Conductance in Co/Al2O3/Si/Al2O3 permalloy with asymmetrically doped barrier

TL;DR

This study investigates conductance and noise in Co/Al2O3/Si/Al2O3 permalloy magnetic tunnel junctions with asymmetric silicon doping, revealing a transition from Coulomb blockade to different tunneling regimes as silicon layer thickness varies.

Contribution

It introduces a model based on the Larkin-Matveev approximation for tunneling through impurity layers, generalized to 3D, accounting for barrier shape variations and impurity localization effects.

Findings

Coulomb blockade breaks down for silicon layers thicker than 1.2 Å.

Tunneling magnetoresistance depends on bias voltage and temperature, with a stable zero-bias magnetoresistance.

Conductance crossover and noise characteristics indicate a monolayer coverage of silicon.

Abstract

Permalloy magnetic tunnel junctions. Complementary low frequency noise measurements are used to understand the conductance results. The obtained data indicate the breakdown of the Coulomb blockade for thickness of the asymmetric silicon layer exceeding 1.2\AA . The crossover in the conductance, the dependence of the tunnelling magnetoresistance with the bias voltage and the noise below 80K correspond to 1 monolayer coverage. Interestingly, the zero bias magnetoresistance remains nearly unaffected by the presence of the silicon layer. The proposed model uses Larkin-Matveev approximation of tunnelling through a single impurity layer generalized to 3D and takes into account the variation of the barrier shape with the bias voltage. The main difference is the localization of all the impurity levels within a single atomic layer. In the high thickness case, up to 1.8\AA, we have introduced a phenomenological parameter, which reflects the number of single levels on the total density of silicon atoms.