Enhanced magnetoresistance by monoatomic roughness in epitaxial Fe/MgO/Fe tunnel junctions

Abstract

Interfacial effects on spin and symmetry filtering in single-crystal Fe(001)/MgO/Fe magnetic tunnel junctions are investigated with the insertion of a Fe monoatomic step at the bottom MgO interface. After annealing, the atomically flat bottom electrode is covered by a fractional part of a Fe monoatomic layer resulting in two-dimensional Fe islands that are separated for low coverages and percolated above around half a monolayer. The magnetotransport properties of the junctions are studied as a function of this Fe sublayer coverage that is varied from 0 to 1 monolayer. Surprisingly, the magnetoresistance ratio exhibits a maximum for a coverage around half a monolayer. Tunneling spectroscopy experiments performed at low temperature allow relating this result to the decrease of the contribution of the interfacial resonance state to the conductance of the junction.

Metalliclike behavior of the exchange coupling in (001) Fe/MgO/Fe junctions

“…In a previous study performed with tunnel transport, a partial quenching of the Fe surface state with atomic islands has been observed [32]. This quenching could indeed increase the density of the conducting electrons, and then corroborate the stronger metallic behavior observed in the present study for the atomic rough interface.…”

“…Nevertheless, and even considering difficult performing statistic measurements in TEM experiments, we do not evidence a higher density of such defects in junctions with Fe islands. These observations corroborate the growth study performed with RHEED [32]. Even with thin MgO layers (3 ML), the thickness of the barrier remains homogeneous when deposited on a Fe surface decorated with atomic Fe islands.…”

“…Finally, we point out is that the graduate properties of the MgO layer on Fe (100) almost coincide with its specific epitaxy: the MgO is totally strained by the underlying buffer, as shown here by HRTEM and previous studies [32,45] until about 5 ML.…”

“…shown that the plastic relaxation occurs in a similar way in both cases for a larger MgO thickness [32]. Moreover, from this study [32], an average island size of 6 nm could be estimated.…”

Giant Tunnel Magnetoresistance with a Single Magnetic Phase-Transition Electrode

“…Here in the case of MPT-TMR, as one may expect the interface roughness may diminish the IRS and therefore it should be detrimental to MPT-TMR and even the sign of MPT-TMR can be reversed. However, a recent work on Fe/MgO/Fe junction shows that the TMR has been surprisingly enhanced with the increasing density of step roughness at Fe/MgO interface [47]. Except the interface disorder, we also consider other possible structural defects for instance the oxygen vacancy inside of MgO barrier, the possible shift of Fermi energy position.…”