  To clarify the instability of the ferrimagnetism which is the fundamental
magnetism of ferrite, numerical-diagonalization study is carried out for the
two-dimensional S=1/2 Heisenberg antiferromagnet with frustration. We find that
the ferrimagnetic ground state has the spontaneous magnetization in small
frustration; due to a frustrating interaction above a specific strength, the
spontaneous magnetization discontinuously vanishes so that the ferrimagnetic
state appears only under some magnetic fields. We also find that, when the
interaction is increased further, the ferrimagnetism disappears even under
magnetic field.
