  Magnetic and magnetocaloric properties of a spin-1 Heisenberg diamond cluster
with two different coupling constants are investigated with the help of an
exact diagonalization based on the Kambe's method, which employs a local
conservation of composite spins formed by spin-1 entities located in opposite
corners of a diamond spin cluster. It is shown that the spin-1 Heisenberg
diamond cluster exhibits several intriguing quantum ground states, which are
manifested in low-temperature magnetization curves as intermediate plateaus at
1/4, 1/2 and 3/4 of the saturation magnetization. Besides, the spin-1
Heisenberg diamond cluster may also exhibit an enhanced magnetocaloric effect,
which may be relevant for a low-temperature refrigeration achieved through the
adiabatic demagnetization. It is evidenced that the spin-1 Heisenberg diamond
cluster with the antiferromagnetic coupling constants J1/kB = 41.4 K and J2/kB
= 9.2 K satisfactorily reproduces a low-temperature magnetization curve
recorded for the tetranuclear nickel complex [Ni4(CO3)2(aetpy)8](ClO4)4 (aetpy
= 2-aminoethyl-pyridine) including a size and position of intermediate plateaus
detected at 1/2 and 3/4 of the saturation magnetization. A microscopic nature
of fractional magnetization plateaus observed experimentally is clarified and
interpreted in terms of valence-bond crystal with either a single or double
valence bond. It is suggested that this frustrated magnetic molecule can
provide a prospective cryogenic coolant with the maximal isothermal entropy
change - Delta S = 10.6 J/(K.kg) in a temperature range below 2.3 K.
