Interfacial electronic transport phenomena in single crystalline Fe-MgO-Fe thin barrier junctions

Abstract

Équipe 101 : Nanomagnétisme et électronique de spinInternational audienceSpin filtering effects in nano-pillars of Fe-MgO-Fe single crystalline magnetic tunnel junctions are explored with two different sample architectures and thin MgO barriers (thickness: 3-8 monolayers). The two architectures, with different growth and annealing conditions of the bottom electrode, allow tuning the quality of the bottom Fe/MgO interface. As a result, an interfacial resonance states (IRS) is observed or not depending on this interface quality. The IRS contribution, observed by spin polarized tunnel spectroscopy, is analyzed as a function of the MgO barrier thickness. Our experimental findings agree with theoretical predictions concerning the symmetry of the low energy (0.2 eV) interfacial resonance states: a mixture of Delta(1)-like and Delta(5)-like symmetries

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

“…Here, the tunneling propagation of Bloch states with k // = 0 and various symmetries is allowed across the MgO(001) barrier and then the complete activation of the surface state is achieved (all its spectral components contribute to tunneling). Moreover, the surface state is indeed observed in the case of a very flat interface, and is completely missing in the case of a rough interface [21]. In the case of the flat interface, in the asymptotic regime corresponding to thick MgO barriers, this surface state has less effect on tunneling than in the case of a thin barrier because of a symmetry-dependent attenuation rate across the (001)MgO barrier [21].…”

“…Moreover, the surface state is indeed observed in the case of a very flat interface, and is completely missing in the case of a rough interface [21]. In the case of the flat interface, in the asymptotic regime corresponding to thick MgO barriers, this surface state has less effect on tunneling than in the case of a thin barrier because of a symmetry-dependent attenuation rate across the (001)MgO barrier [21]. Consequently, no reversal of the TMR is observed, the only effect being a steeper decrease of the TMR with the applied voltage.…”

“…This surface state lies in the minority band, and the enhancement of its 1 spectral density with the presence of carbon leads to a sign reversal of the TMR for a specific bias which activates the conduction channel associated to this surface state. More recently, this reversal has been observed even with an undoped Fe/MgO interface in the thin barrier regime [21]. Here, the tunneling propagation of Bloch states with k // = 0 and various symmetries is allowed across the MgO(001) barrier and then the complete activation of the surface state is achieved (all its spectral components contribute to tunneling).…”

“…It has been previously attributed to the interfacial resonance state surviving in MTJ stacks with a high-quality bottom interface [31]. Previous studies have shown that it can be tuned by interface quality [21] or chemical doping [18]. It is located in the minority band and arrives in the¯ point just above the Fermi level.…”

“…This interface state then dominates the voltage dependence of the antiparallel conductance at negative voltage when it is activated for tunneling. It plays a major role in the case of low MgO thickness, as it is responsible for a reversal of the TMR [21]. We focus now on the differences between the three bottom Fe/MgO interfaces corresponding to different Fe underlayer coverage ratios.…”