  Manganese phosphide MnP has been investigated for decades because of its rich
magnetic phase diagram. It is well known that the MnP exhibits the
ferromagnetic phase transition at $\Tc=292$ K and the helical magnetic phase
below $\TN=47$ K at zero field. Recently, a novel magnetic phase transition was
observed at $T^* = 282$ K when the magnetic field is lower than 5 Oe. However,
the nature of the new phase has not been illuminated yet. In order to reveal
it, we performed the AC and the DC magnetization measurements for a single
crystal MnP at very low field. A divergent behavior of the real and the
imaginary part of the AC susceptibility and a sharp increase of the DC
magnetization was observed at $T^*$, indicating the magnetic phase transition
at $T^*$. Furthermore a peculiar temperature hysteresis was observed: namely,
the magnetization depends on whether cooling sample to the temperature lower
than $\TN$ or not before the measurements. This hysteresis phenomenon suggests
the complicated nature of the new phase and a strong relation between the
magnetic state of the new phase and the helical structure.
