Composition dependence of bulk properties in the Co-intercalated transition metal dichalcogenide <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Co</mml:mi><mml:mrow><mml:mn>1</mml:mn><mml:mo>/</mml:mo><mml:mn>3</mml:mn></mml:mrow></mml:msub><mml:mi>Ta</mml:mi><mml:msub><mml:mi mathvariant="normal">S</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math>

Abstract

Spontaneous Hall conductivity has recently been reported in the triangular lattice antiferromagnet ${\mathrm{Co}}_{1/3}\mathrm{Ta}{\mathrm{S}}_{2}$ under a zero magnetic field. This phenomenon originates from the distinctive noncoplanar triple-Q magnetic ground state, possessing uniform real-space Berry curvature characterized by scalar spin chirality. We investigated the physical properties of ${\mathrm{Co}}_{1/3}\mathrm{Ta}{\mathrm{S}}_{2}$ by judiciously controlling the composition, revealing a drastic change in its bulk properties, even by slight variations in cobalt composition, despite the same crystal structure. For $0.299\ensuremath{\le}x\ensuremath{\le}0.325, {\mathrm{Co}}_{x}\mathrm{Ta}{\mathrm{S}}_{2}$ keeps all the characteristics of the ground state consistent with the previous studies---two antiferromagnetic phase transitions at ${T}_{\mathrm{N}1}$ and ${T}_{\mathrm{N}2}$ ($&lt;{T}_{\mathrm{N}1}$), a large spontaneous Hall conductivity $[{\ensuremath{\sigma}}_{xy}(\mathbf{H}=0)]$, and a weak ferromagnetic moment along the $c$ axis. However, samples with $x\ensuremath{\ge}0.330$ exhibit distinct bulk properties, including the absence of both ${\ensuremath{\sigma}}_{xy}(\mathbf{H}=0)$ and the weak ferromagnetic moment. Our neutron diffraction data reveal that ${\mathrm{Co}}_{x}\mathrm{Ta}{\mathrm{S}}_{2}$ with $x\ensuremath{\ge}0.330$ develops coplanar helical magnetic order with ${\mathbf{q}}_{\mathrm{m}1}=$(1/3, 0, 0). This is entirely different from what has been seen in $x\ensuremath{\le}0.325$, explaining the observed composition dependence.

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