  We study the decay of entanglement of quantum dot electron-spin qubits under
hyperfine interaction mediated decoherence. We show that two qubit entanglement
of a single entangled initial state may exhibit decay characteristic of the two
disentanglement regimes in a single sample, when the external magnetic field is
changed. The transition is manifested by the supression of time-dependent
entanglement oscillations which are superimposed on the slowly varying
entanglement decay related to phase decoherence (which result in oscillatory
behaviour of entanglement sudden death time as a function of the magnetic
field). This unique behaviour allows us to propose the double quantum dot
two-electron spin Bell state as a promising candidate for precise measurements
of the magnetic field.
