the samples [@Yaouanc11; @Ross11b] so that the presence of a transition towards a long-range order might depend on the sample quality.

It has been suggested that the specific heat anomaly, however, does not necessarily correspond to a magnetic ordering [@Ross11b; @DOrtenzio13]. It is therefore essential to probe another thermodynamic quantity, which should be more sensitive to the magnetic nature of the transition: the magnetization. In this letter, we show that the magnetization of [Yb$_2$Ti$_2$O$_7$]{} presents a first-order transition in both a powder sample and a single crystal which was shown to develop additional magnetic intensity on structural peaks [@Chang12]. The first-order nature of the transition suggested in previous studies [@Hodges02; @Chang12; @DOrtenzio13] is proved by the existence of a small thermal hysteresis (of a few millikelvin width). The transition is accompanied by strong time-dependent effects. The magnetization value below the transition temperature is consistent with the stabilization of a ferromagnetic ordering with a reduced spontaneous moment, suggesting a strongly fluctuating spin component. Significantly the first-order behavior occurs below the peak in the specific heat where only a deviation in the susceptibility is observed.

Three samples were measured: i) a compacted powder sample, ii) a crushed powder sample mixed with Cu 