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 (0 01) surface using a tight-binding d-band model with site-dependent Sto ner-like exchange splitting. In the bulk we have studied the relative stability of a ferromagnetic (F) state and antiferromagnetic (AF) stat es in four different configurations as a function of tetragonal distor tion, 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 spi n alignments. For two particular bet lattices, as observed in epitaxia l Mn films on Fe(001) and on Pd(001), we have studied the magnetic str ucture at the (001) surface. Starting from the corresponding ground-st ate magnetic structure in the bulk we have compared several magnetic c onfigurations in the surface layer. The in-plane AF [c(2x2) symmetry] configuration is ground state for both lattices. The relative stabilit y of the different configurations has been interpreted in terms of com peting AF exchange couplings between neighboring surface and subsurfac e sites. This has been done by constructing an effective Ising model f or the surface magnetic order from the energies obtained from the elec tronic structure calculation.