Photoemission study of the itinerant-electron helimagnet Fe_xCo_1-xSi

Fe_xCo_1-xSi is a metal with helical spin structure in the concentration range 0.2<x<0.95 although the end members are a narrow-gap semiconductor (FeSi) and a diamagnetic semimetal (CoSi). We have studied the electronic structure of Fe_xCo_1-xSi (x=0.5, 0.8) using ultraviolet photoemission spectroscopy. With Fe substitution for Co, the structure due to the transition-metal 3d states is shifted toward lower binding energies, qualitatively consistent with the prediction of the rigid-band model. Although the superposition of the spectra of FeSi and CoSi better describes the x dependence of the global spectral features than the rigid-band model, the x dependence near the Fermi level (E_F) is better described by the rigid-band model. The appearance of magnetic order in Fe_xCo_1-xSi may thus be explained by the rigid-band model, which predicts that the density of states at E_F is low or zero for CoSi and FeSi but becomes large for intermediate x. We also find a weak temperature dependence around the Fermi level, qualitatively consistent with the increase in the electrical resistivity below the Néel temperature (T_N).