Fig. 4. Magnetic susceptibility and 2D Ising transition.

a Magnetization versus magnetic field along out-of-plane (z) and in-plane (x) directions, respectively. b Magnetic susceptibility plotted against temperature. The in-plane susceptibility data divided by 8 for clarity. c Effective mobility, ${\mu }_{\mathrm{eff}}=\frac{\mathrm{\Delta }x}{\mathrm{\Delta }H}$, of the DW is plotted against $\tau \equiv 4{\chi }_c/{\chi }_{ab}$, which is the control parameter for the linear wall phase transition. The ${\mu }_{\mathrm{eff}}$ data fits is consistent with the DW mobility predicted by the 2D Ising model (red dotted line)²⁴, but not with the mean-field theory (blue dotted line), which predict a phase transition at ${\tau }^{*}~1$. Error bars represent the uncertainties of least square fitting of data.