Double magnetic transitions and exotic field induced phase in the triangular lattice antiferromagnets Sr${}_3$Co(Nb,Ta)${}_2$O${}_9$

Abstract

Two triangular lattice antiferromagnets Sr${}_3$Co(Nb,Ta)${}_2$O${}_9$ with an effective $j_{\mathrm{eff}}=1/2$ of Co${}^{2+}$ 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 $J/k_{\mathrm{B}}\simeq 4.7$ K and 5.8 K, respectively. Both the compounds feature two-step magnetic transitions at low temperatures [($T_{N1}\simeq 1.47$ K and $T_{N2}\simeq 1.22$ K) and ($T_{N1}\simeq 0.88$ K and $T_{N2}\simeq 0.67$ K), respectively], driven by weak easy-axis anisotropy. Under magnetic field Sr${}_3$CoNb${}_2$O${}_9$ evinces a plateau at $1/3$ magnetization. Interestingly, the high field magnetization of Sr${}_3$CoTa${}_2$O${}_9$ 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 ($C_{\mathrm{p}}\propto T^{\alpha }$) with a reduced exponent $\alpha \simeq 2$. These results demonstrate an unusual field induced state with gapless excitations in the strongly frustrated magnet Sr${}_3$CoTa${}_2$O${}_9$. The complete $T-H$ phase diagram is discussed for both the compounds.

12 pages, 7 figures, 2 tables

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Cited by in corpus (1)

• Evidence of quantum spin liquid state in a Cu${}^{2+}$-based $S=1/2$ triangular lattice antiferromagnet