History-dependent magnetic properties in pure and Zn-doped cupric oxide R.A. Borzi a , S.J. Stewart a , G. Punte a , R.C. Mercader a, * , R.D. Zysler b , M. Tovar b a Departamento de Fõ Âsica, Universidad Nacional de La Plata, C.C. 67, 1900 La Plata, Argentina b Comisio Ân Nacional de EnergI Â a Ato Âmica and Universidad Nacional de Cuyo, Centro Ato Âmico Bariloche and Instituto Balseiro, 8400 San Carlos de Bariloche, Argentina Received 15 June 2000; accepted 3 October 2000 by C.E.T. Gonc Ëalves de Silva Abstract Magnetization curves taken in zero-®eld-cooling and ®eld-cooling regimes on pure CuO and Zn-doped CuO samples demonstrate thermal irreversibility. For the Zn-doped sample, this is present above the Ne Âel temperature (T N ) and is reinforced below T N . Hysteresis is related to inherent frustration of the magnetic structure of CuO. q 2001 Published by Elsevier Science Ltd. Keywords: A. Magnetically ordered materials; D. Exchange and superexchange PACS: 75.50. -y; 75.50.Ee; 75.50.Lk 1. Introduction Cupric oxide (tenorite) has peculiar magnetic properties that have been the subject of many investigations. Above the Ne Âel temperature (T N ) of 230 K, its magnetism exhibits low-dimensional features; short range correlations are revealed by the shape of the thermal dependence of susceptibility measurements performed on poly-crystals above T N [1]. Exchange pathways of both planar and chain-like geometry have been proposed as those with strongest superexchange correlations [2]. Below T N , tenorite antiferromagnetism comprises an incommensurate magnetic phase between 213 and 230 K, and a commensurate one below 213 K [3]. Although the imperfect stoichiometry of CuO has been investigated in several works, there is still no consensus among the different authors about the nature of the native CuO point defects or defect complexes [4]. Their presence, that alters the tenorite magnetism, is likely dependent on the sample preparation conditions. This can be the source of the diversity of results ascribed to defects in the oxygen sub- lattice [5±9]. It has also been stated that Cu 21 ions located close to defects may have antiferromagnetic interactions of different strength with their neighbors, leading to random local canted structures and glassy properties [6]. A spin- glass-like magnetic behavior has been suggested [10,11] to take place in more complex systems containing Cu and O which are often compared to CuO because of their similar magnetic and structural properties. Notwithstanding, no clear evidence of spin-glass behavior in CuO has been given. In this work, we perform magnetization measurements at low ®elds on pure CuO and Zn-doped tenorite samples to investigate the suggested spin-glass-like properties of the array of magnetic moments. 2. Experimental Magnetization measurements were carried out on a high- purity sample of commercial CuO (Cerac 99.999%) and 1.2 at.% Zn-doped CuO samples obtained by chemical methods. The magnetization of an undoped CuO sample prepared in this way was also measured. Zn-doped CuO (Cu 12x Zn x O) was prepared by co-precipi- tation from an aqueous solution of Cu 21 and Zn 21 ions to hydroxides by an alkali (ammonia), followed by heating the precipitate at 5008C in air for 24 h. The aqueous solution Solid State Communications 117 (2001) 311±314 0038-1098/01/$ - see front matter q 2001 Published by Elsevier Science Ltd. PII: S0038-1098(00)00467-1 PERGAMON www.elsevier.com/locate/ssc * Corresponding author. Tel.: 154-221-424-6062; fax: 154-221- 425-2006. E-mail address: rcm@venus.®sica.unlp.edu.ar (R.C. Mercader).