  Injection of spins into semiconductors is essential for the integration of
the spin functionality into conventional electronics. Insulating layers are
often inserted between ferromagnetic metals and semiconductors for obtaining an
efficient spin injection, and it is therefore crucial to distinguish between
signatures of electrical spin injection and impurity-driven effects in the
tunnel barrier. Here we demonstrate an impurity-assisted tunneling
magnetoresistance effect in nonmagnetic-insulator-nonmagnetic and
ferromagnetic-insulator-nonmagnetic tunnel barriers. In both cases, the effect
reflects on/off switching of the tunneling current through impurity channels by
the external magnetic field. The reported effect, which is universal for any
impurity-assisted tunneling process, finally clarifies the controversy of a
widely used technique that employs the same ferromagnetic electrode to inject
and detect spin accumulation.
