Scaling with MgO thickness of the proximity exchange field in a Fe/MgO/Si contact

The scaling of the proximity exchange field, which ferromagnetic Fe exerts on the spin accumulation at the MgO/Si interface of Fe/MgO/Si tunnel contacts, was investigated using the inverted Hanle effect. Data as a function of the applied bias voltage shows that the exchange field varies with bias in the same way as the square of the tunnel spin polarization (TSP). The decay of the exchange field with increasing MgO thickness tMgO is rather weak and nonexponential, which we argue to be related to the improved epitaxial quality of the MgO, and the, consequently, improved spin-filtering properties at larger MgO thickness. Using the TSP data to correct for this yields an exponential decay of the exchange field with tMgO with a decay length of lambda = -0.42 nm. Comparing this to the decay length for the tunnel conductance (lambda = -0.21 nm) suggests that the exchange interaction is dominated by two-step tunneling processes.