Choi et al. (Phys. Rev. B 58 (1998) 5166) using low-energy electron diffrac
tion recently displayed an ordered Mn-Co alloy at the surface of Co for a 0
.3-0.8 equivalent monolayer of Mn on Co(001). From magneto optic Kerr effec
t and X-ray magnetic circular dichroism experiments it was concluded that t
he coupling between Mn and Co in these surface alloys is ferromagnetic in t
ype. Using the tight binding linear muffin tin orbitals scheme within an at
omic sphere approximation and an local density approximation (LDA) in the f
ilm geometry we conclude that Mn in the ordered 1 ML thick Mn0.5Co0.5 surfa
ce alloy is in a high spin state of ca. 3.2 mu(B). Both ferromagnetic and a
ntiferromagnetic couplings between Mn and Co are obtained but the antiferro
magnetic coupling seems to be the preferred type. Going beyond LDA and cons
idering a slightly disordered surface alloy should, therefore, be worthwhil
e. (C) 2000 Elsevier Science B.V. All rights reserved.