transition. In the single crystal, with the field applied along \[100\], the hysteresis is smaller (less than 5 Oe width at 80 mK).

These results are consistent with the ZFC-FC measurements. They imply that the domain pinning is quite weak and that the reversal of the magnetization in an applied field mainly occurs by a continuous rotation of the moments.

Determination of the spontaneous magnetization
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In a ferromagnetic LRO phase, the ferromagnetic ordered component can be associated with the spontaneous magnetization which is in turn deduced from magnetization curves $M$ vs $H$, provided that the field is applied in the direction of the ordered moment.

In [Yb$_2$Ti$_2$O$_7$]{}, the determination of this ordered component appears especially difficult, since the magnetic structure of the LRO phase is not known precisely.

To address this question, we have analysed our $M$ vs $H$ magnetisation curves for the powders and for the single crystal measured along the \[100\] direction, and using a less comprehensive data along the \[110\] direction, as we did not perform precise measurements at very low fields. It is worth noting that the magnetization as a function of field deviates very quickly from the demagnetization line ($=H/N$), in agreement with the 