a signature of a phase transition or rather of a crossover to a field polarized state.

![(color online) $M/H$ vs $T$ for the compacted powder for several fields between 2 and 500 Oe. Inset: $M/H$ vs $T$ for the single crystal for several fields between 30 and 1000 Oe, applied along the \[110\] direction. []{data-label="fig3"}](fig3.pdf){width="7.0cm"}

The dynamics at the transition have been studied via a.c. susceptibility, and are shown for the powder sample in figure \[fig4\]. $\chi'$ shows a sharp peak, which is associated with the onset of the out-of-phase part $\chi''$ of the susceptibility. In the measured frequency range (5.7 mHz - 2.11 kHz), no frequency dependence of the peak position was observed (in both the powder and crystal), but the amplitude of the peak increases a little when the frequency decreases (see figure \[fig4\]a). A small hysteresis is observed in $\chi'$ and $\chi''$ (see figures \[fig4\]b and \[fig4\]c), in the same temperature range as the magnetization. The whole characteristics of the a.c. susceptibility are consistent with the picture of a first-order magnetic transition. The $\chi''$ onset would then be the signature of the dissipation at the transition. The temperature of the $\chi'$ peak gives a transition temperature $T_c=243 