  The magnetic properties of a trigonal prism unit of the spin-2 frustrated
compound Ca3Co2O6 are studied by means of the density-matrix renormalization
group method. A magnetization plateau at $ms/3$ ($ms$ is the saturation
magnetization) with ferrimagnetic structure is observed. By fitting the
experimental data of magnetic curve, an estimation of the couplings gives
J1=-26.84K, J_{2}=0.39K, and J_{3}=0.52K. The local magnetic moments are
unveiled to exhibit an incommensurate sinusoidally modulation along the three
chains of the trigonal prism, which gives a strong theoretical support to the
experimentally observed incommensurate partially disordered antiferromagnetic
state for Ca3Co2O6. The present result suggests that the modulation indeed
originates from the competition of antiferromagnetic and ferromagnetic
couplings.
