Doping dependence of transport properties in Fe1-xCoxSi

The positive magnetoresistance has been investigated for Fe1-xCoxSi single crystals in a wide range of doping (0.05≤x≤0.7). Most of the magnetoconductivity data are found to scale well with the magnetization. This is inconsistent with the quantum interference scenario proposed by Manyala [Nature 404, 581 (2000)]. We have shown that the decrease of density of the minority spin band with high mobility in the course of Zeeman splitting is relevant to the positive magnetoresistance. The nearly half-metallic nature in this system seems to enhance the magnetoresistance. The pressure dependence of resistivity has been measured for Fe0.7Co0.3Si. T-linear behavior has been found in the resistivity above 7GPa, where the helical spin order is completely suppressed. This temperature dependence reproduces that of the hypothetical resistivity of the nonmagnetic state deduced by the analysis of the magnetoresistance. We have investigated the large Hall conductivity in Fe1-xCoxSi (∼40Ω-1cm-1 at a maximum). The doping dependence of the Hall conductivity is almost parallel with those of the critical field and the wave vector of the helical spin state. This suggests that the Hall conductivity is proportional to the effective spin-orbit interaction. We have also observed the doping dependence of the Seebeck coefficient for Fe1-xCoxSi. In the underdoped region (x≤0.1), the negative Seebeck coefficient is enhanced at low temperature below 100K, corresponding to the steep doping variation of the resistivity in this temperature region. In the higher doping region (x≥0.2), the Seebeck coefficient shows a gradual upturn at low temperatures (100K). This is caused by the electronic structural change occurring with the transition from the paramagnetic to the ferromagnetic state.