Transition behavior of La 2 CuO 4d single crystals chemically oxidized in KMnO 4 aqueous solution A.T.M. Nazmul Islam a, * , Ken Ashizawa a , Satoshi Watauchi a,b , Isao Tanaka a,b , Nobukazu Kinomura a a Faculty of Engineering, Institute of Inorganic Synthesis, Yamanashi University, Miyamae 7, Kofu 400-8511, Japan b CREST Project, Japan Science and Technology Corporation JST), Japan Received 16 October 2000; accepted 15 January 2001 Abstract We have studied the magnetic properties of high quality single crystals of La 2 CuO 4d treated in KMnO 4 aqueous solution systematically changing the oxidation conditions, e.g., treatment temperature and time, KMnO 4 concentration of solution, etc. The treated crystals showed two distinct superconducting phases with T c of 15 and 25 K. With in- creasing strength of oxidation condition, transformation of low T c phase into higher-T c phase and decrease of ¯ux trapping was observed. A tendency of the mobile dopants at room temperature to rearrange themselves in few days to show an optimum ¯ux trapping was observed. Some other important conclusions were also drawn from the available results. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 74.72.Dn; 74.25.Ha Keywords: Chemical oxidation; La 2 CuO 4  d single crystals; Flux trapping; Susceptibility 1. Introduction Despite La 2 CuO 4d being one of the simplest and most extensively studied high-T c supercon- ducting materials, the understandings of its prop- erties still remain incomplete. Features like, the easy reversibility of doping, dopants remaining mobile at temperatures as low as 200 K [1], con- sequence of which is the phenomena known as crystallographic phase separation, makes this sys- tem so interesting and complicated. The intercalation of excess oxygen is usually induced by heat treatment under a high oxygen pressure [2] and by electrochemical oxidation [3]. Takayama-Muromachi et al. [4] have shown the eectiveness of chemical oxidation of pow- der samples by an oxidizer KMnO 4 in aqueous solution. There is evidence that the detail phase diagram drawn on the basis of studies on high quality La 2 CuO 4 single crystals diers by a marked degree from those suggested before. In this paper, we report chemical oxidation of large and high quality single crystals of La 2 CuO 4 and discuss on some of the characteristics from its magnetic be- havior. Physica C 357±360 2001) 367±370 www.elsevier.com/locate/physc * Corresponding author. Tel.: +81-55-220-8625; fax: +81-55- 254-3035. E-mail address: g00ds008@ccn.yamanashi.ac.jp A.T.M. Nazmul Islam). 0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0921-453401)00247-7