Dissipation phenomena and magnetic phase diagram of Cu$_2$OSeO$_3$ below 50 K

The magnetic phase diagram of the chiral magnet Cu$_2$OSeO$_3$ has been investigated by dc magnetisation and ac susceptibility over a very large frequency range from 0.1 Hz up to 1 kHz and temperatures below 50 K. Qualitatively different phase diagrams have been obtained by applying the magnetic field along the easy $langle100rangle$ or the hard $langle110rangle$ crystallographic directions. Strong $chi$ appears close to $B_{c2}$, but only below 30 K and is more pronounced when $B$ is applied along $langle100rangle$. In addition, the transition from the helical to the conical phase leads to two adjacent $B_{c1}$ lines below 50 K. The frequency dependence of $chi$ indicates the existence of very broad distributions of relaxation times both at $B_{c1}$ and $B_{c2}$. The associated very long characteristic times eventually prevent the system from reaching thermal equilibrium at low temperatures and lead to thermal hysteresis.