Emergent order in the spin-frustrated system Dy_x Tb _2-x Ti₂ O₇ studied by ac susceptibility measurements

We report the ac susceptibility study of Dy_x Tb_2-x Ti₂ O₇ with x ε [0,2]. In addition to the single-ion effect at Ts (single-ion effect peak temperature) corresponding to the Dy ³⁺ spins as that in spin ice Dy₂ Ti₂ O ₇ and a possible spin freezing peak at T_f (T_f <3K), a new peak associated with Tb³⁺ is observed in χac (T) at nonzero magnetic field with a characteristic temperature T (T_f < T^- < Ts). T increases linearly with x in a wide composition range (0<x<1.5 at 5 kOe). Both application of a magnetic field and increasing doping with Dy³⁺ enhance T. The T peak is found to be thermally driven with an unusually large energy barrier as indicated from its frequency dependence. These effects are closely related to the crystal-field levels and the underlying mechanism remains to be understood.