  We analyze a model of double quantum dot Cooper pair splitter coupled to two
ferromagnetic detectors, and demonstrate the possibility of determination of
spin correlation by current measurements. We use perturbation theory taking
account of the exchange interaction with the detectors, which leads to complex
spin dynamics in the dots. This affects the measured spin and restricts the use
of ferromagnetic detectors to the nonlinear current-voltage characteristic
regime at current plateau, where the relevant spin projection is conserved, in
contrast to the linear current-voltage characteristic regime, in which the spin
information is distorted. Moreover, we show that for separable states the spin
correlation can only be determined in a limited parameter regime, much more
restricted than in the case of entangled states. We propose an entanglement
test based on the Bell inequality.
