Band-theoretical investigation of the magneto-optical Kerr effect in Fe and Co multilayers

A band theory of the magneto-optical (MO) properties of solids, based on the first-principles spin-polarized, relativistic linear muffin-tin orbital method, is described. Two expressions of the relativistic electron-photon-interaction matrix elements are implemented and their accuracies are investigated. It is found that the spin-polarized, relativistic band theory, together with the expression of the photon-absorption matrix elements, is able to give reliable optical conductivity and MO Kerr rotation spectra of bcc Fe that agree well with both experiments and recent calculations using a different method. The MO properties of several Fe and Co multilayers [Fe(Co${)}_2$${\mathrm{Cu}}_6$ fcc-(001), FeCu(Ag${)}_5$ fcc-(001), bcc-Fe/fcc-${\mathrm{Ag}}_5$ (001), bcc-${\mathrm{Fe}}_{\mathit{n}}$/fcc-${\mathrm{Au}}_5$ (001) (n=1,3), ${\mathrm{Co}}_2$${\mathrm{Pd}}_4$ fcc-(111), and ${\mathrm{Co}}_2$${\mathrm{Pt}}_{\mathit{m}}$ fcc-(111) (m=1,4,7)] are then investigated theoretically. The calculated MO Kerr effect in all the multilayers except ${\mathrm{Co}}_2$${\mathrm{Pt}}_1$ fcc-(111) is not larger than in bulk Fe and Co. Nevertheless, the results suggest that materials with larger Kerr rotations can be achieved by making Fe (Co) superlattices with ultrathin layers (few monolayers thick) of heavy elements possessing a partially filled d band (e.g., Pt and Os).

Kerr rotation spectra for Fe(Co)/Cu(Ag, Au, Pd, Pt) bilayers [i.e., a thin layer of Fe (Co) on the Cu (Ag, Au, Pd, Pt) substrate] are also obtained from the calculated optical conductivity tensors of Fe, Co, Cu, Ag, Au, Pd, and Pt. It is found that the MO Kerr effect in the Fe (Co)/noble-metal bilayers is considerably enhanced near the plasma edge of the substrate. This indicates that one can obtain good MO materials by fabricating Fe (Co) bilayers using nonmagnetic metals with a sharp plasma edge (e.g., Ag and Al) as substrates. Calculated Kerr rotation spectra of the Fe and Co multilayers as well as bilayers are in satisfactory agreement with available experiments. The spin and orbital magnetic moments in the Fe and Co multilayers are also calculated, and possible correlations between these and MO properties of the Fe and Co multilayers are discussed.