Low frequency noise in Co/Al2O3⟨δ(Fe)⟩/Ni80Fe20 magnetic tunnel junctions

Abstract

The time dependences, up to 200 s, and the noise power spectrum (0.005-10 Hz) in the electron transport response at bias up to 300 mV of Co/Al2O3/Ni80Fe20 magnetic tunnel junctions (MTJs) and of Co/Al2O3⟨δ(Fe)⟩/Ni80Fe20 (with Fe δ dopants of thickness1.8 Å inside the barrier) were investigated. The magnetic field waschanged between +100 and -100 G in steps of 1 G. The measurements werecarried out at different temperatures between 77 and 300 K for the sampleswith large tunnel magnetoresistance (exceeding 14% at 300 K). We foundthat the magnetization reversal of the Co and permalloy electrodes, asdetected from the time response near the coercive field, occurs viarelaxation on the timescale of about 102 s with sudden jumps in theresistance (ΔR/R~10-2-10-3). We link this noise tothe depinning of the domain walls. In addition to the magnetic noise, insome of the studied MTJs with Fe δ dopants, we observed atwo-level-system telegraph-type noise, which was independent of themagnetic field, indicating its relation to the trapped charges inside theinsulating barrier. For MTJs, the noise power spectrum has 1/fαcharacter for a wide frequency range below a few Hz. At low bias and parallel state the exponent α is close to 1-1.5, but at higher bias or inthe antiparallel state the exponent increases to 2. We link these effects tonon-equilibrium noise in magnetic structure of the electrodes in the antiparallelstate and to non-equilibrium transport inside the barrier at high bias.