each temperature ($\sim 30$ min at each temperature). Inset: isotherm as a function of time $t$ at $T=155$ mK when warming (red circles) and when cooling (blue squares). The lines are fitted to the exponential : $\frac{M}{H}(t)=\frac{M_{\rm eq}}{H} - \frac{\Delta M}{H} \exp(-t/\tau)$. When warming $\tau=207$ s, $\frac{M_{\rm eq}}{H}=0.260$ emu.cm$^{-3}$ and $\frac{\Delta M}{H}=-6.58 \times 10^{-3}$ emu.cm$^{-3}$. When cooling $\tau=165$ s, $\frac{M_{\rm eq}}{H}=0.249$ emu.cm$^{-3}$ and $\frac{\Delta M}{H}=1.08 \times 10^{-2}$ emu.cm$^{-3}$. []{data-label="fig1"}](fig1.pdf){width="7.0cm"}

A detailed study of the magnetization around the transition has been performed. To ensure accurate results, measurements had to be performed with well controlled temperature regulation and extremely slow cooling and warming rates. The protocol was the following: i) regulate at a given temperature. ii) take a large number of measurements (between 40 and 100) so that the magnetization reaches equilibrium at this temperature. iii) change temperature with a step of 5 mK, or 2 mK depending on the measurements. The temperature was ramped between 80 - 400 mK, cooling and warming the sample. The equivalent ramping rate is between 9 and 18 mK/h. The obtained magnetization as a function of temperature for the single crystal is shown in figure \[fig1\]: it can be seen that, at the transition, at a 