Shot noise in Co/Al 2O 3〈M〉/Py (M=Cr, Si) magnetic tunnel junctions

Abstract

Low-frequency noise, including 1/ f and shot noise (SN), and dynamic tunneling resistance have been studied in Co (80 Å)/Al 2O 3 (14 Å)/Py (100 Å) magnetic tunnel junctions (MTJs) without doping and with Cr or Si δ-doping of the insulating barrier. The fluctuations in voltage were measured at frequencies (200< f<2000 Hz), temperatures below 10 K and biases under 150 mV. For the undoped MTJs with the smallest (<10 kΩ) and the highest (>100 kΩ) tunneling resistances, the Fano factor shows “full” SN corresponding to uncorrelated tunneling ( F=1). The SN is reduced for the intermediate resistances with F≈0.65–0.8 indicating correlated electron tunneling, most probably through the localized states formed by the defects inside the barrier. The SN is enhanced for the antiparallel alignment of the ferromagnetic electrodes in the MTJs with tunneling weakly affected by spin-flip scattering. A model, which considers trap-assisted sequential tunneling, qualitatively explains the main experimental results. Cr-doped MTJs typically showed Fano factor close to unity, probably due to some growth condition-induced asymmetry in Cr distribution. Incorporation of a Si δ-layer permits observation of an enhancement of the low-frequency noise, probably induced by Coulomb blockade effects.