Magnetic surface reconstruction

Surface states exist on normal metals as well as on transition metals. Occupied and unoccupied surface states were recently studied experimentally and theoretically on single crystal surfaces of e.g. Ag (Reihl, 1985) Cu (Bartynski et al., 1985) and Ni (Borstel et al., 1985). Surfaces of transition metals are particularly interesting as they not only show a structural relaxation - an effect which is mostly weak on normal metals - but can also exhibit magnetic properties which differ from those of the bulk (Freeman, 1983). A surface enhanced magnetic order as well as magnetic surface reconstruction were observed on Gd(OOOl) (Weller et al., 1985). The magnetic hysteresis loop of the Fe 100) surface, very recently measured by means of the spin polarization of secondary electrons (Fig. 3), shows a softer behaviour within the outermost 5 8 due to reversed domain nucleation (Allens-pach et al., 1986). The structural and electronic differences of surfaces and bulk may manifest themselves as surface core level shifts (Eastman and Himpsel, 1982 Erbudak et al., 1983). In the case of rare earth metals and alloys they may even appear as surface valence transition (Netzer and Matthew, 1986 Kaindl et al., 1982). 

J.A. Dumesic, H. Topspe, and M. Boudart. Surface, Catalytic, and Magnetic Properties of Small Iron Particles. III. Nitrogen Induced Surface Reconstruction. J. Catal. 37 513 (1975). 

The origin of these reconstructions — or perhaps we should say the instability of the ideal (1x1) surface — lies in the peak of the density of states on the (lxl) surface at the Fermi energy (fig. 9) (Singh and Krakauer, 1988). A peak in the density of states at Ep frequently leads to one sort of instability or another, either structural or magnetic, because a change in structure can split the peak and lower the energy of the occupied states. There has been a long discussion, however, about the role of a surface state Fermi surface in this. 

Fig. 60. Schematic view of Fermi surface (FS) reconstruction around magnetic Bragg planes connected by Q according to eq. (114). This leads to strong modification of SC pair potential for conduction electron states with momenta k, — k as indicated.

The antiferromagnetic Fermi surface is thus reconstructed on the basis of the magnetic Brillouin zone. The same procedure mentioned above should be applied to the band 6 hole Fermi surfaces, probably producing small closed Fermi surfaces. They may possibly correspond to dHvA branches with frequencies of 1.5x10 Oe and SxlO Oe as shown in fig. 30b. 

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