  High resolution ultrasonic velocity measurements have been used to determine
the temperature -- magnetic-field phase diagram of the monoclinic multiferroic
CuO. A new transition at TN3 = 230 K, corresponding to an intermediate state
between the antiferromagnetic non-collinear spiral phase observed below TN2 =
229.3 K and the paramagnetic phase, is revealed. Anomalies associated with a
first order transition to the commensurate collinear phase are also observed at
TN1 = 213 K. For fields with B along the b axis, a spin-flop transition is
detected between 11 T - 13 T at lower temperatures. Moreover, our analysis
using a Landau-type free energy clearly reveals the necessity for an
incommensurate collinear phase between the spiral and the paramagnetic phase.
This model is also relevant to the phase diagrams of other monoclinic
multiferroic systems.
