SHOT NOISE SUPPRESSION IN INDIVIDUAL AND SERIES ARRAYS OF MAGNETIC TUNNEL JUNCTIONS

Charge-current shot noise is investigated in single magnetic tunnel junctions and devices having multiple junctions that are connected in series. The ratio of the measured shot noise in single junctions to the expected Poisson value, namely the Fano factor, F, is observed to vary from 1 to well below 0.5. Deviations from F = 1 are attributed to localized states (defects) located in the tunnel barrier or at the interfaces with the magnetic electrodes. For series arrays of junctions, the Fano factor scales inversely with the number (1 <= N <= 30) of junctions in series, even for junctions exhibiting sub-Poissonian (F < 1) shot noise. The 1/N scaling is consistent with the incoherent tunneling of electrons across junctions and indicates that each junction behaves as an individual noise source. The advantages of incorporating series arrays of magnetic tunnel junctions into devices for magnetic field sensing are discussed.