Magnetic structures of bct manganese in the bulk and at the (001) surface

We have studied the magnetic structure of bet Mn in bulk and at the (001) surface using a tight-binding d-band model with site-dependent Stoner-like exchange splitting. In the bulk we have studied the relative stability of a ferromagnetic (F) state and antiferromagnetic (AF) states in four different configurations as a function of tetragonal distortion, cia, for atomic volumes from 12 to 16 Angstrom(3). Between c/a=1 (bcc) and c/a=root 2 (fee) we find three AF phases with different spin alignments. For two particular bet lattices, as observed in epitaxial Mn films on Fe(001) and on Pd(001), we have studied the magnetic structure at the (001) surface. Starting from the corresponding ground-state magnetic structure in the bulk we have compared several magnetic configurations in the surface layer. The in-plane AF [c(2x2) symmetry] configuration is ground state for both lattices. The relative stability of the different configurations has been interpreted in terms of competing AF exchange couplings between neighboring surface and subsurface sites. This has been done by constructing an effective Ising model for the surface magnetic order from the energies obtained from the electronic structure calculation.