Physica B 327 (2003) 385–388 Anomalous magnetization process in exchange-biased MnPd/Co bilayers N.N. Phuoc a , N.A. Tuan a , N.P. Thuy a,b, *, D. Babonneau c , J. Rabier c a International Training Institute for Materials Science (ITIMS), Dai hoc bach khoa, 1 Dai Co Viet, Hanoi, Viet Nam b Cryogenic Laboratory, Faculty of Physics, College of Natural Science, Viet nam National University, Hanoi, Viet Nam c Laboratoire de M! etallurgie Physique, UMR 6630 CNRS-Universit ! e de POITIERS, F-86962 Chasseneuil Futuroscope Cedex, France Abstract MnPd/Co bilayers sputtered onto Si (1 0 0) substrates with varied MnPd thickness have been investigated. Structural characterization of the bilayers was carried out with an X-ray diffractometer and their magnetic properties were measured by a vibrating-sample magnetometer. As-deposited bilayers exhibit large but quite usual exchange-biased coupling, i.e. the shift of the hysteresis loop towards the field direction opposite to the cooling-field direction. This exchange bias vanishes if the antiferromagnetic (AFM) layer thickness is below a certain value. However, for annealed bilayers with an MnPd layer thickness below a critical value, large exchange bias and asymmetric double-shifted loops are unexpectedly observed when the measuring field is along the cooling-field direction. These anomalies in the magnetization process can be explained if one postulates that the orientation of the AFM easy axis with respect to the spin direction of the ferromagnetic layer is changed when the thickness of the AFM layer decreases. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Exchange bias; Magnetic multilayers; Double-shifted loops; Nanostructures The phenomenon of exchange bias between antiferromagnetic (AFM) and ferromagnetic (FM) materials has attracted enormous attention due to its potential application in spin valves. Although it was discovered a long time ago [1] and extensive studies have been carried out recently, the physical origin of exchange bias remains poorly understood [2]. One of the difficulties in solving this problem is the poor understanding of the spin structure at the interface of the FM/AFM bilayers. Earlier models assumed collinear mag- netic structures on the FM and AFM side of the interface [3–5]. However, in a recent calculation by Koon [6], the interfacial energy is minimized for perpendicular coupling between the FM and AFM layers, showing that this kind of coupling is also possible. Recently, a new phenomenon, the double- shifted loops, has been observed in some ex- change-biased systems, such as in Co/NiMn [7–8], Fe/MnPd [9] and NiO/NiFe [10] bilayers. Gen- erally, this phenomenon is observed when measur- ing perpendicular to the cooling-field direction. In this paper, we study the magnetization process in MnPd/Co bilayers and show that double-shifted loops appear at some preparation and measure- ment conditions and can be tuned by changing *Corresponding author. International Training Institute for Materials Science (ITIMS), Dai hoc bach khoa, 1 Dai Co Viet, Hanoi, Viet Nam. Tel.: +84-4-8692518; fax: +84-4-8692963. E-mail address: thuy@itims.edu.vn (N.P. Thuy). 0921-4526/03/$-see front matter r 2002 Elsevier Science B.V. All rights reserved. PII:S0921-4526(02)01751-9