Figure 1. (a) Sketch of the principle behind BENS, presented for the Anti-Parallel state, where EC corresponds to the mobility edge. (b) The energies of these defect states can be inferred from the I-V curve of the sample, and its first (dynamic conductance) and second (IETS) derivatives, but they are detected in a much clearer way though low frequency measurements. (c) Sketch of a band edge (Δ1, Δ5) contribution to the tunneling at −1.2 V.
The interfacial structures between magnetic/non-magnetic films play a crucial rule in spintronic devices. In particular, Tunneling magneto-resistance (TMR) is extremely sensitive to the band structure of ferromagnet/insulator interfaces. In this sense, a number of experimental and theoretical work were devoted to investigate the influence of interfacial structure on TMR, namely in Fe/MgO/Fe magnetic tunnel junctions. For this purpose, we introduce a conceptually new way to gather information on the electron bands of buried metal (semiconductor) /insulator interfaces.
Buried metal (semiconductor)/insulator interfaces are found at the heart of electronics. The current in tunneling devices is determined by the bias, barrier and density of states of the electrodes. Electron states not allowed in bulk could become permitted at the surface leading to topological or interface resonant states.
In this study we investigate the bias dependence of conductance and low frequency noise (LFN) in single barrier tunneling devices in order to determine in-situ the energies of the band edges of the buried interfaces. We unambiguously demonstrate the validity of the band edge noise spectroscopy (BENS) concept by studying seminal Fe/MgO/Fe MTJs with partial doping of the bottom electrode (Fe) with Vanadium (V). Such substitution has been shown to reduce defect states inside the MgO barrier due to improved interface matching between Fe1−xVx and MgO in Fe1−xVxMgO/Fe MTJs. Our simulations confirm that tunneling of band tail electrons, influenced by spin orbit interactions, are responsible for the observed LFN anomalies.
The band edge noise spectroscopy concept permits an investigation of the electron band edges in a wide class of tunneling devices. We demonstrated successfully BENS approach in epitaxial magnetic tunnel junctions. The dependence of the BENS on the relative magnetic alignment of the electrodes allows us to estimate the importance of interband hybridization and spin flips at the FM/I interface. Given the crucial importance of buried interfaces in solid-state devices, the clear need to understand their electronic structure and the limited options available, our work presents a substantial advance in the field of characterizing buried interfaces.
The authors acknowledge A. Gomez-Ibarlucea, D. Herranz, and F. Bonell for their help with the experiments and sample growth. This work has been supported by the Spanish MINECO (MAT2012-32743) and Comunidad de Madrid (P2009/MAT-1726) Grants.
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