  We examine entanglement of thermal states for spin-12 dimers in external
magnetic fields. Entanglement transition in the temperature-magnetic field
plane demonstrates a duality in spin-spin interactions. This identifies dual
categories of symmetric and anti-symmetric dimers with each category classified
into toric entanglement classes. The entanglement transition line is preserved
from each toric entanglement class to its dual toric class. The toric
classification is an indication of topological nature of the entanglement.
