Fig. 2 |. Low-field magnetization and magnetic susceptibility data.

a, Low-temperature Curie–Weiss fit to the constant field magnetic susceptibility ${\chi }_{\text{d.c.}}$ in a temperature range free from Van Vleck contributions from high-energy crystal field doublets (where the majority of trivalent Yb ions are in the $J_{\text{eff}}=1/2$ ground state). A large mean-field interaction strength of −10.3(8) K with an effective local moment, ${\mu }_{\text{eff}}$, of 2.63(19) ${\mu }_{\mathrm{B}}$ is fit with a temperature-independent ${\chi }_0=0.0053\left(3\right)$ e.m.u. mol⁻¹ background term. Inset: EPR data collected at 4.2 K fit to anisotropic $g$-factors of $g_{ab}=3.294\left(8\right)$ and $g_c=1.726\left(9\right)$. b, Isothermal magnetization versus field data reaching only 67% of the expected $1.5{\mu }_{\mathrm{B}}$ per Yb ion polarized moment under ${\mu }_{0}H=9\mathrm{T}$. c, Temperature and frequency dependence of a.c. magnetic susceptibility ${\chi }^{\prime }\left(T\right)$ from 50 mK to 4 K under zero field. d, ${\chi }^{\prime }\left(T\right)$ data collected under applied magnetic fields. A minority fraction of free Yb moments are quenched at low temperatures and high fields, resulting in a peak in ${\chi }^{\prime }\left(T\right)$, and the downward inflection parameterizing this Zeeman splitting is denoted by orange stars. Inset shows field-subtracted $0\mathrm{T}-2\mathrm{T}{\chi }^{\prime }\left(T\right)$ data between 1 K and 3 K and a Curie–Weiss fit quantifying the fraction of free Yb moments in the system, as described in the text. Values in parentheses and error bars indicate one standard deviation. a.u., arbitrary units.