\pm 1$ mK. These results are in strong contrast with a.c. measurements in Yb$_2$Sn$_2$O$_7$ [@Lago14], where a glassy behavior was reported.

![(color online) a.c. susceptibility, in phase $\chi'$ (top) and out-of-phase $\chi''$ (bottom), vs $T$ for the crushed powder sample. Data collected while cooling. The amplitude of the a.c. field is 1 Oe. (a) Magnification of the $\chi'$ peak for frequencies between 5.7 mHz and 211 Hz. (b) $\chi'$ and (c) $\chi''$ vs $T$ at the transition at 21 mHz for the cooling and warming ramps.[]{data-label="fig4"}](fig4.pdf){width="8.0cm"}

Our results show an agreement between thermodynamic measurements, i.e. magnetization and specific heat, supporting a first-order transition in [Yb$_2$Ti$_2$O$_7$]{}, at 245 mK and 150 mK for the powder and the single crystal respectively, and involving a LRO ferromagnetic component. This is qualitatively consistent with neutron measurements performed in the single crystal [@Yasui03; @Chang12], although our results (transition temperature, hysteresis width, value of the ordered moment), do not match quantitatively.

These features, a first-order transition resulting in a small ferromagnetic ordered magnetic moment suggesting a fluctuating component are reminiscent of other pyrochlore compounds which exhibit long-range order [@Gardner10]: Gd$_2$Sn$_2$O$_7$, the archetype of dipolar Heisenberg pyrochlore antiferromagnet [@Wills06; @Bonville03] and Tb$_2$Sn$_2$O$_7$, an ordered spin-ice [@Mirebeau05]. In 