Multiple ferromagnetic structures in an off-center rattling system Eu ₈Ga₁₆Ge₃₀

We performed electrical resistivity ρ, magnetization M and specific heat C measurements on single crystals of a type-I clathrate compound Eu ₈Ga₁₆Ge₃₀, where the guest magnetic ions of Eu²⁺ in the tetrakaidecahedral cages are rattling among off-center positions. The jump of C at the Curie temperature T_C is only one third of the value expected for a uniform ferromagnet with S=7/2 from the mean-field theory. Both ρ(T) and dM(T)/dT show broad humps at T*=20-24 K, where C(T) has no anomaly. With increasing magnetic fields to 5 T, the T* in ρ(T) shifts to 40 K. These observations indicate a crossover from a modulated structure to a uniform one on cooling through T*.