Field-induced magnetic phases in the Kitaev candidate <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>Na</mml:mi> <mml:mn>3</mml:mn> </mml:msub> <mml:msub> <mml:mi>Co</mml:mi> <mml:mn>2</mml:mn> </mml:msub> <mm

We report a rich anisotropic magnetic phase diagram of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:msub> <a:mi>Na</a:mi> <a:mn>3</a:mn> </a:msub> <a:msub> <a:mi>Co</a:mi> <a:mn>2</a:mn> </a:msub> <a:msub> <a:mi>SbO</a:mi> <a:mn>6</a:mn> </a:msub> </a:mrow> </a:math> , a previously proposed cobaltate Kitaev candidate, based on field- and temperature-dependent magnetization, specific heat, and magnetocaloric effect studies. At low temperatures, our experiments uncover a low-lying <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:mrow> <b:msub> <b:mi>j</b:mi> <b:mtext>eff</b:mtext> </b:msub> <b:mo>=</b:mo> <b:mfrac> <b:mn>1</b:mn> <b:mn>2</b:mn> </b:mfrac> </b:mrow> </b:math> state with an antiferromagnetic (AFM) ground state and pronounced in-plane versus out-of-plane anisotropy. The experimentally identified magnetic phases are theoretically characterized through classical Monte Carlo simulations within an extended Kitaev-Heisenberg model with additional ring exchange interactions. The resulting phase diagram reveals a variety of exotic field-induced magnetic phases, including double- <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mi mathvariant="bold">q</c:mi> <c:mo>,</c:mo> <c:mo> </c:mo> <c:mfrac> <c:mn>1</c:mn> <c:mn>3</c:mn> </c:mfrac> </c:math> -AFM, zigzag, and vortex phases.