grease to ensure a good thermal coupling, both obtained from the same synthesis; iii) a single crystal, with the magnetic field applied along the \[100\] and \[110\] axes. These samples are the same as those used in Ref. (and Ref. in the case of the single crystal). Magnetization and a.c. susceptibility measurements were performed down to 70 mK on SQUID magnetometers equipped with dilution refrigerators developed at the Institut Néel [@Paulsen01]. At 70 mK, the magnetization of all the samples reaches about 1.7 $\mu_B$/Yb above 60 kOe [@SupMat]. The susceptibility follows a Curie-Weiss law down to 1.5 K [@SupMat], consistent with previous results above 2 K [@Bramwell01; @Hodges01; @Yasui03; @Thompson11a]. Below 1.5 K, the susceptibility still increases with the temperature decreasing but deviates from the Curie-Weiss law. Then at 250 mK for the powder (165 mK for the single crystal), the magnetization $M$ increases abruptly and reaches a plateau at low temperature. This very sharp increase immediately suggests a magnetic transition, especially as it matches approximately with the anomaly in specific heat [@Chang13].

In an ordered ferromagnetic state, a system has a spontaneous magnetization and the initial intrinsic susceptibility is expected to diverge. The measured value of the bulk susceptibility 