  Ground-state phase diagrams and magnetization curves of a spin-1 Heisenberg
diamond cluster with two different coupling constants and uniaxial single-ion
anisotropy are investigated in a presence of the external magnetic field with
the help of exact diagonalization methods. It is shown that the spin-1
Heisenberg diamond cluster exhibits several remarkable quantum ground states,
which are manifested in zero- and low-temperature magnetization curves as
intermediate plateaus at one-quarter, one-half and three-quarters of the
saturation magnetization. It is found that the width of the fractional
magnetization plateaus depends basically on a relative strength of the coupling
constants as well as uniaxial single-ion anisotropy, which may substantially
shrink or even cause full breakdown of some intermediate magnetization
plateaus. It is evidenced that a relatively weak uniaxial single-ion anisotropy
of the easy-axis type considerably improves a theoretical fit of
low-temperature magnetization curves of the tetranuclear nickel complex
[Ni4(CO3)2(aetpy)8](ClO4)4 in a low-field region without spoiling the previous
fit based on the fully isotropic Heisenberg model in a high-field region.
