IMPORTANCE OF THE EXACT ATOMIC POSITIONS ON THE MAGNETIC-PROPERTIES OF FEX/RUX SUPERLATTICES

Experimentally Fe(x)/Ru(y) superlattices have been found non-magnetic up to x = 4 monolayers and ferromagnetic (FM) beyond, with two dead Fe planes at the interfaces. Preliminary Linear Muffin-Tin Orbitals (LMTO) calculations assuming a perfect hexagonal close packed (hcp) stacking of the atomic planes in the c direction predicted an antiferromagnetic (AF) or ferrimagnetic (FI) ground state. Very recent experiments suggest that in thick Fe layers, some of the hexagonal Fe planes are shifted laterally compared to a hcp position, inducing a much more bcc-like local environment of the Fe atoms in the middle of the layers. The effects of this new atomic ordering and of the relaxation of the interplane distances along the c axis on the magnetic properties of Fe are theoretically studied here by the LMTO method. The main result is a pronounced stabilization of the FM state compared to the AF or FI ones. The effect of the sign of the interlayer coupling between Fe layers has also been studied for Fe2/Ru2. Neither of these effects is sufficient to explain the existence of dead Fe layers.