Magnetic properties in Ca-doped Eu hexaborides

Temperature- and field-dependent magnetization M(T,H) measurements were carried out for the series of Eu 1 - x Ca x B 6 compounds (x=0.0, 0.13, 0.35, 0.54, 0.74, and 0.87). The small Ca doping in Eu 1 - x Ca x B 6 induced a suppression of a ferromagnetic transition temperature from T c 12 K for EuB 6 to T c 5.5 K for Eu 0 . 8 7 Ca 0 . 1 3 B 6 . Interestingly, an antiferromagnetic transition, rather than ferromagnetic transition, was observed in M(T) with H=100 Oe at T N 4.5 K for Eu 0 . 6 5 Ca 0 . 3 5 B 6 and at T N 3 K for Eu 0 . 4 6 Ca 0 . 5 4 B 6 . Temperature-dependent resistivity and specific heat data at low temperatures were presented and discussed in terms of the antiferromagnetic transition. With further increase of Ca doping, paramagnetic state was observed down to T=2 K.