sample, the transition occurs around 245 mK, but the width of the transition and of the hysteresis are similar. Zero Field Cooled - Field-Cooled (ZFC-FC) magnetization shows an irreversibility below the temperature of the transition [@SupMat]. In ordered materials, such irreversibility is often ascribed to domain freezing. The ZFC-FC irreversibility is strongly reduced when the applied field is increased and is suppressed at about 500 Oe. This suggests that small fields are enough to overcome the barriers involved in the freezing.

![(color online)a. $M/H$ vs $T$ for the single crystal in an applied fied $H=5$ Oe parallel to the \[100\] axis, extracted from figure \[fig1\] with only the equilibrium value of the magnetization plotted compared to the specific heat data. b. The equivalent data for the crushed powder.[]{data-label="fig2"}](fig2.pdf){width="7.0cm"}

The effect of the field on the transition has also been investigated. For the powder samples, a field as small as 10 Oe is enough to reach the regime of nonlinear susceptibility, resulting in a smaller value of $M/H$ (see figure \[fig3\]). However, up to at least 20 Oe, the transition temperature and the transition and hysteresis widths remain constant. At 50 Oe, the transition starts to become smoother and shifts to 