A. Borzi, S.J. Stewart, R.C. Mercader*, G. Punte, F. Garcia Departamento de Fn & sica, LANADI, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, CC 67, 1900 La Plata, Argentina Department of Chemistry, The Open University, Walton Hall, Milton Keynes, MK7 AA6, UK Departamento de Fn & sica, LANADI, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, CC 67, 1900 La Plata, Argentina Centro Brasileiro de Pesquisas Fn & sicas/CNPq, Rua X. Sigaud 150, 22290}180 Rio de Janeiro, Brazil Abstract The e!ect of the particle size and ball-mill-induced defects on the magnetic properties of cupric oxide has been assessed by magnetic measurements. Nanoparticles with an average strain "0.035 and average crystallite size D"9 nm show a susceptibility one order of magnitude higher than the starting material. Up to room temperature, instead of the increasing susceptibility found in bulk samples, a Curie}Weiss-like behavior is observed above the ordering temperature of ca. 230 K. 2001 Elsevier Science B.V. All rights reserved. Keywords: Nanocrystalline materials; Transition metal oxides; Low-dimensional systems; Defect e!ects Cupric oxide, CuO, has been extensively studied due to its relation to high-¹ superconductors. The diverse re- sults obtained by di!erent authors when measuring its magnetic properties have been assigned to the existence of intrinsic defects, like cation [1] or anion [2}4] va- cancies, particle-size e!ects [5], uncompensated charges (Cu) [2], etc. Also, CuO has recently been studied in nanostructured form, which is known to have applica- tions in catalysis and optics [6,7]. Notwithstanding, to our knowledge no magnetic measurements performed on CuO nanoparticles are found in the literature. In this work, towards evaluating the e!ect of the par- ticle size and structural defects on the magnetic proper- ties, cupric oxide samples were subjected to ball-milling and their magnetic properties measured. Cupric oxide supplied by CERAC (99.999% purity) was milled for times up to 10 h in an unsealed cylindrical steel vial with one steel ball. A vertical vibratory mixer mill operating at 50 Hz, with 1 cm of amplitude, was used. Electron dispersion X-ray analyses performed on the samples did not show any contamination from the vial or ball within the experimental resolution. X}ray di!raction (XRD) patterns were obtained using a standard Philips PW}1710 di!ractometer. Cu K radi- ation was used to obtain high-statistics data. The Young and Desai model [8] was applied after Rietveld pro"le analyses to assess the grain size and strain of the broadened XRD patterns. The particle size was found to decrease with milling time. Three samples were selected for the magnetic studies; the starting material * with good crystallinity * and two with drastically di!erent particle sizes and strain. The results for the three samples, untreated S ,S , and S , show average grain sizes of 203.5 nm for S , 21.2 nm for S , and 9.0 nm for S , while the average strain obtained was 0.004, 0.005 and 0.035, respectively. The XRD patterns are shown in Fig. 1. Susceptibility curves were obtained using a Quantum Design SQUID magnetometer in zero-"eld-cooled (ZFC) and "eld cooled (FC) conditions in a 50 Oe mag- netic "eld for samples S and S , and a 500 Oe "eld for S . Fig. 2 shows in detail the curves between 77 and 300 K. It can be seen that there is an increase in the susceptibility (¹) for the ball-milled samples; (¹) for S is three times higher than for S and ten times than for S . CuO magnetization (Fig. 2a) exhibits a monotonous behavior in the studied temperature range; an anomaly in the slope at T "230 K shows the change from short- to long-range antiferromagnetic order. On the contrary, 0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII:S0304-8853(00)00943-4