  The incipient quantum phase transitions of relevance to nonzero fluctuations
and entanglement in Heisenberg clusters are studied in this paper by exploiting
negativity as a measure in bipartite and frustrated spin-1 anisotropic
Heisenberg clusters with bilinear-biquadratic exchange, single-ion anisotropy
and magnetic field. Using the exact diagonalization technique, it is shown that
quantum critical points signaled by qualitative changes in behavior of
magnetization and particle number are ultimately related to microscopic
entanglement and collective excitations. The plateaus and peaks in spin and
particle susceptibilities define the conditions for a high/low-density quantum
entanglement and various ordered phases with different spin (particle)
concentrations.
