Anomalous transport properties in Eu1-xLaxB6 (x=0.0, 0.05, 0.1, 0.2, and 0.3): Hall sign reversal in Eu1-xLaxB6 (x=0.2)

Abstract

We have investigated the electric and magnetic properties of series compounds of Eu1-xLaxB6 (x=0.0, 0.05, 0.1, 0.2, and 0.3). It is found that the magnetic ground state changes gradually with increasing La concentration from ferromagnetic to antiferromagnetic, which results in ferromagnetic transition at T-c=15 K for Eu1-xLaxB6 (x=0.0) and antiferromagnetic transition at T-N=9 K for Eu1-xLaxB6 (x=0.3). For Eu1-xLaxB6 (x=0.05, 0.1, and 0.2), the temperature-dependent resistivity rho(T) shows anomalous gaplike increase at low temperatures below approximate to 15 K whereas it is metallic at high temperatures. In addition, the temperature-dependent magnetization M(T) shows spin-glass-type thermal hysteresis, indicating magnetic instability at low temperatures below T-c and T-N. Hall resistivity rho(xy) measurement shows a nonlinear behavior of rho(xy)(B) in the field and temperature ranges, 0 T <= H <= 2 T and 2 K <= T <= 40 K, for Eu1-xLaxB6 (x=0.05, 0.1, and 0.3), which is likely due to asymmetric magnetic scattering and spin-orbit coupling. Remarkably, a sign reversal of temperature-dependent Hall coefficient R-H(T) was observed for Eu1-xLaxB6 (x=0.2):R-H(T) is negative at high temperatures and positive at low temperatures below T approximate to 25 K. The temperature-dependent thermoelectric power TEP S(T) also shows the similar sign reversal at T approximate to 26 K for Eu1-xLaxB6 (x=0.2), whereas it is negative for the whole temperature range 2 K <= T <= 300 K for Eu1-xLaxB6 (x=0.05, 0.1, and 0.3). None of the known models, as we know, was found to describe fully the Hall and TEP sign change. Based on the investigated electric and magnetic properties, we conjectured that the sign reversal of R-H(T) and S(T) would be closely correlated with the magnetic instability.