Journal of Magnetism and Magnetic Materials 298 (2006) 43–47 Coexistence of positive and negative exchange bias in CrMn/Co bilayers Nguyen Nguyen Phuoc a,b,Ã , Nguyen Phu Thuy a,c , Nguyen Anh Tuan a , Le Thanh Hung a , Nguyen Trung Thanh a , Nguyen Thanh Nam a a International Training Institute for Materials Science, Hanoi University of Technology, Hanoi, Vietnam b Information Storage Materials Laboratory, Toyota Technological Institute, Nagoya, Japan c College of Technology, Vietnam National University, Hanoi, Vietnam Received 7 January 2005; received in revised form 19 February 2005 Available online 24 March 2005 Abstract Exchange-biased CrMn/Co bilayers with various thicknesses of Co sputtered onto Si(1 0 0) substrates by the RF sputtering system have been studied. Double-shifted loops have been observed with the thickness of Co layer in a narrow range and become single-shifted loops after some cycles of measurement. Those results are interpreted as the association of positive and negative exchange bias. r 2005 Elsevier B.V. All rights reserved. PACS: 75.70.Cn; 75.70.i; 75.25.+z; 75.30.Gw Keywords: Exchange bias; Magnetic thin film; Double-shifted loop; Training effect 1. Introduction Discovered in 1956 [1], the phenomenon of exchange bias between an antiferromagnet (AF) and ferromagnet (FM) is of great interest due to its widespread application in spin valves and mag- netic tunnel junctions. Nevertheless, its physical origin remains unanswered [2]. Usually, exchange bias is described as an additional unidirectional anisotropy induced by the AF into the FM via exchange coupling at the interface, producing a single magnetic hysteresis loop shifted along the magnetic field axis after field cooling procedure through the Ne´el point of the AF. The magnitude of this shift is named exchange bias field (H E ) and in almost all cases, the magnetic hysteresis loop is shifted in the negative field if one defines the direction of the cooling field (H FC ) as ARTICLE IN PRESS www.elsevier.com/locate/jmmm 0304-8853/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2005.03.006 Ã Corresponding author. Information Storage Materials Laboratory, Toyota Technological Institute, 2-12-1 Hisakata, Tempaku, Nagoya 468-8511, Japan. Tel.: +81 52 809 1872; fax: +81 52 809 1874. E-mail address: nnguyenphuoc@yahoo.com (N.N. Phuoc).