Effects of resonant interface states on tunneling magnetoresistance

Abstract

Based on model and ab initio calculations we discuss the effect of resonant interface states on the conductance of epitaxial tunnel junctions. In particular we show that the ''hot spots'' found by several groups in ab initio calculations of symmetrical barriers of the k ʈ -resolved conductance can be explained by the formation of bonding and antibonding hybrids between the interface states on both sides of the barrier. If the resonance condition for these hybrid states is met, the electron tunnels through the barrier without attenuation. Even when both hybrid states move together and form a single resonance, strongly enhanced transmission is still observed. The effect explains why, for intermediate barrier thicknesses, the tunneling conductance can be dominated by interface states, although hot spots only occur in a tiny fraction of the surface Brillouin zone. DOI: 10.1103/PhysRevB.65.064425 PACS number͑s͒: 72.25.Ϫb, 73.20.Ϫr, 73.40.Rw, 73.40.Gk The tunneling magnetoresistance ͑TMR͒ of magnetic tunnel junctions consisting of ferromagnet͉insulator͉ferro-magnet layers has attracted a strong scientific interest, partly due to their potential application as magnetic random access memories. Miyazaki and Tezuka 1 and Moodera et al. 2 were able to obtain TMR ratios up to 20% in room-temperature experiments and recently room-temperature values of more than 50% were reported by various groups. The understanding of the TMR and of the electronic structure has not progressed equally quickly. Model calculations 3,4 have shed light on various aspects of the effect, but only recently have ab initio calculations of the electronic structure and the spindependent transport been reported. [5][6][7][8] In this paper, we will consider the tunneling through epitaxial junctions, which are characterized by two-dimensional periodicity. Here recent ab initio calculations of the k ʈ -resolved conductance show a very interesting phenomenon: for certain discrete k ʈ values ''hot spots'' or ''spikes'' appear in the transmitted intensity, showing that electrons with such k ʈ values can apparently tunnel through the junction with no or very little attenuation while all other states are very strongly damped. [9][10][11] This effect occurs only in the minority band of the ferromagnet and only for ferromagnetic coupling. If present, it can dominate the tunnel characteristics for intermediate thicknesses. For large thicknesses, in the asymptotic limit, the behavior is determined by the complex band structure of the insulator, 12 i.e., by those metal-inducedgap states, which have the smallest imaginary part of the perpendicular component k z of the Bloch vector. An example for such hot spots is given in Fig. 1, showing the results of ab initio Korringa-Kohn-Rostoker calculations for a junction consisting of two fcc Co͑001͒ half-crystals separated by 4 monolayers ͑ML͒ of vacuum. The results are based on density-functional theory in the local-density approximation and the Landauer formula for the conductance. We have chosen a vacuum layer as the ...

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

“…4,[16][17][18] This is spatially localized at the interface between Fe and MgO and it is resonating at E F . A comparison between the bulk Fe DOS and the DOS for the interface Fe layer ͓Figs.…”

Effect of interface bonding on spin-dependent tunneling from the oxidized Co surface

“…2͑b͔͒ has a narrow crown-shaped ''hot ring'' around the edge of the Fermisurface hole. The analysis of layer and k ʈ -resolved density of states ͑DOS͒ shows that it is not associated with surface states, 26 but is related to the enhancement of bulk k ʈ -resolved DOS near the Fermi-surface edge ͓compare Fig. 2͑b͒ with Fig.…”

Korringa-Kohn-Rostoker Green-function formalism for ballistic transport

“…In recent papers we have applied the method presented here to spin-dependent transport in ferromagnet/ semiconductor systems, 29,30 tunneling through vacuum, 48 and atomic-sized contacts. 45 In this section we confine the calculations to the simplest examples, in order to demonstrate the convergence properties of the method.…”