Electron doping dependence of ferromagnetism in Eu_1-xLa_xB₆

The temperature dependent magnetization M(T) and electrical resistivity p(T) were measured to study the electron doping effect in Eu_1-xLa_xB₆ (x=0, 0.005, 0.03, 0.05, and 0.1). At room temperature, p(T) decreases systematically with an increasing amount of electron doping. For a small doping with x=0, 0.005, and 0.03, M(T) exhibits ferromagnetic transitions and ρ(T) reduces drastically below the transition, which also reveals characteristics of double transition. It is clearly found from both M(T) and ρ(T) that the doped carriers in the region of x<0.03 lower the transition temperature. For the doping level with x=0.05, and 0.01, the ground-state properties look quite different from the ones of EuB₆. While M(T) shows a ferromagneticlike behavior at low temperature, an increase of p(T) around the ferromagneticlike region, rather than a reduction, is observed. This ρ(T) behavior looks similar to the one in SmB₆, which shows the opening of a Kondo insulating gap.