Applied-field magnetic structure and spectroscopy shifts of the effective spin-$\frac{1}{2}$, $XY$-like magnet Li${}_2$CoCl${}_4$

Insulators containing chains of magnetic transition metal cations provide platforms for probing spin-$\frac{1}{2}$ dynamics and quantum critical behavior. Li${}_2$CoCl${}_4$ contains edge-sharing CoCl${}_6$ octahedra that form chains along the crystallographic $c$ axis and orders antiferromagnetically at zero field, but questions remain about its applied-field magnetic structure and the Co${}^{2+}$ spin state. Here, we show with neutron diffraction on a polycrystalline sample how the anti-aligned chains of cobalt moments begin to transition to a ferromagnetic state above 1.6 T. Further, using magnetic resonance absorption measurements and noninteracting spin models, we reveal the strongly anisotropic nature of the Co${}^{2+}$ ion’s $XY$-like magnetic behavior ($g_{\parallel }=2.77$ and $g_{\perp }=5.23$) and its $J=\frac{1}{2}$ ground state. We, therefore, supply the magnetic structures and anisotropic description needed to explore the dynamics of the field-driven magnetic phases, laying the foundation for further experimental and theoretical studies.