Low-temperature transport properties of nanolaminates Ti₃AlC₂ and Ti₄AlN₃

In this paper we report on the electronic and magnetotransport properties of two Ti-based ternaries, Ti₃AlC₂ and Ti₄AlN₃. In order to determine the effective carrier concentrations and their mobilities, the Hall effect, electrical conductivity, thermoelectric power, magnetic susceptibility, and magnetoresistance were measured as a function of temperature between 4 and 300 K and at magnetic fields up to 9 T. For Ti₃AlC₂, the Hall voltage is a linear function of magnetic field at all temperatures. At the lowest temperatures, the Hall coefficient is small but positive; above 100 K it is negative and drops more or less linearly with temperature. The magnetoresistance of Ti₃AlC₂ is dominated by a positive quadratic field dependence. The magnetic susceptibility is nearly constant but displays a weak maximum around the temperature at which the Hall effect changes sign (≈100 K). In contrast, the Seebeck coefficient remains positive up to 800 K, with a maximum at 700 K. The results were analyzed within a two-band framework assuming a temperature-independent charge-carrier density and a hole mobility that is slightly smaller than the electron mobility. The model quantitatively accounts for our observations. The resistivity, magnetoresistance, and Hall coefficient of Ti₄AlN₃, on the other hand, were successfully described within the single-band model, with holes as the dominant charge carriers. This was supported by measurements of the Seebeck coefficient, which is positive and peaks at ≈300 K. The magnetic susceptibility of Ti₄AlN₃ is also quite temperature independent.