First principles studies of modulated Co / Cu superlattices with strongly and weakly exchange biased Co - monolayers

First-principles calculations have been performed in order to determine effective exchange integrals between strongly and weakly exchange-coupled Co monolayers in certain modulated periodic CoCu 2 /CoCu n-type superlattices with three non-equivalent Co planes, which have not yet been studied hitherto. For 3 ≤ n ≤ 6 we find that the two non-equivalent exchange integrals have opposite signs, i.e. the strong coupling is antiferromagnetic and the weak coupling ferromagnetic, and differ for n = 4 from each other by one order of magnitude. It is shown that the results depend on the system as a whole and could not be obtained from separate parts. Finally we suggest that " spin valve " systems of such kind should be considered when trying to obtain good magneto-resistance together with low switching-fields. Magnetic multilayers based on magnetic transition metals with nonmagnetic spacers have been intensively studied for almost five years now, after it was realized that they reveal unusual oscillatory behaviour of the exchange coupling and magnetoresistance [1]. The oscil-latory phenomena have a universal character, do not depend drastically on the kind of metals involved [2] and occur both with the spacer thickness as well as magnetic-slab thickness variations [3, 4, 5], and even depend sensibly on the thickness of an additional non-magnetic cap-layer on top of a Co/Cu/Co trilayers system, e.g. [6]. The explanation of these phenomena e.g. by the essentially equivalent quantum confinement, [7], and Fabry-Perrot like electron reflection theories [8], are also by now clear, and not at the center of our paper. However, recently a great deal of attention has been attracted by exchange-biased " spin-valve " systems of the type AF/F 1 /S/F 2 [9], with one ferromagnetic slab (F 1) strongly coupled to 1 an antiferromagnet (AF) (e.g. M nF e, CoO or N iO) and the other slab (F 2) – almost free – only weakly coupled to the first one via the spacer (S). Systems of this type are not only interesting for fundamental aspects but may also be applied in future magnetic recording devices; in particular, in this way one hopes to obtain systems, where the spin direction of the weakly coupled layer is easily flipped (which explains the name " spin valves "), while at the same time the resistance of the system is sensibly changed by the flipping, which is the magneto-resistive effect mentioned below. The purpose of the present letter is, to …