  A magnetization process in two-dimensional ferrimagnet BIPNNBNO is analyzed.
The compound consists of ferrimagnetic (1,1/2) chains coupled by two sorts of
antiferromagnetic interactions. Whereas a behavior of the magnetization curve
in higher magnetic fields can be understood within a process for the separate
ferrimagnetic chain, an appearance of the singlet plateau at lower fields is an
example of non-Lieb-Mattis type ferrimagnetism. By using the exact
diagonalization technique for a finite clusters of sizes 4x8 and 4x10 we show
that the interchain frustration coupling plays an essential role in
stabilization of the singlet phase. These results are complemented by an
analysis of four cylindrically coupled ferrimagnetic (1,1/2) chains via an
abelian bosonization technique and an effective theory based on the XXZ
spin-1/2 Heisenberg model when the interchain interactions are sufficiently
weak/strong, respectively.
