Shot noise in magnetic tunnel junctions from first principles

Abstract

We compute the shot noise in ballistic and disordered Fe|MgO|Fe tunnel junctions by a wave-function-matching method. For tunnel barriers with 5 atomic layers we find a suppression of the Fano factor as a function of the magnetic configuration. In the antiparallel configuration the shot noise is full up to a threshold bias that indicates the onset of resonant tunneling. We find excellent agreement with recent experiments when interface disorder is taken into account. The statistics of electron transport in mesoscopic systems has been the subject of intensive research in the past decades, leading to important and useful insights.1,2 In a two-terminal conductor with a time-dependent current I (t), the simplest measure is the noise poweriωt dt, where I (t) ≡ I (t) − I denotes the instantaneous fluctuation from the average current and · · · is a time and statistical average. The shot noise S is the zero frequency limit of the noise power when the applied voltage |eV | is sufficiently larger than the thermal energy k B T . The classical shot noise characterized by an uncorrelated Poissonian process is given by the Schottky formula S = 2e I .3 Shot noise contains information about the charge of the elementary excitations, entanglement, the wave versus particle nature of electron transport, and provides a diagnostic for open transport channels. 4 Magnetic tunnel junctions (MTJs) with MgO barriers 5,6 have great potential for applications in magnetic random access memory elements and high-frequency generators. [7][8][9][10] Band structure calculations of isomorphic Fe|MgO|Fe layered structures predicted a large drop in the electric resistance when the relative magnetization direction of the two ferromagnets switches from antiparallel to parallel. 11,12 The subsequently observed large Tunnel Magnetoresistance Ratio (TMR) 5,6 can be explained in terms of the symmetry matching of only the majority-spin states in Fe with the 1 band of MgO, which is by far the least evanescent in the gap. The tunneling ratio of the majority-spin electrons is therefore relatively high while minority-spin states are efficiently filtered out by the MgO barrier. However, a quantitative first-principles description of transport in magnetic tunneling junctions is complicated by defects. The chemical composition of the interface strongly affects the TMR, [13][14][15] and various interfacial defects have been identified to reduce the TMR. [16][17][18] The I-V curves alone cannot discriminate between the possible different origins that reduce the TMR.According to conventional wisdom, shot noise in tunnel junctions is classical, 1 in agreement with earlier experiments.19-21 Recent evidence that shot noise in MTJs is suppressed in the parallel configuration was very surprising. 22In order to resolve this issue we present parameter-free calculations of shot noise in magnetic tunnel junctions. We compute sub-Poissonian shot noise for the parallel magnetic configuration and explain the results in terms of highly transmitting resonant tunneling between stat...

Localized states in advanced dielectrics from the vantage of spin- and symmetry-polarized tunnelling across MgO

“…In this spirit, we can thus ascribe local transport on the electrical hotspot and background of MgO barriers that are sputter-grown with 0% (10%) oxygen in the argon plasma 27 to M and F/F þ (F/F þ and conduction or VB edges) centres. Electrical noise 26,66 and memristance 32,34 studies across MgO-based MTJs can immediately benefit from the clarified defect-mediated tunnelling potential landscape that we propose here. Research groups that can smoothly vary the MgO barrier thickness across a series of MTJs (for example, ref.…”

“…The impact of intrinsic effects such as magnon and phonon generation on TMR has been documented 18,19 . On the other hand, the impact of structural defects in MgO on tunnelling is only emerging in this otherwise widely studied, canonical spintronic system [20][21][22][23][24][25][26] . Indeed, defects have been qualitatively suspected of accounting for much lower tunnelling barrier heights than expected 12,27 .…”

Shot noise in magnetic double-barrier tunnel junctions

“…14,15 It has also been shown experimentally that the Fano factor in a single-barrier magnetic tunnel junction depends on magnetic configuration of the junction, and can be remarkably enhanced for the antiparallel alignment of the electrodes' magnetizations. [16][17][18][19][20] Even more possibilities have been found in double-barrier tunnel junctions with two ferromagnetic electrodes and one ferromagnetic central layer, 21,22 where one can distinguish four magnetic configurations corresponding to different alignments of the magnetic moments of all three magnetic layers. 23 It has also been shown that a simple model based on two well-separated spin channels for electronic transport cannot properly describe the experimental observations without taking into account spin-flip transitions.…”

Controlling Shot Noise in Double-Barrier Magnetic Tunnel Junctions

“…As for MTJs with MgO barriers, full SN (F ¼ 1) independent of the magnetic state was observed in epitaxial Fe=MgO=Fe [19]. Then, the noise was examined for ultrathin (less than 1 nm) MgO barriers, where F ' 0:92 was observed in the parallel state [20,21]. Double-barrier magnetic tunnel junctions (DMTJs), with either nanoparticles [22,23] or a continuous magnetic layer as the central electrode [24], have some advantages in comparison with MTJs.…”