  We present magnetostriction and thermal expansion measurements on
multiferroic (Ni0.93Co0.07)3V2O8. The high field phase diagrams up to 33 T
along the a, b and c directions are built. For H//a, as the magnetic field
increases, two intermediate phases appear between the incommensurate phase and
the paramagnetic phase at about 7 K, and then a magnetically induced phase
appears above the paramagnetic phase. For H//b,thermal expansion measurement
indicates a mutation in the spin lattice coupling of the high field phases. The
interlaced phase boundary suggests a mixed state in the optical high field
phase. For H//c, an intermediate phase between the commensurate phase and the
incommensurate phase is detected. A nonlinear boundary between the intermediate
phase and the low temperature incommensurate phase, and a clear boundary
between the commensurate phase and the paramagnetic phase are found. These
results indicate that doping Co2+ breaks the weak ferromagnetic moment of the
commensurate phase, which exists in the parent compound Ni3V2O8 and
(Ni0.9Co0.1)3V2O8. This nonlinear influence reflects complicated spin
modulation in Ni3V2O8 by doping Co2+.
