  The measure of quantum entanglement is determined for any dimer, either
ferromagnetic or antiferromagnetic, spin-1/2 Heisenberg systems in the presence
of external magnetic field. The physical quantity proposed as a measure of
thermal quantum entanglement is the distance between states defined through the
Hilbert-Schmidt norm. It has been shown that for ferromagnetic systems there is
no entanglement at all. However, although under applied magnetic field,
antiferromagnetic spin-1/2 dimers exhibit entanglement for temperatures below
the decoherence temperature -- the one above which the entanglement vanishes.
In addition to that, the decoherence temperature shows to be proportional to
the exchange coupling constant and independent on the applied magnetic field,
consequently, the entanglement may not be destroyed by external magnetic fields
-- the phenomenon of {\it magnetic shielding effect of quantum entanglement
states}. This effect is discussed for the binuclear nitrosyl iron complex
[Fe$_2$(SC$_3$H$_5$N$_2$)$_2$(NO)$_4$] and it is foreseen that the quantum
entanglement survives even under high magnetic fields of Tesla orders of
magnitude.
