Double magnetic transitions and exotic field-induced phase in the triangular lattice antiferromagnets Sr₃Co(Nb,Ta)₂O₉

Two triangular lattice antiferromagnets Sr₃Co(Nb, Ta)₂O₉ with an effective j_eff = 1/2 ofCo²⁺ are synthesized and their magnetic properties are investigated via magnetization and heat capacity measurements. The leading in-plane antiferromagnetic exchange coupling is estimated to be J/k_B ≃ 4.7 and 5.8 K, respectively. Both compounds feature two-step magnetic transitions at low temperatures [(T_N1 ≃ 1.47 K and T_N2 ≃ 1.22 K) and (T_N1 ≃ 0.88 K and T_N2 ≃ 0.67 K), respectively], driven by weak easy-axis anisotropy. Under magnetic field, Sr₃CoNb₂O₉ evinces a plateau at 1/3 magnetization. Interestingly, the high-field magnetization of Sr₃CoTa₂O₉ reveals an exotic regime (between H_S1 and H_S2) below the fully polarized state in which the heat capacity at low temperatures is governed by a power law (C_p ∝ T^α) with a reduced exponent α ≃ 2. These results demonstrate an unusual field-induced state with gapless excitations in the strongly frustrated magnet Sr₃CoTa₂O₉. The complete T -H phase diagram is discussed for both compounds.