Figure 3: Gapped spin-polarized state above the critical field and T² power-law behaviour in the dielectric constant.

(a) Dielectric constant as a function of temperature above the saturation magnetic field. Black triangles point to anomalies in ε that appear above 40 T. Inset shows the expanded ε(T) curve at 45 T, which show a sudden drop of ε below ~6.5 K. Dotted line is the fit to the equation with a spin gap with 14.7±1.1 K explained in text. (b) Inverse of ε−ε₀ as a function of (T−T_N)² at H≤H_c. at constant magnetic fields (triangles, inverses triangles, and diamonds indicate magnetic field below, at, and above the H_c). The linear behaviour in (ε−ε₀)⁻¹ supports that the T² behaviour is robust within a wide magnetic field range up to 12 K above T_N. Solid lines are guides for the eyes. Inset shows the same plot at H>H_c. The T² behaviour is satisfied when the magnetic field is close to H_c, but deviates quickly as an anomaly in ε is observable at higher magnetic fields.