Experimental evidence of interface resonance states in single-crystal magnetic tunnel junctions

Abstract

All solid-state tunnel spectroscopy experiments performed on single-crystal Fe/MgO/Fe magnetic tunnel junctions show sharp features at 0.2 and 1.1 V. These peaks are observed on the electrical differential conductivity only in the antiparallel magnetic configuration and only for the voltage sign corresponding to the injection of electrons toward the bottom electrode. They are attributed to the conductivity of two different resonant states of the Fe͑001͒/MgO bottom interface. The analysis of the attenuation of these peaks as a function of the insulator thickness provides information on their symmetry. Nowadays, magnetic tunnel junctions ͑MTJs͒ benefit on a strong scientific interest 1 motivated both by their high potential for applications in sensor and storage devices and by the complex fundamental physics of spin dependent tunneling. Theoretical models of tunneling in single-crystal devices 2-4 have been successfully confronted to experimental observations of giant high tunneling magnetoresistance ͑TMR͒ in single-crystal MTJs involving MgO tunnel barrier. 5-8 In these systems, the large TMR effects are determined by the different tunneling mechanisms and symmetry-related decay rates of the Bloch waves for the majority-and the minorityspin channels within the barrier. According to Butler's theory, 2 along the k ʈ = 0 propagation direction, the transport properties of an epitaxial Fe/MgO/Fe stack are governed by the ⌬ i symmetry bulk density of states ͑DOS͒ of the Fe electrodes and their different decay rates into the barrier. For the parallel configuration the transport is governed by the majority ⌬ 1 states which have the lowest decay rate into the barrier. For the antiparallel configuration, there are no ⌬ 1 states available at the Fermi level, the conductance being dominated by the ⌬ 5 states. Moreover, the electronic properties at the metal/barrier interface have been often proven to be important in determining the TMR sign and amplitude. 9-11 Surface states may appear at the ferromagnet/insulator interface through the breakdown of the translational symmetry. 12,13 When coupled to the bulk, they become interfacial resonant states ͑IRSs͒ and are predicted to strongly affect and even dominate the tunnel transmission. 2,4,[14][15][16] Experimentally, the surface states have been already studied for the Fe͑001͒/vacuum system below 17 and above 18 the Fermi level. However, at the Fe͑001͒/MgO interface, the nature of the interfacial electronic structure and its role on the spin dependent transport through the MgO barrier are expected to be more complex. Even in a perfect system, the chemical bondings at the Fe/MgO interface affect the Fe͑001͒ electronic structure and modify the propagation by tunneling of existent IRS. Moreover, the cubic symmetry of the insulator will determine the tunneling attenuation of the IRS as a function of its symmetry. A direct analysis of the Fe͑001͒ surface states covered by MgO above and under the Fermi level ͑E F ͒ is difficult by standard scanning tunneling spectroscopy ͑unoccupied s...

Effect of correlations on electronic structure and transport across (001) Fe/MgO/Fe junctions

“…The local spin density approximation (LSDA) has been used to predict TMR in Fe/MgO/Fe system [13][14][15][16][17] . Reports are largely in agreement with each other but at variance with differential conductance measurements for thin MTJs 12,18 . The LSDA predicts a narrow band of interface resonance states (IRS) of minority electrons ( Fig.…”

Resonant electronic states andI−Vcurves of Fe/MgO/Fe(100) tunnel junctions

“…The most prominent feature in the S͑V͒ curves is the large broad peak at about 1.5 V, found only in the AP configuration and caused by the broad interface states associated to the ⌬ 1 ↓ band edge at the ⌫ point. In experiments such a peak in the AP configuration only is indeed found at about 1 V. 6,7 We note that for the I-V curves calculated in Ref. 19 a corresponding peak in S͑V͒ would appear also in the P configuration.…”

“…7 This is modeled at a simple level by adding a small imaginary part ␦ to the energy when calculating T͑E ; V͒; i.e., it corresponds to a uniform level broadening. 18 The effects of such addition on the I-V, S͑V͒, and TMR are shown in Fig.…”

“…8 Remarkably, at these small MgO thicknesses, the growth mode of the Fe/MgO/Fe stack changes. 7 We investigate theoretically the I-V characteristics of perfectly crystalline Fe/MgO MTJs and demonstrate that its features originate from the sweeping of the ⌬ 1 band edges and of interface states across the bias window. These generic features are emphasized here, as they resemble closely those studied in molecular devices 9 and magnetic point contacts.…”

“…For instance, highquality conventional MTJs ͑2-3 nm MgO thickness͒ show a pronounced broad peak on the S͑V͒ curve at about 1 V for the AP and a number of small peaks at lower voltages in the P configuration. 6,7 Combined with a quantitative theory these measurements can provide a wealth of information and help on the level of device design. We note that very thin MTJs, with a MgO thickness of about 1 nm, have to be used for ultrahigh recording density ͑Ͼ500 Gbit/ in.…”