First-principles Studies of Modulated Co/cu Superlattices with Strongly and Weakly Exchange-biased Co-monolayers Leading to a Ferrimagnetic Ground State

First-principles calculations have been performed in order to determine effective exchange integrals between strongly and weakly exchange-biased Co monolayers in the modulated CoCu 2 /CoCu n superlattices. For 3 ≤ n ≤ 6 it has been found that the respective exchange integrals have opposite signs and differ for n = 4 from each other by one order of magnitude and for n = 4 still by a factor of ∼ 1.7. The obtained phase diagram, with all the relevant magnetic phases, shows that for the n-values considered the ground state configuration is ferrimagnetic. 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]. Recently a great deal of attention has been attracted by exchange-biased systems AF/F 1 /S/F 2 [6], with one ferromagnetic slab (F 1) strongly coupled to 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. It is possible to replace an exchange-biasing antiferromagnet by a trilayer, AF −→ f erromagnet1/spacer/f erromagnet2, 1 provided the thickness of the spacer is chosen such as to ensure strong antiferromagnetic coupling of the two ferromagnetic layers [7]. In an attempt to get more insight into the nature of exchange coupling and possible magnetic phases which may appear, we have studied systematically by the spin-polarized ab initio LMTO-ASA method (linearized muffin-tin orbitals, atomic sphere approximation, scalar-relativistic version, see [5]) the series of multilayers with supercell Cu 2 Co (2) Cu 2 Co (1) Cu 2 Co (2) Cu n Co (3) Cu n of the (001) face-centred tetragonal structure (i.e. Co is grown epitaxially on Cu). In these sytems the monolayers Co (1) couple strongly antifer-romagnetically with Co (2) , while the Co (3) monolayers are only weakly coupled (for n > 2). In contrast to other studies our systems are infinite multilayers, and the sublayer Cu …