  By using the concept of negativity, we investigate entanglement in (1/2,1)
mixed-spin Heisenberg systems. We obtain the analytical results of entanglement
in small isotropic Heisenberg clusters with only nearest-neighbor (NN)
interactions up to four spins and in the four-spin Heisenberg model with both
NN and next-nearest-neighbor (NNN) interactions. For more spins, we numerically
study effects of temperature, magnetic fields, and NNN interactions on
entanglement. We study in detail the threshold value of the temperature, after
which the negativity vanishes.
