Double magnetic transitions and exotic field-induced phase in the triangular lattice antiferromagnets <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>Sr</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:mi>Co</mml:mi><mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mi>Nb</mml:mi><mml:mo>,</mml:mo><mml:mi>Ta</mml:mi><mml:mo>)</mml:mo></mml:mrow><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>9</mml:mn></mml:msub></mml:math>

Abstract

Two triangular lattice antiferromagnets ${\mathrm{Sr}}_{3}\mathrm{Co}{(\mathrm{Nb},\mathrm{Ta})}_{2}{\mathrm{O}}_{9}$ with an effective ${j}_{\mathrm{eff}}=1/2$ of ${\mathrm{Co}}^{2+}$ 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 $J/{k}_{\mathrm{B}}\ensuremath{\simeq}4.7$ and 5.8 K, respectively. Both compounds feature two-step magnetic transitions at low temperatures [(${T}_{\mathrm{N}1}\ensuremath{\simeq}1.47\phantom{\rule{4pt}{0ex}}\mathrm{K}$ and ${T}_{\mathrm{N}2}\ensuremath{\simeq}1.22\phantom{\rule{4pt}{0ex}}\mathrm{K}$) and (${T}_{\mathrm{N}1}\ensuremath{\simeq}0.88\phantom{\rule{4pt}{0ex}}\mathrm{K}$ and ${T}_{\mathrm{N}2}\ensuremath{\simeq}0.67\phantom{\rule{4pt}{0ex}}\mathrm{K}$), respectively], driven by weak easy-axis anisotropy. Under magnetic field, ${\mathrm{Sr}}_{3}{\mathrm{CoNb}}_{2}{\mathrm{O}}_{9}$ evinces a plateau at $1/3$ magnetization. Interestingly, the high-field magnetization of ${\mathrm{Sr}}_{3}{\mathrm{CoTa}}_{2}{\mathrm{O}}_{9}$ reveals an exotic regime (between ${H}_{\mathrm{S}1}$ and ${H}_{\mathrm{S}2}$) below the fully polarized state in which the heat capacity at low temperatures is governed by a power law (${C}_{\mathrm{p}}\ensuremath{\propto}{T}^{\ensuremath{\alpha}}$) with a reduced exponent $\ensuremath{\alpha}\ensuremath{\simeq}2$. These results demonstrate an unusual field-induced state with gapless excitations in the strongly frustrated magnet ${\mathrm{Sr}}_{3}{\mathrm{CoTa}}_{2}{\mathrm{O}}_{9}$. The complete $T\text{\ensuremath{-}}H$ phase diagram is discussed for both compounds.

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